© 2001 by The Society for Integrative and Comparative Biology
Cues for Metamorphosis of Brachyuran Crabs: An Overview1
1 135 Duke Marine Lab Road, Beaufort, North Carolina 28516
2 Florida Institute of Technology, Department of Biological Sciences, 150 West University Blvd., Melbourne, Florida 32901
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The early life cycle of brachyuran crabs has a planktonic dispersal stage consisting of a variable number of zoeal larvae followed by the molt to the megalopa stage. Megalopae undergo horizontal transport to the settlement site where they settle out of the water column and metamorphose to the first crab (juvenile) stage. This review provides an overview of recent laboratory studies of cues that shorten or lengthen the time to metamorphosis (TTM) of the megalopa stage. Megalopae cannot delay metamorphosis indefinitely and have a temporal threshold beyond which metamorphosis occurs without habitat cues. The TTM can be shortened about 15–25% upon exposure to acceleration cues, which include chemical cues and odors from adult substrate, aquatic vegetation, biofilms, conspecifics, estuarine water, humic acids, related crab species, and potential prey. Cues shown to delay metamorphosis include ammonium, hypoxia, predator odor and extreme temperature and salinity conditions. There is no evidence that structural mimics of natural substrate affect TTM.
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INTRODUCTION |
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Most studies of invertebrate larval metamorphosis have been performed with species that are sedentary or sessile as adults. Emphasis has been placed on biofoulers, organisms that are important because they degrade the performance of commercially important structures, such as ship hulls and heat exchangers (e.g., Woods Hole, 1952
; Costlow and Tipper, 1984). The extensive knowledge of metamorphosis of biofoulers provides a context for discussing similar processes in less intensively studied groups and less well understood groups, such as brachyuran crabs.
Biofoulers are of two general types, those with a very short lived (minutes to hours) lecithotrophic planktonic dispersal stage that also functions as the settlement stage (e.g., Clooney and Torrence, 1984; Woolacott, 1984) and those with a longer lived (days to months) planktotrophic, planktonic dispersal stage (Scheltema and Carlton, 1984). The longer-lived feeding forms often change to a final lecithotrophic (e.g., barnacles) or planktotrophic (e.g., mussels) larval stage specialized for settlement in the adult habitat (Crisp, 1984). Usually, there is a period of time before metamorphosis in which the settlement stage is physiologically incapable of responding to settlement cues. When larvae become capable of responding to these cues, they are referred to as being "competent" (for review see Crisp, 1984).
Once larvae become competent, settlement and metamorphosis are frequently triggered by specific chemical and physical cues (Crisp, 1974, 1984; Burke, 1986; Butman, 1987; Pawlik, 1992; Hadfield and Paul, 2001). Settlement is defined as a behavioral process and includes movement out of the water column to a potential settlement site. Virtually all biofouling larvae respond to shear forces due to water moving past a surface and are small enough (often less that 500 µm in one major dimension) to move out of flow into the boundary layer over the surface (e.g., Crisp, 1984). Once in contact with the substrate, larvae exhibit "exploratory behaviors" which expose them to surface associated stimuli that include texture, surface chemistry, flow, vibration and pheromones (reviewed by Crisp, 1984; Rittschof et al., 1998). If sufficient stimuli are present, the physiological process of metamorphosis is initiated within the larvae. In most biofouling organisms, settlement and metamorphosis are so tightly coupled that there is some debate as to whether they are actually separate processes (Clare et al., 1992b).
Alternatively, competent larvae of many sessile invertebrate species do not progress toward metamorphosis if stimulatory cues are absent. They return to the plankton and repeat the process involved in locating and selecting suitable habitat. This capability is referred to as "delay of metamorphosis" (reviewed by Crisp, 1984) and provides an interval in which competent larvae of sessile invertebrate species essentially stop (Miller and Hadfield, 1990) or slow the developmental clock until a habitat that stimulates metamorphosis is encountered (reviewed by Pechenik, 1990). Delay of metamorphosis is thought to have high ecological costs because it extends the time in the plankton, and may or may not have physiological costs. For example, a delay in metamorphosis of barnacle cyprids induces a depression in the post-metamorphic growth rate (Pechenik et al., 1993).
Cues for settlement and metamorphosis of invertebrate species with motile benthic adults are less well studied. Among brachyuran crabs, most studies have occurred within the last decade and have focused on identifying cues that shorten or lengthen the time to metamorphosis (TTM). All of the brachyuran species studied thus far live in estuaries or along coasts and have the same general life cycle. They have planktotrophic, planktonic dispersal stages consisting of a variable number of zoeal larvae. They then molt to a specialized planktotrophic settlement stage (post-zoeal larva) called a megalopa, which is often referred to as post-larva in the literature.
Megalopae are similar to most biofouler larvae in that they respond to shear forces at boundaries (e.g., Welch et al., 1999). However, their dimensions are too large (1–4 mm) to enable them refuge within boundary layers used by the smaller larvae of biofoulers (Crisp, 1984). Megalopae are strong swimmers, which frequently undergo diel vertical migration and during selective tidal-stream transport actively leave and return to the water column in synchrony with tidal currents (e.g., De Vries et al., 1994). Thus, the settlement stage of brachyuran larvae is very different from larvae of most sessile or fouling organisms in that it is a large, swimming zooplankter that can influence horizontal transport by vertical swimming.
Within the brachyuran larval cycle there are pronounced morphological changes upon the molt from the last zoeal stage to the megalopal stage and upon molt from the megalopal stage to the first crab stage. Although both molts could be considered metamorphic, investigators have consistently considered metamorphosis to occur when megalopae molt to the first crab or first juvenile stage (e.g., Weber and Epifanio, 1996). Although some species, such as the portunid Callinectes sapidus, can swim as adults most brachyurans assume a totally benthic existence upon metamorphosis. Thus, associated with metamorphosis is a transition from a planktonic to a benthic existence and transformation to the adult body form.
The classification system for the different stages in the molt cycle of crustaceans was established by Drach (1939) and reviewed in detail by Stevenson (1985). Sequentially there is progression from postmolt through intermolt to premolt (proecdysis) followed by molting (ecdysis). Although there are a number of stages within each of these general steps (e.g., Anger, 1983; Stevenson, 1985), studies of the effect of cues on brachyuran metamorphosis have, at best, separated megalopae into two distinct categories, intermolt and premolt. Thus, this review will use these broad categories and designate megalopae as intermolt if they have not begun morphological changes leading to ecdysis and as premolt if these morphological changes are apparent. General techniques, times to metamorphosis, and cues that accelerate and delay metamorphosis of brachyuran megalopae will be considered.
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A variety of procedures have been used to test the effects of factors on time to metamorphosis (TTM) of brachyuran megalopae. For Callinectes sapidus (Wolcott and De Vries, 1994
; Forward et al., 1994, 1996, 1997; Brumbaugh and McConaugha, 1995), Carcinus maenas (Zeng et al., 1997) and Cancer magister (Fernandez et al., 1994), megalopae were collected from the plankton for experimentation. One problem with this procedure is that a cohort (i.e., megalopae collected at the same time or in the same plankton sample) can contain megalopae in different molt stages, which will likely have different times to metamorphosis independent of any experimental condition. For C. sapidus, this problem was reduced for megalopae collected in offshore areas (Wolcott and De Vries, 1994; Brumbaugh and McConaugha, 1995) because they were all in intermolt. However, these megalopae may have molted to the megalopa stage at different times and therefore be in different stages of intermolt. Alternatively, C. sapidus megalopae in very different molt stages are collected as they entered estuaries. A partial solution to this problem is careful, visual segregation of megalopae by molt stage and the use of only intermolt animals in experiments (e.g., Forward et al., 1996, 1997). Nevertheless, even with sorting, there are still differences in TTM between cohorts collected on different nights (Forward et al., 1996). These differences probably result because of differences in prior environmental conditions and variation in the relationship between visual appearance and physiological age. Thus, comparisons of TTM between cohorts are an imprecise measure of the effects of different test conditions and comparisons are best made within a cohort (Forward et al., 1996). Nevertheless, it is possible to estimate inter-cohort effects by including it as a separate factor or predictor variable in the statistical analyses (see below).
Alternatively, larvae were reared in the laboratory to the megalopal stage for Chasmagnathus granulata (Gebauer et al., 1998, 1999), Panopeus herbstii (Weber and Epifanio, 1996; Rodriguez and Epifanio, 2000; Andrews et al., 2001), Uca pugilator (Christy, 1989; O'Connor, 1991) and Uca pugnax (O'Connor and Judge, 1997; O'Connor and Gregg, 1998). Laboratory rearing minimizes variability because (1) larval development occurs under controlled environmental (e.g., temperature, salinity, and photoperiod) and feeding conditions, (2) the time of molt to the megalopal stage is known and (3) megalopae can be separated by broods making it possible to factor out possible inter-brood effects. In some cases, broods were tested separately (e.g., O'Connor, 1990; Gebauer et al., 1998), while in other studies megalopae from different broods were mixed together (e.g., Weber and Epifanio, 1996; Fitzgerald et al., 1998) for experimentation. Since TTM does differ among broods (O'Connor, 1990; Gebauer et al., 1998), suggesting a genetic or maternal effect (Holm, 1990), testing separate broods is perhaps preferable since it enables inter-brood effects to be estimated and separated from main treatment effects.
TTM is measured for groups of megalopae placed in glass finger bowls or aquaria (Forward et al., 1994, 1996; Weber and Epifanio, 1996; Gebauer et al., 1998, 1999; Rodriguez and Epifanio, 2000; Andrews et al., 2001) or when megalopae are placed individually in compartmentalized plastic boxes (Wolcott and De Vries, 1994; Brumbaugh and McConaugha, 1995; Forward et al., 1997; Fitzgerald et al., 1998). When tested in groups, early juvenile (first) crabs are removed from the groups upon metamorphosis to minimize cues and cannibalism of megalopae by juveniles. Although the water volume during incubation can differ between experiments, there is no evidence that volume affects the TTM (Forward et al., 1996; Gebauer et al., 1998).
Testing individually in compartmentalized boxes has several advantages including (1) increased sample size and possible statistical power, since each megalopae can be treated as a separate "experimental unit" in the analysis (see below), and (2) elimination of potential problems due to density (number of megalopae per unit volume of water) effects. Gebauer et al. (1998) did not observe an effect of density on TTM, yet both Forward et al. (1996) and Fernandez et al. (1994) found that TTM increased as density increased. For Callinectes sapidus, the density effect was not due to chemical cues but seems to be cued by physical interaction between megalopae (Forward et al., 1996). Considering the possible functional significance, Fernandez et al. (1994) speculated that the delay resulted from the vulnerability of megalopae to cannibalism as they molt.
The density effect presents a problem for assessing the effects of different experimental conditions on TTM for C. sapidus. For example, let us assume two similar size groups of megalopae are exposed to a control condition (e.g., clean seawater) and an experimental condition that decreases the TTM (e.g., seagrass odor). As megalopae in the presence of seagrass odor metamorphose and are removed, the decrease in density will cause a decrease in the TTM. Thus, density is a confounding variable, since both the seagrass odor and the decrease in density shorten the TTM as compared to the control. These considerations suggest that the best experimental procedure consists of testing megalopae individually. However, it is worth noting that metamorphosis can be affected by plasticizers (e.g., pthalates) and catalysts (e.g., dibutyl tin), which leach into seawater from the plastics in the compartmentalized boxes (Jobling et al., 1995; Zou and Fingerman, 1999). These compounds could impact steroid metabolism directly as steroid mimics or antagonists or indirectly by altering enzyme levels for steroid metabolism (Clare et al., 1992a; Jobling et al., 1995). It is well know that steroids play a central role in metamorphosis of crustaceans (e.g., Dauphin-Villemant et al., 1999; Durica et al., 1999). This potential problem can be removed by testing megalopae in individual glass vials.
To standardize the test procedure, megalopae are usually fed brine shrimp nauplii daily and their water changed every day or every other day. However, for Callinectes sapidus neither water changes nor feeding are necessary for the megalopae to metamorphose and these factors did not affect the TTM (Forward et al., 1996; Brumbaugh and McConaugha, 1995). Nevertheless, changing the test water does renew any chemical cues and prevent the accumulation of waste products, which may influence the TTM. Also, since test conditions are not axenic, renewing the water reduces the accumulation of bacteria and breakdown products from bacterial action.
The procedures used for monitoring the TTM depended on whether megalopae were collected from the plankton or reared in the laboratory. In tests where megalopae were collected from the plankton, monitoring typically began immediately, and megalopae were observed for metamorphosis 2–4 times each day (Wolcott and De Vries, 1994; Forward et al., 1994, 1996, 1997; Brumbaugh and McConaugha, 1995). Observations continued until all megalopae either metamorphosed or died. However, mortality was typically very low.
For laboratory-reared megalopae, monitoring of metamorphosis began some time after the molt to the megalopal stage because preliminary trials indicated that there is a minimum time before metamorphosis began. These times were 6 days for Uca pugilator (O'Connor, 1991) and U. pugnax (O'Connor and Judge, 1997), 10–11 days for Panopeus herbstii (Weber and Epifanio, 1996; Rodriguez and Epifanio, 2000; Andrews et al., 2001) and 3 days for Rhithropanopeus harrisii (Fitzgerald et al., 1998). Metamorphosis was either monitored at one day intervals until all megalopae molted or the proportion of megalopae that molted was determined after a predetermined period of time, such as 3, 6 and 9 days (O'Connor, 1991).
Statistical analyses used to compare the TTM under control and treatment conditions depended upon the experimental design and the method used to assess TTM (i.e., dependent variable). If metamorphosis was monitored continuously at regular intervals until all or most megalopae molted to the first crab stage, then mean or median TTMs for each treatment were compared using parametric ANOVAs and associated post-hoc multiple comparison tests or their nonparametric analogs if the data failed to meet the assumptions of the parametric tests (Forward et al., 1994, 1996, 1997; Wolcott and De Vries, 1994; Brumbaugh and McConaugha, 1995; O'Connor and Judge, 1997; Fitzgerald et al., 1998; Gebauer et al., 1998; O'Connor and Gregg, 1998; Rodriguez and Epifanio, 2000). This method is comparable to using probit analysis to calculate and compare the times to 50% metamorphosis from plots of the cumulative percentage of megalopae metamorphosing vs. time.
When megalopae are maintained in groups (e.g., in plastic or glass bowls), then the group is the smallest unit to which the treatment is applied and individual megalopae cannot be treated as independent replicates in the analysis. Consequently, either the results for each group are averaged (Forward et al., 1994, 1996) or a nested design is employed (Rodriguez and Epifanio, 2000). The TTM for each megalopae is considered a "subsample" rather than an "experimental unit" (i.e., megalopae nested within group) and used to test for possible group effects.
An alternative approach has been to compare the proportion of animals metamorphosing after a predetermined period of time (e.g., 72 hr, 5, or 10 days), rather than regularly monitor the TTM of individual megalopae (Weber and Epifanio, 1996; O'Conner, 1991; Gebauer et al., 1998). Results are compared using either parametric ANOVA on transformed (arcsine) values or comparable nonparametric tests (e.g., Mann-Whitney U-test). This method has the advantage of being less time consuming since it does not require checking megalopae at regular intervals, yet the selection of the time to compare the proportion of megalopae metamorphosing among treatments is often arbitrary and may not reflect the optimal time for detecting a difference among treatments.
A third approach uses failure-time analysis (Cox Proportional Hazards Model) to compare the TTM among treatments (Tankersley and Wieber, 2000). Time to metamorphosis is substituted for "time until an event occurs" in the analysis and the Cox Proportional Hazards Model is used to calculate the "hazard function" for each treatment group (Muenchow, 1986; Kleinbaum, 1996). The hazard function gives the instantaneous potential that a megalopa would metamorphose during the next time interval, given that it had not metamorphosed since the experiment began. Thus, the Cox model can be used to test for differences among metamorphosis rates over the entire time period of the study rather than at a single point in time. Moreover, additional variables can be included as covariates in the model and used to calculate metamorphosis curves (cumulative percent metamorphosis vs. time) adjusted for the effects of extraneous or confounding variables such as cohort effects. Unlike analyses which compare mean or median TTMs among treatments, this approach has the added benefit of allowing observations for which complete TTM values are unavailable, such as megalopae that die during the experiment or fail to metamorphose by the end of the study period, to be included in the analysis as "right-censored" data (Kleinbaum, 1996).
Most experiments compare TTM or proportion that metamorphoses under control condition to test conditions. Control conditions are control seawater in a glass bowl, aquarium or compartmentalized boxes. Control seawater consists of water that is presumed to be free of chemical cues, which influence metamorphosis. However the origin of the "control water" varies among studies and included water that was (1) collected offshore beyond the influence of estuaries (Forward et al., 1994, 1996, 1997; Weber and Epifanio, 1996; Fitzgerald et al., 1998; Rodriguez and Epifanio, 2000; Andrews et al., 2001), filtered (1 µm, Gebauer et al., 1998), aged and filtered (1.2 µm, O'Connor, 1991; 0.45 µm, O'Connor and Gregg, 1998), or filtered artificial seawater (1.2 µm, O'Connor, 1990).
Metamorphosis-accelerating cues are those that when added to control seawater reduce the TTM as compared to times in control seawater. Alternatively, the power of testing for cues that increase (delay) the TTM is improved by a different experimental design. Since control seawater is a condition that approximates the longest experimental TTM, the assay described above is insensitive to the addition of another cue that may delay metamorphosis and increase the TTM. Thus, delaying cues have been tested by combining them with a cue known to increase the TTM, and TTM in the presence of the accelerating cue alone is the control condition. Through this procedure, a delaying cue is identified if it reverses the effects of the accelerating cue.
The procedure for exposure to acceleration and delaying cues varied with species. For all of the species examined, except Panopeus herbstii, megalopae were exposed to different experimental treatments either as they molted to the megalopal stage if reared in the laboratory, or immediately after collection from the plankton. In the case of P. herbstii, preliminary experiments found that metamorphosis did not begin until 10 days after the molt to the megalopal stage regardless of experimental treatments. Thus, exposure to experimental conditions did not begin until megalopae were 10 days post-molt (Weber and Epifanio, 1996; Rodriguez and Epifanio, 2000; Andrews et al., 2001).
Duration of the megalopal stage
Although some invertebrates can delay metamorphosis for extremely long periods of time (e.g., months) in the absence of habitat cues (e.g., Pechenik, 1990) or will die without metamorphosing in the absence of specific cues (e.g., Morse, 1984), the brachyuran crustaceans lack this ability. To determine the maximum TTM (i.e., delay) megalopae are reared in the laboratory and exposed to control seawater alone. The mean TTM varies (Table 1), but all species studied thus far metamorphose within a number of days (longest 
20 days). This result indicates there is a temporal threshold beyond which metamorphosis occurs even in the absence of habitat cues (Weber and Epifanio, 1996).
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If the maximum times to metamorphosis are compared to mean times when megalopae are exposed to cues that accelerate metamorphosis, the times shorten by only a few days (Table 1). With the exception of Christy's (1989)
study, in which there was a 67% reduction in the TTM, the average decrease was 15 to 25%. Since multiple factors in the field (see below) can affect metamorphosis, past laboratory studies of TTM using clean seawater in glass bowls most likely over estimate the duration of the megalopal stage and may have little relevance to actual duration of this stage in the field (Weber and Epifanio, 1996).
Biological rhythms in metamorphosis
The time of metamorphosis is related to the light:dark and tidal cycles, but there is little consistency between species studied thus far. Cancer magister has an endogenous rhythm in metamorphosis as most megalopae molt at the time of night whether they are maintained under a light:dark cycle, constant light, or constant dark (Fernandez et al., 1994). Since megalopae are more vulnerable to predation during metamorphosis, molting at night would reduce the risk from visual predators, such as fish (Fernandez et al., 1993).
In contrast, the predation argument is not supported by studies with Callinectes sapidus. When maintained on a light:dark cycle, C. sapidus metamorphoses during the day (Forward et al., 1996). This pattern agrees with observations of transport in estuaries, because megalopae actively enter the water column during flood tide at night but are absent during the day and presumed to be on or near the bottom (e.g., De Vries et al., 1994; Tankersley et al., 1995). This pattern results from light inhibition of swimming during the day (Forward and Rittschof, 1994). Thus, during the day megalopae do not actively swim in the water column and are on or near the bottom. Since swimming is difficult during molting, metamorphosis occurs at a time and location when swimming is reduced (Forward et al., 1996).
Zeng et al. (1997) measured the time of metamorphosis of Carcinus maenas megalopae in the presence of a light:dark cycle, constant light, and constant dark. There was a clear circatidal rhythm in which megalopae molted at the time of high tide in the field. Zeng et al. (1997) speculated that metamorphosis around the time of high tide may enhance settlement in the upper intertidal zone.
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Temperature and salinity
Since many species undergo larval development in offshore areas but metamorphose in estuaries, it was hypothesized that metamorphosis should be accelerated by environmental factors characteristic of estuaries (e.g., Wolcott and De Vries, 1994
). Salinities are lower in estuaries due to river inflow. During the summer, temperatures are higher in estuaries as compared to offshore areas, but the temperature pattern reverses in the winter.
Many older studies (e.g., Panopeus herbstii, Costlow et al., 1962) considered the effects of temperature and salinity on the duration of larval development and lethal limits. For example, the duration of the megalopal stage of Callinectes sapidus decreased as temperature increased from 15 to 30°C, but salinity did not affect the TTM between 20–40 psu (Costlow, 1967). The contour plots of the combined effects of salinity and temperature on survival and metamorphosis of C. sapidus megalopae indicated the optimum conditions for development were between 15–35 psu and 24–30°C (Costlow, 1967).
Recent studies of Callinectes sapidus found that TTM decreased as the salinity decreased, but the effect was very small (Wolcott and De Vries, 1994; Forward et al., 1994, 1997). The reverse effect was seen with Rhithropanopeus harrisii, in which TTM increased as salinity decreased (Costlow et al., 1966; Fitzgerald et al., 1998). Finally, lowering the salinity had no effect on TTM of Carcinus maenas (Zeng et al., 1997).
Estuarine chemical cues
Chemical cues present in estuarine water are known to affect metamorphosis. Forward et al. (1994) found that in Callinectes sapidus, the TTM decreased in estuarine water, and the effect was still evident when estuarine water was diluted to a 10% solution with offshore water (Forward et al., 1996). This result agreed with the hypothesis of Wolcott and De Vries (1994) that metamorphosis progresses slowly in offshore water but is accelerated by chemical cues associated with estuaries.
In contrast, the entire life cycle of Rhithropanopeus harrisii is completed within estuaries. Adults occur in brackish water (Williams, 1984) and all larval stages are retained within estuaries near the adult population (Cronin, 1982). Since larvae are rarely exported to coastal areas for development, differentiation between oceanic and estuarine areas as sites for metamorphosis is unnecessary. This situation lead to the hypothesis that TTM of R. harrisii should be unaffected by exposure to offshore and estuarine waters (Fitzgerald et al., 1998). This hypothesis was not supported, as TTM decreases upon exposure to estuarine water as compared to times in offshore water at salinities from 5–25 psu. Thus, differentiation between offshore and estuarine areas for metamorphosis is common among estuarine crabs regardless of their pattern of larval development (Fitzgerald et al., 1998).
The active molecules in estuarine waters that accelerate metamorphosis are <10 kDa in size and data support the hypothesis that they include humic acids (Forward et al., 1996, 1997). Estuarine humic acids are a heterogeneous group of chemical species, which are decomposition products of structural elements of terrestrial plant material and enter estuaries through freshwater inflow (Stevenson and Butler, 1969). Similar humic substances are produced by degradation of estuarine plants such as Spartina alterniflora (Moran and Hodson, 1994). Humic acids are attractive as possible cues for metamorphosis because they are abundant in estuaries (e.g., Fox, 1981) and more important, they precipitate as the salinity increases (e.g., Sieburth and Jensen, 1968; Hair and Bassett, 1973). Thus, their concentration decreases from the head to the mouth of an estuary and they are in very low concentrations in offshore areas. For Callinectes sapidus, the TTM decreases upon exposure to increasing concentrations of humic acids extracted from river water and upon exposure to commercial humic acids at environmental levels (Forward et al., 1997). Thus, dissolved humic acids can serve as an unequivocal cue for being in an estuary.
Brumbaugh (1996) suggested that ammonia/ammonium could serve as an additional chemical cue for metamorphosis in estuaries. This suggestion was reasonable considering studies of other invertebrate larvae. Ammonia induces settlement behavior in oyster larvae (Bonar et al., 1990; Coon et al., 1990; Fitt and Coon, 1992) and metamorphosis in echinoderm (Gilman, 1991) and Japanese scallop (Kingzett et al., 1990) larvae. Similarly, ammonium induces metamorphosis in hydroid larvae (Berking, 1988). Among crustaceans, ammonia accelerates molting in the shrimp Penaeus monodon (Chen and Lin, 1992). Alternatively, Zimmer-Faust and Tamburri (1994) found ammonia/ammonium does not induce settlement of oyster larvae.
Forward et al. (1997) tested Brumbaugh's (1996) hypothesis that ammonia/ammonium serves as a cue for metamorphosis in Callinectes sapidus. In contrast to Brumbaugh's (1996) suggestion, initial experiments indicated that ammonium delays the TTM. Since both estuarine water and humic acids in offshore water are positive cues, which decrease the TTM, the delaying effect of ammonium chloride was tested in combination with these solutions. At concentrations of 20 µM and greater, the accelerating effects of humic acids and estuarine water were reversed. The possible functional significance of the inhibition of C. sapidus metamorphosis by ammonium chloride is that ammonium levels are inversely related to oxygen levels (Fitt and Coon, 1992), and at high concentrations ammonium is toxic to marine invertebrates. Thus, high ammonia/ammonium levels may signal low oxygen levels, which could be detrimental to megalopae or metabolically stresses megalopae.
Considering oxygen, C. sapidus megalopae are sensitive to hypoxia as 50% mortality occurs after 13 hr of exposure to oxygen at 20% of saturation and after 1 hr in anoxic conditions (Tankersley and Wieber, 2000). Upon chronic exposure to hypoxic conditions in estuarine water (accelerating cue), the TTM increased as the percent saturation of oxygen declined from 100% to 40%. Upon acute exposure, in which megalopae were subjected to 4 hr of 20% saturation and then returned to normoxic conditions each day, survival was high but the TTM also increased (Tankersley and Wieber, 2000). Thus, the effects of hypoxia and ammonia/ammonium on metamorphosis suggest that conditions in sub-optimal settlement habitats inhibit estuarine cues that accelerate metamorphosis, presumably delaying metamorphosis so that megalopae have time to locate other more suitable settlement areas. The physiological mechanism could be that both ammonia and hypoxia slow down metabolism, which in turn slows down the TTM.
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HABITAT CUES |
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Since many species of brachyurans settle in specific locations, it is possible that cues from these habitats induce metamorphosis. For most species substrate from the adult habitat reduces the TTM (Table 2). Perhaps the most surprising result is that substrate from the typical settlement habitat of Carcinus maenas (i.e., gravel; Zeng et al., 1997
) and Cancer magister (i.e., mud and shells; Fernandez et al., 1994) do not affect the TTM. Megalopae of many species are clearly able to differentiate between habitat substrates, because some natural substrates do (Table 2) and others fail to affect the TTM (Table 3). One of the few consistent results among all species tested thus far is that clean structural mimics of natural substrate do not influence the TTM (Table 4).
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A possible reason for the inactivity of clean substrate mimics is that they lack chemical cues from a biofilm. This hypothesis was tested by Rodriguez and Epifanio (2000)
for metamorphosis of Panopeus herbstii. Clean glass fragments and marbles do not affect the TTM for this species (Weber and Epifanio, 1996). However, if glass microscope slides were conditioned in the area of the adult habitat, a biofilm developed which decreased the TTM. The cue from the biofilm increased in concentration with longer exposure times. Although Weber and Epifanio's (1996) results suggested that the cue was water soluble, futher experiments are needed for confirmation. Normal biofilms contain both bacteria and microalage. Since TTM were similar when megalopae were exposed to biofilms containing bacteria and microalgae and only bacteria, the active component in biofilms appears to be come from bacteria. Also, biofilms differ between incubation sites because an active biofilm did not develop if slides were incubated in a non-adult habitat, such as a sandy intertidal area, but did develop on slides placed at the adult habitat (Rodriguez and Epifanio, 2000). The surface upon which biofilms develop may also be important. Andrews et al. (2001) found that biofilms on Plexiglas did not produce a chemical cue that reduced the TTM in P. herbstii.
Aquatic vegetation
The presence of aquatic vegetation either reduces the TTM or has no effect (Table 5). There are no studies indicating that metamorphosis was delayed in the presence of vegetation, but Forward et al. (1996) found that Callinectes sapidus megalopae died very quickly in the presence of the green alga Codium fragile subsp. tomentosoides, which suggests the presence of allomones. Among non-algae, the TTM of C. sapidus was reduced in the presence of salt marsh grasses Spartina alterniflora (Forward et al., 1996) and Phragmites australis (Tankersley et al., 2001) and sea grasses (Forward et al., 1994, 1996). The response to seagrass (e.g., Zostera marina) is expected because seagrass beds serve as the primary nursery areas for Callinectes sapidus (Orth and van Montfrans, 1987; Olmi et al., 1990).
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Results with macroalgae vary with algal species (Table 5). Some species are very effective at reducing the TTM, while metamorphosis is unaffected by other species. Moreover, there is no consistent relationship between the type of algae (green, brown or red) and the tendency to affect TTM. The most effective algal species occur in typical settlement habitats. For example, Panopeus herbstii settles in areas where Fucus verticulus is common and the TTM is reduced by exposure to this species (Weber and Epifanio, 1996
). Alternatively, the failure of some algae to shorten TTM is consistent with this pattern. For example, Callinectes sapidus undergo larval development offshore where the floating alga Sargassum natans is common, but they metamorphose in estuaries. The presence of S. natans has no effect on the TTM for C. sapidus (Forward et al., 1996).
The effect of various plants appears to be due to the release of chemical cues either directly from the plant or from biofilms associated with the plants. If plants are incubated in clean seawater and then removed, the resulting water is effective at reducing the TTM (Forward et al., 1996; Weber and Epifanio, 1996). The effect is related to chemical concentration, as the TTM of Callinectes sapidus decreases as the concentration of Spartina alterniflora increases (Tankersley et al., 2001). This result indicates that a range of concentrations should be tested in order to conclude that a plant odor is ineffective. The absence of an effect of Spartina alterniflora odor on metamorphosis of Uca pugnax (O'Connor and Gregg, 1998) is either due to the concentration tested or the ineffectiveness of the plant odor.
Structural mimics of effective algae such as ribbons (Forward et al., 1996), black plastic sheeting strips (Weber and Epifanio, 1996) and nylon threads (Gebauer et al., 1998) in control seawater have no effect on the TTM. The ineffectiveness of structure at inducing metamorphosis is further indicated by the observation that the TTM is the same in the presence of plant odor water and plant odor water plus a structural mimic (Forward et al., 1996). Considering functional significance, activation of metamorphosis from a variety of plants is useful if megalopae are transported to different areas that have different aquatic vegetation.
Conspecific odor
The effect of water-soluble conspecific odor on metamorphosis was determined by either exposing megalopae to water in which adults were incubated (Forward et al., 1994; Weber and Epifanio, 1996; Fitzgerald et al., 1998; O'Connon and Gregg, 1998; Rodriguez and Epifanio, 2000; Andrews et al., 2001) or to water containing adults (O'Connor, 1991; Gebauer et al., 1998, 1999). In the latter case, adults and megalopae were placed in the same water but were physically separated so that they did not come in contact with one another. For most species (Table 6), odor from conspecifics reduced the TTM. The exceptions were Callinectes sapidus (Forward et al., 1994) and Carcinus maenas (Zeng et al., 1997) where adult odor had no effect on TTM. The effect of conspecific odor on TTM appears to be dose dependent, since TTM in R. harrisii decreased as concentration of adult odor increased (Fitzgerald et al., 1998).
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Attempts to characterize the adult chemical cues are limited and present an area for future study. O'Connor and Gregg (1998)
found that the odor from Uca pugnax remained effective after freezing and thawing. Similarly, the odor from Panopeus herbstii remained active after exposure to low (–20°C) and high (100°C) temperatures and has a molecular weight of <1,000 Da (Andrews et al., 2001).
Tests with odors from non-conspecific species have produced mixed results. O'Connor and Gregg (1998) reported that odor from Uca minax had no effect on TTM of U. pugnax. The TTM of Panopeus herbstii was not affected by odors from the fishes Tautoga omitis and Fundulus heterclitus or the crabs Callinectes sapidus and Uca pugnax (Weber and Epifanio, 1996; Rodriguez and Epifanio, 2000). However, there was a slight reduction in the TTM when Panopeus herbstii was exposed to odor from the crab Dyspanopeus sayi, oyster Crassostrea virginica (Rodriguez and Epifanio, 2000) and the crab Hemigrapsus sanguineus (Andrews et al., 2001). Rodriguez and Epifanio (2000) speculated that the response to D. sayi resulted because of its close taxonomic relationship to P. herbstii. Similarly, the absence of a response to odor from C. sapidus and U. pugnax may result because these species are more taxonomically distant. The response to odor from the oyster may result because P. herbstii prey on oyster. If so, then this is the first report that a prey-associated cue affects crab metamorphosis (Rodriguez and Epifanio, 2000). Such relationships are well documented for other invertebrates (for reviews see, Crisp, 1974; Pawlik, 1992; Hadfield and Paul, 2001).
An area for future research is the effect of predator odors on metamorphosis. In theory settlement and metamorphosis should be inhibited by predator odors, which would allow megalopae to migrate to other areas where predator risk is lower. This possibility is supported by a study of settlement by Callinectes sapidus megalopae on hogs-hair collectors containing odors from different sources (Welch et al., 1997). Settlement was significantly reduced in the presence of odors from the adult crabs Uca pugilator, U. pugnax, Panopeus herbstii, and grass shrimp Palaemonetes pugio, all of which actively prey on C. sapidus megalopae. In addition Tankersley et al. (2001) found that the reduction in the TTM of C. sapidus by exposure to odor of the salt marsh cord grass Spartina alterniflora could be reversed by the addition of P. herbstii odor.
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DISCUSSION |
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TOP
SYNOPSISINTRODUCTIONTECHNIQUESENVIRONMENTAL FACTORSHABITAT CUESDISCUSSIONReferences |
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Among brachyuran crabs, zoeal larvae are the planktonic dispersal stage. They then molt to the megalopal stage, which undergoes horizontal transport to the settlement site, settlement out of the water column and finally metamorphosis to the first crab stage. When compared to the general larval sequence of sessile invertebrates (e.g., Crisp, 1984
), the zoeal larvae represent the early larval stage that are incapable of responding to settlement cues, whereas the megalopal stage becomes competent, since it can respond to these cues. Postmolt and intermolt megalopae are incapable of metamorphosing, but, at least, intermolt megalopae can settle out of the water column in areas with cues for settlement (e.g., Welch et al., 1997). Nevertheless, it is not known how early in the megalopal stage (postmolt or intermolt) megalopae become competent and respond to settlement cues. Megalopae then enter premolt and progress toward metamorphosis. If megalopae settle in an inappropriate area, they can reenter the water column for further transport because they remain highly mobile. Thus, in contrast to biofouling organisms (Clare et al., 1992b), settlement and metamorphosis are probably not tightly coupled, as megalopae can move between sites before metamorphosis. Collectively, the past studies considered in this overview indicate that the time to metamorphosis (TTM) of brachyuran crabs can be either accelerated or delayed in response to a variety of cues (Table 7). Cues from sources that reduce the TTM are chemical, and there is no definitive evidence that structural cues are important. All of the acceleration cues are from sources found in nursery areas or adult habitat. Alternatively, cues that delay metamorphosis are adverse environmental factors, such as hypoxia, ammonia/ammonium, and chemical odor from potential predators.
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Although young megalopae are probably competent, it is unclear when in the progression through postmolt, intermolt and premolt that the cues that affect TTM are active. Weber and Epifanio (1996)
found that early exposure to cues (presumably during postmolt and intermolt) did not change the TTM. O'Connor and Gregg (1998) observed that if acceleration cues were given 6 days after molt to the megalopal stage, the TTM was reduced, but the cue was ineffective if applied 8 or 10 days after this molt. These results suggest that like crustacean biofoulers (Rittschof et al., 1984), cues are effective in a specific time interval for older megalopae, but studies designed specifically to address this topic are needed.
The maximum TTM is presumed to occur when megalopae are exposed to control seawater in a glass container (Table 1). This may be true, but it could be argued that the glass container provides a structural cue that induces metamorphosis and that greater delay is expected by megalopae that remain free swimming in the plankton. This argument is probably incorrect because megalopae do routinely contact substrate prior to settlement and metamorphosis. For example, megalopae of Callinectes sapidus, Uca spp. (e.g., De Vries et al., 1994), and Carcinus maenas (Zeng and Naylor, 1996) utilize flood-tide transport for movement to nursery areas, in which they are in the water column during flood tide and on the bottom in contact with substrate during ebb tide. Rhithropanopeus harrisii megalopae also move between the bottom and water column prior to metamorphosis (Cronin, 1982). Thus, the TTM in glass containers with clean seawater is probably a reasonable measure of the maximum duration of the megalopal stage.
The difference in time between the maximum TTM in control water and TTM upon exposure to acceleration cues is a measure of how fast metamorphosis can be accelerated if megalopae settle in a suitable habitat. The average difference in time varies with species but ranges over about 1–5 days (Table 1). These are very modest times compared to the length of the megalopal stage, which indicates that if a megalopa settles in a suitable habitat with acceleration cues early in the megalopal stage, metamorphosis may not occur for several days. Alternatively, the effects of acceleration cues can be overridden by exposure to adverse habitat cues, such as hypoxia, which would allow time for megalopae to find a more suitable settlement site.
Past studies have tested very general cues from natural environments and clearly indicate that these cues can affect the TTM. Areas for future study include consideration of species that live in different habitats, such as subtidal coastal and deep sea areas. Further characterization of the chemical cues includes studies of effective concentration ranges and the relation, if possible, to levels of cues in the settlement areas. Considering studies of sessile animals, it is unlikely that the chemical cues will be specifically identified, but they can be generally characterized as to molecular weight, temperature stability, polarity, and general type (e.g., peptide, protein, carbohydrate, etc.).
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ACKNOWLEDGMENTS |
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This material is based in part on research supported by the National Science Foundation grants No. OCE-9819355/9901146/0096205 and No. OCE-0095092/0094930.
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FOOTNOTES |
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1 From the Symposium Ontogenetic Strategies of Invertebrates in Aquatic Environments presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 3–7 January 2001, at Chicago, Illinois.
2 E-mail: rforward{at}duke.edu
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References |
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|
TOP
SYNOPSISINTRODUCTIONTECHNIQUESENVIRONMENTAL FACTORSHABITAT CUESDISCUSSIONReferences |
|---|
Andrews, W. R., N. M. Targett, and C. E. Epifanio. 2001. Isolation and characterization of the metamorphic inducers of the common mud crab, Panopeus herbstii. J. Exp. Mar. Biol. Ecol, 261:121-134.[CrossRef][Web of Science][Medline]
Anger, K. 1983. Moult cycle and metamorphosis in Hyas araneus larvae (Decapoda, Majidae), reared in the laboratory. Helgolander Meeresunters, 36:285-302.[CrossRef]
Berking, S. 1988. Ammonia, tetramethylammonium, barium and amiloride induce metamorphosis in the marine hydroid Hydractinia. Roux's Arch Dev. Biol, 197:1-9.[CrossRef]
Bonar, D. B., S. L. Coon, M. Walsh, R. M. Weiner, and W. Fitt. 1990. Control of oyster settlement and metamorphosis by endogenous and exogenous cues. Bull. Mar. Sci, 46:484-498.
Brumbaugh, R. D. 1996. Recruitment of blue crab Callinectes sapidus postlarvae to the back-barrier lagoons of Virginia's Eastern Shore. Ph.D. Diss., Old Dominion Univ., Norfolk, Virginia.
Brumbaugh, R. D., and J. R. McConaugha. 1995. Time to metamorphosis of the blue crab Callinectes sapidus megalopae: Effects of benthic macroalgae. Mar. Ecol. Prog. Ser, 129:113-118.
Burke, R. D. 1986. Pheromones and the gregarious settlement of marine invertebrate larvae. Bull. Mar. Sci, 39:323-332.
Butman, C. A. 1987. Larval settlement of soft-sediment invertebrates; The spatial scales of patterns explained by active habitat selection and the emerging role of hydrodynamic processes. Oceanogr. Mar. Biol. Ann. Rev, 25:113-165.
Chen, J. C., and C. Y. Lin. 1992. Effects of ammonia on growth and molting of Penaeus monodon juveniles. Comp. Biochem. Physiol, 101C:449-452.[CrossRef]
Christy, J. H. 1989. Rapid development of megalopae of the fiddler crab Uca pugilator reared over sediment: Implications for models of larval recruitment. Mar. Ecol. Prog. Ser, 57:259-265.
Clare, A. S., D. Rittschof, and J. D. Costlow Jr. 1992a. Effects of the non-steroidal ecdysone mimic RH5849 on larval crustaceans. J. Exp. Zool, 262:436-440.[CrossRef]
Clare, A. S., D. Rittschof, D. J. Gerhart, and J. S. Maki. 1992b. Molecular approaches to non-toxic antifouling. Invert. Reprod. Dev, 22:67-76.
Clooney, R. A., and S. A. Torrence. 1984. Ascidian larvae: Structure and settlement. In J. D. Costlow and R. C. Tipper (eds.), Marine biodeterioration: An interdisciplinary study, pp. 141–147. U.S. Naval Institute, Annapolis.
Coon, S. L., W. Walch, W. K. Fitt, R. M. Weiner, and D. B. Bonar. 1990. Ammonia induces settlement behavior in oyster larvae. Biol. Bull, 179:297-303.[Abstract]
Costlow, J. D. 1967. The effect of salinity and temperature on survival and metamorphosis of megalopae of the blue crab Callinectes sapidus. Helgolander wiss. Meeresunters, 15:84-97.[CrossRef]
Costlow, J. D., C. G. Bookhout, and J. Monroe. 1962. Salinity-temperature effect on larval development of the crab Panopeus herbstii Milne-Edwards, reared in the laboratory. Physiol. Zool, 35:79-93.
Costlow, J. D., C. G. Bookhout, and J. Monroe. 1966. Studies on the larval development of the crab, Rhithropanopeus harrisii (Gould). I. The effect of salinity and temperature on larval development. Physiol. Zool, 39:81-100.
Costlow, J. D., and R. C. Tipper. 1984. Marine biodeterioration: An interdisciplinary study. U.S. Naval Institute, Annapolis.
Crisp, D. J. 1974. Factors influencing settlement in marine invertebrate larvae. In P. T. Grant and P. T. Machie (eds.), Chemoreception in marine organisms, pp. 177–265. Academic Press, New York.
Crisp, D. J. 1984. Overview of research on marine invertebrate larvae, 1940–1980. In J. D. Costlow and R. C. Tipper (eds.), Marine biodeterioration: An interdisciplinary study, pp. 103–125. U.S. Naval Institute, Annapolis.
Cronin, T. W. 1982. Estuarine retention of larvae of the crab Rhithropanopeus harrisii. Coast. Shelf Sci, 15:207-220.[CrossRef]
Dauphin-Villemant, C., D. Boecking, M. Tom, M. Maiebeche, and R. Lafont. 1999. Cloning of a novel cytochrome P450 (CYP4C15) differentially expressed in the steroidogenic glands of an arthropod. Biochem. Biophys. Res. Commun, 264:413-418.[CrossRef][Web of Science][Medline]
De Vries, M. C., R. A. Tankersley, R. B. Forward Jr.,, W. W. Kirby-Smith, and R. A. Luetttich. 1994. Abundance of estuarine crab larvae is associated with tidal hydrologic variable. Mar. Biol, 118:403-413.[CrossRef]
Drach, P. 1939. Etude preliminaire sur le cycle Crustaces Decapodes. Annls. Inst. Oceanogr. Monaco, 19:103-391.
Durica, D. S., A. C.-K. Chung, and P. M. Hopkins. 1999. Characterization of EcR and RXR gene homologs and receptor expression during the molt cycle in the crab Uca pugilator. Amer. Zool, 39:758-773.
Fernandez, M., O. Iribarne, and D. Armstrong. 1993. Habitat selection by young-of-the-year Dungeness crab Cancer magister and predation risk in intertidal habitats. Mar. Ecol. Prog. Ser, 92:171-177.
Fernandez, M., O. Iribarne, and D. Armstrong. 1994. Ecdysial rhythms in megalopae and first instars of the Dungeness crab Cancer magister. Mar. Biol, 118:611-615.[CrossRef]
Fitt, W. K., and S. L. Coon. 1992. Evidence for ammonia as a natural cue for recruitment of oyster larvae to oyster beds in a Georgia salt marsh. Biol. Bull, 182:401-408.[Abstract]
Fitzgerald, T. P., R. B. Forward Jr.,, and R. A. Tankersley. 1998. Metamorphosis of the estuarine crab Rhithropanopeus harrisii: Effect of water type and adult odor. Mar. Ecol. Prog. Ser, 165:217-223.
Forward, R. B. Jr.,, M. C. De Vries, D. Rittschof, D. A. Z. Frankel, J. P. Bischoff, C. M. Fisher, and J. M. Welch. 1996. Effects of environmental cues on metamorphosis of the blue crab Callinectes sapidus. Mar. Ecol. Prog. Ser, 131:165-177.
Forward, R. B. Jr.,, D. A. Z. Frankel, and D. Rittschof. 1994. Molting of megalopae from the blue crab Callinectes sapidus: Effects of offshore and estuarine cues. Mar. Ecol. Prog. Ser, 113:55-59.
Forward, R. B. Jr., and D. Rittschof. 1994. Photoresponses of crab megalopae in offshore and estuarine waters: Implications for transport. J. Exp. Mar. Biol. Ecol, 182:183-192.[CrossRef]
Forward, R. B. Jr.,, R. A. Tankersley, D. Blondel, and D. Rittschof. 1997. Metamorphosis of the blue crab Callinectes sapidus: Effects of humic acids and ammonium. Mar. Ecol. Prog. Ser, 157:277-286.
Fox, L. E. 1981. The geochemistry of humic acids and iron during estuarine mixing. Ph.D. Diss., Univ. of Delaware, Lewes.
Gebauer, P., K. Paschke, and K. Anger. 1999. Costs of delayed metamorphosis: Reduced growth in early juveniles of the estuarine grapsid crab, Chasmagnathus granulata. J. Exp. Mar. Biol. Ecol, 238:271-281.[CrossRef]
Gebauer, P., I. Walter, and K. Anger. 1998. Effects of substratum and conspecific adults on the metamorphosis of Chasmagnathus granulata (Dana) (Decapoda: Grapsidae) megalopae. J. Exp. Mar. Biol. Ecol, 223:185-198.[CrossRef]
Gilmour, T. H. J. 1991. Induction of metamorphosis of echinoderm larvae. Amer. Zool, 31:105A.
Hair, M. E., and C. R. Bassett. 1973. Dissolved and particulate humic acids in an east coast estuary. Estuar. Coast. Mar. Sci, 1:107-111.[CrossRef]
Hadfield, M. G. 1998. The D. P. Wilson lecture: Research on settlement and metamorphosis of marine invertebrate larvae: Past, present and future. Biofouling, 12:9-29.
Hadfield, M. G., and V. J. Paul. 2001. Natural chemical cues for settlement and metamorphosis of marine invertebrate larvae. In J. McClintock and P. Baker (eds.), Marine chemical ecology. (In press).
Holm, E. R. 1990. Attachment behavior in the barnacle Balanus amphitrite (Darwin): Genetic effects on metamorphosis. J. Exp. Mar. Bio. Ecol, 135:85-98.[CrossRef]
Jobling, S., T. Reynolds, R. White, M. R. Parker, and J. P. Sumpter. 1995. A variety of environmentally persistent chemicals, including some pthalate plasticizers, are weakly estrogenic. Envir. Health Pers, 103:582-687.
Kingzett, B. C., N. Bourne, and K. L. Leask. 1990. Induction of metamorphosis of the Japanese scallop Patinopecten yessoensis Jay. J. Shellfish Res, 9:119-124.
Kleinbaum, D. G. 1996. Survival analysis: A self-learning text. Springer, New York.
Miller, S. E., and M. G. Hadfield. 1990. Developmental arrest during larval life and life-span extension in a marine mollusc. Science, 248:356-358.
Moran, M. A., and R. E. Hodson. 1994. Dissolved humic substances of vascular plant origin in a coastal marine environment. Limnol. Oceanogr, 39:762-771.
Morse, D. E. 1984. Biochemical control of larval recruitment and marine fouling. In J. D. Costlow and R. C. Tipper (eds.), Marine biodeterioration: An interdisciplinary study, pp. 133–140. U.S. Naval Institute, Annapolis.
Muenchow, G. 1986. Ecological use of failure time analysis. Ecology, 67:246-250.[CrossRef]
O'Connor, N. J. 1990. Morphological differentiation and molting of juvenile fiddler crabs (Uca pugilator and U. pugnax). J. Crust. Biol, 10:608-612.[CrossRef]
O'Connor, N. J. 1991. Flexibility in timing of the metamorphic molt by fiddler crab megalopae Uca pugilator. Mar. Ecol. Prog. Ser, 68:243-247.
O'Connor, N. J., and A. S. Gregg. 1998. Influence of potential habitat cues on duration of the megalopal stage of the fiddler crab Uca pugnax. J. Crust. Biol, 18:700-709.[CrossRef]
O'Connor, N. J., and M. L. Judge. 1997. Flexibility in timing of molting of fiddler crab megalopae: Evidence from in situ manipulation of cues. Mar. Ecol. Prog. Ser, 146:55-60.
Olmi, E. J. III,, J. van Montfrans, R. N. Lipcius, R. J. Orth, and P. W. Sadler. 1990. Variations in plankton availability and settlement of blue crab megalopae in the York River, Virginia. Bull. Mar. Sci, 46:230-243.
Orth, R. J., and J. Montfrans. 1987. Utilization of a seagrass meadow and tidal marsh creek by blue crabs Callinectes sapidus: 1. Seasonal and annual variation in abundance with emphasis on post-settlement juveniles. Mar. Ecol. Prog. Ser, 41:283-294.
Pawlik, J. R. 1992. Chemical ecology of the settlement of benthic marine invertebrates. Oceanogr. Mar. Biol. Annu. Rev, 30:273-335.
Pechenik, J. A. 1990. Delayed metamorphosis by larvae of benthic marine invertebrates: Does it occur? Is there a price to pay? Ophelia, 332:63-94.
Pechenik, J. A., D. Rittschof, and A. R. Schmidt. 1993. Influence of delayed metamorphosis on survival and growth of juvenile barnacles Balanus amphitrite. Mar. Biol, 115:287-294.[CrossRef]
Rittschof, D., E. S. Branscomb, and J. D. Costlow. 1984. Settlement and behavior in relation to flow and surface in larval barnacles, Balanus amphitrite Darwin. J. Exp. Mar. Biol. Ecol, 82:131-146.[CrossRef]
Rittschof, D., R. B. Forward Jr.,, G. Cannon, J. M. Welch, M. M. McClary, E. R. Holm, A. S. Clare, S. Conova, L. M. McKelvey, P. Bryan, and C. L. Van Dover. 1998. Cues and context: Larval responses to physical and chemical cues. Biofouling, 12:31-44.
Rodriguez, R. A., and C. E. Epifanio. 2000. Multiple cues for induction of metamorphosis in larvae of the common mud crab Panopeus herbstii. Mar. Ecol. Prog. Ser, 195:221-229.
Scheltema, R. S., and J. T. Carlton. 1984. Methods of dispersal among fouling organisms and possible consequences for range extension and geographical variation. In J. D. Costlow and R. C. Tipper (eds.), Marine biodeterioration: An interdisciplinary study, pp. 127–132. U.S. Naval Institute, Annapolis.
Sieburth, J. Mc. N., and A. Jensen. 1968. Studies of algal substances in the sea. 1. Gelbstoff (humic material) in terrestrial and marine waters. J. Exp. Mar. Biol. Ecol, 2:174-189.[CrossRef]
Stevenson, F. J., and J. H. A. Butler. 1969. Chemistry of humic acids and related pigments. In G. Eglinton and M. T. J. Murphy (eds.), Organic geochemistry, pp. 534–557. Springer-Verlag, New York.
Stevenson, J. R. 1985. Dynamics of the integument. In D. E. Bliss (ed.), The biology of Crustacea, Vol. 9, pp. 1–42. Academic Press, New York.
Tankersley, R. A., L. M. McKelvey, and R. B. Forward Jr. 1995. Behavioral responses of crab megalopae to hydrostatic pressure, salinity and light. Mar. Biol, 122:391-400.[CrossRef]
Tankersley, R. A., R. B. Forward Jr.,, K. A. Smith, T. A. Ziegler, and T. Marshal. 2001. Habitat selection and development of post-larval blue crabs Callinectes sapidus:. Response to chemical cues from Spartina vs. Phragmites. (In press).
Tankersley, R. A., and M. G. Wieber. 2000. Physiological responses of postlarvae and juvenile blue crabs Callinectes sapidus to hypoxia and anoxia. Mar. Ecol. Prog. Ser, 194:179-191.
Weber, J. C., and C. E. Epifanio. 1996. Response of mud crab (Panopeus herbstii) megalopae to cues from adult habitat. Mar. Biol, 126:655-661.[CrossRef]
Welch, J. M., R. B. Forward Jr.,, and P. A. Howd. 1999. Behavioral responses of blue crab Callinectes sapidus postlarvae to turbulence: Implications for selective tidal stream transport. Mar. Ecol. Prog. Ser, 179:135-143.
Welch, J. M., D. Rittschof, T. M. Bullock, and R. B. Forward Jr. 1997. Effects of chemical cues on settlement behavior of blue crab Callinectes sapidus postlarvae. Mar. Ecol. Prog. Ser, 154:143-153.
Williams, A. B. 1984. Shrimps, lobsters, and crabs of the Atlantic coast of the Eastern United States, Marine to Florida. Smithsonian Inst. Press, Washington, D.C.
Wolcott, D. L., and M. C. De Vries. 1994. Offshore megalopae of Callinectes sapidus: Depth of collection, molt stage and response to estuarine cues. Mar. Ecol. Prog. Ser, 109:157-163.
Woods Hole Oceanographic Institution., 1952. Marine fouling and its prevention. U.S. Naval Institute, Annapolis.
Woollacott, R. M. 1984. Environmental factors in bryozoan settlement. In J. D. Costlow and R. C. Tipper (eds.), Marine biodeterioration: An interdisciplinary study, pp. 149–153. U.S. Naval Institute, Annapolis.
Zeng, C., and E. Naylor. 1996. Occurrence in coastal waters and endogenous tidal swimming rhythms of late megalopae of the shore crab Carcinus maenas: Implications for onshore recruitment. Mar. Ecol. Prog. Ser, 136:69-79.
Zeng, C., E. Naylor, and P. Abello. 1997. Endogenous control of timing of metamorphosis in megalopae of the shore crab Carcinus maenas. Mar. Biol, 128:299-305.[CrossRef]
Zimmer-Faust, R. K., and M. N. Tamburri. 1994. Chemical identity and ecological implications of a waterborne larval settlement cue. Limnol. Oceanogr, 39:1075-1087.
Zou, E., and M. Fingerman. 1999. Effects of exposure to diethyl phthalate, 4-(tert)-octylphenol, and 2,4,5-trichlorobiphenyl on activity of chitobiase in the epidermis and hepatopancreas of the fiddler crab, Uca pugilator. Comp. Biochem. Physiol, 122C:115-120.
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