© 2000 by The Society for Integrative and Comparative Biology
Tadpole Locomotion: Axial Movement and Tail Functions in a Largely Vertebraeless Vertebrate1
1 Department of Biological Sciences, University of Nevada, Las Vegas, Las Vegas, Nevada 89154
2 Department of Anatomy and Neurobiology, Sir Charles Tupper Medical Building, 5859 University Avenue, Dalhousie University, Halifax, Nova Scotia B3H 4H7, Canada
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SYNOPSIS.Tadpoles are exceptional among vertebrates in lacking vertebrae along most of their body axis. Their caudal myotomes are also unusually simple for free-living vertebrates. This overall morphological simplicity, in theory, makes tadpoles good models for exploring how vertebrates control undulatory movements. We used electromyography (EMG), high speed ciné, computational fluid dynamics (CFD), and mechanical tissue testing to understand how Rana tadpoles regulate their locomotion.
Bullfrog (Rana catesbeiana) tadpoles have several patterns of muscle activity, each specific to a particular swimming behavior. Ipsilateral muscles in the tail were active either in series or simultaneously, depending on the tadpole's velocity, and linear and angular acceleration. When R. catesbeiana larvae swam at their natural preferred tail beat frequency, muscles at the caudal end of their tail were inactive. Mechanical tests of tissue further suggest that the preferred tail beat frequency closely matches the resonance frequency of the tail thus minimizing the energetic cost of locomotion.
CFD modeling has demonstrated that the characteristically high amplitude oscillations at a tadpole's snout during normal rectilinear locomotion do not add to drag, as might be supposed, but rather help generate thrust. Mechanical testing of the tadpole tail fin has revealed that the fin is viscoelastic and stiffer in small rather than large deformations. This property (among others) allows the tail to be light and flexible, yet stiff enough to generate thrust in the absence of a bony or cartilaginous skeleton.
Many recent studies have documented predator-induced polyphenism in tadpole tail shape. We suggest that this developmental plasticity in locomotor structures is more common in tadpoles than in other vertebrates because tadpoles do not need to reform skeletal tissue to change overall caudal shape.
Tadpole tail fins and tip, in the absence of any skeleton, are fragile and often scarred by predators. Based on the high incidence of tail fin injury seen in tadpoles in the wild, we suggest that the tadpole tail fin and tip may play an ecological role that goes beyond serving as a propeller to help tadpoles stay beyond predators' reach. Those soft tissue axial structures, by failing under very small tensile loads, may also allow tadpoles to tear free of a predator's grasp.
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INTRODUCTION |
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SYNOPSISINTRODUCTIONREGULATING UNDULATORY LOCOMOTIONTADPOLE MECHANICS AND THE...ECOLOGICAL IMPLICATIONS OF...CONCLUSIONSReferences |
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The last systematic review of tadpole locomotion appeared ten years ago, coincidentally also in American Zoologist (Wassersug, 1989)
. That essay explored the implications of having a tail with neither cartilage nor bone to the overall biology of anurans. The thesis presented there was that tadpole morphology and kinematics are obligatorily linked to the need for the tadpole to have an agile, undulatory premetamorphic "propeller," yet one that can be quickly "jettisoned" (viz, resorbed) at metamorphosis, so as not to handicap saltatory locomotion in the frog.
Here we update what has been learned in the intervening decade about the form and function of the tadpole tail in regard: 1) how tadpoles regulate axial movements and 2) anuran larval ecology. We start with a review of the basic morphological problem. Unlike other aquatic vertebrates, tadpoles are remarkable in lacking in their tails the solid, segmental skeletal elements that give vertebrates their name. For most of their length, tadpoles are vertebraeless vertebrates. Tadpoles have a notochord but, except at the base of the tail where a few vertebral elements develop (which later fuse to form the frog's urostyle), the tadpole's tail completely lacks either osseous or cartilaginous elements (see Fig. 1 in Wassersug, 1989). Not only vertebrae but fin rays, ribs, hemal arches, and scales (i.e., a potential exoskeleton; cf., Long et al., 1996) are all similarly absent. Whereas the tail of most other aquatic vertebrates expands, like a fan, into a caudal fin, the tadpole tail tapers to a point, providing little surface for the generation of thrust. Lastly the myotomes in the tail of tadpoles are but simple chevrons, without the complex nesting and zigzag pattern of fish axial musculature (e.g., Jayne and Lauder 1994, 1996; plus older papers cited therein for descriptions of teleost musculature).
The present paper returns to the question of how the tadpole tail works, given its simple design. Although basic kinematic have been described for tadpoles (Wassersug and Hoff, 1985; Hoff and Wassersug, 1986), understanding the mechanisms controlling body bending and the production of thrust in these organisms requires new approaches. In the following pages, we review recent results from computational fluid dynamic (CFD) modeling, biomechanical tissue testing, ablation studies, and electromyography (EMG). The new data reveal diverse mechanisms for controlling bending in tadpoles which rival those of agile fishes. However tadpoles make a variety of functional tradeoffs in being largely vertebraeless vertebrates.
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REGULATING UNDULATORY LOCOMOTION |
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Tadpoles, like most fishes, swim by lateral undulations of the body axis during which waves of bending pass caudally as the animals move forward (Wassersug and Hoff, 1985
; Hoff and Wassersug, 1986). At one time it was presumed for both amphibian larvae and fishes that such waves of bending were generated by waves of muscle contraction that traveled down the animal's body in direct one-to-one registration with body bending (e.g., Gray, 1936). However, in a now classic set of papers, Blight (1976, 1977) suggested that there are at least three patterns of muscle activity that could produce travelling waves of bending: 1) ipsilateral axial muscles contract serially and alternately with contralateral muscles, 2) ipsilateral axial muscles contract simultaneously and 3) waves of bending are transmitted passively along a portion of the body. In the last decade there have been many studies on muscle activity during swimming for a wide array of vertebrates with body plans more complex than tadpoles' (e.g., Jayne and Lauder, 1996; Jayne and Lauder, 1994, 1995; Gillis, 1998). Our EMG data on bullfrog tadpoles (Rana catesbeiana) suggest that a similarly wide array of muscle activity patterns occur during swimming in tadpoles, despite the simplicity of their axial musculoskeletal system.
Starting, turning and stopping
Bullfrog tadpoles can initiate swimming with either: 1) low amplitude caudal undulations that correspond to low amplitude EMGs and have a clear rostro-caudal lag (see Steady swimming, below), or 2) bending the body sharply into a C-shape, with EMG signals showing high amplitude both rostrally and caudally, and with little or no measurable rostro-caudal lag at the onset of activity (Figs. 1, 2). Initiation of the C-shaped kinematics during rapid starts is thought to be Mauthner cell-mediated in amphibians (Bullock, 1978) but, as has been suggested for fishes (Zottoli et al., 1998), those cells may not be necessary for abrupt C-starts in all species. Mauthner cells, for example, are very small or absent in larvae of some species (e.g., Bufo; Stefanelli, 1951). They are, however, large in Rana, and Rock (1980) was able to correlate Mauthner cell activity with early contralateral muscle contractions in R. catesbeiana tadpoles. Anatomical and kinematic studies by Will (1991) also support the involvement of Mauthner cells in the startle responses of Rana, as well as Xenopus tadpoles.
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R. catesbeiana tadpoles turn in two ways. They can make tight turns, with turning radius less than 10% of bodylength (0.1 L; where L = tadpole length from snout to tail tip), in response to a mechanical stimulus delivered when they are stationary. In this situation, EMG signals show high amplitude spikes both rostrally and caudally (as in C-starts, Fig. 2A), with no contralateral muscle activity and no detectable rostro-caudal lag at the onset of muscle activity. The tadpoles can also turn in much wider arcs, which they do spontaneously, when they initiate swimming from rest, and in the course of normal locomotion. Spontaneous turns are of variable duration, forming a continuum with the very tight turns and taking up to 1 s to complete.
The major difference between tightest and widest turns is that during wide turns muscles on both sides of the body are active throughout most of the turn. The EMG signals recorded during these turns are of lower amplitude and of greater duration on the convex than on the concave side. These spontaneous wide turns are frequently (70% in our study) continuous with the next tail beat. Regardless of angular velocity (= speed of the turn), none of the tight and wide turns we recorded (n = 20) showed a measurable rostro-caudal lag in initial muscle activity on the turning side. Although the resolution of this study was not sufficient to preclude the possibility of rostro-caudal lag in initial muscle activity, the pattern of EMG activity among starts and turns was consistent and distinctive and was not continuous with or overlapping the easily discernable rostro-caudal lag in EMG pattern seen in steady swimming, below.
Weihs (1972) considered sharp turns to be part of the same kinematic pattern that produces fast starts in fishes and speculated that they could be controlled by the same neuromuscular program. However, our work with tadpoles and much recent work on fast-starts in fishes (e.g., Wakeling and Johnston, 1998, 1999) indicate a greater diversity of movement and neuromuscular pattern. For example, the EMG pattern for turns shares a defining feature (large amplitude, no or low rostro-caudal lag ipsilateral activity) with the C-start EMG pattern (see Fig. 2B). However low amplitude EMG activity on the side opposing the turn, concurrent with muscle activity on the turning side, was clearly present and distinctive in the slower and wider turns. This contralateral muscle activity may stiffen and straighten the tail, increasing the radius of the turn. Indeed, turning radii for R. catesbeiana tadpoles can vary from 0.05 L in a C-start to more than 1.0 L in a slow wide turn (Hoff, 1987). It is thus likely that tadpoles—in the absence of a skeletal system to stiffen the terminus of the tail—rely on muscle activity to vary stiffness, as well as the duration of muscular contraction, in order to control both the radius of a turn and the amount of rotation.
Unlike fishes, anuran larvae do not have large enough exposed lateral appendages to aid in slowing down, or stopping (at least not until near metamorphosis; Wassersug, 1989). We frequently observed bullfrog larvae simply gliding to a stop, without detectable EMG activity in axial musculature. However, during the glide before a stop, we have observed in some of our EMG recordings muscle activity simultaneously on both sides of the caudal end of the tail. This unlikely action presumably stiffens the tail and dampens caudal oscillations (Fig. 2C). It may extend the tadpoles glide before the animal comes to a complete halt.
Tadpoles can also bend the posterior tail to the side sharply to form a J-shape for active braking. (J-stops are also used by salamander larvae [Hoff et al., 1985]). It is notable that this is the only swimming activity documented in Rana tadpoles in which muscle activity is initiated caudal to the midtail; that is, muscles are active at 0.8 L, but not at 0.7 L (Fig. 2C).
Steady swimming
For very slow, steady swimming tadpoles recruit only a fraction of their axial muscle. We have observed that when bullfrog tadpoles swim with a velocity less than 1.0 L·s–1, the rostral portion of their body and tail does not bend. Here both travelling waves and muscle activity are confined to the caudal portion of the tadpole. Specifically EMG activity is present at 0.7 and 0.8 L, but not at 0.45 L (Fig. 3). This EMG pattern nicely matches that seen in eels, similarly swimming at low speed (Gillis, 1998).
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At slightly higher (>1.0 L·s–1; i.e., intermediate) swimming velocities, tadpoles "shift gears" and use only rostral axial muscles (at 0.45 L and 0.7 L, but not 0.8 L; Fig. 3). By passive manipulations of anesthetized Rana tadpoles, we have demonstrated that muscle activity in the caudal half or so of the tail is not necessary for normal waves of bending during steady swimming at these speeds (Wassersug and Hoff, 1985
). For these experiments, the anesthetized tadpoles were mounted on a mechanical oscillator through their center of axial muscle mass, which is at the base of their tails (Fig. 4). When the tail base was oscillated from side to side with an appropriate amplitude for that segment of the animal during normal swimming at low to moderate tail beat frequencies (i.e., up to approx. 5 Hz), the kinematic wave form in the rest of the tail completely mimicked that seen during unconstrained swimming. Indeed, at swimming velocities between 1 and 6 L·s–1, we observed no EMG activity at 0.8 L.
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To achieve higher swimming speeds, however, more axial muscles must be recruited. Our passively oscillated, anaesthetized tadpole could not transmit travelling waves of normal kinematics to the end of the tail at tail beat frequencies greater than 6 Hz. And, as expected, normal tadpoles swimming at higher tail beat frequencies showed EMG activity at all electrode sites (Fig. 3).
In 33 of 36 swimming sequences, we found a rostro-caudal lag in EMG activity (measured from EMG onset) between electrodes at 0.45 L and 0.7 L. In the remaining 3 sequences no rostro-caudal lag in EMG activity was detected for at least 3 tail beats suggesting that ipsilateral axial muscles could be active simultaneously. Where rostro-caudal lag was detectable, muscle activity lag matched the speed of mechanical bending wave for tail beat frequencies above 3 Hz (Fig. 5A). However, at tail beat frequencies at or below 3 Hz, there was a clear phase shift of as much as 1° per 1% body length between the wave of EMG activity and the bending wave (Fig. 5B). For all steady swimming ipsilateral axial muscles contracted serially and alternately with contralateral muscles, that is electrodes placed on the right and left sides at both 0.45 L and 0.7 L showed a lag between onset of EMG signals of 0.5 T (where T = period of the tail beat).
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The duration of muscle activity at rostral, midtail and caudal sites was variable; however, midtail (0.7 L) sites were generally active for longer (duration = 0.30 to 0.41 T) than were rostral (0.45 L) sites (duration = 0.30 to 0.34 T) during steady swimming at moderately low speed (i.e., tail beat frequency = approx. 3 Hz; equivalent to velocity of 1–2 L·s–1, see Fig. 5B). The greater duration of muscle activity at the more caudal sites is unusual among swimming vertebrates studied to date (see Gillis, 1998
) and gives emphasis to the diverse uses of the caudal portion of the tail in tadpoles. Although the musculature at 0.8 L is not active during steady swimming at moderate speeds, it is active at both high speed and very low speed, as well as during starting, turning and occasionally stopping. This extensive use of the most caudal musculature has not been documented in other swimming vertebrates.
The apparent differences in phase shift and duration of EMG activity between slow swimming and faster swimming may be due in part to the relatively low time resolution (±5 ms) in our EMG recordings, and also to the fact that the electrodes in the very thin portions of the tail (at 0.7 L and 0.8 L), where there is very little muscle, are probably recording activity in both the superficial red fibers and the deeper white muscle fibers (see Fig. 1), while the electrodes at 0.45 L almost certainly recorded from white fibers only. From previous work on eels (cf., Gillis, 1998) we would expect that red fibers would be active during slow and fast swimming and would show a rostro-caudal phase shift, while white fibers (active at faster swimming speeds) may not show a phase shift. The variability we observed in EMG activity, as well as in the tadpoles' kinematics, also reminds us that tadpoles rarely swim steadily. Rather, they are starting, stopping or turning in almost every tail beat (Wassersug and Hoff, 1985; Wassersug, 1989).
In sum, R. catesbeiana larvae appear to make use of all of Blight's possible patterns of axial muscle activity during normal swimming behaviors. The most easily identifiable difference in motor patterns between tadpoles and more anatomically complex swimming vertebrates is that the terminal portion of the tadpole tail has a large and distinctive role in swimming. We speculate that such regional activity within the terminal portion of the tail would be less likely, if the tail were stiffened by a limited number of articulating vertebrae and fin rays along its total length.
Neural control vs. mechanical factors regulating tadpole locomotion
By differential recruitment of caudal muscle, tadpoles can regulate their swimming speed over a range from less than 1 L·s–1 to almost 30 L·s–1, depending on the species (see Table 1 in Wassersug, 1989; see also discussion of tadpole swimming performance in Wassersug, 1997a). We have found, however, that unrestrained tadpoles have preferred swimming speeds and tail beat frequencies. For example, 10 cm R. catesbeiana at room temperature swim at tail beat frequencies that cluster around either 3 Hz or 10 Hz (Fig. 6). Xenopus laevis larvae overwhelmingly used frequencies of approximately 10 Hz independent of size over a fairly broad range (3.4–4.6 cm) of L (Hoff and Wassersug, 1986), with slightly higher frequencies seen at younger developmental stages and at higher temperatures (van Mier, 1986).
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2 for each) shows a bimodal distribution of preferred tail beat frequencies. Frequencies of approximately 3 Hz and 10 Hz were selected far more often than other tail beat frequencies in response to mechanical stimulation. B) During spontaneous swimming 19 of 20 tadpoles selected tail beat frequencies of approximately 3 Hz. Each bar represents a frequency range of 0.5 Hz.Undoubtedly intrinsic neural rhythms account, in part, for preferred tail beat frequencies. The central pattern generator in Xenopus embryos has been particularly well studied by Roberts and his colleagues (e.g., Roberts, 1989
; Arshavsky et al., 1993; Yoshida et al., 1998). Less attention has been given to Rana. However, spontaneous bursts of rhythmic activity have been recorded in vitro from the ventral motor roots of R. catesbeiana larvae (Stehouwer and Farel, 1980) and those authors observed a typical pattern almost identical to the EMG activity pattern we observed in the muscles innervated by the same ventral roots in our R. catesbeiana larvae.
As a cautionary note, the rates reported for ventral root bursts in R. catesbeiana varied both among and within preparations: the interval between bursts ranged from 100 to 500 ms (approx. frequency = 2–5 Hz; Stehouwer and Farel, 1980). This range is so large that it seems unlikely that spontaneous rates of activity intrinsic to a CNS pattern generator alone dictate the preferred tail beat frequency of bullfrog tadpoles.
An alternative (or complementary) hypothesis examined by Oxner et al. (1993) is that preferred tail beat frequency for a tadpole is influenced by the intrinsic properties of the tadpole's tissues. Storage of energy in muscles, skeletal elements, and skin was suggested by various authors in the 1970s and 1980s (see Wainwright 1983 for an introduction to the literature) and has since been documented for a variety of undulating and oscillating organisms (see various papers in previous American Zoologist symposia "Aquatic Locomotion" 36:535–735 [1996] and "Muscle Properties and Organism Function" 38:697–792 [1998], plus this symposium). Such studies all suggest that elastic storage can contribute to greater locomotor efficiency, if the animal's movements are matched to the optimum rates of energy loading and release; i.e., the resonant frequency of its structures.
In this vein, Oxner et al. (1993) modeled the swimming tadpole as a flexible beam with variable moment of inertia and cross-sectional area. They sectioned tadpole tails and compressed the sections with a strain regime that simulated the maximum compressive strain of a swimming animal in order to obtain empirically the elastic modulus for the tail as a whole. It was gratifying to see that the lowest fundamental frequency (resonant frequency) predicted by their model was in good agreement with our observed 3 Hz tail beat frequency preferred by tadpoles of the same size and species. These observations suggest that tissue mechanics may be as important as motoneuron firing patterns in the regulation of tadpole locomotion. We have only recently begun to examine how much energy can be stored in the various tissues of the tadpole tail (infra vide "Mechanical testing of tail fin tissue"), but of particular interest is the notochord (cf. Koehl et al., 2000). In R. catesbeiana the notochord is continuously covered by elastic tissue, with the proximal and distal halves of the notochord surrounded by separate single layers of elastin that overlap at approximately midtail (0.65–0.7 L; Bruns and Gross, 1970). This construction guided the original placement of electrodes for our EMG study, since preliminary midtail recordings around this area made it clear that axial muscles rostral to the midtail are used differently from those caudal to that region. The tadpole tail thus seems to be divided into two functional subunits in both the structure of the notochord and muscle recruitment during swimming.
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TADPOLE MECHANICS AND THE GENERATION OF THRUST |
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Computational fluid dynamic modeling of tadpole locomotion
Perhaps the key problem identified, but not solved, in the Wassersug 1989
review was how tadpoles manage to generate thrust with virtually no axial skeleton, high amplitude oscillations at the snout and tail tip, and approximately half the propulsive musculature mass of carangiform and subcarangiform fishes of similar size. The most substantial advance on this problem comes from CFD studies of the unsteady flow around swimming tadpoles, obtained using a finite volume method (FVM; Liu, 1995; Liu and Kawachi, 1999; also introduced in Wassersug, 1997b). This method assesses the momentum transferred from the undulating organism to the surrounding water. Tadpoles are the first undulatory vertebrates to be realistically modeled by this powerful technique. Such CFD models allow for both flow visualization and the direct calculation of thrust/power production (Fig. 7).
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Liu et al.'s FVM, like other finite element techniques, requires that the fluid space around the tadpole be discretized; that is, represented by a grid that matches the moving body and deforms with each time iteration. The accuracy of a CFD result from the FVM depends on the product of grid density (approx. 7 x 105 grids in Liu et al.'s 1997
case, with the grid density highest adjacent to the tadpole) and the number of iterations run for each simulation.
Accuracy in CFD modeling also depends on the geometric realism of the model swimmer and its kinematics. The geometry of the undulating object, as it continually changes shape, must be describable by manageable equations. To meet this requirement Liu et al. digitized images of a bullfrog tadpole in dorsal and lateral view and constructed a "virtual" tadpole from a series of ellipsoidal cross-sections, derived from those images. A spline fit was then used to smooth the surface between ellipsoids. Next Liu et al. incorporated kinematic data on the wave form in the tail of a swimming tadpole, taken directly from Wassersug and Hoff (1985). Those data were for a 4.7 cm tadpole of the same species swimming at a constant velocity (Reynolds number = 7,200) on a straight path.
A few simplifying assumptions were necessary. For example, the Liu et al. "virtual" tadpole lacked an oral disc, protruding eyes, and a spiracle. These structures provide only slight surface irregularities in live tadpoles and were assumed to have little impact on flow at the level resolved by the CFD simulations.
A small portion of the tail tip was truncated in the "virtual" tadpole to avoid a problem with overlapping grid lines around a sharply pointed tail. This altered the tadpole's surface area by approximately 1%, but did not affect the realism of the CFD models. In fact, R. catesbeiana tadpoles drawn from a lake in eastern Canada were found lacked (presumably due to predators; see below) on average 2% of their tail surface area, most commonly around the tail the tip (n = 98; Blair and Wassersug, 2000). Neither force production nor flow were significantly affected by this slight caudal truncation in the model bullfrog tadpole (Liu et al., 1997), which is not surprising since our EMG studies had already suggested that the tip of a Rana's tail generates little or no thrust during rectilinear, constant velocity swimming at the Reynolds number used in our calculations.
Lastly the Liu et al. (1997) study assumed that at every instant the tadpole had a smooth but firm skin; i.e., that the tadpole deformed the water and not visa versa. Surprisingly, despite the fact that the tadpole's tail fin lacks an internal or external skeleton, both high speed videography and mechanical testing of the tail fins (see below) suggest that this assumption was valid.
As an aside, the importance of using realistic morphologies in CFD modeling is underlined by the recent effort of Carling et al. (1998) to model in 2D anguilliform locomotion. On one hand, those authors criticize Liu et al. (1997) for using a "tethered" tadpole. Specifically Liu et al. (1997) fixed their frame of reference onto the tadpole's body, which, in their study of constant velocity swimming, had no effect on the biological accuracy of the results. On the other hand, for their own analysis Carling et al. used an attenuate isosceles triangle with a blunt snout as their model organism, rather than the profile of an eel. Since pressure at the front end of undulating aquatic vertebrates can be very high (DuBois et al., 1976), fluctuate greatly, and produce major cross flows with each tail beat (Lighthill, 1993; Liu et al., 1997), minor changes in rostral geometry lead to major changes in CFD results. (The unrealistic flows demonstrated by the Carling et al. [1998] model can be understood in that light.)
The Liu et al. (1997) study answered several longstanding questions about thrust generation and efficiency during tadpole locomotion. It indicated, for example, that large lateral oscillations at the snout, which have previously been considered inefficient in tadpoles (Wassersug and Hoff, 1985; see also discussion in Wassersug, 1989) are, to the contrary, important for the efficient generation of thrust. This was established by testing locomotor performance when the CFD model tadpole was swum according to alternative, namely subcarangiform fish, kinematics (taken from Videler, 1993). Subcarangiform fishes show little lateral displacement of the snout during rectilinear locomotion compared to tadpoles. Without the lateral oscillations at the snout, Froude efficiency for the tadpole plummeted.
Isocontour pressure plots on the body of the undulating virtual tadpole (Fig. 7a) confirm that the bodies of tadpoles help propel water caudally and that oscillations at the snout contribution to the generation of thrust rather than drag. Although no deformation (lateral flexion) of the body anterior to the tail has been observed in tadpoles while they swim, the fact that they use not just their tails but also their rotund bodies to generate thrust suggests that axial muscles within the torso and at the base of the tail must be contracting when tadpoles swim at moderate to high speeds. This ineluctable conclusion from the CFD analysis independently confirms the EMG results discussed earlier.
In the context of the greater mystery of how tadpoles generate sufficient thrust with a vertebraeless and not particularly muscular tail, the CFD and EMG results now lead to one partial answer: Tadpoles "cheat." Approximately half of the axial musculature in tadpoles lies hidden from view in their bodies anterior to the tail (Fig. 4). Those anterior muscles arise from vertebrae within the globose body of the tadpole and are, indeed, used in both acceleration and sustained swimming at higher velocities.
Part and parcel of the high lateral oscillations of the rostrum during tadpole swimming is flow separation off the body, which then reattaches around the high point of the tail fin (Fig. 7a). The streamlines and pressure pattern on the tail indicate that the majority of thrust is generated anterior to the last quarter or so of the tail.
As the tail tapers to a point, it contributes less and less to the generation of thrust during constant velocity, rectilinear locomotion. That does not mean, though, that the tail tip is strictly decorative. As noted above, its core muscle is active during many swimming behaviors. It also acts as added mass to resist excessively high oscillations more rostrally in the tail (Wassersug and Hoff, 1985), which would raise the Strouhal number and decrease locomotor efficiency (cf. Liu et al., 1996).
Mechanical properties of tail fin tissue
Another part of the mystery of how tadpoles manage to generate thrust with their unsupported tail fins has been partially resolved by Doherty et al. (1998), who exposed tissue samples from the dorsal fin of bullfrog tadpoles to the following mechanical tests: large deformation cyclic load, small-deformation forced vibration (both at 1 and 3 Hz), and stress relaxation.
Tissue strips, 0.5 x 1.5 cm, were taken from the dorsal margin of the dorsal fin, where the tail is deepest and, as noted above, most thrust is generated. In this region the fin is essentially a double layer of skin, underlain by a crossed array of collagen fibers that then surrounds a loose (gelatinous) connective tissue core.
All of Doherty et al.'s tests revealed that R. catesbeiana fins are strikingly viscoelastic compared to other connective tissues. In stress relaxation tests, for example, the fin samples, when loaded with just 3 g (19 kpa stress), lost half of the stress at initial extension in a single second and 85% after 100s.
Doherty et al. further demonstrated that these viscoelastic fins are three times stiffer in small vibration than large cyclic deformation at the same frequency and strain. Since the frequency and amplitude of the loads in their tests realistically encompassed the loading regime for unconstrained, freely swimming bullfrog tadpoles (Hoff, 1987; Oxner et al., 1993), these results suggest that the fin should remain stiff when the tadpoles swim naturally.
Doherty et al., however, also found that their fin strips were exceptionally weak under tension; i.e., failing with loads as small as 5 g (32 kpa stress). Could there ever be a situation where it would be to the advantage of a swimming vertebrate to have such fragile fins? Doherty et al. speculated that tadpoles can escape the grasp of predators easily, if the fins deform viscoelastically and then tear easily. The tadpoles will end up with lacerated fins, but will still be alive. One might even suppose that escape would be less likely if the tadpoles had firm skeletal elements in their tails for predators to latch on to.
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ECOLOGICAL IMPLICATIONS OF HAVING A TAIL WITHOUT A SKELETON |
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A tail without a skeleton can change rapidly. Wassersug (1989)
argued that the vertebraeless tadpole tail allows for rapid resorption at metamorphosis. In the intervening years, it has been discovered that the mere proximity of tadpoles to natural predators can induce changes in their tail in many species (e.g., Smith and Van Buskirk, 1995; McCollum and Van Buskirk 1996; McCollum and Leimberger 1997; Van Buskirk and Relyea, 1998), well before metamorphosis. This process is known as predator-induced polyphenism and, like metamorphosis, happens quickly (A. McCollum, personal communication). We suggest that the developmental plasticity that is essential for predator-induced polyphenism, could not be so rapid, if the tadpoles had to either deposit or resorb minerals in vertebrae and/or fin rays before they could reshape their tails. Consistent with that speculation is the fact that many more cases of polyphenism in locomotor structures have been documented for tadpoles than fishes.
The most commonly reported predator-induced change in tadpole tails is an increase in relative tail height. One might suppose, by comparison with fishes (e.g., Webb, 1984, 1986), that a proportionately taller tail would specifically enhance maneuverability—i.e., angular and linear acceleration—rather than either endurance or maximum velocity, but that has not been assessed. It may also be true that if the tail fin provides some protection to core caudal muscle, then predator-induced increases in fin size may be important to tadpoles for a reason other than improved locomotor performance. Enlarged fins could aid tadpoles in escaping a predator, if they limited the predator's grasp to the fin and blocked the predator from planting claws or jaws into core muscle (Doherty et al., 1998).
Sacrificing fins to predators
The above speculation is predicated on the belief that tadpole fins are commonly grasped and torn by predators, with the tadpoles often escaping otherwise lethal injury. CFD models (Liu et al., 1997) and high speed ciné studies (Wassersug and Hoff, 1985) suggest that the tail tip is the most dispensable (i.e., least essential for generation of thrust) portion of the tail in tadpoles. But how often is it damaged in nature? And is there evidence that it serves a protective role for tadpoles independent to its role in locomotion?
Recently we surveyed the pattern and amount of tail damage found in wild caught tadpoles for a variety of species of anurans that differed in larval ecology (Blair and Wassersug, 2000). As expected, the level of injury was high. The percentage of wood frog (Rana sylvatica) tadpoles, for example, with damaged tails ranged from 37 to 47% for five ponds in Nova Scotia (n = 100 tadpoles per pond) with each tadpole having lost on average 2.6 ± 0.3% of its tail area, mostly from the tip. A few tadpoles (
2%) had even lost over 25% or their tail! In staged experiments in enclosures, Semlitsch and his colleagues (see Figiel and Semlitsch, 1991, plus older papers cited therein) found that tadpoles could, in fact, endure that much tail loss without increasing their susceptibility to predators. We, however, found evidence of a long term cost to young tadpoles that received that much tail damage. The percentage of injury found in older (stage 31; Gosner, 1960) R. sylvatica tadpoles was less than a quarter that of younger (stage 26) individuals. This could be accounted for by many factors, such as stage-specific differences in the rate of fin regeneration and repair. However, the most parsimonious explanation, we believe, is that the more severely injured a tadpole's tail tip is in early encounters with predators, the less likely that tadpole is to survive later encounters.
There are some interesting exceptions to this inverse relationship between the amount of caudal injury and the developmental stage of tadpoles, as seen in R. sylvatica. In toad (Bufo americanus) tadpoles, which are unpalatable to many predators (Wassersug, 1973), the average amount of injury per individual tadpole was only 0.6% (n = 100), or less than a 
th that found in R. sylvatica. That level of injury in Bufo was also constant across developmental stages suggesting that Bufo larvae can endure this small amount of injury without significantly increasing their susceptibility to further injury.
Another exception may be tadpoles that have colored markings on their tail tip (Caldwell 1982, 1986; McCollum and Van Buskirk, 1996; McCollum and Leimberger, 1997; Skelley, 1997). These markings can be induced in some species by predators and thus constitute another form of predator-induced polyphenism. Caldwell in 1982 (also 1986), and others since, have suggested that such tail "flags" act as a lure to draw predator attacks away from the head end of the tadpole. If that is true, then we would expect tadpoles with such markings to: 1) have tail injuries highly concentrated on that portion of the tail; and 2) the longer tadpoles with these conspicuous tail tips are in the ponds, the more predator "hits" their conspicuous tail tips would receive.
Our colleague, Joanna Blair, has confirmed these two hypotheses with a population of Ascaphus truei tadpoles from Oregon. Ascaphus tadpoles have a conspicuous white dot surrounded by a black band at the tip of their tails. Despite the fact that most (51%; n = 81) of the tadpoles had some tail damage, only one individual had >5% (but still less than <10%) of its tail absent. The average amount of the tail lost per individual was just 1.1 ± 0.2% of tail area, but the injury was strongly focused on the tail tip (Fig. 7b). Furthermore the amount of injury found in individuals increased significantly with age, consistent with the hypothesis that colorful tail tips draw predator attacks.
Collectively the above observations on tail injury suggest that the attenuated tail tips of tadpoles may be more adapted for helping tadpoles survive the grasp of predators than helping keep them forever beyond any predator's reach. It is important to reiterate, though, that the end of the tadpole tail cannot be sacrificed without some cost. Ablation of the most caudal portion of the tail fin results in a clear reduction in maximum swimming velocity (Fig. 8). As already noted from our EMG studies, the musculature near the tip of the tail is not recruited when tadpoles swim naturally at their preferred speed. However it may be called into play during starting, turning, stopping, and during both very slow and very fast swimming.
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In our previous studies of the tadpole tail, we suggested that the absence of a skeleton in the tail permitted: 1) high maneuverability and 2) rapid metamorphosis. To this we can now add the fact that the tail plays a role in how tadpoles interact with their predators, which goes beyond simply swimming out of range. The tails of tadpoles from several species exhibit polyphenism, which allows them to rapidly change shape (and color) in response to predators. We suggest that this developmental plasticity is facilitated by the fact that the tadpoles do not need to buildup or breakdown mineralized tissue in order to change tail shape. One might suppose that what tadpoles lose in mechanical efficiency by having such a flexible tail (Liu et al., 1996
, 1997), they make up for in developmental flexibility.
Axial movements in tadpoles are regulated by a diverse array of muscle activity in a manner similar to anguilliform fishes (Gillis, 1998). Under various circumstances, tadpoles manifest all of the patterns of muscle recruitment noted by Blight (1976, 1977) in his studies of axial muscle activity.
When we combine the results from our studies on the regulation of axial propulsion in tadpoles with what we currently know about predator influences on tadpole tails, we arrive at a picture of an axial structure—the tadpole tail—that is functionally and developmentally as adaptable as the caudal fin of most teleosts. Overall our studies demonstrate that, despite their relatively simple morphology, tadpoles have an elegant array of mechanisms for controlling their axial locomotion. But in the absence of a solid skeleton, the soft tissue—be it the loose connective tissue of the fins or the muscles themselves—must, literally, take up the load.
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ACKNOWLEDGMENTS |
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We thank J. Blair, P. Doherty, K. Kawachi, J. M. Lee, and H. Liu for collaborating with us on various studies of tadpole functional morphology. Manuscript production was greatly facilitated by T. Lownds and S. Whitefield. In addition we thank J. P. Caldwell, B. C. Jayne, G. B. Gillis, J. H. Long, Jr., R. W. Marlow, S. A. McCollum and G. W. Thiemann for helpful discussion and encouragement along the way. This research was supported by funds from the National Science Foundation (USA) and the Natural Science and Engineering Research Council of Canada.
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FOOTNOTES |
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1 From the Symposium The Function and Evolution of the Vertebrate Axis presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 6–10 January 1999, at Denver, Colorado.
3 E-mail: tadpole{at}is.dal.ca
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