© 2001 by The Society for Integrative and Comparative Biology
Introduction to the Symposium: Ontogenetic Strategies of Invertebrates in Aquatic Environments1
1 Laboratoire d'Ecophysiologie des Invertébrés, EA 3009 Adaptation Ecophysiologique au cours de l'Ontogenèse, Université Montpellier II, Place Eugène Bataillon, 34095 Montpellier cedex 05, France
2 Marine, Earth and Atmospheric Sciences, Box 8208, North Carolina State University, Raleigh, North Carolina 27695-8208
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This symposium presents different ecological and physiological strategies used by invertebrates to successfully adapt to aquatic environments. Adaptation has been studied mainly in adult animals, but the papers comprising the symposium emphasize ontogenetic strategies, starting from the principle that natural selection acts on all stages of development. Adaptive strategies may thus differ strikingly between developmental stages of the same organism. Invertebrates offer a wide array of ecophysiological models for study, and these are exemplified by the contributions to the symposium, which are briefly summarized. Future research in the field will 1, expand the number of models for comparative purposes; 2, examine the strategies, not only of larvae and juveniles, but also of embryos, eggs and reproductive cells; and 3, investigate the genetic basis of ontogenetic strategies.
The symposium aims at a better understanding of ontogenetic strategies of invertebrates in aquatic environments, and of their evolutionary significance. Particular emphasis is given to ecophysiological, and to a lesser extent behavioral, aspects of adaptation. In this introduction we comment on the key words that define the objectives of the symposium, briefly indicate the contributions of the participants, and propose areas for future research.
Ecophysiology developed as a blending of studies conducted in animals under natural conditions and of comparative physiology, in order to examine the physiological diversity of organisms in relation to the environments in which they live. Comparing animals from different habitats and finding out the solution each species uses to solve its problem elucidated general principles in adaptive physiology (Feder et al., 1987
; Schmidt-Nielsen, 1997; Bradley and Zamer, 1999). The accumulation of knowledge from comparative ecology and physiology clarified that organisms have morphological and physiological features, acquired through variation and natural selection, that increase their fitness in their environment (Bartholomew, 1987; Schmidt-Nielsen, 1997). Ecophysiology thus deals with Darwinian evolution, hence the use of the term evolutionary physiology (Bradley and Zamer, 1999), itself a part of evolutionary biology (Futuyma, 1998). Several presentations of the symposium, indicated below, illustrate these ecophysiological aspects.
The term strategies points to the different but coordinated ways, both ecological and physiological, an animal uses to adapt to its habitat. Hence a strategy covers all (in an ideal situation of knowledge) or several adaptations, and their interrelations, used by an animal in its environment. The word adaptation has been widely used and discussed for several decades. As stated by Bennett (1987) and Bartholomew (1987), it would be misleading to consider that the aim of ecophysiology is to demonstrate that animals are adapted to their habitats, or that they are using good strategies, which is close to demonstration of evidence, or tautology. The objectives of studies aimed at deciphering adaptations and strategies are thus not to show that animals can live in their habitats, but "to describe how animals function in their natural environments and how they adapted to them evolutionarily" (Bennett, 1987, p. 6). Strategies encompass morphological, physiological, ecological and behavioral adaptations, and only the first leaves a trace in the fossil record, enhancing the difficulty of deciphering the evolution of the others. The evolutionary approach has shown that adaptive change is driven by chance variation and by natural selection, which lead to systems that function adequately and not perfectly with regard to the environmental conditions (Bartholomew, 1987).
The strategies addressed in the following articles concern invertebrates, particularly but not exclusively crustaceans, in aquatic environments. Vertebrates were excluded because the large volume of information on them would warrant a separate symposium. Many invertebrates live in aquatic environments. They have developed strategies including adaptations to such factors as temperature, pressure, density, currents, available oxygen, light, salinity, and pH, the two latter being exclusive features of the aquatic media.
Strategies permitting invertebrates to successfully occupy aquatic environments have long been studied almost exclusively on adult stages. In contrast, this symposium emphasizes ontogenetic strategies, i.e., the adaptations of the different stages of development of diverse taxa. The rationale behind an ontogenetic approach to adaptive strategies is simply that natural selection acts on all stages of development. As Bartholomew (1987, p. 18) stated, "... an organism must not only be adapted to meet major environmental challenges, it must also be capable of carrying out essential functions during its entire life..." In a landmark article on the importance of an ontogenetic perspective in physiological studies, Burggren (1992, p. 236) insisted that "Developmental studies of physiological traits... should be included wherever possible in studies of physiological adaptation" (see also Metcalfe and Stock, 1987; Wood et al., 1992).
Aquatic invertebrates are excellent subjects for such investigations because most produce abundant reproductive cells, eggs and larvae, and they use water as a dispersal medium for their offspring. Depending on the ecology of the adults and of each of the young stages, which may live in water bodies where environmental conditions vary greatly in space and time, the environment may exert different selection pressures on each developmental stage. Successful strategies will thus differ between stages. Therefore, ecological and ecophysiological studies aimed at deciphering the adaptive strategies of a species should be conducted at all stages of its life cycle from egg to adult.
Transitional periods such as hatching or metamorphosis (in those species where it occurs) are generally accompanied by high rates of mortality observed both in the field and under experimental conditions. A well-known example of such events is the critical megalopal stage of brachyuran decapod crustaceans (Anger, 1996). Particular attention should be directed to these periods or stages which may correspond to changes in ecological relationships associated with morphological, behavioral and physiological changes. Several presentations of the symposium deal with such topics by selecting examples of ontogenetic strategies of invertebrates in aquatic habitats ranging from fresh to brackish to sea water, and they range from ecophysiology to behavior and ecology.
The first four articles deal with the ontogeny of physiological functions and their adaptive relations with environmental factors. N. B. Terwilliger and M. Ryan address the ontogeny of respiration in crustaceans through the structure and function of respiratory proteins and the expression of their genes, in relation to external factors such as food availability, temperature and hypoxia. J. I. Spicer gives particular attention to the ecological relevance of development of cardiac function in crustaceans. G. Charmantier and M. Charmantier-Daures deal with the ontogeny of osmoregulation in embryos of crustaceans and its relation to salinity tolerance. E. S. Chang, S. A. Chang, and E. P. Mulder then treat the appearance of hormonal coordination in crustaceans, with emphasis on ecdysteroid structure and functions.
The following article illustrates examples of ontogenetic adaptations to extreme environments. T. W. Cronin and R. N. Jinks describe the development of vision and the larval and adult adaptations to different ontogenetic habitats in a hydrothermal vent crab whose successive developmental stages are exposed to strikingly different light conditions.
The next two articles deal with ontogenetic adaptations leading to larval settlement. R. B. Forward, R. A. Tankersley, and D. Rittschoff analyze ontogenetic behavioral changes in crustacean larvae during tidal transport, settlement and metamorphosis in response to environmental cues. Finally, from studies conducted in annelids and molluscs, M. G. Hadfield, E. J. Carpizo-Ituarte, K. del Carmen, and B. T. Nedved propose the hypothesis that rapid metamorphosis in most marine invertebrate larvae requires little or no de novo gene action and permits a fast settlement and the accompanying change in habitat.
As the symposium presentations and numerous other studies illustrate, a significant body of knowledge is already available on the ontogenetic strategies of invertebrates in aquatic environments, and yet much remains to be done, with at least three main pathways for progress. One is continuing efforts to collect information from new models of the ecophysiology of ontogeny. The comparative method so useful in general ecology and physiology is also profitable in ontogenetic studies. Some invertebrate species are known to hatch in seemingly improbable habitats, for example in empty snail shells or hollow leaves containing a limited amount of water, and the mechanisms permitting the development of eggs and/or larvae in such temporary and harsh conditions will open new insights in the general knowledge of how animals adapt to their habitats.
Secondly, ontogenetic studies have mainly concentrated on larval stages, with a lesser body of information in embryos and reproductive cells. Further work will ideally encompass all ontogenetic stages. Finally, new techniques adapted to the small size of the stages have begun and will have to be applied. In particular, molecular techniques can illuminate the genetic basis of ontogenetic strategies, for example of different proteins such as neuropeptides, gas carriers and ion transporters, and their regulators, according to the physiology-to-genes and the gene-to-physiology approaches (Arnold, 1987; Bennett and Lenski, 1999). Respectively, these examine the genetic basis of physiological or ecophysiological traits and the influence of single genes on organismal characters that change with development.
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This symposium was held during the annual meeting of the Society for Integrative and Comparative Biology (SICB), hosting the winter meeting of The Crustacean Society (TCS), at Chicago, IL, 3–7 January 2001. It was funded by the National Science Foundation (NSF), the Division of Invertebrate Zoology and the Division of Ecology and Evolution of SICB, and TCS. We thank several people who were instrumental in the advent of the symposium, Martin E. Feder, President of SICB, John Pearse, Program Officer of SICB, Jens T. Hoeg, President of TCS, Jack O'Brien, Program Officer of TCS.
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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: charmantier{at}univ-montp2.fr
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Anger, K. 1996. Salinity tolerance of the larvae and first juveniles of a semiterrestrial grapsid crab, Armases miersii (Rathbun). J. Exp. Mar. Biol. Ecol, 202:205-223.[CrossRef]
Arnold, S. J. 1987. Genetic correlation and the evolution of physiology. In M. E. Feder, A. F. Bennett, W. W. Burggren, and R. B. Huey (eds.), New directions in ecological physiology, pp. 189–215. Cambridge University Press, Cambridge.
Bartholomew, G. A. 1987. Interspecific comparison as a tool for ecological physiologists. In M. E. Feder, A. F. Bennett, W. W. Burggren, and R. B. Huey (eds.), New directions in ecological physiology, pp. 11–37. Cambridge University Press, Cambridge.
Bennett, A. F. 1987. The accomplishments of ecological physiology. In M. E. Feder, A. F. Bennett, W. W. Burggren, and R. B. Huey (eds.), New directions in ecological physiology, pp. 1–8. Cambridge University Press, Cambridge.
Bennett, A. F., and R. E. Lenski. 1999. Experimental evolution and its role in evolutionary physiology. Amer. Zool, 39:346-362.
Bradley, T. J., and W. E. Zamer. 1999. Introduction to the symposium: What is evolutionary physiology? Amer. Zool, 39:321-322.
Burggren, W. W. 1992. The importance of an ontogenetic perspective in physiological studies. Amphibian cardiology as a case study. In S. C. Wood, R. E. Weber, A. R. Hargens, and R. W. Millard (eds.), Physiological adaptations in vertebrates: Respiration, circulation and metabolism, pp. 235–253. Marcel Dekker, New York, New York.
Feder, E. F., A. F. Bennett, W. W. Burggren, and R. B. Huey. 1987. New directions in ecological physiology. Cambridge University Press, Cambridge.
Futuyma, D. J. 1998. Evolutionary biology. 3rd ed. Sinauer Associates, Sunderland, Massachusetts.
Metcalfe, J., and M. K. Stock. 1987. Physiological changes during ontogeny. In M. E. Feder, A. F. Bennett, W. W. Burggren, and R. B. Huey (eds.), New directions in ecological physiology, pp. 328–341. Cambridge University Press, Cambridge.
Schmidt-Nielsen, K. 1997. Animal physiology: Adaptation and environment. 5th ed. Cambridge University Press, Cambridge.
Wood, S. C., R. E. Weber, A. R. Hargens, and R. W. Millard. 1992. Physiological adaptations in vertebrates: Respiration, circulation and metabolism. Marcel Dekker, New York, New York.
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