The Development of Form and Function in Fishes and the Question of Larval Adaptation

Epilogue

John Jeffrey Govoni

doi: https://doi.org/10.47886/9781888569582.ch9

In preparing reviews of the development of organs, or organ systems, the authors of the symposium were asked to address two principal questions: (1) what is the timing of the onset of development of organs, or organ systems, and in what capacity do they function in embryos and larvae; and (2) are any specializations, unique to embryos and larvae, apparent? In answer to the first question, some authors took a classical structuralist approach, others a functionalist approach. Whether a feature or trait that appears in evolutionary time enables an embryo or larva to meet a physiological need or environmental challenge, or whether need or challenge requires the evolution of a feature or trait to ensure survival of individuals within populations, is a philosophical issue outside of the scope of this symposium. What is important for the considerations of this symposium is simply how do embryonic and larval fishes manage within the environments into which they are released. The authors of this symposium responded directly to the first question with detailed accounts. As for the second question, few authors identified specializations unique to embryos and larvae that apparently solve specific physiological needs or environmental challenges.

Across the broad phylogenetic tree that represents fish diversity, it is now apparent that the development of most organs, or organ systems, matches the physiological and ecological needs (i.e., functional demands) of fish embryos and larvae. Aside from hatching, two junctures in development coincide with major changes in organs or organ systems: the completion of yolk and oilglobule absorption and metamorphosis. Although the many organs, or organs systems, considered herein do not become fully differentiated until metamorphosis, most organs appear to function adequately in embryos and larvae. There are no apparent gross developmental deficits or lags, unless induced by environmental conditions, principally temperature (e.g., Rombough 2003; Johnston and Hall 2004, this volume). The completion of yolk and oil absorption requires active feeding by larvae, and the sensory and signal processing equipment necessary to meet this challenge are typically functional to the necessary extent at this juncture. A singular exception might be in the production of digestive enzymes for the alimentary canal; enzyme production and activity appears to stall or decline briefly after first feeding in some fishes with altricial development (Hoehne-Reitan and Kjørsvik 2004, this volume). Metamorphosis is typically concomitant with shifts in habitat, a shift from the three-dimensional space of the pelagic habitat of some eggs and larvae to two-dimensional space of bentho-pelagic or benthic habitats of juveniles and adults, or the reciprocal shift. (Some fishes, of course, remain benthopelagic or pelagic throughout life, from embryos through adulthood). New habitats have different physiological or ecological demands and challenges.