Body size in freshwater planktonic crustaceans: an overview of extrinsic determinants and modifying influences of biotic interactions |
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Authors: | R C Hart E A Bychek |
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Institution: | (1) School of Biological and Conservation Sciences, University of KwaZulu-Natal, Scottsville, Private Bag X01, Pietermaritzburg, 3209, South Africa;(2) Department of Zoology & Entomology, School of Botany & Zoology, University of Natal, Scottsville, Private Bag X01, Pietermaritzburg, 3209, South Africa |
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Abstract: | In view of its fundamental and pervasive influences and impacts on organism physiology and ecology, body size is recognized
as a key component of evolutionary fitness and serves as the cornerstone of a seminal contribution in freshwater zooplankton
ecology—the Size Efficiency Hypothesis (SEH) of Brooks & Dodson (Science 150:28–35, 1965). While the roles and implications of body size in predation and competition—central tenets of the SEH—have been widely considered
and reviewed, no broader integrated synthesis exists of the collective array of body size determinants and their implications
in the ecology in crustacean zooplankton—a numerically and functionally dominant group of aquatic organisms. Focusing on planktonic
Cladocera and Copepoda in inland waters, in particular, we provide a wide-ranging overview of the direct and/or indirect effects
of environmental conditions, consumable resources and biotic interactions that independently and/or collectively influence
the phenotypic expression of body size (particularly as length), both within and between species. Some indirect ultimate evolutionary
consequences of body size are considered, and we identify some controversies and unresolved issues related to this biologically
crucial trait. While by no means exhaustive, our overview reveals a complex nexus of extrinsic proximate abiotic and biotic
factors and interactions that influence body size, the phenotypic expression of which in natural systems commonly reflects
contrasting outcomes related to conflicting direct and/or indirect selective pressures. In general, however, body size (both
inter- and intra specifically) declines with rising temperature and increases with rising food supply (depending on its quality),
although both temperature and food supply exert contrary influences on particular taxa (or life history stages) under certain
environmental circumstances. Predation undoubtedly has an overriding influence on body size selection. Depending on its mechanistic
basis (visual, tactile or both in tandem), it selectively favours either small or large body size, both within (adults vs.
juveniles) and between prey species, which are accordingly often ‘size-trapped’ between contrasting selective pressures, with
consequent indirect effects. The bioenergetics of fundamental physiological processes undoubtedly set constraints on body
size and serve as the primary determinant. However, within such constraints, the phenotypic expression of body size reflects
its adaptive modification in response to the prevailing abiotic and biotic environment. As such, body size represents an emergent
ecological property, reflecting the outcome of specific circumstances and conditions, which vary both temporally within and
spatially between different ecosystems, and are accordingly context dependent. Nevertheless, underlying physiological advantages
of larger size (within and between species) among crustacean zooplankters—lower mass-specific metabolic rates (although recently
challenged), higher individual feeding rates (at least among cladocerans), potentially wider food size-ranges, better starvation
tolerances, higher potential fecundity, etc.—collectively favour the selection of increased body size, as predicted by the
SEH. Although competitive superiority of large size (measured in terms of minimal food requirements) has been confirmed experimentally,
this cannot be generalized to natural conditions, where conflicting and temporally variable pressures apply, and contribute
to generally mixed, and temporally variable body size compositions. Complex underlying ecological interactions and influences
ultimately determine the phenotypic expression of body size in directions consistent with fitness optimization under prevailing
circumstances. Certain specific and general deficiencies in information are identified. In particular, the overwhelming emphasis
on daphniid cladocerans as model study taxa in freshwater ecosystems has marginalized the acquisition of a comparably broad
and penetrating understanding of specific features both of non-daphniid cladoceran and copepod life histories and body size
selection. Among daphniid cladocerans, contemporary definitive understanding devolves largely from reductionist laboratory
approaches. Holistic re-integration of these mechanistic findings into natural system circumstances presents a difficult challenge
that is attracting increasingly attention. With regard to copepods, synthetic integration of the expansive marine knowledge
base appears crucial to inform and direct future investigations on freshwater taxa. The question of intrinsic body size regulation
in copepods and cladocerans, especially in regard to final phenotypic plasticity in body size expression, awaits resolution.
Overall, body size remains a multi-facetted and complex topic, offering promising challenges for further investigation. |
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