Life histories of the Harpacticoida (Copepoda, Crustacea): a comparison with meiofauna and macrofauna

The Harpacticoida reside in a wide range of habitats--from all marine situations to most freshwater habitats, and even moist terrestrial substrata like moss and litter. They exhibit free-living to parasitic life-styles and larval development with plankto- and lecithotrophy. Harpacticoid life cycles as a rule comprise six nauplius larvae and six copepodids, the sixth being the adult. Harpacticoid life history traits known from the field or from cohort analyses include differential fecundity, stage duration and survival, sex ratio, development and life span. Understanding life history traits, i.e. assessing the importance of various biotic and abiotic factors in regulating life cycle patterns, is essential for the interpretation of seasonal variations of population size. Evolutionary stable strategies (EES) of reproductive resource allocation and timing are determined by trade-offs between environmental conditions as well as by intrinsic factors. The latter are the genetic base, depending on the evolutionary history and relatedness of the taxa under consideration which determine their phenotypic plasticity. Most interesting are ontogenetic niche shifts, especially in habitat, food and stage duration, including dormancy phenomena. Harpacticoid life history traits will be discussed in the light of meio- and macrofauna life cycles in general. It seems as if small meiofaunal taxa are not able to proliferate pelagic larvae of sufficient quantity, because larvae need a certain size to become functional. Instead, meiofaunal taxa switch to a trait of direct development and even brood care in some cases. It is hypothesized that the Harpacticoida are the only meiofaunal taxa to develop larval stages because their nauplii share the same habitat as the adults and no risky habitat transition will occur as in the pelagic larvae of the macrofauna.