TIMING OF METAMORPHOSIS IN A FRESH-WATER CRUSTACEAN - COMPARISON WITHANURAN MODELS

Many crustaceans have complex life cycles characterized by a metamorph osis, yet variation in metamorphic traits, and the causes and conseque nces of this variation, have rarely been examined. Food concentrations were changed during specific larval stages of the freshwater copepod Mesocyclops edax Forbes (Copepoda: Cyclopoida) to examine whether age and size at metamorphosis remain flexible or become fixed during the l arval period. Results were compared to predictions of both flexible (t he Wilbur-Collins model) and fixed (Leips-Travis model) rate models fo r the timing of amphibian metamorphosis. Age and size at metamorphosis were variable in all treatments, and age was always more variable tha n size. Changes in food concentration early in larval development resu lted in significant differences in age at metamorphosis among treatmen ts, but changes initiated when 60% of the larval period had passed had no effect on age at metamorphosis. Development appeared to become fix ed later in the larval period, before the ultimate larval stage was re ached. These results support predictions of the Leips-Travis model. Ea rly changes in food concentrations had significant effects on size at metamorphosis, but changes initiated during the penultimate larval sta ge (50-60% of larval development) had no effect on metamorph size. Siz e at metamorphosis in M. edax also appeared to be fixed before the ult imate larval stage was reached. Fixation of size at metamorphosis duri ng development is not predicted by either model and may be unique to o rganisms with rigid exoskeletons that constrain growth within any stag e. Patterns of covariation between age and size at metamorphosis sugge st that food conditions early in larval development exert a large effe ct on metamorphic traits, in contrast to patterns observed in several amphibian species. The Wilbur-Collins model places a fitness premium o n delaying metamorphosis to achieve a maximum size, when growth condit ions are favorable; it thus may not apply to crustaceans. Selection pr essures on the timing of metamorphosis in crustaceans may differ subst antially from those identified for amphibians and other organisms. Bec ause of these differences, incorporating crustaceans into studies of m etamorphosis will help to clarify the factors affecting this life cycl e transition.