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Marine invertebrate larvae are diverse in size and shape. Understanding the functional differences among these larvae and their ecological consequences is critical for understanding the evolution of larval forms and predicting responses to varying environmental conditions. This study compared the growth physiology of two morphologically distinct feeding larvae of echinoid echinoderms: the typical echinopluteus of Strongylocentrotus purpuratus, and the less well-known echinopluteus transversus larva of Centrostephanus coronatus. Larvae of both types were reared at 16°C with 20,000 algal cells/ml of food. Despite striking differences in morphology (with late larvae of the two species bearing eight or two arms, respectively), ciliary band length grew at similar rates in both types of larvae. Significant differences between larval types were observed in growth in midline body length and stomach size. Physiologically, the typical echinopluteus larvae of S. purpuratus exhibited higher rates of protein growth, daily ingestion of algae, and aerobic respiration than did the echinopluteus transversus larvae of C. coronatus. The relationships between ingestion rate and ciliary band length or stomach size differed as well, with ingestion rate in larvae of S. purpuratus increasing much more rapidly than that of larvae in C.coronatus in relation to both morphological features. The higher physiological rates in S. purpuratus were not at the expense of increased physiological efficiency: larvae of S. purpuratus had digestive and growth efficiencies that were up to threefold higher than those in larvae of C. coronatus. Taken together, these results demonstrate fundamental differences in performance that go beyond the readily observable differences in morphology of these two larval forms. The reduced physiological capacity and growth efficiency of transversus larvae of C. coronatus provide a physiological basis for their long planktonic duration and possible teleplanktonic dispersal capabilities. The high physiological capacity and efficiency of the typical larvae of S. purpuratus may be adaptive for growing in areas of high seasonal algal abundance.

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