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Echinoid larvae can express food-conditioned morphological plasticity at ecologically relevant culture densities

Peter Nilsson, Bruno Pernet*

*Corresponding author:

ABSTRACT: The feeding larvae of many echinoids develop long postoral arms relative to body length when food is sparse but relatively short postoral arms when food is abundant, a response thought to adaptively adjust feeding capability. However, in an important recent study, larvae of Dendraster excentricus exhibited this food-conditioned plasticity only when reared at a high density typical of laboratory cultures; when reared at a lower density more representative of larval densities in nature they did not exhibit this plastic response. This suggests that laboratory results cannot be easily extended to make inferences about phenotypic plasticity in nature. We replicated this study and extended it to an even lower larval culture density and to a second species, Lytechinus pictus. Larvae of D. excentricus developed longer arms adjusted for body length when fed the lower of two food rations at all culture densities, though differences were only marginally significant at the lower culture density in one experiment. Larvae of L. pictus tended to develop longer arms adjusted for body length at lower food rations, though differences only approached statistical significance at the highest culture density in one experiment. For both species, contrasts between food rations almost always showed an inverse relationship between postoral arm length and stomach length, consistent with prior work demonstrating trade-offs in investment in these two features characteristic of phenotypic plasticity. These results suggest that the feeding larvae of echinoids may exhibit food-conditioned plasticity of postoral arm length even at low natural densities.