Search for: All records

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 finding 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 2 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 2 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. 

Rates of development of the feeding larvae of marine invertebrates may often be limited by inadequate food, extending the length of the larval period and increasing overall larval mortality. A better understanding of the frequency and importance of this phenomenon requires knowledge of the food concentration below which larvae are limited, and above which they are not, as well as estimates of how strongly food supply affects length of the planktonic period. We addressed these issues using larvae of the sand dollar Dendraster excentricus as a model and chl a concentration as a metric of food abundance. We reared larvae in natural seawater collected from coastal southern California (USA), as well as in reduced and supplemented food treatments created from this natural seawater, 6 times from 2017 to 2019 to take advantage of temporal variation in chl a concentration. Larvae showed morphological responses indicative of low food in nature in only 1 of 6 experiments and showed delayed time to 50% metamorphic competence in 2 of 6 experiments. Larvae appeared to be food limited below chl a concentrations of ~2.4-3.0 µg l -1 , but developed at maximal rates at higher food concentrations. Low natural food supplies delayed time to 50% competence by up to 1.25 d. An 11 yr record of chl a concentration in waters of coastal southern California suggests that larvae of D. excentricus are likely food limited in developmental rate throughout much of the year except for late winter to late spring.