Virtually all studies investigating morphological plasticity have focused on how organisms change in response to treatments that are constant throughout the experiment but which have different means. In this study, we investigated the possibility that organisms can morphologically respond to other environmental parameters like the amount of environmental variability or environmental maximum. Sea urchin larvae adjust the length of their feeding structure, a band of cilia, in response to different mean food concentrations. We investigated whether sea urchin larvae are also capable of responding to environmental variability or maxima by rearing larvae on four fluctuating diets, where all treatments had the same mean concentration of food. Larvae reared on a low variable diet produced longer larval arms (i.e. a longer ciliary band) than larvae reared on more variable diets. This response is similar to the morphological change that occurs when the mean food concentration is reduced (small mean = long arms), and indicates that organisms can morphologically respond to environmental parameters other than the environmental mean – such as the amount of environmental variability or the environmental maxima. We also quantified the shape of the relationship between larval arm length and fixed food concentrations to determine whether our results might be explained by nonlinearity in this relationship (i.e. Jensen's inequality). The shape of the relationship was inconsistent with a Jensen's inequality explanation. In addition, sea urchin larvae were unable to track fluctuations in food concentrations. This inability to track our imposed environmental fluctuations indicates that there was a time delay greater than 2 days in the response of larvae to changes in food concentrations. Since plutei likely experience fluctuations in food concentrations at least once a day, it is possible that larvae cannot track natural fluctuations in food concentration. We discuss the importance of our results in light of adaptive interpretations of plasticity and predictions of morphological response.
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