Waterborne cues from crabs induce thicker skeletons, smaller gonads and size-specific changes in growth rate in sea urchins
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Indirect predator-induced effects on growth, morphology and reproduction have been extensively studied in marine invertebrates but usually without consideration of size-specific effects and not at all in post-metamorphic echinoids. Urchins are an unusually good system, in which, to study size effects because individuals of various ages within one species span four orders of magnitude in weight while retaining a nearly isometric morphology. We tracked growth of urchins, Strongylocentrotus droebachiensis (0.013–161.385 g), in the presence or absence of waterborne cues from predatory Jonah crabs, Cancer borealis. We ran experiments at ambient temperatures, once for 4 weeks during summer and again, with a second set of urchins, for 22 weeks over winter. We used a scaled, cube-root transformation of weight for measuring size more precisely and for equalizing variance across sizes. Growth rate of the smallest urchins (summer: <17 mm diameter; winter: <7 mm diameter) decreased by 40–42% in response to crab cues. In contrast, growth rate of larger urchins was unaffected in the summer and increased in response to crab scent by 7% in the winter. At the end of the 22-week experiment, additional gonadal and skeletal variables were measured. Cue-exposed urchins developed heavier, thicker skeletons and smaller gonads, but no differences in spine length or jaw size. The differences depended on urchin size, suggesting that there are size-specific shifts in gonadal and somatic investment in urchins.
KeywordsNominal Diameter Spine Length Gonad Index Interambulacral Plate Crushed Conspecific
We thank M. Wright, M. Pratt, M. Devin and R. Russell for help with collecting or providing sea urchins; J. Allen, P. Dickinson, M. Pratt, T. Edgell and two anonymous reviewers for feedback on the manuscript; and M. Murray, J. Hauptman and A. Garfield for technical support. Crabs were donated by Allen’s Seafood, Harpswell, ME. This work was funded by a Beckman Foundation Fellowship to R. Selden, by a planning grant from Maine SeaGrant to A. Johnson and O. Ellers, and through Bowdoin College by two Faculty Fund Research awards, a Rusack Project Initiation Grant and a Faculty Leave Fellowship to A. Johnson. The project described was supported by NIH Grant Number P20 RR-016463 from the INBRE Program of the National Center for Research Resources. The experiments comply with the current laws of the country in which the experiments were performed.
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