Morphological plasticity and resource allocation in response to food limitation and hyposalinity in a sea urchin

This study examined how the purple sea urchin, Anthocidaris crassispina, responded to food limitation and hyposalinity, two common stressors it may experience in the field. Young adults were reared under a combination of two food (adlibitum feeding vs. fed once biweekly) and two salinity (31.5‰ to 33.5‰ natural seawater versus 25‰ diluted seawater) levels in a laboratory for 24 wk. Within the ranges examined, both stressors had a significant negative effect on test diameter, total weight, test weight, gonad weight, and gonad index. Food limitation led to marked urchin mortality (20.9% to 37.5%), but had no significant affect on lantern weight, demipyramid length, or lantern index. Hyposalinity alone did not alter urchin survival, lantern weight, or demipyramid length, but it had a significant negative effect on total weight, test weight, gonad weight, and gonad index, whereas it had a significant positive effect on lantern index. Our results indicate that this sea urchin has substantial adaptability to food limitation and hyposalinity, and such adaptation can have important consequences in resource allocation and body allometry. In response to food limitation, this sea urchin appears to allocate energy to body maintenance, rather than to increasing the relative size of the lantern (a strategy different from many other sea urchins). In response to hyposalinity, it reduces test weight, thereby increasing the relative weight of the lantern.