Developmental flexibility in a variable environment: lessons from sand dollars and sea urchins

Despite recent reports of intraspecific developmental plasticity in marine invertebrates, exceptions to the rule of species-specific developmental patterns remain rare. Here we describe unusual intra-clutch variation in the development of an echinoid echinoderm. To generate this variation we exposed sand dollar and sea urchin embryos to increased temperature and low salinity environments. For these types of nearshore animals, the intertidal and shallow subtidal environment is a place of high variability in salinity and temperature. We found that under moderate levels of salinity and temperature stress, the sand dollar, Echinarachnius parma, exhibits the unusual developmental pattern of producing multiples (twins, triplets and quadruplets). For echinoderms, this is only the second report of the production of multiples under conditions embryos experience in the real world; the first described briefly by Mortensen 75 years ago. Multiple production is much more frequent in E. parma than in the other nearshore echinoids examined: Strongylocentrotus droebachiensis and Lytechinus variegatus. We hypothesize that the differences we observed in the propensity to produce multiples are due to differences among echinoids in the strength of the hyaline layer that surrounds blastomeres during early development. We plan to test this hypothesis in other echinoids known to have frail hyaline layers, notably Eucidaris tribuloides. Whether the production of multiples is an adaptive response to a variable environment, or simply an interesting developmental aberration remains to be demonstrated. However, novel developmental responses to present-day fluctuations in salinity suggest that ongoing environmental shifts may drive substantial changes in marine invertebrate developmental patterns