Developmental Consequences of Temperature and Salinity Stress in the Sand Dollar Dendraster excentricus .

Animals that reside, reproduce, and develop in nearshore habitats are often exposed to strong fluctuations in abiotic conditions, including temperature and salinity. We studied the developmental response of the sand dollar Dendraster excentricus to increased temperature and reduced salinity at levels comparable to those induced by summer freshwater input into the San Juan Archipelago, Washington. We observed that embryos exposed to temperature and salinity stress exhibited polyembryony (the splitting of one embryo into multiple independent individuals), and we subsequently tested the competency of twin and normal embryos to reach metamorphosis. We found that twin embryos generated from a single egg are each capable of reaching metamorphosis. To begin investigating the mechanisms underlying polyembryony, we tested whether osmotic stress caused swelling of the fertilization envelope, thus allowing embryos the physical space to produce multiples within a single envelope. We also tested whether reduced calcium levels in low-salinity seawater reduced cell-cell adhesion and allowed cells to separate and develop as multiple embryos within a fertilization envelope. However, neither osmotic stress nor reduced calcium levels alone appear sufficient to induce polyembryony. We hypothesize that changes in the properties of the hyaline layer that lies beneath the fertilization envelope facilitate polyembryony.