Comparative sensitivity of gametes and early developmental stages of a sea urchin species (Echinometra mathaei) and a bivalve species (Isognomon californicum) during metal exposures.

Bioassays were developed using sperm of a sea urchin (Echinometra mathaei), and sperm, embryos, and larvae of a bivalve species (Isognomon californicum). Sea urchin spawning was restricted to only a few months of the year and viability of sperm throughout the year varied from 5 to 75%. Sea urchin fertilization assays were affected by temporal variation in sperm viability. Spawning in this bivalve species occurs year-round and there is little temporal variation in sperm viability. Since stringent sperm:egg ratios are not required for the bivalve embryo and larval assays, these were less affected by variation in gamete quality. The relative sensitivity of the various assays were compared during exposure to three different metal pollutants: cadmium, copper, and tributyltin. Gametes and embryos were relatively resistant to cadmium toxicity, but larvae were very sensitive. With copper and tributyltin, sea urchin and bivalve fertilization assays were the least sensitive; and bivalve growth assays were the most sensitive, followed closely by the bivalve embryo assays. On the basis of sensitivity, ease and time required to conduct the assay, and salinity tolerance, the bivalve embryo assay was recommended as the overall single most reliable toxicity bioassay. However a multispecies, multidimensional approach using sperm fertilization assays as well as embryo assays should be employed, perhaps in a hierarchal manner. Larval growth assays were deemed too tedious and time consuming to be used routinely, but due to their high sensitivity, should still be considered as a valuable comparative tool.