Effect of sperm cell density on measured toxicity from the sea urchin Tripneustes gratilla fertilization bioassay.

The sea urchin sperm fertilization bioassay is an approved U.S. Environmental Protection Agency (U.S. EPA) biomonitoring tool for regulatory and compliance purposes. The method adapted for the Hawaiian species Tripneustes gratilla recommends a sperm-to-egg ratio of 2,500. Using a fixed ratio for all tests does not sufficiently account for seasonal and individual variabilities of this organism. Our studies showed that gametes must be present at specific concentrations, which are unique for each batch. Parallel experiments were conducted to compare tests using a constant 2,500 sperm-to-egg ratio with tests using an optimized sperm dosage determined prior to each analysis. The recommended 2,500 sperm-to-egg ratio often gave overfertilized controls and reduced toxicological response. The risks of oversperming and concomitant toxicity masking were minimized when the bioassays were performed with optimum sperm-to-egg ratio.