Meeting Abstract
In marine environments, most solitary eggs are shed into the plankton, whereas most benthic eggs are grouped into protective capsules, gel masses, or broods. Presumably the seafloor is usually more dangerous than the plankton for a solitary embryo, but field experiments on benthic versus planktonic mortality during early development have been with tethered larvae and larval mimics, not solitary embryos. Here, a field experiment compared survival of embryos offered different degrees of protection from the seafloor. A suspension of embryos at early cleavage stages was introduced into conical chambers whose basal surfaces at the seafloor differed in mesh openings at the sediment surface or in distance from the sediment surface. All embryos were exposed to water that entered through side windows with a 0.08 mm mesh. Surviving embryos hatched as blastulae and swam upward into an apical collection tube, which was later removed for counting. The test site was a coastal lagoon in the NE Pacific. The test embryos were of a sand dollar. Mean proportion of embryos retrieved was 0 and near 0 in chambers floored with 0.9 mm and 0.08 mm meshes at the sediment surface. Mean proportion retrieved was greater (mean = 0.40) in chambers floored with a 0.08 mm mesh several cm above the sediment surface and still greater (mean = 0.68) in chambers floored with a complete barrier at the sediment surface. The mean retrieval in the treatment with the complete basal barrier was close to the retrieval when chambers were in laboratory aquaria without sediment. In this experiment survival therefore ranged from zero without protection from marine sediments to high with protection from the sediment. Most of the earth is covered by marine sediments. This method for field experiments can be employed at varied sites and with varied embryos to test the generality of these results.
