RAHMAN, Y.J.*; STRATHMANN, R.R.; Univ. of California, Riverside; Univ. of Washington: Does scarcity of sites for egg deposition limit reproductive success?

Estimated mortality rates are higher for planktonic larvae than for benthic embryos of marine invertebrates, but these estimates do not include difficulties associated with deposition of a benthic egg mass. One limitation on deposition may be availability of substratum for attaching eggs. We examined this possibility for the opisthobranch snail Haminaea vesicula in False Bay, San Juan Island, USA. The snail attaches its egg masses to firm substrata, not to mud or sand. The most common substrata in False Bay are bivalve shells and algae, but even these are scarce. Eelgrass (Zostera marina) is scarcer in most of the bay, occurring as small clumps of a few shoots or as drifting blades. We tested the hypothesis that substratum availability limits egg deposition by H. vesicula by adding artificial eelgrass made of duct tape to tidepools. The number of egg ribbons laid per square meter in the experimental areas was 21 to 428 times that in control areas. Also, the number of adult snails was significantly greater in the experimental areas. In multiple-choice experiments, the snails preferred to deposit eggs on branched red algae (Ceramium) most, followed by eelgrass, then bivalve shells (Macoma), then green algal blades (Ulva growing on shell). The order of preference for these substrata was the inverse of the order of substratum abundance in the bay. In a pairwise comparison of two substrata commonly used by H. vesicula, the snails strongly favored eelgrass to bivalve shell. H. vesicula prefers to lay its eggs on Z. marina, but the rarity of that substratum in the bay limits reproductive output.