STRATHMANN, R.R.: Selection of Sites for Deposition of Benthic Eggs

Collaborators are SY Henderson (1st author) ML Grace, SL Graham, LS Hong, TC Kang, CLM Altman (formerly CL Martin), MA Morris, and C Vogel. Estimated mortality rates indicate that embryos on the sea floor are safer than planktonic larvae; but safety of benthic embryos depends on maternal selection of sites for egg deposition, and that can require travel by sedentary animals. We examined selective deposition by two snails, Lacuna variegata and L. vincta, that have similar egg masses (small gelatinous mounds or rings), times for development to hatching (14 to 17 d at 9 deg C), size at hatching (0.21 mm shell length), and broadly overlapping habitats (both seagrasses and algae). L. variegata showed a greater preference than L. vincta for deposition on seagrass rather than algae and deposited masses closer to tips and edges of seagrass blades. L. variegata, unlike L. vincta, also deposited on shells of other snails, especially in the absence of preferred substrata. Mothers must travel to the preferred sites, and greater specificity implies greater travel. Tips of blades may be especially safe for egg masses. M Friedman and MP Russell (pers comm) found fewer losses to predation for masses deposited higher on grass blades. Solar UV radiation can kill the embryos, but survival of embryos in masses on blades of Zostera marina was high if plants were rooted and continuously submerged; survival was slightly less and more variable on rooted seagrass exposed at low tide and on drifting seagrass. Female L. variegata preferentially deposited egg masses near other masses. A previously deposited mass may be a cue that the site is safe for embryos or perhaps for a depositing mother. A mother could be at risk during the 1 to 3 h of egg deposition. Hatchlings disperse as pelagic larvae, however, ending further possible benefits of gregarious deposition.