Extended Pre-Feeding Period in Planktotrophic Larvae of the Bathyal Echinoid Aspidodiadema Jacobyi

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Abstract
The long-standing hypothesis (Thorson, 1946, 1950; Mileikovsky, 1971; Jablonski & Lutz, 1983) that deep-sea invertebrates should reproduce by direct development or by non-feeding (lecithotrophic) larvae is beginning to fall in the light of recent data. Traditional reasoning maintained that planktonic food should be limiting at great depths, and that microscopic, ciliated larvae should be incapable of migration to the euphotic zone. However, in recent years, planktotrophic larvae of two deep-sea gastropods have been collected in surface waters, and planktonic larval development has been inferred from shell morphology and chemistry in several other species (Bouchet & Warren, 1979; Killingley & Rex, 1985). Planktonic larvae have also been collected in the water column of the deep sea (Berg et al., 1985; Smith, 1985; Berg & Van Dover, 1987), particularly near hydrothermal vents. Relatively diverse benthopelagic plankton populations relying primarily on suspended detritus for food are now known from the benthic boundary layer of the deep sea (Wishner, 1980a, b; Gowing & Wishner, 1986). Thus, it is increasingly apparent that planktotrophy may be a common option for deep-sea larval development.