Dangers for Unprotected Embryos on the Benthos

One of the arguments for the persistence of planktonic larval stages in the life cycles of benthic marine invertebrates is that pelagic larvae experience a release from predation when they move off the benthos and into the water column. However, it is difficult to substantiate this hypothesis because we lack comparative data on mortality rates of unprotected (e.g., not in egg capsules or gelatinous masses) embryos and larvae on the benthos and in the water column. The aim of this study was to provide some of this missing information, by comparing development success in benthic and pelagic environments. Dendraster excentricus embryos were placed in enclosed development chambers under three treatment conditions: 1) directly on the substratum (unscreened benthic), 2) on a screen on the substratum (screened benthic), and 3) on a screen raised 3 cm above the substratum (screened pelagic). Development success for pelagic embryos was consistently higher than those on the benthos, substantiating the hypothesis that embryos experience a release from predation as they move off the benthos. In addition, the screened benthic treatment had significantly higher development success than the unscreened benthic treatment, indicating that macro-predators (>55 μm) may significantly reduce development success for unprotected larvae. Higher development success of protected pelagic treatments may have been due to lower abundances of pelagic micro-predators or anoxic conditions on the benthos