A field experiment demonstrating risk on the seafloor for planktonic embryos

Abstract

Most solitary marine eggs are shed into the plankton. Presumably the seafloor is more dangerous than the plankton for small solitary embryos, but estimates of benthic mortality of solitary embryos are few. To assess risk, we introduced suspensions of sinking, early stage embryos into conical chambers whose basal surfaces differed in mesh size and distance of mesh from the sediment surface. Surviving embryos hatched as blastulae and swam upward into an apical collection tube, later removed for counting. Test embryos were of a clypeasteroid echinoid. The two test sites, within a coastal lagoon in the NE Pacific, differed in sediments. At both sites, 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, but greater in chambers floored with a 0.08 mm mesh about 6 cm above the sediment (0.40 and 0.42) and also with a different chamber design with finer (0.055 mm) mesh at the sediment (0.42). Mean proportion retrieved was still greater (0.68 and 0.67) with chambers floored with a complete barrier at the sediment surface and similar to retrieval with chambers in laboratory aquaria without sediment. Estimated mortality rates for embryos on the sediment exceeded published estimates from the plankton. The results support the hypothesis that solitary eggs are released to the plankton because of benthic risks. This method can be used at varied sites on the seafloor, with varied embryos, and with varied protective barriers to test the generality of these results.

The association of coloniality with parental care of embryos

“…(1) The foremost advantage of brooding is that a small and immobile embryo gains an adult's protection. Although there can be substantial mortality within broods whether external, as in crustaceans (Kuris, 1991), or internal, as in a holothuroid (Sewell, 1996), the estimated mortality rates of embryos in broods are less than mortality rates estimated for solitary embryos in the plankton or on the seafloor (Bi, Rose, & Benfield, 2011; Ohman, Runge, Durbin, Field, & Niehoff, 2002; Zacher & Strathmann, 2018) and also less than estimated mortality rates for planktonic larvae (Rumrill, 1990; Strathmann, 2007, 2018). For sessile marine animals, the mother's body is the nearest and usually the safest place to protect a group of embryos.…”