Recovery, behaviour and evolutionary implications of live Monoplacophora

Abstract

THE Monoplacophora are molluscs with bilateral serially repeated structures¹. Their shells occur among the oldest mineralised hard parts in the early Cambrian, and because they are traceable from there only to the mid-Devonian they had been considered extinct^2,3. However, in 1957 Lemche⁴ reported an extant species in the deep sea. By 1972 six further extant species and several taxonomically unassigned specimens had become known from abyssal depths^5–10, showing that the Monoplacophora continued to exist in post-mid-Devonian time in the deep sea. These finds also established that such molluscs are widely distributed on the deep-sea floor^5–10. Two specimens were obtained about 1,800 m deep, but all others came from between 3,000 and 6,500 m (refs 5–10). McLean has reported a new, minute species obtained from cobbles at depths of 174, 229 and 388 m on the Cortes Ridge off southern California^11,12. He is assigning it to the genus Vema. This is the first record of an extant monoplacophoran which, like early Palaeozoic species, inhabits shallow bathyal depths and, like some Silurian species, occurs on a hard substrate¹³. I report here the recovery of live specimens of this monoplacophoran.