MOLLUSCS OF BRITISH COLUMBIA

 Class Monoplachopora

by

James A. Cosgrove

A single (mono) cap-shaped shell (placo = tablet, phoros = bearing) characterizes the members of the Class Monoplachopora.  On first seeing a specimen from this class one would immediately think that it was a limpet and should belong in the same class as the limpets (Class Gastropoda).  Study has shown, however, that despite the visual similarity a number of significant differences place these animals into their own Class.

Until 1952, the Class Monoplacophora was thought to consist of only extinct species.  A Danish biologist dredging in the deep marine sediments off the Pacific coast of Costa Rica discovered a living monoplacophorn and named it Neopilina.  Currently eight genera, most containing a single species, have been described.

All monoplacophorans are small (0.5 – 3.5 cm) and live in the mud in very deep water. They probably feed on the organic components of the mud.  The shell is often very thin and fragile and is rounded bilaterally.  Unlike the limpets, the monoplacophorans have a serial repetition of kidneys, gills and reproductive structures but this is not true segmentation such as you would find in annelid worms.

Monoplacophorans have a single rounded foot for locomotion and a reduced head with no eyes or tentacles.

As with the members of the Aplacophora, the fact that the Monoplacophora also live in the mud of the deep oceans has limited our understanding of them and their place in the ecology of the benthic community.  Time and technology will aid in increased scientific investigation and understanding.