‘Pearls’ from the common mussel

October 26th, 2007

Mussels containing inclusions or ‘pearls’ were found in Loch Long in Scotland. Mytilus edulis, known as the common mussel, is a bivalve; 80-100mm in length and dark blue to purple and sometimes brown in colour. It attaches firmly to rocks and stones etc. by byssus threads. The pearls found in these mussels are formed around parasitic worm larvae that infect the mussels. The parasite is usually the flatworm Gymnophallus bursicola, which enters as larvae and lodges itself in the mantle of the mussel. In order to protect itself, the mussel forms layers of a shell-like nacreous (iridescent) material over the worm to form a pearl. Mytilus edulis, alongside other bivalves, can also be infected by the pea crab (Pinnotheres pisum) which commonly inhabits the mantle cavity of the mussel.

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Violet sea snails found in Ireland

September 15th, 2007

In August and September, there have been several reports of violet sea snails sighted in Ireland. The violet sea snails, Janthina pallida were discovered on a strand in County Sligo and Janthina pallida and Velella velella were both found on Portnoo beach in Donegal.

A mass stranding (>200) of violet sea snails (Janthina pallida) was discovered in County Sligo. These snails grow to about 25mm high and spend their adult life floating on the surface of the sea. This is achieved by creating a ‘raft’ of air bubbles that is encased in a thin layer of chitin produced by the foot. They are often washed ashore during storms and they feed primarily on the siphonophore, Velella.

In Donegal, both Janthina pallida, some of which had bubble floats with eggs attached, and the purple sails, Velella velella, were found washed up on the beach. These organisms usually live on the open sea but can become stranded on beaches if the prevailing winds drive them onto the shore. Velella velella, also known as ‘Jack Sail by-the-Wind’, is deep blue to blue/violet in colour. They are about 100mm in length and are eaten by the violet sea snails, Janthina. Velella velella feeds on young fish, crustaceans and other organisms caught by the nematocysts (stinging cells) on the tentacles.

Photo: Steve Trewhella

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