An episodic epitoke

Many marine creatures become more mobile or congregate when breeding, often travelling considerable distances. Such behaviour stimulates reproduction and increases genetic diversity. Examples are brittlestars, cuttlefish and spider-crabs. This behaviour is also seen in some species of polychaete worms, a well-known example being the swarming behaviour of the palolo worms in a single night over the reefs of the Caribbean and South Pacific (Eunice spp.) This is triggered by the light of the moon in its last quarter and local medicine men have historically gained much respect by their predictions of this spectacular event, perhaps the worms supplemented a dull diet?

In the UK swarms are short-lived and it is a matter of chance that they are observed.

In the afternoon of 23 May 2004, about an hour after low tide, such a chance fell to Chris, Clare and Tamsin Page whilst swimming at Prisk Cove, south of Rosemullion Head near the entrance to Helford River and within the HVMCA. They found themselves surrounded by a swarm of hundreds of deep pink coloured worms, measuring up to 18 cm long all swimming frantically at the edge of the water or wriggling amongst the rocks and seaweed. Each worm was dorsally rounded but flat below and paddled vigorously using a row of deeper red projections along each side. One unfortunate individual was seen to be clipped in half by a hungry crab although one part escaped!

As no specimen was collected, we can not be sure of the species but we know that some polychaete worms including members of the ragworm family (Nereidae) have a reproductive swimming stage. Either most of or just the hinder end of the body of both male and female develops paddle-like processes enabling the worms, which normally crawl amongst weed and rock, to swim freely. These are known as epitokes and were even described originally as a separate species - Heteronereis). The ripe sexual elements - sperm and ova - erupt from the adult epitoke body which then dies leaving numerous larvae to start life in the plankton.

Both the lower shore rocky habitat and the description of the worm given by the Page family suggest a ragworm Nereis pelagica which is known to swarm when breeding at about 3 years old. The larvae are mainly free-swimming and laboratory studies suggest a pelagic life of some 18 days before settling. A similar occurrence at Exmouth in about 1993 had been reported to Dr Tegwyn Harris, a national polychaete expert, but although he visited the site soon afterwards, he was disappointed to find nothing. Neither Drs David George or Peter Olive, both national experts on polychaete worms, had ever come across witnesses to this spectacular event and the literature is unclear as to the frequency of such social spawning. This shows the value of observations and recording - we are lucky to have this report from our local shores.

Stella Turk and Pamela Tompsett

Extract from HVMCA newsletter No.29 Autumn 2004