The occurrence of Cercaria patellae Lebour (Trematoda) and its effects on the host; with notes on some other helminth parasites of British limpets.

Abstract

During investigations of the helminth parasites of the British limpets Patella vulgata, P. intermedia and P. depressa in relation to a possible bearing of parasitism on mortality and sex proportions, three larval trematodes, Cercaria Patellae Lebour, 1911; an unidentified Xiphidiocercaria, Cercaria B; an unidentified Cercaria C; and one larval cestode were found. Of these, C. patellae was the commonest.Each of the three trematodes occurs independently in the digestive gland, the cestode being encysted on the surface of the visceral mass. The Cercaria C is visible only in smear preparations or sections, but infestations with the other three parasites can usually be seen on removing the limpet's shell. The sporocysts of the Xiphidiocercaria, Cercaria B, are restricted to a limited portion of the digestive gland or, very rarely, of the kidney; the rediae of C. patellae are loosely distributed throughout the visceral mass. No sporocysts of the Cercaria C have been observed.C. patellae is commonest in Patella depressa, but since in all three species it is more common in populations inhabiting rock pools the apparent preference for P. depressa may be because this species occurs in pools or at low water marks, whereas the other species frequent ‘dry’ habitats.Most limpets infested with Cercaria Patellae are in the larger size (age) groups. For example, the percentage infestation in Patella depressa at Port St Mary, Isle of Man, is very low in specimens less than 40 mm. in length, increasing to 100% in those of more than 65 mm.Cercaria Patellae escapes from the limpet by way of the veinlets in the mantle skirt. It is suggested that migration of cercariae within the haemocoele, and subsequent escape from the mantle, may be a common occurrence in other molluscs infested with larval trematodes.The number of cercariae given off by the limpet in vitro is greatest immediately after immersion in water, the rate of escape then steadily falls and finally ceases after about 30 hr. of immersion. Under natural conditions there is probably a rhythm imposed by tidal fluctuations, the cercariae escaping only when the host is covered by the tide.