The European Flat Oyster, Ostrea edulis L., as a Host for Mytilicola intestinalis Steuer

Abstract

Baird, Bolster & Cole (1951) reported the occurrence of Mytilicola intestinalis Steuer in oysters from the Penryn River, Cornwall, and from the River Blackwater, Essex. So far as is known this was the first time this host-parasite relation had been observed, although in 1946 Odlaug had reported the occurrence of Mytilicola orientalis Mori from Ostrea lurida Carpenter on the American Pacific coast. Since then infection of a number of molluscs, including oysters, with Mytilicola intestinalis larvae has been achieved under laboratory conditions (Hepper 1953). Since Mytilicola intestinalis has been shown by Cole & Savage (1951), Korringa (1951, 1953, 1954), and others, to cause a serious loss of condition in mussels, Mytilus edulis L., and Odlaug (1946) has concluded that Mytilicola orientalis causes loss of condition in Ostrea lurida, it may be assumed that any widespread and heavy infection of oysters by Mytilicola intestinalis would have serious consequences for the British oyster industry. If the oyster is susceptible to heavy infection this could easily occur, as the mussels in the region of all the major oysterages are already heavily infected with Mytilicola. Another disturbing possibility is that if oysters infected with Mytilicola were relaid near mussel beds free from the parasite, this could, and in fact almost certainly would, introduce the parasite to the mussel beds, possibly with disastrous consequences. Should this happen in, say, the Wash or North Wales the effect on the British mussel industry would be serious. With a view to investigating these possibilities a survey was made in 1954-55 of the degree to which oysters from various parts of Britain were infected with Mytilicola. Some results obtained in 1953 are also included. Samples of twenty oysters each from the Yealm, Crouch, Helford and Fal estuaries had been examined in 1952, but Mytilicola was not found. It is considered now that these samples were too small to show the degree of infection and they are not included. A sample normally consisted of about fifty-sixty oysters, but in some cases it was not possible to get so many, particularly in earlier surveys. The oysters were opened by severing the adductor muscle, followed by removal of the flat valve; the digestive gland and gonad were then carefully sliced away with a sharp scalpel until the gut was exposed. The gut was opened with a needle, thus exposing any parasites present. In practice this is a simple operation, and the parasites, being bright red, show up very clearly; it is unlikely that any were missed, except perhaps a few very early stages. Since this investigation was concerned with established infections, this slight loss, if it occurred, was unimportant.