VARIATION IN ARENICOLA MARINA (L.) AND THE STATUS OF ARENICOLA GLACIALIS MURDOCH (POLYCHAETA)

Abstract

Summary. An account is given of ninety‐four anomalous specimens of A. marina, collected from the Firth of Clyde (sixty‐six specimens) and the Firth of Forth (twenty‐eight specimens). More than half of the specimens show anomalies of segment number. The commonest is the presence of an incomplete extra (twentieth) segment at the base of the tail. The highest segment number is twenty with an incomplete twenty‐first; the lowest is eighteen of which the last is incomplete. About 7 per cent of the natural population shows anomalies of segment number. Variation in segment number may be caused in more than one way. It is not generally correlated with other anomalies, but there is a group of worms with eighteen segments and several other peculiarities, suggesting a genetic condition with manifold effects. The collection also includes the following anomalies: vestigial gills on segment vi associated with headward displacement of the hearts through one segment; a nephridium opening on segment iii; a nephridium opening on segment x; absence of the usual hindmost pair of nephridia opening on ix; three oesophageal glands. Fourteen specimens of A. glacialis from Point Barrow, Alaska, were examined. Two had vestigial gills on segment vi with headward displacement of the hearts. The more representative specimens differed from typical A. marina only (a) in lacking segments xviii and xix, and (b) in a forward displacement of the level at which the inner end of the dorsal septal vessel (branchial efferent) moves from the dorsal to the subintestinal vessel. The latter displacement occasionally occurs in A. marina. Several of the anomalous A. marina showed departures from the typical condition that were at least as striking at those of A. glacialis. It is suggested that A. glacialis should be given subspecific status as A. marina glacialis Murdoch. A. marina then includes three subspecies differing in segment number and geographical distribution. These are: A. m. marina (L.) Nineteen trunk segments. North Atlantic, north of about 40°, the neighbouring parts of the Arctic Ocean, Hudson Bay, the northern part of the western Mediterranean; probably also in cold water on the Pacific Coast of North America. A. m. schantarica Zachs. Eighteen trunk segments. Sea of Okhotsk. A. m. glacialis Murdoch. Seventeen trunk segments. Point Barrow, Bernard Harbour (N.W. Territories), Bering Strait. There is evidence that A.m. marina tends to separate into much smaller local populations with some degree of genetic differentiation.