Sibling species in the marine pollution indicator genus Pontonema Leidy (Nematoda : Oncholaimidae), with a description of P-mediterranea sp nov.

Species of the marine nematode genus Pontonema have been found to dominate the macrobenthos under abnormally high conditions of particulate organic en richment. Populations from organically enriched habitats in six localities( Kiel fjord, Germany; the Garroch Head sewage-sludge dump ground in the Firt h of Clyde, Scotland; Cornelian Bay, N.E. England; the Tyne estuary, N.E. E ngland; Sete, N.W. Mediterranean, France; Blanes Bay, N.W. Mediterranean, S pain) have been compared. Multivariate analyses of 16 morphometric characte rs in males, and 13 in females, shows that each population is significantly different morphologically from every other population. However, there is m orphological overlap between the populations, and it is considered pragmati c to recognize three species in this group, based on a few stable morpholog ical characters: P. vulgare (Bastian, 1865) from the Baltic, P. alaeospicul a Bett and Moore 1988 from the UK sites, and P. mediterranea sp. nov. from the Mediterranean. P. mediterranea sp. nov. is described. The species were not found to have established sympatry in this study, unlike many other opp ortunistic taxa from organically enriched habitats, and the significance of this is discussed.