GENETIC-EVIDENCE FOR THE OCCURRENCE OF A CRYPTIC SPECIES WITH THE LITTORAL NEMERTEANS LINEUS RUBER AND L-VIRIDIS (NEMERTEA, ANOPLA)

Lineus ruber and L. viridis are common intertidal heteronemerteans fou nd on the coasts of northern Europe and the east coast of North Americ a. The two species resemble each other morphologically and were synony mised for many years prior to being separated on the basis of larval d evelopment. In this study, specimens oft. ruber and L. viridis were co llected from sites along the west and southwest coast of Britain, nort hern France and the east coast of North America. The external morpholo gy and allele frequencies of isozymes of the two species were compared from all sites. The external morphology of L. ruber and L. viridis wa s similar but they could generally be separated by colour. Allele freq uencies for up to 13 enzyme loci between sympatric populations of L. r uber and L. viridis indicated that these two species are genetically v ery different (Nei's genetic identity = 0.090 to 0.083). Allele-freque ncy data also indicated the presence of a third genetic type occurring sympatrically with populations of both L. ruber and L. viridis on the coasts of Britain and France but not on the coast of North America. F ixed differences in allele frequencies between populations of the thir d genetic type and sympatric populations of L. ruber and L. viridis we re observed across multiple loci. Genetic identity between the third g enetic type and sympatric populations of L. ruber and L, viridis were extremely low (Nei's genetic identity = < 0.078). Such large genetic d ifferences between populations indicate a barrier to gene flow and rep roductive isolation. The aberrant type, which exhibits sufficient morp hological variation to prevent individuals being distinguished from th ose of both L. ruber and L. viridis, therefore represents a separate s pecies. The low genetic identities found in intrageneric comparisons o f species found in this study have been found in other studies on neme rteans. They may indicate systematic problems within these groups or o ther phenomena such as morphological stasis.