A reassessment of the systematics and a proposal for the phylogeny of some cosmopolitan Lineus species (Nemertea)

Morphological, behavioural and biochemical characters of 9 cosmopolitan, benthic species of the nemertean genus Lineus are used both to clarify their taxonomy and to discuss their phylogenetic relationships. Data analysis shows that all fissiparous species of Lineus collected in the world seas up to date can be placed in a single species, L. sanguineus, with three recognisable subspecies, L. s. sanguineus, L. s. nigricans and L. s. pseudolacteus. These taxa were originally described as separate species mainly according to their geographic origin under the names L. sanguineus (shores of European seas), L. socialis (Atlantic North American shores), L. vegetus (Pacific North American shores), L. pseudolacteus (shores of the English Channel) and L. nigricans (Mediterranean coasts of Italy and France). Examination of the character value matrix suggests a phylogenetic tree developing from two ancestral branches. The first branch includes the two related species L. ruber and L. viridis, the second branch the three species L. longissimus, L. lacteus and L. sanguineus. This approach to the taxonomic and phylogenetic relationships of Lineus nemertean species is in agreement with the intra- and interspecies histocompatibility data in nemerteans: (i) grafts transplanted from donors to recipients of the same species succeed and grafts transplanted from donors to recipients of different species fail; (ii) grafts are rejected more rapidly when the donor and recipient species diverged earlier in evolution and, a contrario, grafts are accepted better when the donor and recipient species have been more recently isolated.