Delimiting species in the hoplonemertean genus Tetrastemma (phylum Nemertea): morphology is not concordant with phylogeny as evidenced from mtDNA sequences

The marine genus Tetrastemma contains monostiliferous hoplonemertean (phylum Nemertea) species which mostly undergo direct development with no free-swimming stages of larvae. The lack of a pelagic phase, and the fact that many benthic species lay eggs attached to the bottom substrate, are obvious restrictions on dispersal and gene flow. Nevertheless, some of the species, for example T. candidum and T. melanocephalum , are described as ubiquitous and are reported from waters all over the world. We studied genetic variation and evolutionary relationships in order to assess whether they are concordant with external characters in samples of nine morphologically distinct forms formally named as different Tetrastemma species, from different geographical localities. We estimated the phylogeny and species network based on 539 base pairs of the mitochondrial protein-coding gene cytochrome oxidase-1 (CO1) for 30 ingroup specimens. From this, we assessed the evolutionary history and phylogenetic relationships between these forms. We conclude that in most cases there was no correspondence between evolutionary lineage and morphotype. Our results thus indicate that morphological species delimitation in nemerteans may be questionable, and that this in turn may have a profound effect upon estimates of species diversity within the phylum.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 86 , 201–212.