Systematics and evolution of smiling worms: Family Prosorhochmidae (Hoplonemertea; Nemertea)

Prosorhochmidae is a peculiar family of monostiliferous hoplonemerteans (phylum Nemertea) occupying a variety of habitats from fully marine to fully terrestrial and display a variety of unusual among nemerteans morphological and life history characteristics (e.g. modified nephridial system, hermaphroditism and viviparity) that make them an interesting subject for comparative studies. Origins of terrestriality within a marine phylum have spawned the initial interest for the group, however, lack of a robust phylogeny precluded further testing of evolutionary hypotheses. I present the first phylogenetic analysis of the group based on the combined analysis of morphological characters and partial sequences of Cytochrome Oxidase Subunit I and 16S rDNA mitochondrial genes. A specimen-based revision of the morphological characters previously used in prosorhochmid systematics literature demonstrated that many characters were misinterpreted and exposed a fundamental problem of nemertean (soft-bodied) systematics associated with quantitative nature of morphological characters, lack of methods to accurately quantify variation and lack of appreciation for intraspecific variation. Sequence data appear to be especially important in such "morphology-poor" taxa as they offer an independent source of information and the combined analysis allows an ultimate test of homology of morphological characters. The hypotheses of monophyly of the family and included genera were tested and a taxonomic revision based on the cladistic analysis presented. Alternative hypotheses about the evolution of terrestriality and adaptive significance of modified nephridial systems and hermaphroditism are discussed.