Turbeville, J. M.: Recent Progress in Nemertean Biology: Phylogeny, Development, Ecology

Phylum Nemertea contains approximately 1000 species of unsegmented worms that possess an eversible proboscis. The phylum has a global distribution and includes marine, freshwater and terrestrial species. Although relatively common, nemerteans have been understudied. However, in recent years significant advances in nemertean biology have been made and contributions to three areas will be reviewed: 1) phylogeny, 2) development and 3) ecology. Nemerteans are generally considered to be primitive acoelomate bilaterians most closely related to platyhelminths, and their HOX genes recently have been examined to gain insight into the evolution of these regulatory genes in Bilateria. Nevertheless, their phylogenetic position remains uncertain. One recent cladistic analysis of morphological characters places nemerteans in a spiralian coelomate clade, whereas another supports a Nemertea + Platyhelminthes clade. These conflicting phylogenies result from different a priori interpretations of nemertean body cavities and other morphological features. Analyses of 18S rDNA sequences suggest that nemerteans are most closely related to coelomate “lophotrochozoans”. Preliminary comparisons of mitochondrial gene rearrangements support monophyly of spiralians inclusive of playthelminths and nemerteans, but the placement of nemerteans among the spiralians is unresolved. Additional molecular and morphological characters are currently being evaluated for estimating nemertean relationships. Recent studies of nemertean development have clarified the origin of mesoderm and larval structures, have revealed differences in regulation between direct and indirect developing species and have elucidated calcium dynamics during fertilization. Progress in nemertean ecology will also be addressed.