Phylogenetic study of pelagic nemerteans (Pelagica, Polystilifera)

Abstract

Pelagic polystiliferous nemerteans, often referred to as Pelagica, represent one of the most enigmatic groups of nemerteans. The group includes 98 valid species assorted into 41 genera and 3–11 families, depending on the classification. Pelagica inhabit the water column of the world oceans, occupying depths from several hundred to several thousand meters. As is the case with most meso- or bathypelagic soft-bodied animals, specimens are few and numerous difficulties are associated with obtaining and preserving them, resulting in incomplete descriptions and, therefore, an obscure classification. Most genera and families of pelagic nemerteans are based upon unique combinations of two or three characters. Here, we present results of the first cladistic analysis of pelagic nemertean phylogeny. The analysis is based on morphological character data available from the primary literature and personal observations. A large percentage of missing entries in the data (21.5%), due to incompleteness of descriptions, results in a large number of maximum parsimonious cladograms, which translates into a drastic lack of resolution on the strict consensus tree. Traditional families supported by the cladistic analysis are Armaueriidae Brinkmann, 1917 and Pelagonemertidae (sensu Korotkevitsch, 1955). We propose a new diagnosis for Armaueriidae and discuss morphological characters traditionally used in pelagic nemertean classification. A few potentially informative characters are suggested for greater attention in future studies of specimens.