Diversity & Evolution 2 (2002)
Electronic Supplement 5
Morphological phylogenetics of the sea spiders (Arthropoda: Pycnogonida).
print version: Org. Divers. Evol. 2(2): 119-126. 2002 (full article)
Part 1. Species used for the analysis and other material examined. pdf-format, 120 KB
Part 2. Diagrams of proboscis shapes in the Pycnogonida. pdf-format, 190 KB
Part 3. NEXUS-file for phylogenetic analysis. txt-format, 4 KB
Pycnogonids or sea spiders are a group of marine arthropods whose relations to the chelicerates have been an issue of controversy. Higher-level phylogenetic relationships among the lineages of sea spiders are investigated using 36 morphological characters from 37 species from all extant families and a Devonian pycnogonid fossil. This is one of the first attempts to analyze the higher-level relationships of the Pycnogonida using cladistic techniques. Character homoplasy (implied weights) is taken into account to construct a polytomous, most-parsimonious tree in which two major clades within Pycnogonida are obtained. Clade A includes Ammotheidae paraphyletic with Colossendeidae, Austrodecidae and Rhynchothoracidae, and clade B is formed by Nymphonidae, Callipallenidae (apparently paraphyletic), Pycnogonidae and Phoxichilidiidae. The analysis of equally weighted data is presented and helps to identify those characters less consistent. The reduction of the chelifores, palps and ovigers - shown independently within each of the clades as parallel evolution events - challenges the assumption of a gradual mode of reduction within the group, according to analysis of unordered vs ordered characters. Most of the phylogenetic affinities proposed here are compatible with traditional classifications. However, traditional taxonomic characters need to be complemented by sets of anatomical, molecular and developmental data, among others, to produce more robust phylogenetic hypotheses on the higher- and lower-level relationships of the sea spiders.
Key words: Pycnogonida, phylogeny, morphology, character evolution, gradual reduction, sea spiders