MOLECULAR PHYLOGENETICS OF FORCIPULATACEAN STARFISH (ECHINODERMATA; ASTEROIDEA)

The Forcipulatacea is one of three major lineages of modern crown-group Asteroidea, including such familiar genera as the ecologically important Pisaster. Living forcipulataceans occur primarily in cold-water settings-polar and deep-sea environments at depths ranging from the intertidal (0-2 m) to the abyss (>1000 m). In spite of a highly diverse range of body plans, forcipulataceans are morphologically distinctive, displaying several immediately recognizable diagnostic features, including 3 piece crossed pedicellariae, four rows of tube feet, fused proximal adambulacral ossicles (called an adoral carina) and a reticulate skeleton. Although the group is easy to identify, classification among certain forcipulatacean taxa, especially within the Asteriidae has been historically problematic.

12 and 16S sequence data was sampled and sequenced from 50 taxa representing all major forcipulatacean groups, including the Asteriidae, Pedicellasteridae, Labidiasteridae, Pycnopodiidae, Heliasteridae, Zoroasteridae, and the Brisingida were analyzed using Bayesian and parsimony analyses resulting in largely similar tree topologies.

The tree topology is strongly tied with regional diversification. A derived lineage composed of individual Holarctic, Antarctic, and tropical equatorial clades is supported as a sister group to more basal members which occur primarily in the southern hemisphere and in deep-sea regions. Zoroasterid and brisingid lineages are supported as monophyletic although exact relationships between these and other basal forcipulataceans is ambiguous. The Labidiasteridae, which has long been perceived as a non-natural grouping, is shown to be polyphyletic and should be synonymized. The Pedicellasteridae, which has been perceived as intermediate between forcipulataceans and other asteroids is not supported as monophyletic with taxa supported in the derived and basal lineages.

Asteriid taxa in the basal Southern Hemisphere/deep-sea lineage are morphologically most similar to Jurassic asteriids described by Blake (1990) and Hess (1972) whereas fossils and geological events, such as isolation of the Southern Ocean suggest the derived lineage as occurring more recently. Brooding of juveniles is observed only from the Antarctic and Holarctic lineages on distantly related clades.