ASTERIIDAE (Fisher, 1940)

Mah, Christopher & Foltz, David, 2011, Molecular phylogeny of the Forcipulatacea (Asteroidea: Echinodermata): systematics and biogeography, Zoological Journal of the Linnean Society 162 (3), pp. 646-660 : 653-654

THE ASTERIIDAE : BOREAL CLADE

Among the most diverse of the asteriid lineages is a boreal clade that includes primarily shallow-water, continental-shelf genera from the Arctic, temperate to high-latitude North Atlantic, and North Pacific, inclusive of cold and temperate waters in Asia ( Japan and Taiwan) and Russia. Several familiar genera of Asteriidae , including the intertidal Asterias and Pisaster , are supported as monophyletic. These two genera show a sister-group relationship in the twogene tree ( Fig. 2 ), but a more distant relationship in the three-gene tree. Stephanasterias and Plazaster are united as sister taxa on the three-gene tree ( Fig. 3 ) with 68% bootstrap support, and are shown as closely related on the two-gene tree, which further suggests close affinities between the two genera.

The largest clade, containing the greatest number of species within the holarctic Asteriidae , is the one upholding the genus Leptasterias , a highly diverse lineage that has shown extensive diversification across the Arctic and temperate to high-latitude North Atlantic and North Pacific.

All included species of Leptasterias were recovered on a single clade, which also included the North Pacific Evasterias and the Southern Hemisphere Perissasterias as sister taxa within the Leptasterias lineage. Previous phylogenetic studies of the genus Leptasterias employing more taxa and longer sequence reads than those analyzed here ( Hrincevich, Rocha-Olivares & Foltz, 2000; Foltz et al., 2008) have reported somewhat different results from those in Figures 2 and 3 : (1) Evasterias was strongly supported as the sister group to Leptasterias ; (2) Leptasterias fisheri + Leptasterias stolacantha + Leptasterias muelleri was strongly supported as the sister group to the six-armed subgenus Hexasterias ; (3) Leptasterias polaris ( Polaris section of Fisher, 1930) was strongly supported as the sister group to the remaining Hexasterias (i.e. Fisher’s Camtschatica section); (4) Leptasterias leptodoma was supported as the sister group to the remaining members of the Camtschatica section. Leptasterias hylodes , Leptasterias ochotensis , Leptasterias squamulata were not sampled in the earlier studies. These differing results probably reflect the difficulty of inferring phylogenetic relationships within a recent, rapid radiation ( Foltz et al., 2008).

Two Southern Hemisphere genera, Perissasterias and Taranuiaster , were present among the predominantly Northern Hemisphere boreal Asteriidae . Further data are needed to make any definitive conclusions, but if the results are taken at face value, then two interpretations are possible for one or both of these genera. Perissasterias and Taranuiaster may belong to mostly extinct lineages of Asteriidae , which suggests a greater number of taxa with a more widespread distribution in the past, especially into the Southern Hemisphere. Alternatively, larval or adult dispersal may have transported ancestors of these taxa to the Southern Hemisphere, providing suitable environmental conditions to settle, thrive, and establish an adult population. Fisher (1930) noted that Perissasterias was one of the few Southern Hemisphere genera to share characters with Northern Hemisphere Asteriidae . Also, Perissasterias and Taranuiaster are deep-sea taxa, and live in environments that may parallel cold-water settings similar to those of Leptasterias and other boreal Asteriidae .

Pycnopodia was supported as the sister taxon to Rathbunaster . Pycnopodia has been classified as one of two genera (the other being Lysastroma) within the Pycnopodiidae (see Clark & Mah, 2001). However, Lysastroma was not included in our treatment, which for the moment, prevents us from completely testing the monophyly and validity of the Pycnopodiidae .

Figures

Figure 2. Maximum-likelihood tree for 95 forcipulate taxa and nine velatidan taxa, rooted on 111 taxa belonging to the Valvatida, Paxillosida, and Notomyotida (these taxa have been omitted for clarity), and based on 261 bp of sequence data for the 12S rDNA gene and 437 bp for the 16S rDNA gene. Bootstrap support values are based on 250 pseudoreplicates and are shown as percentages when ± 50%. Named clades correspond either to traditional taxonomic groups or to geographically restricted lineages.

Figure 3. Maximum-likelihood tree for 78 forcipulate taxa and five velatidan taxa, based on 327 bp for the early-stage histone H3 gene plus the same rDNA sequences that were used in Figure 2. Bootstrap support values are based on 200 pseudoreplicates. Other details are as described in Figure 2.