ASTERINIDAE, Gray, 1840

Mah, Christopher & Foltz, David, 2011, Molecular phylogeny of the Valvatacea (Asteroidea: Echinodermata), Zoological Journal of the Linnean Society 161 (4), pp. 769-788 : 777-778

THE ASTERINIDAE , GANERIIDAE, AND SOLASTERIDAE

The Asterinidae has occupied a controversial position in the history of asteroid classification, having been placed in Spinulosida in Fisher (1911) and others (e.g. Sladen, 1889) but later independently reassigned to the Valvatida by Blake (1987) and Gale (1987b). Classification and revision within the Asterinidae has been a constant source of activity (e.g. A.M. Clark, 1983; O’Loughlin & Waters, 2004), but the distinctive morphology of asterinids has generally suggested a derived and potentially monophyletic assemblage with only limited subdivision (e.g. the Tremasterinae Sladen, 1889 ; Anseropodinae Fisher, 1906; summary in Spencer & Wright, 1966). Waters et al. (2004a) provided the first molecular phylogenetic treatment of the Asterinidae that included taxa beyond Asterina + Patiriella , using mitochondrial COI, 12S, and 16S rRNA data, primarily from Australasian taxa. Their phylogenetic tree did not support monophyly for the Asterinidae , but was the basis for subsequent far-reaching taxonomic revisions ( O’Loughlin & Waters, 2004).

Basal relationships in the large clade subtended by node P in Figure 1 were mostly unresolved. Somewhat better resolution was obtained in the two-gene tree ( Fig. 2 ) for some groups, such as Parvulastra and Kampylaster + Anseropoda antarctica . However, support for all basal nodes in the asterinid clade of our two-gene tree (i.e. those subtended by node 1 in Figure 2 ) was also relatively poor (<50%), suggesting that basal relationships within this group will require further study. However, when placed into the context of greater taxonomic sampling, our phylogenetic results mirror the results of Waters et al. (2004a) and cast substantial doubt on the monophyly of the Asterinidae . This strongly suggests that the Asterinidae , as traditionally defined, is a paraphyletic assemblage. The stemward positions on node P of all the asterinid taxa suggest that they occupy a relatively plesiomorphic condition, forming more of a grade relative to more derived taxa, such as the solasterids and the ganeriids. This represents a significant shift in perception of the Asterinidae , which has historically been supported by distinct morphological autapomorphies and has been perceived more as a derived, terminal branch (e.g. Blake, 1987; Gale, 1987b) rather than as an ancestral grouping.

Several of the other phylogenetic and taxonomic results produced by Waters, O’Loughlin & Roy (2004a) and O’Loughlin & Waters (2004), especially the monophyly of the higher branches, such as Meridiastra , Patiria , Paranepanthia , Aquilonastra , and Parvulastra , are upheld by our results. The basal branches in Waters, O’Loughlin & Roy (2004a) are clarified in our results. Dermasterias , which was shown embedded within the ingroup Asterinidae , is clearly removed from the Asterinidae by our trees. Our examplar species of Nepanthia , Nepanthia grangei , differs from that of Waters, O’Loughlin & Roy (2004a) who used Nepanthia troughtoni but our result shows Nepanthia as the sister to the ganeriid Tarachaster australis .

Tremaster has traditionally been placed in the Asterinidae but has been separated by some workers (e.g. Sladen, 1889; Smith & Tranter, 1985) into the Tremasterinae , which included Tremaster , Stegnaster , and two fossil tremasterines ( Spencer & Wright, 1966). Jangoux (1982) disagreed with this classification and removed Stegnaster from the Tremasterinae , a conclusion that is consistent with our mitochondrial-only tree ( Fig. 2 ). Available data support Tremaster as a divergent lineage within the Asterinidae with uncertain affinities.

Derived members of the ‘asterinid’ clade include primarily former members of the Solasteridae and the Ganeriidae , plus the Leilasteridae . The solasterid + asterinid relationship, as supported by molecular data, was first observed by Wada, Komatsu & Satoh (1996) and later developed by Matsubara et al. (2004). The presence of ganeriid taxa, such as Cycethra and Perknaster , within the asterinid clade is consistent with historical classifications that have demonstrated affinities between asterinids and ganeriids. The Ganeriidae have always been supported as similar to or morphologically close to the Asterinidae . Early classifications (e.g. Sladen, 1889) included Cycethra and Ganeria as members of the Ganeriinae, part of a subfamily within the Asterinidae . This affinity has been further supported by contemporary morphology-based phylogenies ( Blake, 1987; Gale, 1987b). Further sampling will be necessary to test the monophyly and affinities of the Ganeriidae .

The Solasteridae, sensu A. M. Clark (1996) was largely supported as monophyletic with the sole exception of the Antarctic Cuenotaster involutus , which was included as the sister clade to the ganeriid Perknaster . Although much work remains to be completed regarding this question, some morphological characters are consistent with Cuenotaster as a member of the Ganeriidae .

Jurassic fossils are known for taxa within the clade subtended by node P ( Solasteridae + Asterinidae + Ganeriidae + Leilasteridae ) but are limited to the Asterinidae and the Solasteridae . Asterinid fossils are limited to those showing affinities with Tremaster in the subfamily Tremasterinae .

Tremaster is supported on a long branch as sister group to a clade containing Nepanthia and Tarachaster ( Fig. 2 ). Tremasterine fossils, such as Mesotremaster felli Hess, 1972 and Mesotremaster zbindeni Hess, 1981 from Germany and Protremaster uniserialis Smith & Tranter, 1985 from Antarctica have been collected from Jurassic strata. Nontremasterine asterinid fossils are unknown.

Plesiosolaster (described as Brachisolaster ) moretonis was re-described and discussed by Blake (1993) and further discussed by Villier, Charbonnier & Bernard (2009) from the Jurassic of England and is the only well-preserved fossil solasterid known. Although a full phylogenetic analysis was not undertaken, Blake hypothesized that the multiarmed condition for solasterids was derived relative to a more ancestral Lophaster or Rhipidaster -like morphology. Blake’s hypothesis is consistent with our phylogenetic tree ( Figure 1 ). This may imply that the lineage containing multiarmed taxa ( Solaster and Crossaster ) may be closely associated with the Jurassic occurrence of multiple arms in the Solasteridae .

Figures

Figure 1. Maximum likelihood tree of valvatacean sea stars rooted against forcipulatacean + velatid sea stars. The analysis is based on 1084 bp of concatenated sequence data from two mitochondrial rDNA genes and one nuclear protein-coding gene, early-stage histone H3. As discussed in the text, solasterids are treated as part of the ingroup. The analysis was carried out in RAxML with bootstrap percentages indicated for nodes that had> 50% support. Boxed letters A–P are referred to in the text. Traditional family names are shown at the right; all families are in the Valvatida except as noted. The scale bar shows the expected number of substitutions.

Figure 2. Maximum likelihood tree of asterinid + solasterid sea stars rooted against closely related valvatid sea stars. The analysis is based on 1417 bp of concatenated sequence data from two mitochondrial rDNA genes. Circled numbers 1 and 2 are referred to in the text. Other details as in Figure 1.