A new species of Xyloplax (Echinodermata: Asteroidea: Concentricycloidea) from the northeast Pacific: comparative morphology and a reassessment of phylogeny

Xyloplax janetae n. sp. is described from the northeast Pacific Ocean. This is the third species recognized for the monogeneric Concentricycloidea. Skeletal structures are elaborated by scanning electron microscopy and compared with those of Xyloplax niedusiformis from New Zealand and Xyloplax turnerae from the Bahamas. Critical-point-dried specimens show fibrous connective tissue emerging from openings on the abactinal surface, on the abactinal spine bases, and at broken cross sections of the adambulacral spines. Knoblike structures emerging through the stereom openings of the abactinal spine base are also observed. Tube feet have round, swollen knobs and show few striations relative to other asteroids. Cosmopolitan bathyal•abyssal echinoderm species tend to exhibit few morphological differences over broad geographic ranges; this generalization is true of Xyloplax. The Concentricycloidea is proposed as an infraclass within the Asteroidea, as the sister branch to the Neoasteroidea (the crown-group asteroids) within the subclass Ambuloasteroidea. The hypothesis presented is compatible with recent phylogenetic data supporting affinities between Xyloplax and the Asteroidea. Characters plesiomorphic in basal neoasteroids are consistent with a sister-group relationship to concentricycloids. Actinal plate presence, an important synapomorphy for the Neoasteroidea, is absent from concentricycloids. The substantial morphological departure of Xyloplax is considered to be associated with the postPaleozoic diversification of crown-group asteroids rather than from modification of an established and conservative morphology. Additional key words: neoasteroidea, evolution, extraxial-axial theory The study of environments in the bathyal and potheses (summarized below), including discussions abyssal zones has presented unique opportunities to of homology and the definition of class-level reladocument morphological novelty in the evolution of tionships within the Echinodermata. marine biodiversity (Gage & Tyler 1991; Mooi & Xyloplax janetae n. sp. MAH 2006 is described David 1996). Significant new perspectives on existfrom specimens collected in the north Pacific at ing paradigms in invertebrate evolution and system2675 m (Voight 2005). This is the third described speatics have resulted from observations of novel cies of Xyloplax and the second from the Pacific morphologies and body plans in select deep-sea Ocean. Xyloplax medusiformis BAKER et al. 1986, taxa. One such taxon is the genus Xyloplax, a tiny the type species, was collected 1057•1208 m off the (~l-8mm diameter) discoidal echinoderm living at New Zealand coast. This was followed by the deabyssal depths on sunken wood. When originally description of Xyloplax turnerae ROWE et al. 1988 colscribed, Xyloplax displayed sufficient morphological lected at 2066 m from the Tongue of the Ocean, disparity relative to other echinoderms that it was Bahamas. considered a sixth extant class of Echinodermata, the Baker et al. (1986) and Rowe et al. (1988) referred Concentricycloidea (Baker et al. 1986). Xyloplax has Xyloplax to a new class, the Concentricycloidea, parinspired discussions and alternative phylogenetic hyallel to the Asteroidea and the Ophiuroidea, based on unique characters such as the development of the tube feet and mouth frame into a uniserial ring. Smith (1988) and Belyaev (1990) argued placement ''Author for correspondence. oï Xyloplax within the Caymanostellidae (Velatida, E-mail: [email protected] Asteroidea; see Table 1 for asteroid classification). New species of Aj/o/j/ax 137 Table 1. Classification of the Asteroidea sensu Blake (1987) and Blake & Hagdorn (2003) featuring taxa mentioned in the text. Class Asteroidea Subclass Ambuloasteroidea Incertae sedis Family Calliasterellidae: genus Calliasterella Family Compsasteridae: genus Compsaster Infraclass Neoasteroidea Infraclass Concentricycloidea Superorder Forcipulatacea Superorder Peripodacea Order Trichasteropsida Order Peripodida Family Trichasteropsidae Family Xyloplacidae Genus Trichasteropsis Genus Xyloplax Genus Berkhemeraster X. janetae n. sp. Order Brisingida X. medusiformis Family Brisingasteridae X. turnerae Genus Brisingaster Order Forcipulatida Family Zoroasteridae Family Asteriidae Genus Asterias Genus Rathbunaster Superorder Spinulosacea Order Velatida Family Caymanostellidae Family Pterasteridae Order Spinulosida Superorder Valvatacea Order Notomyotida Order Paxillosida Family Porcellanasteridae Other pertinent echinoderm classes Class Ophiuroidea The brittle stars•sister group to the asteroids Class Somasteroidea Paleozoic-only grouping of taxa considered ancestral to asteroids and ophiuroids. challenging the distinctiveness of the Concentritween concentricycloids and the Asteroidea had not cycloidea as a separate class. Janies & McEdward yet been found. However, Janies & Mooi (1999) pre(1994a,b) derived Xyloplax from the velatid asteroid sented a tree that combined morphological and body plan and argued that concentricycloids were molecular data (rDNA) that placed Xyloplax on progenetic velatidan asteroids. distal branches within the Asteroidea. Further elabPearse & Pearse (1994) presented a cladistic analoration of these data was presented by Janies (2001). ysis ofmorphological characters comparing Aj/o/7/ax Structural and population studies oí Xyloplax inwith other echinoderm taxa, including ophiuroids elude those of Rowe (1988), who presented additional and all orders of crown-group asteroids. They armorphological data on X. turnerae and reinterpreted gued that phylogenetic affinity with caymanostellids the Paleozoic cyclocystoids (class Cyclocystoidea) or ophiuroids of similar appearance (e.g.. Astrobased on observed features in Xyloplax. Healy et al. phiura) could alternatively be explained through (1988) and Rowe et al. (1994) reported on spermioconvergence. They emphasized that available evigenesis, and spermatozoon and body microstructure, dence was insufficient to clearly identify affinities respectively. and retained Xyloplax as a separate class. Mooi A new collection of 103 specimens initially reportet al. (1998) applied their extraxial-axial theory ed by Voight (2005) is described herein. These spec(EAT) to inferred homologies in Xyloplax, questionimens represent the first concentricycloids collected ing proposed affinities with the caymanostellids, and since those included in Rowe et al. (1988). The prepointed out that unequivocal synapomorphies bevious absence of fresh material limited opportunities Invertebrate Biology vol. 125, no. 2, spring 2006