Meridiastra O’Loughlin, 2002

O’Loughlin, P. Mark & Waters, Jonathan M., 2004, A molecular and morphological revision of genera of Asterinidae (Echinodermata: Asteroidea), Memoirs of Museum Victoria 61 (1), pp. 1-40 : 22-23

Meridiastra O’Loughlin

Figures 1 (clade III), 2i, 4c, 5a, 10g –i

Meridiastra O’Loughlin, 2002: 280 .— Waters et al., 2004: 874, 875, 877.

Diagnosis. Rays predominantly 5 or 6 or 7 or 8; interradial margin straight or incurved, rays not discrete; flat actinally, low to high convex abactinally; abactinal plates in longitudinal series along rays, carinal series present for at least part ray length; papulae small, lacking longitudinal series of large papulae along sides of rays; abactinal spinelets granuliform; lacking pedicellariae; abactinal plates with glassy convexities; actinal plates in longitudinal series; actinal spines in mid-interradius digitiform or tapering or conical, 1–3; lacking superambulacral and superactinal plates, distal interradial margin supported by contiguous interior projections from abactinal and actinal plates.

Type species. Asterina atyphoida H.L. Cark, 1916 (original designation).

Other species. M. calcar (Lamarck, 1816) ; M. fissura O’Loughlin, 2002 ; M. gunnii ( Gray, 1840) (junior synonym: Patiriella brevispina H.L. Clark, 1938 , by O’Loughlin et al., 2003); M. medius ( O’Loughlin et al., 2003) ; M. modesta ( Verrill, 1870) (junior synonym: Asterina agustincasoi Caso, 1977 by O’Loughlin, 2002); M. mortenseni ( O’Loughlin et al., 2002) ; M. nigranota O’Loughlin, 2002 ; M. occidens ( O’Loughlin et al., 2003) ; M. oriens ( O’Loughlin et al., 2003) ; M. rapa O’Loughlin, 2002 .

Material examined. See O’Loughlin (2002), O’Loughlin et al. (2002, 2003).

M. calcar . Australia, Victoria, Cape Paterson , NMV F73126 (4).

Description with species variations. Rays predominantly 5 ( atyphoida , modesta , mortenseni , nigranota , rapa ), or 6 ( gunnii , medius , occidens , oriens ), or 7 ( fissura ), or 8 ( calcar ); interradial margin straight to incurved, rays sometimes distinct, narrowly rounded to pointed distally; integument noticeable; size medium ( gunnii up to R = 56 mm) to very small ( rapa up to R = 6.0 mm); pedicellariae absent; gonopores actinal ( nigranota ) or abactinal; fissiparity in one species ( fissura ).

Abactinal plates in longitudinal series; carinal series present to variable extent; abactinal appearance of two types, with papulate areas extensive, papular spaces large with numerous small papulae and secondary plates, plates distinctly doubly or singly notched with projecting proximal edge crescentiform except in distal plates ( calcar , gunnii , medius , mortenseni , occidens , oriens ), or with papulate areas small, papular spaces small with few small papulae and secondary plates, plates not distinctly notched and projecting proximal edge not crescentiform ( atyphoida , fissura , modesta , nigranota , rapa ); lacking distinct longitudinal series of large papulae along sides of rays; disc variably distinct with border of continuous radial and interradial plates; abactinal plates sometimes with low spinelet-bearing elevation; cleared abactinal plates with glassy convexities; abactinal spinelets over projecting anterior edge of plates, not in tufts, granuliform, and stout, opaque and firmly attached ( calcar group above) or fine, glassy and weakly attached ( atyphoida group above); series of subequal, spinelet-covered, superomarginal and inferomarginal plates; margin formed by projecting inferomarginal plates.

Actinal plates in longitudinal series, sometimes oblique ( atyphoida ).

Actinal spines per plate: oral 4–7; suboral 0–2; furrow 2–5 proximally, webbed; subambulacral 1–3; actinal 1–3 in mid-interradius, up to 4 distally; adradial row of actinal plates with variably complete series of spines; interradial actinal spines thick, bulbous to short tapered to conical to digitiform.

Lacking superactinal and superambulacral plates; interradial margin supported by contiguous internal projections of the abactinal and actinal plates.

Distribution. Southern Australia, New Zealand, central and eastern South Pacific, Mexico, Panama; 0– 59 m.

Remarks. Meridiastra , erected by O’Loughlin (2002) on morphological evidence only, is supported by the molecular phylogenetic relationships of the nine species included in clade III of Waters et al. (2004). The key morphological features of these nine species are: straight to shallow incurved interradial margins, rays not discrete; longitudinal series of abactinal plates; granuliform abactinal spinelets; lack pedicellariae; presence of conspicuous glassy convexities on plates; actinal plates in longitudinal series; low numbers of thick, tapering actinal spines; lack superambulacral and superactinal plates; have internal contiguous projections of distal interradial abactinal and actinal plates supporting the margin. Most of the species of Meridiastra were previously assigned to Patiriella which has irregularly arranged abactinal plates, actinal plates in predominantly oblique series, and superambulacral and superactinal plates present. The molecular phylogeny supports the separation of Meridiastra and Patiriella . Meridiastra is closest morphologically to Asterina which also lacks internal skeletal plates but which has more discrete rays, opaque spinelets which are elongate, and actinal plates in predominantly oblique series. The molecular phylogeny also supports the separation of Meridiastra (Indo-Pacific) from Asterina (NE Atlantic). The morphological similarities of these two genera could reflect convergent evolution, or the retention of ancestral character states, or a true sister relationship; the phylogeny of Waters et al., 2004 was uninformative in this regard.

Molecular clade III provides evidence of two groups of closely related species within the clade: M. gunnii , M. oriens , M. mediu s and M. occidens ; and M. fissura , M. nigranota and M. atyphoida . These closer molecular relationships are reflected in the morphological description above, where these two sets fall within the M. calcar and M. atyphoida groups. Our decision to combine the two groups into a single genus is subjective but supported by the significant internal skeletal similarities.

Figures

Figure 1. Phylogenetic relationships of asterinid mtDNA sequences (after Waters et al., 2004). Support for particular reconstructions is indicated by bootstrap values (>50%; in bold) and by decay indices (in italics). MtDNA Clades I–VI are identified on the basis of high bootstrap support. Generic assignments made in this paper appear on the right.