Dichrometra sp.

Goh, Shawne, Quek, Randolph Z. B., Foo, Sze Hui, Tay, Teresa Stephanie, Messing, Charles G. & Huang, Danwei, 2023, Phylogeny and morphology of Himerometroidea (Echinodermata: Crinoidea) feather stars in Singapore, Raffles Bulletin of Zoology 71, pp. 92-105 : 100-101

Dichrometra sp.

( Fig. 5 )

Material examined. Sta. DR 111, Outside Eastern Boarding Ground A, start 01°12.989′N, 103°53.062′E, end 01°12.862′N, 103°52.852′E, 136– 125 m, 29 May 2013 (n = 1; SS-3634, ZRC.ECH.1968); sta. DR125, Sister’s Island , start 01°12.416′N, 103°49.858′E, end 01°12.433′N, 103°49.817′E, 30.8– 25.3 m, 30 May 2013 (n = 1; SS-3643- 4, ZRC.ECH.1973); sta. TB28, Singapore Port Limit, start 01°13.036′N, 103°52.820′E, end 01°13.058′N, 103°52.746′E, 97.6– 94.3 m, 22 May 2013 (n = 1; SS-0967, ZRC.ECH.0371); sta. TB96, Eastern Bunkering A, start 01°18.140′N, 104°04.22′E, end 01°18.329′N, 104°04.396′E, 25.1– 22.4 m, 28 May 2013 (n = 4; SS-2538-1, SS-2538-2, SS-2538-3, SS-2538-4, ZRC.ECH.0442) .

Diagnosis. Dichrometra with P 2 the longest or stoutest ( Fig. 5D ); cirri XI–XXIV, slender, or short and stout, with 13–28 cirrals; basal cirrals always broader than long; middle to distal cirrals aborally carinate ( Fig. 5B ), spinose ( Fig. 5C ), or each bearing a distally-directed triangular projection; brachials on undivided arm plain; between 10–30 arms; IIBr either consists of 2 ossicles or 4(3+4) ( Fig. 5A ); proximal pinnules generally thin and slender, but can possess spinular side projection ( Fig. 5E ) or carinations on distal edge of distal pinnulars in juveniles.

Description. Centrodorsal hemispherical to discoidal; basal diameter from 0.75 to 2.56 mm in juveniles, to 3.45 mm in adults; aboral pole flat or convex, smooth with no granulation, although bead-like tubercles observed on four juveniles. Cirri XIII–XVIII, 13–28 cirrals in juveniles; cirral ornamentation variable, with middle to distal cirrals aborally carinate, or possessing spines or triangular projections; on one specimen, middle cirrals bearing blunt spines progress into sharper spines on distal cirrals (SS-2538-3, ZRC.ECH.0442). Brachitaxes mostly of two ossicles joined by synarthry ( Fig. 5A ); two juvenile specimens with IIBr4(3+4) observed; brachials on undivided arm aborally plain; proximal pinnules distally flagellate and tapered to a point with no projection in the adult specimen, but variable in juveniles; smooth with no projections, or possess small spines on distal edges.

Remarks. This species is clearly distinguished from all other Dichrometra spp. based on the phylogeny obtained. Nevertheless, it resembles D. palmata in its superior size of P 2 ( Taylor et al., 2018), ranging from 1.07 mm to 4.33 mm in length and 0.1 to 0.36 mm in width (with the exception of SS-2538-1, ZRC.ECH.0442, which has P 3 as the longest and stoutest pinnule). The size range of P 2 in D. palmata is not known. Dichrometra palmata possesses distally flagellate proximal pinnules, and its distal cirrals are usually aborally carinate, occasionally with a spine ( Messing & Tay, 2016; Taylor et al., 2018). This species has distal projections on proximal pinnules and variable cirral ornamentation (see above). Adult descriptions may be subject to further clarification as only one adult specimen was examined in the study. Furthermore, given that the type material of species synonymised with D. palmata in AH Clark (1941) are unexamined, and the uncertainty of proximal pinnule relative sizes as diagnostic characters, this species cannot be confidently placed in D. palmata . Depth range in Singapore 136– 22 m, inhabiting substrates comprising clay, laterite gravel, or sand. Myzostomid worm observed on one specimen (SS-2538-3, ZRC.ECH.0442).

Figures

Fig. 5. Dichrometra sp. morphological characters. A, division series illustrating two ossicles each of IBr2 (IBr1–IBr2), IIBr2 (IIBr1–IIBr2) and IIIBr2 (IIIBr1–IIIIBr2) division series; B, cirri with distal cirrals aborally carinate (red arrow); C, cirri with middle to distal cirrals bearing sharp aboral spines (red arrow); D, proximal pinnules distally flagellate and tapering to a point. P2 longer and basally stouter than P1 and P3; E, spinular side projection on distal edge of distal pinnulars on proximal pinnules on one juvenile (SS-2538-2, ZRC.ECH.0442).