Homalometra crenulata ( Carpenter, 1882 )

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 : 96

Homalometra crenulata ( Carpenter, 1882)

( Fig. 2 )

Antedon crenulate Carpenter, 1882: 507.

Antedon decipiens Bell, 1882: 534 .

Antedon irregularis Bell, 1882: 534 .

Antedon denticulata Carpenter, 1888: 130 .

Antedon dubia Carpenter, 1888: 258 .

Craspedometra aliena AH Clark, 1909c: 31 .

Amphimetra crenulata AH Clark, 1913: 16 .

Craspedometra aruensis Reichensperger, 1913: 99 .

Heterometra crenulata AH Clark, 1918: 79 .

Homalometra denticulata Foo et al., 2021: 638 .

Holotype. EXEMS 263 /1903/B/57 (fragments from specimen ZMH 487-526 [507] which is missing).

Material examined. Sta. SUB07 , SE of St. John’s Island, start 01°12.671′N, 103°51.232′E, end 01°12.607′N, 103°51.145′E, 53.7– 34.8 m, 19 April 2016 (n = 1; TR0322, ZRC.ECH.1637); sta. SUB19 , S of St. John’s Island, start 01°12.533′N, 103°51.097′E, end 01°12.497′N, 103°51.120′E, 43.8– 40.6 m, 2 June 2016 (n = 1; TR0951, ZRC.ECH.1679); sta. SUB21 , SE of St. John’s Island, start 01°12.519′N, 103°51.399′E, end 01°12.516′N, 103°51.486′E, 60.8– 56.2 m, 2 June 2021 (n = 3; TR0952i, ZRC.ECH.1639; TR0952ii, ZRC.ECH.1636; TR0952iii, ZRC.ECH.1638); sta. SUB36 , SW of St. John’s Island , start 01°12.649′N, 103°50.659′E, end 01°12.617′N; 103°50.613′E, 40.6– 38.3 m, 30 June 2016 (n = 2; TR2137, ZRC.ECH.1632; TR 2144iii, ZRC. ECH.1628) .

Diagnosis. A highly variable species. First three outer pinnules enlarged and are moderately to strongly prismatic. P 1 almost always the smallest, P 2 and P 3 equally likely to be the largest (where Pn denotes the nth exterior pinnule from the base of each undivided arm). Proximal pinnulars of the proximal pinnules bear carination ( Fig. 2A ) but might be absent in juveniles ( Fig. 2B ). From the third or fourth pinnular onwards, their distal corners on the abambulacral side always ornamented with serrated triangular processes which may be conspicuous and broadly rounded ( Fig. 2C ) or small, compact, and in tight apposition to the pinnules ( Fig. 2D ). The distal margin of the distal pinnulars often bordered with minute spines that form a partial rim across the abambulacral surface, giving the distal edges of the pinnulars a serrated profile ( Fig. 2E ). Radials may be partially or completely concealed in the midradial lines. When exposed, the distal edges are most likely bordered with bead-like tubercles ( Fig. 2F ), though exceptions of smooth radials were seen in some specimens including in one juvenile.

Description. Centrodorsal discoidal to hemispherical, with the polar area either flat and barren* or appearing uneven with indentations which may resemble obsolete cirral sockets ( Fig. 2G ) (* = adapted from AH Clark, 1941). Cirri as little as XI* in small specimen and as many as XXXV* in adults (XVII–XXX in examined specimen), arranging marginally in two alternating rows (number of cirri denoted in Roman numerals). Number of cirrals range between 17 and 46* (usually 30–40). Cirral mostly broader than long or as broad as long except in the middle cirrals (~6–11) of some specimens. Middle to distal cirrals bear carinae with varying morphology which may be absent in immature specimens. In typical examples, the carinae project distally into a triangular extension ( Fig. 2H ) while in other cases, they are divided into 2–3 teeth ( Fig. 2J ). Note that different types of carinae could be present in the same individual on different cirri. The cirri of immature specimens may appear smooth, slender, and tapered to a point with the lack of carination, as seen in some of the western Australian specimens, including in the holotype of Homalometra denticulata ( Carpenter, 1888) , now a junior synonym. Opposing spine always present on the penultimate cirral. Brachitaxes up to three series, with IBr2 united by synarthry, IIBr either 2 or 4(3+4), and IIIBr chiefly absent or two ossicles when present, but IIIBr4(3+4)* has also been reported in some specimens examined by AH Clark (1941) (Roman numerals with “Br” and an Arabic numeral refer to a particular brachitaxis, e.g., IBr2 = primibrachial series comprising two ossicles; particular ossicles are identified with a lower-case “br” followed by an Arabic numeral, e.g., Ibr2 = second ossicle of the primibrachial series; plus sign denotes syzygy articulation between specific ossicles, e.g., IIBr4(3+4) = secondibrachial series comprising four ossicles with syzygy between ossicles IIbr3 and IIbr4). The first two segments beyond each axillary bear a weak synarthrial tubercle at the middle line of their junction. Proximal brachials discoidal. The following brachials triangular to wedge-shaped, with a variable tendency to overlap dorsally, and their broader ends project alternately on opposite sides of the arm to a greater or less extent*. The sides of the proximal brachials may be flattened or swollen. First syzygial pair on the undivided arms always between brachials 3+4, subsequent syzygial position variable. Articular tubercles present.

Remarks. Habitat generally with rocky substrate, coarse sand, broken shells, shelly mud, laterite rock and/or gravel. Depth range extends from shoreline to 111 m (60.8– 34.8 m for the local specimens).

Figures

Fig. 2. Homalometra crenulata morphological characters. Red arrows indicate specific features described for each image (A–F, H–J). A, carination of proximal pinnulars of proximal pinnules; B, lack of carination on the proximal pinnulars of juvenile proximal pinnules; C, broad, round triangular processes on distal corners of ambulacral side of proximal pinnules; D, unique pinnular with triangular processes of proximal pinnulars are small, compact, and tightly appose to the pinnulars; E, serration on the distal margin of distal pinnulars; F, bead-like tubercles on the distal edge of a radial; G, polar area of centrodorsal bumpy and uneven, with indentations that resemble obsolete cirri sockets; H, cirrals with a distally-directed triangular carinate extension; J, aboral projections compressed laterally on distal cirrals, terminating in 2–3 weak or strong teeth, or almost a flattened tip (lower-right arrow).