Stephanometra

Rankin, Dana L. & Messing, Charles G., 2008, A revision of the comatulid genus Stephanometra AH Clark with a rediagnosis of the genus Lamprometra AH Clark (Echinodermata: Crinoidea), Zootaxa 1888, pp. 1-35 : 3-5

Stephanometra AH Clark, 1909

Antedon (part) Lütken, 1874:190 (in Carpenter, 1879).

Comatula (part) Smith, 1876:406.

Himerometra (part) AH Clark, 1907:356.

Dichrometra (part) AH Clark, 1909a:13.

Stephanometra AH Clark, 1909a:9; 1941:407–409.— Gislén, 1924:59, 64, 66, 89, 99, 100, 235.— Zmarzly, 1985:352 – 353.— Chen et al., 1988:78.— Kogo, 1998:61 –63.

Lamprometra (part) HL Clark, 1915b:104.—AH Clark 1941:472 –475.— Utinomi & Kogo, 1965:274 –276.— Zmarzly, 1985:352.— Chen et al., 1988:78.— Kogo, 1998:61 –63, 65–67.

Diagnosis. A genus of Mariametridae having the centrodorsal convex discoidal with gently sloping sides, and with cirrus sockets encroaching on aboral pole; brachitaxes well-separated; brachitaxis ossicles bearing rounded adambulacral processes oriented parallel or oblique to longitudinal axis of ossicle and producing characteristically scalloped or knobbed lateral margins; cirrals <40; distal cirrals with aboral ornamentation ranging from prominent spine to slight carination; one or more pairs of oral pinnules with reduced ambulacral groove, flattened articular facets, reduced tissue between pinnulars, conical tip and with LW of middle pinnulars 1.5–4.0; P2 of 8 to 18 pinnulars.

Geographic distribution. From the Red Sea to Tanzania in the west to the Republic of the Marshall Islands and Fiji in the east, including tropical Australia as far south as the Capricorn Channel, Queensland, and as far north as southern Japan.

Bathymetric range. Littoral to 62 m.

Remarks. AH Clark (1941:407) distinguished Stephanometra from other mariametrid genera chiefly on the basis of “one or more of the oral pinnules enlarged, greatly stiffened, sharp pointed, and spinelike…” His diagnosis of the most closely similar genus, Lamprometra , referred to its longest stoutest pinnule (P2) as very stout to slender. Unfortunately, his species descriptions included substantial overlap between the two. He referred to P2 of S. indica in comparison with other members of S tephanometra as “somewhat less enlarged and stiffened, usually more or less strongly recurved distally becoming slender and delicate, though not flagellate” (1941:409). P2 of Lamprometra palmata palmata , “though tapering... may be straight and almost spinelike.” As a result, AM Clark (1972) found it difficult to distinguish L. palmata from S. indica and thought that the two genera intergraded. She noted that a specimen from Muhlos, Maldives, appeared twice in AH Clark (1941), once as S. indica (p. 453) and once as L. palmata (p. 502). She compared P2 of this specimen, composed of elongated pinnulars, tapering to a fine point and identified as L. p. palmata (her Fig. 10 e), with P2 of the holotype of S. indica (her Fig. 10 g) to distinguish flagellate versus spine-like forms. However, the Mulhos specimen is actually a much smaller specimen of S. indica than the holotype, which bears the third largest P 2 in the current study. Figure 1 more clearly illustrates the difference between “stiff and spinelike” (Figs. 1b–d) and “flagellate” (Figs. 1e–g).

After noting the unreliability of the pinnules as a diagnostic character, AM Clark (1972) suggested that the lateral adambulacral margins of the brachitaxes ossicles might distinguish the two genera: Stephanometra had lateral processes, while Lamprometra tended to have smooth margins. AH Clark (1941:407) described the brachitaxes of Stephanometra as well separated, with component ossicles bearing rounded ventrolateral (=adambulacral) extensions. Those of Lamprometra were typically in close lateral contact and composed of flat-sided ossicles (or, rarely, just in contact with sides slightly or not at all flattened). According to AM Clark (1972), the lateral adambulacral margins in Lamprometra had a straight continuous edge and were blunter than those in Stephanometra , in which flanged margins rounded off at each end produced a scalloped appearance. However, she also noted specimens in which these margins were slightly scalloped as in Stephanometra , while P2 had a flagellate tip as in Lamprometra . She concluded that, because Stephanometra tended to integrate with L. palmata , it should be synonymized with Lamprometra , together with Liparometra and Dichrometra .

The current study reveals that, while specimens of both genera may bear robust, straight oral pinnules, several additional features consistently distinguish the two. In Stephanometra , the enlarged oral pinnules bear middle pinnulars chiefly 2–3 times longer than wide. The tight articulations between these ossicles, characterized by flattened, often almost smooth facets joined by reduced tissue, generate the stiffness and styliform appearance (Fig. 2a–c, 3a, c). The terminal pinnular is short and conical (Fig. 2d). Scallop-like processes along the lateral adambulacral margins of the brachitaxes orient obliquely and may produce elongated knobs proximolaterally on the axils (Fig. 3b, e, f, 4a). In addition, the centrodorsal is convex with sloping sides, and apical cirrus sockets encroach on a reduced aboral pole (Fig. 3e, f, 4b–c).

By contrast, the pinnulars of the robust and occasionally spine-like P 2 in Lamprometra are proportionally shorter (LW of middle pinnulars 1.0–1.5, rarely longer) and bear well-developed articular facets (Fig. 2e–f). The pinnule tapers to a slender flagellate tip, and the distal pinnulars bear small spines (Fig. 1h). The lateral adambulacral margins of the brachitaxis ossicles may be weakly thickened but lack scalloped processes ( Fig. 4 d); ossicles of adjacent brachitaxes range from flattened against each other to separate ( Fig. 5 a). The centrodorsal bears a broad aboral pole with cirrus sockets restricted to the margin ( Fig. 4 e–f).

A plot of aboral pole diameter against centrodorsal diameter for Stephanometra and Lamprometra indicates that, for a given aboral pole diameter, L. palmata specimens have a greater centrodorsal diameter (Fig. 6a). Although slight overlap exists among several juvenile specimens, the two genera otherwise fall out as separate character spaces.

In plots of the length of pinnular 6 of P2 against several growth-related characters [axil (Ibr2) width, maximum cirrus length and arm radius], the two genera form distinctly separate character spaces with little overlap. Only the plot of pinnular length versus axil width is shown (Fig. 6b). Of the two species of Stephanometra recognized here (see below), S. tenuipinna forms a continuum with S. indica , reflecting the close relationship of these species.

Finally, AH Clark (1941) indicated that the IIIBr series was more frequently internal than external in Stephanometra . However, the specimens in this study develop IIIBr either externally or both internally and externally.

FIGURE 1. a–d. Stephanometra indica with P2-P4 stiff and spine-like, IRSNB/CRI 396. a. P1, b. P2. c. P3. d. P4. e–g. Lamprometra palmata with flagellate pinnules, IRSNB/CRI 413. e. P1. f. P2. g. P3. h. Distal pinnulars of P2, NSUOC 345. Scale bars: upper (a–g) 2 mm; lower (h) 0.5 mm.

Figures

FIGURE 5. Lamprometra palmata. a. Specimen from Ngederrak Reef, E entrance to Malakal Harbor, Palau, 07 º 17.900 ’ N, 134 º 28.340 ’ E; 1 m, sea-grass bed; showing flat-sided bases of rays (arrows). b – c. Specimens from Iro Lake Tunnel, W of Ngeruktabel I., Palau, 07 º 17.390 ’ N, 134 º 25.020 ’ E; 1 m; showing concentric color patterns. d. Same, L. palmata cf. form brachypecha. No scales recorded.

FIGURE 10. Stephanometra indica (original identifications in parentheses). a – d. Centrodorsal, aboral view. a. NSUOC 313 (S. spinipinna). USNM 1094093 (S. indica). c. FLMNH 6822 (S. spicata). d. IRSNB / CRI 407 (S. oxyacantha). e – g. Centrodorsal, lateral view. e. IRSNB / CRI 409 (S. indica). f. NSUOC 313. g. USNM E 5269 (S. spicata). h – o. Proximal brachitaxes. h. NSUOC 323 (S. spicata). i. USNM 1094116 (S. indica). j. NSUOC 313 (S. spinipinna). k. IRSNB / CRI 409 (S. indica). l. USNM 1094092 (S. spicata). m. NSUOC 316 (S. spicata). n. IRSNB / CRI 400 (S. oxyacantha). o. Same, adjacent lateral adambulacral margins of brachitaxes. Scale bars: upper left (b, d, e), right (a, f, g), lower left (h – o), all 2 mm.

FIGURE 4. a – c. Stephanometra indica. d – f. Lamprometra palmata. a, d. Ray base with brachitaxes IBr 2 and IIBr 2. b, e. Centrodorsal aboral view. c, f. Centrodorsal lateral view. Scale bars: upper left (a, d), upper right (b, c), lower right (e, f), all 2 mm.