Ostracotheres caramica sp. nov., 2024

Ahyong, Shane T. & Mendoza, Jose Christopher E., 2024, Confirmation of Ostracotheres H. Milne Edwards, 1853 from Southeast Asia in a new species from Singapore and Indonesia (Crustacea: Decapoda: Brachyura), Zootaxa 5476 (1), pp. 9-16 : 10-14

Ostracotheres caramica sp. nov.

( Fig. 1 , 2 , 3A–G )

Ostracotheres cynthiae .— Tesch 1918: 263–264.— Schmitt et al. 1973: 6, 28 [part, Indonesian record only]. [Not O. cynthiae Nobili, 1906 ].

Type material. HOLOTYPE: ZRC 2022.0969 , ovigerous female (cl 6.1 mm, cw 6.6 mm), eastern reef of Kusu Island , 1.225571°N, 103.859833°E, 7 m, stn KD15, from ARMS deployed December 2019, retrieved 25 November 2020, coll. I. Daud, J.J. Ong & Z. Jaafar, 25 November 2020 . PARATYPE: ZRC 2022.0968 , spent female (cl 2.9 mm, cw 3.0 mm), Big Sister Island , 1.212253°N, 103.834955°E, southern reef, 6.5 m, stn SD8, from ARMS deployed November 2019, retrieved 25 November 2020, coll. I. Daud, J.J. Ong & Z. Jaafar, 25 November 2020 .

Other material examined. NBC ZMA. CRUS.D.242068, 1 ovigerous female (cl 3.8 mm, cw 4.2 mm), Tual, Kai Islands, Indonesia, 22 m, dredge, Lithothamnion , sand and coral, from Styela pneumonodes Sluiter, 1895 , Siboga Expedition Sta. 258, 10 December 1899.

Diagnosis of female. Epistome buccal margin between antennal article 1 straight, without triangular median projection. Pereopod 2 length about 1.5 × cl. Pereopods 2–4 dactyli length equal, shorter than pereopod 5 dactylus; proximal three-fourths or more of flexor margin lined with corneous spinules. Pereopod 5 dactylus longer than propodus.

Description of female. Carapace subcircular, slightly wider than long, surface with short, sparsely scattered setae anteriorly and laterally; strongly vaulted longitudinally, rounded in lateral view; lateral surface flattened; front weakly produced, shallowly concave; anterolateral margins defined; dorsal surface smooth, regions weakly indicated, near absent. Epistome buccal margin (posterior margin of epistome) between antennal article 1 straight, without triangular median projection. Antennular sinus larger than orbit; antennules folded slightly obliquely. Antenna short, free antennal articles not extending dorsally beyond eye; antennal articles 1 and 2 fused to epistome. Eyes filling orbit, cornea pigmented.

Maxilliped 3 ischiomerus length about 2.4 × width; surface finely and sparsely tomentose, marginal setae longer, irregular; inner margin sinuous, proximal two-thirds weakly concave, almost straight, distomesial margin bluntly rounded, produced distally slightly beyond palp articulation; outer margin strongly convex. Maxilliped 3 palp composed of carpus and propodus. Carpus longer than half propodus length. Propodus spatulate, length about twice width, subquadrate, apex bluntly rounded. Exopod margins convex; flagellum unsegmented.

Cheliped (pereopod 1) surface finely tomentose. Dactylus and pollex curved, apices crossing distally, without gape, irregular, setose distally. Dactylus longer than dorsal margin of propodus palm; with low, irregular occlusal teeth along proximal quarter and triangular tooth at proximal one-quarter, remaining margin straight, densely setose, distally with small corneous denticles proximal to apical tooth. Pollex occlusal margin weakly crenulate, with large, irregularly triangular proximal tooth, small tooth at distal two-thirds, margins between teeth densely setose, distally with small corneous denticles proximal to apical tooth; inner ventral margin setose. Propodus palm dorsal margin length about as long as high; outer surface with irregular smooth, setose; ventral margin distinctly sinuous, concave at base of pollex. Carpus mesial margin with setal tuft, unarmed; merus unarmed, stout, shorter than propodus.

Walking legs (pereopods 2–5) similar, slender, longest leg (pereopod 3) about 1.5 × cl; relative lengths: pereopod 3> pereopod 2> pereopod 4> pereopod 5; surfaces finely and sparsely setose. Pereopods 2–5 with row of short setae along extensor margins of propodus to merus and short distal row of longer setae on distal flexor meral margin; pereopods 3 and 4 propodus with row of long natatory setae near extensor margin, extending onto dorsal surface of carpus; each article with row of natatory setae on flexor margin. Meri unarmed, length 3.8–4.6 × height (pereopods 2–4), 3.7–3.9 × height (pereopod 5). Propodi unarmed, length 3.8–4.9 × height (pereopods 2–4), 2.8–3.3 × height (pereopod 5). Dactyli of pereopods 2–4 similar, slender, length equal; evenly curved, falcate, evenly tapering, apices spiniform, corneous; surfaces and extensor margin sparsely setose; proximal three-fourths or more of flexor margin lined with corneous spinules; 0.6–0.7 × propodus length. Pereopod 5 dactylus slender, gently curved, 1.2–1.3 × propodus length; flexor margin straight, lined with short corneous spinules, largely concealed by general setation; apex spiniform, corneous; setation similar to that of pereopods 2–4.

Thoracic sternum anterior margin concave medially; sternites 1–3 indistinguishably fused.

Pleon of 6 free somites and telson, extending to buccal region, covering coxa and basis of walking legs. Telson width 3.4 × length, posterior margin weakly concave medially, recessed into pleonite 6.

Egg diameter 0.37–0.48 mm (in preservative).

Host and habitat. Tesch’s (1918) Kai Islands record of O. cynthiae (attributed here to O. caramica sp. nov.) was hosted by Polycarpa aurata (Quoy & Gaimard, 1834) (under the name Styela pneumonodes Sluiter, 1895 ( Styelidae ). The hosts of the Singaporean specimens were not recorded but the collectors were able to observe encrusting invertebrates, including solitary ascidians, on the ARMS units. Unfortunately the ascidians could not be identified further as they were not collected and preserved (Z. Jaafar, pers. comm.). Nonetheless, in Singapore (viz. Millar 1975; Lee et al. 2013), although P. aurata has thus far not been recorded, the solitary ascidian genus Polycarpa Heller, 1877 is represented by other species— P. aurita ( Sluiter, 1890) , P. argentata ( Sluiter, 1890) , P. captiosa ( Sluiter, 1885) , P. decipiens Herdman, 1906 , P. cf. olitoria ( Sluiter, 1890) , P. papillata ( Sluiter, 1885) , and P. procera ( Sluiter, 1885) . These ascidians could potentially serve as hosts for O. caramica sp. nov.

Kusu Island and Big Sister Island , where the holotype and paratype of O. caramica sp. nov., respectively, were collected, are small islands surrounded by fringing coral reefs located south of mainland Singapore at the edge of the Singapore Strait. The latter island is part of a marine protected area, the Sisters’ Islands Marine Park.

Etymology. The new species is named caramica , an arbitrary combination of “amica carissima” (L., dear friend), in honour of our late dear friend and colleague, Ng Ngan Kee. Ngan Kee’s kindness and generosity was evident to all who knew her, especially other staff and colleagues whom she took under her wing; it is fitting that one of the localities from which type material was collected is Big Sister Island. Used as a noun in apposition.

Remarks. Ostracotheres caramica sp. nov., the third member of the genus, most closely resembles O. cynthiae in sharing the comparatively slender dactyli of pereopods 2–4, and the elongated, almost straight pereopod 5 dactylus that is as long as or longer than the propodus as well as the pereopods 2–4 dactyli. In these features, O. caramica and O. cynthiae differ from O. tridacnae , which has comparatively stout, subequal pereopod 2–5 dactyli that are all shorter than their corresponding propodi. As in O. cynthiae , the carapace of O. caramica is also only sparsely setose in contrast to that of O. tridacnae , which is evenly covered with a fine, short tomentum. The new species differs from O. cynthiae in: 1) the median buccal margin of the epistome (straight in O. caramica [ Figs. 1C , 3C ]; with triangular median projection in O. cynthiae [ Fig. 3L, N ]); 2) the ornamentation of the flexor margin of pereopod 2–4 dactyli (lined with corneous spinules for at least the proximal three-fourths in O. caramica ( Figs. 2G–I, N–P ), but not more than two-thirds in O. cynthiae ; Fig. 3D–F ); and 3) the proportionally longer walking legs (pereopod 3 about 1.5 × cl in O. caramica , 1.1 × cl in O. cynthiae ). Only females of O. caramica are currently known, but the first two distinguishing features are consistent between sexes in other species of Ostracotheres and the related genus Austrotheres Ahyong, 2018 ( Ahyong 2018) and can therefore be inferred as diagnostic for both sexes. The walking legs in pinnotherids, however, are typically sexually dimorphic, being proportionally longer in males, which, in male O. cynthiae , are reportedly about 1.5 cl ( Laurie 1915); male walking leg distinctions between O. cynthiae and O. caramica must await discovery of males of the latter.

The two type specimens of O. caramica , despite the major difference in body size (holotype cl 6.1 mm, paratype cl 2.9 mm) are both mature and agree well in most respects. The extent of flexor marginal spination on the pereopod 2–4 dactyli in O. caramica , however, slightly varies allometrically, with more spines overall and with the distalmost spine being placed slightly more distad in the holotype than the paratype and Indonesian specimen ( Fig. 2G–I vs 2N–P, 3D–F). Such variation is paralleled in O. cynthiae , although the distalmost flexor spine of pereopods 2–4 is placed no further than the proximal two-thirds of the flexor margin ( Fig. 3H–J, M ) (vs three-fourths or more in O. caramica ). Note that the pereopods 2–4 dactylar spine row of O. caramica extends to three-fourths of the length of the flexor margin even in the smallest specimen examined (cl 2.9 mm; Fig. 2N–P ), compared with two-thirds in the largest known specimen of O. cynthiae (cl 7.8 mm; Ahyong 2018: fig. 4E–G). Therefore, despite allometric changes in the development of the pereopods 2–4 dactylar flexor spine row in O. caramica and O. cynthiae , the character remains diagnostic across the known size range of the two species. The very recent discovery of O. caramica in Singapore, despite historical and expanding knowledge of the ascidian fauna (e.g., Millar 1975; Lee et al. 2013), raises the question of why the new species has not been detected until now, and accordingly, the possibility that the new species could be a recent introduction. Ascidians are common in vessel biofouling and the port of Singapore is a high volume international maritime port, so O. caramica could plausibly have arrived in transported ascidians (and this would not be the first decapod to have been first discovered in biofouling on artificial habitats, e.g., Yeo et al. 2009, 2011; Ng & Ahyong 2013). On the other hand, O. caramica could simply be a rare native species whose specific hosts have been insufficiently investigated for symbionts. For example, a close congener from the Red Sea, O. cynthiae , remains known from a total of only eight specimens collected on six occasions since 1855 ( Ahyong 2018), despite occurring in a region with a long history of active marine research ( Vine 1986), In such a context, the recent discovery of O. caramica in Singapore is not inconsistent with natural occurrence there. Also, many pinnotherids appear to be naturally rare, with several species in Southeast Asian waters known only from very few individuals or from single isolated collecting events ( Ahyong & Ng 2007, 2020; Ng & Ahyong 2022). Thus, although the possibility that O. caramica has exotic origins cannot be excluded, we currently consider it to be a rarely observed, native species.

Tesch (1918) reported an ovigerous female from the Kai Islands, Indonesia, as O. cynthiae , providing a description but no figure. Ahyong (2018) had previously excluded this record from O. cynthiae sensu stricto on account of Tesch’s description of it having a dense dorsal tomentum and strongly hooked pereopods 2–4 dactyli, in contrast to the relatively more straightened pereopod 5 dactylus. The differing oceanography between the Singapore Strait and the Kai archipelago, including lower salinity in the former, differentially influences the development of coral reefs, affecting faunal composition ( Tomascik et al. 1997). Thus, the Kai Islands specimen is of particular interest for comparison with the Singaporean material. Re-examination of the Kai Islands specimen showed that the possible curvature differences in P2–5 dactyli drawn from Tesch’s (1918) account do not hold, being similar to that in O. cynthiae and the Singaporean type specimens of O. caramica . The Kai Islands specimen, nevertheless, agrees in all respects with the Singaporean specimens, including the degree of spination of the pereopod 2–5 dactyli ( Fig. 3D–G ), length of pereopod 3 (1.5× cl) and in the straight buccal margin of the epistome ( Fig. 3C )—it is referable to O. caramica , suggesting that the species’ distribution spans much of southern Southeast Asia.

Distribution. Singapore and Kai Islands, Indonesia; 6.5– 22 m.

Figures

FIGURE 1. Ostracotheres caramica sp. nov.: ovigerous female holotype, cl 6.1 mm, cw 6.6 mm, Singapore, ZRC 2022.0969. A, habitus, dorsal view; B, habitus, anterior view.

FIGURE 2. Ostracotheres caramica sp. nov.: A–K, ovigerous female holotype, cl 6.1 mm, cw 6.6 mm, Singapore, ZRC 2022.0969; L–Q, spent female paratype, cl 2.9 mm, cw 3.0 mm, Singapore, ZRC 2022.0968. A, dorsal habitus; B, carapace, right lateral view; C, cephalothorax, anterior view; D, anterior thoracic sternum, ventral view; E, M, telson and pleonite 6, external view; F, right chela, external view; G–J, right pereopods 2–5; K, right maxilliped 3, external view; L, front, dorsal view; N–P, left pereopod 2–4 dactylus (plumose setae omitted); Q, left pereopod 5 dactylus and propodus (plumose setae omitted). Scale: A, B, E = 2.0 mm; C–J, L = 1.0 mm; K, N–Q = 0.5 mm.

FIGURE 3. A–G, Ostracotheres caramica sp. nov., Kai Islands, Indonesia, NBC ZMA.CRUS.D.242068, ovigerous female, cl 3.8 mm, cw 4.2 mm; H–L, Ostracotheres cynthiae Nobili, 1906, Gulf of Suez, ZRC 2017.0227, spent female, cl 5.6 mm, cw 5.7 mm; M–N, Ostracotheres cynthiae Nobili, 1906, Gulf of Suez, ZRC 2017.0227, spent female, cl 4.0 mm, cw 4.1 mm. A, right maxilliped 3, external view; B, telson; C, L, N, epistome, median buccal margin; D–G, outline of right pereopod 2–5 dactylus and propodus (setae omitted); H–K, outline of right pereopod 2–5 dactylus (setae omitted); M, outline of left pereopod 2 dactylus (setae omitted). Scale: A = 0.5 mm; B = 2.0 mm; C–N = 1.0 mm.