Dorometra sesokonis, Obuchi, Masami, Kogo, Ichizo & Fujita, Yoshihisa, 2009

Obuchi, Masami, Kogo, Ichizo & Fujita, Yoshihisa, 2009, A new brooding feather star of the genus Dorometra (Echinodermata: Crinoidea: Comatulida: Antedonidae) from the Ryukyu Islands, southwestern Japan, Zootaxa 2008, pp. 61-68 : 62-67

Dorometra sesokonis n. sp.

[New Japanese name: Sesoko-hime-umishida] ( Figures 1 and 2)

Types. Holotype: OMNH Iv 3445, off Sesoko Beach, Sesoko-jima Island (northwest of Okinawa-jima Island), the Ryukyu Islands, Japan, 12.9 m depth, under dead coral rubble, 6 June 2008, SCUBA, coll. M. Obuchi.

Paratypes: OMNH Iv 3428 (1 specimen), off Maeda-misaki, Okinawa-jima Island, the Ryukyu Islands, Japan, 21.7 m, under dead coral rubble, 29 June 2003, SCUBA, coll. Y. Fujita; OMNH Iv 3430-1 (1 specimen) and Iv 3430-4 (1 specimen), off Magaigua, Sesoko-jima Island, the Ryukyu Islands, Japan, 15–25 m, under dead coral rubble, 11 December 2003, SCUBA, coll. Y. Fujita; OMNH Iv 3444-1 (1 specimen), off Sesoko Beach, Sesoko-jima Island, the Ryukyu Islands, Japan, approx. 20 m, under dead coral rubble, 5 June 2007, SCUBA, coll. M. Obuchi; RUMF-ZE-00002 (1 specimen) and ZE-00011 (1 specimen), off Magaigua, Sesoko-jima Island, the Ryukyu Islands, Japan, 12 m, under dead coral rubble, 19 February 2008, SCUBA, coll. M. Obuchi.

Diagnosis. An extremely small species with arm length not exceeding 30 mm, rarely over 20 mm. Centrodorsal low hemispherical, 1.3 mm in diameter. Cirri slender, up to 30 in number, 10 segments, 3.5 mm long; elongate cirrals approximately 3.5 times longer than median width; distal end flared, overlapping base of following segment; dorsal spine absent; opposing spine and terminal claw prominent. Brachials entirely smooth. Pinnules relatively small and slender. Proximal pinnulation incomplete; P1, P3 and P c always present; but P2 and P4 sometimes rudimentary or absent; P a, P b and P d more frequently absent; pinnulation not uniform on each ray of a specimen. P3 longest proximal pinnule, up to 1.7 mm in length, 8 segments, slightly longer than P1, about as long as or occasionally shorter than middle and distal pinnules. P2 and P4 (if present) much shorter than P3. Pattern of comparative length of pinnules as follows: P1>P2<<P3>P4<P5. P3 to up to P13 bearing gonads, and spawned eggs frequently attached to such pinnules. Hermaphroditic.

Description of holotype. OMNH Iv 3445. Centrodorsal low hemispherical, 1.2 mm in diameter, 0.3 mm high. Polar area without papillae, 0.4 mm across.

Cirri compactly arranged in 2 marginal rows, 30 in number, each composed of 8 or 9 segments, up to 3.2 mm long; proximal cirral cylindrical; succeeding cirrals gradually elongated; longer medial cirrals, constricted medially, flared distally, overlapping basal end of following segments, resulting in very knobby appearance; distal cirrals decreasing in length. First and second cirrals approximately 0.3 as long as broad; third approximately 2.0 as long as broad; fourth to sixth longest, approximately 3.0 or 4.0 times longer than median width; dorsal spine wholly absent; penultimate cirral approximately as long as broad with a prominent opposing spine. The terminal claw curved, sharply pointed distally, nearly as long as penultimate cirral.

Radials visible only in interradial angles or completely hidden by centrodorsal. IBr series connected by synarthry; Ibr1 with very short distal margin, concave distally with aboral rim of Ibr2; 0.2 mm long, 0.4 mm wide. Ibr2 (axillary) bearing large aboral rim proximally, rhombic or hexagonal with short lateral margins, 0.3 mm long, 0.6 mm wide.

Arms very slender, fragile, 10 in number, 15.7 (shortest regenerating arm)– 21.7 mm long, 0.3 mm wide at first syzygy. Proximal brachials comparatively short; br1 oblong, concave distally with aboral rim of br2, much shorter than broad; br2 pentagonal with aboral protuberance proximally, as long as broad; br3 and br4 articulated by syzygy, approximately as long as broad; succeeding brachials gradually more elongated, nearly 2.0 times as long as middle width; middle brachials forming a weak zigzag line in aboral view because of oblique articulations with prominent ligaments, each 2.0 or 3.0 times longer than median width; distal brachials excessively slender, rod-like, 4.0 or 5.0 times longer than median width, constricted medially with flared distal ends. All brachials uniformly smooth.

Syzygial pairs at br3+4, br9+10, br14 +15, br18+ 19, and at intervals of 3 (occasionally 2 or 4) muscular articulations. Longest arm 21.7 mm long, bearing 17 pinnules with rudimentary P4 on outer side of paired arms, and 16 pinnules (P a lacking) on inner side.

Pinnules relatively small and slender. Complements of proximal pinnules, P1 to P d, incomplete, not uniform on all rays; outer pinnules P1, P2 and P3 present on all 10 arms; P4 present on 9 arms but rudimentary on remaining arm. Pinnulars flared distally with minute distal spine. P1 most slender, bearing on br2, of 7 or 8 segments, 1.2–1.5 mm long, no gonads; first pinnular shorter than wide; second about as long as wide; succeeding 4 or 5 pinnulars each much more elongated, up to 5.0 times longer than median width; distal 1 or 2 pinnulars decreasing in length; aboral surface of second to third, or occasionally to proximal part of fourth pinnular, covered with fine spines. P2 much shorter than P1, of 5 or 6 segments, 0.6 mm long, no gonads. P3 longest proximal pinnule, of 8 segments, 1.3–1.6 mm long; one gonad on second to fifth segments; 1 or 2 spawned eggs attached on the surface. P4 shorter than P3, of 6 or 7 segments, 0.6–1.3 mm; 5 cases with gonads. P5 longer than preceding proximal pinnules including P3, of 8 or 9 segments, 1.4–1.7 mm, always with gonads and up to 4 (usually 1 or 2) attached eggs. P6–P11 of 10 segments, 2.1–2.4 mm, usually with gonads and 1–4 attached eggs. Distal pinnules (P12–P 18 in this specimen) extremely slender, of 8–11 segments, up to 2.7 mm, no gonads; fourth to sixth segments longest, 5.0 or 6.0 times longer than median width. Proximal pinnulation on inner side of arms more incomplete than on outer side; P c and P d developed on all 10 arms, P b present on 6 arms but budlike on remaining 4 arms, and P a present on only one arm. P a of 6 segments, 0.7 mm, no gonads. P b of 5 segments, up to 0.7 mm, no gonads. P c of 8 segments, 1.2–1.5 mm, always with gonads and 1–3 attached eggs. P d of 7 or 8 segments, up to 1.3 mm, gonads in 6 cases. P e of 8–9 segments, with mature gonads and 1 or 2 attached eggs. Total number of gonads 145.

. Dorometra sesokonis :, oral view of entire animal (RUMF-ZE-00011);, variation of proximal pinnulation (P1–P4) on outer side of the arm; proximal arm with perfect pinnulation (B, OMNH Iv 3430-1), without P4 (C, RUMF-ZE-00002) and without P2 and P4 (D, RUMF-ZE-00002), brachials on which pinnules are absent arrowed; E, cirrus (OMNH Iv 3445); F, embryos attached on genital pinnule (OMNH Iv 3445). Scale bars: A, 10 mm; B–F, 0.5 mm.

Disk diameter 2.0 mm. Mouth central. Anal cone sub-central, beside mouth.

Notes on paratypes. OMNH Iv 3428 (slightly larger than holotype): Morphological features almost same as in holotype. Ten arms 17.0–24.0 mm long. Centrodorsal 1.0 mm in diameter, with polar area of 0.5 mm across. Cirri compactly arranged in 1 or 2 rows, 18 in number, composed of 9 segments, up to 3.5 mm long. P2, P4, P b and P d present, but P a completely absent; P1 of 8 segments, 1.4 mm, no gonads; P2 of 5 segments, 0.5 mm, no gonads; P3 up to 8 segments, 1.7 mm, always with gonads; P4 mostly absent, 8 segments, 1.4 mm, no gonads; P5 of 9 segments, 2.0 mm, always with gonads; P6–P11 of 10 segments, 2.3 mm, with gonads; distal pinnules up to 11 segments, 2.5 mm, no gonads. P3 longest of the proximal pinnules, though smaller than P5 and other middle pinnules. Gonads over 71 in total number; no attached eggs. OMNH Iv 3430-1 (mediumsized paratypes): Ten arms, 8.5–12.5 mm in length. Centrodorsal 0.6 mm in diameter, with 18 cirri of 7–8 segments up to 2.3 mm long. Proximal pinnulation incomplete; P2, P4, P a, P b and P d absent; P1 of 7 segments, 0.9 mm long, no gonads; P3 of 7 segments, 1.1 mm, with gonads in 4 cases; P5 of 8 segments, 1.1 mm, usually with gonads; P6–P8 of 7 or 8 segments, up to 1.6 mm, with gonad in 5 arms of 10 arms. Total number of gonads greater than 31; no attached eggs. OMNH Iv 3430-4 (juvenile): Ten very short arms, 4.5 mm long, of 22 brachials. Centrodorsal 0.3 mm in diameter. Cirri 10 in number, 6 or 7 segments, up to 1.5 mm long; cirrals without dorsal spine; distal end of second to fourth segments flared. Proximal pinnulation incomplete; P1 of 4 or 5 short segments, 0.3 mm long, bud-like; other proximal pinnules (including P3 and P c) completely absent; P5 of 6 segments, 0.7 mm; P6 of 6 or 7 segments, 0.9 mm; P7 and P8 (distal pinnules of this specimen) of 7 segments, up to 1.0 mm. No gonads in any pinnules.

Coloration in life. Ground body color generally pale and whitish brown, with brown blotches on surface of arms and disk. Cirrals pale brown with bright bands on both ends; elongated segments medially colorless and transparent. Arms sometimes with 2 pale brown, longitudinal aboral lines; elongated brachials translucent. Pinnules generally translucent with brown blotches; genital pinnules with reddish or pale pink ovaries within proximal part. Tips of somatic parts also translucent.

Specimens fixed in 70% ethanol retain almost the same coloration as in living specimens, except for the eggs in the ovaries, which lose their reddish color.

Habitat. The new species was found at depths of 10–25m, on reef slopes characterized by dead coral rubble, with dense populations on the undersides of the rubble. On the coasts of Sesoko-jima Island, population density reached more than 40 individuals per 0.25 m 2. Such aggregations consisted of variously sized individuals, not only adults but also settled cystideans and pentacrinoids.

Distribution. So far known only from Sesoko-jima Islands and Maeda-misaki, Okinawa, Ryukyu Islands, Japan.

Reproductive biology. Individuals with a maximum arm length greater than 10.9 mm possessed swollen genital pinnules (P3, P5 to up to P13 and rarely P4), including mature gonads that contain either an ovary or a testis throughout the year. By contrast, animals with arm length of less than 5 mm have not reached gonogenesis. This species has both ovarian and testicular pinnules on the same animal, and therefore shows simultaneous hermaphroditism. The testes are filled with spermatozoa, and the ovaries usually contain egg cells (mostly 2–5, up to 12) at various oogenetic stages. Embryos develop on the surface of the parental genital pinnule until the doliolarian stage. Therefore, this species is considered as an external brooder. The released doliolariae swim freely, and transform into cystideans after settlement on the substratum. The reproductive biology will be described in greater detail elsewhere (Obuchi et al. in preparation).

Etymology. This new species is named for its type locality, Sesoko-jima Island.

Remarks. The new species shows incomplete proximal pinnulation: P1, P3 and P c are present on mature specimens, while P2, P4, P a, P b and P d are frequently absent or rudimentary regardless of maturity of specimens. The complements of pinnules differ among specimens or and among rays of individual specimens. P4 is more often absent than P2. Interior pinnules, P a, P b and P d, are absent more frequently than the exterior proximal pinnules. These pinnules tend to be better developed on larger specimens, although not in all cases.

Morphologically, Dorometra sesokonis n. sp. is unique in the family Antedonidae in having juvenile characters such as excessively short and slender arms, undeveloped proximal pinnulation, and elongated colorless cirrals, even when sexually mature. Such features may cause taxonomic problems, because the comparative length of proximal pinnules diagnoses some antedonid genera, especially within the subfamily Antedoninae to which Dorometra belongs.

The new species was assigned to Dorometra based on the fact that P3 is the longest proximal pinnule, and that the number of cirrals is a few, never exceeding 13. According to Clark & Clark (1967), “P3 is much the longest and stoutest pinnule on the arm” in Dorometra . However, although P3 is always the longest and stoutest of the proximal pinnules (P1 to P4) in D. sesokonis , it is only as long as or even slightly shorter than the middle and distal pinnules. This suggests that the diagnosis of genus Dorometra as currently used may not be strictly applicable to D. sesokonis . On the other hand, Clark & Clark (1967) stated P3 as almost the same length as the distal pinnules in D. mauritiana and D. nana , and shorter than the distal pinnules in D. briseis (2.9–5.0 versus 4.0– 6.5 mm). Therefore, the generic diagnosis of Dorometra may be modified as follows: P3 the longest and stoutest proximal pinnule, either longer or shorter than middle or distal pinnules.

With the addition of the new species, the genus Dorometra now contains nine species. Dorometra sesokonis n. sp. is closely allied to D. nana and D. briseis in sharing P1 longer than P2, and fragile cirri with the longest cirrals more than three times as long as broad. These two congeners also have been reported from Okinawa Island ( Kogo & Fujita, 2005), and especially D. nana has been rarely found on the coasts of Sesokojima Island. Although the new species is easily distinguished from the other two species by its shorter arms (<30 mm long in D. sesokonis ; up to 60 mm long in D. nana , and up to 45 mm long in D. briseis ), Dorometra sesokonis n. sp. is possibly identified as small-sized specimens of two congeners. For comparative purposes, we examined the specimen of D. nana (OMNH Iv 3432) collected from same locality of the new speies. Dorometra sesokonis n. sp. is distinguished from small-sized D. nana by comparative length of P3 to P1 (the ratio of P3/P1 ≤ 1.8 in D. sesokonis ;> 1.8 in the same sized D. nana ). As for D. briseis , the description of type specimen (originally described as Antedon briseis ) with the longest arm 25 mm long made by Clark (1907) and the specimen “sp. 2” with the longest arm 20 mm long made by Gislén (1922) can be referred. Dorometra sesokonis n. sp. is distinguished from D. briseis by its shorter cirrus (≤ 3.5 mm long in D. sesokonis ; 7 mm long in the same sized D. briseis ). Additionally, at the same size, arm length 20–25 mm, D. sesokonis n. sp. is distinguished by having shorter pinnules, especially P3 (≤ 2 mm long in D. sesokonis ;> 2 mm long in D. nana and D. briseis ).

Dorometra sesokonis n. sp. may appear similar to Antedon parviflora (A. H. Clark, 1912) (originally described as Compsometra parviflora ), described by Gislén (1922) from the Bonin Islands (now the Ogasawara Islands, Japan) as an extremely small species with arms 5–25 (mostly 15–25) mm long, and P2 absent in mature sized specimen, as in D. sesokonis . Although Gislén (1922) gave no detailed information about the lengths of P3 and the following pinnules of his specimen, P3 of A. parviflora is usually shorter than P1, unlike Dorometra species (see Clark & Clark, 1967). Clearly, more detailed research is needed on this new species and other known Dorometra species to further clarify the true levels of species diversity within this understudied genus.

Figures

FIGURE 1. Dorometra sesokonis n. sp., OMNH Iv 3444 - 1: A, aboral view of centrodorsal and proximal arms; B, oral view of disk and proximal arms; C, distal arm with pinnules (aboral view); D, large and small cirri (side view). Parenthesized pinnules are absent. Abbreviations represent at: anal tube, cd: centrodorsal, ci: cirrus, eg: attached egg, mo: mouth, re: ripe egg in gonad, sy: syzygy. Scale bar 1.0 mm.