Galathea ryuguu, Osawa, Masayuki, 2015

Osawa, Masayuki, 2015, A new strikingly-colored species of the genus Galathea Fabricius, 1793 (Crustacea: Decapoda: Anomura: Galatheidae) from the Ryukyu Islands, Japan, Zootaxa 4039 (4), pp. 591-598 : 592-596

Galathea ryuguu n. sp.

( Figs. 1–3 )

Type material. Holotype: ovigerous female (pcl 2.8 mm), RUMF-ZC 3910, Apogama, Onna, Okinawa Island, Ryukyu Islands, 35 m, on colony of Muricella sp., 17 May 2010, SCUBA, coll. N. Shirakawa & Y. Yamada. Paratype: 1 male (pcl 2.1 mm), RUMF-ZC 3911, same data as holotype.

Diagnosis. Carapace with short, interrupted ridges and no spines on surface; cardiac region with long, transverse ridge; lateral margin with small spine between anterolateral spine and anteriormost spine of branchial margin; frontal margin unarmed. Rostrum triangular, 1.3 times as long as broad, with 3 or 4 moderately small teeth on lateral margin. Pterygostomian flap unarmed. Ocular peduncle 1.8 times longer than broad in dorsal view. Abdominal somites 2–4 each with 2 blunt, transverse ridges on tergite. Antennular article 1 with 3 distal spines; distomesial spine much smaller than other 2 spines. Mxp3 merus with 2 or 3 spines of subqual size on flexor margin, disto-extensor spine present. P1 1.6–1.8 times longer than entire carapace including rostrum; carpus 0.8 length of palm; dactylus 0.8–0.9 times length of palm. P2–4 meri each with a single spine at lateral distoflexor angle, lateral surfaces with obsolete transverse ridges; dactyli each with 3 or 4 teeth on flexor margin. Epipods absent from P1–3.

Description. Carapace ( Fig. 1 A, B) with ridges bearing row of sparse plumose setae, some setae much longer and stouter; postorbital carapace longer than broad; dorsal surface unarmed. Cervical groove moderately distinct, laterally bifurcated. Gastric region with short, interrupted ridges; epigastric ridge not delimited. Anterior branchial region with few short or small ridges. Mid-transverse ridge interrupted, preceded by shallow cervical groove, followed by 6 ridges including posterior submarginal ridge when counted along lateral margin; anterior second ridge only uninterrupted medially, ridges on intestinal and posterior cardiac regions short. Lateral margins slightly convex, with 6 (paratype) or 7 (holotype) spines (2 in front of and 4 or 5 behind anterior cervical groove); first spine anterolateral, second spine small, additional small spine ventral to between first and anterior branch of cervical groove; 2 spines on anterior branchial margin and 2 or 3 spines on posterior branchial margin. Frontal margin unarmed. External orbital angle rounded, infraorbital margin with 1 minute spine ( Fig. 1 C). Rostrum ( Fig. 1 A, B) broad, triangular, 1.3 times as long as broad, 0.3 times entire carapace length and 0.4 times breadth of carapace, nearly horizontal in lateral view; distance between distalmost lateral incisions 0.3 times distance between proximalmost lateral incisions; dorsal surface with scattered, very short setae; lateral margin with 3 or 4 moderately small teeth.

Pterygostomian flap with long, longitudinal ridges, unarmed; anterior margin narrowly rounded ( Fig. 1 E).

Thoracic sternum slightly longer than broad, lateral extremities gently divergent posteriorly. Sternite 3 ( Fig. 1 D) oblong in general outline, slightly narrower than anterior margin of sternite 4; anterior margin with median Vshaped cleft; surface smooth. Sternite 4 ( Fig. 1 D) with anterior margin slightly concave; surface with 2 granular, transverse ridges, anterior ridge interrupted medially. Sternites 5–7 nearly smooth.

Abdomen ( Fig. 1 A) with row of sparse, short and moderately long plumose setae on each transverse ridge. Somite 1 with short, slightly elevated ridge on each posterolateral side. Somite 2–4 each with 2 blunt, transverse ridges on tergite. Somite 5 with 2 transverse rows of short striae. Somite 6 with sparse, short striae; posteromedian margin straight. Telson incompletely subdivided into 7 platelets by shallow sutures, weakly calcified posteriorly; surface with short ridges on anterior half; lateral margins convex proximally, without spines; posterior margin with distinct median notch. Male with pair of gonopods each on somites 1 and 2.

Ocular peduncle ( Fig. 1 A, E) 1.8 times longer than broad in dorsal view; cornea not dilated, maximum diameter 0.5 times rostrum width; eyelashes composed of sparse short, simple setae and 2 tufts of moderately long, plumose setae.

Antennular peduncle with article 1 ( Fig. 1 E) armed with 3 distal spines, distodorsal spine largest, distomesial spine much smaller than other 2 spines; lateral margin with 2 minute spines near base of distodorsal spine. Ultimate article with sparse, short setae on dorsal surface; no distinct tuft of setae on dorsodistal margin.

Antennal peduncle ( Fig. 1 A, E, F) short. Article 1 armed with small distomesial spine barely reaching distal margin of article 2. Article 2 with 2 small distal spines, distolateral spine longer than distomesial spine and reaching midlength of article 3. Article 3 with small distomesial spine. Article 4 unarmed.

Mxp3 ( Fig. 1 G) relatively slender. Ischium with small distoflexor and disto-extensor spines; crista dentata with about 20 denticles. Merus shorter than ischium in maximum length; flexor margin with 2 or 3 spines of subqual size, proximal spine slightly larger and located at midlength, distal spine at terminal end; extensor margin with distal spine and short transverse ridge medially. Carpus with 3 slightly elevated ridges on extensor margin. Propodus convex on medioflexor margin. Dactylus moderately elongate. Exopod overreaching distal margin of merus.

P1 ( Fig. 2 A) 1.6–1.8 times longer than entire carapace including rostrum, with short to moderately long, noniridescent, simple and plumose setae arising from short ridges and bases of spines. Merus 0.6–0.7 times carapace length, 2.0–2.1 times as long as carpus, with spines arranged roughly in longitudinal rows, mesial and distal spines prominent. Carpus 0.8 length of palm, 2.0–2.4 times as long as broad; dorsal surface with 2 longitudinal rows of irregularly-arranged, small spines; mesial surface with 1 small median spine ventrally; lateral surface unarmed except for small ventrodistal spine. Palm 2.2–3.0 times longer than broad, with lateral and mesial margins subparallel; dorsal surface with 2 rows of small spines along mesial margin and laterally to midline; lateral surface with 1 small subdistal spine and with or without 1 ventromedian spine. Fingers distally spooned, each with 3 blunt terminal teeth; opposable margins nearly straight, with row of low, blunt or subacute teeth; ventral margins each with row of small, obsolete teeth ( Fig. 2 B, C). Dactylus 0.8–0.9 times length of palm, without spines.

P2–4 ( Fig. 2 D, F, G) moderately stout, with sparse, short to moderately long, simple and plumose setae, decreasing in length posteriorly; P2 0.9–1.0 times length of entire carapace including rostrum. Meri successively shorter posteriorly (P3 merus 0.9 times length of P2 merus, P4 merus 0.9 times length of P3 merus); P2 merus 0.4 times carapace length, 3.3–3.6 times as long as broad, 1.2–1.3 times longer than P2 propodus; P3 merus 2.9–3.1 times longer than broad, 1.1times longer than P3 propodus; P4 merus 2.7–2.8 times as long as broad, 0.9–1.0 times length of P4 propodus; extensor margins each with row of 5 or 6 spines, distalmost spine prominent, spines weaker on P2 than on P3 and P4; lateral surfaces with obsolete transverse ridges, distoflexor margins each terminating in strong spine, followed proximally by some slightly elevated ridges, on P2, additional 1 or 2 smaller spines present. Carpi each with 4 spines on extensor margin, proximally diminishing in size; extensor distal margin with minute spine; lateral surfaces each with 1 small spine and some short ridges sub-parallel to extensor margin; flexor distal margins each with few granules. Propodi 4.0–4.9 (P2 and P3) and 5.0–5.3 (P4) times as long as broad; lateral surfaces nearly smooth; extensor margins each with 2–4 small spines on proximal half; flexor margins each with 3 or 4 corneous spines; flexor distal margins each with another smaller spine on mesial side. Dactyli 0.5–0.6 times length of propodi, each terminating in strong claw; flexor margins each with 3 or 4 proximally diminishing teeth, each tooth bearing slender corneous spine ( Fig. 2 E).

Epipods absent from P1–3.

Variation. The rostrum has three or four lateral teeth in each type specimen. The posterior branchial margin of the carapace is armed with three spines in the holotype, whereas there are only two spines in the paratype.

Coloration in life. ( Fig. 3 ) Carapace and abdomen light blue, with irregular streaks of dull yellow and pale red. Corneas pale yellow, with red small spots. P1–5 generally translucent, with irregular blue and red blotches.

Distribution. At present, known only from Okinawa Island, Ryukyu Islands; 35 m.

Habitat. The type specimens were collected from a colony of generally pink and gray-colored sea fan of the genus Muricella Verrill, 1868 (Anthozoa: Alcyonacea : Acanthogorgiidae ) at the depth of 35 m ( Fig. 3 ). Because of its coloration and the presence of a pygmy seahorse, Hippocampus bargibanti Whitley, 1970 , on the colony, the sea fan is probably similar to Muricella sp. 3 reported by Reijnen et al. (2011: fig. 2a). The seahorse species was observed in the host sea fan from which Galathea ryuguu n. sp. was obtained.

Etymology. The specific name is derived from the Japanese “ Ryuguu ” (Sea God’s Palace), in reference to the vivid colors of the new species and its host sea fan presenting an image of secret beauty in the sea. The name is used as a noun in apposition.

Remarks. Galathea ryuguu n. sp. is most closely allied to G. squamea Baba, 1979 , known only from New Caledonia. These species are characterized by the anterior half of the carapace only with interrupted, short ridges; the rostrum with four lateral teeth at least on one side; the ocular peduncles short, clearly less than twice longer than broad; and the P1 without an epipod. However, the new species is distinguished from G. squamea by the following characters: 1) the epigastric ridge on the carapace is not distinctly delimited as in G. squamea ; 2) the frontal margin of the carapace does not have a small spine just outside of the external orbital angle, while this spine is present in G. squamea ; 3) the antennular basal article possesses a much smaller distomesial spine than in G. squamea ; 4) the Mxp3 merus is armed with much smaller spines on the flexor margin than in G. squamea , and with a distinct disto-extensor spine, which is absent in the latter species; 5) the P2–4 meri have one instead of two spines at each lateral distoflexor angle; and 6) the P2–4 dactyli possess three or four teeth instead of about six teeth on the flexor margin. The absence of epigastric spines on the carapace may also distinguish the new species from G. squamea , although Macpherson & Robainas-Barcia (2015) noted some of their examined specimens of the latter species have very minute or no epigastric spines.

Galathea ryuguu n. sp. also somewhat resembles G. bimaculata Miyake & Baba, 1966 and G. f o r m o s a De Man, 1902 in the comparatively broad rostrum, general arrangement of transverse ridges on the gastric region, and absence of epigastric spines on the carapace. Nevertheless, the new species is clearly different from the latter two species in the rostrum having smaller lateral teeth, no distinct epigastric ridge, the P1 fixed finger without lateral marginal spines, and the epipod being absent from P1, as well as the coloration (see De Man 1902; Miyake & Baba 1966; Baba 1977; Macpherson & Robainas-Barcia 2015).

The new Japanese name for G. ryuguu n. sp. is “Ryuguu-koshioriebi”.

Figures

FIGURE 1. Galathea ryuguu n. sp., holotype, ovigerous female (pcl 2.8 mm), RUMF-ZC 3910 (A, C – G); paratype, male (pcl 2.1 mm), RUMF-ZC 3911 (B). A, carapace, eyes, and antennal peduncles, dorsal view (setae omitted from left side); B, carapace, dorsal view (setae omitted); C, left anterolateral part of carapace, dorsal view; D, thoracic sternites 3 and 4, ventral view (setae omitted); E, left eye, antennular article 1, antennal peduncle, and anterior part of pterygostomian flap, ventral view (setae omitted); F, left anterolateral part of carapace and antennal articles 1 and 2, ventrolateral view (setae omitted); G, left Mxp 3, lateral view (setae omitted). Scales = 1.0 mm.

FIGURE 2. Galathea ryuguu n. sp., holotype, ovigerous female (pcl 2.8 mm), RUMF-ZC 3910. A, right P 1, dorsal view (setae omitted); B, same, fingers, dorsal view; C, same, tips of fingers, ventral view; D, right P 2, lateral view; E, same, dactylus, lateral view (setae omitted); F, right P 3, lateral view lateral view (setae omitted); G, right P 4, lateral view (setae omitted). Scales = 1.0 mm.

FIGURE 3. Galathea ryuguu n. sp., holotype, ovigerous female (pcl 2.8 mm), RUMF-ZC 3910. Living animal on colony of unidentified sea fan of Muricella Verrill, 1868, in situ (photo: Y. Yamada).