Sternostylus iocasicus, Dong & Gan & Li, 2021

Dong, Dong, Gan, Zhibin & Li, Xinzheng, 2021, Descriptions of eleven new species of squat lobsters (Crustacea: Anomura) from seamounts around the Yap and Mariana Trenches with notes on DNA barcodes and phylogeny, Zoological Journal of the Linnean Society 192 : -

STERNOSTYLUS IOCASICUS SP. NOV.

( FIGS 13, 14; SUPPORTING INFORMATION, FIG. S3B, C)

Zoobank registration: urn:lsid:zoobank.org:act:4D4E9E12-9435-48CD-8F0E-CAD01208392A

Material examined: Holotype, MBM189202 , one ovig. female ( PCL 18.5 mm), stn. FX-DIVE 141, M4 seamount located on the Caroline Plate , West Pacific, 10°31.37’N, 140°10.91’E, 1246 m, coll. Faxian ROV on R / V Kexue, 24 August 2017 . Paratype: MBM286651 , one male ( PCL 12.5 mm) , stn. FX-DIVE 177, Kocebu Guyot of the Magellan seamounts, West Pacific, 17°20.88’N, 153°08.30’E, 1366 m, coll. Faxian ROV on R / V Kexue, 11 April 2018 .

Description: Carapace (excluding rostrum) 1.2 times longer than broad. Front margin concave; outer orbital angle with tiny acute spine. Lateral margins divergent posteriorly on anterior one-third part and convex at posterior branchial margin; anterolateral spine strong; branchial margin with two prominent anterior spines followed by numerous small spines. Posterior margin slightly concave and ridged, armed with a row of spines. Dorsal surface hairless, with regions well delimitated; cervical grooves distinct, mid-cervical groove on midlength of carapace. Gastric region elevated, with six prominent spines in hexagonal arrangement and another spine in center. Anterior branchial region with two or three spines on each side. Cardiac region and posterior branchial region with prominent spines arranged roughly in four longitudinal rows, and interspersed with additional small spines. Rostrum spiniform, 1.4 times longer than broad, approximately 0.3 times of remaining carapace length, distinctly upturned distally. Pterygostomial flap with distinct spine on anterior end; surface covered with numerous small spines; median part of surface with subtriangular concavity having ridged posterior and anterior edges.

Sternal plastron slightly broader than long. Sternite 3 with median anterior margin produced and separated with lateral lobes with shallow grooves, bearing pair of small spines on ventral surface. Sternite 4 broad, separated with sternite 3 with depressed constriction; anterolateral margins oblique, with two pairs of strong marginal spines; ventral surface smooth, grooved along midline, bearing two pairs of small spines, anterior pair located between (somewhat posterior to) anterior marginal spines, and posterior pair between constriction points of posterolateral margins. Sternites 4–7 each with median groove, and separated with one another by transverse ridges.

Abdominal segments sparsely setose on surfaces; tergites of segments 2–6 each with pair of oval, bumplike elevations on lateral ends; pleurae transversely hollowed on surfaces, pleurae of segments 2–5 laterally tapering, with spinose lateral ends. Segment 1 with transverse row of seven or eight spines on elevated ridge; pleura indistinct, with strong lateral spine. Segment 2 having tergite with broad, transverse ridge bearing row of four spines (tubercle-like spines on larger individual), lateral elevations each with one or two acute spines (tubercle-like spines on larger individual); pleura with one to four spines on anterolateral margin, and few spines or unarmed on posterolateral margin, surface smooth or with few tubercles. Segment 3 having tergite unarmed; pleura with one to four spines on posterolateral margin, surface smooth or with pointed tubercles. Segment 4 similar to segment 3. Segment 5 having tergite with pair of distinct spines on lateral elevations, accompanied with few small spines on dorsal surface and posterior margin; pleura having anterolateral margin somewhat rounded, armed with one to six spines on posterolateral margin, and several distinct spines on surface (tubercle-like spines on smaller individual). Segment 6 with rounded lateral margin; tergite bearing two pairs of mesial spines, lateral elevations indistinct, two or three spines present on each elevation and pleura; posterior margin with three or five spines; posterolateral margin with one or two spines.

Telson approximately 1.4 times broader than long; distal portion 2.5 times broader than long, 1.4 times proximal portion length and 1.1 times proximal portion breadth.

Eyestalk reaching distal 0.2 of rostrum; lateral and mesial margins of peduncles concave on median point; cornea hemispheric, approximately 0.4 times of remaining peduncle length.

Antennal peduncle reaching distal end of rostrum. Article 2 with strong distolateral spine. Article 4 with distoventral spine. Article 5 approximately 1.6 times of article 4 length, with distinct distoventral spine. Antennal scale rudimentary.

Third maxilliped with dense bristle on flexor margins of merus, carpus, propodus and dactylus. Coxa with strong ventral spine. Ischium shorter than merus, crista dentata with about ten denticles. Merus with straight flexor and extensor margin; extensor margin with strong distal spine. Carpus bearing distinct distal spine and small proximal tubercle on extensor margin. Propodus short and distally broaden.

P1 spinose, conspicuously elongate, approximately 8.1 times PCL, bearing sparse setae. Coxa with two strong ventral spines longitudinally arranged. Ischium approximately 0.6 times PCL, with distinct dorsodistal spine and scattered spines on surface. Merus approximately 3.0 times PCL, slightly broader than carpus and palm, roughly hexagonal in cross-section; surface with slender spines closely spaced and regularly arranged in seven rows. Carpus approximately 2.0 times of PCL, similar to merus in shape and armature. Palm slightly compressed, with seven rows of spines. Fingers approximately 0.6 times palm length, subcylindrical, distally tapering and medially gaping; occlusal margins bearing brush-like setae, with rows of tubercular teeth, and prominent tooth near base of movable finger opposed to two small tooth on fixed finger.

P2–4 slender and spinose, bearing sparse setae; P2 approximately 4.7 times PCL. Coxa with two distinct ventral spines. Ischia short, with irregular rows of spines. Meri subcylindrical, P2 merus approximately 2.0 times PCL, P3 and P4 meri 0.9 times or equal to P2 merus length; surface with five regular rows of spines usually intersperse with few small spines. Carpi subcylindrical, P2 carpus approximately half of P2 merus length, P3 and P4 carpi 0.9 times P2 carpus length; surface bearing six (P2) or five (P3 and P4) rows of spines. Propodi elongate, increasing in length from P2 to P4; P2 propodus 1.3 times PCL, P2 propodus 0.8 times P4 propodus length, P3 propodus 0.9 times of P4 propodus length; propodus/ merus length ratios on P2–4 being 0.6, 0.7 and 0.8, respectively; surface bearing five (P2) or five (P3 and P4) rows of spines excluding row on flexor margin; flexor margin with a row of 17 to 19 movable cornea spines including distal pair, spines closely set distally, proximal one or two spines fixed. Dactyli subequal in length from P2 to P4, 0.2 to 0.3 times of propodus length, terminating in strong corneous claw; ventral margin straight, with eight or nine corneous spines along entire length.

Coloration: Median part of carapace whitish; rostrum, lateral parts of carapace, abdominal segments and P1–4 generally orange red.

Habitat: Associated with gorgonian species of Primnoidae .

Distribution: Magellan Seamounts and Caroline Plate, West Pacific; depth 1246–1366 m.

Etymology: The specific name commemorates the 70 th anniversary of the Institute of Oceanology, Chinese Academy of Sciences (IOCAS) in the year 2020.

Remarks: The new species closely resembles S. defensus ( Benedict, 1902) , S. iaspis ( Baba & Haig, 1990) from the eastern Pacifc and S. niwa Schnabel, 2020 from the Kermadec Ridge and Tasmania in the following morphological characters: the gastric region with six prominent spines in hexagonal arrangement and one centre spine; the abdominal segments hairless; and the abdominal tergites without numerous small spines. The new species has a distinct subtriangular concavity on the pterygostomial flap with ridged posterior and anterior edges. However, this character is absent in S. iaspis ( Baba & Haig, 1990; Baba, pers. comm., 2019) and is not recorded in S. defensus ( Benedict, 1902; Baba & Haig, 1990; Baba & Wicksten, 2019). Schnabel (2020) did not mention this character in S. niwa , but the photograph (fig. 224B) showed that it seems has such a concavity on the pterygostomial flap. The new species further differs from S. iaspis in having distinct spines on the surface of tergite 5 (at least on the lateral elevations) instead of being totally unarmed, and more prominent spines and fewer interspersed small spines on the posterior dorsal surface of the carapace ( Baba & Haig, 1990). The new species further differs from S. defensus in having tergites 3 and 4 unarmed instead of with spines, at least on the lateral elevations, and sternite 4 with two small spines between the first lateral marginal spines instead of lacking such spines ( Benedict, 1902; Baba & Haig, 1990). The new species differs from S. niwa in having tergites 3 and 4 unarmed instead of armed with spines at least on the lateral elevations, and P2–4 dactyli with stout flexor spines instead of distinctly slender spines.

The subtriangular concavity on the pterygostomial flap links S. iocasicus to S. cavimurus ( Baba, 1977b) from the eastern Pacific. However, the new species can be distinguished from the latter by having the gastric region with prominent spines in hexagonal arrangement, whereas the gastric region of S. cavimurus has numerous spines that are not arranged in hexagonal shape.

The new species is also morphologically similar to S. investigatoris ( Alcock & Anderson, 1899) in the armature of the abdominal segments and the hexagonal arrangement of spines on the gastric region. Few studies have described the concavity on pterygostomial flap of S. investigatoris . Re-examination of the holotype of S. investigatoris [according to photos (Supporting Information, Fig. S7) provided by Dr Kandasamy Valarmathi in Zoological Survey of India (ZSI), Kolkata, India, 2019] showed that there is no distinct concavity on the pterygostomial flap. However, such concavity is present on specimens of S. investigatoris from Western Australia ( McCallum & Poore, 2013) and the Ryukyu Islands ( Osawa & Higashiji, 2019), although their anterior and posterior edges are somewhat sloping and not as ridged as those in the new species (Osawa, pers. comm., 2019; McCallum, pers. comm., 2019). These S. investigatoris specimes, including the holotype, have numerous interspersed small spines on the gastric and posterior branchial regions; in contrast, S. iocasicus has much fewer interspersed small spines, especially on the gastric region where no small spines are observed among the prominent spines. Sternostylus iocasicus can also be distinguished from S. investigatoris in having relatively glabrous instead of noticeably setose abdominal segments.

Another two Sternostylus specimens were collected from the Magellan Seamounts and the Caroline Plate, which were morphologically similar to S. iocasicus but differ in having unarmed third maxilliped carpus and unarmed antennal peduncle article 4. However, the two specimens also varied significantly from each other in the armature of the sternite 4 and abdominal pleurae 2–4. Considering the variation and uncertainty of the morphological characters, these two specimens were placed under Sternostylus sp. More specimens are needed to verify their taxonomic status in the future.

No DNA sequences of S. iaspis and S. defensus were available for comparative analysis. The two type specimens of the new species have intraspecific genetic distances of 0.9% for COI and 0.4% for 16S. The genetic distances between the new species and S. investigatoris from north-western Australia ( Tan et al., 2018) are 16.1–16.5% for COI and 4.5–4.7% for 16S. The genetic distances between the new species and S. niwa from the New Zealand are 5.4%–5.6% (Schnabel, pers. comm., 2021).The genetic distances between two specimens of Sternostylus sp. are 2% for COI and 0.4% for 16S, while the interspecific distances between Sternostylus sp. and S. iocasicus are 4.9–6.1% for COI and 1.3–1.7% for 16S, showing noteworthy genetic differentiation.

SUPERFAMILY GALATHEOIDEA SAMOUELLE, 1819