Trichopagurus spinibrachium Nakajima & Fujita & Osawa 2024, n. sp.
Creators: name: Nakajima, Hiroki
name: Fujita, Yoshihisa
name: Osawa, Masayuki
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Paguridae
Trichopagurus
Trichopagurus spinibrachium

description: Trichopagurus spinibrachium n. sp.
[New Japanese name: Doukutsu-yawakuda-yadokari]
(Figs. 1–4)
Type material. Holotype: RUMF-ZC-7664, male (sl 1.4 mm), a semi-submerged marine cave on Onna Village, Okinawa Island, Ryukyu Islands, about 1 m depth, collected by H. Nakajima, T. Sato, M. Mizuyama and Y. Fujita, 2 July 2023.
Description. Eleven pairs of biserial gills, including 1 pleurobranch above pereopod 4 (thoracomere 7) and 2 arthrobranchs on each maxilliped 3 to pereopod 4.
Shield (Fig. 1A) 1.06 times longer than wide; anterior margin between rostrum and lateral projections slightly concave; anterolateral margins sloping; posterior margin rounded; dorsal surface polished, convex, with some short setae anteriorly. Rostrum triangular, slightly falls short of proximal half of ocular acicles, terminating in spinule. Lateral projections with small submarginal spine.
Ocular peduncles (including cornea) (Fig. 1A) stout, cylindrical, 0.47 length of shield; dorsal surface with tufts of stiff setae near base of cornea; lateral margin somewhat inflated medially; cornea not dilated, rounded, corneal width less than half length of peduncle. Ocular acicles subtriangular, moderately widely separated, each with small submarginal spine terminally.
Antennular peduncles (Fig. 1A) moderately long, when fully extended, overreaching distal corneal margins by half length of article 2. Article 3 2.2 times as long as high, widened distally in lateral view, with tuft of long setae distally and few short setae on dorsal surface. Article 2 without setae. Article 1 with lateral margin distally produced into small spine; statocyst lobe slightly inflated, with spinule on lateral face.
Antennal peduncles (Fig. 1A) overreaching distal corneal margins by nearly full length of article 5. Articles 4 and 5 subcylindrical, with few setae. Article 3 with spinule at mesiodistal margin. Article 2 dorsolateral angle produced, nearly reaching distal margin of article 4, terminating in simple spine, bearing few setae; dorsomesial distal angle with acute spine. Article 1 with small distal spine on lateral surface. Antennal acicle reaching midlength of article 5, overreaching anterior margin of cornea by half length, terminating in acutely or bluntly. Antennal flagellum 3.7 times length of shield; each article with few short to moderately long setae.
Mouthparts not dissected. Maxilliped 3 (Fig. 1D, E) moderately slender. Dactylus, propodus and carpus unarmed, with numerous setae on ventral margins. Merus with strong dorsodistal spine and small ventromedian spine; surfaces with spares setae. Ischium with well-developed crista dentata composed of very small corneous teeth and 2 accessory teeth; ventral surface with sparse setae. Basis with single spinule distally on mesial margin. Exopod slightly overreaching distal margin of merus.
Chelipeds strongly unequal and dissimilar. Right cheliped (Fig. 2A–E) elongate, moderately slender. Chela suboval in dorsal view, 2.45 times as long as wide (widest on proximal part of palm). Dactylus 0.53 length of palm, only slightly curved ventrally, terminating in small corneous claw; dorsal surface smooth, convex; dorsomesial margin not delimited; all surfaces with scattered short setae; occlusal margin with 2 large, triangular calcareous teeth medially. Palm slightly shorter than carpus, entirely smooth, with sparse setae (longer on ventral surface); lateral margin weakly convex; dorsal surface gently convex from side to side, dorsomesial and dorsolateral margins not delimited; ventral surface moderately convex. Fixed finger with scattered short setae on dorsal and ventral surfaces, terminating in small, blunt calcareous claw; occlusal margin with 2 distinct obtuse teeth, median tooth much larger. Carpus flattened dorsoventrally, medially widened in dorsal view, distinctly longer than merus; dorsomesial margin distinctly convex and delimited with 6 irregular-sized spines on proximal two-thirds; dorsolateral margin delimited with 3 spines on median one-third, spines increasing in size distally and larger than dorsomesial spines; ventrolateral margin with short distal spine; dorsal surface gently convex, with scattered short setae; ventral surface weakly convex in lateral and mesial views, with sparse short and long setae. Merus smooth, unarmed on dorsal surface; ventromesial margin with 4 spines, distal 3 spines subequal in size and much larger than proximalmost spine; ventrolateral margin somewhat sinuous, with 7 spines subequal in size; ventral surface concave, with sparse long setae. Ischium unarmed, dorsodistally with tuft of plumose setae. Coxa with small ventrodistal spine.
Left cheliped (Fig. 3A–D) slender, reaching half length of right chela. Chela 3.52 times longer than wide (widest on bases of fingers). Dactylus 1.20 times longer than palm, terminating in small corneous claw; dorsomesial margin rounded; surfaces nearly smooth, with scattered short setae; occlusal margin with row of small corneous teeth. Palm about half as long as carpus; dorsal surface slightly convex, dorsomesial margin not delimited, dorsolateral margin weakly delimited on proximal one-fourth; mesial and lateral surfaces smooth, only with sparse setae. Fixed finger with sparse setae on surfaces, terminating in small corneous claw; occlusal margin with row of small corneous teeth. Carpus longer than merus; dorsal surface flattish, dorsomesial margin with 5 small spines on proximal two-thirds, dorsolateral margin with 4 spines including distal spine, spines stronger than dorsomesial spines; ventromesial margin with blunt, small distal spine; surfaces with sparse setae, ventral setae much longer. Merus with 3 and 4 slender spines on ventromesial and ventrolateral margins, respectively; mesial and lateral surfaces with sparse setae distally and ventrally; ventral surface with few long setae. Ischium with small distal spine on mesial surface and plumose setae on dorsal surface. Coxa unarmed.
Ambulatory legs (Fig. 3E–H) moderately slender, sparsely setose. Dactyli 8.3 (pereopod 2) and 9.0 (pereopod 3) times longer than high, 0.72 (pereopod 2) and 0.79 (pereopod 3) length of respective propodus, in dorsal view straight, in lateral view nearly straight with slightly curved distal claw; dorsal and ventral margins each with irregular row of setae; lateral and mesial faces non-sulcate; ventral margins each with 9 (pereopod 2) and 10 (pereopod 3) slender corneous spines. Propodi with sparse, short and moderately long setae on surfaces; dorsal surfaces unarmed; ventral surface with 4 (pereopod 2) and 3 (pereopod 3) slender corneous spines, distal margin with pair of slender corneous spines. Carpi with 2 tiny proximal spines (pereopod 2) and unarmed (pereopod 3) on dorsal surfaces. Meri somewhat compressed laterally; dorsal margins with sparse setae including some bristle-like setae; ventrolateral margin with 3 small spines on median one-third (pereopod 2) and unarmed (pereopod 3). Ischia unarmed.
Pereopod 4 (Fig. 1B) semichelate. Dactylus nearly straight, with tufts of setae, distal setae much longer. Propodal rasp consisting of single row of small corneous scales. Carpus with distal tuft of long setae; lateral surface ventrally with sparse setae.
Pereopod 5 chelate. Male coxae (Fig. 1F, G) slightly asymmetrical; right coxa with short sexual tube directed posterolaterally (about 1.4 times longer than width of right coxa), with distal part filamentous, recurved mesially; left sexual tube very short, papilla-like.
Thoracic sternite 3 with anterior margin slightly produced medially, unarmed. Anterior lobe of thoracic sternite 6 (Fig. 1C) weakly marked, oblong, with row of sparse short setae on anterior margin. Thoracic sternite 8 (Fig. 1G) with pair of obsolete lobes, each with few short setae.
Pleon dextrally twisted. Male with 3 unpaired, unequally biramous left pleopods. Uropods markedly asymmetrical; protopods unarmed.
Telson (Fig. 1H) with distinct lateral indentations; posterior lobes somewhat asymmetrical, rounded; terminal margins oblique, each with 4 tiny spinules posteriorly; lateral indentations and posterolateral margins with tufts of setae, posterolateral setae much longer.
Female unknown.
Color in life (Fig. 4). Shield and posterior carapace generally white, with scattered pale red blotches (numerous on posterolateral parts). Ocular, antennule and antennal peduncles, chelipeds and ambulatory legs entirely orangish brown in general, paler or yellowish orange on distal parts. Corneas black, with scattered small whitish yellow spots.
Distribution. So far known only from the type locality, Okinawa Island, Ryukyu Islands, southwestern Japan.
Habitat. The holotype was obtained from a semi-submerged marine cave, at 30–35 m from the entrance and about 1 m deep. The collection environment is anchialine (slightly lower salinity) and completely darkness. The hermit crab species co-occurring with the present new species include Pagurixus longipes Osawa, Fujita & Okuno, 2006 (Paguridae) and Paguristes jalur Morgan, 1992 (Diogenidae). Additionally, a new plagusiid crab species, Caligoplagusia okinawa Fujita & Naruse, 2024, was recently described from the inner most part of the same cave (Fujita & Naruse 2024).
Etymology. The specific name is derived from a combination of the Latin, spina (= spine) and brachium (= arm), referring the possession of strong spines on the carpi and meri of both chelipeds in the new species. Used as a noun in apposition.
Remarks. Although only a single male specimen is available for study, the present new species closely matches the generic diagnosis of Trichopagurus revised by Komai & Osawa (2005). Within the genus, T. spinibrachium n. sp. may be distinguished from all known congeners by the stronger armature of the carpi and meri of both chelipeds.
Trichopagurus spinibrachium n. sp. most closely resembles two congeners, T. macrochela known from the south Japan, Yap Islands in Micronesia, and the Philippines, and T. tenuidactylus known only from the Philippines (Komai 2013). Shared characters of the three species are: chelipeds and ambulatory legs only sparsely setose; carpus of right cheliped with spines on dorsomesial margin; palm of right cheliped almost smooth on dorsal surface. However, the new species is distinguished from T. macrochela and T. tenuidactylus by the shape and armature of the ocular, antennular and antennal peduncles, chelipeds, and ambulatory legs. The cornea is narrower than the basal width of ocular peduncle in T. spinibrachium n. sp. (Fig. 1A), rather being slightly wider than or equal in width to the ocular peduncle in T. macrochela and T. tenuidactylus (Komai & Osawa 2005: fig. 1B; Komai 2013: fig. 1A). The antennular and antennal peduncles are proportionally longer in T. spinibrachium n. sp. than in T. macrochela and T. tenuidactylus. The antennular peduncle is comparatively longer and overreaches the distal corneal margins by half the length of article 2 in T. spinibrachium n. sp. (Fig. 1A), instead by 0.5–0.7 length and 0.5 length of article 3 in T. macrochela and T. tenuidactylus, respectively (Komai & Osawa 2005: fig. 1B; Komai 2013: fig. 1A). The antennal peduncle overreaches the distal corneal margins by nearly the full length of article 5 in T. spinibrachium n. sp. (Fig. 1A), instead by at most only 0.2–0.3 length of article 5 in T. macrochela and T. tenuidactylus (Komai & Osawa 2005: fig. 1B; Komai 2013: fig. 1A). The dorsolateral distal angle of the antennal peduncle article 2 nearly reaches the distal margin of article 4 in T. spinibrachium n. sp. (Fig. 1A), whereas it slightly falls short of the midlength of article 4 in T. macrochela and T. tenuidactylus (Komai & Osawa 2005: fig. 1B; Komai 2013: fig. 1A). The carpi of both chelipeds are much more weakly convex on the ventral surfaces in T. spinibrachium n. sp. (Figs. 2A, B, 3A, B) than in T. macrochela and T. tenuidactylus (Komai & Osawa 2005: fig. 3B, C, E, F; Komai 2013: fig. 2A–D). The carpus of the right cheliped is convex on the median part of the dorsomesial margin in the new species (Fig. 2D), but it is widened toward the distal in T. macrochela and T. tenuidactylus (Komai & Osawa 2005: fig. 3A; Komai 2013: fig. 1B). The dorsolateral margin of the same article has three distinct spines in T. spinibrachium n. sp. (Fig. 2A, D), whereas it possesses a row of low tubercles or protuberances in T. macrochela and T. tenuidactylus (Komai & Osawa 2005: fig. 3A, C; Komai 2013: figs. 1B, 2B). The ventromesial margin of the right cheliped merus is armed with three strong spines and one small spine in T. spinibrachium n. sp. (Fig. 2B), instead of only a blunt spine proximally and no spines in T. macrochela and T. tenuidactylus, respectively (Komai & Osawa 2005: fig. 3B; Komai 2013: fig. 2A). The carpus of the left cheliped is longer than the merus in T. spinibrachium n. sp. (Fig. 3A, B), rather than shorter in T. macrochela and T. tenuidactylus (Komai & Osawa 2005: fig. 3E, F; Komai 2013: fig. 2C, D). The dactylus of the ambulatory legs is shorter than the propodus (about 0.7–0.8 times longer than the propodus) in the new species (Fig. 3E, G), instead of longer in T. macrochela (1.1–1.3 times) and T. tenuidactylus (1.1–1.2 times) (Komai & Osawa 2005: fig. 2H, G; Komai 2013: fig. 3A, C).
In addition to the distinctions mentioned above, T. spinibrachium n. sp. differs from T. macrochela in the right cheliped palm being entirely smooth on all the surfaces, the right cheliped carpus having three distinct spines on the median one-third, and the carpus of the left cheliped with a row of distinct spines on both the dorsomesial and dorsolateral margins (Figs. 2A–C, 3A, B, D). In T. macrochela, the right palm is minutely granular at least on the mesial and lateral surfaces, the dorsolateral margin of the right cheliped carpus is weakly delimited by a row of low tubercles or protuberances, and the carpus of the left cheliped has no conspicuous spines on the dorsal surface (Komai & Osawa 2005: fig. 3A–F).
The proportionally shorter dactylus of the ambulatory legs further distinguishes the present new species from T. tenuidactylus; the dactyli are 8.3–9.0 versus 11–14 times longer than the height (Fig. 3E, G; Komai 2013: fig. 3A, C). The ventral margins of the same articles also have nine or ten corneous spines in T. spinibrachium n. sp. (Fig. 3E–H), instead of only eight spines in T. tenuidactylus (Komai 2013: fig. 3A–D).
The new species can be immediately distinguished from T. asper by the dorsal surface of the palm of the right cheliped being smooth (Fig. 2C), instead of distinctly tuberculate (Komai & Poupin 2012: fig. 6A–C), and from T. trichophthalmus by both the chelipeds and ambulatory legs being much less setose (Figs. 2, 3), instead of distinctly setose (Komai & Osawa 2005: figs. 5, 6E –G).
The fresh coloration of the entire body is generally similar in all of T. macrochela, T. trichophthalmus, and T. spinibrachium n. sp., but the pereopods are pale brown in T. macrochela, rather than orangish brown in T. trichophthalmus, and T. spinibrachium n. sp. (Arima 2014; present study).
Although the sole present specimen of T. spinibrachium n. sp. was collected from a semi-submerged marine cave, the new species may also occur on exposed shallow reef walls and nearby environments, as in three known congeners, T. asper, T. macrochela, and T. trichophthalmus. Actually, T. macrochela and T. trichophthalmus have also been collected from coral reefs and lagoons as well as submarine caves in the Philippines and the Ryukyu Islands, respectively (Komai 2013). Trichopagurus macrochela, T. trichophthalmus, and T. spinibrachium n. sp. have been recorded from Japanese waters (Komai & Osawa 2005; Arima 2014; present study), whereas T. asper is known only from Mayotte Island in the southwestern Indian Ocean (Komai & Poupin 2012). As mentioned in the Habitat section above, two hermit crabs, Paguristes jalur and Pagurixus longipes, were observed in the nearby site in the same semi-submerged marine cave where T. spinibrachium n. sp. was found. Paguristes jalur and Pagurixus longipes have been also recorded from non-cave but dark environments (cf. Osawa & Fujita 2019), suggesting that the new species may have a relatively wide habitat preference, as in the two species.
description: Published as part of Nakajima, Hiroki, Fujita, Yoshihisa & Osawa, Masayuki, 2024, A new species of the genus Trichopagurus de Saint Laurent, 1968 (Crustacea: Decapoda: Anomura: Paguridae) from a semi-submerged marine cave in Okinawa Island, southwestern Japan, pp. 121-129 in Zootaxa 5419 (1) on pages 122-128, DOI: 10.11646/zootaxa.5419.1.5, http://zenodo.org/record/10782044