Pagurus rectidactylus Komai, Saito & Myorin, 2015, n. sp.
Creators: name: Komai, Tomoyuki
name: Saito, Yuma
name: Myorin, Eiji
Publisher: name: Zenodo
Year: 2015
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Paguridae
Pagurus
Pagurus rectidactylus

description: Pagurus rectidactylus n. sp.
[New Japanese name: Etajima-hon-yadokari] (Figs. 1–5)
Material examined. Holotype: Okadaio, Okimi, Etajima, Hiroshima Prefecture, Seto Inland Sea, shallow subtidal, about 50 cm at low tide, seaweed bed, 15 June 2014, coll. E. Myorin & Y. Saito, male (sl 5.6 mm), CBM-ZC 12623.
Paratypes: Okadaiou, Okimi, Etajima, Hiroshima Prefecture, Seto Inland Sea, intertidal, 2 February 2014, coll. Y. Saito, 3 males (sl 4.0– 4.6 mm), 1 female (sl 3.5 mm), 3 ovigerous females (sl 4.2–4.4 mm), CBM-ZC 12624; same locality, 31 March 2014, shallow subtidal, seaweed bed, 1 male (sl 5.1 mm), 1 ovigerous female (sl 4.2 mm), coll. Y. Saito, CBM-ZC 12625; same data as holotype, 1 female (sl 5.2 mm), CBM-ZC 12626. Takahama, Ohi County, Fukui Prefecture, shallow subtidal, seaweed bed, 2 May 2014, coll. K. Oribe, 1 male (sl 4.6 mm), CBM- ZC 12627.
Description. Eleven pairs of biserial phyllobranchiate gills.
Shield (Fig. 1 A) as long as wide; anterior margin between rostrum and lateral projections slightly concave; anterolateral margins sloping; posterior margin roundly truncate; dorsal surface slightly convex transversely, with several tufts of long setae arranged symmetrically; paragastric grooves obsolescent. Rostrum broadly triangular, reaching beyond level of lateral projections, with paired tufts of moderately short setae. Lateral projections obsolete, each with or without small submarginal spine. Posterior carapace (not figured) about 0.8 length of shield; carapace lateral lobe moderately narrow, well calcified; cardiac sulci subparallel, extending to midlength of posterior carapace; sulci cardiobranchiales slightly diverging posteriorly, extending to midlength of posterior carapace; median area flanked by sulci cardiobranchiales slightly calcified, with some tufts of moderately short setae arranged symmetrically; branchiostegites membranous, with scattered tufts of short to moderately long setae.
Ocular peduncles (including cornea) (Fig. 1 A) moderately slender, about 0.6 times as long as shield; inflated basally, basal width greater than corneal width; cornea not dilated, its width about 0.2 of peduncular length; dorsal surface with longitudinal row of tufts of short to moderately long setae mesially, but no setal tuft at base of cornea. Ocular acicles roundly subtriangular or subovate, separated basally by width of one acicle, each with small submarginal spine distally; dorsal surface nearly flat. Interocular lobe clearly visible, medially concave.
Antennular peduncles (Fig. 1 A), when fully extended, overreaching distal corneal margins by 0.5 length of ultimate segment. Ultimate segment about 1.5 as long as penultimate segment, slightly widened distally in lateral view; dorsal surface with few short individual setae, including 1 setae at dorsolateral distal angle. Basal segment with distolateral margin produced as short process; lateral surface unarmed; ventromesial distal angle slightly produced, blunt.
Antennal peduncles (Fig. 1 A) slightly overreaching distal corneal margins, with supernumerary segmentation. Fifth and fourth segments with few short stiff setae on lateral and mesial faces. Third segment with small spine at ventromesial distal angle obscured by tuft of moderately long setae. Second segment with dorsolateral distal angle sligytly produced, terminating in simple acute spine; dorsomesial distal angle with minute spine; mesial surface with several short to moderately short setae. First segment with minute spine on lateral face distally; ventrodistal margin unarmed lateral to small process bearing excretory pore. Antennal acicle slightly falling short of or slightly overreaching distal corneal margin, slightly arcuate, terminating in small spine partially obscured by tuft of long setae; dorsal surface and mesial margin with row of tufts of short to moderately long stiff setae. Antennal flagellum very long, exceeding 8 times of shield length; each article with few minute setae.
Mandible, maxilla, first and second maxillipeds without distinctive features. Endopod of maxillule (Fig. 1 B) with recurved external lobe; inner lobe obsolete, with single apical seta. Third maxilliped (Fig. 1 C) moderately stout; carpus unarmed on dorsodistal margin; merus unarmed on dorsodistal margin, ventromesial margin with 1 small spine at midlength; ischium with crista dentata consisting of closely spaced corneous teeth increasing in size proximally, and with 1 moderately strong accessory tooth (Fig. 1 D); basis-ischium fusion incomplete; basis with 1 spinule on mesial margin (Fig. 1 D); exopod reaching midlength of carpus, bearing well-developed flagellum.
Chelipeds distinctly unequal and dissimilar; surfaces with numerous tufts of long setae, partially obscuring armature (Fig. 5 A, B). Right cheliped (Fig. 2 A–E) somewhat elongate. Chela elongate subovate in dorsal view, about twice as long as wide; no distinct hiatus between fingers; spines on dorsal surface generally acute, subconical, directed slightly distally, vast majority non-corneous-tipped. Dactylus slightly shorter than palm, slightly curved ventrally, overlapped by fixed finger distally; dorsal surface almost unarmed, dorsomesial margin not delimited, with few very small spines or tubercles or row of very small, blunt tubercles; mesial surface with few very small spines or tubercles proximally; ventral surface unarmed; cutting edge with row of rounded or subtriangular calcareous teeth of various sizes in proximal 0.8 and with row of minute corneous teeth in distal 0.2, terminating in small corneous claw. Palm subequal in length to carpus; dorsal surface gently convex, armed with several small spines or tubercles arranged in 5 irregular longitudinal rows, lateral rows extending onto proximal part of fixed finger; dorsomesial margin not delimited, dorsolateral margin delimited by row of small spines decreasing in size distally and proximally, extending onto fixed finger; lateral, mesial and ventral surfaces without conspicuous armature. Fixed finger nearly straight; cutting edge with low, rounded calcareous teeth, terminating in small calcareous claw. Carpus slightly widened distally, subequal in length to merus; dorsal surface with several small to moderately large spines (spines of dorsomesial row larger than other spines) arranged in 4 or 5 irregular longitudinal rows; dorsolateral and dorsomesial margins not delimited; lateral surface without conspicuous armature, ventrolateral margin with few tiny spines adjacent to distal angle; mesial surface with few short transverse ridges or very low protuberances, and few tiny spines or tubercles adjacent to ventromesial distal margin; ventral surface convex, without armature. Merus with dorsal surface bearing very low transverse ridges, each ridge bearing tuft of short to long setae; dorsodistal margin with few small spines; lateral surface with few, very low, short transverse ridges or very low protuberances distally, ventrolateral margin with row of small spines; mesial surface also with very row, short transverse ridges distally, otherwise smooth, ventromesial margin with row of small spines; ventral surface with few to some tiny spines laterally. Ischium with row of minute tubercles or denticles on ventromesial margin, otherwise unarmed. Coxa unarmed.
Left cheliped (Fig. 3 A–C) moderately slender. Chela about 2.8 times as long as wide (greatest width at base of dactylus); no hiatus between fingers. Dactylus about 1.6 times as long as palm, slightly curved ventrally; all surfaces without conspicuous armature; cutting edge with comb-like row of small, slender corneous teeth on distal 0.7–0.8, terminating in small corneous claw. Palm about half length of carpus; dorsal surface gently convex transversely (midline not particularly elevated), with 2 longitudinal row of small to moderately small spines adjacent to midline, otherwise almost unarmed; dorsomesial margin not delimited; dorsolateral margin delimited by row of tiny spines extending onto midlength of fixed finger; lateral, mesial and ventral surfaces without armature, ventral surface moderately convex. Fixed finger slightly depressed dorsoventrally; cutting edge with row of minute, acute calcareous teeth interspersing minute corneous teeth, terminating in moderately small corneous claw. Carpus subequal in length to merus, slightly widened distally; dorsal surface moderately narrow, unarmed; dorsolateral and dorsomesial margins each with row of widely spaced, moderately small to large spines; lateral surface with scattered, very low protuberances, ventrolateral margin feebly denticulate; mesial surface without conspicuous armature; ventral surface moderately convex, unarmed. Merus with dorsal surface nearly smooth or with very low protuberances, dorsodistal margin unarmed; lateral surface almost smooth, ventrolateral margin with row of small spines; mesial surface also nearly smooth, ventromesial margin with row of small spines; ventral surface unarmed or few tiny spines laterally. Ischium with row of minute spines or denticles on ventromesial margin. otherwise unarmed. Coxa unarmed.
Ambulatory legs (Fig. 4 A–F) long and slender, right second pereopod reaching tip of extended right cheliped. Dactyli 0.9–1.0 times as long as propodi, 8.2–10.8 times longer than broad in right second (mean 9.47; n = 9), 8.6–12.9 times in left third (mean 10.2; n = 11), in dorsal view almost straight, in lateral view nearly straight or slightly curving ventrally; dorsal margins each with row of tufts of short to moderately long setae decreasing in length distally; lateral surfaces not sulcate medially, with row of short setae medially; mesial surfaces not sulcate medially, with double row of slender corneous spinules adjacent to dorsal margin (second; Fig. 4 B) or with double row of slender corneous spinules adjacent to dorsal margin and single row of similar corneous spinules ventral to midline (third; Fig. 4 E); ventral margins each with 14–23 (rarely 12 or 13) long corneous spines (these spines narrowly to moderately spaced and increasing in length distally). Propodi slightly narrowing distally, nearly straight or very slightly curving ventrally, 5.0–6.5 times as long as greatest width; dorsal surfaces without armature, but with row of tufts of long setae; lateral and mesial surfaces with sparse tufts of short to long setae; ventral margins each with row of 4–9 (rarely 2 or 3) corneous spines increasing in size distally, ventrodistal margins each with 2 or 3 (rarely 0 or 1) corneous spines. Carpi usually each with dorsodistal spine, but otherwise unarmed on dorsal surface; dorsal and ventral margins and lateral faces with tufts of long setae. Meri with numerous tufts of long setae on dorsal and ventral margins, but without conspicuous armature except for tiny ventrolateral distal spine in second; lateral surfaces also with tufts of short to long setae adjacent to dorsal and ventral margins. Ischia with dorsal and ventral tufts of long setae, unarmed. Female with paired gonopores on coxae of third pereopods.
Fourth pereopods (Fig. 1 E) semichelate, subequal and similar from left to right, stiff long setae on dorsal margins of dactyli to meri and ventral margin of meri. Dactyli slightly curved, terminating in small corneous claw, each with comb-like row of minute, closely spaced corneous teeth on ventral margin; no preungual process. Propodal rasp consisting of 2 or 3 rows of corneous scales.
Fifth pereopods chelate. Coxae (Fig. 1 G) of males each with gonopore accompanied with tuft of short setae.
Third thoracic sternite with anterior margin nearly straight, with spinule on either side of midpoint; ventral surface with prominent tuft of setae medially. Anterior lobe of sixth thoracic sternite (Fig. 1 F) subsemicircular, slightly skewed to left, unarmed, but with numerous long setae on anterior margin. Eighth thoracic sternite (Fig. 6 G) weakly divided in 2 symmetrical lobes, each lobe with numerous long setae on anteroventral surface.
Pleon (Fig. 5 A) dextrally twisted. Male with 3 (third to fifth) unpaired, unequally biramous left pleopods (endopods greatly reduced, bud-like). Female with 4 (second to fifth) unpaired, unequally biramous left pleopods (in fifth, endopod small, about 0.2 length of exopod). Uropods markedly asymmetrical; protopods unarmed.
Telson (Fig. 1 H) with distinct lateral indentations; posterior lobes slightly unequal or subequal, median cleft shallow; each terminal margin nearly horizontal, with row of 6 or 7 minute corneous spines including 1 spine at lateral angle (Fig. 1 I).
Variation. The number of corneous spines on the ventral margin of each ambulatory dactyli fairly varies, included in the range of 12 to 23; mean value is 17.9 (n = 40). Similarly, the number of corneous spines on the ventral margin of each propodus also varies from two to nine (except for spines on the ventrodistal margin).
Coloration in life. (Fig. 5) Shield mottled with brown; posterior carapace with minute brownish dots on translucent background. Ocular peduncle pale brown, with scattered brown dots, but without distinct markings; cornea grayish brown, with 2 transverse dark brown bands. Antennular peduncle pale brown generally, ultimate segment with 2 brown longitudinal stripes dorsally. Antennal peduncle also pale brown generally; ultimate segment with brown longitudinal stripe laterally and mesially. Chelipeds mottled with brown, with grayish markings on distal parts of carpus and merus; dactyli with 2 or 3 brown longitudinal stripes on dorsal surface; carpi each with brown longitudinal stripe on midline of mesial face. Ambulatory legs generally pale brown with brown longitudinal stripes on dactylus; dactyli each with 1 dorsal, 1 ventral, 1 lateral (median) and 1 mesial (median) stripes; propodi each with 1 dorsal and 1 ventral and 3 lateral and mesial longitudinal stripes; carpi each with 3 longitudinal stripes on lateral surface; meri each with 4 longitudinal stripes. Pleon light brown.
Distribution and habitat. Presently known only from Etajima Island, Hiroshima Prefecture (Seto Inland Sea) and Takahama, Ohi County, Fukui Prefecture (Sea of Japan); intertidal to shallow subtidal zone. Specimens from Etajima were collected from sea grass beds of Zostera marina. All specimens inhabited carcinoecia formed by a hydrozoan Stylactaria misakiensis.
Remarks. Pagurus rectidactylus n. sp. appears closest to P. quinquelineatus in the general shape of ocular peduncles, antennular peduncles and antennal peduncles, and the general armature and setation on chelipeds and ambulatory legs. In particular, both species have relatively numerous and strong corneous spines on the dactyli and propodi of the ambulatory legs. The general coloration is strikingly similar between the two species, although living coloration is often used to differentiate morphologically similar or cryptic species in the Paguroidea (e.g., Rahayu & Forest 1999; Komai 2003a, 2003b; Malay et al. 2012). The new species is distinguishable from P. quinquelineatus by the much more elongate ambulatory legs with more numerous ventral corneous spines on dactyli and propodi (cf. Fig. 4 A, B and Fig. 6 A, B; Table 1). For example, the dactylus of the second pereopod is 8.2–11.4 times (mean 9.4; n = 9) as long as wide in P. rectidactylus n. sp. versus 5.9–6.9 times (mean 6.9; n = 6) as long in P. quinquelineatus (no overlap of values between the two species). The propodus of the same pereopod is 5.0–6.5 times (mean 5.7; n = 11) as long as wide in P. rectidactylus n. sp. versus 3.6–4.3 times (mean 3.9; n = 6) as long in P. quinquelineatus (also no overlap of values between the two species). The dactylar spines are more numerous in the new species than in P. quinquelineatus, though the ranges sometimes partially overlap (Table 1); for example, the dactylus of the right second pereopod is armed with 15–21 (mean 17.6; n = 9) ventral spines in P. rectidactylus, rather than 11–13 (mean 12; n = 6) in P. quinquelineatus. The propodal ventral spines also tend to be more numerous in the new species than in P. quinquelineatus, although the ranges greatly overlap (for example, in the second pereopod, the maximum is nine in P. rectidactylus n. sp., whereas four in P. quinquelineatus). Furthermore, the armature of the right chela is generally weaker in P. rectidactylus n. sp. than in P. quinquelineatus. In particular, the dactylus of the right cheliped is unarmed or only with a few proximodorsal or dorsomesial tubercles in the new species (Fig. 2 D), rather than armed with a dorsal and mesial rows of conspicuous spines in P. quinquelineatus (Fig. 6 A). The general coloration is strikingly similar between the two species, but the markings on the mesial surfaces of cheliped meri seem to be much darker in P. quinquelineatus than in P. rectidactylus n. sp.
Difference in the carcinoecia between the two species is also remarkable. The new species was found to exclusively use carcinoecia formed by a hydrozoan Stylactaria misakiensis (cf. Fig. 5 B), while P. quinquelineatus always uses empty gastropod shells without association with coelenterates. The new species cannot fully retract the chelipeds and ambulatory legs when disturbed, whereas the protection is complete in P. quinquelineatus (personal observation). However, the third author (EM) observed that one specimen of the new species, kept alive in an aquarium, changed the carcinoecia of hydrozoan to a normal gastropod shell. It is not clear if the association of the new species with hydrozoan is so restricted. Anyway, the available data suggest that the carcinoecia preference differs between the new species and P. quinquelineatus. With regard to species of Pagurus, similar close association with carcinoecia formed by hydrozoan is known in P. constans (Stimpson, 1858) (e.g., Makarov 1938; Miyake 1982; Komai 1999a), although P. constans also rarely utilize sponges or gastropod shells for housing (Komai 1999a).
The discovery of a new species from relatively well-investigated inshore waters is rather surprising. Careful sampling would be necessary for documenting the marine fauna even for intertidal to shallow subtidal zone.
Etymology. From the combination of the Latin rectus (= straight) and dactylus (claw), in reference to the dactyli of ambulatory legs with the greater part straight and curving ventrally only in the distal part.
description: Published as part of Komai, Tomoyuki, Saito, Yuma & Myorin, Eiji, 2015, A new species of the hermit crab genus Pagurus Fabricius, 1775 (Crustacea: Decapoda: Anomura: Paguridae) from shallow coastal waters in Japan, with a checklist of the East Asian species of the genus, pp. 224-238 in Zootaxa 3918 (2) on pages 225-234, DOI: 10.11646/zootaxa.3918.2.4, http://zenodo.org/record/235286