Pagurus pectinidactylus, Almón & Cuesta & García-Raso, 2022

Almón, Bruno, Cuesta, Jose A. & García-Raso, J. Enrique, 2022, A new species of hermit crab of the genus Pagurus Fabricius, 1775 (Crustacea: Anomura: Paguridae) from north-western Africa, Zootaxa 5209 (4), pp. 426-440 : 430-436

Pagurus pectinidactylus n. sp.

( Figs. 2–4 )

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Type material. Holotype: ♂ sl. 3.2 mm ( IEOCD-BR/2979 ), Mauritania, off Lemhaisrat , 19°00′09.7″N 16°23′38.0″W, sand, 21 m depth, 03 June 2012 . Paratypes: 2 ovigerous ♀, sl. 3.9 and 2.9 mm ( IEOCD-BR/2978 ), Mauritania, north of Nouakchott, 18°36′55.4″N 16°18′32.0″W, sand, 30 m depth, 03 June 2012 .

Description. Shield ( Fig. 2A ) flattened dorso-ventrally, broader than long, with tufts of short setae along shallow paragastric grooves and numerous low blister-like tubercles only visible when stained with methylene blue. Eleven pairs of phyllobranchia ( Fig. 2B ) with arthrobranchia on Mxp3, on cheliped and pereiopods 2–4, with one well-developed pleurobranchia on pereiopod 4.

Rostrum broadly triangular, tip slightly sloping downwards, with small tubercle distally and produced to or slightly beyond level of obtusely triangular lateral projections; frontal margin between lateral projections thickened; lateral projections each armed with minute spine.

Posterior carapace ( Fig. 2D ) membranous except for weakly calcified median and submedian parts, both with low blister-like tubercles and tufts of short setae; branchial region with tufts of simple setae.

Ocular peduncles 0.7–0.8 length of shield, slightly swollen medially, with row of short setae medially; conspicuous oblique calcified scale covering the latero-proximal part; corneas slightly dilated; ocular acicles ovate, separated basally by 0.5 width of 1 acicle, terminating subacutely, with small submarginal spine and some setae on distal margin; dorsal surface shallowly concave medially; lateral extensions subquadrate, well-developed.

Antennular peduncles ( Fig. 2A, C ) overreaching ocular peduncles by 0.3–0.4 length of ultimate segment; ultimate segment almost twice as long as penultimate segment, slightly widened distally, with tufts of long setae, one near distal margin, second medially; additional simple setae on proximal 1/3, one dorsal and one lateral; penultimate segment with few short setae; basal segment with lateral margin produced into blunt projection with narrow, blunt distal end; ventromesial distal margin produced as rounded projection; statocyst lobe unarmed; ventromesial distal angle unarmed.

Antennal peduncles ( Fig. 2A ) overreaching ocular peduncles by approximately 0.3–0.4 length of fifth segment, and slightly longer than antennular peduncles. Second segment with dorsolateral distal angle strongly produced, reaching to 3/4 of fourth peduncular segment, terminating in strong spine, mesial margin with at least 10 additional small spines; dorsomesial distal angle with acute spine. Antennal acicles arcuate, reaching to proximal 0.4–0.5 of ultimate peduncular segment, with small terminal spine; mesial margin with row of rather stiff setae.

Third maxiliped ( Fig. 2E ) moderately stout; dactylus about 0.8 length of propodus; carpus and merus unarmed; ischium with crista dentata with about 8 corneous teeth increasing in size and more widely spaced proximally, with 1 strong accessory tooth; basis with 2 minute denticles on mesial margin; exopod reaching distal margin of carpus.

Chelipeds unequal and dissimilar, right appreciably larger than left. Male right cheliped ( Fig. 3A–C ) moderately stout, appreciably elongated. Chela subquadrate in dorsal view, about twice as long as wide. Dactylus approximately same length as palm, not overlapped by fixed finger when closed; dorsal surface not elevated, smooth except for short row of small spines proximally; longitudinal rows of tufts of setae on dorsal surface and longitudinal row of stiff setae along cutting edge; dorsomesial margin defined by single row of small, obtuse tubercles, with associated tufts of moderately long stout setae; ventral surface smooth, with 3 rows of tufts of long stout setae; cutting edge with row of 7 rounded calcareous teeth, smallest medially, terminating in small corneous claw. Palm including fixed finger 1.7 times longer than carpus; with row of blunt spines on dorso-mesial margin; dorsal surface flat, smooth, with clumps of stout short setae forming circular tufts arranged in longitudinal rows ( Fig. 3F ); dorsolateral margin elevated, clearly delimited by row of closely-spaced acute to subacute spines with associated short setae; space between spines widening distally, disappearing near tip; dorsomesial margin elevated, delimited by single row of spines with associated tufts of long setae; ventral surface slightly convex, smooth, with sparse tufts of long setae arranged in longitudinal rows. Cutting edge of fixed finger with 8 or 9 calcareous teeth, smallest proximally, terminating in small corneous claw. Carpus elongated, widened distally, about twice as long as maximum width and similar in length to merus; dorsal surface smooth, with tufts of stout short setae arranged in rows more dense on lateral and mesial margins; dorsomesial margin slightly elevated and clearly delimited with row of oblique moderately strong blunt or acute spines; dorsodistal margin unarmed, with rows of setae of different size; dorsolateral margin not well-delimited, with several rows of stout short setae; lateral surface slightly convex, perpendicular, smooth and almost glabrous; mesial face slightly concave, with scattered tufts of long simple setae, ventromesial margin forming a wind-like protuberance with spines continuing the dorsomesial row; ventral surface convex, with scattered tufts of moderately long setae. Merus subtriangular; dorsal surface with low protuberances with associated short setae; dorsodistal margin with 2 large and 1 smaller acute spines with associated short setae; lateral surface slightly convex, smooth, with few very short scattered setae; mesial surface slightly concave with scattered long setae; ventromesial margin smooth except for small spinulose area distally; ventrolateral margin not well-defined, rounded; ventral surface smooth, with scattered long setae. Ischium unarmed. Female right cheliped differs from males in length, being less elongated but similar in shape and armature to male.

Male left cheliped ( Fig. 3D, E ) generally similar to right. Dactylus approximately 1.5 length of palm, dorsomesial margin with row of closely-spaced acute tubercles; dorsal surface smooth, with rows of tufts of moderately long setae; masticatory border toothless, except some very small proximally; rest of masticatory border bearing closelyspaced stout setae, forming a well-developed brush-like structure ( Fig. 3D inset). Palm with row of subacute spines on dorsomesial margin, ending distally in two conspicuous spines; dorsal surface almost flat, unarmed; dorsolateral margin slightly raised and with row of closely-spaced subacute tubercles not extending to tip of fixed finger. Carpus with row of subacute oblique spines on dorsomesial margin; dorsal surface smooth, with longitudinal ridges of tufts of stiff setae; dorsodistal margin with several small spines. Merus with 2 spines on dorsodistal margin; ventromesial and ventrolateral margins each with row, or only few, very small blunt spinules or tubercles. Female left cheliped similar in shape to male, with more tubercles especially evident at dorsolateral surface of carpus, which is slightly elevated bearing some low tridentate tubercles with associated stiff setae, and merus, with dorsal surface crossed by rows of spinose sulcus, continuing along lateral and mesial surfaces, especially distal ones.

Ambulatory legs ( Fig. 4A, B ) short and stout. Dactylus about same length (second) to 1.2 times longer than propodus (third) and about 5 times longer than broad, flattened laterally and gently curved ventrally terminating in strong corneous claw; lateral surface smooth, with very shallow longitudinal sulcus and few tufts of short setae on lower half; dorsal margin with irregular rows of tufts of short, thick setae, ending in row of long setae occupying distal 1/5; ventro-inner margins each with row of 5 or 6 (second) to 7 or 8 (third) strong corneous spines increasing in size distally, evenly distributed except most proximal of second pereiopod, which is more separated, and associated tufts of long setae. Propodus approximately 1.4 times longer than carpus (second and third); dorsal surfaces unarmed, each with multiple rows of short thick setae; lateral surface smooth, with longitudinal rows of short setae; ventral margins each with 3 corneous spines and tufts of short setae, denser near distal margin. Carpus with dorsal margin defined by row of small spines and associate tufts of short setae, with strong spine at dorsodistal margin; several rows of tufts of setae on lateral faces, short at upper half and near lower margin, longer at midline; ventral margin unarmed and glabrous. Merus with dentate dorsal margin, with associated rows of setae; lateral surface smooth and glabrous; ventral margins unarmed or with low protuberances or spinules, particularly on second pereiopods, and row of moderately long setae.

Fourth pereiopods ( Fig 4E ) semichelate, subequal in length from left to right; dactylus with subterminal tuft of short setae on dorsal margin; carpus ending in conspicuous spine at distodorsal margin; stiff long setae and tufts of setae on dorsal margin of propodus to carpus and distodorsal and ventromesial margin of merus. Dactylus nearly straight, with convex dorsal margin, terminating in small, strongly-curved corneous claw; ventral margin with closely-set, microscopic corneous teeth; propodal rasp consisting of single row of corneous scales.

Fifth pereiopods chelate, propodal rasp occupying distal half of segment.

Sternite of somite XII in males (thoracomere 6, third pereiopods) with roundly subrectangular anterior lobe ( Fig. 2F ), bearing setae of different sizes on anterior margin. Sternite of somite XIV (thoracomere 8, fifth pereiopods) with widely separated lobes in males, each with moderately long setae ( Fig. 4C ); coxae of male each with gonopore partially masked by tuft of setae. Sternite of somite XII of females, with subquadrate single central lobe bearing moderately long simple setae ( Fig. 4D ); coxa of females each with gonopore, and associated tufts of setae above, without masking them. Males with 3 uniramous unpaired left pleopods; females with slightly unequally biramous 2-4, fifth uniramous..

Telson ( Fig. 4F ) with lateral incision indicating anterior and posterior portions; posterior lobes slightly asymmetrical, subquadrate, left slightly larger than right, separated by very small median cleft; terminal margins each with row of calcareous large teeth alternating with small ones, extending into lateral margin on left lobe and to posterior half of right; few short setae associated with spines and some tufts of longer setae on lateral margins.

Distribution. So far only know from two off-shore stations, 21–30 m depth, within Mauritanian waters, north of Nouakchott.

Etymology. Derived from Latin pecten (= comb) and Greek δάχτμλος (dáctilos = finger), referring to the characteristic row of closely-spaced stout setae on the masticatory border of left cheliped dactylus, forming a welldeveloped brush-like structure.

Taxonomic remarks. The new species is assignable to the informal Pagurus anachoretus group (cf. Forest 1978) as it concurs with the general definition of the group ( Forest & Ngoc-Ho 1992; McLaughlin & Forest 1999; Komai & Rahayu 2004). This group includes 12 species at present: Pagurus anachoretus from the Mediterranean, P. anachoretoides , P.gordonae , P.laurentae and P.souriei from the tropical eastern Atlantic, P.emmersoni and P.liochele from South Africa, and the Indo-West Pacific P. decimbranchiae Komai & Osawa, 2001 , P. fungiformis Komai & Rahayu, 2004 , P. hedleyi ( Grant & McCulloch, 1906) , P. kulkarnii Sankolli, 1961 and P. moluccensis Haig & Ball, 1998 (see Forest 1978; Forest & Ngoc-Ho 1992; McLaughlin & Forest 1999). Four other species from the tropical eastern Atlantic, P. alcocki , P. dartevellei ( Forest, 1958) , P. fimbriatus and P. triangularis , also fit the definition of the P. anachoretus group except for the cheliped form ( Forest 1978; Forest & Ngoc-Ho 1992; McLaughlin & Forest 1999; Komai & Rahayu 2004). Pagurus fraserorum described by Landschoff (2018) also shares the main characters of the group except for the shape of the right cheliped, and based on molecular results, it has been suggested that it may constitute a separate group, more related with P. boriaustraliensis and P. pitagsaleei .

Notwithstanding, the new species shows enough morphological differences to avoid any confusion with the species of the group or any other congeners known so far. The oblique calcified scale present in the base of the peduncles, is not known to occur in any other species of Pagurus . The shape and armature of the chelipeds are diagnostic, with a subquadrate shape in dorsal view, clearly elongate. The palm surface is flat and smooth, but with numerous distinctive clumps of stout short setae forming circular tufts arranged in longitudinal rows. The masticatory border of the left cheliped dactylus is also very conspicuous, lacking teeth, but bearing instead a row of closely-spaced stout setae, forming a well-developed brush-like structure, which has not been described for any other species of Pagurus in the study area.

Although those characters are sufficiently distinctive to avoid any confusions, the new species shows a closer morphological resemblance with some species of the anachoretus group, specifically with Pagurus liochele , P. emmersoni , P. triangularis , P. fimbriatus and P. similimanus . The first two are considered endemic to South African waters. The new species differs from the two South African congeners in having broader carapace and rostrum, longer ocular peduncles, and more ovate ocular acicles. The antennules are shorter in P. emmersoni , overreaching distal margin of corneas by approximately 0.2 length of antennular segment 3, while they are similar in P. pectinidactylus n. sp. and P. liochele , overreaching the ocular peduncles by approximately 0.3–0.4 length of antennular segment 3. Another distinctive character can be found in the antennal segment 2 dorsolateral distal angle, which is strongly produced in both P. liochele and P. pectinidactylus n. sp., with mesial margin more densely dentate in the new species with at least 10 small spines, while in P. liochele there are 1–5. Moreover, the new species has a longer palm, with the characteristic clumps of setae arranged in circles on dorsal surface instead of any tubercles or spines, character that has not been described in any of the other species. The pereiopods are stout but more elongated in the new species, with the dactylus longer than propodus, bearing 5 (second) to 7 (third) strong corneous spines on ventral margin. The sternite of third pereiopods anterior lobe is roundly rectangular in P. pectinidactylus n. sp. and P. emmersoni , although with differences in setation, while in P. liochele the shape of the sternite is subovate.

The other three species share also some morphological characters, the last two even with a distribution relatively close to that of the new species (see introduction). P. fimbriatus differs in the rostrum, which is obtuse, not pointed and not thickened. The shield is as long as broad while in the new species is broader than long. The antennal scales are shorter in P. fimbriatus , reaching only the base of the cornea, while in the new species reach distal end of corneas. Another important character can be found at dorsolateral distal angle of the second antennal segment, which is strongly produced in P. pectinidactylus n. sp., reaching to 3/4 of fourth peduncular segment, terminating in strong spine and with mesial margin bearing at least 10 additional small spines; based on the descriptions and figures provided by Forest (1966), this character is not present in P. fimbriatus . Pagurus triangularis and the new species share a pointed rostrum, but the former (like P. fimbriatus ) has a narrower shield and shorter antennal acicles, which does not reach the distal margin of the cornea (at most it reaches its base), and also has a shorter dorsolateral distal process at the second antennal segment. P. similimanus also has a narrower shield, and a rostrum that is less projected than in the new species. The ocular peduncles are also shorter in P. similimanus , reaching to midlength of antennular segment 3, and proximal 1/3 of antennal segment 5, while in the new species reach the distal 1/3 of the antennular segment 3 and antennal segment 5. The antennal acicle is much longer, clearly exceeding the cornea in P. similimanus while in the new species reaches the distal margin of the cornea. The dorsolateral distal process of the second antennal segment has 6–8 little teeth on mesial margin, while in the new species the number is higher (up to 10). The shape of the chelipeds is also different in all these species, with a variable degree of elongation that never reaches that of the new species. The dorsal surface of the chelipeds is flat and unarmed in P. pectinidactylus n. sp., and the characteristic pattern formed by the clumps of setae is not mentioned in any of the other Eastern Atlantic species, neither the diagnostic masticatory border of the dactyl of the left cheliped formed by strong, densely arranged setae instead of teeth.

Figures

FIGURE 2. Pagurus pectinidactylus n. sp., Mauritania, ♂ sl. 3.2 mm, holotype, (IEOCD/2979): (A) Anterior part of carapace and cephalic appendages, dorsal view (arrow highlighting the position of the distinctive oblique calcified scales); (B) branchial number and disposition on body appendages; (C) right antennule, lateral view; (D) carapace, dorsal setae and blister-like tubercles general disposition; (E) left maxilliped 3; (F) sternite of somite XII (third pereiopods), ventral view. (Scales: A–F = 1 mm).

FIGURE 3. Pagurus pectinidactylus n. sp., Mauritania, ♂ sl. 3.2 mm, holotype, (IEOCD/2979): (A) Right cheliped, dorsal view; (B) right cheliped, mesial view; (C) anterior part of the right cheliped, lateral view; (D) left cheliped, dorsal view; anterior part of left cheliped, lateral view (inset: detail of the dactylus masticatory border); (E) anterior part of the left cheliped, lateral view (F) detail of circular clumps of setae. (Scales: A, B = 3 mm; C = 1 mm; D, E = 2 mm; F = 0.5 mm).

FIGURE 4. Pagurus pectinidactylus n. sp., Mauritania, ♂ sl. 3.2 mm, holotype, (IEOCD/2979): (A) right pereiopod 2, lateral view (inset- dactylus mesial view); (B) right pereiopod 3, lateral view (inset- dactylus mesial view); (C) Sternite of somite XIV and coxa of fifth male pereiopods, ventral view. Pagurus pectinidactylus n. sp. Mauritania, ♀ sl. 3.9 mm, paratype, (IEOCD/2978): (D) Sternite of somite XII and coxa of third female pereiopods, ventral view; (E) left pereiopod 4, lateral view; (F) Telson, dorsal view. (Scales: A–F = 1 mm).