Sargassocarcinus Ward, 1936

Ng, Peter K. L., Lin, Chia-Wei & Naruse, Tohru, 2016, Revision of the spider crab genus Sargassocarcinus Ward, 1936 (Crustacea: Brachyura: Epialtidae), Zootaxa 4162 (2), pp. 313-330 : 314-320

Genus Sargassocarcinus Ward, 1936

Sargassocarcinus Ward, 1936: 8 ; Sakai 1965: 75; Griffin & Tranter 1986: 97; Davie 2002: 267. Type species: Sargassocarcinus foliatus Ward, 1936 , by original designation.

Diagnosis. Surfaces of carapace, chelipeds, ambulatory legs covered by clavate setae, sometimes dense, foliose, spongiform along margins; dorsal surface of carapace with regions not well defined; proto-, metagastric regions elevated, each with median tubercle; cardiac region distinctly raised, with median tubercle; pseudorostrum short, bilobed, not deflexed, with distinct elongated, bifurcated rostral-like structure (pararostral teeth) below pseudorostrum formed by distal rim of antennular fossae; preorbital lobe large, triangular, directed anteriorly; postorbital lobe low to almost undiscernible; hepatic, lateral regions strongly expanded laterally to form wing-like lateral expansion with 2 broadly triangular lateral lobes; posterior carapace margin cristate. Eye dorsoventrally flattened, fused to orbital margin, immobile. Third maxilliped merus subtriangular, anteroexternal angle subauriculiform, carpus inserted on inner subdistal margin of merus, propodus, dactylus short. Chela short, with dorsal, ventral margins of palm distinctly cristate, dorsal margin of dactylus flattened, fingers of large males strongly curved forming prominent median gape when closed, cutting edge of dactylus with distinct subproximal tooth. Ambulatory legs covered with clavate setae, forming dense pubescence, expanded, foliose, along dorsal margins of merus, carpus, propodus forming “pseudo-cristae”; ventral margin of dactylus lined with pair of short subdistal spines, small denticles usually present posteriorly. Thoracic sternum wide, sternites 1 – 4 completely fused, surface of sternites 1 – 3 distinctly concave. Male abdomen with somites 3 – 5 separated by distinct sutures but somites immobile. G1 elongated, gently sinuous, distally bent outwards, distal part dilated but tapering to sharp tip; G2 short. Adult female abdomen with somites 1–3, telson free, somites 4–6 fused, domed outline.

Remarks. The entire surface of all three Sargassocarcinus species are covered with unusual clavate setae. The setae are low and appear squamiform or reticulate on the dorsal surface of the carapace and thoracic sternum, especially when dried ( Fig. 3 ). The setae form crest-like, lobiform or dentiform structures on the ambulatory legs and chelipeds (e.g., Fig. 6 ). For the setae to form crest-like structures is unusual, even among majoids. The pubescence is particularly low at the sutures of the thoracic sternites and abdomen, exposing the underlying structures ( Fig. 5A, B ). This pubescence is prominently denser and expanded on the dorsal and ventral margins of the ambulatory legs (more so in the former), with the structure appearing distinctly cristate (e.g., Fig. 6 A) even though the surfaces are actually rounded ( Fig. 6 B). Ward (1936: 9) remains the only author to have remarked on this pubescence, but only for the surface of the carapace: “covered with fine tomentum that imparts a punctate appearance”. Ward (1936: 9) described the legs of S. foliatus as “sharply carinated” without mentioning the setae, suggesting he may have thought these were actually part of the exoskeleton. Rathbun (1916: 536) also noted that the ambulatory meri of S. sublimis are cristate but this is actually not the case when denuded ( Fig. 6 B), although the dense clavate setae make it appear so ( Fig. 6 A, C–H). Balss (1924: 28) describes the dorsal surface of the carapace of S. cristatus as smooth but it is not so. In the smaller male (8.2 × 7.4 mm, USNM 48248), the setae are present but conspicuously short and sparse, being slightly denser on the lateral parts ( Fig. 3 E). The setae are more spread out but still visible ( Fig. 3 F) in the larger male (14.4 × 11.7 mm, ZRC 2015.288).

An unusual feature of the species of Sargassocarcinus is the dorso-laterally flattened eyes, which are completely fused to the orbital margins ( Figs 5D ; 7A, B, K; 10B, C). While the ocular peduncle is still clearly visible, it is completely fused to the carapace and immobile. This is unusual for epialtids. Even the similarly looking members of Huenia have mobile eyes. The eye itself is peculiar in form, with the lateral edges cornea appearing cristate ( Fig. 7A, B , I, J).

The carpus of the third maxilliped of most majoids is inserted on the distal margin of the merus. In the case of Sargassocarcinus species, the carpus is attached to the inner subdistal margin of the merus. As a result, when the palp (carpus, propodus, and dactylus) is appressed against the merus, most of it is covered ( Fig. 10 B, C), and only part of the dactylus is visible from the outer view ( Fig. 7 D, E).

The structure of the front in Sargassocarcinus is noteworthy and could be unique among epialtids. The actual pseudorostrum is actually short, not deflexed, divided into two relatively low triangular lobes by a median concavity and clearly visible from dorsal view ( Fig. 7A , I, J). Below this pseudorostrum is a pair of slender triangular lobes that are actually the expanded and projecting distal rims of the antennular fossae ( Fig. 7B ). These are here referred to as pararostral teeth, and are longer than the actual pseudorostral ones. Sakai (1965: text-fig. 11b) clearly noticed this as he illustrated this unusual structure although he did not discuss it. This is a feature not known for other epialtids, even Huenia . Rathbun (1916: 536) describes the pseudorostrum (as a rostrum) as “deflexed and cut halfway back into two small triangular teeth separated by a triangular sinus”.

Griffin & Tranter (1986: 98) observed that the male abdomen consists of seven free somites (including the telson). While the sutures between somites 3–5 are visible ( Figs 5A, B ; 8A–D), they are unable to articulate and are effectively fused. In females, only somites 4–6 are fused, with the sutures between the somites in adults barely visible, forming a domed brood pouch for the eggs ( Figs 8 E, F; 10B, C).

The carapace of Sargassocarcinus superficially resembles genera like Huenia De Haan, 1837 , and Menaethius H. Milne-Edwards, 1834 , in having wing-like expansions of the lateral margin. The flattened and fused eyes and the unusual pseudorostral and pararostral structures, however, argue against a close relationship between them (cf. Griffin & Tranter 1986; Takeda & Marumura 2010). The wing-like expansions of Sargassocarcinus also resemble the recently described Pteromaja Ng & Anker, 2014 , from Papua New Guinea, but in that genus the carapace is far more pyriform, the pseudorostrum, eyes, chelipeds, and ambulatory legs are completely different (cf. Ng & Anker 2014).

Sargassocarcinus foliatus Ward, 1936

( Figs 2 A–C; 3A–C, 4; 7A, B, D–F; 8A; 9A–E)

Sargassocarcinus foliatus Ward, 1936: 9 , pl. 3 figs 4–6; Springthorpe & Lowry 1994: 100. Sargassocarcinus sublimis Griffin & Tranter 1986: 97 (part); Davie 2002: 287; Ng et al. 2008: 101 (part) (not Peltinia sublimis Rathbun, 1916 ).

Material examined. Holotype: female (7.8 × 6.8 mm) (QM W739), Whitsunday Passage , Lindeman Island, Queensland , Australia, coll. 1930s; 2 males (6.4 × 6.0 mm, 5.7 × 5.4 mm) (QM W14482 ), in subtidal branching coral, Home Beach, Lindeman Island, northeast Queensland, coll. P.J.F. Davie & J. Short, 25 March 1987 .

Diagnosis. Carapace with distinct median tubercle on protogastric region; sub-branchial regions gently convex, obliquely raised ( Figs 2 A–C; 3A–C); pararostral teeth long, clearly visible from dorsal view ( Figs 3 A–C; 7A, B); eye relatively short ( Figs 4 A; 7B); postorbital lobe low to almost undiscernible, confluent with wing-like lateral expansion ( Fig. 3 A–C); lateral expansion lamelliform, with relatively deep, symmetrical concavity separating 2 subequal lobes, posterior margin of posterior lobe reaching beyond posterolateral margin of carapace as short cristate ridge, joining sub-branchial region ( Figs 2 A–C; 3A–C). Ambulatory legs (notably merus) relatively longer ( Figs 2 A–C; 4A, B, D, E; 7F). G1 relatively shorter, stouter, tapering bent distal part relatively short ( Fig. 9 A – D).

Description. Carapace subhexagonal; wider than long; surfaces covered by short scale-like setae which appears reticulated; relatively smooth when denuded; regions not well defined, delimited by shallow grooves; proto-, metagastric regions elevated, each with median tubercle; cardiac region distinctly raised, with median tubercle; sub-branchial regions gently convex, forming obliquely raised regions ( Figs 2 A–C; 3A–C). Pseudorostrum short, triangular, visible in dorsal view, forming 2 low rounded lobes with gentle median concavity; distinct pararostral teeth present below pseudorostrum, directed obliquely anteriorly, gently bent downwards, with deep median cleft, clearly visible from dorsal view ( Figs 2 A–C; 3A–C; 7A, B). Preorbital lobe large, triangular, directed anteriorly, separated from pseudorostrum by prominent cleft ( Figs 3 A–C; 7A). Postorbital lobe low, visible, confluent with lateral expansion in males ( Fig. 3 B, C); undiscernible in females ( Fig. 3 A). Hepatic, lateral regions strongly expanded laterally to form lamelliform, wing-like expansion; each expansion directed obliquely upwards with concave ventral surface, with 2 broadly triangular subequal lateral lobes, separated by relatively deep, broad, symmetrical concavity; posterior margin of posterior lobe reaching beyond posterolateral margin of carapace as short cristate ridge, joining raised oblique convexity on sub-branchial region ( Fig. 3 A–C). Posterolateral margin sinuous, subcristate; posterior carapace margin sinuous, distinctly cristate, partially directed upwards ( Figs 2 A–C; 3A–C). Orbits small; supraorbital margin short, entire, concave; suborbital margin entire, gently crenulated; eye short, dorsoventrally flattened, fused to orbital margin, immobile ( Figs 3 A–C; 4A; 7A, B). Proepistome narrow, not spiniform or dentiform ( Fig. 7B ). Antennular fossa large, subtriangular; antennules folding obliquely ( Fig. 7B ). Basal antennal article large, subrectangular, completely fused to carapace without trace of sutures except near orbit; flagellum inserted medially ( Fig. 7B ). Epistome transversely rectangular; posterior margin forming raised rim, sinuous, with median fissure; lateral edges of posterior margin of epistome, pterygostomial region raised ( Fig. 7B ).

Third maxilliped with outer surfaces covered with short pubescence; relatively short, stout, completely covering buccal chamber when closed; basis separated from ischium by distinct suture; ischium slightly longer than broad, distal, proximal margins subparallel; inner margin with crista dentata, without median sulcus; merus subtriangular, proximal quarter folds under distal part of ischium, anteroexternal angle subauriculiform; carpus broad, inserted on inner subdistal margin of merus, articulating obliquely inwards; propodus, dactylus short; exopod stout, outer margin sinuous, distal third tapering, inner margin with low subdistal triangular tooth, flagellum long which extends almost across width of merus ( Figs 4 B; 7D, E).

Chelipeds relatively short; most of surface (except for fingers) covered with low pubescence, denser, longer on margins, forming “pseudo-cristae” ( Fig. 4 A–C). Basis, ischium, merus fused; basis-ischium short, suture between ischium and merus visible; merus long, trigonal in cross-section, with subdistal tooth on dorsal margin, rest of margin uneven to weakly spinous. Carpus subovate, short, outer margin with low cristae, dorsal median part with distinct concavity and low rounded tubercle behind it ( Fig. 2 A–C). Chela with fingers shorter than palm; dorsal margin of palm with strong crista along entire length; ventral margin cristate along entire length to pollex; dactylus curved, dorsal margin distinctly flattened with lateral margins subcristate; pollex curved; cutting edges of fingers evenly lined with small teeth in smaller specimens ( Fig. 4 A–C); in large males, fingers arcuate, forming distinct gape when closed, distal part of cutting edges with teeth, median parts unarmed except for subproximal tooth on dactylus ( Fig. 4 B, C); in females, chela relatively more slender, fingers not gaping even in adults ( Fig. 4 A).

Ambulatory legs similar to those in Sargassocarcinus species, short, stout; leg decrease progressively in length posteriorly; covered with clavate setae, forming pubescence which is densest, expanded along dorsal margins of merus, carpus, propodus foliose, forming “pseudo-cristae”; surfaces generally smooth when denuded ( Figs 2 A–C; 4A, B, D, E; 7F). Merus laterally flattened, margins not cristate, appears to be so because of dense pubescence; dorsal margin uneven, may be lined with small scattered granules, with large, rounded distal tooth; ventral margin uneven, may be lined with scattered granules ( Figs 4 D, E; 7F). Carpus with tooth on dorsal margin, appears larger because of pubescence before denuding ( Figs 4 D, E; 7F). Propodus subrectangular; dorsal margin with 2 low lobes, appears larger because of pubescence before denuding ( Figs 4 D, E; 7F). Dactylo-propodal articulation, lock well developed, prominent; dactylus covered with low pubescence except for pectinated tip, gently falcate, ventral margin lined with pair of short subdistal spines, usually with several small denticles behind them ( Figs 4 D, E; 7F).

Male thoracic sternum wide, covered with low pubescence; smooth when denuded; sternites 1 – 4 completely fused except for low, rounded ridge demarcating sternites 3, 4; surface of sternites 1 – 3 distinctly concave; low curved rim demarcating sterno-abdominal cavity; sterno-abdominal cavity reaching to just before demarcation between sternites 3 and 4, with prominent rim on sternite 4; prominent tubercle of male abdominal locking mechanism on distal edge of sternite 5 ( Fig. 7 C). Penis short, coxal, coming out below condyle. Female thoracic sternum conspicuously wide, sterno-abdominal cavity round, deeply concave, with low anterior rim, completely covered by abdomen ( Fig. 10 B–D). Sternites medially interrupted; sternite 8 separated into 2 halves by median gap. Vulva with operculum, positioned on raised mount on sternite 6, produced obliquely forwards, pushing suture between sternites 5 and 6 anteriorly.

Surfaces of male, female abdomens covered by low pubescence, smooth when denuded. Male somites 1, 2 transversely narrow, free, reaching bases of coxae of fifth ambulatory leg, completely covering sternite 8 when closed; somites 3 – 5 separated by distinct sutures, immobile; somite 6 subrectangular with convex lateral margins; telson triangular with convex lateral margins ( Fig. 8A ). Female somites 1–3 free, reaching to bases of coxae of last ambulatory legs; somites 4–6 fused, with sutures barely or not visible in adults, forming domed structure; telson semicircular, free ( Fig. 4 A).

G1 elongated, gently sinuous, distally bent outwards, distal part dilated but tapering to sharp tip; groove for G2 ventrally positioned ( Fig. 9 A – D). G2 short, distal segment very short, subspatuliform ( Fig. 9 E).

Live colour. Unknown.

Remarks. Ward (1936) established this species from nine specimens. One female (carapace width 8.0 mm) was designated as the holotype; making the remaining four females (carapace widths 8.5 to 10.5 mm) and four males (carapace widths 7.5 to 10 mm) the paratypes.

Distribution. Known only from the type locality, Lindeman Island, Queensland, Australia.

Figures

FIGURE 2. Overall habitus. A, Sargassocarcinus foliatus, holotype female (7.8 × 6.8 mm) (QM W 739), Queensland, Australia; B, S. foliatus, male (6.4 × 6.0 mm) (QM W 14482), Queensland, Australia; C, S. foliatus, male (5.7 × 5.4 mm) (QM W 14482), Queensland, Australia; D, S. sublimis, holotype male (13.3 × 10.8 mm) (USNM 48247), Philippines; E, S. cristatus, male (14.4 × 11.7 mm) (ZRC 2015.288), Taiwan; F, S. cristatus, male (12.5 × 10.1 mm) (NMMBCD 4065), Taiwan.

FIGURE 3. Dorsal view of carapace. A, Sargassocarcinus foliatus, holotype female (7.8 × 6.8 mm) (QM W 739), Queensland, Australia; B, S. foliatus, male (6.4 × 6.0 mm) (QM W 14482), Queensland, Australia; C, S. foliatus, male (5.7 × 5.4 mm) (QM W 14482), Queensland, Australia; D, S. sublimis, holotype male (13.3 × 10.8 mm) (USNM 48247), Philippines; E, S. cristatus, male (8.2 × 7.4 mm) (USNM 48248), Japan; F, S. cristatus, male (14.4 × 11.7 mm) (ZRC 2015.288), Taiwan.

FIGURE 4. Sargassocarcinus foliatus. A, D, holotype female (7.8 × 6.8 mm) (QM W 739), Queensland, Australia; B, C, E, male (6.4 × 6.0 mm) (QM W 14482), Queensland, Australia. A, B, ventral view of cephalothorax; C, left cheliped; D, right fourth ambulatory leg; E, right fourth ambulatory leg (partially denuded). Scales = 1.0 mm.

FIGURE 6. Ambulatory legs and cheliped. A – H, Sargassocarcinus sublimis, holotype male (13.3 × 10.8 mm) (USNM 48247), Philippines [all same scale]; I – N, S. cristatus, male (8.2 × 7.4 mm) (USNM 48248), Japan [all same scale]. A, left first ambulatory leg; B, right first ambulatory leg (denuded); C, J, left second ambulatory leg; D, right second ambulatory leg; E, K, left third ambulatory leg; F, L, right third ambulatory leg; G, left fourth ambulatory leg; H, M, right fourth ambulatory leg; I, right first ambulatory leg; N, left cheliped. Scales = 1.0 mm.

FIGURE 9. Gonopods. A – E, Sargassocarcinus foliatus, male (6.4 × 6.0 mm) (QM W 14482), Queensland, Australia; F – I, S. sublimis, holotype male (13.3 × 10.8 mm) (USNM 48247), Philippines; J – N, S. cristatus, male (8.2 × 7.4 mm) (USNM 48248), Japan; O – R, S. cristatus, male (14.4 × 11.7 mm) (ZRC 2015.288), Taiwan. A, F, J, O, left G 1 (ventral view); B, G, K, P, left G 1 (dorsal view); C, H, L, Q, distal view of left G 1 (ventral view); D, distal part of left G 1 (dorsal view); E, I, N, R, left G 2; M, distal half of right G 1 (ventral view). Scales: A, B, F, G, I, J, K, M, N, O, P, R = 0.5 mm; C – E, H, L, Q = 0.1 mm.

FIGURE 10. Sargassocarcinus cristatus. A, B, female (10.8 × 8.9 mm) (NMMBCD 4065), Taiwan; C, D, ovigerous female (16.2 × 12.1 mm) (ZRC 2015.289), Taiwan. A, overall habitus; B, ventral view of cephalothorax showing buccal cavity, epistome, eyes and abdomen; D, sterno-abdominal cavity showing vulvae. Abbreviation: v, vulvae.

FIGURE 5. A – E, H, Sargassocarcinus sublimis, holotype male (13.3 × 10.8 mm) (USNM 48247), Philippines; F, S. cristatus, male (14.4 × 11.7 mm) (ZRC 2015.288), Taiwan; G, S. cristatus, male (8.2 × 7.4 mm) (USNM 48248), Japan. A, anterior male thoracic sternum and abdomen; B, male thoracic sternum and abdomen; C, left cheliped; D, orbits, antennules, antennae and epistome; E, F, frontal views of cephalothorax; H, posterior view of cephalothorax. Scales = 1.0 mm.

FIGURE 7. A, B, D – F, Sargassocarcinus foliatus, holotype female (7.8 × 6.8 mm) (QM W 739), Queensland, Australia; C, S. foliatus, male (6.4 × 6.0 mm) (QM W 14482), Queensland, Australia; G, I, S. sublimis, holotype male (13.3 × 10.8 mm) (USNM 48247), Philippines; H, S. cristatus, male (8.2 × 7.4 mm) (USNM 48248), Japan; J, S. cristatus, male (14.4 × 11.7 mm) (ZRC 2015.288), Taiwan; K, S. cristatus, ovigerous female (16.2 × 12.1 mm) (ZRC 2015.289). A, I, J, frontal margin and orbits; B, frontal margin, orbits, epistome, antennules and antennae; C, thoracic sternum and sternoabdominal cavity; D, inner view of left third maxilliped; E, outer view of left third maxilliped; F, G, dactylus of first ambulatory leg (denuded); K, ventral view of left eye. Scales: A – C, F – J = 1.0 mm; D, E = 0.5 mm.

FIGURE 8. A, Sargassocarcinus foliatus, male (6.4 × 6.0 mm) (QM W 14482), Queensland, Australia; B, S. sublimis, holotype male (13.3 × 10.8 mm) (USNM 48247), Philippines; C, S. cristatus, male (8.2 × 7.4 mm) (USNM 48248), Japan; D, S. cristatus, male (14.4 × 11.7 mm) (ZRC 2015.288), Taiwan; E – H, S. cristatus, ovigerous female (16.2 × 12.1 mm) (ZRC 2015.289), Taiwan. A – D, male abdomen (somites 3 – 6 and telson); E, female fused abdominal somites 4 – 6 and telson; F, female abdominal somites 2 – 4; G, female abdominal somite 1; H, female vulvae (arrow points to buccal cavity). Scales = 1.0 mm.