Ectaesthesius bifrons Rathbun, 1898

Ng, Peter K. L., Ahyong, Shane T. & Castro, Peter, 2023, Re-appraisal of the families and subfamilies of Trapezioidea Miers, 1886, with establishment of a new family, Ectaesthesiidae (Crustacea: Decapoda: Brachyura), Raffles Bulletin of Zoology 71, pp. 606-631 : 616-618

publication ID	https://doi.org/ 10.26107/RBZ-2023-0047
publication LSID	lsid:zoobank.org:pub:821BC4EC-5AF9-4727-84A3-C44839DFBE28
DOI	https://doi.org/10.5281/zenodo.10271575
persistent identifier	https://treatment.plazi.org/id/038EC92D-7978-7E52-FF5B-6B4B24CEFAF2
treatment provided by	Felipe
scientific name	Ectaesthesius bifrons Rathbun, 1898
status

Treatment

Ectaesthesius bifrons Rathbun, 1898 View in CoL

( Figs. 1–4 View Fig View Fig View Fig View Fig )

Ectaesthesius bifrons Rathbun, 1898: 591 View in CoL , pl. 42 figs. 12–14, 1930: 460, text-fig. 76a–c; Garth, 1946: 466, pl. 58, figs. 1–7; Hickman & Zimmerman, 2000: 116, unnumbered figure; Ng et al., 2008: 202; Poore & Ahyong, 2023: 728, 729, 730, fig. 14.139b.

Material examined. 1 male (5.4 × 4.0 mm) ( LACM 1934.220 View Materials ), station 346-35, mud and shell bottom, between Seymour and Daphne islands, Galápagos Islands, 0°24.4′S, 90°21.8′W, 101 m, coll. R / V Velero III, 1934 GoogleMaps ; 2 males (larger 12.8 × 9.0 mm), 1 female (11.2 × 8.0 mm), 2 juveniles ( LACM 1938.56 View Materials ), station 795-38, rock and sand substrate, Bahía Sullivan (= Sullivan Bay ), San Salvador (= James) Island, Galápagos Islands, 0°16.2′S, 90°34.8′E, 66–73 m, coll. R / V Velero III, 21 January 1938 GoogleMaps ; 1 juvenile female (3.9 × 3.3 mm), 1 female (10.1 × 7.0 mm) ( ZRC 2013.0341 View Materials ), station 795A-38, volcanic sand substrate, Bahia Sullivan (= Sulivan Bay ), San Salvador (= James) Island, Galápagos Islands, 0°16.13′S, 90°34.6′W, 91–110 m, coll. R / V Velero III, 21 January 1938 GoogleMaps ; 1 young female (6.8 × 5.0 mm) ( LACM 1938.80 View Materials ), station 814-38, north of Española (= Hood) Island, Galápagos Islands, 1°21.9′S, 90°40.1′E, 37–73 m, coll. R / V Velero III, 28 January 1938 GoogleMaps .

Diagnosis. As for family.

Description of male. Carapace transversely subovate, distinctly wider than long, widest near midlength ( Figs. 1A View Fig , 2A View Fig , 3A View Fig ); dorsal surface smooth, gently convex in frontal view, regions poorly demarcated, H-shaped gastrocardiac groove shallow but visible ( Fig. 2A, C View Fig ). Front gently sinuous to almost entire, smooth, weakly rimmed; with very shallow median concavity separating low lobes; lateral lobe low, visible, with rounded margin, barely demarcated from supraorbital margin; supraorbital margin gently concave, without fissure or cleft; outer orbital tooth small but distinct, sharp ( Figs. 2A View Fig , 3A View Fig ). Antero- and posterolateral margins clearly demarcated, margin very finely granulated; anterolateral margin just shorter than posterolateral margin, with 2 triangular teeth, first tooth obtuse, second tooth (marking junction with posterolateral margin) acute, sharp ( Figs. 2A View Fig , 3A View Fig ); anterior part of posterolateral margin gently convex before curving to become concave along posterior half ( Fig. 2A View Fig , 3A View Fig ). Posterior carapace margin short, gently convex to almost straight ( Figs. 1A View Fig , 2A View Fig , 3A View Fig ). Suborbital, subheptic, pterygostomial, sub-branchial regions smooth ( Figs. 2C View Fig , 3B View Fig ). Suborbital margin evenly concave, finely granulated, appearing smooth, suborbital tooth low, obtuse, not pronounced ( Fig. 3B View Fig ). Posterior margin of epistome entire, separated from pterygostomial region by distinct fissure, margin with low median triangular lobe, lateral margin relatively short, entire, gently concave ( Figs. 2C View Fig , 3C View Fig ). Basal antennal article quadrate, mobile, peduncle and flagellum entering hiatus of orbit, flagellum longer than width of orbit ( Figs. 2C View Fig , 3B View Fig ). Antennules transverse, flagellum folding transversely ( Figs. 2C View Fig , 3A View Fig ). Eyes short, completely filling orbit, cornea large, black, round; peduncle short, smooth ( Figs. 1A View Fig , 2A, C View Fig , 3A, B View Fig ).

Maxillipeds 3 closing almost completely with only narrow gap between ischia ( Fig. 2E View Fig ); merus subquadrate, anteroexternal part distinctly auriculiform, wider than subrectangular ischium; ischium rectangular, with oblique submarginal sulcus. Exopod relatively stout, with subdistal triangular lobe on inner margin, distal edge reaching to distal edge of merus; flagellum long, exceeding width of merus ( Fig. 3D View Fig ).

Chelipeds heterochelous, relatively long, about 1.7× carapace width; ischiobasis short, smooth, with low protuberance on median part of flexor margin; merus short, quadrate in dorsal view, with sharp granule on subdistal edge of flexor margin, otherwise smooth; extensor margin smooth; carpus with strong tooth on inner angle; propodus outer surface smooth, without tubercles; fingers of major chela shorter than palm, cutting margins unarmed, tips recurved; fingers of minor chela as long as palm, with low teeth along distal third of cutting edges or almost smooth ( Figs. 1A View Fig , 2D View Fig , 3A, E, F View Fig ).

P2–5 short; P3 longest, P5 shortest; numerous and short stiff setae on surface and margins of carpus, propodus, dactylus but do not obscure margins, setae on merus more scattered, more concentrated on distal part; merus not elongated, relatively stout, not foliaceous, unarmed on dorsal margin, at most with low angle on subdistal part ( Fig. 1A View Fig , 3A View Fig ); carpus short, smooth ( Fig. 1A View Fig ); propodus laterally compressed, dorsal margin uneven but not serrate, ventral margin with spines on distal half ( Figs. 1A View Fig , 3H View Fig ); dactylo-propodal lock well-developed ( Fig. 3H View Fig ); dactylus margins with ungrouped, robust, movable spines, without comb-like rows of feeding setae, unguis corneous, gently curved ( Figs. 1A View Fig , 3H View Fig ).

Thoracic sternum smooth; sternites 1, 2 completely fused to form triangular structure without trace of fissure; sternites 2, 3 fused without trace of lateral fissures; sternites 3, 4 fused without trace of median suture, groove or depression except for short lateral fissure; sternopleonal cavity relatively shallow, poorly demarcated, reaching anteriorly to level of midlength of P2 coxae ( Figs. 2E, F View Fig , 3G View Fig ). Sternite 4 without longitudinal groove; sternites 5–8 complete with all sutures distinct ( Figs. 2E, F View Fig , 3G View Fig ). Rounded tubercle of pleonal-locking mechanism small, on posterior edge of thoracic sternite 5, near suture with sternite 6 ( Figs. 2F View Fig , 3G View Fig ).

Pleon narrow; all somites and telson free, without trace of fusion; somite 1 trapezoidal, reaching to coxae of P5; somite 2 subrectangular; somite 3 widest, wider than somite 1; lateral margins distinctly convex; somites 4, 5 decreasing in width; somite 6 quadrate, wider than long, lateral margin gently sinuous; telson semicircular ( Figs. 2E View Fig , 3I View Fig ).

G1 arcuate, relatively slender, gently curving outwards, gradually tapering distally; margins of distal half lined with numerous short spinules; distal part dilated with wide opening, forming low dorsal flap ( Fig. 4A–D View Fig ). G2 relatively slender, elongated, about four-fifths length of G1; distal portion prominent but short, filiform ( Fig. 4E View Fig ).

Females. Female pleon longitudinally ovate, covering most of thoracic sternum but not along lateral parts; somite 1 trapezoidal, reaching to coxae of P5; all somites and telson free; telson triangular, wider than long, lateral margins gently convex ( Figs. 2G View Fig , 3J View Fig ). Sternopleonal cavity shallow ( Fig. 2G, H View Fig ). Vulva flat, subcircular, positioned on proximal part of sternite 6, adjacent to suture with sternite 5, positioned close to median line, covered by thin membrane ( Fig. 2H View Fig ).

Remarks. Ectaesthesius bifrons was described from one ovigerous female (9.7 × 7.0 mm; USNM 21586) from off Chatham Island, Galápagos Islands, from a depth of 45 fathoms (82 m). Garth (1946: pl. 58) redescribed this species from a good series of specimens from the Galápagos Islands and figured a male and a female, but no indication was given as to which specimens was used, but they seem to match with those in LACM 1938.56 ( Fig. 3 View Fig ).

Very little is known about the ecology of E. bifrons , and it is somewhat surprising that it has not been recollected since the Velero cruises in the late 1930s, even though it is not a deep-water species. It appears to be endemic to the Galápagos Islands, being known only from three of the islands: Chatham, James, and Hood.