Trionectes, Koch & Spiridonov & Ďuriš, 2023

TRIONECTES View in CoL GEN. NOV.

( FIGS 3B, 4C, 16)

Z o o b a n k r e g i s t r a t i o n: u r n: l s i d: z o o b a n k. org:act: BCC68580-6F16-44F1-8DDC-D41241E34F7B

Type species: Portunus (Amphitrite) tenuipes De Haan, 1835 , by present designation.

Included species: Seven.

Trionectes andersoni (De Man, 1887) comb. nov.

= Neptunus (Hellenus) andersoni De Man, 1887 Trionectes mariei (Guinot, 1967) comb. nov.

= Portunus (Hellenus) mariei Guinot, 1957 View in CoL Trionectes pseudotenuipes (Spiridonov, 1999) View in CoL comb. nov.

= Portunus pseudotenuipes Spiridonov, 1999 View in CoL Trionectes rugosus (A. Milne-Edwards, 1861) View in CoL comb. nov.

= Neptunus rugosus A. Milne-Edwards, 1861 View in CoL

Trionectes spiniferus (Stephenson & Rees, 1967) View in CoL comb. nov.

= Portunus spiniferus Stephenson & Rees, 1967 Trionectes tenuipes (De Haan, 1835) comb. nov.

= Amphitrite tenuipes De Haan, 1835 )

Trionectes tridentatus (Yang, Dai & Son, 1979) comb. nov.

= Portunus tridentatus Yang, Dai & Song, 1979

Diagnosis: Carapace ( Fig. 16A) broadly hexagonal, width ~1.5 times as long as broad (without lateral teeth); dorsal surface variously granulate, with regions moderately demarcated. Front ( Fig. 16B) with three distinct triangular or rounded lobes, median lobe subequal but more slender than lateral ones, all three lobes distinctly projecting beyond tip of inner supraorbital lobe. Orbit with inner supraorbital angle truncated, with oblique distal margin; dorsal orbital margin with two short fissures; infraorbital margin with deep ‘V’-shaped lateral notch. Anterolateral margin with nine spiniform, triangular or lobiform teeth, irregular in size; last tooth distinctly larger, lateral. Posterolateral junction of carapace angular, pointed, often distinctly upturned, sharp. Sutures of thoracic sternum unclear ( Fig. 16C); thoracic sternites may be granular. Merus of third maxilliped elongate, distally markedly produced to triangular, trapezoidal or rounded lobe. Chelipeds robust, with merus bearing three or four spines on anterior border and one or two spines distally on posterior border; carpus with single spine on outer face, which, in some species ( T. pseudotenuipes ), is obsolete; upper surface of palm with one or two distal teeth. Chelae ( Fig. 16D) slightly inaequal, heterodontic, with moderate molariform tooth at cutting edge of dactylus in larger chela. Dactyli of pereiopods 2–4 cultriform or styliform, costate, with short setose emargination on ventral margin. Merus of pereiopod 5 a little longer than broad, without posterodistal spine. Male pleon ( Figs 3B, 16C) quasi-triangular. Pleomeres 2 and 3 with laminar crests. Lateral margins of pleomere 3 straight or convex; terminal part of posterior thoracic episternite fills interspace between anterior margin of pleomere 3 and thoracic sternite 8. Pleomeres 3–5 fused without remaining sutures, but indistinct keels may be present. Pleomere 6 with lateral margins sinuate. First male gonopod ( Fig. 16E) short, curved, tapering distally to slender tip; pair of first gonopods lying obliquely directed inwards but isolated, not touching medially, with apices curved anterolaterally. Female vulva ( Fig. 4C) (examined in T. mariei and T. tenuipes ) in form of an oblique widened slit, located in medial part of proximal portion of sternite.

Etymology: Given that species were previously included in Xiphonectes , the new generic name is a combination of the Greek ‘τρί’, three (pointing on trilobed front) and ‘νήκτης’, swimmer, thus a ‘trilobed swimmer’; gender masculine.

Systematic position: All species of Trionectes , together with the new genera Eodemus , Incultus and the lupocycline genus Alionectes , were previously considered members of Xiphonectes (see listing by Ng et al., 2008). All these taxa are similar in overall appearance, with a more or less strongly depressed carapace, usually with a long pair of lateral teeth and elevated patches of granules on the dorsum and with a produced or subacute posterolateral angle.

Based on the morphological comparison and the present molecular analysis, Trionectes is most closely related to Xiphonectes (s.s.; present paper), and Incultus is more basal (see remarks for Xiphonectes , below). Trionectes differs from these genera by: (1) the front with three subequal triangular or blunt teeth in Trionectes (vs. four distinct triangular or rounded teeth, with submedian ones smaller in Xiphonectes , or four shallow subequal teeth in Incultus ); (2) feebly demarcated dorsal regions of the carapace (vs. distinctly elevated in Incultus ); (3) predominantly sharply triangular anterolateral teeth on the carapace (vs. lobate in Incultus ); (4) the infraorbital margin deep, ‘V’-shaped, in Trionectes (vs. with narrow lateral notch in Xiphonectes , or with broad lateral notch in Incultus ); (5) the male pleon elongately trapezoid (vs. same in Incultus , but with the sixth pleomere distinctly constricted subdistally in Xiphonectes ); (6) the chelipeds stout (vs. same in Incultus , but comparatively slender in Xiphonectes ), with fingers subequal to palm length (vs. same in Xiphonectes but distinctly shorter in Incultus ); (7) the first male gonopods short, stout, subdistally bent, not meeting medially (vs. similar but more slender, smoothly out-curved in midlength in Xiphonectes , but relatively long, bent in midlength, with pair of gonopods overlapping each other medially in Incultus ); and (8) the female vulva, which is located at the mesial or posteromesial margin of the sternite in Incultus and has a different position in Trionectes and Xiphonectes .

From the lupocycline Alionectes , Trionectes can be distinguished easily by the tridentate front, the presence of a dorsally incised orbital margin, which is entire in Alionectes , the posterodistally unarmed merus of the swimming leg, and by the smooth, entire margin of the transverse crest on the second pleonal somite in males (vs. serrated on the second pleomere in Alionectes ).

Trionectes is also easily distinguished from Eodemus by the tridentate front, but also by the subtriangular male pleon (vs. a ‘T’-shaped pleon) and by the distally produced triangular or circular rounded, not distolaterally produced, third maxilliped merus (vs. a distinctly laterally produced anterolateral lobe). The genera also differ by the position and shape of the female vulva, which is in the form of a widened slit in Trionectes and somewhat distant from the margin of the sternite, and chinking located close to the mesial margin in Eodemus .

Remarks: Three species differ from the remaining ones in the genus. The examined MNHN and ZMMU specimens of T. mariei and MNHN specimens of T. spiniferus and the species T. tridentatus , based on published reports ( Dai & Yang, 1991; Yang et al., 2012), all have distinct unequal and sharp anterolateral teeth on the carapace, the third maxillipeds with a rounded distal projection, more robust chelipeds and three spines on the upper surface of the chela palm, but they all share the trilobed front, and the pleon bears a distinct laminar crest on the second pleomere typical for Trionectes . Only T. spiniferus was available for the present molecular analysis but, based on the morphological similarity of all three species and its deeply nested position in the Trionectes clade ( Fig. 3), the inclusion of T. mariei and T. tridentatus in the genus appears to be well founded.

Size: Small to medium-sized crabs; maximum known size ranges from CW 16 mm in T. rugosus (A. Milne-Edwards, 1861) to CL × CW: 24.5 mm × 53.3 mm in T. tridentatus ( Yang et al., 2012) .

Ecological notes: The species with better-documented records (i.e. T. tenuipes and T. pseudotenuipes ), occur mostly in the upper subtidal zone, to a depth of 35 m, on sand or muddy sand ( Ramadan, 1936; Monod, 1938; Spiridonov, 1999). The few known records of T. mariei are also from upper subtidal sand patches on reefs ( Guinot, 1957; present study: ZMMU Ma 3408 and SMF 47928 specimens).

Geographical range: Indo-West Pacific: from the Red Sea ( Ramadan, 1936; Monod, 1938; present study) and the western Indian Ocean to the Philippines and New Caledonia ( Stephenson, 1972a, b; Spiridonov, 1999; Poupin, 2010; Yang et al., 2012).