Alionectes, Koch & Spiridonov & Ďuriš, 2023

Koch, Milan, Spiridonov, Vassily A. & Ďuriš, Zdeněk, 2023, Revision of the generic system for the swimming crab subfamily Portuninae (Decapoda: Brachyura: Portunidae) based on molecular and morphological analyses, Zoological Journal of the Linnean Society 197, pp. 127-175 : 142-144

ALIONECTES GEN. NOV.

( FIGS 3A, 4A, 6)

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: 5946851F-6789-4AF3-8DB9-B609146580ED Type species: Neptunus (Hellenus) pulchricristatus Gordon, 1931 , by present designation.

Included species: Two. Alionectes pulchricristatus ( Gordon, 1931) comb. nov. = Neptunus (Hellenus) pulchricristatus Gordon, 1931 = Neptunus (Hellenus) alcocki Gordon, 1930 [pre-occupied name, primary junior homonym of Neptunus (Hellenus) alcocki Nobili, 1906 ]

Alionectes spinipes ( Miers, 1886) comb. nov. = Neptunus (Amphitrite) spinipes Miers, 1886

Diagnosis: Carapace ( Fig. 6A) broadly hexagonal, with outline of anterior half semicircular; width (without last lateral teeth) ~1.5 × length; dorsal surface convex; regions feebly developed. Front ( Fig. 6B) with four blunt triangular lobes projecting beyond tip of inner supraorbital lobe. Orbits large, circular, with inner supraorbital lobe rounded; upper border of orbit deep, without fissures; infraorbital margin with subrectangular shallow lateral notch. Anterolateral margin of carapace with nine teeth: eight anterior ones low, triangular or almost reduced, unequal in size; last tooth distinctly larger, lateral. Posterolateral junction of carapace subrectangular, slightly produced, pointed. Sutures of thoracic sternum well expressed; suture between thoracic sternum 6 and 7 developed in males and interrupted in females; suture 7/8 interrupted in males and almost fused in females. Thoracic sternites granular ( Fig. 6C). Merus of third maxilliped rounded distally, not produced anterolaterally. Chelipeds relatively robust; merus bearing three spines on anterior border and two spines distally on posterior border; carpus with outer spine. Chelae ( Fig. 6D) moderately unequal, heterodontic; palm with longitudinal serrated carinae, inner dorsal carina with one or two distal spines, proximal spine over articulation with carpus sharp or obsolete. In larger chela, well-developed molariform tooth present. Dactyli of pereiopods 2–4 cultriform, setose with fine plumose setae; merus of pereiopod 5 distinctly longer than broad; posterior margin with distal spine. Male pleon ( Figs 3A, 6C) triangular, elongate, regularly tapering distally; pleomeres 2 and 3 forming low subequal crests, with at least second pleomere with crest marginally serrated. Lateral part of pleomere 3 in coaptation with terminal part of last thoracic episternite; third to fifth pleomere terga fused, without trace of sutures and keels; sixth pleomere elongate, tapering distally. First male ( Fig. 6E) gonopod relatively short and robust proximally; arched and steeply tapering distally to the slender tip; pair of first gonopods lying obliquely directed inwards, touching subdistal parts, with apices directed anterolaterally. Female vulva (examined in Alionectes pulchricristatus ; Fig. 3A) located in medial part of proximal portion of sternite, rounded, occupying more than one-third of sternite length, with long axis nearly perpendicular to anterior edge of sternite.

Etymology: An arbitrary combination of the suffix of the name ‘ Xiphonectes ’ [from Greek, νεκτες, received] and the Latin alio, other, to point it out as ‘another genus derived from Xiphonectes ’; gender masculine.

Systematic position: The species of Alionectes were previously included in Xiphonectes , sharing with the latter, and with the new genera Eodemus , Incultus and Trionectes , a general body appearance with the carapace flattened and the similarly produced posterior anterolateral tooth, and the produced or subacute angle formed by the posterolateral and posterior margins. Alionectes is unique among these five genera mainly by the large circular orbits without dorsal fissures, by the distinct posterodistal tooth on the merus of the swimming leg and by the serrated margin of the transverse crest of the second (or also third) pleonal somites in males. The species of this genus, together with Eodemus , possess two posterodistal spines on the cheliped merus (vs. a single spine in Incultus and Xiphonectes , or one or two spines in Trionectes ). The first male gonopods are, unlike those in the other four genera compared here, relatively short and proximally robust, and obliquely touch along their out-curved subdistal parts (for comparison of the gonopod shape and position, see the respective paragraph regarding Incultus ).

In the present phylogenetic analysis ( Fig. 1), the representative of Alionectes occupies a position remote from the morphologically similar genera listed above, but in a sister position to Lupocycloporus . Both genera share a posterodistal spine on the merus of the last pereiopod (a character possessed by Lupocyclus Adams & White, 1848 and, despite being reduced, also by Achelous , along with the subfamily Thalamitinae ; see Apel & Spiridonov, 1998), the subtriangular shape of the male pleon, the coaptation of the terminal part of the last thoracic episternite and the anterolateral margin of the third pleomere (shared also with Lupocyclus , but in a perfect state in Alionectes ), and the stout, strongly hooked first male gonopods. Lupocycloporus is clearly distinguishable by its remarkably slender chelipeds and by the carapace bearing transverse ridges dorsally (instead of granulate patches on the dorsal regions in Alionectes ) and rounded corners distolaterally (see also: Stephenson & Campbell, 1959; Spiridonov, 2020). Based on both morphological and molecular evidence, Alionectes is thus placed here in the subfamily Lupocyclinae , which previously contained only two genera, Lupocyclus and Lupocycloporus .

Remarks: Although Alionectes spinipes was not included in the present molecular analysis, this species is easily included in the new genus because of its close morphological similarity to Alionectes pulchricristatus . Both species share orbits without any notch on the upper margin, the serrated margin of the blade-like ridge of the second pleonal somite, a spine on the merus of the swimming legs and almost identical gonopodes. Alionectes spinipes differs from Alionectes pulchricristatus by the triangular shape of the anterolateral teeth of the carapace (vs. reduced in size, tuberculiform), by the distinctly shorter lateral tooth (i.e. less than the width of the front; vs. distinctly longer than the width of the front in Alionectes pulchricristatus ) and by a smooth, not serrated, ridge on the third pleonal somite (vs. serrated ridges on both the second and third pleomeres).

Size: Relatively small crabs; the maximum reported size (CW) in Alionectes spinipes (male) is 26 mm ( Stephenson & Rees, 1967a) and in Alionectes pulchricristatus (female) 32 mm ( Gordon, 1931).

Ecological notes: The existing information for Alionectes pulchricristatus indicates it to be a subtidal species living mostly within the upper 100 m but known to a depth of ~ 290 m on various types of substrates ( Stephenson & Rees, 1967a; Apel & Spiridonov, 1998).

Geographical range: Indo-West Pacific, so far known from the northern Indian Ocean (Gulf of Oman and eastwards) and in the western tropical Pacific from China and Vietnam to the Philippines, Indonesia and Australia ( Stephenson, 1972a, b; Apel & Spiridonov, 1998; present study).