Ezaxius, De Matos-Pita, Susana S. & Ramil, Fran, 2015

De Matos-Pita, Susana S. & Ramil, Fran, 2015, Additions to thalassinidean fauna (Crustacea: Decapoda) off Mauritania (NW Africa) with the description of a new genus and a new species, Zootaxa 4020 (3), pp. 571-587: 578-579

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n. gen.

Genus Ezaxius  n. gen.

Diagnosis. Carapace faintly pitted. Rostrum triangular with upturned tip, laterally spinose, extending posteriorly as lateral carinae. Gastric region elevated, bearing 5 carinae: median, submedian and short lateral; submedian carinae converging anteriorly to form a horseshoe-like shape. No linea thalassinica. Cervical groove distinct only on dorsal surface, postcervical carina wide and short, weakly visible only over posterior median lobe. Eyes short, unpigmented, one-third rostrum length. Antennal acicle elongate, protruding straight forward. Anterolateral margin broadly rounded, unarmed. Maxilla 2 exopod posterior lobe with one long seta. Maxillipeds 1–3 with exopod and single epipod; maxilliped 2 with single podobranchia; maxilliped 3 with double podobranchia and single arthrobranchia. Pereiopods 1 and 2 chelate, pereiopods 1–4 with single epipod and double arthrobranchia, pereiopods 1–3 with double podobranchia. Pleurobranchia absent. Pleomeres unarmed; first somite with pleura acute; second broad, anteriorly and posteriorly rounded; 3–5 anteriorly rounded, declining to a rounded angle posteriorly; pleura on somite 6 rounded. Pleopod 1 uniramous, unsegmented; pleopods 2–5 alike, slender, with appendix interna; pleopod 2 without appendix masculina. Uropod exopod with spinose transverse suture. Telson longer than wide, with posterior margin rounded, without posteromedian tooth. Hermaphrodite.

Type species. Ezaxius ferachevali  new species, by present designation.

Etymology. Generic name is a combination of the name Eza from the Spanish R/V Vizconde de Eza, on which the Maurit surveys were conducted, and the genus name Axius  . Gender masculine.

Remarks. Ezaxius  n. gen., described here with pereiopod 2 chelate, should be included in the infraorder Axiidea/Callianassidea (see Table 1). Within this infraorder, the following combination of characters separates the new genus from all except the Axiidae  and related families (see Table 1): the absence of linea thalassinica, the presence of a conspicuous rostrum and the presence of a long seta on maxilla 2 exopod posterior lobe. The presence of a uropodal exopod transverse suture, epipods on pereiopods 1–4 and an appendix masculina attached mesially on pleopod 2 endopod exclude the new genus from ‘Sakai et al. ’ families Eiconaxiidae  and Eiconaxiopsididae  (both without uropodal suture), Coralaxiidae (lacking epipods on pereiopods 1–4) and from Calocarididae  (in which the appendix masculina is attached terminally). Therefore, we subsume Ezaxius  n. gen. to the family Axiidea.

Since the first attempt to identify valuable characters to define axiid genera by Kensley & Gore (1981: Table 1), the various authors have grouped the species mainly based on the morphology of the rostrum, length ratio rostrum/eyestalk, carapace carinae, antennal acicle, maxilliped 3, symmetry of the chelipeds, pleonal pleura, telson and uropods, as well as on the presence or absence of maxilla 2 posterior lobe long setae, epipods, exopods, branchiae, pleopods 1 and 2, appendix masculina, appendix interna and/or uropodal exopod transverse suture (e.g. Poore 1994; Ngoc-Ho 2003; Sakai 2011).

Some of these characters must be taken with caution because they have been demonstrated to be ambivalent. Thus, although lateral and median carapace carinae is usually constant, the submedian carina can be poorly defined or blurred in different specimens of the same species, such as in Calaxius pitatucensis  (de Man, 1925) (see Sakai 2011: 104; as Colemanaxius pitatucensis  ), in Allaxius aethiopicus ( Nobili, 1904)  and in Axiopsis serratifrons ( A. Milne-Edwards, 1873)  (see Sakai 2011: 41, 56, respectively); moreover, postcervical carina can be posteriorly distinct in larger specimens but indistinct in smaller mature specimens, such as in Eutrichocheles modestus ( Herbst, 1796)  (see Sakai 2011: 112). The presence or absence of pleopod 1 can also be confusing, mainly in species descriptions based on only one specimen, because of the absence of this appendage in young specimens of some species, such as Paraxiopsis brocki  (de Man, 1888), in which pleopod 1 is a segmented flagellum in large ovigerous females but lacking in small females ( Sakai 1992 b: 216). Variations in the armature of several body structures, including rostrum, gastric submedian carinae, telson, third maxilliped, chelipeds, second pereiopods, uropods and the length of the scaphocerite, were also observed between holotype and paratype specimens of Formosaxius  dorsum Komai, Lin & Chan, 2010 by Komai et al. (2010: 3).

Our material does not share all the foregoing characters with the 19 axiid genera cited by Poore (1994), with the updated 21 axiid genera listed by Komai & Tachikawa (2007), with the 44 genera included in the Axiidae  by Sakai (2011), or with the features of the axiid genus Formosaxius Komai, Lin & Chan, 2010  . A triangular rostrum with lateral teeth continuous with lateral carinae, the presence of five gastric anterior carinae, the elongate antennal acicle, pleopods 3–5 endopods with appendix interna and the presence of a transverse suture on uropodal exopod link Ezaxius  n. gen. with Albatrossaxius Sakai, 2011  , Axiorygma Kensley & Simmons, 1988  , Balssaxius Sakai, 2011  , Guyanacaris Sakai, 2011  and Leonardaxius Sakai, 2011. However, all of them present a conspicuous posteromedian tooth on telson (vs no tooth in Ezaxius  n. gen.) and none of them was described as being hermaphrodite. Moreover, Ezaxius  n. gen. can be separated from Albatrossaxius  , Guyanacaris  and Leonardaxius by the presence of appendix masculina (vs absent in our specimen); from Balssaxius  by the cervical groove distinct along the entire length (vs dorsally but not fully laterally distinct) and from Axiorygma  by the absence of postcervical carina and pleopod 1 (vs presence). Therefore, while generic definitions in this family continue to be unclear or vague ( Kensley & Simmons 1988; Poore 1994; see above), we describe Ezaxius  n. gen. here in order to accommodate the new species Ezaxius ferachevali  n. sp.