Strianassa, Anker, 2020

Anker, Arthur, 2020, Strianassa lerayi gen. et sp. nov., a new laomediid mud-shrimp from the eastern Pacific, with new records of Axianassa ngochoae Anker, 2010 and Heteroaxianassa heardi (Anker, 2011) in the western Pacific (Malacostraca: Decapoda: Gebiidea), Zootaxa 4820 (3), pp. 523-539 : 524-525

Genus Strianassa gen. nov.

Diagnosis. Body weakly sclerotised, with relatively thin integument. Carapace elongate; cervical groove deep, crescent-shaped in dorsal view; linea thalassinica complete. Rostrum broadly rounded-triangular; anterior margin with strong apical tooth; lateral margins with strong, widely spaced teeth; dorsal surface with several rows or fields of more or less strong teeth, mid-dorsal area and areas adjacent to lateral margins shallowly depressed. Eyestalks short, anteriorly rounded, with relatively small cornea, not visible in dorsal view, visible in lateral view. Pleon moderately elongate and slender; first pleuron with blunt ventrolateral process; remaining pleura ventrolaterally rounded. Telson lacking teeth or spines, broadly rounded posteriorly. Antennular peduncle with third article extremely elongate, slender; dorsal flagellum much more robust and longer than ventral. Antennal scale (scaphocerite) short, daggerlike; fourth article of peduncle extremely elongate, slender. Mandible with three-articulated palp; incisor and molar processes partly fused; molar process with few cusps; incisor process well developed, square-shaped, with numerous distal teeth. Maxillule with slender, two-articulated palp. Maxilla with scaphognathite bearing several long setae extending posteriorly into branchial chamber. Third maxilliped with short, non-subdivided exopod; ischium with well-developed crista dentata; ventrolateral margin of merus with small spaced teeth distally. First pereiopods (chelipeds) somewhat unequal in length and asymmetrical in shape; ischium and merus with spaced teeth on ventral and ventromesial margin; carpus with blunt teeth on ventrolateral surface. Third to fifth pereiopods with dactyli strongly flexed, their posterior margin being in dorsal position; extensor margin with rows of stout, corneous, spiniform setae. Fifth pereiopod subtly subchelate, its base not covered by carapace. First pleopod absent in male. Second to fifth pleopods biramous, with narrowly lanceolate exopod and endopod, latter lacking appendices internae. Uropodal exopod and endopod both lacking transverse suture; exopod with teeth on lateral margin and dorsal surface; endopod with teeth on median dorsal ridge. Gill-exopod formula provided in Table 1.

Etymology. The generic name combines the official abbreviation of the Smithsonian Tropical Research Institute / Instituto Smithsonian de Investigaciones Tropicales / STRI (Stri-) and the last six letters of the genus Axianassa (- anassa), a superficially similar and phylogenetically putatively closely related genus. Gender feminine.

Type species. Strianassa lerayi sp. nov., by monotypy and present designation.

Distribution. Presently known only from the tropical eastern Pacific.

Discussion. Within the family Laomediidae , Strianassa gen. nov. is morphologically closest to Axianassa , Heteroaxianassa and Saintlaurentiella , but can be easily separated from them by the dorsally toothed rostrum and the presence of a short exopod on the third maxilliped. Other differentiating features include the presence of what appears to be a set of five setobranchs on the third maxilliped (see below) and the presence of a small podobranch on the fourth pereiopod. In all other morphological characters, e.g., the general configuration of the carapace, pleon, telson, mouthparts, chelipeds and other pereiopods, Strianassa gen. nov. is similar to Axianassa and Heteroaxianassa , and perhaps to a lesser extent, to Saintlaurentiella .

The fronto-rostral area of Strianassa gen. nov. is somewhat reminiscent of that of many members of the family Upogebiidae . The presence of strong teeth on the dorsal surface of the rostrum, where most of them are arranged in longitudinal rows between shallow longitudinal depressions, is a novel character for the family Laomediidae , although in some species of Naushonia , the most proximal rostral area and/or the adjacent post-rostral area may bear short rows of small teeth (e.g. Anker 2014: fig. 2C; Komai & Anker 2015: figs. 4A, 7A). In all other laomediid genera, including Laomedia and the large and more heavily calcified Jaxea , the dorsal surface of the rostrum is unarmed ( Bouvier 1940; Le Loeuff & Intès 1974; Kensley & Heard 1990; Ngoc-Ho 1997; Anker 2010, 2011; Komai 2014; Anker & Pachelle 2016).

The presence of an exopod on the third maxilliped appears to be an ancestral feature within the Laomediidae as it is well developed, typically composed of three articles, in Laomedia , Jaxea and Naushonia ( Selbie 1914: pl. XV, fig. 8; Sakai 1962: pl. VI, fig. 14; Anker 2014: fig. 3F), absent in Axianassa , Heteroaxianassa and Saintlaurentiella ( Le Loeuff & Intès 1974; Kensley & Heard 1990; Anker 2010, 2011; Komai 2014; Komai & Fujita 2019), and relatively short, but far from rudimentary, in Strianassa gen. nov. Thus, in the development of the third maxilliped exopod, the new genus seems to be in an intermediate position between the genera Laomedia , Jaxea and Naushonia , all three with a well-developed, three-articulated exopod, and the genera Axianassa , Heteroaxianassa and Saintlaurentiella , in which the exopod was presumably lost on this appendage.

The five long flexible setae on the coxa of the third maxilliped are here interpreted as setobranchs, based on their shape and position. They may aid in grooming of the hypertrophied podobranch of the second maxilliped and/or other gills. Setobranchs are present on the third maxilliped of Jaxea nocturna Nardo, 1847 ( Selbie 1914: pl. XV, fig. 8) and Laomedia astacina De Haan, 1841 ( Batang et al. 2001: figs. 3, 4A, B), but to the author’s best knowledge, have not been reported in other laomediid genera, although they may actually be present but concealed by the voluminous epipodal complex. They seem to be absent in Axianassa and two species assigned to Heteroaxianassa by Sakai (2016). In the rather disorganised and confusing redescription of L. astacina, Sakai (1962) reported a “trichobranch” on the coxa of the “second pereiopodite” (= second maxilliped?) and illustrated what looks like a set of setobranchs on the coxa of the “seventh pereiopodite” (= fourth pereiopod?, cf. Sakai 1962: pl. VII, fig. 23), but did not mention nor illustrate such structures on the coxa of the third maxilliped (cf. ibid.: pl. VI, fig. 14). However, the presence of setobranchs on the coxa of the third maxilliped in L. astacina was confirmed by Batang et al. (2001), who called them setobranch setae or SB-setae, following Bauer (1998). According to these authors, the typical setobranch setae, such as those of caridean shrimps, arise from small coxal papillae (= setobranchs in Bauer 1998) and are multidenticulate, i.e. bearing “digitate scale setules”. However, in Laomedia and apparently also in Strianassa gen. nov., they arise from depressions or “sunken sockets”, not from papillae. If the setae on the coxa of the third maxilliped of Strianassa gen. nov. are shown to be microscopically multidenticulate (e.g. by SEM), they may well be homologous to the setobranch setae of L. astacina . The tuft of setae on the fourth pereiopod (?) of L. astacina illustrated by Sakai (1962) likely correspond to one of the two other types of coxal setae present on the pereiopods of this species and also in Thalassina anomala Herbst, 1804 (Thalassinidae) , discussed and illustrated by Batang & Suzuki (1999) and Batang et al. (2001).