Crosnierocaris Anker 2022, gen. nov.

Crosnierocaris gen. nov.

Diagnosis. Body not significantly compressed, not elongate. Carapace smooth, unarmed dorsally and laterally. Rostrum elongate, slender, about as long as half of carapace length, unarmed dorsally, with subdistal ventral tooth. Orbital teeth present as stronger sharp extra-corneal teeth and weaker blunt infra-corneal teeth. Pterygostomial angle produced in stout sharp tooth. Cardiac notch well developed. Third to fifth pleura armed with two to four small, sharp, well-spaced teeth on distoventral margins; sixth pleuron with articulated plate. Telson slender, with two or more pairs of stout cuspidate setae on dorsal surface; posterior margin rounded, with two pairs of spiniform setae; anal tubercles absent. Eyes well developed, largely visible in dorsal and lateral views; cornea large, well pigmented. Antennules with well-developed, distally acute stylocerite, distinctly overreaching distal margin of first article of peduncle. Antenna with sharp distolateral tooth on basicerite; scaphocerite well developed, slender. Mandible with two-articulated palp; molar and incisor processes well developed. Third maxilliped slender, with long exopod; coxa with small lateral plate; ultimate article tapering into corneous tip, with two stout subapical spiniform setae. First pereiopods (= chelipeds) not particularly enlarged in both sexes, stouter but shorter than walking legs, equal, symmetrical; ischium unarmed; carpus with setal rows on mesial surface; chela small, simple; fingers with cutting edges unarmed, without snapping mechanism. Second pereiopod with ischium unarmed; carpus with five subdivisions, first longest; chela simple. Third to fifth pereiopods slender; ischia of third and fourth pereiopods and meri of third to fifth pereiopods with cuspidate setae; propodus of third pereiopod with numerous spiniform setae disposed in two rows; propodi of fourth and fifth pereiopods with single row of spiniform setae, that of fifth pereiopod with well-developed cleaning brush; dactyli biunguiculate. Second male pleopod with appendix masculina reaching far beyond appendix interna and exceeding endopod; second female pleopod with appendix interna only. Uropodal protopod with lateral lobe projecting as long sharp tooth; exopod with lateral portion of diaeresis strongly dentate. Gill-exopod formula as given in Table 1.

* represented by the coxal lateral plate on the third maxilliped.

** refers to sets (tufts) of typically two or three individual setae.

Etymology. The new genus is named after the eminent French carcinologist, Dr. Alain Crosnier (1930–2021), in recognition of his immense contributions to taxonomy of decapod crustaceans. The generic name is composed of Dr. Crosnier’s last name and the Greek word Karis (Latinised caris), for shrimp, with the letter “o” added between them for euphony. Gender feminine.

Type species. Crosnierocaris athanasoides sp. nov., by monotypy and present designation.

Distribution. Currently known only from the south-western Indian Ocean (Mozambique Channel).

Remarks. Crosnierocaris gen. nov. remarkably combines features of three alpheid genera that are not particularly closely related, viz. Athanas, Potamalpheops and Yagerocaris, although the latter two genera share a number of plesiomorphic characters and were recovered in a “basal” position in the phylogenetic analysis of the Alpheidae by Anker et al. (2006a). In the more recent molecular analysis of alpheid shrimps (Chow et al. 2021), in which Crosnierocaris gen. nov. was not included, Potamalpheops and Athanas (+ several satellite athanoid genera) were recovered in the same clade of “higher” alpheids (part of Clade S), whereas Yagerocaris was recovered as a “basal” lineage distant from Clade S.

The frontal region of Crosnierocaris gen. nov. is remarkably similar to that of many species of Athanas (Banner & Banner 1960, 1973; Anker 2003). The similarities include the general shape of the rostrum, especially in dorsal view, the presence of extra- and infra-corneal teeth, as well as the largely exposed, well-pigmented eyes. The rostrum of Crosnierocaris gen. nov. reaches half of the carapace length in most individuals of the type series and is therefore longer than the rostra of Athanas spp. The long and slender rostrum of the new genus in fact represents the proportionally longest rostrum relative to the total body length among alpheid shrimps. Only a few other alpheid shrimps have elongate rostra that approach that of Crosnierocaris gen. nov., such as some species of Triacanthoneus Anker, 2010, Salmoneus Holthuis, 1955 and Synalpheus Spence Bate, 1888 (De Man 1911; Anker 2010; Alvarez et al. 2012; De Grave et al. 2020).

The presence of non-enlarged chelipeds is another important diagnostic feature of the new genus. Similar, feebly enlarged chelipeds are known in some species of Athanas (especially in females) and in the closely related Acanthanas Anker, Poddoubtchenko & Jeng, 2006 (Banner & Banner 1960, 1973; Anker et al. 2006b). The presence of a triangular articulated flap on the sixth pleonite is another feature shared by Crosnierocaris gen. nov. and Athanas, but also many other, morphologically less similar alpheid genera (Anker et al. 2006a). In several other characters, Crosnierocaris gen. nov. differs from Athanas and does not seem to belong to the athanoid clade recovered by Anker et al. (2006a: fig. 4, clade AP). The most important differences between Crosnierocaris gen. nov. and Athanas are the rostrum with a subdistal ventral tooth (tooth absent in Athanas and related genera); the third to fifth pleura with distoventral margins armed with teeth (unarmed in Athanas and all other alpheid genera, except for the acute posterolateral angle in some, see below); the mesial surface of the cheliped carpus with several rows of short serrulate setae (lacking in Athanas and related genera); the presence of stout cuspidate setae on the meri of the third to fifth pereiopods (absent in Athanas and related genera, except for Athanopsis Coutière, 1897); the uropodal exopod with a strongly dentate diaeresis (never dentate in Athanas and related genera, although armed with stout spiniform setae in Pseudathanas Bruce, 1983); and the presence of a small arthrobranch at the base of the third maxilliped (absent in Athanas and related genera).

The pterygostomial angle of Crosnierocaris gen. nov. is produced into a strong sharp tooth, one of the most important diagnostic characters of the genus. In the majority of species of Athanas, the pterygostomial angle is rounded (Banner & Banner 1960, 1973) or armed with a minute acute tooth (Anker 2003). However, in one Japanese species, A. squillophilus Hayashi, 2002, the pterygostomial angle is anteriorly projecting, forming a sharp tooth as large as the one in Crosnierocaris gen. nov. (Hayashi 2002). Similarly, the gill formula of Athanas typically typically does not include a mastigobranch on the coxa of the fourth pereiopod and a set of setobranchs on the coxa of the fifth pereiopod (Anker & Jeng 2007), in contrast to Crosnierocaris gen. nov. However, since the presence of these structures was reported (perhaps erroneously) in at least one species of Athanas, it should be used only as an additional distinguishing character between the two genera.

Crosnierocaris gen. nov. appears to share more features with Potamalpheops, a relatively small (currently 15 described species), but widely distributed and ecologically diversified genus (e.g. Powell 1979; Anker 2003; Soledade et al. 2014; Christodoulou et al. 2019). These features are the presence of an articulated plate on the sixth pleonite; the presence of cuspidate setae on the meri of the third to fifth pereiopods; the uropodal exopod with a dentate diaeresis; and the identical gill-exopod formula, including the presence of an arthrobranch at the base of the third maxilliped and the presence of a mastigobranch and setobranchs on the coxae of the fourth and fifth pereiopods, respectively. In most (but not all) species of Potamalpheops, the mesial surface of the cheliped carpus is furnished with several rows of short serrulate setae (Anker 2003; Anker et al. 2006a; Christodoulou et al. 2019), which represents another feature linking Potamalpheops to the new genus. In addition, the rostrum of some species of Potamalpheops is armed with a subdistal ventral tooth, which is also present in Crosnierocaris gen. nov. However, Crosnierocaris gen. nov. and Potamalpheops can be separated by the pterygostomial angle projecting as a strong sharp tooth in the new genus versus rounded or rounded to angular in Potamalpheops; the distoventral margins of the third to fifth pleura armed with teeth in the new genus versus unarmed in Potamalpheops; the third pereiopod propodus armed with a double row of numerous spiniform setae (20 or more in both rows) in Crosnierocaris gen. nov. versus armed with a single row of at most 10 (usually much less) spiniform setae in Potamalpheops; the dactyli of the third to fifth pereiopod biunguiculate in Crosnierocaris gen. nov. versus simple in Potamalpheops; and the diaeresis of the uropodal exopod armed with eight or so large triangular teeth versus finely serrated, i.e. with minute, more numerous teeth, in Potamalpheops. It is also important to note that all 15 presently known species of Potamalpheops occur in marine shallow-water or freshwater habitats, such as mangrove mudflats, estuaries, brackish lagoons, small rivers, streams and lakes, anchialine and freshwater caves (Powell 1979; Hobbs 1983; Yeo & Ng 1997; Anker 2003; Christodoulou et al. 2019), whilst all specimens of Crosnierocaris athanasoides sp. nov. were collected in deeper offshore waters (240–255 m, see below).

Crosnierocaris gen. nov. also shares some features with the monotypic genus Yagerocaris, presently known only from marine caves in Mexico (Kensley 1988). Among these characteristics are the anteriorly projecting pterygostomial angle (in Yagerocaris forming a more posteriorly originating, distinct tooth); the pleura of the third to fifth pleonites with distoventral margins armed with at least one acute tooth; the meri of the third to fifth pereiopods armed with cuspidate setae; and the presence of a complete gill-exopod formula, with a mastigobranch on the fourth pereiopod (the latter two features also shared by Potamalpheops). On the other hand, the armature of the distoventral margin of the third, fourth and fifth pleura of Crosnierocaris gen. nov. differs from that of Yagerocaris. In the new genus, the distoventral margin of each of these pleura typically bears two, sometimes up to four, small, acute, well-spaced teeth. This armature appears to be unique within the family Alpheidae. In contrast, in Yagerocaris, the distoventral angles of the third to fifth pleura are each produced into a single sharp tooth (Kensley 1988: fig. 4). In addition, Crosnierocaris gen. nov. can be separated from Yagerocaris by the very different configuration of the frontal margin of the carapace and telson, as well as by the presence of an articulated plate on the sixth pleonite, which is lacking in Yagerocaris (Kensley 1988: figs. 4, 6F, G).

Although the exact phylogenetic position of Crosnierocaris gen. nov. presently remains unknown, the new genus clearly represents one of the morphologically least derived lineages within the family Alpheidae, with a number of plesiomorphic features. Some of the most notable plesiomorphies (or presumed plesiomorphies) of the new genus are (1) the unusually long rostrum; (2) the incomplete protection of the eyes; (3) the very long exopodites of the first and second maxillipeds; (4) the feebly developed chelipeds, with setal rows on the mesial surface of the carpus; and (5) the dorsal surface of the telson occasionally with more than two pairs of spiniform setae (very rare in alpheid shrimps, but also known in a few other genera). Based on these features, the phylogenetic position of Crosnierocaris should be close to Potamalpheops and allied genera in the morphology-based phylogeny of Anker et al. (2006a) and somewhere between Potamalpheops and the athanoid genera in the molecular phylogeny of Chow et al. (2021).

Noteworthy is the presence of an unusually elongate appendix masculina in Crosnierocaris gen. nov., Yagerocaris and some species of Athanas and Potamalpheops (Coutière 1911; Kensley 1988; Yeo & Ng 1997; Christodoulou et al. 2019). In these taxa, the appendix masculina is at least twice as long as the adjacent appendix interna and sometimes even longer, reaching far beyond the end of the endopod. The question remains open as to why the elongation of the appendix masculina is relatively more common in these “lower” alpheids, although it is also known in a few “higher” alpheid taxa (Anker et al. 2006a).