Arete cornutus, Anker, 2025

Anker, Arthur, 2025, Three new species of the shrimp family Alpheidae from the south-western Indian Ocean (Decapoda: Caridea), Zootaxa 5659 (1), pp. 51-68 : 52-56

publication ID	https://doi.org/10.11646/zootaxa.5659.1.3
publication LSID	lsid:zoobank.org:pub:CF273210-0DEA-48FC-95D0-8181EE3BC596
persistent identifier	https://treatment.plazi.org/id/3777878E-AD02-2B76-8CA1-FF6204CEFEFC
treatment provided by	Plazi
scientific name	Arete cornutus
status	sp. nov.

Treatment

Arete cornutus sp. nov.

( Figs. 1–3 View FIGURE 1 View FIGURE 2 View FIGURE 3 )

Type material. Holotype: 1 male (cl 3.0 mm), MNHN-IU-2010-5221, Madagascar, ATIMO VATAE sta. TB12, Taolagnaro , off Pointe Flacourt, 25°01.5’S 47°00.0’E, depth 4–5 m, leg. MNHN team, 14.05.2010 GoogleMaps . Paratypes: 1 male (cl 2.9 mm), MNHN-IU-2010-5208, same collection data as for holotype; 1 ov. female (cl 4.3 mm) [left P1 missing, specimen dissected], MNHN-IU-2019-5847, Madagascar, ATIMO VATAE sta. TB13, Taolagnaro, off Pointe Flacourt , 25°01.5’S 47°00.0’E, depth 2–4 m, leg. MNHN team, 15.05.2010 GoogleMaps ; 1 ov. female (cl 3.7 mm) [both P1 missing], MNHN-IU-2010-2876, Madagascar, ATIMO VATAE sta. TS21, Taolagnaro, off Pointe Flacourt , 25°01.5’S 46°60.0’E [probably 47°00.0’E], depth 2–4 m, leg. MNHN team, 15.05.2010 GoogleMaps ; 1 ov. female (cl 3.0 mm) [both P1 and tail fan missing], MNHN-IU-2019-5735, Madagascar, ATIMO VATAE sta. TB07, Taolagnaro , S of Libanona beach, 25°02’27.7”S 46°59’40.0”E, depth 4–5 m, leg. MNHH team, 09.05.2010 GoogleMaps ; 1 ov. female (cl 3.5 mm) [both P1 missing], MNHN-IU-2010-3013, Madagascar, ATIMO VATAE sta. TM21 , Ankobanalabe Bay S of Mitriaky, 25°08.9’S 46°45.4’E, depth 0–1 m, leg. MNHN team, 14.05.2010 GoogleMaps ; 2 males (cl 3.5 mm, 4.1 mm), 1 female (cl 3.4 mm), 1 ov. female (cl 3.5 mm) [all with both P1 missing, however, several loose P1 present in the vial], MNHN-IU-2018-5690, Madagascar, Toliara, depth not recorded, sea urchins (“oursins”), together with specimens of A. indicus , leg. R. Hipeau-Jacquotte , 13.02.1968 .

Description. Small-sized alpheid shrimp (type material: cl 2.9–4.1 mm). Body stout, bulky. Carapace subcylindrical, smooth, not setose. Rostrum long, acuminate, slightly lanceolate, dorsoventrally compressed, with some erect setae; tip reaching or extending slightly beyond distal margin of second article of antennular peduncle; lateral margins straight to slightly convex; dorsal carina barely distinct ( Fig. 1A, B View FIGURE 1 ). Orbit evenly and deeply concave; supracorneal teeth strong, acute, reaching half-length of cornea; extracorneal teeth prominent, acute, almost reaching distal margin of cornea; infracorneal teeth absent ( Fig. 1A, B View FIGURE 1 ). Pterygostomial angle with acute tooth ( Fig. 1B View FIGURE 1 ). Cardiac notch deep.

Pleon smooth, stout, subcylindrical, glabrous; first to fourth pleonites with pleura posteroventrally rounded; fifth pleuron posteroventrally angular; sternite of fifth pleonite without median lobe; sixth pleonite with large, triangular, articulated plate posteroventrally, posterolateral angle blunt ( Fig. 1C View FIGURE 1 ), posterior margin of sternite produced into long, blunt or subacute tooth. Telson moderately broad, tapering posteriorly, 1.7 times as long as greatest width; dorsal surface with two pairs of slender spiniform setae (one spiniform seta missing in the illustrated specimen) both inserted submarginally in posterior half; posterior margin about half as long as anterior margin, broadly rounded, with one pair of slender spiniform setae at each posterolateral angle, mesial spiniform setae much longer than lateral ones ( Fig. 1D View FIGURE 1 ).

Cornea of eye large, well pigmented, largely exposed in dorsal and lateral views ( Fig. 1A, B View FIGURE 1 ). Antennular peduncle stout; first article with distodorsal margin conspicuously toothed, ventromesial carina with stout, anteriorly directed tooth; stylocerite prominent, acuminate, reaching past mid-length of third article; second article about 1.2 times as long as wide, with distodorsal margin conspicuously toothed; third article shortest; dorsolateral flagellum with short accessory ramus and numerous groups of aesthetascs along ventral surface, fused portion composed of five subdivisions ( Fig. 1A, B, E View FIGURE 1 ). Antenna with basicerite rather stout, armed with strongly protruding, acute tooth on distolateral margin; scaphocerite slightly overreaching antennular peduncle, blade broad, distolateral tooth prominent, reaching far beyond distal margin of blade and slightly past distal end of antennular peduncle, lateral margin straight; carpocerite short, subcylindrical; flagellum (broken in most specimens) slightly thickened proximally ( Fig. 1A, B View FIGURE 1 ; see also Fig. 3 View FIGURE 3 ).

Mouthparts as figured ( Figs. 1F–K View FIGURE 1 ). Mandible with incisor process greatly expanded, armed with more than 20 small teeth; molar process rather slender; palp with three articles ( Fig. 1F, G View FIGURE 1 ). Maxillule with bilobed palp, each lobe furnished with thick seta ( Fig. 1H View FIGURE 1 ). Third maxilliped stout; coxa with strap-like epipod below somewhat protruding, distally rounded lateral (coxal) plate; antepenultimate article (ischio-merus) about 3.2 times as long as wide, with distodorsal margin conspicuously protruding in form of triangular lobe; penultimate article (carpus) almost squarish, about 1.1 times as long as wide; ultimate article (propodo-dactylus) about 0.6 times as long as antepenultimate article, noticeably tapering distally, mesial surface with dense transverse rows of microserrulate setae, apex armed with crown of four stout corneous spiniform setae; exopod long, slender, overreaching distal margin of antepenultimate article (not counting distodorsal triangular lobe); arthrobranch absent ( Fig. 1L–N View FIGURE 1 ).

Chelipeds (= first pereiopods) subsymmetrical in shape, subequal to somewhat unequal in size, carried directed forwards with dactylus in lateral to ventrolateral position, stouter in males; coxa with blunt ventral process; basis unarmed, with small ovate exopod near base; ischium with one small proximodorsal protuberance armed with stout spiniform seta, one more prominent distodorsal protuberance armed with stout spiniform seta near apex, and one prominent ventrodistal protuberance on mesial side, latter more pronounced in larger individuals together with blunt mesial lobe; merus stout, swollen in larger individuals, about 1.9–2.3 times as long as greatest width, with acutely produced distodorsal angle; carpus large, broad, somewhat square shaped, with blunt ventral process and bluntly produced distoventral angle on mesial side; palm somewhat compressed mesiolaterally, smooth, subrectangular, 1.5–1.7 times as long as wide, with blunt distolateral margins; fingers 1.3–1.5 times as long as palm, gently curved, with crossing tips; dactylus with occlusal margin evenly crenulated or armed with low teeth in proximal half; pollex with occlusal margin evenly crenulated, similarly to that of dactylus, or armed with some low teeth in proximal half and large, bulging, distally truncate tooth ranging from mid-length of pollex to about beginning of its distal third ( Fig. 2 View FIGURE 2 ).

Second pereiopod moderately stout, short; coxa with strap-like epipod and single setobranch seta; basis with rudimentary ovate exopod; ischium slightly shorter than merus; carpus with four subarticles, first about as long as remaining three combined; chela simple, about as long as first carpal subarticle ( Fig. 1O View FIGURE 1 ). Third pereiopod moderately stout; coxa without strap-like epipod, with single setobranch seta; ischium with small spiniform seta on ventrolateral surface (sometimes only trace of it); merus 3.8 times as long as wide, distal angle of ventrolateral margin protruding, forming small tooth; carpus about half-length of merus, more slender; propodus slightly longer than merus, as wide as carpus, with dozen of spiniform setae along ventral margin and three longer and stouter spiniform setae (two lateral and one mesial) on distoventral margin near propodo-dactylar articulation; dactylus stout, gently curved, about 0.25 as long as propodus, biunguiculate, secondary unguis subparallel to main (terminal) unguis ( Fig. 1P View FIGURE 1 ). Fourth pereiopod generally similar to third, slightly more slender; coxa without setobranch; distal tooth on ventrolateral margin of merus less pronounced ( Fig. 1Q View FIGURE 1 ). Fifth pereiopod generally similar to fourth; ischium unarmed; merus about 3.5 times as long as wide; propodus evenly and slightly curved towards ventral (flexor) margin, with eight spiniform setae on ventral margin, two longer and stouter spiniform setae distally, near propodo-dactylar articulation, and three rows of microserrulate setae forming grooming brush on distolateral surface; dactylar similar to those of third or fourth pereiopod ( Fig. 1R, S View FIGURE 1 ).

Male second pleopod with endopod about as long as exopod; appendix masculina slightly longer than appendix interna, with few setae on apex, one much longer ( Fig. 1T, U View FIGURE 1 ). Uropod with lateral portion of protopod faintly bilobed distally; exopod with small distolateral tooth flanked mesially by small slender spiniform seta, latter not reaching distal margin of exopod, distolateral angle not curved mesially; diaeresis sinuous, not reaching mesial margin of exopod; endopod ovate, as long as exopod, without specific features ( Fig. 1V View FIGURE 1 ). Developing embryos in ovigerous female large, about 0.7 mm x 0.5 mm in diameter ( Fig. 1W View FIGURE 1 ).

Gill/exopod formula typical for genus (e.g., Anker & Jeng 2007).

Colour pattern. Body greyish-bluish with purplish tinge due to numerous red chromatophores; white or pinkish mid-dorsal line running from rostral tip to sixth pleonite; each carapace flank with four incomplete colourless longitudinal bands (two originating from anterior margin and two from posterior margin and disappearing near carapace mid-length) and one more ventral, complete colourless longitudinal band (originating from anterior margin and running full length of carapace), most bands with streaks of white chromatophores; pleon with continuous colourless band on pleura; antennules and antennae bluish-purplish with reddish chromatophores; telson bluish with reddish chromatophores; chelipeds overall bluish-purplish with red chromatophores arranged in diffuse transverse bands or patches on merus and carpus, fingers darker, chela apparently with white longitudinal bands on palm and fingers (see below); remaining pereiopods bluish-purplish with reddish chromatophores; uropods dark blue ( Fig. 3 View FIGURE 3 ) [Note: both specimens were photographed post-mortem and out of water, resulting in artificial white light reflections from the camera flashes; with a single photograph available for each specimen, these light reflections are difficult to distinguish from the white pigmental bands that seem to be present on the cheliped palms and fingers].

Etymology. The new species’ name refers to the strong, almost horn-like ( cornutus, Latin for horned) supra-orbital teeth; used as an adjective.

Type locality. Pointe Flacourt , Taolagnaro (Port-Dauphin), southern Madagascar .

Distribution. South-western Indian Ocean: presently known only from southern and south-western Madagascar (Taolagnaro, Ankobanalabe Bay and Toliara).

Ecology. All specimens of the new species were collected on rocky and coral reefs, from the lower intertidal (ATIMO VATAE sta. TM21; R. Hipeau-Jacquotte’s material from Toliara) to about 4–5 m (ATIMO VATAE sta. TB12). The specimens from Toliara were associated with sea urchins, possibly Echinometra sp. (see below).

Remarks. Arete cornutus sp. nov. is unique within the genus Arete , as redefined by Anker & Jeng (2007), by the presence of strong, sharp supra-orbital teeth, which are reaching 0.4 length of the cornea ( Fig. 1A, B View FIGURE 1 ). In most other features, including the general shape and armature of the chelipeds, A. cornutus sp. nov. appears to be closest to A. acanthocarpus Miya & Miyake, 1968 , originally described from Japan ( Miya & Miyake 1968). In A. acanthocarpus , the supra-orbital teeth are also present but are blunt and not nearly as long as in A. cornutus sp. nov. (cf. Fig. 1A, B View FIGURE 1 ; Miya & Miyake, 1968: fig. 10D, H). The new species can be additionally separated from A. acanthocarpus by the pterygostomial angle of the carapace acutely protruding (vs. blunt in A. acanthocarpus ); the strongly toothed distal margins of the first and second articles of the antennular peduncle (vs. straight in A. acanthocarpus ); and the stronger spines on the dorsal surface and posterior margin of the telson (cf. Fig. 1D View FIGURE 1 ; Miya & Miyake 1968: figs. 10D, H, J; see also Suzukli 1970: figs. 9, 10). The colour pattern of A. cornutus sp. nov. is different from that of A. acanthocarpus in lacking the dark-brown spots on the cheliped fingers, which are characteristic of the latter species (cf. Fig. 3 View FIGURE 3 ; Suzuki 1970: fig. 8; Minemizu 2013: 111, colour photograph).

Correspondingly, A. cornutus sp. nov. can be easily separated from A. indicus Coutière, 1903 and A. dorsalis Stimpson, 1860 (both species complexes in need of revision), but also from A. kominatoensis Kubo 1942 and A. amboinensis De Man, 1910 (both species with uncertain taxonomic status), by the presence of prominent supra-orbital teeth and strongly toothed distodorsal margins of the first and second articles of the antennular peduncle (cf. Fig. 1A, B View FIGURE 1 and illustrations in De Man 1910; Kubo 1942; Suzuki 1970; Banner & Banner 1973, Bruce 1989, 1990), and from A. indicus and A. dorsalis also by colour pattern (cf. Fig. 3 View FIGURE 3 ; Gherardi 1991: fig. 2; Minemizu 2013: 110, 111, colour photographs). However, it must be noted here that the identity of most of the material reported under A. indicus and A. dorsalis by Suzuki (1970) and Banner & Banner (1973) is highly questionable ( Bruce 1989, 1990; Anker & Jeng 2007). A long overdue revision of Arete is currently impeded by lack of fresh material (with colour photographs and reliable echinoid host data) from type localities of numerous forms currently considered as junior synonyms of either A. indicus or A. dorsalis (e.g., Banner & Banner 1960, 1973; Miya & Miyake 1968; Suzuki 1970).

In the closely related genus, Athanas Leach, 1814 , which differs from Arete mainly by the presence of five subarticles in the second pereiopod carpus ( Anker & Jeng 2007), the only species with robust, non-foldable chelipeds and prominent supra-orbital teeth is the wide-ranging A. areteformis Coutière, 1903 (presumably a species complex). However, A. areteformis is not known to associate with sea urchins and has a very different colour pattern ( Minemizu 2013: 109, colour photograph), in addition to numerous other morphological differences with the new species (cf. Coutière 1903; Banner & Banner 1973). On the other hand, two other species of Athanas with chelipeds carried extended are known to be symbionts of sea urchins in temperate waters, viz. A. granti Coutière, 1908 (southern Australia) and A. mendax Ahyong, 2015 (Kermadec Islands, north-east off New Zealand) ( Coutière 1908; Banner & Banner 1973; Ahyong 2015). Both species can be readily distinguished from A. cornutus sp. nov. by the absence of supra-orbital teeth and several other details of morphology (see also below).

Even though none of the ATIMO VATAE specimens of A. cornutus sp. nov. was accompanied by notes on specific sea urchin hosts, R. Hipeau-Jacquotte’s specimens from Toliara (MNHN-IU-2018-5690) were found in a larger lot together with specimens of A. indicus , suggesting that the host of both species may be Echinometra mathaei ( Blainville, 1825) or a closely related species (cf. Gherardi 1991; Gherardi & Calloni 1993). A likely association of A. cornutus sp. nov. with Echinometra sp. is also supported by the striped type of colour pattern, which also characterises A. indicus and A. acanthocarpus ( Suzuki 1970; Gherardi 1991; Minemizu 2013). Nevertheless, only collection of specimens of A. cornutus sp. nov. in association with Echinometra sp. could confirm the herein presumed symbiosis.

As noted by Ahyong (2015), A. mendax presents several intermediate characters between Athanas and Arete . In the general shape of the chelipeds, telson (including size and position of spiniform setae), third maxilliped, antennular peduncle, third to fifth pereiopods (including stout biunguiculate dactylus), as well as the greatly expanded incisor process of the mandible and posteriorly acuminate sternite of the sixth pleonite, A. mendax approaches species of Arete . On the other hand, the second pereiopod carpus with five subarticles in adults and the presence of a strap-like epipod (mastigobranch) on the third pereiopod, both features being present in A. mendax , are diagnostic characters of Athanas . Therefore, the generic assignment of A. mendax and more generally the recognition of Arete as a separate genus from Athanas (for detailed discussions see Banner & Banner 1960 and Anker & Jeng 2007) would require confirmation by a molecular phylogenetic analysis of the entire athanoid clade ( Anker et al. 2006; Chow et al. 2021; Anker 2022a). Within this clade, two or three instances of ecological convergence, i.e., symbioses with echinoids and the resulting convergent morphological adaptations, are exemplified by (1) species currently assigned to Arete , (2) A. granti (which appears to be more closely related to A. areteformis and A. nitescens ( Leach, 1814) than to A. mendax , despite the shared presence of a flap on the sternum of the fifth pleonite in A. granti and A. dorsalis sensu Banner & Banner 1973 ), and perhaps (3) A. mendax (if shown to be neither a sister taxon to nor nested within Arete ).