Metabetaeus mcphersonae, Anker, Arthur, 2010

Anker, Arthur, 2010, Metabetaeus Borradaile, 1899 revisited, with description of a new marine species from French Polynesia (Crustacea: Decapoda: Alpheidae), Zootaxa 2552, pp. 37-54 : 40-48

publication ID	https://doi.org/ 10.5281/zenodo.196825
DOI	https://doi.org/10.5281/zenodo.6206640
persistent identifier	https://treatment.plazi.org/id/797C87A3-FFC3-6224-FF25-A1F9FAF6FD35
treatment provided by	Plazi
scientific name	Metabetaeus mcphersonae
status	sp. nov.

Treatment

Metabetaeus mcphersonae View in CoL n. sp.

Figs. 1–7 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7

Material examined. Holotype: ovigerous female (cl 3.0 mm, tl 8.1 mm), FLMNH UF Arthropoda 23247 , French Polynesia, Society Islands, Moorea, in front of Hilton Hotel , 149.84247 W, 17.47564 S, depth: 13-17 m, fore-reef, digging and fanning of coarse sand and small-sized rubble, collector S. McPherson (processed by A. Anker), 5 December 2009 (fcn BMOO 9789). GoogleMaps Paratypes: 1 male (cl 2.1 mm, tl 6.3 mm), FLMNH UF Arthropoda 23249 (fcn BMOO 9791); GoogleMaps 1 ovigerous female (cl 2.8 mm, tl 8.3 mm, dissected), FLMNH UF Arthropoda 23250 (fcn BMOO 9788); GoogleMaps 1 ovigerous female (cl 2.4 mm, tl 7.1 mm), FLMNH UF Arthropoda 23248 (fcn BMOO 9790); all same collection data as for holotype. GoogleMaps

Description. Carapace smooth, not setose. Frontal margin with rostrum and extra-corneal teeth; rostrum variable in shape, triangular or subtriangular, as broad as long or longer than broad, reaching to mid-length of first segment of antennular peduncle; orbital teeth well developed, acute distally, directed more or less mesially ( Figs. 1 View FIGURE 1 A, 4A). Pterygostomial angle rounded, not protruding anteriorly. Posterior margin with welldeveloped cardiac notch. Epistomial sclerites not projecting ventrally. Ocellar beak obscure, not visible in lateral view. Eyestalks completely concealed in dorsal and lateral views; cornea well developed, ovalelongate, in lateral position ( Figs. 1 View FIGURE 1 A, 4A, 7A).

Pleura of second to fourth abdominal somites rounded; fifth pleuron with posteroventral angle produced into small subacute tooth; sixth somite without distinct articulated plate at posteroventral angle (indistinct suture visible in living specimens); preanal plate rounded. Telson subrectangular, slightly tapering posteriorly; dorsal surface with two pairs of strong spiniform setae inserted first at mid-length and second at 0.7–0.8 length of telson, at some distance from edges; posterior margin rounded medially, each angle with two pairs of strong spiniform setae, mesial distinctly longer than lateral; central area with three long plumose setae ( Fig. 1 View FIGURE 1 G).

Antennular peduncle reaching to end of antennal carpocerite, greatly overreachiung scaphocerite; first segment with stylocerite reaching to 0.3 length of second segment, with acute tip; mesioventral carina with strong, anteriorly directed tooth ( Fig. 1 View FIGURE 1 C); second segment shorter than dorsally visible portion of first segment, somewhat longer than wide ( Fig. 1 View FIGURE 1 A, 4A). Antenna with basicerite bearing strong, sharp distoventral tooth ( Fig. 1 View FIGURE 1 B); carpocerite significantly overreaching scaphocerite; scaphocerite ovate, with strong, sharp distolateral tooth and convex distal margin, latter slightly exceeding distolateral tooth ( Figs. 1 View FIGURE 1 A, 4A).

Mouthparts typical to genus. Mandible with two-segmented palp, well-developed molar process and incisor process bearing at least six teeth distally and large dark-brown spot proximally ( Fig. 2 View FIGURE 2 A, B). Maxillule with bilobed palp, each furnished with one seta; basal endite with particularly strong spine-like setae ( Fig. 2 View FIGURE 2 C). Maxilla with simple endopod; basal endite rounded and notched distally ( Fig. 2 View FIGURE 2 D). First maxilliped with non-segmented endopod; caridean lobe of exopod moderately developed ( Fig. 2 View FIGURE 2 E). Second maxilliped without specific features, as illustrated ( Fig. 2 View FIGURE 2 F). Third maxilliped with blunt lateral plate above mastigobranch; antepenultimate segment long, flattened lateroventrally, ventromesial margin setose; penultimate segment about twice as long as wide; ultimate segment tapering distally, with minute spine-like seta near blunt tip ( Fig. 2 View FIGURE 2 G, H), with transverse rows of finely serrulate setae dorsomesially ( Fig. 2 View FIGURE 2 H); arthrobranch present, small, composed of a few lamellae.

First pereiopods (= chelipeds) somewhat polymorphic, especially in the development of chelae; ischium slender, with usually four slender spiniform setae on dorsal margin ( Figs. 3 View FIGURE 3 A, 4B); merus flattened on ventral side, ending bluntly distoventrally and distodorsally, ventromesial margin with row of five to ten slender spiniform setae ( Figs. 3 View FIGURE 3 A, B, 4B); carpus variable in shape from subcylindrical, more than twice as long as wide ( Fig. 3 View FIGURE 3 A), to vase-shaped, widening distally ( Fig. 4 View FIGURE 4 B); chela varying in proportions from slender, not swollen, subcylindrical, with palm about three times as long as high ( Fig. 3 View FIGURE 3 A, C), to somewhat swollen, more ovoid, with palm twice or slightly more as long as high ( Fig. 4 View FIGURE 4 C, E, F); palm smooth, setose along dorsal and ventral margins; length ratio fingers to palm variable approximately from 0.8–0.9 in non-enlarged chelae to 0.6–0.7 in enlarged chelae; fingers unarmed in non-enlarged chelae ( Fig. 3 View FIGURE 3 C) or armed with blunt teeth on proximal half of cutting edge in enlarged chelae ( Fig. 4 View FIGURE 4 C–F), with elongate stiff setae.

Second pereiopod long, slender, ischium and merus subequal in length; carpus five-segmented, with ratio of segments being approximately equal to 2: 1: 1.5: 0.7: 1.2; chela simple, with palm longer than fingers ( Fig. 3 View FIGURE 3 D). Third pereiopod slender; ischium with two spiniform setae on ventrolateral surface; merus slightly more than six times as long as wide, with three spiniform setae on ventrolateral surface; carpus more slender than merus, unarmed; propodus more slender than carpus, with three to five spiniform setae on ventral margin; dactylus simple, conical, faintly curved, about 0.25 length of propodus, with slightly thickened setae inserted at about 0.7 of dactylar length. Fourth pereiopod very similar to third except for being more slender; merus with one or two spiniform setae on ventrolateral surface ( Fig. 3 View FIGURE 3 G). Fifth pereiopod as slender as fourth; ischium and merus unarmed; propodus with spiniform setae and some rows of setae distally ( Fig. 3 View FIGURE 3 H).

First pleopod with endopod less than half-length of exopod ( Fig. 1 View FIGURE 1 D); female second pleopod with appendix interna ( Fig. 1 View FIGURE 1 E); male second pleopod with appendix interna and appendix masculina, latter elongate, reaching beyond endopod, furnished with at least three strong setae ( Fig. 4 View FIGURE 4 G). Uropod with lateral lobe of protopod obscurely bifid, main ramus somewhat projecting, acute; exopod with diaeresis bearing large subtriangular tooth adjacent to strong, long distolateral spiniform seta; endopod with conspicuous tooth on distolateral margin ( Fig. 1 View FIGURE 1 F).

Colour pattern. Semitransparent with reddish chromatophores forming more or less distinct bands, two on the carapace and six on the abdomen; scattered red chromatophores also on antennular peduncles, chelipeds, walking legs and tail fan; eggs / embryos dark brown-red ( Figs. 5–7 View FIGURE 5 View FIGURE 6 View FIGURE 7 ).

Size range. Females at cl 2.4–3.0 mm, tl 7.1–8.3 mm; male at cl 2.1 mm, tl 6.3 mm.

Etymology. This very interesting new species is named after its collector, Sarah McPherson (FLMNH).

Ecology. All specimens were collected by digging and fanning of coarse sand and rubble on a fore-reef, at a depth range of 13– 17 m.

Biological notes. All three ovigerous females of M. mcphersonae n. sp. (holotype and two paratypes) were carrying a low number (4–18) of large embryos (relative to body size, see Figs. 5 View FIGURE 5 A, 6A; diameter of embryos ~ 0.6–0.8 mm, see Fig. 1 View FIGURE 1 D). Some embryos hatched into relatively advanced zoeae ( Fig. 7 View FIGURE 7 G), suggesting abbreviated larval development.

Type locality. Moorea, Society Islands, French Polynesia.

Distribution. Presently known only from the type locality in French Polynesia (see Fig. 11 View FIGURE 11 ).

Remarks. Metabetaeus mcphersonae n. sp. differs from both M. minutus and M. lohena by the better developed cornea (compare Figs. 1 View FIGURE 1 A, 4A, 8A; see also Banner & Banner 1960, figs. 1a, 2a); the more conspicuous distolateral tooth on the uropodal endopod (compare Figs. 1 View FIGURE 1 F, 8F); the propodus of the third and fourth pereiopods armed with only a few spiniform setae (vs. numerous spiniform setae in M. minutus and M. lohena ; compare Figs. 3 View FIGURE 3 E, G, 8D); and the presence of a small arthrobranch at the base of the third maxilliped (absent in M. minutus and M. lohena ). In addition, in most specimens of M. mcphersonae n. sp., the chela palm is stouter and more swollen, while the cutting edges of finger have a fairly strong proximal dentition ( Fig. 4 View FIGURE 4 D); in M. minutus and M. lohena , the palm is more slender, while the cutting edges are either straight or bearing minute teeth proximally ( Figs. 9 View FIGURE 9 D, 10B, D). Furthermore, in M. lohena , the fingers are more slender and elongate, usually longer than the palm, and somewhat twisted ( Fig. 10 View FIGURE 10 ). The length of the stylocerite is apparently variable in both M. minutus and M. lohena ( Banner & Banner 1983) ; nevertheless, in all available specimens of M. mcphersonae n. sp., the stylocerite always exceeds the distal margin of the first segment of the antennular peduncle, reaching to at least 0.2 of the second segment ( Fig. 1 View FIGURE 1 A, 4A). The new species also differs from M. minutus and M. lohena by its much smaller size (maximum cl 3 mm vs. cl 5.8 in M. minutus and cl ~6.0 in M. lohena ), and ecologically by its association with marine coral reef habitats rather than with land-locked anchialine pools and caves, which are typical habitats of M. minutus and M. lohena .

Banner & Banner (1983) reported as Metabetaeus sp. five incomplete specimens (all missing their chelipeds) from Tuléar, southwestern Madagascar. These specimens were collected by an airlift suction sampler on the outer reef front ("gravely sedimentary accumulation at the foot of a vertical cliff along the edges of an outer creed [= outer re-entrant] of the outer reef front”), at a depth of about 7 m. The whereabouts of these specimens are unknown and their taxonomic identity remains unclear. Banner & Banner (1983) tried to provide a plausible explanation for the presence of Metabetaeus in rubble substrate of a coral reef, e.g., by the possible proximity of karstic systems (according to B. Thomassin) or the tolerance of saltwater by hypogeal euryhaline shrimps (according to J. Maciolek). However, the finding of M. mcphersonae n. sp. on a fore-reef in Moorea, where no karstic systems are known, suggests that the ancestors of the three recent species of Metabetaeus were probably truly marine shrimps associated with rubble and/or deep reef matrix. Following this hypothetical scenario, M. mcphersonae n. sp. remained a truly marine shrimp, while ancestors of M. minutus and M. lohena invaded anchialine habitats. A similar evolutionary scenario was proposed for another anchialine cave alpheid, Bermudacaris harti Anker & Iliffe, 2000 (see Anker & Iliffe 2000). An alternative scenario, i.e. M. mcphersonae n. sp. having evolved from anchialine ancestors, is less likely considering the marine ancestry of the vast majority of alpheid shrimps, but cannot be totally excluded.