Nennalpheus gabonensis, Anker, 2019

Anker, Arthur, 2019, The alpheid shrimp genus Nennalpheus Banner & Banner, 1981 in the tropical eastern Atlantic, with description of a new species from Gabon and new records of N. sibogae (De Man, 1910) in the Indo-West Pacific (Malacostraca: Decapoda: Caridea), Zootaxa 4646 (1), pp. 87-100 : 88-92

publication ID	https://doi.org/ 10.11646/zootaxa.4646.1.5
publication LSID	lsid:zoobank.org:pub:9DBCFAB6-5D6A-40E8-AC85-617E02879679
persistent identifier	https://treatment.plazi.org/id/B074274D-FFBF-FFFD-FF31-FCB6FDF10529
treatment provided by	Plazi
scientific name	Nennalpheus gabonensis
status	sp. nov.

Treatment

Nennalpheus gabonensis View in CoL sp. nov.

Figures 1–4 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 , 5A View FIGURE 5

Type material. Gabon. Holotype: male (cl 8.4 mm), OUMNH. ZC. 2018.01.061, Sahoué, off Cap Esterias, 0°40.474’N 9°16.243’E, scuba diving, rocky reef, under rock, depth: 11 m, leg. P. Wirtz, 27.11.2017. GoogleMaps

Description. Medium-sized alpheid shrimp (holotype at cl 8.4 mm). Carapace glabrous. Rostrum well developed but relatively short, subtriangular, about 1.5 times as long as broad at base, acute distally, falling short of mid-length of first article of antennular peduncle; rostral carina not distinct; lateral margin not setose ( Fig. 1A, B View FIGURE 1 ). Orbital hoods moderately developed, slightly swollen, covering most of eyes in dorsal view, except for anterior-most portion, and only partly concealing eyes laterally; anterior margin of each orbital hood projecting in form of broad blunt tooth, latter not reaching far beyond base of rostrum; margin between tooth and rostrum shallowly concave ( Fig. 1A, B View FIGURE 1 ). Pterygostomial angle rounded, not protruding ( Fig. 1B View FIGURE 1 ); cardiac notch well developed.

Pleon with first to fourth pleura broadly rounded; fifth pleuron with blunt posteroventral angle; sixth pleonite with well-developed articulated flap ( Fig. 1E View FIGURE 1 ); sternites of first to fifth pleonites each with strong median process, first four processes sharp diminishing in size towards posterior, fifth process shortest but also broadest of all, with bifid blunt tip ( Fig. 1C, D View FIGURE 1 ).

Telson moderately broad, subrectangular, tapering distally, more than twice as long as maximum width; dorsal surface with two pairs of stout spiniform setae both inserted at some distance from lateral margins, first pair at about telson mid-length, second pair between 0.7 and 0.8 of telson length; posterior margin protruding as broadly rounded lobe; posterolateral angles each with one pair of spiniform setae, mesial ones six to seven times longer and much stouter than lateral ones ( Fig. 1F, G View FIGURE 1 ).

Eyestalk with very large well-pigmented cornea; anteromesial angle with short blunt process visible in dorsal view ( Fig. 1A View FIGURE 1 ), but not in lateral view. Epistomial sclerite with short blunt process.

Antennular peduncle relatively slender; stylocerite long, slender, with sharp tip reaching well beyond distal margin of first article, however, not reaching mid-length of second article; ventromesial carina with anteriorly directed tooth; second article elongate, about 2.5 times as long as wide; lateral antennular flagellum with short fused portion, consisting of five subdivisions, and well-developed secondary ramus, latter with numerous groups of aesthetascs ( Fig. 1B, H View FIGURE 1 ). Antenna with stout basicerite bearing slender sharp distoventral tooth; scaphocerite elongate, with straight lateral margin and well-developed blade, latter not overreaching small sharp distolateral tooth; carpocerite reaching well beyond scaphocerite and slightly exceeding end of antennular peduncle ( Fig. 1A, B View FIGURE 1 ).

Mouthparts typical for genus in external observation (see below under N. sibogae ). Third maxilliped slender, pediform; coxa with ear-shaped, distally subacute lateral plate; antepenultimate article slightly flattened ventrolater- ally; penultimate article about 3.5 times as long as wide; ultimate article with numerous rows of short serrulate setae and longer simple setae, apex armed with four (one apparently broken) stouter spiniform setae; arthrobranch well developed ( Fig. 2A, B View FIGURE 2 ).

Chelipeds symmetrical, very similar in size and shape, carried extended, inverted, with dactylus in ventral position; ischium very short, stout; merus moderately slender, noticeably widening distally, smooth, its distodorsal and distomesial margins unarmed; carpus shorter than high, somewhat cup-shaped, with strong acute distodorsal tooth and two rounded distomesial lobes; chela elongate, with palm about twice as long as fingers, smooth, subcylindrical in cross-section; dactylus slightly shorter than pollex, articulated in the same axis as palm, not twisted; cutting edge of dactylus with one stout bulging tooth at about mid-length, excavated distally, and two much smaller, simple, rounded teeth on proximal half, in addition to shallow proximal bump; pollex with similarly positioned, distally excavated bulging tooth, opposed to dactylar tooth, and two additional smaller, simple, rounded teeth proximally; fingertips and adjacent distal areas of dactylar cutting edges corneous; distoventral surface of palm and proximal surface of dactylus with small adhesive disks ( Fig. 3 View FIGURE 3 A–G).

Second pereiopod slender; ischium and merus subequal in length; carpus with five subdivisions, first longer than sum of others, with ratio approximately equal to 6.0/1.2/1.0/1.2/1.3; chela as long as two distal-most carpal subdivisions combined ( Fig. 4A View FIGURE 4 ). Third pereiopod slender; ischium with two spiniform setae on ventrolateral surface, inserted in deep pits; merus about 10 times as long as wide, unarmed; carpus about half-length of merus, noticeably more slender than merus, unarmed; propodus much longer than carpus but shorter than merus, with four spaced small spiniform setae on ventral margin in addition to one pair of longer spiniform setae near dactylar base, distodorsal margin with single spiniform seta near dactylar base, remaining dorsal margin unarmed; dactylus about 0.6 length of propodus, slender, gradually curving, conical, simple, subdistal portion with two minute notches on dorsal margin each marked by tuft of thick setae ( Fig. 4 View FIGURE 4 B–D). Fourth pereiopod generally similar to third, more slender; ischium with two spiniform setae on ventrolateral surface, inserted in deep pits; merus about 12 times as long as wide, unarmed; carpus slightly less than half-length of merus, somewhat more slender than merus, unarmed; propodus with four spaced small spiniform setae on ventral margin in addition to one pair of longer spiniform setae near dactylar base, distal half of dorsal margin with row of five long stout spiniform setae, most-distal above dactylar base; dactylus about 0.5 length of propodus, slender, gradually curving, conical, simple, subdistal portion with two minute dorsal notches each with setal tuft ( Fig. 4 View FIGURE 4 E–G). Fifth pereiopod as slender as fourth pereiopod; ischium with two spiniform setae on ventrolateral surface, inserted in deep pits; merus about 12 times as long as wide, unarmed; carpus slightly less than half-length of merus, somewhat more slender than merus, unarmed; propodus with two spaced small spiniform setae on proximal half of ventral margin and 10 short rows of serrulate setae forming cleaning brush on distal half of ventrolateral surface; dactylus (broken on the right side in the holotype, present on the left side) slightly less than 0.5 length of propodus, slender, gradually curving, conical, simple, subdistal portion with two minute dorsal notches each with setal tuft ( Fig. 4 View FIGURE 4 H–J).

Male first pleopod with small elongate endopod, latter fringed with long setae and some stouter and shorter setae on anterior margin ( Fig. 2C, D View FIGURE 2 ). Male second pleopod with appendix masculina slightly shorter than appendix interna, with stiff setae on apex and subapical area ( Fig. 2E, F View FIGURE 2 ). Uropod with lateral lobe of protopod ending in blunt tooth; exopod broadly ovoid, with strong distolateral tooth; diaeresis strongly sinuous, with subtriangular tooth adjacent to stout spiniform seta; endopod somewhat narrower than exopod, ovoid, without specific features ( Fig. 1I View FIGURE 1 ).

Gill formula typical for genus: five pleurobranchs above first to fifth pereiopods; one arthrobranch at third maxilliped; no podobranch; mastigobranchs (strap-like epipods) on coxae of third maxilliped and first to fourth pereiopods; setobranchs (setiferous tubercles) on coxae of first to fifth pereiopods.

Colour. Most of body, tail fan and chelipeds semitransparent, uniformly speckled with fairly large, bright red chromatophores; chelae more intense red distally, with conspicuous whitish area on distal fingers, finger tips semitransparent, dark yellow-amber; walking legs with some red chromatophores proximally, mostly colourless distally; antennal flagella whitish; eyes with dark cornea ( Fig. 5A View FIGURE 5 ).

Distribution. Eastern Atlantic: presently known only from the type locality in Gabon.

Habitat. The holotype specimen was collected on a rocky reef on top of a small seamount off Cap Esterias, at about 11 m. Underwater photographs supplied by Dr. P. Wirtz show a heavily silted rocky reef with mediocre visibility (organic and non-organic matter in suspension), with rocky and sandy bottom with some algae, flat corals, sponges and gorgonians ( Fig. 5B, C View FIGURE 5 ). The shrimp was found dwelling under a large rock.

Etymology. After the country of the type locality, the Gabonese Republic; used as adjective.

Remarks. Nennalpheus gabonensis sp. nov. is easily distinguishable from both N. sibogae (see below) and N. inarticulatus by the different armature of the cheliped fingers and the presence of strong median processes on the pleonal sternites. In the new species, the dactylus and the pollex each possess a well-defined bulging tooth that is deeply excavated on its distal surface, in addition to two more proximal simple teeth ( Fig. 3E View FIGURE 3 ). In contrast, in N. sibogae and N. inarticulatus , the pollex possesses a shallow depression opposed to a very shallow bulge—rather than a distally excavated tooth—on the dactylus, in addition to five or so simple teeth on the proximal half of the finger cutting edges ( Fig. 7C, E View FIGURE 7 ; see also Banner & Banner 1981: fig. 1f). In N. gabonensis sp. nov., each of the first five pleonal sternites is armed, at least in males, by a very strong median process; these processes are gradually diminishing in size from the first to the fifth ( Fig. 1C, D View FIGURE 1 ). Such processes are absent in both males and females of N. sibogae (see below) and also in N. inarticulatus , according to the very detailed description of the latter species provided by Banner & Banner (1981). Another diagnostic feature of N. gabonensis sp. nov. that is absent in the two previously described congeners is the presence of spiniform setae on the dorsal surface of the propodus of the fourth pereiopod ( Fig. 4F, G View FIGURE 4 ). The new species differs specifically from N. sibogae by the presence of blunt orbital projections ( Fig. 1A, B View FIGURE 1 ) rather than distinct acute orbital teeth as in N. sibogae ( Fig. 6A, B View FIGURE 6 ), and from N. inarticulatus by the presence of an articulated flap on the sixth pleonite ( Fig. 1E View FIGURE 1 ), which is absent in the latter species, as indeed indicated by its scientific name ( Banner & Banner 1981: fig. 1q).

As mentioned above, the present description of N. gabonensis sp. nov. significantly extends the previously known distribution range of the genus Nennalpheus from the Indo-West Pacific (with new material of N. sibogae form the southwestern Indian Ocean, see below) to the eastern Atlantic Ocean. In addition, the two “novel” morphological characters of N. gabonensis sp. nov. that should be added to the generic diagnosis of Nennalpheus in Banner & Banner (1981) are: (1) the pleonal sternites unarmed or armed with strong median processes (at least in males, condition in females presently unknown) ( Fig. 1C, D View FIGURE 1 ), and (2) the armature of the cheliped fingers consisting either of a abroad shallow bulge on the dactylus opposed to a fairly deep depression (fossa) on the pollex and five additional teeth on proximal half ( Fig. 7C, E View FIGURE 7 ), or two stout opposing tooth-like bulges, each deeply excavated distally, and two proximal teeth ( Fig. 3B View FIGURE 3 ).