Triacanthoneus alacranes, Anker, Arthur, 2010

Anker, Arthur, 2010, A new genus and three new species of alpheid shrimps (Crustacea, Decapoda, Caridea) from the tropical American coasts, Zootaxa 2652, pp. 47-63 : 57-62

Triacanthoneus alacranes n. sp.

( Figs. 7–9 , 10 G)

Type material. Holotype: ovigerous female (CL 3.7 mm), UNAM-CNCR 26177, Mexico, Yucatán, Arrecife Alacranes ( Alacranes Reef), sta ALN39, La Anegada, 22°27'37.2''N, 89°36'40.6''W, shallow flat with sand and abundant rocks and coral rubble, near channel, depth 0.5 m, in crevices under rocks, coll. N. Simões and J. Duarte-Gutiérrez, 11 August 2009, fcn JD-061B.

Description. Small-sized alpheid shrimp. Carapace with one strong sharp mediodorsal tooth situated in posterior third of carapace and two strong sharp dorsolateral teeth situated slightly ventral and slightly anterior to mediodorsal tooth, and slightly posterior to mid-length of carapace, significantly posterior to hepatic area ( Figs. 7 , 8 B–E). Rostrum straight, more than 1.5 times as long as broad at base; lateral margins slightly concave; tip overreaching distal margin of first segment of antennular peduncle; ventral margin with small, anteriorly directed subapical dent; rostral carina strong, reaching posteriorly to mediodorsal tooth; orbital teeth present, small, sharp, lateral to eye ( Fig. 8 A–C). Pterygostomial angle not protruding anteriorly, broadly rounded ( Fig. 8 B).

Pleura of first to fourth abdominal somites rounded posteroventrally; fifth pleuron with posteroventral angle subacute ( Fig. 7 ). Telson relatively narrow, at least 3.2 times as long as wide in proximal third, tapering distally; dorsal surface with two pairs of small submarginal spiniform setae situated both in posterior half of telson, at about 0.7 and 0.8 telson length, respectively; posterior margin straight, each posterolateral angle with two pairs of spiniform setae, lateral being 0.8 times as long as mesial, and six plumose setae between mesial spiniform setae ( Fig. 8 L, M).

Eyes partly exposed in dorsal and lateral views; cornea not particularly reduced, well pigmented, occupying entire anterior portion of eyestalk, anteromesial margin without process ( Fig. 8 A, B).

Antennular peduncle stout, with second segment approximately as long as wide; first segment with distally acute stylocerite reaching to 0.7 length of second segment; ventromesial carina of first segment with anteriorly directed tooth; lateral flagellum biramous, fused portion composed of at least three segments; accessory ramus composed of three or so partly fused segments with groups of aesthetascs ( Fig. 8 A, B). Antenna with stout basicerite bearing sharp distoventral tooth; scaphocerite not reaching to end of antennular peduncle, ovate; blade broad, with anterior margin convex, not reaching beyond strong distolateral tooth; carpocerite not reaching beyond 0.5 length of scaphocerite ( Fig. 8 A, B).

Mouthparts not dissected, appearing typical for genus in external view. Third maxilliped with coxa bearing dorsally slightly produced lateral plate above mastigobranch; antepenultimate segment elongate, flattened; penultimate segment elongate, slender; ultimate segment tapering distally, ending in corneous tip, with small setiform seta on apex; arthrobranch moderately developed ( Fig. 8 G, H).

Chelipeds very unequal in size and asymmetrical in shape, carried folded ( Fig. 9 A, D). Major cheliped enlarged, elongate, slender; ischium moderately elongate, slender, ventrolateral surface with two stout spiniform setae; merus almost twice as long as ischium, slender, smooth, ventrally flattened; carpus distinctly shorter than merus, vase-shaped, distally broadening, smooth; chela subcylindrical, smooth; fingers about same length as palm, compressed, slightly twisted, cutting edges evenly serrated, with at least 12 subtriangular teeth ( Fig. 9 A–C). Minor cheliped not particularly enlarged, slender; ischium long, with two stout spiniform setae on ventrolateral surface; merus longer than ischium, somewhat compressed, smooth; carpus about 0.6 length of merus, subcylindrical, slightly widening distally, smooth; chela simple, smooth, with fingers slightly shorter than palm, finger cutting edges straight, except for a few small teeth in distal half ( Fig. 9 D, E).

Second pereiopod slender; ischium about as long as merus, with one spiniform seta on ventrolateral surface; carpus five-segmented, first segment longest, longer than sum of four other segments, ratio of carpal segments (from proximal to distal) more or less equal to: 6: 1: 1: 1.5: 1.8; chela simple, longer than mostdistal carpal article ( Fig. 9 F). Third pereiopod slender, compressed; ischium elongate, with two spaced spiniform setae on ventrolateral surface; merus elongate, at least nine times as long as wide, smooth; carpus slightly shorther than merus; propodus with two small spiniform setae on ventral margin and one larger distal spiniform seta adjacent to dactylus; dactylus slightly less than 0.4 propodus length, simple, conical, slender, gradually curving distally, furnished with setae on dorsal margin ( Fig. 9 G, H). Fourth pereiopod nearly identical to third pereiopod. Fifth pereiopod longer and more slender than third or fourth pereiopods; ischium unarmed; carpus as long as merus, propodus apparently without spiniform setae on ventral margin, propodal bruish reduced to one distal row of serrulate setae at base of dactylus ( Fig. 9 I, J).

Second pleopod (of ovigerous specimen) with appendix interna and appendix masculina, latter much shorter than appendix interna, with stiff apical and subapical setae ( Fig. 8 I). Uropod with lateral lobe of protopod ending in small blunt tooth; exopod and endopod slender; exopod with sinuous diaeresis and obscure, blunt distolateral tooth adjacent to slender spiniform seta ( Fig. 8 J, K).

Gill-exopod formula as described for genus (see above).

Colour in life semitransparent-white, gonads yellow ( Fig. 10 G).

Etymology. Specific name derived from Alacranes Reef, the type locality of this new species; used as a noun in opposition.

Habitat. Shallow flat with sand and rocks, near slope to a deeper channel; the single specimen was found under a rock at a depth of 0.5 m.

Type locality. Alacranes Reef, about 130 km north of Progreso, Yucatán Peninsula.

Distribution. Presently known only from the type locality in the southern Gulf of Mexico, off Yucatán.

Remarks. Triacanthoneus alacranes n. sp. differs from the other two species of the genus by the more posterior and also more dorsal position of the dorsolateral teeth on the carapace; the more posterior position of the mediodorsal tooth on the carapace; the more pronounced rostral carina; the presence of orbital teeth; the presence of only two spiniform setae on the ischium of the major and minor chelipeds (vs. three in the other two species); the reduced propodal brush on the fifth pereiopod; and the wider telson, with much smaller dorsal spiniform setae, all four situated in the posterior half of the telson.

Figures

FIGURE 7. Triacanthoneus alacranes n. gen., n. sp., holotype, ovigerous female (CL 3.7 mm) from Alacranes Reef, Yucatán, Mexico, UNAM-CNCR 26177: habitus, lateral.

FIGURE 8. Triacanthoneus alacranes n. gen., n. sp., holotype, ovigerous female (CL 3.7 mm) from Alacranes Reef, Yucatán, Mexico, UNAM-CNCR 26177: A, frontal region, dorsal; B, carapace and frontal appendages, lateral; C, carapace, dorsal; D, post-hepatic tooth of carapace, lateral; E, mediodorsal tooth of carapace, lateral; F, distal rostrum, lateral; G, third maxilliped, lateral; H, same, tip of ultimate segment; I, second pleopod, appendix masculina and appendix interna; J, uropod, dorsal; K, same, posterior region of exopod; L, telson, dorsal; M, same, posterior region.

FIGURE 9. Triacanthoneus alacranes n. gen., n. sp., holotype, ovigerous female (CL 3.7 mm) from Alacranes Reef, Yucatán, Mexico, UNAM-CNCR 26177: A, left (major) cheliped, lateral; B, same, carpus and chela, mesial; C, same, chela fingers; D, right (minor) cheliped, lateral; E, same, chela, mesial; F, second pereiopod, lateral; G, third pereiopod, lateral; H, same, distal propodus and dactylus; I, fifth pereiopod, lateral; J, same, distal propodus and dactylus, mesial.

FIGURE 10. Life colouration of three species of Triacanthoneus n. gen.: A – C, Triacanthoneus toro n. sp., paratype, ovigerous female from Bocas del Toro, Caribbean coast of Panama, USNM 1145376; D – F, Triacanthoneus pacificus n. sp., holotype, ovigerous female from Playa Venao, Pacific coast of Panama, USNM 1145374 [D, E], non-type ovigerous female from Bahía Malaga, Pacific coast of Colombia, UVC [F]; G, Triacanthoneus alacranes n. sp., holotype, ovigerous female from Alacranes Reef, Yucatán, Mexico, UNAM-CNCR 26177 [G by J. Duarte-Gutiérrez, all other photographs by author].