Prionocrangon brasiliensis, Anker, Arthur, Pachelle, Paulo P. G. & Tavares, Marcos, 2014

Anker, Arthur, Pachelle, Paulo P. G. & Tavares, Marcos, 2014, Two new species and two new records of deep-water caridean shrimps from Brazil (Decapoda: Pandalidae, Palaemonidae, Crangonidae), Zootaxa 3815 (2), pp. 263-278 : 273-276

Prionocrangon brasiliensis sp. nov.

Figs. 7 , 8

Material examined. Southwestern Atlantic Ocean off Brazil: holotype, male (cl 4.8 mm), TAAF MD 55 / BRÉSIL 1987, station 54 CB 93, depth: 707–733 m, 19º36’S 38º53’W, 30.V.1987 ( MZUSP 31147); paratype, male (cl 5.5 mm), TAAF MD 55 / BRÉSIL 1987, station 54 CB 93, depth: 707–733 m, 19º36’S 38º53’W, 30.V.1987 ( MZUSP 31146); paratype, ovigerous female (cl 7.6 mm), TAAF MD 55 / BRÉSIL 1987, station 54 CB 93, depth: 707–733 m, 19º36’S 38º53’W, 30.V.1987 ( MZUSP 31498).

Description. Rostrum relatively long, usually reaching beyond base of branchiostegal (= pterygostomial) tooth, directed obliquely upwards, distally subacute, about 0.11–0.12 times as long as carapace ( Fig. 7 a, h, l). Middorsal carina of carapace armed with 10–13 teeth, two or three of them beyond carapace mid-length, anterior-most tooth distinctly removed from rostrum ( Fig. 7 a, h, l). Antennal tooth relatively small, subtriangular, marginal; pterygostomial tooth very strong, falling short of tip of distolateral tooth of antennal basicerite, not reaching beyond distal margin of eyestalks ( Fig. 7 a, b).

Fourth and fifth abdominal somites without dorsal carina; fourth and fifth somites faintly pitted; sixth somite about 1.6 times as long as fifth (measured along mid-dorsal line); posterolateral margin with short, truncated lobe, without sharp tooth; ventrolateral angle with conspicuous depression, without deep incision ( Fig. 7 d). Telson subequal to sixth abdominal somite in length, much shorter than uropods; lateral margins more or less straight in anterior portion of telson, with strong convexity at about mid-length, and strongly convergent posterior to this convexity; dorsal surface with three pairs of spiniform setae, one pair near mid-length convexity and two pairs in posterior half of strongly tapering portion of telson, most-posterior pair adjacent to posterior margin; posterior margin rounded, about 0.25 length of proximal width of telson, with shallow median depression and two long, thick, plumose setae, without minute median denticle; ventral surface with strong teeth, forming telson-uropodal locking mechanism ( Fig. 7 e, f).

Eyestalks elongate, cylindrical, with blunt extremities, somewhat twisted, curving ventrally in-between antennular peduncles; ventral surface with one more or less developed denticle ( Fig. 7 b, c, i). Antennular peduncle with long proximal article, about 0.6 times as long as carapace; stylocerite with elongate, acute tip, not reaching distal margin of eyestalks ( Fig. 7 a, b). Antenna with basicerite armed with strong distolateral tooth; scaphocerite equal to or slightly shorter than proximal article of antennular peduncle, with extremely narrow blade and small distolateral tooth ( Fig. 7 b, j, tip sometimes broken). Third maxilliped with rudimentary exopod ( Fig. 8 a).

First pereiopod with merus bearing small, sharp distodorsal tooth; palm moderately stout, 4.0–4.2 times as long as wide; pollex situated between 0.6 and 0.7 length of palm, stout, ending in sharp point; dactylus sickleshaped, not reaching proximal base of pollex when fully closed ( Fig. 8 b–d, j). Second and third pereiopods typical for genus ( Fig. 8 e, f). Fourth and fifth pereiopods with dactyli expanded, with minute terminal hook, 0.6 and 0.5 times as long as propodi, respectively ( Fig. 8 g–i).

Endopods and protopods of pleopods without lateral lobe or distoventral projection; endopod of female second pleopod about 0.25 times as long as exopod; male second pleopod with appendix masculina much shorter than endopod, with few robust setae apically ( Fig. 7 k). Uropod without specific features ( Fig. 7 e).

Colour pattern unknown.

Etymology. Refers to the new species’ occurrence on the continental slope of Brazil; used as an adjective.

Distribution. Western Atlantic: presently known only from the type locality off Espírito Santo, Brazil, at a depth of 707– 733 m.

Remarks. Prionocrangon is morphologically a very homogeneous genus, making separation of the eight currently recognised species ( Kim & Chan 2005; Hendrickx & Ayón-Parente 2012) extremely difficult. As Chace (1984) noted, “one feature that seems to be most useful in distinguishing the species, from prevailing knowledge, is the form of the degenerate eyes and eyestalks”. Kim & Chan (2005) used this character throughout their revision of the genus, distinguishing between species with triangular eyestalks inhabiting deeper waters (1033–2556 m) and species with the eyestalks “drawn out to bluntly cylindrical or villiform extremities” from somewhat shallower depths (200–891 m).

Prionocrangon brasiliensis sp. nov. is morphologically very close to Prionocrangon pectinata Faxon, 1896 , the only other Atlantic species of the genus, known from depths of 516–1236 m in the Gulf of Mexico and 200–1033 m in the Caribbean Sea ( Faxon 1896; Cruz et al. 2002; Felder et al. 2009), differing from it most conspicuously by the elongate, cylindrical, distally broad and blunt eyestalks ( Fig. 7 b, c) vs. much shorter, triangular, i.e. distally narrowing eyestalks in P. pectinata ( Cruz et al. 2002: fig. 4; Kim & Chan 2005: fig. 3B, C). In this respect, the new species resembles the Indo-West Pacific species Prionocrangon ommatosteres Wood- Mason in Wood-Mason & Alcock, 1891, Prionocrangon dofleini Balss, 1913 , Prionocrangon curvicaulis Yaldwyn, 1960 , and Prionocrangon formosa Kim & Chan, 2005 ( Kim & Chan 2005) . The Brazilian species differs from P. ommatosteres by the shorter antennal scaphocerite and much broader and shorter telson, the latter also with a different configuration of dorsal spines; from P. do f l e i n i by the absence of mid-dorsal carina on the fourth and fifth abdominal somites and broader telson; from P. curvicaulis by the much shorter antennal scaphocerite (reaching far beyond distal margin of first article of antennular peduncle in P. c u r v i c au l i s vs. not reaching this margin in P. brasiliensis sp. nov.); and from P. formosa by the fewer teeth on the carapace (6 in P. f o r m o s a vs. 10 –13 in P. brasiliensis sp. nov.) and more convex lateral margin of the telson (cf. Figs. 7 , 8 and Kim & Chan 2005: figs. 1, 4–8). The new species also differs from the recently described Prionocrangon incisum Hendrickx & Ayón- Parente, 2012 from the eastern Pacific, by the absence of a deep incision on the posteroventral margin of the sixth abdominal somite, apically broad (not narrowing) eyestalks, and absence of a distinct U-shaped groove on the carapace flanks (cf. Figs. 7 a, b, d and Hendrickx & Ayón-Parente 2012: 1A, C, E, G, H).

The authors are aware that the shape of the eyestalks and some of the other characters currently used in taxonomy of Prionocrangon ( Chace 1984; Kim & Chan 2005) may eventually be shown to be variable. In addition, the scarcity of the material makes taxonomic decisions even more challenging. However, assigning the Brazilian specimens to one of the previously described species, for instance, to the Atlantic P. pectinata with much shorter, triangular eyestalks, or to one of the Indo-West Pacific taxa with similarly elongate eyestalks, but with one or two differences in other features, would seriously destabilise the current taxonomic concept of Prionocrangon ( Kim & Chan 2005) . Future research on this rare and bizarre genus, including comparison of DNA sequences from fresh specimens, will show whether P. brasiliensis sp. nov. is a distinct southwestern Atlantic taxon or conspecific with P. pectinata or one of the Indo-West Pacific (and hence pantropical) species.

The ecology and biology of Prionocrangon brasiliensis sp. nov. remains largely unknown. The type series was found syntopically with another crangonid shrimp, Parapontophilus gracilis , recently reported from Brazil by Cardoso (2013). Interestingly, P. gracilis , unlike all species of Prionocrangon , has large and well-pigmented corneas. The very large egg size of P. brasiliensis sp. nov. ( Fig. 7 g) suggests that this species may have an abbreviated larval development, i.e. with a few lecithotrophic larvae.

Figures

FIGURE 7. Prionocrangon brasiliensis sp. nov.: paratype, ovigerous female from TAAF station 54, off Espírito Santo, Brazil (MZUSP 31498) (a – g); holotype, male from the same locality (MZUSP 31147) (h – k); paratype, male from the same locality (MZUSP 31146) (l); a, carapace and frontal appendages, lateral; b, frontal region, dorsal; c, eyestalks, lateral; d, abdomen, lateral; e, tail fan, lateral; f, telson, dorsal; g, egg; h, carapace, lateral; i, eyestalks, lateral; j, antennal scaphocerite, dorsal; k, second pleopod, mesial; l, carapace, lateral.

FIGURE 8. Prionocrangon brasiliensis sp. nov.: paratype, ovigerous female from TAAF station 54, off Espírito Santo, Brazil (MZUSP 31498) (a – i); holotype, male from the same locality (MZUSP 31147) (j); a, third maxilliped, lateral; b, first pereiopod, lateral; c, same, distal merus and carpus; d, same, chela, ventral; e, second pereiopod, lateral; f, third pereiopod, lateral (dactylus missing, indicated by broken line); g, fourth pereiopod, lateral; h, same, propodus and dactylus, mesial; i, fifth pereiopod, lateral; j, first pereiopod chela, dorsal.