Triacanthoneus chapelianus, Alvarez, Fernando, Iliffe, Thomas M. & Villalobos, José Luis, 2014

Alvarez, Fernando, Iliffe, Thomas M. & Villalobos, José Luis, 2014, A new species of the alpheid shrimp genus Triacanthoneus Anker, 2010 (Crustacea: Alpheidae) from Belize, Zootaxa 3768 (1) : -

Triacanthoneus chapelianus sp. nov.

( Figs. 1–4 )

Type material. Holotype: ovigerous female, cl 4.3 mm, tl 17.3 mm; Belize, marine cave off Caye Chapel (17º42'46" N, 88º02'05" W), depth 8–10 m, 18 February 1989, coll. T.M. Iliffe and S.M. Sârbu, CNCR 28284.

Description. Small-sized shrimp (tl less than 18 mm), carapace length approximately one fourth of total length. Carapace with one pair of acute teeth immediately posterior to extra-corneal angle and sharp tooth along medio-dorsal line, on anterior half of carapace ( Figs. 1 A, B, 2A, B). Rostrum reaching distal half of second antennular article, pointing forwards, dorsal margin smooth, ventral margin with one tooth ( Fig. 2 B). Cardiac notch reduced to small incision ( Fig. 2 B).

Eyes not completely covered by carapace, visible in dorsal and lateral views ( Figs. 2 A, B). Cornea not faceted, with dark granules of pigment.

Ventral margins of pleura of first three abdominal somites rounded, posterolateral angle of fourth somite subacute, and that of fifth somite acute. Third and sixth somites subequal in length ( Fig. 1 A).

Telson 1.5 times as long as sixth abdominal somite, 2.2 times as long as its anterior width. Dorsal surface with two pairs of spines, anterior pair on proximal half, posterior pair on distal third. Posterior half with shallow median groove, becoming more evident posteriorly, ending in U-shaped notch on posterior margin. Posterior margin with two pairs of strong spines, subequal in length; one medial pair of plumose setae, shorter than adjacent spines, on lateral margins of median notch ( Figs. 2 D, E). Uropods with exopod and endopod subequal in length, slightly longer than telson ( Fig. 2 D).

Antennule with well developed stylocerite, tip acute, reaching middle portion of second article of antennular peduncle ( Fig. 2 A). First article of antennular peduncle longer than second and third. Lateral flagellum divided after third article, accessory branch with five articles and series of long aesthetascs ( Figs. 3 A–C). Antenna with strong carpocerite bearing large tooth on lateroventral angle; scaphocerite ovate, as long as antennular peduncle, distal margin rounded, reaching beyond distolateral tooth ( Fig. 2 A).

Mouthparts not dissected, of typical appearance for the genus in external observation. Third maxilliped pediform, last article with transversal rows of short setae, exopod reaching distal margin of merus ( Fig. 4 A). First pair of pereiopods strongly asymmetrical in shape and size. Major cheliped about half as long as total body length; ischium with three lateral spines, 0.5 times length of merus; merus cylindrical, 1.5 times length of carpus; carpus becoming wider distally; propodus longest article, 1.4 times length of merus; dactylus 0.6 times length of propodus; palm somewhat inflated, thicker than fingers, fingers closing completely except for distal fifth, cutting edges with minute teeth alternating when fingers completely closed, finger tips strongly crossing ( Fig. 3 D). Minor cheliped less than half length of major cheliped; ischium 0.7 times length of merus, with three spines on lateroventral margin; merus longest article, 1.1 times length of carpus; carpus becoming wider distally; chela with fingers subequal in length to palm, fingers closing completely, not gaping, tips crossing ( Fig. 4 B, C).

Second pair of pereiopods slender; ischium 1.2 times as long as merus, with three spines on ventrolateral margin; merus simple; carpus 5-articulated, first carpal article longer than half-length of carpus, remaining carpal articles subequal in length; chela simple, with fingers about half length of palm ( Fig. 3 E). Pereiopods 3–5 simple, increasing in length posteriorly, carpi longest articles in third and fourth pereiopods, propodus longest in fifth; ischia unarmed; meri unarmed, slender; propodi unarmed, with some stiff setae; brush on propodus of P5 reduced to distal tuft; dactylii simple, slender, curved, about 0.35 length of propodi in third and fourth pereiopods, about 0.25 length of propodus in fifth pereiopod ( Figs. 1 , 3 F–H, 4D–F).

Second pleopod with endopod bearing appendix interna and appendix masculina ( Fig. 4 G). Appendix masculina straight, stout, apical portion with four slender spinules oriented laterally; appendix interna subequal in length to appendix masculine, curved mesially, with coupling hooks on apical portion.

Holotype with approximately 70 eggs, ovoid in shape, measuring 0.5 x 0.35 mm.

Etymology. The specific epithet is derived from the type locality, Caye Chapel, Belize.

Distribution. Presently known only from the type locality, a marine cave off Caye Chapel, Belize.

Habitat. Caye Chapel Cave is located 0.5 km east (seaward) of the northern tip of Caye Chapel, Belize. The entrance consists of two vertical shafts located about 15 m apart on a seagrass bed in 3 m depth. At a depth of 8 m in the cave, the two entrance shafts join in a low room from where a sandy restriction under a ledge leads to a large descending rift, well decorated with stalactites and floored with white sandy sediments. At 35 m depth, the rift levels off and begins to ascend to a point at which the cave ends in collapse 150 m from the entrance. The water is clear with visibility about 20– 30 m. The deeper waters of the cave appear relatively more sterile in comparison with regions closer to the entrance where encrusting sponges, orange mysids, and red shrimps (possibly Janicea or Parhippolyte ) are conspicuous. Speleonectes cokei Yager, 2013 , previously known only from Caye Caulker Cave, is the second species of Remipedia to be collected from a completely submarine cave. The harpacticoid copepod Novocrinia trifica Huys & Iliffe, 1998 , from the new family Novocriniidae and the calanoid copepod Enantiosis belizensis Fosshagen, Boxshall & Iliffe, 2001 , from the family Epacteriscidae , as well as representatives of the primitive calanoid genera Ridgewayia Thompson & A. Scott , and Pseudocyclops Brady have been recorded from this cave (Audun Fosshagen, pers. comm.).

Remarks. Triacanthoneus chapelianus sp. nov. can be distinguished from the other four known species of the genus by the position of the dorsolateral teeth on the carapace, which are in a submarginal position posterior to the extra-corneal (orbital) angle ( Figs. 2 A, B); in the other four species, these teeth are in a more or less hepatic position (cf. Anker 2010; Alvarez et al. 2012). Other characters of the new species are a reduced cardiac notch on the posterodorsal margin of the carapace and the presence of a U-shaped notch on the posterior margin of the telson ( Fig. 2 E).

The holotype of T. chapelianus sp. nov., described as an ovigerous female, has a well-developed appendix masculina on the second pleopod, a characteristic already reported for the three species of Triacanthoneus described by Anker (2010) ( T. toro , T. pacificus , T. alacranes ), but not found in all specimens of the type series of T. akumalensis , where one ovigerous individual, designated as “female” did not have appendix masculina, while another “ovigerous specimen” had a well-developed appendix masculina ( Alvarez et al., 2012).

Egg size is remarkably uniform in Triacanthoneus regardless of the two types of habitats: the eggs of the shallow coastal species T. toro and T. alacranes measure 0.65 x 0.36 mm and 0.6 x 0.38 mm, respectively ( Anker 2010); while they measure 0.5 x 0.33 mm in T. akumalensis and 0.5 x 0.35 mm in T. chapelianus , the two cave species ( Alvarez et al. 2012; present study). This suggests that the larval development of the cavernicolous species of Triacanthoneus is not advanced as observed in many other cave shrimps.

Although all species of Triacanthoneus are found in coastal areas, T. toro , T. pacificus and T. alacranes were collected in shallow/intertidal habitats in substrates with sand, seagrasses or rubble ( Anker 2010), whereas, in contrast, T. chapelianus and T. akumalensis were collected in marine caves at shallow depths, 8–10 m for the former, and moderate depths, 25–40 m for the latter ( Alvarez et al. 2012; present study).

Figures

FIGURE 1. Triacanthoneus chapelianus sp. nov., ovigerous female, holotype (CNCR 28284): A, total lateral view; B, photograph of a preserved individual. Scale bar = 1 mm.

FIGURE 2. Triacanthoneus chapelianus sp. nov., ovigerous female, holotype (CNCR 28284): A, carapace, dorsal view; B, carapace, lateral view; C, detail of rostrum and submarginal dorsolateral teeth; D, telson and uropods, dorsal view; E, detail of posterior margin of telson. Scale bars = A, B, D, 1 mm; C, 0.5 mm; E, 0.25 mm.

FIGURE 3. Triacanthoneus chapelianus sp. nov., ovigerous female, holotype (CNCR 28284): A, antennular flagella; B, detail of accessory branch of lateral antennular flagellum; C, detail of tip of accessory branch of lateral antennular flagellum; D, major cheliped; E, second pereiopod; F, third pereiopod; G, fourth pereiopod; H, fifth pereiopod. Scale bars = A, B, C, 0.5 mm; D – H, 1 mm.

FIGURE 4. Triacanthoneus chapelianus sp. nov., ovigerous female, holotype (CNCR 28284): A, third maxilliped; B, minor cheliped; C, detail of chela of minor cheliped; D – F, detail of propodi and dactylii of third through fifth pereiopods; G, second pleopod showing endopod, appendix masculina, and appendix interna (exopod missing). All scale bars = 1 mm.