Holotype: COBERPES 3, sta α10, 19°40.066N, 92°45.490W, 780 m, 19 November 2011: 1 male 8.9 mm (EPC 201310). Paratype: COBERPES 5, sta A11, 19°32.108N, 93°08.373W, 827 m, 24 May 2013: 1 male 8.1 mm (USNM 12376795).

Thirteen pairs of quadriserial gills. Arthrobranchs on third maxilliped and somite X (thoracomere 4, or cheliped) well developed. Pleurobranchs well developed on each of somites XI–XIII (thoracomeres 5–7, above second to fourth pereopods).

Shield (Figs 1A, B, 2A) slightly longer than broad, well calcified, with short transverse rows of setae arranged longitudinally on each side from near lateral projections to near posterior margin of shield. Rostrum broadly triangular, ending roundly or in sharp spine. Anterior margins between rostrum and lateral projections concave. Lateral projections ending in sharp spine. Anterolateral margins sloping. Posterior carapace with lateral lobes (“accessory portions” of McLaughlin and Lemaitre 2001) fused to shield; with well calcified and delimited posteromedian and posterolateral figs (Figs 2A). Branchiostegite (Figs 1C, 4A) with 2 narrow calcified figs: 1 anterodorsal, and 1 posterior curving down adjacent to sulcus verticalis and bifurcated ventrally.

Ocular peduncles (Figs 1A, B, 2A) less than half length of shield, inflated basally and diminishing in width distally, with dorsomesial, longitudinal row of long setae; corneas reduced not dilated. Ocular acicles subtriangular, terminating in strong spines with mesial margins nearly contiguous.

Antennules (Fig. 2A) relatively robust, exceeding distal margin of corneas by one-fifth length of penultimate segment. Ultimate segment about one-fourth its length longer than penultimate, with scattered setae dorsally. Basal segment with strong laterodistal spine. Ventral flagellum with 8–12 articles.

Antennal peduncles (Fig. 2A) long, exceeding distal margins of corneas by full length of fifth segment. Fifth and fourth segments unarmed, sparsely setose. Third segment with strong ventromesial spine. Second segment with dorsolateral distal angle prominently produced, with long setae dorsomesially, and terminating in strong spine with 3 or 4 small spines mesially. First segment with small spine on lateral face distally. Acicle exceeding distal margin of corneas by about half length of acicle, reaching to about midpoint of fifth antennal segment; terminating in minutely bifid spine; with dorsomesial row of long bristle-like setae extending to tip of acicle. Flagellum exceeding chelae, densely setose, with setae 2–8 flagellar articles in length.

Mandible with edge of incissor process weakly sinuous, sharp, chitinous. Maxillule with external lobe slender, straight, internal lobe with 1 or 2 long bristles. Maxilla with endopodite not exceeding distal end of scaphognathite. First maxilliped with endopodite not exceeding distal margin of exopod. Second maxilliped without distinguishing characters. Third maxilliped with merus and carpus each armed with small dorsodistal spine; ischium with crista dentata consisting of about 15 small, subequal corneous teeth, with 1 accessory tooth (or as in holotype, with 2 teeth on one side); coxa with 2 sharp mesial teeth. Sternite IX (of third maxillipeds) with sharp spine on each side of midline.

Chelipeds not markedly asymmetrical, subequal in length; carpi and chela with dense, long bristle-like setae on dorsal surfaces, setae less dense and present only on distal two-thirds of ventral surfaces. Right cheliped (Fig. 2B) with fingers having numerous tufts of setae on dorsal surfaces, much less numerous on ventral surfaces, each finger terminating in inwardly curved, sharp corneous tip, armed with small blunt spines on dorsal surfaces basally and on lateral and mesial faces; cutting edges each consisting of fused corneous teeth on distal third, and 2 large, unequal rounded, calcareous teeth on proximal two-thirds, with additional smaller calcareous tooth on fixed finger. Right chela dorsal surface with scattered small spines, lateral and mesial margins rounded, with dorsomesial margin armed with irregular rows of small blunt or sharp spines; ventral face smooth. Carpus smooth except for few small blunt spines on dorsomesial margin distally, and distinct dorsodistal spine near distal margin; ventral surface smooth. Merus nearly naked, with row of setae on dorsodistal margin; smooth except for distal spine on ventromesial margin, and small, well-spaced blunt spines on ventral surface. Ischium unarmed except for ventromesial row of setae and irregular row of small blunt spines or tubercles on ventral surface.

Left cheliped (Fig. 2C) similar in setation and armature to right, except carpus with 3–5 sharp spines on dorsodistal margin.

Ambulatory legs or pereopods 2 and 3 (Fig. 3A–D) with dense, bristle-like setae or tufts of bristle-like setae, on dorsal and ventral margins of meri, carpi, propodi and dactyls, setae arranged in short transverse rows on dorsal margins of meri, carpi and propodi, denser on propodi. Dactyls broadly curved, each about 1.8 times as long as propodi, terminating in sharp corneous claw, and armed with 12–20 small corneous spinules on ventromesial margin; with row of bristle-like setae on ventromesial margin arranged in short transverse or oblique rows. Propodi through ischium unarmed except for: bristles and small dorsodistal spine on each carpus, and also merus of first ambulatory leg (pereopod 2), irregular row of small spines on ventral margin of merus of first ambulatory leg, and row of small spines on ischium of first ambulatory leg; ischium of second ambulatory leg (pereopod 3) with ventral margin lacking spines but with row of setae.

Pereopod 4 (Fig. 3E) weakly semi-chelate, with long, bristle-like setae dorsodistally on merus, and on dorsal margins of carpus, propodus and dactyl. Dactyl subtriangular, terminating in sharp, corneous claw, lacking preungual process; with ventrolateral row of minute, fused corneous teeth. Propodal rasp consisting of 2 rows of lanceolate corneous scales distally, 3 rows proximally.

Pereopod 5 chelate. Propodal rasp well developed, occupying nearly half of lateral face of propodus.

Sternite XII (Fig. 1D) distinctly divided into anterior and anterior portions by membranous hinge. Anterior portion with semi-subcircular, setose lobe.

Sternite XIV (pereopod 5) weakly subdivided anteriorly into 2 setose lobes, with low posteromedian rounded ridge (Fig. 4C).

Pleon with first somite not fused to last thoracic somite. First somite with tergite consisting of pair of small calcareous figs anteriorly and partially calcified posterior portion (Fig. 4B); sternite calcified, with median lobe (Figs 1E, 4C).

Uropods strongly asymmetrical. Telson (Fig. 4D) nearly symmetrical, longer than broad; with distinct lateral indentations separating anterior and posterior portions; posterior lobes separated by small, shallow V-shaped sinus (more visible in holotype male 8.9 mm), distal margins of posterior lobes armed with small, mostly blunt spines with corneous tips.

Male with paired gonopores, each with slightly protruding vas deferens; lacking pleopods 1; pleopods 2–5 present on left side, biramous. Female unknown.

The species name is derived from the Mayan “Ah-Kin-Pech” (meaning “place of snakes and ticks”), given by that civilization to a settlement where the city of Campeche, Mexico, is now located. The Mayan name was hispanicized and used for the modern city and adjacent Campeche Bank, in the vicinity of which this new species was collected.

So far known only from the southwestern Gulf of Mexico, off the Campeche Bank; 780 to 827 m.

Gastropod shells.

In the holotype male, the rostrum terminates in a sharp spine whereas in the paratype the rostrum terminates bluntly (Fig. 1A, B). The carpus of the right cheliped is more spinose than the paratype, having a dorsodistal margin armed with a distinct mesial spine and four small blunt spines laterally, the dorsomesial margin has irregular row of five distinct spines increasing in size distally and surrounded by irregularly placed small, blunt spines.

This new species is curiously more similar morphologically to a congener from Indonesian waters, Tomopaguropsis crinita, than to the other only known congener from the western Atlantic, T. problematica. In the new species and T. crinita, the ocular peduncles diminish in width distally, and corneas are reduced or weakly dilated; the chelipeds and ambulatory legs are covered with dense, long bristle-like setae most frequently arranged in tufts, but otherwise are unarmed or at most with scattered small tubercles; and males lack paired pleopods 1 (at least in the known male specimens of both species).

While this new species can be placed in Tomopaguropsis as currently defined (McLaughlin 1997, 2003), it is unusual and differs from other congeners in the development of calcified structures on the branchiostegite, posterior carapace, and pleon, at least based on the only two known male specimens. On the branchiostegite, all species of Tomopaguropsis have a narrow, calcified anterodorsal fig, but in T. ahkinpechensis sp. n. there is also a narrow, calcified posterior fig that curves down following the sulcus verticalis and bifurcates ventrally (Figs 1C, 4A). In the new species, the posterior carapace has three calcified, well developed dorsal figs anteriorly (Fig. 2A): one dorsomedian or (“posteromedian fig”), and two dorsolateral (“posterolateral figs”). Also, in T. ahkinpechensis sp. n., the first pleonal somite is not fused to the last thoracic somite, has a completely calcified sternite with a median lobe, and a tergite with a pair of fully calcified anterior figs and partially calcified posterior portion (Fig. 4B, C). The sternite of the first pleonal somite in T. ahkinpechensis sp. n. is similar to that of T. crinita in having a median lobe, but in T. crinita the sternite is fused to the last thoracic somite and not as strongly calcified as in T. ahkinpechensis sp. n.. The unique morphology of the branchiostegite and first pleonal somite in T. ahkinpechensis sp. n., could well justify the placement of this new species, and possibly T. crinita as well, in a separate new genus. Given that only two specimens of a single sex of this new species, and only four of T. crinita, are known, it is best to wait for additional material or obtain molecular data, in order to better evaluate whether these characters reflect or not the evolution of a separate clade that could justify their placement in a new genus.

The only two known male specimens of Tomopaguropsis ahkinpechensis sp. n. lack pleopods 1. As noted by McLaughlin (1997), male paired pleopods 1 could possibly be a variable condition in species of Tomopaguropsis. McLaughlin (1997) placed her two species T. crinita and T. miyakei in Tomopaguropsis, despite the absence of paired pleopods 1 in males of the former and that males of the latter were unknown. The presence of pleopods 1 in males was one of the main characters used by Alcock (1905) when he established the genus Tomopaguropsis for his T. lanata and A. Milne-Edwards and Bouvier’s (1893) Eupagurus ? problematicus (= T. problematica, with spelling corrected). McLaughlin (1997) noted that A. Milne-Edwards and Bouvier did not mention male pleopods 1 in the description of their taxon, and presumed that Alcock’s assignment of E. ? problematicus to Tomopaguropsis was based on remarks by A. Milne-Edwards and Bouvier’s that implied the presence of pleopods 1 in E. ? problematicus. In order to determine whether or not males of that species had paired pleopods 1, McLaughlin (1997) examined male specimens of T. problematica in museum collections and found them all to lack paired pleopods 1. However, we have examined one specimen (USNM 103420, 3.9 mm, from off Honduras, Caribbean Sea) which has paired pleopods 1 present and both male (paired) and female (unpaired right) gonopores, lending support to McLaughlin’s assertion that male paired pleopods 1 may be a variable condition (possibly the result of parasitic feminization) in at least two species of Tomopaguropsis, T. lanata, and now T. problematica.

There is a striking similarity in sternite XII (of third pereopods) and first pleonal sternite and tergite of T. ahkinpechensis sp. n. with the two species of the family Pylojacquesidae (Pylojacquesia colemani McLaughlin & Lemaitre, 2001, and Lemaitreopsis holmi McLaughlin, 2007). In the new species and the two pylojacquesids, the anterior and posterior portions of sternite XII are distinctly separated by a membranous “hinge”; and the first pleonal somite has a pair of calcified figs anteriorly on the tergite, and a median lobe (in T. ahkinpechensis sp. n.) or posteriorly directed projection (in Pylojacquesidae). The similarity of these unusual characters conceivably can be interpreted as evidence of a close shared ancestry between Tomopaguropsis, as a member of the Paguridae, and the Pylojacquesidae, a phylogenetic relationship previously suggested by McLaughlin et al. (2007) and McLaughlin (2007). A full phylogenetic evaluation of Tomopaguropsis and its five species, however, must await the study and discovery of additional specimens, ideally combined with genetic analyses.

When McLaughlin (2003) updated the generic diagnosis of Tomopaguropsis she stated that a preungual process was absent in species of this genus, even though she (McLaughlin 1997) had previously reported the presence of a prominent preungual process in T. miyakei. Furthermore, examination of USNM specimens has shown that a preungual process is also present in T. problematica.