Nanoplax thomai, Felder, 2020

Felder, Darryl L., 2020, A new crab of the genus Nanoplax from the Gulf of Mexico, and assignment of Micropanope pusilla to a new genus (Crustacea, Brachyura, Pseudorhombilidae), Zootaxa 4810 (3), pp. 531-545 : 533-540

Nanoplax thomai n. sp.

(Figs 1A–H; 2A–I; 3A–F)

Micropanope truncatifrons, Thoma et al., 2014: 88 , table 1, 95, 100. [Not Micropanope truncatifrons Rathbun, 1898 ].

Type materials. Male holotype, cw 11.6 mm ( ULLZ 5794 = USNM 1542142 ) northwestern Gulf of Mexico, Ewing Bank , 28˚ 06.30’ N, 91˚ 02.11’ W, 57–71 m, 1 August 2002 .

Paratypes: Northeastern Gulf of Mexico . 1 female, cw 5.9 mm ( ULLZ 13191 = USNM 1546792 ) off Alabama, muddy shell of bank slope, 29˚ 15.512’ N; 88˚ 20.269’ W, 85 m, 3 December 2010 . Northwestern Gulf of Mexico . 1 female, cw 7.4 mm ( ULLZ 10610 = USNM 1545268 ) off Louisiana, muddy shell hash bank, 28˚ 05.11’ N, 9˚ 58.61’ W, 54–55 m, 1 July 2001 ; 1 male, cw 10.1 mm ( ULLZ 4645 = USNM 1540470 ) off Louisiana, deep bank rubble, 28˚ 06.03’ N, 91˚ 01.77’ W, 57 m, 27 May 2000 ; 1 male, cw 10.1 mm ( ULLZ 15179 = USNM 1548208 ) off Loui- siana, deep bank rubble, 28˚ 05.62’ N, 91˚ 01.84’ W, 56–58 m, 19 October 2013 ; 2 males, cw 7.9, 10.4 mm ( ULLZ 4646 = USNM 1540471 ) off Louisiana, rubble of deep bank, 28˚ 05.99’ N, 91˚ 02.29’ W, 57–59 m, 27 May 2000 ; 1 male, cw 8.1 mm, 1 ov female, cw 8.7 mm ( ULLZ 6754 = USNM 1541951 ) off Louisiana, 27˚ 55.72’ N, 92˚ 22.58’ W, 55–150 m, 22 June 2005 ; 1 male, cw 6.4 mm ( ULLZ 13841 = USNM 1547306 ) off Louisiana, deep bank, 28˚ 05.95’ N, 91˚ 1.683’ W, 54–56 m, 29 August 2011 ; 1 male, cw 11.8 mm ( ULLZ 11904 = USNM 1545979 ) off Loui- siana, 27˚ 55.02’ N, 92˚ 23.08’ W, 71– 58 m, 22 June 2005 ; 1 male, cw 9.9 mm, 1 female, cw 5.3 mm ( ULLZ 10641 = USNM 1545291 ) off Louisiana, rubble of deep bank, 28˚ 03.606’ N, 92˚ 27.665’ W, 62 m, 30 June 2001 ; 1 male, cw 11.6 mm, 1 female, cw 8.8 mm ( ULLZ 10946 = USNM 1545382 ) off Louisiana, hard bank, 27˚ 48.28’ N, 92˚ 03.96’ W, 64–66 m, 22 August 2008 ; 1 male, cw 9.1 mm, 1 female, cw 16.4 mm ( ULLZ 7528 = USNM 1543091 ) off Louisiana, hard bank rubble, 28˚ 05.95’ N, 91˚ 01.34’ W, 68.5 m, 4 June 2005 ; 1 ov female, cw 10.4 mm ( ULLZ 8133 = USNM 1543391 ) off Louisiana, deep bank rubble, 28˚ 05.57’ N, 91˚ 01.27’ W, 56–58 m, 4 June 2005 ; 1 male, cw 7.4 mm, 3 females (1 ov), cw 6.7–12.9 mm ( ULLZ 7359 = USNM 1541680 ) off Louisiana, deep bank rubble, 28˚ 6.12’ N, 91˚ 2.45’ W, 62–112 m, 4 June 2005 ; 1 juvenile male, cw 6.1 mm, female, cw 12.7 mm ( ULLZ 13195 = USNM 1546796 ) off Louisiana, deep bank rubble, 28˚ 05.67’ N, 91˚ 01.31’ W, 56–62 m, 4 December 2010 ; 4 males, cw 4.7–6.0 mm, 4 females, cw 4.7–7.4 mm ( ULLZ 12282 = USNM 1546309 ) off Louisiana, deep bank rubble, 28˚ 05.67’ N, 91˚ 01.31’ W, 57–60 m, 1 July 2001 ; 1 male, cw 9.7 mm ( ULLZ 10985 = USNM 1545402 ) off Louisiana, deep bank rubble, 27˚ 58.977’ N, 91˚ 39.426’ W, 66–71 m, 21 August 2008 ; 1 male, cw 12.0 mm ( ULLZ 11900 = USNM 1545976 ) off Louisiana, deep bank rubble, 27˚ 58.95’ N, 91˚ 39.38’ W, 66–79 m, 21 August 2008 ; 1 female, cw 7.6 mm ( ULLZ 17500 = USNM 1604120 ) off Louisiana, deep bank rubble, 27˚ 58.15’ N, 91˚ 36.66’ W, 54 m, 19 May 2019 ; 1 male, cw 6.7 mm ( ULLZ 18311 = USNM 1604121 ) off Louisiana, 27˚ 55.19’ N, 92˚ 22.61’ W, 62–77 m, 19 May 2019 ; 1 male, cw 10.4 mm, 1 ovi female, cw 10.1 mm ( ULLZ 4706 = USNM 1540506 ) off Louisiana, deep bank, 27˚ 48.88’ N, 93˚ 03.10’ W, 55–65 m, 25 May 2000 .

Diagnosis. Carapace fronto-orbital width less than three-fourths maximum width of carapace (including lateral extent of anterolateral teeth), front wider than orbit; dorsal surface slightly convex, distinct areolations separated by smooth furrows, dorsal surface otherwise with uniform coarse granulation broken by a few transverse ridges and enlarged granules; short epigastric ridge oblique, hepatic ridge longer, weakly arched, transverse epibranchial ridge well developed, broadly arched; anterolateral margin usually with 5 well-defined teeth, first (= outer orbital angle) small, angular, well-separated from lobiform subangular to rounded second; third tooth strongly developed, lateral margin arched, sharply angular to spiniform tip directed anteriorly; fourth tooth and much smaller fifth sharply angular to spiniform, tips directed anterolaterally (fifth sometimes obsolescent in young); subhepatic tubercle below margin usually formed by one or more enlarged, raised, rounded or spiniform granules. Eyestalk slightly elongate, heavy, not narrowed terminally, ending in globular, rounded cornea, corneal diameter equal to or exceeding greatest width of eyestalk shaft. Chelipeds distinctly heterochelous in most specimens, dorsally covered by acute to subacute granules, especially on ridges and article margins, often densest on minor cheliped, variably extended onto at least proximal upper half of palm external surface, becoming weaker distally and ventrally; propodus upper surface with longitudinal furrow of palm positioned between inner superior crest of enlarged granules and outer crest of slightly smaller granules; carpus with depressed furrow distolaterally, superior surface with cover of regularly spaced coarse acute to subacute large granules or short spines, inner corner with two strong spines, distal the larger, major chela with dactylus opposable margin bearing enlarged, basal tooth. Ambulatory pereopod merus with superior margin armed by defined row of well separated short but erect (often weakly hooked) spinules. Male first gonopod (G1) with patch of minute subterminal spinules, and another of enlarged subterminal spines directed proximally, apex hooked, directing opening of terminal infundibulum laterally to posterolaterally in young to strongly proximally in mature. Female gonopore (= vulva) large, occupying most of subpleonal length of sixth thoracic sternite. Applicable GenBank sequence accession numbers from Thoma et al. (2014) are as follow for paratype ULLZ 7359 = USNM 1541680: (12S) GU144406 ; (16S) GU144434 ; (18S) KF682934 ; (COI) KF682809 ; (ENO) KF682661 ; (H3) GU144490 .

Description. Carapace (Fig. 1A) weakly convex, distinctly wider than long, dorsal regions well defined by shallow grooves, raised surfaces granulate, granules dense and largest in hepatic region and anterior of frontal region, forming weakly defined rows; frontal margin bilobed, lined by dense row of granules, median fissure distinctly V-shaped, lobes subtruncate to very broadly convex, lateral tooth subangular to lobiform, terminated laterally in deep antennal sinus offset ventrally from supraorbital margin; supraorbital margin granulate, forming distinct tooth mesially above antennal sinus, median and lateral fissures shallow, obscure, marked by distinct breaks in marginal granulation. Anterolateral teeth well developed, first (outer orbital) acutely angular, separated by shallow sinus from lobiform second; third and fourth broadly triangular, some granules of margins often enlarged to form small ancillary teeth, tips corneous, tip of slightly larger third curved to direct anteriorly, tip of fourth less curved, directed anterolaterally; fifth tooth distinct, much smaller than preceding, variably developed, acute to subacute. Pterygostomial and subhepatic regions (Fig. 1B, C) coarsely granulate, distinct but variable subhepatic tubercle evident as raised area below margin, usually surmounted or formed by one or more enlarged subacute to spiniform granules; pterygostomial ridge slightly raised, granulate. Branchiostegite ventrolateral margin, below anterolateral teeth, weakly concave above coxa of each ambulatory leg.

Eyestalk slightly elongate, anterior crest coarsely granulate, raised to form granulate tubercle distally. Antenna with long flagellum, distalmost fused basal article distally produced to form granulate prominence, first article of base separated from anterior edge of pterygostomium by exposed narrow plate (Fig. 1C).

Third maxilliped (Figs 1B–D) protopod elongate, subcuneate, narrowing to rounded tip laterally, anterior surface deeply grooved to receive ventral edge of carapace, antero-internal surface with two unequal projections on anterior margin, proximal to tight bundle of lamellae forming small podobranch gill positioned above pair of much longer, lamellate, arthrobranch gills. Epipod thin, strap-like, distally fringed with long simple setae. Basis subtriangular, suture with ischium obscurely fused; ischium broadly subrectangular, proximal portion deflected laterally, external surface mostly smooth, mesial margin with fringe of short to medium length simple setae; merus subquadrate, anterolateral corner produced laterally, protruding, lateral margin weakly concave, distal margin truncate, distomesial corner obliquely excavate to accommodate carpus, external surface with scattered granules, coarsest in lateral half and near distomesial margins, depression with very fine microgranulation filling much of mesial half proximal to articulation with carpus, internal surface excavate to accept endopod of second maxilliped, excavation distally with fringe of short setae, raised setose ridge and prominent setose boss near mesial margin proximal to articulation with carpus, mesial margin lined by closely set granules and sparse fringe of medium length setae; carpus appearing subcylindrical externally, subovate internally, internal surface with distal submarginal field and distal fringe of elongate stiff setae overlying propodus; propodus cylindrical, internal surface with several transverse submarginal rows of elongate, stiff, distally directed setae; dactylus subcylindrical, tapering distally, length about 1.3 times that of propodus, flexor margin with few tufts of short, fine setae, tip bearing dense tuft of long stiff setae exceeding length of dactylus. Exopod subrectangular, comparatively short, broad, heavy, internal edge of mesial margin produced to form strong subtriangular projection in distal third, projection fringed with short setae, external surface mesial margin minutely crenulate, lateral margin with sparse minute setae, small tuft of plumose setae distally at articulation of flagellum, flagellum multi-articulate, bearing numerous long, distally directed setae.

Chelipeds (first pereopods) (Figs 1A, B, E–H) unequal, sparsely setose, dense fields of subacute granules covering much of superior and extensor surfaces, some forming variably defined lines or rows on superior surface of carpus and propodus; ischium with line of slightly hooked acute to subacute spines or teeth along inner margin; merus superolateral margins armed with large, broad, acute to subacute spines or teeth, tips slightly hooked to point distally, enlarged granules near inner margins and distally on inferior margin; carpus densely covered by coarse, acute to subacute granules, supero-external surface marked by distinct groove parallel to distal margin, superior sur- face with granules enlarged along internal margin proximal and distal to pair of strong acute spines arming internal angle, tips of both spines corneous, weakly hooked to point distally, distalmost of two spines the larger, positioned slightly above the smaller proximal spine on internal margin, both spines armed with acute to subacute granules on their shoulders.

Major chela propodus coarsely granulate proximally along superior surfaces, weak evidence of superior longitudinal furrow, several granules enlarged along inner superior margin, upper and lower surfaces of palm otherwise appearing smooth to microgranulate; fixed finger of major chelae short, stout, inferior margin sinuous, apex distinctly curved upwards, opposable margin bearing three to four prominent subtriangular teeth with several smaller rounded teeth between, row of teeth set between grooves to internal and external sides; dactylus arched, slightly longer than fixed finger, opposable margin with strong, lobiform, enlarged basal tooth proximally, three somewhat shouldered subtriangular teeth extending from there distally, decreasing in size, subacute tip strongly curved to cross to internal side of fixed finger tip when flexed.

Minor cheliped propodus similar to that of major, including having several enlarged granules among others along the upper flexor margin, though with stronger definition of superior longitudinal furrow; fixed finger of minor chela distinctly longer and more slender than that of major, slender cutting edge on opposable margin bearing 5 or 6 subacute triangular teeth; dactylus of minor chela less arched than that of major, longer than fixed finger, opposable margin lacking enlarged basal lobiform tooth, cutting edge dentition much weaker than in major chela, 2 or 3 broad, low, broadly shouldered subtriangular teeth proximally, several smaller triangular teeth distally, subacute tip crossing to internal side of fixed finger tip when flexed.

Ambulatory pereopods 2–5 (= ambulatory legs) long and narrow, similar in form (Fig. 1 A–B); pereopods 2–4 subequal in size; pereopod 5 smaller, though with relatively broader, more flattened and spatulate propodus; merus length about four times width at widest point, extensor (superior) margin bearing single longitudinal row of irregular small acute teeth or spinules and a few setae, distal extensor margin ending in subacute tooth beyond subdistal notch, flexor margin granulate with several simple setae, longest proximal; carpus strongly bent in proximal third, extensor margin with scattered long and short setae along ridge surmounted by low granules, ridge roughly parallel to second ridge on superoposterior surface, shallow sulcus between ridges most developed in distal three-quarters; propodus extensor margin bearing weakly defined row of enlarged subacute granules, long and short plumose setae along extensor and flexor margins, long setae dominating extensor margin, dense short setae dominating flexor; dactylar-propodal locking mechanism not developed; dactylus narrowly elongate, flattened, weakly arched over most of length, tapering distally, flexor and extensor margins densely lined with short plumose setae forming pubescence intermixed with few longer setae, corneous tip weakly falciform, lacking subterminal, calcareous, raptorial tooth.

Thoracic sternum of male (Figs 1B; 2A–C) wide, length from acute anterior apex to suture between fourth and fifth thoracic sternites (measured at edge of pleon) 0.60–0.62 times greatest width of fourth sternite (including episternites), pleonal depression in fourth sternite forming pair of distinct submedian pockets fitting distal ends of first gonopods below flexed pleon; fifth thoracic sternite with large press button to each side of pleonal depression; fourth and fifth episternites angular, terminally rounded posteriorly, sixth and seventh episternites broadly rounded posteriorly, eighth thoracic sternite exposed as small subtriangular area between lateral margin of flexed second pleonite and condyle of fifth pereopod coxa.

Pleon of male (Figs 1B; 2A, E, F) with third through fifth pleonites fused, internal surfaces with submedian tracts of very fine elongate setae, tracts converging beneath telson; first pleonite widest at articulation with carapace, narrowing to articulation with second pleonite; second pleonite narrowest distally, widest proximally near articulation with first pleonite; third pleonite widest; fused third through fifth pleonites widest at lateral flange of third, narrowing distally, width at articulation with sixth pleonite about half that at articulation with second pleonite, sutures between fused pleonites evident near slight indentations on lateral margins; sixth pleonite rectangular, slightly broadened distally near articulation with telson to accept press-button internally; telson terminally rounded, widest in proximal third.

Pleon of female with first pleonite narrowing distally, margins weakly convex, widest where transverse ridge abuts carapace, second pleonite slightly widening distally, margins weakly concave; third with convex margins, widening distally, lengths of third through sixth pleonites slightly increasing incrementally, telson exceeding length of sixth; fourth pleonite widest, widths of fourth through sixth pleonites decreasing, all with convex lateral margins, sixth slightly wider than telson greatest width, telson subtriangular, terminally rounded, length about three-fourths width.

Mature male first gonopod (G1) ( Fig. 2B , F–H) trunk flattened, somewhat twisted along length, sinuous, broad proximal flange extended posterolaterally over male coxal gonopore and papilla, distally with apex hooked posterolaterally (minimally so in juveniles), directing opening of terminal infundibulum strongly proximally in mature male (in juveniles, anterolaterally or laterally to posterolaterally), patch of minute subterminal spinules and another of enlarged subterminal spines on sternal side directed proximally. Male second gonopod (G2) ( Fig. 2I ) slightly more than one-third length of first gonopod, terminating in simple spiniform tip. Female mature gonopore (= vulva) ( Fig. 2D ) large, centered on and spanning over two-thirds of subpleonal length of sixth sternite, broadly crescentic to ovoid, raised lip forming anterolateral edge.

Color. Dorsal carapace coloration primarily reddish orange of varied intensity over off-white background, color darkest anteriorly and on raised areas, sometimes with almost complete coverage of reddish to reddish orange pigmentation ( Fig. 3 A–F); cheliped proximal articles and palm reddish orange dorsally, fading ventrally, fingers intense deep red to reddish brown; ambulatory pereopod meri with two dorsally evident bands of reddish orange separated by off white, distal articles primarily reddish orange except for off white margins near joints. Embryonated eggs rose red to reddish maroon.

Habitat. Abundant on offshore reefs and deep banks, especially in small cavities and interstices of eroded hard substrates, including rhodoliths and molluscan shells, as well as among sponges, bryozoans, corals, encrusting algae, and other epibiota of fouling communities. Confirmed depth records range from 54– 155 m.

Size. The maximum carapace width was 16.4 mm for a large female among available collections. The largest male in these collections had a carapace width of 12.0 mm. Juveniles of neither sex could be definitively identified by structural morphology alone at less than 5.5 mm carapace width, though scarlet pigmentation below the posterior edge of the carapace clearly marked some as small as cw 4.7 mm. Embryonated eggs (most showing eyespots) in sampled clutches of alcohol preserved ovigerous females ranged from 0.39 to 0.46 mm in greatest dimension.

Distribution. Known from middle to outer continental shelf waters of the northeastern to northwestern Gulf of Mexico, Alabama to western Louisiana.

Etymology. The species is named for Brent P. Thoma in recognition of his substantial contributions to understanding of xanthoid crab phylogenetic relationships and in gratitude for the many ways in which he has facilitated the present author’s work over the last decade.

Remarks. Nanoplax thomai n. sp. and Micropanope truncatifrons are similar in general habitus, which led to their previous confusion. While the latter species was named for its truncate carapace front, its immature male paratype (USNM 8530, cw 3.6 mm, the only known male) has slightly convex frontal lobes, not unlike those in many specimens of N. thomai n. sp. Furthermore, the frontal lobes in N. thomai n. sp. are sometimes rather truncate, with the lateral corners of the front in some specimens simply angular and in others developed as a very slightly elevated tooth. With no mature males of M. truncatifrons available for comparisons, neither gonopod characters nor those based upon the relative widths of the male first through third pleonites could be used to support separation of the species. However, gonopores of the apparently mature female holotype of M. truncatifrons (USNM 9497, cw 10.4 mm) do not bear close resemblance to those of Nanoplax spp., instead being smaller, more narrowly ovoid, and more obliquely oriented. Centered within the anterior three-fifths of the sixth thoracic sternite, its linear cross-section occupies less than two-thirds of the sternite length, it is not notably ear-shaped or crescentic in shape, and there is no obvious development of a raised marginal lip.

The materials thought to represent M. truncatifrons in molecular phylogenetic analyses by Thoma et al. (2014), clearly did not group with Micropanope sculptipes Stimpson, 1871 and M. lobifrons A. Milne-Edwards, 1880 , the only two species that are known to definitively represent Micropanope s.s. ( Guinot 1967, 1971). This was not unexpected given the long-questioned generic assignment of M. truncatifrons (see also Ng et al. 2008; Felder et al. 2009), even if the species had been correctly identified in that analysis. However, distinction of the sequenced specimen (= N. thomai n. sp.) from M. truncatifrons became clear in recent synoptic comparisons to types of the latter (USNM 9497, 9530, 20718), which confirmed that a subhepatic tubercle was indeed lacking in all three specimens of M. truncatifrons , as had been indicated in a brief diagnosis offered by Rathbun (1930: 433). While previously discounted as a definitive character because of its variable development, a subhepatic tubercle of some form was found in all the misidentified specimens of the species, though it was sometimes manifest as little more than a collection of enlarged granules or teeth. In addition, while the type series for M. truncatifrons was collected from 238–355 m depths, almost all of the misidentified specimens came from 55–80 m depths, with the maximum known bathymetric record reaching to no more than 155 m.

The gene-sequenced specimen misidentified as “ Micropanope truncatifrons –ULLZ 7359” (Thoma et al. 2014: fig. 1; herein = USNM 1541680) was shown in phylogenetic analyses to share a clade with Nanoplax xanthiformis . Morphological characters now also support this relationship, underpinning assignment of the new species to the formerly monotypic genus, Nanoplax . Among these are shared features in carapace anterolateral dentition (Fig. 1A–C; A. Milne-Edwards 1873 –1881: pl. 53 fig. 4a; Williams 1965: fig. 176; Hendrickx 1995: fig. 2B), shape of the third maxilliped merus (Fig.1B–D; A. Milne-Edwards 1873 –1881: pl. 53 fig. 4a), shape of the male first gonopod ( Fig. 2B , F–H, J, K; Guinot 1967: fig. 16; Martin & Abele 1986: fig. 3M, 194; Hendrickx 1995: fig. 3E, F), and features of the male pleon related to the fifth pereopod coxa and relative exposure of the eighth thoracic sternite ( Fig. 2 A–C, J); Hendrickx 1995: fig. 1B). In addition, the pleonal depression of the fourth sternite of mature males (beneath the pleonal telson when flexed) is in both species well-marked by deepened pockets to either side of the median line, apparently to accommodate distal ends of the first gonopods when the pleon is tightly flexed ( Fig. 2B, J ). While depression of the sternum was mentioned in the Guinot (1967) diagnosis of Nanoplax , this feature has not been previously illustrated, and it varies in development among present and former members of Micropanope . Initially, comparison of the first gonopods in the two species suggested striking differences in their terminal development, though close comparison revealed much in common. In both species of Nanoplax , the first gonopod trunk is broadly flattened and similarly twisted along its length, originating from a broad base articulated to the basis itself, which forms a subtriangular posterolaterally directed flange. This flange fully covers the male gonopore and papilla on the proximal extreme of the fifth pereopod coxa ( Fig. 2 A–C, J). Male first gonopods of both species have broadly open terminal vestibules or infundibula, distal to a patch of long, somewhat curved, spines that are directed proximally, these being present in addition to some microspinulation. However, an obvious difference in the first gonopods of mature males is found in the direction toward which the terminal opening faces and the degree to which it is enclosed by surrounding lips. In mature males of Nanoplax xanthiformis , the opening is typically vestibular, loosely framed by the surrounding lips (“flange” of Hendrickx 1998), and directed distomesially, with a patch of elongate proximally directed setae immediately proximal to the opening, these being evident in view on the pleonal surface of the first gonopod when the pleon is lifted. In mature males of Nanoplax thomai n. sp., the opening is more fully enclosed by overlapping lips to create a tubular funnel that flares terminally, with the entire apex of the first gonopod being hooked to direct the opening proximally (though less hooked and ranging from an anterolaterally to proximolaterally directed opening in immature males), with a somewhat smaller patch of elongate proximally directed setae immediately proximal to the hooked apex, these being largely concealed on the sternal side of the first gonopod when the pleon is lifted. Key characters articulated by Hendrickx (1998: 642) for recognition of the genus Nanoplax need only to encompass the alternative direction of the male first gonopod terminal opening in order to accommodate the second species.

For most specimens of both sexes, the carpus of the major and minor cheliped in N. thomai n. sp bears a pair of conspicuous spines on its inner angle, these being distinctly more acute and elongate than others that may trail ventrally or proximally from them. Both are readily evident in dorsal view, even though the larger of the two is set distally and slightly above the smaller. By contrast, in N. xanthiformis only one enlarged spine is readily evident on the internal angle of the carpus in dorsal view of the cheliped, though sometimes there is a second spine or tubercle set ventroproximally to it, usually only slightly larger than those around it. While not absolute, this character appears to separate most mature specimens.

In all presently known examples, fresh specimens of Nanoplax thomai n. sp. can be readily separated from N. xanthiformis (as well as from all other present and former members of Micropanope ) by unique, striking, scarlet red patches marking dorsolateral extremes of seventh and eighth sternites, just above the coxae of fourth and fifth pereopods ( Fig. 3 B–D). However, these are not evident dorsally without lifting or pushing forward the posterior edge of carapace. Of unknown function, nothing comparable in this anatomical location has been found in fresh specimens of N. xanthiformis or any other regionally sympatric pseudorhombilid. However, a very similar scarlet coloration can be seen on the third maxilliped propodite and epipod of N. xanthiformis and Micropanope lobifrons when the ventral edge of the anterior carapace is lifted above the coxae of the first pereopods in fresh specimens. This intense pigmentation is reminiscent of similar red to reddish-maroon coloration found consistently or occasionally on the internal surface of the third maxilliped ischium in many species of offshore pseudorhombilids, including but not limited to fresh specimens of Nanoplax xanthiformis , Micropanope sculptipes , Panoplax depressa Stimpson, 1871 , Milnepanopeus lobipes (A. Milne-Edwards, 1880) , Euphrosynoplax campechiensis Vázquez-Bader & Gracia, 1991 , and E. clausa Guinot, 1969 .

FIGURE. 1. Nanoplax thomai n. sp., male holotype, cw 11.6 mm (ULLZ 5794 = USNM 1542142) northwestern Gulf of Mexico. A, left side of body, dorsal surfaces; B, left side of body, ventral surfaces; C, left side, anterolateral quadrant, ventral surfaces; D, right third maxilliped, internal surface; E, major (left) cheliped, external surface; F, major (left) cheliped, internal surface; G, minor (right) cheliped, ventro-external surface; H, minor (right) cheliped, dorso-internal surface. Scale bars = 3 mm.

Previous records for Nanoplax thomai n. sp. may include more than are listed in the short synonymy above, though this remains uncertain. Distributional records for Micropanope truncatifrons compiled by Felder et al. (2009), therein listed under the now abandoned combination Nanocassiope truncatifrons ( Rathbun, 1898) , included Rathbun’s original records of Micropanope truncatifrons in addition to later records of several specimens reported as this species by Soto (1986: 35). Unfortunately, the latter specimens, all of which were indicated to be females, cannot be located, and it is uncertain whether they might in part represent Nanoplax thomai n. sp. Notably, one of those was from a depth of only 103 m on Cay Sal Bank, Bahamas, and thus within the known bathymetric range for N. thomai n. sp.

Figures

FIGURE 2. Nanoplax thomai n. sp., A–C, E–I = male holotype, cw 11.6 mm (ULLZ 5794 = USNM 1542142) northwestern Gulf of Mexico; D, female paratype, cw 8.8 mm (ULLZ 10946 = USNM 1545382) northwestern Gulf of Mexico. Nanoplax xanthiformis, J–L = male, cw 9.1 mm (ULLZ 17499 = USNM 1604271) northwestern Gulf of Mexico. A, male pleon, sternum, and pereopod coxae, posteroventral; B, male sternum and gonopods, pleon removed, ventral; C, male left posterior sternites, coxae, gonopore, and genital papilla, pleon and gonopods removed, ventral; D, fifth and sixth sternites, pleon extended to show female gonopores, ventral; E, male pleon, detached, ventral; F, male pleon and gonopods, internal (sternal) surface; G, male right first gonopod tip, pleonal surface; H, male right first gonopod tip, sternal surface; I, male right second gonopod, pleonal surface; J, male sternum and gonopods, pleon removed, ventral; K, male left first gonopod tip, mesial surface; L, male left first gonopod tip, pleonal surface. Scale bars A, B, D–F, J = 2 mm; C = 3 mm; G–I, K, L = 1 mm.

FIGURE 3. Nanoplax thomai n. sp., A, female paratype, cw 16.4 mm (ULLZ 7528 = USNM 1543091) northwestern Gulf of Mexico; B, ov female paratype, cw 12.9 mm (ULLZ 7359 = USNM 1541680) northwestern Gulf of Mexico; C, female paratype, cw 12.7 mm (ULLZ 13195 = USNM 1546796) northwestern Gulf of Mexico; D, E, male holotype, cw 11.6 mm (ULLZ 5794 = USNM 1542142) northwestern Gulf of Mexico; F, male paratype, cw 9.7 mm (ULLZ 10985 = USNM 1545402) northwestern Gulf of Mexico.