Robertsella mystica Guinot, 1969

Gouvêa, Ariane, 2013, A new species of Robertsella Guinot, 1969 (Crustacea: Brachyura: Panopeidae) from the southwestern Atlantic, Zootaxa 3734 (1) : -

Robertsella mystica Guinot, 1969

( Figs. 1 E–H; 2E–H; 3B, D; 4C–D)

Robertsella mystica Guinot, 1969: 716 , pl. 5, figs. 1, 132–133.

Robertsella mystica— Powers 1977: 115–117; Soto 1985: 482, 1986: 37, 1991: 626; Wenner & Read 1982: 189; Hendrickx 1998: 643; Števčić 2005: 52; Ng et al. 2008: 189; Felder et al. 2009: 1084; Castro et al. 2010: 15. Robertsella nr. mystica— Wenner & Read 1982: 189.

Material examined. United States: South Carolina, off Charleston, 37o27'42"N – 79o34'30"W, B. B. Boothe coll. 26.i.1976, A. B. Williams det., 1976, 22 m: male, cl 18 mm, cw 23 mm (USNM 170418). Gulf of Mexico, Florida, Monroe County, south of Dry Tortugas, RV Oregon, stn 1330, 10.vii. 1955, 366 m: holotype male, cl 20 mm, cw 15 mm (USNM 99490). Miami, RV Albatross, stn 2644, 25 o40'00"N – 80o00'00"W, 9.iv. 1886, 353 m: paratype male, cl 8 mm, cw 11 mm (USNM 11407).

Description. Carapace distinctly wider than long, maximal width at fourth anterolateral tooth, minutely granular near margins, granulation slightly more distinct posterolaterally; mesogastric, cardiac, intestinal regions poorly defined by faint grooves; cardiac and intestinal grooves little more distinct. Granulation more distinct on suborbital, subhepatic and branchiostegal regions. Pterygostomian region granular near moulting suture, smooth near mxp3, with distinct transverse row of coarser granules on low crest (plectrum) above inhalant channel. Branchiostegal margin lined with granules, above distinct row of granules all along margin. Fronto-orbital width distinctly more than half of maximal width of carapace; frontal margin V-shaped with median cleft; frontal margin smooth; just behind frontal margin inconspicuous scattered granules, pits. Supraorbital margin interrupted by 2 small notches, lined with distinct granules, granulation coarsest on lateral half, continued to outer orbital tooth (first anterolateral). Inner supraorbital tooth low, blunt, separated from front by wide, shallow gap; outer orbital tooth lobe-like. Suborbital margin lined with small granules, mesially with broadly triangular lobe directed toward front, laterally with broad lobe, V-shaped cleft between suborbital lateral lobe and outer orbital tooth. Anterolateral margin projecting in 5 distinctly granular teeth (outer orbital included), increasing successively in size posteriorly from first to fourth tooth: second tooth blunt; third broad triangular; fourth spiniform, blunt; fifth (last anterolateral tooth) very small but distinct. Gap between anterolateral teeth 3–4 much wider than between remaining teeth. Posterolateral margin well defined, rather straight behind last anterolateral tooth, slightly shorter than anterolateral margin. Limit between epistome and endostome well defined, forming pronounced, sinuous lip, interrupted by 2 Vshaped notches, one at each side of shallow mesial notch. Ocular peduncle cylindrical, widely scattered granules dorsally, freely movable, thick, constricted subproximally, fully retractable into orbital cavity; cornea occupying little less than half-length of whole eye, brownish; ommatidia poorly recognizable. Antennules prominent; proximal article thickest laterally, with row of transverse granules; second article smooth, elongated, subcylindrical, articulated with proximal article at mesial end of antennular fossa; third article nearly equal in length to second, swollen distally, tapered to proximal articulation with second article, terminally with long marginal setae at either side of dorsal flagellum. Antennal article 2+3 immovable, filling orbital gap; articles 4, 5 freely movable, subcylindrical. Thoracic sternum regularly moderately punctuate, pits rather faint; sternoabdominal cavity deep, smooth anteriorly, lateral margins with scattered, minute granules more discernible posteriorly. Abdominal locking system functional in male, thoracic sternal button small, directed forward, placed about mid-length between thoracic sternal sutures 4/5, 5/6. Third maxillipeds inserted wide apart from each other; ischium smooth, pitted, with faint longitudinal furrow, mesial margin granular; merus distinctly shorter than ischium, evenly granular; palp smooth. Chelipeds heterochelous, right P1 strongest. Merus of major P1 trigonal, dorsal margin with strong, blunt, subdistal spine; surfaces evenly covered with small granules. Carpus with blunt spine on inner margin; surfaces granular, granules stronger anteriorly and ventrolaterally. Propodus stout, surfaces minutely granular, granules forming reticular patterns. Fingers closing terminally only; cutting edges bluntly dentate proximally, teeth becoming more coalescent posteriorly; teeth stronger on fixed finger. Minor cheliped similar to major cheliped except for its slender fingers. Ambulatory legs (P2–P5 detached from body) long, slender, similar to each other. P2–P5 meri spinulous dorsally, granular ventrally. P2 – P5 carpi with dorso-lateral, longitudinal, smooth depression, on each side a row of distinct granules; ventral surface smooth. P2–P5 propodi with scattered granular proximally. P2–P5 dactyli corneous-tipped, 6 longitudinal rows of long setae, 2 dorsal, 2 ventral, 2 lateral. Abdomen of 4 somites and telson; abdominal somites 1–2 markedly short, somite 1 much broader than somite 2; somites 3–5 fused together; abdominal somite 3 expanded laterally, partially covering penis distally; abdominal suture 3/4 faint, still recognizable; suture 4/5 indistinct, its position indicated laterally only by faint cleft. Telson semicircular. G1 distinctly curved outward, just reaching to thoracic sternal suture 4/5. G2 short, just reaching to sternal suture 7/8.

Remarks. Robertsella mystica was erected by Guinot (1969: 716) for two males, one previously assigned to Pilumnoplax elata (A. Milne-Edwards, 1880) by Rathbun (1918), and the second collected by R.V. Oregon in 1955 and identified as Pilumnoplax by F. Chace. Guinot (1969) provided a preliminary diagnosis and illustration for R. mystica and referred to Rathbun (1918) for the description of the dorsal surface of the carapace and appendages. Rathbun's description is too brief to allow confident identification, thus R. mystica is redescribed and illustrated herein based on the holotype. Rathbun (1918: 24) noticed growth variation in the first and last anterolateral teeth of the carapace and the merus of the cheliped, which is less developed in younger specimens. In the large male USNM 170418 the last tooth of carapace is indeed distinctly more produced than in the smaller holotype male (USNM 99490), even though the first carapace teeth are equivalents in both specimens. In the male (USNM 170418) the cheliped merus bear a strong, subdistal acute spine on the lateral margin and the carpus an acute, subproximal spine. In the younger holotype male both merus and carpus possess a strong tubercle instead of a spine.

The strong faunistic relationships between the southwestern Atlantic and the Caribbean Sea and the eastern Gulf of Mexico has ultimately concealed the patterns of diversity in the southwestern Atlantic. In most groups of marine benthic invertebrates the number of species in common with the southwestern Atlantic, on one hand, and the Caribbean Sea and the eastern Gulf of Mexico, on the other hand, is conspicuously high (over 80%; see for instance Boschi 2000). Although critical for understanding marine diversity in the southwestern Atlantic, direct comparative studies between southern specimens and those collected further north are highly infrequent and, therefore, the knowledge of the marine benthic diversity in the southwestern Atlantic is largely the result of identifications based on literature alone (see also Tavares 2011: 260). The recognition of southern counterparts for northern species that have been historically believed to be widely distributed in the western Atlantic (either continuously or disrupted) has nevertheless been the subject of several reports (e.g., Manning & Holthuis 1989; Manning et al. 1989; Brandão et al. 2010, 2012; Tavares 1991, 1993; Tavares & Melo 2005, 2010; Santana & Tavares 2008; Tavares & Pinheiro 2011), of which Robertsella meridionalis n. sp., the southern counterpart of R.

mystica , is one more example. The largely untested assumption of a widely distributed western Atlantic decapod fauna (e.g., Coelho & Ramos, 1972; Melo, 1996; Boschi, 2000) has perhaps been too easily accepted as factual, although to which extent is yet to be investigated.

Figures

FIGURE 1. A – D, Robertsella meridionalis sp. n., male holotype cl 22 mm, cw 30 mm (MZUSP 21772). E – H, Robertsella mystica Guinot, 1969, male cl 18 mm, cw 23 mm (USNM 170418). A, E, frontal view of cephalothorax. B, F, dorsal view of carapace. C, G, anteroventral view of cephalothorax. D, H, posteroventral view of cephalothorax. Note abdominal suture 5 / 6 located well above thoracic sternal suture 6 / 7 in D and sutures aligned to each other in H (arrows). Also note cheliped ischial ridge coarsely tuberculate in C (arrow). Ab 3, third abdominal somite. CxP 5, coxa of fifth pereiopod. St 6 - 8, thoracic sternites 6 to 8. Scale bars 5 mm.