Aratus pacificus Thiercelin & Schubart, 2014, n. sp.
Creators: name: Thiercelin, N.
name: Schubart, C. D.
Publisher: name: Zenodo
Year: 2014
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Sesarmidae
Aratus
Aratus pacificus

description: Aratus pacificus n. sp.
(Figs. 3 C, D, G, H, K, L, O, P; 4C, D, G, H, K, L; 8C, D, E, F, I, J, K, L, O, P, Q, R, U, V, W, X; 9C, D, E, F, I, J, K, L, O, P, Q, R)
Aratus pisonii Smith 1871: 92.— Kingsley 1879: 402.— Rathbun 1918: 322 -325, pl. 96, fig. 1 & 2.— Crane 1947: 86.—Cuesta et al. 2006: 175-196, fig. 1–7.
Type material. Holotype male (17.3 × 16.6mm) (SMF 43530), Ecuador, Puerto Morro, 2°36.498'S – 80°18.245'W, coll. C.D. Schubart & R. Landstorfer, 10 Oct.2007.
Paratypes. Ecuador. Puerto Morro, 2°36.498'S – 80°18.245'W, 2 ♂ 1 ♀ (14.7 × 14.5, 15.3 × 14.9, 14.6 × 14.5mm) (RMNH. CRUS.D.55082-55084)— 2 ♂ 1 ♀ (13.45 × 13.0, 13.4 × 13.35, 15.05 × 14.9mm) (MNHN-IU- 2009-3091, MNHN-IU-2009-3092, MNHN-IU-2009-3093)— 1 ♂ 2 ♀ (12.35 × 12.25, 12.55 × 12.45, 13.25 × 13.25mm) (NHMW 25472-25474) - 1 ♂ 1 ♀ (12.9 × 12.55, 11.05 × 10.7mm) (ZRC.2013.0452) - 2 ♂ 2 ♀ (10.7 × 10.55, 8.9 × 8.9, 9.75 × 9.65, 9.25 × 9.5) (SMF 43531), coll. C.D. Schubart & R. Landstorfer, 10 Oct.2007. Costa Rica. Puntarenas, Mata de Limón, 9°55.422'N- 84°42.759'W, 1 ♂ 2 ♀ (12.59 × 11.93, 14.48 × 14.4, 12.39 × 12.18mm) (ZSMA 20130010-20130012) - 3 ♂ 2 ♀ (18.11 × 17.82, 11.6 × 11.39, 11.55 × 11.55, 13.52 × 13.44, 11.26 × 10.70mm) (MZUCR 3173-01) - 3 ♂ 3 ♀ (15.49 × 15.38, 9.62 × 9.48, 6.36 × 6.19, 13.48 × 13.21, 13.06 × 12.98, 6.10 × 6.10mm) (SMF 43532), coll. N. Thiercelin & T. Poettinger, 29 Apr. & 2 May 2011.
Description. Carapace trapezoidal, frontal margin conspicuously wide, orbits extending to anterolateral margin; front vertical, four lobes, about 4 times as long as deep; lateral margins subparallel, upper margin with row of granules, clear median groove separating wider outer lobes from inner lobes. Lower margin divided by median incision. Postorbital angles acute; lateral margins carinate, convergent, posterior margin slightly rounded. Presence of 5 or 6 subparallel carinae running obliquely along posterior half of lateral margin in direction to postlateral angle; carapace regions sharply delineated; granular frontal region, gastric lobes, sides finely punctate, remaining upper surface smooth, shiny. Pterygostomial region finely, regularly beaded. Seven-segmented male and female abdomens, subcircular, except for last somite abruptly narrowed with distal margin rounded. Transversely folded antennules under frontal margin. Small antennae excluded from orbit by broad, rounded lobe. Short eyestalk; large cornea, terminal, well developed. Well separated external third maxillipeds, with distance between them near equal to width of one ischium maxilliped. Unequally suboval ischium, inner side more convex than outer side; distinct median channel on ischium of outer face; oblong oval merus, scarcely one-sixth longer than ischium, with distinct groove subparallel to outer margin, deeper groove obliquely channelling median region; narrow palp elongate, setiferous; maxillipeds inner lateral margins fringed with long, stiff setae.
Chelipeds sexually dimorphic. Small cheliped with sharp subdistal tooth on upper ischial margin; three-sided merus, granular on lower surface margins; convex carpus, granular. Greatly dilated male propodus, convex, granular, furnished with clusters of stiff bristles; fingers nearly as long as palm, gaping widely in males, tips meeting, inner edges weakly dentate; bristles present on proximal half of both fingers. Ambulatories long, slender, decreasing in length anteriorly from fourth to second; meral joint oblong, suboval, one-third as wide as long, anterior lateral margin with acute subdistal tooth; upper surface roughly granular; elongated carpus, narrowed basally, dilated distally with 2 longitudinal carinae; distally narrowed propodus about twice carpus length; short dactyli, stout, less than one-third of propodi length, conspicuously acuminate.
First male gonopod (Figs. 3 C, D, G, H, K, L, O, P; 4C, D, G, H, K, L; 8C, D, E, F, I, J, K, L, O, P, Q, R, U, V, W, X; 9C, D, E, F, I, J, K, L, O, P, Q, R) elongated, slender. Stem triangular in cross section; basal diameter wider than medial diameter. Mesial, ventral faces flat; slanted suture (sperm channel) clearly marked on dorsal face with terminal section in strongly marked depression; endpiece larger than medial part, easily noticeable in mesial, lateral views, endpiece bulging in lateral part; dorsal part almost spoon-shaped in mesial view. Mesial endpiece ventral, dorsal parts at same level (horizontal in mesial view); tip short, chitinous and slightly curved (approx. 90°) in medial position of apex, with pore on dorsal margin; suture aligned with pore on chitinous tip; mesial face of chitinous tip/mesial face approx. 130°; small denticles on lateral face of chitinous process. Large number of long setae on apex around chitinous tip, lateral, dorsal sides of endpiece lateral bulge. Row of setae on endpiece at edge between mesial, dorsal faces. Few short setae on gonopod.
Etymology. The epithet pacificus refers to the geographical distribution of the new species, which is distributed in the eastern Pacific Ocean, in contrast to Aratus pisonii, which is restricted to the western Atlantic Ocean.
Biology. Found in mangroves from estuaries and lagoons, in saline to hypersaline (e.g. SMF 43530, RMNH.CRUS.D.55083-55084) or brackish waters (e.g. MZUCR 3173-01, ZSMA20130011). Rebolledo-Navarro & Wehrtmann (2012) and Wehrtmann & Rebolledo-Navarro (2012) compared the ecology of A. pisonii and A. pacificus n. sp. in Costa Rica. They observed 1) smaller egg volume in A. pacificus n. sp. than in A. pisonii, and 2) differences in the population structure of the two species, with females found in an intermediate size class in A. pacificus n. sp., whereas in A. pisonii females are found in larger size classes.
Coloration. Wehrtmann & Rebolledo-Navarro (2012) pointed out differences between the coloration of Atlantic and Pacific populations in Costa Rica, with the Atlantic population having reddish chelae, whereas the Pacific populations have red-orange chelae. Similar patterns were observed in Panama (R. Lasley and C.S. McKeon, personal communication). We remain cautious concerning the reliability of the live coloration, however, as variability was observed in the field between specimens from north and central Brazil. Photographs of Florida specimens even exhibit a different coloration with purple coloration on the external side and reddish on the inner side of the chelae, which differs from the Brazilian populations which present only reddish pattern on both sides of the chelae. It remains difficult to use the color pattern to distinguish between the two species, as long as the variability is not clearly established between the different populations of Aratus pisonii sensu stricto along its entire distribution range. The carpi of the ambulatory legs of alcohol-preserved specimens often exhibit stripes in Aratus pacificus n. sp., whereas they are of a homogeneous colour in Aratus pisonii.
Geographical distribution. Tropical eastern Pacific coastline, from Baja California to Peru (Beever et al. 1979), although only specimens between Costa Rica and Ecuador have had their identity confirmed in the present study.
Remarks. Aratus pacificus n. sp. can be distinguished from A. pisonii by their respective male first gonopods. The gonopod of A. pacificus n. sp. is slightly more slender than the gonopod of A. pisonii. In A. pisonii the ventral mesial part of the terminal end is higher than the dorsal part (angle approx. 25° in mesial view), whereas both parts are at the same level in A. pacificus n. sp. (Figs. 3 A–D, 6A–F and 8A–F). The chitinous tip is located ventrally and is U-shaped in A. pisonii (Figs. 4 A, B, E, F, 7A–L, 9A, B, G, H), whereas the tip has a central position on the apex and is curved not more than 90° in A. pacificus n. sp. (Figs. 4 C, D, G, H, 9C–F and 9I –L). A small depression at the base of tip is present in A. pisonii, but absent in A. pacificus n. sp. (Figs. 4 A–D, 7A–F and 9A–F). In A. pisonii, the suture is aligned with the edge of the chitinous tip on the apex (Figs. 3 E–F, 5, 6G–L and 8G, H). In A. pacificus n. sp., in contrast, the suture is aligned with the pore on the chitinous tip (Fig. 3 G, H, 5 and 8I –L). This also implies that it is possible to measure the angle in dorsal view between the mesial face of the chitinous tip and the suture in A. pisonii (approx. 115°), which is not possible in A. pacificus n. sp. (Fig. 5). The angle between the mesial side of the gonopod and the mesial side of the chitinous tip is approx. 110° in dorsal view in A. pisonii and approx. 130° in A. pacificus n. sp. (Figs. 3 E–H, 6G–L and 8G–L). Allometric changes in the gonopod shape can be observed between the smallest and largest specimens in both species (as in specimens NHMW25475 and NHMW25477 for A. pisonii; MZUCR 3173-01 (18.11 × 17.82mm) and MZUCR 3173-01 (11.60 × 11.39mm) for A. pacificus n. sp.). These changes are more marked in dorsal and ventral views, where the gonopods appear more slender for the smallest specimens in both species, and remains clearly visible in preserved specimens regardless of their size.
description: Published as part of Thiercelin, N. & Schubart, C. D., 2014, Transisthmian differentiation in the tree-climbing mangrove crab Aratus H. Milne Edwards, 1853 (Crustacea, Brachyura, Sesarmidae), with description of a new species from the tropical eastern Pacific, pp. 545-560 in Zootaxa 3793 (5) on pages 553-556, DOI: 10.11646/zootaxa.3793.5.3, http://zenodo.org/record/229915