Megalobrachium pacificum Gore & Abele, 1974

Ferreira, Luciane Augusto De Azevedo & Anker, Arthur, 2021, An annotated and illustrated checklist of the porcelain crabs of Panama (Decapoda: Anomura), Zootaxa 5045 (1), pp. 1-154 : 28

Megalobrachium pacificum Gore & Abele, 1974

( Figs. 9–11 )

Megalobrachium pacificum Gore & Abele 1974: 568 , fig. 3F; Gore & Abele 1976: 17; Gore 1982: 6; Hiller et al. 2004: 3; Lazarus-Agudelo & Cantera-Kintz 2007: 229; Hiller & Lessios 2019: 414, fig. 1e.

Megalobrachium poeyi .— Haig 1957b: 14; Haig 1960: 214, pl. 16, fig. 4, pl. 39, fig. 1; Haig 1962: 188 (part.); Haig 1968: 71; Gore 1971: 404 (part., Pacific specimens) [not Megalobrachium poeyi Guérin-Méneville, 1855 )].

Material examined. Panama [Pacific]: 1 female, cl 5.6, cw 6.1 ( FLMNH UF 57702 ), Coiba Is ., Isla Coiba , Bahía Santa Cruz , 7°37’18.3”N, 81°46’06.8”W, mudflat with large rocks exposed at low tide, leg. M. Leray et al., 23.02.2019 (fcn PAN-250); 1 male, cl 7.3, cw 7.9 ( MZUSP 33141 ), Río Mar , rocky-sandy intertidal, leg. A. Anker & J.F. Lazarus-Agudelo, 23.03.2015; 1 male, cl 4.0, cw 4.4 ( MZUSP 33579 ), Río Mar, rocky-sandy intertidal, leg. A. Anker, J.F. Lazarus-Agudelo & T. Kaji, 19.03.2015; 1 female, cl 4.5, cw 4.9 ( MZUSP 33472 ), Las Perlas Is ., Isla Contadora, north end of Playa Larga, rocky-sandy intertidal, under rocks and in rock crevices, leg. A. Anker, J. Luque, A.R. Palmer & T. Kaji, 15.04.2015; 1 male, cl 9.3, cw 9.5 ( MZUSP 33474 ), same collection data as for previous specimen; 1 male, cl 7.4, cw 8.3 ( FLMNH UF 57120 ), Las Perlas Is ., Isla Saboga , rocky shore near “Beach Club”, 8°37’29.3”N, 79°04’13.6”W, rocky intertidal, under rocks, leg. P.P.G. Pachelle & M. Leray, 19.03.2019 (fcn PP 19-013); 1 male, cl 8.7, cw 9.3 ( FLMNH UF 57121 ), same collection data as for previous specimen (fcn PP 19-014). Extra-limital material . Colombia [Pacific]: 1 ov. female, cl 7.8, cw 8.4 (CRBM-UV-cr:2009-102), Bahía Málaga , Los Negros, 03°59’N, 77°17’W, large boulders on sand, under rocks, leg. A. Anker, 25.04.2009 (fcn COL- 149); 1 male, cl 7.5, cw 8.0 (CRBM-UV-cr:2009-104), same collection data as for previous specimen (fcn COL- 160) .

Previous records from Panama. Haig (1957b, 1960, 1962, 1968, all as M. poeyi ( Guérin-Méneville, 1855)) ; Gore & Abele (1974); Gore & Abele (1976).

Distribution. East Pacific: Mexico, Costa Rica, Panama (Coiba, Río Mar, Taboga Is., vicinity of Ciudad de Panamá, Las Perlas Is.), Colombia and Ecuador ( Haig 1960, 1962, 1968, as M. poeyi ; Gore & Abele 1974, 1976; Hiller et al. 2004; Lazarus-Agudelo & Cantera-Kintz 2007; present study).

Ecology. Rock-sand intertidal and subtidal, presently known depth range: 0–46 m; under rocks on muddy sand or in rock crevices, individuals often coated with fine silt ( Haig 1960, as M. poeyi ; Lazarus-Agudelo & Cantera- Kintz 2007; present study).

Remarks. Megalobrachium pacificum is the eastern Pacific sibling sister of the western Atlantic M. poeyi ( Guérin-Méneville, 1855) (Hiller & Lessios 2019) , with which it was treated as conspecific for many years (e.g. Boone 1931; Haig 1960, 1968) until the formal species description in 1974. Gore & Abele (1974) separated the two transisthmian sister species by several subtle morphological characters. In M. pacificum , the protogastric region of the carapace is indistinct and not higher than the frontal and hepatic regions, whereas in in M. poeyi , this region is distinct and clearly elevated above the frontal and hepatic regions. The granules on the carapace and P1 of M. pacificum are smaller and less distinct than those of M. poeyi . In addition, the P2–P4 are much more robust in M. pacificum than in M. poeyi ; this is especially apparent in the proportions of the propodi. The colour patterns of M. pacificum ( Figs. 9–11 ) and M. poeyi ( Figs. 12 , 13 ) are somewhat variable, but generally very similar. The Pacific species often has a pinkish or dark reddish patch on the ventral side of the palm (= manus), which is usually absent in the Atlantic species (cf. Figs. 9B , 12B ).

Hiller & Lessios (2019) recovered two genetically separated clades within M. pacificum , viz. the more southern Colombian-Panamanian clade and the more northern Panamanian-Costa Rican clade. The authors provided a colour photograph of a specimen from Bahía Solano, Colombia, with a highly contrasting colour pattern, which is also seen in one relatively young specimen collected near Ciudad de Panamá in 2007 ( Fig. 10A , material not deposited). However, most specimens from Panama, e.g. from Coiba, Río Mar and Las Perlas ( Figs. 9 , 10 ), as well as from Bahía Málaga, Colombia ( Fig. 11A ), display a less contrasting, more homogenous reddish colour. A more rigourous collecting with photographic vouchering will be necessary to show whether the southern and northern clades of M. pacificum are also distinguishable by colouration.

Figures

FIGURE 9. Megalobrachium pacificum Gore & Abele, 1974: A, B, male (cl 7.3 mm, cw 7.9 mm) from Río Mar, Panama (MZUSP 33141), dorsal (A) and ventral (B) views [photographs by AA].

FIGURE 10. Megalobrachium pacificum Gore & Abele, 1974: A–C, female (cl 5.6 mm, cw 6.1 mm) from Coiba Island, Panama (FLMNH UF 57702), dorsal (A), ventral (B) and frontal (C) views [photographs by AA].

FIGURE 11. Megalobrachium pacificum Gore & Abele, 1974: A, young specimen (sex, cl cw indet.) from Ciudad de Panamá, Panama (not deposited), dorsal view; B, extra-limital specimen, male (cl 7.5 mm, cw 8.0 mm) from Bahía Málaga, Colombia (CRBM-UV-cr:2009-104), dorsal view [photographs by AA].

FIGURE 12. Megalobrachium poeyi (Guérin-Méneville, 1855): A, B, female (cl 7.0 mm, cw 7.4 mm) from Bocas del Toro, Panama (MZUSP 33522), dorsal (A) and ventral (B) views [photographs by AA].

FIGURE 13. Megalobrachium poeyi (Guérin-Méneville, 1855): A, male (cl cw indet.) from Portobelo, Panama (not deposited), dorsal view; B, male (cl cw indet.) from Bocas del Toro, Panama (not deposited), dorsal view; C, specimen of unknown sex, possibly female (cl cw indet.), from Golfo de San Blas, Guna Yala, Panama (not deposited), frontal view [photographs by AA].