Percnon gibbesi

Percnon gibbesi (H. Milne Edwards, 1853)

(Fig. 52A–F)

Acanthopus gibbesi H. Milne Edwards, 1853: 180 [Type locality: Antilles].

Trindade and Martin Vaz specimens. 1 male (MZUSP 40247), 1 male (MZUSP 40249), Trindade Island, Ilha da Racha, 20°30’26.5”S, 29°20’48.0”W, J.B. Mendonça coll., 30.vii.2018, 23 m. 1 juvenile male (MZUSP 40244), ibidem, J.B. Mendonça coll., 16.vii.2013, 20.8 m. 1 juvenile male (MZUSP 40329), ibidem, J.B. Mendonça coll., 12.vii.2012, 30.3 m. 1 juvenile female (MZUSP 40166), ibidem, Enseada da Cachoeira, Farrilhões, 20°31’29.8’’S, 29°19’52.0”W, J.B. Mendonça coll., 21.xi.2017, 11.3 m. 1 male, 1 juvenile female (MZUSP 33827), ibidem, 20°31’22.4’’S, 29°19’52.0”W, J.B. Mendonça coll., 9.vii.2013, 10.4 m. 1 male (MZUSP 40164), ibidem, Mendonça coll., 23.vii.2015, 17 m. 1 ovigerous female (MZUSP 33824), ibidem, Enseada das Orelhas, 20°29’40.2’’S, 29°20’32.9”W, J.B. Mendonça coll., 1.xi.2014, 12 m. 1 male (MZUSP 39608), ibidem, J.B. Mendonça coll., 8.viii.2018, 9 m. 1 ovigerous female (MZUSP 40260), ibidem, Enseada dos Portugueses, SECOM/ ECIT, 20°30’20.9’’S, 29°18’43.7’’W, J.B. Mendonça coll., 4.vii.2016, 11.4 m. 1 ovigerous female (MZUSP 42002), ibidem, Enseada dos Portugueses, 20°30’17.7’’S, 29°18’56.7’’W, J.B. Mendonça coll., 11.vii.2012, 11 m. 1 male (MZUSP 40168), ibidem, Enseada da Cachoeira, Praia do M, 20°30’53.8’’S, 29°20’19.2’’W, J.B. Mendonça coll., 27.xi.2017, 10.4 m. 1 male (MZUSP 41240), 3 males, 1 female (MZUSP 41241), ibidem, Ponta do Monumento, 20°30’53.8’’S, 29°20’19.2’’W, J.B. Mendonça coll., 13.vii.2012, 8.1 m. 2 females (1 ovigerous) (MZUSP 41242), ibidem, 20°30’10.3’’S, 29°20’36.1’’W, J.B. Mendonça coll., 16.vi.2012, 12.1 m. 1 male (MZUSP 33837), ibidem, Praia das Tartarugas, 20°30’54.1’’S, 29°18’18.2’’W, J.B. Mendonça coll., 5.vii.2013, tide pool. 1 ovigerous female (MZUSP 39614), ibidem, 20°29’46.4’’S, 29°20’35.4’’W, J.B. Mendonça coll., 6.viii.2018, 9.5 m. 1 megalopa (MZUSP 40569), ibidem, Ponta Norte, Crista do Galo, 20°29’14.8’’S, 29°20’13.9”W, J.B. Mendonça coll., 21.v.2014. 1 male (MZUSP 40255), Martin Vaz Archipelago, 20°30’45.7’’S, 29°18’21.9”W, J.B. Mendonça coll., 23.vii.2013, 13 m. 1 ovigerous female (MZUSP 40169), ibidem, Ilha do Norte, 20°28’11.51’’S, 28°51’25.01”W, J.B. Mendonça coll., 25.i.2019, 19.1 m.

Size of largest male: cl 21 mm, cw 19 mm; largest female: cl 18.9 mm, cw 17.8 mm.

Comparative material examined. Percnon gibbesi: Western Atlantic: Barbados: 1 male (MZUSP 28062), Christ Church, Bridgetown, Turtle Beach, 13°04’30.77’’S, 59°36’16.88”W, L.R.L. Simone coll., 13.x.2012. Brazil: Rocas Atoll: 1 male (MZUSP 12473), 1 female (MZUSP 12477), R.L. Moura and R.S. Rosa coll., vii.1995. Fernando de Noronha Archipelago: 1 juvenile female (MZUSP 41243), inside sponge, iii.1998, no further details. 2 males (1 juvenile) (MZUSP 8333), ibidem, Ponta Air France, P.S. Young coll., 2.xii.1986. 1 female (MZUSP 7236), ibidem, Buraco do Inferno, S.A. Rodrigues coll., 30.x.1985. 1 juvenile male (MZUSP 20569), ibidem, Praia das Caieiras, 3°50’11’’S, 32°23’53.4”W, L.R.L. Simone and C.M. Cunha coll., 8.iii.2009. Central Atlantic: Ascension Island: 1 male (USNM 256690), south of Collyer Point, stn 6A–76, R.B. Manning coll., 14.vii.1976. 1 female (USNM 256691), ibidem, McArthur Point, stn ASC-1B–76, R.B. Manning coll., 11.vii.1976. Venezuela and Brazil (Rocas Atoll, Fernando de Noronha, and Trindade and Martin Vaz) (Pocock 1890, as Leiolophus planissimum Herbst; Coelho & Ramos 1972, as Percnon planissimum (Herbst); Rodriguez 1980; Williams 1984; Melo 1996; Alves et al. 2008; Felder et al. 2009). This is the first record of Percnon gibbesi from Rocas Atoll and Trindade and Martin Vaz. Central Atlantic: Ascension and Saint Helena islands (Miers 1881; Manning & Chace 1990). Eastern Pacific: Baja California to Chile, including the Galapagos Islands (Rathbun 1918; Garth 1946; Williams 1984). Eastern Atlantic: one occasional record from off the west coast of Portugal, near Sesimbra (Paula & Hartnoll 1989); Azores, Cape Verde slands, Canary Islands and Morocco to Angola (d’Udekem d’Acoz 1999; Manning & Chace 1990; González 2016; 2018). Invasive to the Mediterranean Sea, through the Strait of Gibraltar, from Oran Bay (Algeria) eastward to the Levantine coast (Relini et al. 2000; Yokes & Galil 2006; Ilan et al. 2015; Stasolla et al. 2016; Hussein et al. 2020, and references therein).

Ecological notes. Percnon gibbesi (Fig. 52A–F) inhabits rocky, stone beaches and reefs from the splash zone down to 30 m in its native range (Rathbun 1918; Garth 1946; this study). It is commonly found in association with the sea urchin Diadema ascensionis Philippi, sometimes two crabs per host (Williams 1984; Hayes et al. 1998; this study), Echinometra (Manning & Chace 1990), and occasionally inside sponges (see above under material examined). Urchins and their boreholes can provide structurally complex microhabitats (Schoppe & Werding 1996), especially in areas which tend to be poor in reef builders, such as Trindade and Martin Vaz. In Trindade, Diadema ascensionis is known to provide microhabitat and shelter for several species, including other decapods (Tavares et al. 2017 as D. antillarum) and fishes (Giglio et al. 2018). There, P. gibbesi has been found living independently of D. ascensionis or both underneath the urchin on the bottom of the borehole and alongside the hole’s wall (Fig. 52F).

Ovigerous females are known from May to September in the Gulf of Mexico (Williams 1984); February in Galapagos (Garth 1946); February, March, April and August off West Africa (Manning & Holthuis 1981); and June 2012, November 2014, July 2016, August 2018 and January 2019 either in Trindade or Martin Vaz (see above un- der material examined). A megalopa (MZUSP 405969) was recovered in Trindade in May 2014. The zoeal stages, megalopa and first crab instar were described by Paula & Hartnoll (1989). The larval development goes through six stages zoeae (perhaps seven) in P. gibbesi and the late zoeal stages and megalopa are apparently more concentrated near the sea surface (Paula & Hartnoll 1989). This behavior lend support to the hypothesis that the currents that enter the Mediterranean through the Strait of Gibraltar transported P. gibbesi larvae that ultimately settled (Pipitone et al. 2001).

In the Mediterranean basin (see above under distribution), P. gibbesi has been found frequently in small aggregations of 2 to 5 individuals, between 0 and 8 m. Gently sloping rocky bottoms with crevices and encrusting algae are apparently preferred (Deudero et al. 2005; Félix-Hackradt et al. 2018). Puccio et al. (2006) suggested that P. gibbesi is strictly herbivorous based on a dietary analysis from stomach contens of a population from Sicily, while others have found that it feeds on macroalgae (brown, green and red) and animal materials including pagurids and other crustaceans, polychaetes, gastropods, and presumably jellyfish (Deudero et al. 2005; Sciberras & Schembri 2008). Ovigerous females are known from May to October in the Maltese Islands (Sciberras & Schembri 2008). Estimates based on pleon morphology suggested that the sexual maturity in P. gibbesi is reached at cl 15–16 mm in Maltese waters, although an ovigerous female at cl 14 mm has been reported from southern Italy. Egg mass was found to range from 254 to 32,040 281 eggs. Percnon gibbesi and Pachygrapsus marmoratus were found to be syntopic on the Maltese coast, but agonistic interactions between the two species were rarely observed in the field (Sciberras & Schembri 2008, and references therein). Contrary to the western Atlantic populations (including Trindade and Ascension), P. gibbesi has not been reported associating with sea urchins in the Mediterranean.

Remarks. Percnon lacks the characteristic grapsoid rhomboidal gap between the third maxillipeds which exposes the mandibles when closed. However, the genus was removed from the grapsoid family Plagusiidae (see Ng et al. 2008) by Schubart & Cuesta (2010) essentially based on molecular data and larval morphology (see also Guinot et al. 2018).

In Percnon, the male gonopore opens so close to the coxo-sternal articular condyle that it appears to be appendicular (Guinot et al. 2013; Kienbaum et al. 2018). However, no dissections have been carried out so far to find whether the male ejaculatory duct passes between the coxal muscles (heterotreme condition) or opens straight on the thoracic sternite 8 without connecting the P5 coxa (thoracotreme condition). Our dissections based on fresh mate- and hence devoid of any sclerotized protective covers. The soft penial tissues are only protected by the pleon, which completely covers the penis and closely seals the sterno-pleonal cavity. The lateral margin of the fused pleonal somites 3–5 expands laterally at the level of the P5 coxa to cover the penis almost completely. It additionally expands laterally into a small triangular protrusion which fits close into the space left between the sternite 8 and the coxosternal articular condyle of P5. This triangular protrusion helps to cover and hence protect the penis. The sternite 8 is shallowly concave laterally to receive the lateral end of the pleonal somite 2 expanded laterally to cover the P5 coxa partially.

In Percnon gibbesi, the vulva opening on thoracic sternite 6 is not obliterated by an operculum or a sternal vulvar cover (see also Guinot et al. 2013 and references therein).

Small males (e.g. MZUSP 40247, cl 6.2 mm, cw 5.2 mm) and females of P. gibbesi lack the characteristic patch of feathered setae along dorsomesial side of the adult male cheliped merus.