Omalacantha interrupta

Omalacantha interrupta (Rathbun, 1920)

(Fig. 22A)

Trindade specimens. 1 male (MZUSP 40196), Trindade Island, Praia das Tartarugas, 20° 31’1.3”S, 29°17’56.9”W, J.B. Mendonça coll., 12.vii.2013, 10.8 m. 1 male (MZUSP 41171), ibidem, 30.iv.2014, 9.8 m. 1 male (MZUSP 41168), ibidem, Ponta do Monumento, 20°30’10.3”S, 29°20’36.1”W, J.B. Mendonça coll., 3.vii.2015, 15.5m. 2 females, 3 juveniles (MZUSP 41169), ibidem, Ponta da Calheta, 20°30’18.7”S, 29°18’31.6”W, J.B. Mendonça coll., 30.iv.2014, 15.6 m. 1 juvenile female (MZUSP 39920), ibidem, Praia da Calheta, 20°30’29.5”S, 29°18’37.0”W, J.B. Mendonça coll., 21.vi.2016, 12.5 m. 1 female (MZUSP 41172), ibidem, 20°30’20.9”S, 29°18’43.7”W, J.B. Mendonça coll., 18.vii.2012, 11.6 m. 1 male, 1 female (MZUSP 41170), ibidem, Paredão, 20°31’36.9”S, 29°18’14.3”W, J.B. Mendonça coll., 19.vi.2012, 19 m. 1 male (MZUSP 39842), ibidem, Enseada Praia do Príncipe, Ilha Sul, 20°31’36.9”S, 29°18’14.3”W, J.B. Mendonça coll., 10.vii.2015, 22.2 m. 1 male (MZUSP 41403), ibidem, Enseada das Orelhas, 20°29’40.2”S, 29°20’32.9”W, J.B. Mendonça coll., 6.vii.2013, 14 m. 1 juvenile female (MZUSP 39691), ibidem, Praia das Cabritas, 20°29’32.0”S, 29°19’46.5”W, J.B. Mendonça coll., 28.iv.2014, 9.2 m. 1 male (MZUSP 41002), ibidem, Praia do Lixo, 20°31’29.8”S, 29°19’43.9”W, J.B. Mendonça coll., 4.xi.2014,. 15.4 m.

Size of largest male: cl 9.4 mm, cw 7.1 mm; largest female: cl 9.5 mm, cw 8.0 mm.

Comparative material examined. Omalacantha antillensis (Rathbun, 1920): Brazil: Bahia: 2 males (MZUSP 5928), Santa Cruz de Cabrália, Ponta do Mutá, M. L. Christoffersen coll., 16.x.1982, in coral reef. Omalacantha bicornuta (Latreille, 1825): São Paulo: 2 males, 1 ovigerous female (MZUSP 7531), Ubatuba, Praia do Lázaro, A. Fransozo coll., 24.v.1986. Omalacantha garthi (Lemos de Castro, 1953): Espírito Santo: 1 male (MZUSP 8515), Santa Cruz, H.R. da Costa [presumably] coll., vii.1973 [G.A.S de Melo det. ii.1987]. Omalacantha interrupta: Maranhão: 3 males (MZUSP 9279), probably R/V “Almirante Saldanha”, stn 1817, 2°12’S, 42°25’W, 28.xi.1967, 60 m. Pernambuco: 2 males (MZUSP 9278), Fernando de Noronha Archipelago, Praia de Santo Antônio, P. S. Young coll., 27.i.1987.

Distribution. Gulf coast of Mexico (Veracruz), Antilles (Cuba, Antigua, Barbados), Caribbean coasts of Costa Rica and Colombia (near Cartagena), Venezuela (Isla Margarita) and Brazil (from Piauí to Alagoas) (Lemaitre 1981; Moran & Dittel 1993; Melo 1996; Lira et al. 2013; Diez García 2014; Vassallo et al. 2014). Brazilian oceanic islands: Fernando de Noronha (Melo 1996) and Trindade (this study).

Ecological notes. Omalacantha interrupta has been reported to inhabit soft and hard bottoms from the tide mark down to 50 m, including sandy-rocky bottoms, rocky coasts, and stone reefs (Santos et al. 2012; Lira et al. 2013; De la Cruz-Francisco et al. 2017). It has also been found in association with calcareous algae (Melo 1996; Melo & Veloso 2005), Thalassia (Murina et al. 1969) and banks of the bivalve Arca zebra Swainson (Lira et al. 2013). Omalacantha interrupta is a heavy decorator, which uses a diverse array of materials. In Trindade, it has been found with the carapace and legs completely covered (P2–P5 dactyli less so) with algae, sponges, and organic debris mixed with mineral fragments. Occasionally, females have been reported carrying polyps of the octocoral Carijoa riisei (Duchassaing & Michelotti) in northeastern Brazil (Bruto-Costa et al. 2014). The fouling material is attached by means of long straight setae and hooked setae, both present in O. interrupta (see Berke & Woodin 2009).

Remarks. Omalacantha interrupta (Fig. 22A) stands apart from O. bicornuta in having one blunt tooth on the infraorbital margin outside the basal article of the antenna, and more prominent tubercles on the branchial region (vs one small, rounded tubercle on the infraorbital margin outside the basal article of the antenna) (Fig. 22D).

Omalacantha antillensis (Fig. 22B) can be recognized on the basis of the absence of a tooth or tubercle on the infraorbital margin outside the basal article of the antenna (rarely a minute bump next to it, Fig. 22C) and, consistently, two spines at the widest part of the carapace branchial region (instead of one spine as in the precedent two species) (Fig. 22A, B).

Contrary to Melo (1996), O. bicornuta and O. garthi have in common a large tooth at the anterolateral angle of the basal article of the antenna and one small tubercle just outside it on the infraorbital margin (Fig. 22D), and therefore neither of which characters can be used to distinguish between the two species. The holotype of O. garthi (female, cl 13 mm, cw 11 mm, originally deposited at the Museu Nacional, Rio de Janeiro) has not been located for study, but from its original description and illustrations (Lemos de Castro 1953) no other characters are available to date to separate O. garthi from O. bicornuta, so that the former species is here placed under the synonymy of O. bicornuta.