Rhacostoma atlanticum L. Agassiz

Rhacostoma atlanticum L. Agassiz

(Fig. 13)

References consulted. Kramp 1955: 266. Kramp 1959a: 168, fig. 237. Kramp 1961: 210. Moreira 1975: 557. Goy 1979: 290–291. Bouillon 1999: 423, fig. 3.81.

Material. Municipality of Pontal do Paraná, Shangrilá Beach (25°39–40’S; 48°21–26’W): 08/08/1998 — 1 specimen; 17/09/1999 — 16 specimens; 16/11/1999 — 2 specimens; 30/10/2004 — 1 specimen; 14/01/2005 — 2 specimens; Municipality of Guaratuba (25°54’S; 48°23’W): 21/04/2001 — 1 specimen; 29/07/2001 — 1 specimen; 08/08/2003 — 1 specimen; 20/06/2004 — 1 specimen; 23/07/2004 — 1 specimen; 18/08/20004— 4 specimens; 25/11/ 2004 — 3 specimens; Municipality of Guaraqueçaba, Superagüí Island (25°20–27’S; 48°07’W): 23/07/2005 — 1 specimen; 29/10/2005 — 24 specimens; Municipality of Paranaguá, Mel Island (25°33–36’S; 48°07–17’W): 27/10/ 2004 — 4 specimens; Municipality of Matinhos, Matinhos Beach (25°45–49’S; 48°24–30’W): 30/10/2004 — 106 specimens; 25/07/2005 — 2 specimens. Matinhos Beach (25°45–49’S; 48°24–30’W): 30/10/2004 — 25 specimens; 14/01/2005 — 1 specimen; 03/05/2005 — 47 specimens; 25/07/2005 — 86 specimens.

Reference specimens deposited. MZUSP 904, 1 specimen, MZUSP 905, 1 specimen.

World distribution. Atlantic Ocean (Bouillon 1999), coasts of North America, Africa, and Brazil (Kramp 1955, 1959a, 1961; Bouillon 1999; Migotto et al. 2002).

Distribution in Brazil. From the states of São Paulo to Rio Grande do Sul (Navas-Pereira 1981; Tronolone 2001; Nogueira & Haddad 2006b; Nogueira et al. 2010; Nogueira 2012).

Description. Rather flat umbrella ranging from 22 to 63 mm in diameter. Manubrium short, with fringed edges, wide mouth, about half the diameter of the umbrella. Simple radial canals, in number from 70 to 110, without centripetal canals. With distinct rows of gelatinous papillae (Fig. 13), parallel between each canal. Gonads along most of the canals, usually both ends free.

Systematic remarks. The genus Rhacostoma L. Agassiz is monotypic and easily distinguishable from other Aequoreidae: from Aequorea, by having gelatinous papillae rows between the canals; from Zygocanna Haeckel, by having simple canals; and from Gangliostoma Xu, by the absence of circular rows of papillae at the base of the manubrium (Kramp 1955, 1959a, 1961; Bouillon & Boero 2000). These distinctive features are readily observable, even in nearly destroyed specimens. The specimens studied differ somewhat from the descriptions of North Atlantic specimens. The mature animals are much smaller and have fewer tentacles in relation to the canals, despite the possible presence of rudimentary bulbs that may develop later, or under different environmental conditions. This difference seems to be a pattern in Brazilian populations, and has been mentioned by other authors (Moreira 1975; Navas-Pereira 1981).

Biological data. Moreira (1975) suggested that the species is invasive in Brazilian waters, appearing only since the 1970s. If so, it seems to have developed well-established populations in the region. It can occur in large aggregates in the middle section of the southeastern Brazilian shelf, representing the majority (68%) of the biomass in organic carbon of macrozooplankton (Mianzan & Guerrero 2000).