On-line version ISSN 1982-436X
Braz. j. oceanogr. vol.57 no.4 São Paulo Oct./Dec. 2009
Stomach contents of the pelagic stingray (Pteroplatytrygon violacea) (elasmobranchii: dasyatidae) from the tropical atlantic
Dráusio Pinheiro VérasI,III; Teodoro Vaske JúniorII; Fábio Hissa Vieira HazinIII; Rosangela Paula LessaII; Paulo Eurico TravassosIV; Mariana Travassos TolottiI,IV; Taciana Martins BarbosaII
IUniversidade Federal de Pernambuco - Departamento de Oceanografia (Cidade Universitária, 50670-901 Recife, PE, Brasil) E-mail: firstname.lastname@example.org
IIUniversidade Federal Rural de Pernambuco -DIMAR/DEPAQ (Av. Dom Manuel de Medeiros s/n, Dois Irmãos, 52171-900 Recife, PE, Brasil) E-mail: email@example.com; firstname.lastname@example.org; email@example.com
IIIUniversidade Federal Rural de Pernambuco -LOP/DEPAq (Av. Dom Manuel de Medeiros s/n, Dois Irmãos, 52171-900 Recife, PE, Brasil) E-mail: firstname.lastname@example.org
IVUniversidade Federal Rural de Pernambuco -LEMAR/DEPAq (Av. Dom Manuel de Medeiros s/n, Dois Irmãos, 52171-900 Recife, PE, Brasil) E-mail: email@example.com; firstname.lastname@example.org
The pelagic stingray Pteroplatytrygon violacea (Bonaparte, 1832) is the only pelagic dasyatid ray occurring in tropical and subtropical waters of the Atlantic, Indian and Pacific Oceans (BIGELOW; SCHROEDER, 1953; MOLLET, 2002; ELLIS, 2007). This poorly known species was originally described from the Mediterranean Sea, and was first reported from the Atlantic by Bigelow & Schroeder (1962). The species has no commercial value, but is fairly abundant off the Southeastern United States representing about 2.5% in number of the catches by pelagic longlines between 1992-2000 (BEERKIRSHER et al., 2004).
The first record in southern Brazilian waters was reported by Sadowski; Amorim (1977) and Mazzoleni; Schwingel (2002) subsequently recorded the pelagic stingray as a bycatch species regularly caught by tuna longliners off southern Brazil. Menni et al. (1995) reported on the presence of the pelagic stingray in northeastern Brazilian waters. Although the pelagic stingray is caught regularly by tuna longliners operating along the Brazilian coast, few biological data are available on the species. In this context, the stomach contents of the pelagic stingray were analyzed to provide more specific information on its feeding habits in the southwestern equatorial Atlantic Ocean.
The sampled area was located between 40º25ºW and 5ºN-20ºS (Fig. 1). All specimens were caught by the Research Vessel Riobaldo (CEPENE-IBAMA), in the years 1993, 1994 and 1995 through the Ecotuna Project, and by the Brazilian tuna longline fleet, in the years 2005 and 2006 (SEAP, Onboard Observer Program), in waters of 2000 to 5000 m local depth, with hook depth between 50 and 250 m along the longline. The specimens were stored on ice onboard, and at the laboratory, they were sexed and had their disc width measured to the nearest centimeter.
A total of 106 specimens were analyzed (69 males, 26 females, and 11 with no sex information). Females were slightly larger, ranging between 40.0 cm and 60.0 cm, with a mean disc width of 49.0 cm. Males varied from 32.0-50.0 cm, with a mean disc width of 43.5 cm (Fig. 2).
The stomachs were removed and preserved in 10%formalin, with the food items subsequently identified to the lowest possible taxon. The importance of each food item in the diet was obtained by the Index of Relative Importance (IRI) (PINKAS et al., 1971), utilizing weight data:
IRIi = %FOi x (%Ni x %Wi)
Where %FOi is the relative frequency of occurrence of each item; %Ni is the proportion in prey number of each item in the total food; and %Wi is the proportion in weight of each item in the total food. Longline baits (Loligo sp. and Scomber japonicus) were not included. Preys in very good condition had their total length measured to investigate preferential prey sizes ingested in the size range of the rays sampled.
Two species of fish were observed (Diodon hystrix and Gempylus serpens), although most fish remains could not be identified. Various cephalopods were observed, including glass squid (Cranchiidae), ommastrephid squids and the octopod Japetella diaphana. Crustaceans were well represented in the diet, especially in terms of hyperiid amphipods (Phronima sedentaria, Phronimopsis sp., Phrosina semilunata and platyscelids), with shrimps (Heterocarpus ensifer), brachyuran megalopae, and squillids also consumed. Other prey included tunicates, pteropods and heteropods (Table 1).
The five most important prey were included among the hyperiid amphipods, teleosts, brachyuran megalopae and pteropods. Although prey size ranged from 1.0 to 140.0 mm length, most prey were between 1.0 and 40.0 mm, being represented by small crustaceans, pteropods and heteropods (Fig. 3). The largest prey species were fish, cephalopods and polychaetes.
References in the literature describe the pelagic stingray as epipelagic (SCOTT; TIBBO, 1968; WILSON; BECKETT, 1970; SADOWSKI; AMORIM, 1977; BRANSTETTER; MCEACHRAN, 1983; PRATT et al, 1990; MENNI et al., 1995), although Nakaya (1982) suggested that they may be a benthopelagic species, utilizing both benthic and pelagic habitats, since captures of pelagic stingray were observed between 330 and 381 m depth, with occasional incursions to superficial waters. However, Bañón et al.(1997) had observed and suggested that the P. violacea was probably caught in the top 100 m in the bottom trawl fishery, when the bottom trawl at 800 m was hauled up. Siqueira and Sant'Anna (2007) obtained specimens from the artisanal fishery, caught by handline, operating at depths from 30 to 45 m in the adjacent areas of Father Island and Shallow Island, Rio de Janeiro. Therefore, the distribution of P. violacea in the water column is probably related to the geographical location and environmental parameters of the region.
Probably due to the hook selectivity of the longline, the smallest individual had 32.0 cm of disc width. The individuals analyzed by Mazzoleni and Schwingel (2002) in Trindade and Martin Vaz islands ranged from 30.0-66.0 cm of disc width, which are, respectively, the smallest and the largest sizes observed for Brazilian waters. Ribeiro-Prado and Amorim (2008) analyzed individuals in São Paulo that ranged from 35.0-65.0cm. Mazzoleni and Schwingel (2002) pointed out that size differences between males and females could be related to reproduction. Although other studies have reported a predominance of females in the sex ratio (MOLLET, 2002), males predominated in this study, corroborating with what was observed by Forselledo et al., (2008), but it is not clear if it is a population characteristic or due to some kind of depth or spatial segregation related to reproduction or feeding habits, or if vertical distribution is in part due to the influence of the sea water temperature. Sexual segregation has been widely observed in sharks by geographical location, intraspecific competition, and requirements of reproductive choices associated with pre-or post-mating strategies (SPRINGER, 1967; MENNI et al., 1979; SIMS et al., 2001; ODDONE et al., 2007). Experiments with captive specimens at the Monterey Bay Aquarium (USA) showed that females reach 20.0 to 30.0 cm more than males at the same age, and that females were more abundant than males (MOLLET et al., 2002).
The pelagic stingray is an epipelagic predator that in the southwestern equatorial Atlantic feeds mainly upon small crustaceans, especially hyperiid amphipods, brachyuran megalopae, and also pteropods, fish and cephalopods. The pelagic feeding habits are further confirmed by the presence of pelagic stingrays in drift gill-net fisheries off southern California (HANAN et al., 1993) and in the Gulf of California (DÁVALOS-DEHULLU; GONZÁLEZ-NAVARRO, 2003).
The stingray catches the bait by wrapping their wings around it and nibbling it until it is gone or until they are hooked (MOLLET, 2002). Bigelow and Schroeder (1962) found two seahorses Hippocampus sp., two small shrimps, and fragments of squid in one specimen. SCOTT; TIBBO (1968), found parts of a thalassinoid decapod in a specimen from the northwestern Atlantic, and Wilson and Beckett (1970) found sargassum weed, squid beaks, seahorses, unidentified fish and coelenterates in 16 specimens from the North Atlantic. Two semi-digested skulls of Scomber japonicus with a standard length of 235 mm were found in the stomach of a female P. violacea in the Gulf of California (DÁVALOS-DEHULLU; GONZÁLEZ-NAVARRO, 2003), although the authors did not comment on the possibility of these S. japonicus originating from longline bait. Ribeiro-Prado and Amorim (2008) found mollusca as the most common group, noting also Actinopterygii fishes and Crustaceans in stomachs of P. violacea. Siqueira and Sant'Anna (2007) observed only unidentified teleost remains (vertebrae fragments and crystallines).
The dorsal and ventral dark violet color may help to camouflage the pelagic stingrays while they seek pelagic prey in the water column. Besides, the teeth of the pelagic stingray differs from those of most other rays of the Dasyatidae by the presence of cuspidate cutting teeth in both male and female jaws, which contrast with the crushing dentitions of their demersal relatives. These cutting teeth are probably more efficient for grasping small crustaceans. Differences found in the dentition and the swimming behavior of this species, in relation to other Dasyatis spp., appear to be autapomorphic (character state that is unique to a particular species or lineage in the group under consideration) functional adaptations to a pelagic lifestyle and a diet of fish and squid (ROSENBERGER, 2001), although prey preferences may vary, as observed in this study, where small crustaceans were more important than fish and squid.
The data and comments above suggest that the P. violacea alter their food items according to their geographical location and also shows how the species, despite belonging to the Dasyatidae family, is adapted to the pelagic environment.
The authors are grateful to the CEPENE-IBAMA, to the Department of Fisheries and Aquaculture of the Federal Rural University of Pernambuco, Brazil, where the stomachs were examined. The present study was also made possible by funding from the Special Secretariat for Aquaculture and Fisheries of the Presidency of the Republic of Brazil (SEAP) and the Brazilian Onboard Observer Program, also of SEAP.
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(Manuscript received 31 July 2008; revised 26 February 2009; accepted 07 July 2009)