Massive Consumption of Gelatinous Plankton by Mediterranean Apex Predators

Abstract

Stable isotopes of carbon and nitrogen were used to test the hypothesis that stomach content analysis has systematically overlooked the consumption of gelatinous zooplankton by pelagic mesopredators and apex predators. The results strongly supported a major role of gelatinous plankton in the diet of bluefin tuna (Thunnus thynnus), little tunny (Euthynnus alletteratus), spearfish (Tetrapturus belone) and swordfish (Xiphias gladius). Loggerhead sea turtles (Caretta caretta) in the oceanic stage and ocean sunfish (Mola mola) also primarily relied on gelatinous zooplankton. In contrast, stable isotope ratios ruled out any relevant consumption of gelatinous plankton by bluefish (Pomatomus saltatrix), blue shark (Prionace glauca), leerfish (Lichia amia), bonito (Sarda sarda), striped dolphin (Stenella caerueloalba) and loggerhead sea turtles (Caretta caretta) in the neritic stage, all of which primarily relied on fish and squid. Fin whales (Balaenoptera physalus) were confirmed as crustacean consumers. The ratios of stable isotopes in albacore (Thunnus alalunga), amberjack (Seriola dumerili), blue butterfish (Stromaeus fiatola), bullet tuna (Auxis rochei), dolphinfish (Coryphaena hyppurus), horse mackerel (Trachurus trachurus), mackerel (Scomber scombrus) and pompano (Trachinotus ovatus) were consistent with mixed diets revealed by stomach content analysis, including nekton and crustaceans, but the consumption of gelatinous plankton could not be ruled out completely. In conclusion, the jellyvorous guild in the Mediterranean integrates two specialists (ocean sunfish and loggerhead sea turtles in the oceanic stage) and several opportunists (bluefin tuna, little tunny, spearfish, swordfish and, perhaps, blue butterfish), most of them with shrinking populations due to overfishing.

Foraging dichotomy in loggerhead sea turtles Caretta caretta off northwestern Africa

“…The dispersal pattern of young loggerhead sea turtles from Cape Verde has recently been estimated from particle tracking models and drifter trajectories (Monzon- 72 0 0 74 3 0 76 2 0 78 13 0 80 6 0 82 12 0 84 11 0 86 7 1 88 3 1 90 0 1 92 0 3 94 0 1 96 0 0 98 0 2 100 1 1 102 0 0 Table 1 (Bjorndal 1997, Revelles et al 2007, Frick et al 2009, Cardona et al 2012) and a recent study on leatherback sea turtles Dermochelys coriacea revealed low macrozooplankton biomass and poor forage success to the west of Cape Verde compared to the east (Fossette et al 2010). In this scenario, the immature loggerhead sea turtles returning to Cape Verde after their developmental migration certainly had gained previous knowledge about the existence of a food-poor region west of the archipelago and about the existence of a more productive area east of the archipelago, but were likely unaware of the existence of a food-rich continental shelf 500 km east of the archipelago.…”

Benthic scavengers and predators of jellyfish, material for a review

“…Nevertheless, jellyfish continue to be depicted occasionally (see Arai 2005 for examples) as dead ends in the food chains of the sea and/or as a rare source of food for predators (e.g. Condon et al 2011), despite an increasing number of species of fishes (Redeke 1911, Luttenberger 1981, Gorelova & Grudtsev 1986, Kelley 1987, Ates 1988, Arai 1988, Zann 1988: 208, Cerqueira & Haimovici 1990, Hall 1992, Harbison 1993, Massuti et al 1998, Purcell & Arai 2001, Bonaldo et al 2004, Arai 2005, Orsi Relini et al 2010a, Orsi Relini et al 2010b, Chaves et al 2010, Thaler 2012, Cardona et al 2012, Milisenda et al 2014, Battaglia et al 2014, Sweetman et al 2014, Dias & Almeida 2015, birds, reptiles and mammals (Dathe 1989, Ates 1991, Gronert 1992, Shiomi & Ogi 1992, Bell 1996, Zonfrillo 1997, Peglow 1998, Corsi 2000, Arai 2005, Suazo 2008, Cardona et al 2012, Jarman et al 2013, Melville 2013, Jones & Seminoff 2013, other cnidarians and ctenophores …”

Consumption of pelagic tunicates by cetaceans calves in the Mediterranean Sea

“…However, the scarcity of records about gelatinous zooplankton in cetaceans strongly suggests that they would seldom be eaten as prey; this conclusion is further supported in striped dolphins by stable isotope analysis results (e.g. Cardona et al, 2012). Moreover, the fact that most records about tunicates in the stomach contents of cetaceans involve suction-feeder species may suggest that tunicates are accidentally eaten along with the actual prey (see Walker et al, 2002).…”