Predation on the zoanthid Palythoa caribaeorum Duchassaing & Michelotti, 1860 (Anthozoa: Sphenopidae) by vertebrates has been recorded in the literature for reef fishes (Francini-Filho & Moura, Reference Francini-Filho and de Moura2010) and sea turtles (Stampar et al., Reference Stampar, Silva and Luiz2007), despite this zoanthid being loaded with palytoxin (PTX), an anti-predator chemical defence (Moore & Scheuer, Reference Moore and Scheuer1971; Gleibs et al., Reference Gleibs, Mebs and Werding1995). Such records, however, do not provide information on the zoanthid's reproductive stage (e.g. presence of reproductive structures or eggs) which might represent an increase in the organism's nutritional value (Villinski, Reference Villinski2003) and imply a higher potential predation risk (Duffy & Paul, Reference Duffy and Paul1992; Rotjan & Lewis, Reference Rotjan and Lewis2005).
Here we report, for the first time, a predation on a reproductive P. caribaeorum by a reef fish. Although some reef fishes are known to eventually predate these zoanthids (Randall, Reference Randall1967; Gleibs & Mebs, Reference Gleibs and Mebs1998; Francini-Filho & Moura, Reference Francini-Filho and de Moura2010), this is the first record of P. caribaeorum predation that provides information on the zoanthid's reproductive stage and discusses its potential implications.
MATERIALS AND METHODS
Observations were taken during fieldwork in the coral reefs of the Abrolhos Archipelago, north-east Brazil (17°58′S 38°42′W) at the Santa Bárbara Island (Portinho Norte) in March 2010. These fringing reefs are not massive coralline formations but are constituted of reef organisms (e.g. corals, sponges and coralline algae) growing on rocky substrates around the islands, being defined as embryonic fringing reefs (Pitombo et al., Reference Pitombo, Ratto, Belém, Choat, Barnes, Borowitzka, Coll, Davies, Flood, Hatcher, Hopley, Hutchings, Kinsey, Orme, Pichon, Sale, Sammarco, Wallace, Wilkinson, Wolanski and Bellwood1988). Benthic cover at the observation site was mainly algal turf, coralline algae, the scleractinian coral Mussismilia braziliensis and the zoanthid Palythoa caribaeorum. Photographs were taken using a Canon 5D camera with macro lens.
One individual of the whitespotted filefish Cantherhines macrocerus Hollard, 1853 was observed feeding on a reproductive P. caribaeorum colony at about 3 m deep (Figure 1A) and there were several fresh bite marks in other colonies around, suggesting that other individuals and probably other species were also feeding on the zoanthid. The colony's reproductive stage was visually assessed in the bite scar immediately after the observed predation, when it was possible to observe egg-bearing polyps (Figure 1B). There was no other morphological trait on the colony that could previously indicate its reproductive stage.
Few vertebrates are known to feed on Palythoa caribaeorum colonies (e.g. Gleibs & Mebs, Reference Gleibs and Mebs1998; Stampar et al., Reference Stampar, Silva and Luiz2007; Francini-Filho & Moura, Reference Francini-Filho and de Moura2010) and most parts of them are reef fishes. The first reef fish species recorded feeding on Palythoa sp. was the confamilial scribbled leatherjacket filefish Aluterus scriptus in Puerto Rico (Randall, Reference Randall1967). Most recently, predation on P. caribaeorum was recorded for other reef fish species, including the whitespotted filefish Cantherhines macrocerus at the same study site in the Abrolhos Archipelago (Francini-Filho & Moura, Reference Francini-Filho and de Moura2010). This relatively rare species (<0.1% of fish biomass at the reefs of Abrolhos (Francini-Filho & Moura, Reference Francini-Filho and de Moura2010) is regarded as a sessile invertebrate feeder, with sponges comprising the major part of its diet (Randall, Reference Randall1967). Therefore, the novelty of this record is for discussing the potential effect of P. caribaeorum reproductive stages on predators' choice, since none of the previous records acknowledged the zoanthid reproductive stage at the time of predation.
The zoanthid P. caribaeorum is abundant at the study area and widely known for containing PTX, one of the most poisonous marine toxins known to date, acting as an anti-predation chemical defence (see Gleibs et al., Reference Gleibs, Mebs and Werding1995). Although primarily found on Palythoa spp., evidences also suggest a bacterial origin for the PTX production (see Ramos & Vasconcelos, Reference Ramos and Vasconcelos2010) and the presence of this toxin in organisms living in close association with colonial zoanthids (Gleibs & Mebs, Reference Gleibs and Mebs1999). Additionally, it is known that organisms from different trophic levels might be differentially affected by PTX, some of them even accumulating the toxin in their tissues (see Ramos & Vasconcelos, Reference Ramos and Vasconcelos2010). The entrance, diffusion and sequestration of PTX into the food chain of coral reefs are further discussed by Gleibs & Mebs (Reference Gleibs and Mebs1999).
Even though there is little available information on PTX geographical and/or seasonal variation in Palythoa species (Gleibs et al., Reference Gleibs, Mebs and Werding1995), a positive correlation between the PTX content and the reproductive cycle of polyps was found for Palythoa tuberculosa in the Pacific (Kimura et al., Reference Kimura, Hashimoto and Yamazato1972). Conversely in Palythoa species from the Caribbean Sea (P. caribaeorum and P. mammillosa), PTX is distributed among polyps independently of their reproductive stage (Gleibs et al., Reference Gleibs, Mebs and Werding1995). If this is true, potential predators would benefit more by selectively targeting reproductive colonies since egg-bearing polyps might increase the predator benefit in the predation trade-off.
Similarly to our observation, parrotfishes may selectively target areas of Montastrea annularis with higher reproductive allocation and thus increased nutritional value (Rotjan & Lewis, Reference Rotjan and Lewis2005). The predation on reproductive cnidarians by reef fishes suggests that fishes might detect a prey's reproductive stage, increasing the predation pressure on these benthic cnidarians loaded with eggs.
Differences in nutritional quality within the natural range occurring among coral reef organisms are suggested to be among the main forces modelling predator–prey interactions in such habitats (Duffy & Paul, Reference Duffy and Paul1992). Therefore, reproductive stages of chemically defended organisms should be recorded in predation events, since they might shape the frequency and intensity of such interactions.
Financial support was provided by CNPq through the Grants MCT-Jovens Pesquisadores (#571295/2008-8) and SISBIOTA-Mar: Rede Nacional de Pesquisa em Biodiversidade Marinha (#381235/2011-4) awarded to S.R. Floeter (PI). We also thank S. Stampar, another anonymous referee and the Executive Editor Dr A. Pulsford for valuable comments to improve this manuscript. G.O. Longo was granted a scholarship from CAPES, Brazilian Ministry Educational Council.