Abstract
In the eastern tropical Atlantic, the orangeback flying squid Sthenoteuthis pteropus (Steenstrup 1855) (Cephalopoda, Ommastrephidae) is a dominant species of the epipelagic nekton community. This carnivore squid has a short lifespan and is one of the fastest-growing squids. In this study, we characterise the role of S. pteropus in the pelagic food web of the eastern tropical Atlantic by investigating its diet and the dynamics of its feeding habits throughout its ontogeny and migration. During three expeditions in the eastern tropical Atlantic in 2015, 129 specimens were caught by hand jigging. Stomach content analyses (via visual identification and DNA barcoding) were combined with stable isotope data (∂15N and ∂13C) of muscle tissue to describe diet, feeding habits and trophic ecology of S. pteropus. Additionally, stable isotope analyses of incremental samples along the squid’s gladius—the chitinous spiniform structure supporting the muscles and organs—were carried out to explore possible diet shifts through ontogeny and migration. Our results show that S. pteropus preys mainly on myctophid fishes (e.g. Myctophum asperum, Myctophum nitidulum, Vinciguerria spp.), but also on other teleost species, cephalopods (e.g. Enoploteuthidae, Bolitinidae, Ommastrephidae), crustaceans and possibly on gelatinous zooplankton as well. The squid shows a highly opportunistic feeding behaviour that includes cannibalism. Our study indicates that the trophic position of S. pteropus may increase by approximately one trophic level from a mantle length of 15 cm to 47 cm. The reconstructed isotope-based feeding chronologies of the gladii revealed high intra- and inter-individual variability in the squid’s trophic position and foraging area. These findings are not revealed by diet or muscle tissue stable isotope analysis. This suggests a variable and complex life history involving individual variation and migration. The role of S. pteropus in transferring energy and nutrients from lower to higher trophic levels may be underestimated and important for understanding how a changing ocean impacts food webs in the eastern Atlantic.
Highlights
Oceanic squids are muscular, fast moving opportunistic molluscan predators that feed on a variety of prey [1,2,3] to sustain high metabolic and growth rates [1,4,5,6]
Our results show that S. pteropus preys mainly on myctophid fishes (e.g. Myctophum asperum, Myctophum nitidulum, Vinciguerria spp.), and on other teleost species, cephalopods (e.g. Enoploteuthidae, Bolitinidae, Ommastrephidae), crustaceans and possibly on gelatinous zooplankton as well
Our study indicates that the trophic position of S. pteropus may increase by approximately one trophic level from a mantle length of 15 cm to 47 cm
Summary
Fast moving opportunistic molluscan predators that feed on a variety of prey [1,2,3] to sustain high metabolic and growth rates [1,4,5,6]. Squids generally have a short lifespan of about 1–2 years and are semelparous; there is one reproductive cycle after which the individual dies [4] They play a key role in the trophic structure of marine pelagic ecosystems [7,8] due to their large and in some areas increasing biomass [9] and their importance in the diet of marine predators such as fishes and marine mammals [10,11]. In order to obtain high resolution data on feeding ecology and individual migration behaviour, we analyzed stable isotope ratios of the cephalopod gladius This archival hard structure, which consists of chitin and proteins, is present in the hatchlings and grows continuously with no metabolic turnover after synthesis. Using a combination of visual and molecular stomach content and stable isotope analysis we investigated the diet and the position of S. pteropus in the pelagic food web of the eastern
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