Predatory babies: the potential of Pelagia noctiluca ephyrae feeding on larval tuna

Abstract

The Atlantic bluefin tuna, Thunnus thynnus migrate long distances to reproduce in relatively unproductive areas like the offshore waters off the Balearic archipelago in the Mediterranean Sea, where the predatory pressure on eggs and larvae is thought to be low due to a near-absent community of filter-feeding fishes. However, such low predatory pressure remains largely unverified as the effect of planktonic predators like gelatinous organisms has not been quantified. Pelagia noctiluca is the most abundant jellyfish in the Mediterranean Sea and remains its entire life cycle in the pelagic environment. Previous research has shown that fish larvae and eggs are important food sources of P. noctiluca ephyrae, and we observed up to 73 ephyrae m-3 in the Balearic waters during the tuna spawning season. Here we modeled the distribution of ephyrae and larval tuna and estimated their quantitative overlap in Balearic waters over five spawning seasons. Larvae and ephyrae co-occurred in only 46% of the sampled stations and overlap values were generally low. In the sites where overlap was exceptionally high, ephyrae contained tuna larvae in their gut content. These findings suggest either predator avoidance or prey depletion, highlighting a need to further investigate the predatory potential of ephyrae on larval tuna.