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Jellies delicious for this fish

By JW Dowey - 14 Apr 2014 6:45:0 GMT
Jellies delicious for this fish

Above Neptune grass, this little yellow-striped brogue forms shoals near European and African coasts and is commercially fished for fresh consumption; brogue shoal image; Credit: © Shutterstock

It is time that fish were regarded as jellyfish predators with the interest on jellyfish populations and their control now rising. Turtles can be almost exclusive jellyfish predators and there is other predation, but scientific opinion has generally been that there is not enough nutrition in these primitive animals.

The mistake was that if jellyfish are numerous, their very nature makes them easily digestible and able to form a major part of a fish's diet as "fast food." The little sea bream called the brogue, Boops boops, has often been observed eating the Mediterranean's commonest jelly, Pelagia noctiluca. There is an annual cycle in the Pelagia population, following sea surface temperature fluctuations.

The fish specialise in certain events such as jellyfish blooms in the spring, but when the female jelly has lipoprotein-rich mature oocytes during summer, the Boops really has its feast day. During winter, the fish rarely eat jellies, even though the prey's population is still high. Probably, the water soluble molecules of the jellies' gonads attract the fish's attention. There is also the possibility that the females' purple-red gonads make them more visible, so they are predated more for that reason. The fish shoal along inshore waters near the Straits of Messina, catching the jellyfish where they accumulate near the surface.

The method of research was to video the predation and analyse the jellyfish remains to assess those parts that had been eaten. Only the bream was observed with other abundant near-shore fish never observed to eat the jellies. The parts eaten were usually the oral arms or the gonads. Energy content of the parts was carried out, with five times the energy available in the female gonads. Similar results have been fund in the male and female gonads of Chrysaora hysoscella , and several other species.

The conclusions from Giacomo Milisenda and his colleagues from the Universities of Salento, Messina and Western Washington (US) as well as the Italian Institute of Marine Sciences and Institut de Ciencies del Mar-CSIC, Barcelona are presented in - PloS One.

Conclusions must be that this jellyfish is an important food resource for gelativorous predators. The available biomass, even of oral arms from immature jellies, is very large. Turtles and other important predators may be needed to maintain a balance, while the dynamics of jellyfish blooms needs to be understood. We will be able to see whether fish or other creatures such as turtles can influence breeding, locomotion and the population size of those enormous blooms that could be caused partly by global warming.