Saturday 1 November 2014

Scavengers on Jellyfish carcasses on the ocean floor.

Jellyfish (and other gelatinous organisms) form a major part of the marine ecosystems, and in parts of the world have been shown to be increasing due to human activities, which often remove species that would otherwise control Jellyfish numbers. Many Jellyfish species undergo periodic ‘blooms’ in which very large numbers appear due to some environmental stimulus, then after briefly dominating the local environment, dying en masse and sinking to the ocean floor. This represents a major relocation of nutrients from the pelagic zone (i.e. open water) to the seafloor, and is likely to have some impact on life there. While the rapid consumption of Fish carcasses on the deep ocean floor is well documented, the fate of Jellyfish in a similar environment is unclear, and there have been sightings of masses of Jellyfish on the seafloor off the coast Côte d’Ivoire and in the Gulf of Oman which were untouched by scavengers, leading to speculation that such carcasses would be broken down only by bacterial action, with the nutrients then incorporated into marine sediments and lost to the ocean ecosystem.

In a paper published in the Proceedings of the Royal Society SeriesB: Biological Sciences on 15 October 2014, Andrew Sweetman of the InternationalResearch Institute of Stavanger, Craig Smith of the University of Hawaii atManoa, Trine Dale of the Norwegian Institute for Water Research and DanielJones of the National Oceanography Center at the University of Southampton describe the results of a series of experiments to determine the fate of Jellyfish carcasses on the ocean floor.

Sweetman et al. took frozen and thawed specimens of two Jellyfish species, Periphylla periphylla and Cyanea capillata and of the Mackerel, Scomber scombrus, and placed them on plates with attached cameras that could be lowered to the ocean floor. Fresh (unfrozen) specimens of Periphylla periphylla were also used. The samples were then lowered to the seafloor in the Sognefjorden off the coast of Norway in October 2012, an area where the water is about 1250 m deep. To ensure that the different experiments did not interfere with one-another (deep marine scavengers are thought to hunt largely by sense of smell), the samples were deposited at least 2 km apart.

All of the samples were rapidly consumed, being visited by a number of scavengers including the Atlantic Hagfish, Myxine glutinosa, a Galatheid Crab, Munida tenuimana, a Decapod Shrimp, probably Pontophilus norvegicus, and large numbers of Lysianassid Amphipods, Orchomenella obtusa, showing that nutrients in the bodies of Jellyfish falling to the seafloor in the Sognefjorden are clearly not lost to the ecosystem, being instead recycled by marine scavengers.

 (a) Myxineglutinosa scavengers swarming at the Scomber scombrus bait. (b) Myxine glutinosa voraciously feeding on thawed Periphylla periphylla bait. (c) Myxine glutinosa and Munida tenuimana feeding on a single fresh Periphylla periphyllacarcass. (d)Munida tenuimana and Decapod Shrimp feeding on thawed Cyanea capillata bait. The black bait plate is 50 by 50 cm with gridlines separated by 5 cm. Sweetman et al. (2014).

However the different scavengers did show preferences for different prey, suggesting that changes in Jellyfish abundance does have an impact on the seafloor ecosystem. The Mackerel, Scomber scombrus, samples attracted large numbers of Hagfish, Myxine glutinosa, who consumed the majority of the high nutrient flesh, followed by a second wave of invertebrate scavengers, primarily Galatheid Crabs and Lysianassid Amphipods who consumed the rest of the corpse. Hagfish were also the first to arrive at the Periphylla periphylla sites, preferentially consuming some tissues (probably the high energy gonads) before leaving. They were then replaced by a phase of consumption by Galatheid Crabs and Lysianassid Amphipods, then finally a third phase of consumption by the Decapod Shrimps, which had shown little interest in the Mackerel bait. The Hagfish largely avoided the Cyanea capillatabait, possibly due to the large amount of mucus this species produces, which is toxic to fish and may be noxious to Hagfish even after the Jellyfish has been dead some time.

These findings are significantly at odds with observations made previously off the coast Côte d’Ivoire and in the Gulf of Oman. Sweetmanet al. suggest that in these instances large build-ups of inorganic carbon, sulphide compounds and ammonium may have made the local environment hostile to the scavengers, resulting in the persistence of Jellyfish carrion on the seafloor. Alternatively this may be the result of seasonality; the Sognefjorden experiments were carried out in October, a time of year when Jellfish (and other plankton species) die-offs are expected to occur, and when deep-sea scavengers may be primed to look for this source of nutrients. It is possible that Jellyfish carcasses persist for longer at other times of year because scavengers are not seeking them, or are engaged in other behaviour.

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