Eating living jellyfish is hard enough to imagine – they are mostly water, they usually have piles of sting cells, and seem neither nourishing or appetizing. Unusually, Loggerhead Sea Turtles and Ocean Sunfish eat them out of choice. Even some people claim to: there are quite a few recipes for things like jellyfish crisps to go with your beer. But really, who really would eat them if there were decent alternatives around?
Jellyfish usually come in blooms, a nice name for the immense aggregations that frequently occur in coastal waters around the world. So what happens when a bloom of jellyfish matures, sheds its eggs and sperm, and dies? The dead mass of jellyfish sinks to the bottom of the sea and we have assumed it decays there, slowly consumed by bacteria, smothering the sea bottom, rendering it unfit for most other organisms.
We’re wrong. An elegant study published several months ago has surprised everyone. Dead jellyfish were fastened to 50x50cm ‘landers’ and sunk, along with similar landers laden with yummy bits of mackerel, to the bottom of Sognefjord in southern Norway, 4000 feet below the surface. The results were filmed.
Surely the community of bottom scavengers would selectively eat the mackerel, and pay little attention to the jellyfish carcasses.
Instead, dense aggregations of scavengers moved in on the jellyfish just as quickly as they did on the mackerel. First came Atlantic Hagfish, attractively know as slime eels, which burrowed into the mass of dead jellyfish and selectively ate the spent gonads.
Then came the crustaceans – particularly a long-clawed crab called a galatheid which in the video look to be aggressive, each protecting its piece of jellyfish from others, spaced out over the lander. Then came a decapod shrimp and lyssianasid amphipods. The scavengers eliminated the jellyfish in 2 1/2 hours, which is extraordinarily fast.
In fact the whole event is quite extraordinary.
When a whale dies and sinks the bottom – we call it a whale fall – it becomes a major source of nourishment for the bottom scavenging community. Now it seems that when a jellyfish fall occurs, the same thing happens. The dead jellyfish contribute to the food web in ways we did not expect – and carbon is transported from pelagic organisms near the sea surface to the scavengers foraging on the sea bottom.
This is all good to learn. In recent years jellyfish blooms appear to be larger and more frequent, often in places already stressed by low oxygen or overfishing. But they remain part of the food web instead of smothering part of it when they die, and we did not know this. It makes them less of a threat to marine ecosystem stability than we thought.
By why any crab – or hagfish for that matter – would pass up mackerel flesh for dead jellyfish jelly remains a mystery.