JELLYFISH AND THEIR PREDATORS

Jellyfish are amazing animals. Almost entirely made of water, without a brain or central nervous system, they manage to get around the oceans very successfully.

One of the common species in the eastern North Pacific is the sea nettle, Chrysaora fuscescens. This species is among the better known Pacific coast jellyfish because it can be kept in aquaria. It can become superabundant in our waters, presumably when the zooplankton on which it feeds are similarly abundant, but it also may be because of the vagaries of ocean currents. Jellyfish generally swim upcurrent so they encounter a regular supply of the tiny animals on which they feed.

Not very many animals feed on jellyfish because of a combination of their fairly effective antipredator adaptions and their very low nutrient content. Two of these that do so are actually specialists, large animals that roam slowly around the world’s oceans and eventually run into single or even concentrations of jellyfish.

The Ocean Sunfish (Mola mola) is the more common of these two species. This is the largest and best-known species of its family and in fact the heaviest bony fish in the world, with an average weight of 1,000 kilograms. It looks about like a head with fins, swimming with a sculling motion of the big dorsal and anal fins. A long fin waving at the water surface is usually a good indication of one of these two-meter monsters.

Ocean sunfish are usually found floating at the surface on their side, perhaps taking advantage of the warmest surface water to more effectively digest the great amount of jellyfish they have to eat to gain adequate nutrition. We see them when they are at the surface, but in fact they spend much of their time well below the surface and perhaps come up just to warm up!

Ocean Sunfish are known to lay the most eggs of any animal in the world, up to 300 million eggs at one time. The larvae look nothing like the adults but are more like the larvae of other members of their order, including puffers and porcupine fish. It is rare to sight groups of juveniles, but five such groups were seen off Grays Harbor in September 2013; presumably like other fish, they school for protection from predators, among them sharks and sea lions.

The other main medusivore, as a jellyfish-eater should be called, is the Leatherback Turtle (Dermochelys coriacea). This is also huge for its group, the largest living turtle at an average weight of 400 kilograms. The largest ever recorded had a carapace length of over 2.2 meters. These animals are very different from the sunfish in that they have to go to shore to nest, and the ones in our waters are migrants from, amazingly, the Southwest Pacific.

Of all the sea turtles, this is the one most capable of living in cold waters, even north to the Gulf of Alaska. They generate metabolic heat by swimming, and they are insulated by fat as well as warmed by counter-current heat exchange in their blood vessels. Although “cold-blooded” like other reptiles, their body temperature has been recorded as up to 18° C. warmer than the water in which they swam.

Northern Fulmars (Fulmarus glacialis) also eat large jellyfish, among the few birds that do so. The cnidarians because of their watery nature are poorly represented in stomach contents, but observers have seen them picking at jellies. They appear to go after the gonads, which are doubtless more nutritious and oil-rich than the rest of the animal.

And all these animals eat their jelly without peanut butter!

Dennis Paulson