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

THE SHOREBIRDS ARE BACK

In fact, southbound migratory shorebirds have been back in the Pacific Northwest since the last week of June, but it is timely to write about them, as they are probably at their peak at the beginning of September.

The adults come back as soon as their young fledge, but of course some nests fail, and those adults are the first to return. Why stay in the Arctic, with all those mosquitoes and arctic foxes, when where you really should be is on a mud flat in Grays Harbor or a sandy beach in Sinaloa? Some of them are going farther, well into South America, so they had better get an early start for that long flight.

In quite a few shorebird species, one sex deserts the other adult and the offspring soon after the eggs hatch. The majority of these are females, presumably because females have expended much energy producing the eggs, so to balance parental investment, the males are left to raise the young. Most shorebirds don’t feed their young, so raising young shorebirds consists of shepherding them around to feeding sites and warning them about potential predators. It’s still a lot of work (imagine keeping track of four kids when you can’t see them much of the time).

Perhaps because they are big enough to potentially ward off predators, large shorebirds such as curlews and godwits divide parental responsibility, and the sexes migrate together. This is also true of most plovers. But the first Western Sandpipers you see in fall are probably those that failed at nesting, then a large wave of females that have left their families, then the males.

Many of these species undergo body molt while they are migrating, so in the fall we see birds in breeding plumage, in nonbreeding plumage, and at all stages in between. In addition, another plumage complicates the issue. These are the juveniles, young of the year that migrate after the adults. The peaks of their migration are often about a month apart, so in some species that continue south after passing through our region, we see a lot of adults and then a lot of juveniles, but not much mixing.

When trying to identify unknown shorebirds, it is extremely important to place them in a plumage, or at least an age stage. In fall, the adults have very worn body feathers until they are all replaced, and many of them don’t replace all their feathers until some time in the winter. Of the flight feathers, both the primaries and the tertials (the feathers of the inner wing that cover the primaries when the wing is folded) become very worn, and that wear is easily seen. Juveniles, on the other hand, have neat unworn feathers, including the primaries and tertials.

Get out to the coast and savor the shorebirds. You can easily see one to two dozen species on a good day, and identification is much facilitated because they are often in mixed-species flocks.

Dennis Paulson