Jellyfish--Planktonic Predator
Tentacles and Stinging Harpoons Make Jellyfish Formidable Predators
Jellyfish inhabit all the world's oceans and are often seen by swimmers and beachcombers alike. Here is a simple, informative look at a fascinating creature.
In the marine world, the sovereign of slipperiness is the jellyfish. Despite having no brain and limited sense organs, jellyfish are one of the most numerous and effective predators in the sea.
Pylum Cnidaria
Jellyfish are not fish, nor are they composed of jelly. When zoologists divide the animal kingdom into groups called phyla, jellyfish—along with animals like coral and sea anemones—belong to the phylum Cnidaria.
Many cnidarians have complex life histories. These include alternating generations of both stationary bottom dwellers, like anemones, and pelagic drifting medusoid individuals, like jellyfish. Alternating forms ensures that different life stages of cnidarians don’t compete with one another for the same food resources.
Morphology
Overall, the jellyfish’s body plan is simple, and organized along the lines of radial symmetry. This means that they display a general form like a cylinder, with one main axis around which the various body parts are arranged. For a jellyfish, this consists of a bowl shaped, often transparent contractile bell for propulsion, a digestive cavity, spoke- like canals radiating out from the gut for nutrient transport; and tentacles armed with cells that fire poisonous harpoons. Light sensors and “tilt” sensors line the rim of the bell, and a net-like nervous system crosshatch its body.
One adaptively significant feature of radial symmetry is that it allows an animal to confront their environment in numerous directions. A radially-symmetrical animal has no front or back end. This body form is most common in sessile and drifting species. Given those lifestyles, it is clearly advantageous to be able to confront the environment equally in a variety of directions.
Hunting Tactics
Jellyfish live a drifting life, and troll endlessly for food. They trail their tangle of tentacles and paralyze whatever small prey they touch, reeling it up to their mouth once it's subdued.
Living at the mercy of currents can leave a jellyfish high and dry at the tide line, but it has its advantages. Jellyfish move with their planktonic food without wasting much energy. They simply forage by jetting upward in the water column, sink slowly down to capture whatever prey they might encounter, then jet upward again. Some jellyfish can change direction as they swim, and many are attracted to light.
Jellyfish are often very numerous in areas where temperature or salinity discontinuity layers occur in the sea. They are concentrated there by the currents, and feed on the zooplankton and fish also congregating at these boundaries.
Self Defense
Jellyfish themselves make a nasty meal for most predators due to their toxic tentacles and watery tissues. Even so, at least several hundred species eat jellyfish, including creatures like tuna and sunfish, leatherback turtles, and oceanic birds such as fulmars and phalaropes. Jellyfish also appear to be a food source for chum salmon. Overall, however, it is difficult to determine their dietary importance to other species. Their tissues digest quickly, leaving little evidence of being eaten.
Body Composition
What, exactly, are jellyfish composed of? In the ocean, gelatinous slime is a very simple and effective construction material. It is cheap and easy, consisting of no more than a few proteins together with a lot of polysaccharides, or long sugar chains that soak up water. The result is an animal that is 95% liquid and 5% organic matter. Consequently, jellyfish easily convert food into mass. When there is plenty of prey they quickly regenerate damaged parts, grow in size, and reproduce like crazy. When food becomes scarce, a jellyfish shrinks down and waits for conditions to change.
A transparent, gelatinous body makes for a good marine life strategy. It camouflages the animal, nullifying a predator’s eyesight. And a sturdy body isn’t necessary in the ocean, because there isn’t much to run into and gravitational force is less of a factor. Yet jellyfish are structurally tougher than one might think. Cnidarians cells are grouped into tissues, but there are just two layers to those tissues—an outer skin layer ribbed with muscle, and an inner gut layer. Sandwiched between them is an amorphous layer of incompressible jelly. Since muscle can only contract, movement poses the jellyfish with a structural challenge: how, after contracting their bodies to shoot out a jet of water, do they yank themselves back into shape? Their adaptive answer: add collagen fibers to their tissues to make themselves tough and rubbery. When the jellyfish contracts its muscles, the fibers bend with them—then spring the entire jellyfish back to shape when the muscles relax.
Overall, jellyfish are simply structured predators, yet are well adapted to the marine environment. They possess their own practical elegance.
(For a look at another fascinating marine predator, read North Pacific Salmon Sharks.)
References
Brusca, R. and G. Brusca. 1990. Invertebrates. Sinauer Associates, Inc. Sunderland, Massachusetts.
