Jelly fish are likely to be more than just an opportunistic prey to many organisms. It is true that a predator does not get much from eating a single jelly fish, but if it eats many, it will make a difference and provide the predator with valuable fatty acids.
In other words: Low food quality can be weighed up by high food quantity. As an example, researchers have observed a salmon eat a jelly fish 20 times faster than it took for it to eat a shrimp. So, if the predator doesn’t have to spend much energy on eating loads of jelly fish, this preying strategy begins to make sense. Jellyfish often come in shoals and they move slowly through the water. They can’t really swim away when predators start eating them.
On a global scale, marine environments are changing, and an increasing abundance of jelly fish is thought to replace other prey items in the oceans. Several essential fatty acids were found in the German moon jellies. Among them are the polyunsaturated fatty acids arachidonic acid, eicosapentaenoic acid and docosahexaenoic acid.
About Jelly Fish
Jellyfish are invertebrates that, together with corals, gorgonians and anemones belong to a group called the cnidarians (knidé = nettle, from the Greek). This animal group has stinging cells which they use both to capture their prey and as a form of defense. These cells contain a capsule with a rolled-up filament and a poison. When a prey animal makes contact with the jellyfish, the capsule opens and the filaments are ejected and stick onto the prey, injecting their poison.
Jellyfish are pelagic animals they live in the open seas from tropical to Artic waters and, although they can propel themselves with rhythmic motions of their umbrella, they are basically at the mercy of the currents of the sea.
Because their body is 95% water, they are perfectly camouflaged. The body of a jellyfish exhibits radial symmetry and is divided into three main parts: the umbrella, the oral arms (around the mouth) and the stinging tentacles. They have an internal cavity, in which digestion is carried out. This cavity has a single aperture which functions both the mouth and the anus.
Jellyfish are carnivores and can increase in size rapidly and procreate in large numbers when food is abundant. However, if food is scarce, they can become smaller. These animals, of a gelatinous consistency, have a very unsophisticated anatomy which is nevertheless very effective. They feed mainly on zooplankton, small crustaceans, and in some cases, small fish and other jellyfish also form part of their diet. It is a strange sight to see the jellyfish’s latest prey inside its body before it is digested.
The tentacles, with their stinging cells, serve as defense and as a powerful weapon for capturing prey. When they come into contact with their victims, the nematocysts (cells loaded with poison) present in the tentacles release their harpoons or filaments and release a toxic substance that paralyses the prey. The oral arms help in the capture and ingestion of the captured animal.
Among the predators of the jellyfish, the following have been identified: ocean sunfish, grey triggerfish, turtles (especially the leatherback sea turtle), some seabirds (such as the fulmars), the whale shark, some crabs (such as the arrow and hermit crabs), some whales (such as the humpbacks).
Some other cnidarians such as anemones, certain nudibranches (small molluscs without shells) also feed on jellyfish. Some of these may even take over their stinging cells to use in their own defense.
The marine turtle species that preys on jellyfish the most is the leatherback, named so because it lacks the hard, external carapace that is characteristic of its relatives. The world’s largest turtle with lengths of up to 6 feet, the leatherback has jaws with sharp edges that are not designed for crushing prey with hard shells, as other sea turtles do. Leatherbacks therefore feed mainly on jellyfish and other creatures with soft bodies. Downward-pointing spines inside the leatherback’s esophagus keep jellyfish from slithering out before the turtle has a chance to swallow them. Other species of sea turtles also prey on jellyfish, but to a lesser extent.
At around 5,000 pounds, there is no heavier bony fish in the world than the ocean sunfish, or mola, for which jellyfish are the food of choice. This enormous and odd-looking denizen of the deep, whose shape National Geographic describes as “bullet-like,” consumes a variety of jellyfish species, including moon and comb jellies. Inside the mola’s relatively tiny mouth are two teeth plates that are often likened to a bird’s beak. Because they do not chew, ocean sunfish break down jellyfish by sucking them in and out of their mouths repeatedly. The ocean sunfish’s digestive tract has a slimy, viscous lining that is believed to prevent it from being stung by its prey’s tentacles.
The bearded goby, a fish species found in the waters off the coast of South Africa and Namibia, consumes jellyfish in what scientists believe is a recent adaptation to drastic changes in its environment. In doing so, the bearded goby has helped prevent an ecosystem overrun by jellies from becoming a dead zone. Jellyfish also account for part of the diet of a few fish considered generalist feeders, including spiny dogfish, butterfish, swordfish and some species of salmon and tuna.
Porcupine Fish – Diodon Hystrix
This widely distributed species is frequently found throughout the reefs of the Bahamas. The fish gets his name for the 20 or so spines that cover its body, when the fish is threatened it inhales water to expand its body like a balloon causing the spines to stick out. The fish dines on snails, sea urchins and hermit crabs with the use of its strong beak-like mouth.