Jellyfish

Overview

Found in every ocean and some freshwater, the jellyfish isn’t really a fish at all. For this reason, scientists often refer to jellyfish as “jellies.” It’s an invertebrate, meaning it has no bones. Its body is comprised of roughly 95 percent water. It’s a bell-shaped jelly-like structure that encloses its innards, from which tentacles extend, sometimes as long as 100 feet.

Jellyfish bodies aren’t hydrodynamic, so they don’t swim all that fast. Instead, jellyfish tend to passively drift along, creating a current that sucks prey into stinging tentacles. Jellyfish don’t have brains either and but do have a very basic nervous system. They react to odor and light stimuli and are able to react quickly.

Jellyfish are eaten in Chinese and other Asian cuisine. Because of their high water content, they are generally a low calorie food with a crispy, crunchy texture.

Growth and Reproduction

Over 200 species are known to exist. Some jellies spawn in a similar way to other ocean animals, with males and females releasing their sperm and eggs into the water. When these collide, the fertilized egg grows into a young jelly that looks more like an anemone. This “polyp” settles on the seafloor where it will stay for some time. Later in its life the polyp releases miniature jellies that develop into male and female adults. Other jellies change sex from male to female during their lives, and some may be both male and female at the same time.

Jellies sometimes come together to form massive jellyfish “blooms” of up to hundreds of organisms. Being able to swarm, though, is dependent on myriad factors: ocean currents, water temperature and nutrient as well as oxygen levels in the water. It is while in these large blooms that jellies reproduce in mass quantities. Ecological concerns are often raised at times like this, potential jellyfish “outbreaks.”

The absence of other fish in the food chain due to destructive fishing practices such as overfishing and bottom trawling is one factor that has enabled jellyfish to multiply exponentially. In fact, some scientists are concerned that if humans continue fishing at current unsustainable levels, jellyfish will be among the few creatures left in the oceans within the next half century.

Why Jellyfish Sting

Jellyfish feed on microscopic animals known as zooplankton and also small fish. Most have barbed tendrils covered in poisonous cells called nematocysts, which are activated by tiny mechanical “triggers”. When a fish brushes up against the jelly’s tendrils, a stinging filament coiled inside launches into the unlucky animal and injects the toxin. Once disabled, the jellyfish may then pull small prey into its mouth.

For the most part, jellyfish are not dangerous to humans. A small number can be deadly, however. They have little control over where they swim, so when they sting a human it’s an accident. Many find a jellyfish’s sting to be quite painful, and some experience allergic reactions to the toxin.

Threats

Few creatures pose a threat to jellyfish because few are able to penetrate the stinging tentacles. Predators that are impervious to a jelly’s sting include sea turtles, crabs, and some fish.