… mostly harmless, but scary

There are jellyfish in all the world’s warm oceans – and some of the cold ones.  The basic jellyfish body (the bell) is a thin-skinned bag filled with goo that’s mostly water.  In the middle of the bell are a mouth and stomach, eye spots and gonads – no heart, no brain, just enough to eat and reproduce.  The eye spots don’t really see, but can sense a wide range of light.

Hanging down from the bell are some strings (the tentacles) and several thicker arms to carry food to the mouth.  The rim of the bell usually has a short, hair-like fringe (the cilia).

Jellyfish depend on waves and currents to carry them through the water.  That’s why they often end up on beaches after a storm.  Fluttering the edges of the bell gives the jellyfish a little control, and if it needs to move faster it can close the bell, pushing water out.

Jellyfish feed mostly on zooplankton – tiny shrimp and insect-like creatures and the larvae of other marine animals.  Fluttering the cilia helps to push prey toward the jellyfish’s tentacles, where stinging cells will stun it until the arms can carry it to the mouth.  In turn, jellyfish are eaten by sea birds, spadefish and sunfish, and all seven species of sea turtles.

The jellyfish commonly found near Jamaica are named for their shapes and habits.  The pale tan, yellow or purple upside-down jellyfish, the laziest of the lot, is often found on a mud bottom or near mangroves resting on its back with its tentacles (ruffles rather than strings) facing up.

The moon jellyfish has an almost-clear disk that can be more than a foot across.  Its internal organs make a four-leaf clover pattern in the middle.  Moon jellies are not shy but will keep their distance, making them very good subjects for pictures.  Both moon and upside-down jellies are safe to be near if you don’t grab them.

The little brown thimble jellyfish is a nuisance.  From late spring until early fall, large groups of thimbles can be found over shallow reefs, often near moorings.  Both adult thimbles and their larvae (called “sea lice”) can produce a nasty itching rash.  They make no attempt to stay away from swimmers.  Their small size and large numbers make them hard to avoid, especially since sea lice are easily trapped in and under clothing.

Box jellyfish (sea wasps) are dangerous.  They usually stay in deep water during the day, but at night they come near the surface closer to shore.  They are attracted to the lights of night divers and fishing boats, and through the summer they gather in breeding swarms about ten days after each full moon.  Sea wasps deliver a sting that looks and feels like a bad burn.  It isn’t deadly, but anyone who is sensitive to insect stings or gets stung several times should get immediate medical treatment.

Reports of jellyfish swarms around the Caribbean are increasing.  Even the harmless ones frighten tourists and foul beaches, docks and fishing gear.  Also, this is a strong sign of a system badly out of balance.  Jellyfish can thrive in warm, polluted, oxygen-poor water where most other things can’t.  The things that eat them – especially sea turtles – are increasingly rare or endangered.  If coral reefs are the “canary in the coal mine” for the marine environment, jellyfish are the rats and cockroaches.