Sometimes referred to as jellyfish, the sea jelly is not a fish at all as it lacks vertebrae.

Jellyfish or Sea Jellies?

True sea jellies (or scyphozoans) are single animals that have a bowl or bell-shaped body, fringed with tentacles and oral arms. Largely at the mercy of ocean currents, sea jellies drift in the oceans, feeding on plankton and small fish. There are many different kinds of sea jellies; some more common ones include the moon jelly and Australian spotted jelly.

The Australian spotted jellyfish is a true sea jelly and are commonly found in the western pacific from Australia to Japan. They can often be seen in the Swan River.

The box jellies (or cubozoans) are named due to their square shape when viewed from above. Single tentacles or clusters of tentacles are found on each corner at the base of the cube-shaped bell. Box jellies are unusually strong swimmers and therefore active hunters. The northern Australian stinger, Chironex fleckeri, is one of the deadliest animals in the world!

The bluebottle is a type of siphonophore, which is not a single animal but a colony of animals that depend on each other for survival. Made up of four kinds of highly modified individuals known as zooids, dependant on one another for survival. The float or pneumatophore is a single individual, and predomoninately blue. This float is living, and secretes its own gas! The tentacles or dactylozooids detect and capture prey, taking the prey to digestive polyps known as gastrozooids. The gonozooids or the reporductive polyps make up the final part of interesting colony of animals! Drifting on the ocean currents, feeding on small fish and other animals, the main tentacle can range up to 10 m in length.

A blue bottle washed up on a beach. They can still sting, so look, don’t touch!

True sea jellies, box jellies and bluebottles are related to corals and anemones, and are in a group of animals called cnidarians (pronounced nigh-dare-ee-ans). The name comes from the Greek word ‘cnidos’, meaning sting nettle. A key feature of these animals is the presence of nematocyst or sting cells, found mainly in the tentacles. These specialized stinging cells aid in defence and the capture of prey and can sometimes cause a nasty sting to humans.