upside-down jellyfish

An upside down jellyfish (Cassiopeia andromeda) at uShaka Marine World in South Africa.

These jellyfish live in the intertidal area on sandy bottoms, mud flats and in mangroves. Their color is variable, often they are dull brown but they can be colorful. They are usually found on the bottom with their mouth upwards, hence the name upside-down jellyfish. They are able to swim but prefer to remain stationary.


As with many other jellyfish they have a symbiotic relationship with various types of algae (zooxanthellae) which provide them with some of their nutrition. They also capture small fish and Zooplankton with their stinging cells.


Upside down jellyfish do not have what we would traditionally call a brain or centralized concentration of nerve cells. Instead they have a nerve net distributed throughout the body which has various simple sensors on it and operates in a decentralized fashion. Information is transmitted through the net and via a circular net that runs around the bell with rhopalia on it, small structures containing sensors to sense light and gravity.

The circular net around the bell also controls the contraction of the bell for propulsion. Jellies use their light sensing cells to orientate themselves against the light. They use pulsations of their bell to propel themselves forward and having no sight, have limited control over where they will end up, their directional logic being generally limited to either moving away from light or towards it. Recent research indicates that some jellies may orientate their direction on a current but this is not yet proven.


Jellyfish or sea jellies can reproduce through both sexual and asexual reproduction. During the Medusa stage specialized organs known as gonads form either eggs or sperm. In  some species, female jellies release eggs into the water table. These are fertilized by sperm released by the males. In others the eggs are fertilized in the stomach of the female when the sperm released by the males swims into the females stomach and then fertilize them there.

Some jellies such as the moon jelly have special areas on the oral arms which function as brood chambers. The eggs hatch into larvae known as planula with cilia on them allowing them to swim and these attach themselves onto a firm substrate and develop into polyps called scyphistomae.  These polyps produce numbers of ephyra, that detach and swim off to grow into the medusa stage. The polyps can also bud asexually producing other polyps.


Phylum: Cnidaria
Class: Scyphozoa
Order: Rhizostomae
Family: Cassiopeidae
Genus: Cassiopea
Species:C. andromeda