Saturday, 13 April 2013

New Arrivals: Upside-Down Jellyfish

Adult Cassiopea
Now on show in the marine invertebrate section of Bug World is a small group of unusual jellyfish, the Upside-Down Jellyfish Cassiopea sp. As you might guess from their name, these creatures spend their adult lives resting upside down on the sea bottom, instead of swimming in the supper surfaces of the sea as most jellyfish do although if disturbed they can swim with contractions of the bell as with their more mobile relatives.

Cassiopea is only found in shallow waters with high light levels on the sea floor, usually lagoons, mud flats and mangrove swamps. They do this because like corals they harbour algae in their tissues, which provide food to their host by photosynthesis. As well as their algae, they can also feed on smaller organisms as they retain the nematocysts, the stinging cells which predatory jellyfish use to kill prey, although to humans at least the sting is very weak. The group of jellyfish they belong too has modified their internal anatomy – the primary mouth at the center of the bell is closed and instead secondary openings into the digestive cavity open at the edge of the body.
A different morph - colour is due to symbiotic algae
Jellyfish (Scyphozoa is the technical name) exhibit alternation of generations during their life cycle. The adult reproductive phase is called the medusa, and the adults are either rmale or female. Eggs and sperm cells are released into the digestive cavity and expelled through the mouths into the surrounding water, where they meet and the egg is fertilised. After spending a short period in the plankton, the egg become a planula larva (so called because it resembles a planarian flatworm) only a few millimetres long, which settles on a suitable substrate to become a polyp with tentacles that feeds and grows for a few months. How the planula selects a suitable substrate is not entirely clear – in the Bermudan species C. xamachana they seem to prefer the shady side of decaying Red Mangrove leaves. Presumably other species (the genus is found worldwide) have similar requirements, but I have not seen much study of differences between them. At this point they also acquire their symbiotic algae through feeding on plankton.
Lateral view showing the feeding arms
After growing for a few months, the small polyp begins to strobilate. This involves budding off tiny medusae, which then settle down on the sea floor to grow into the adult phase over the next few months. As neither the adults nor the polyps are especially mobile, how many actual species of Cassiopea exist is subject to debate and is probably more than are currently recognised – they certainly seem to come in different colour morphs in parts of their range but whether these are actually separate species is unclear. They do not seem to have many natural enemies – possible specialised flatworms prey on them as they do on corals, but I have not found any specific documentation on line on these. None of the forms is currently listed as threatened, and this is unlikely to change in the near future.

(images from wikipedia)

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