Despite their gelatinous resemblance to jellyfish (cnidarians), comb jellies are ctenophores and do not possess stinging nematocysts to capture prey and nor do they have a sessile, polyp phase. Instead they have developed other food capture strategies, such as sticky tentacles and oral lobes. Comb jellies have a transparent body of two layers cells which is characterised by eight rows of comb-like plates, comprised of several thousand cilia which beat in a wave pattern moving from the aboral to the oral end. This allows the animal to move in a smooth motion, unlike that of the typical jellyfish stop-and-go pattern. A careful look at the cilia will show a shimmering rainbow of pulsating light which is produced by the diffraction of light passing between the cilia. At night the combs are luminescent. Comb jellies vary in body shape, but many found in the south-west are oval shaped. A number of genera and species occur in south-western Australia.
Comb jellies have a central mouth and a number of anal pores. Ctenophores are carnivorous and prey on a variety of planktonic animals. Some species of comb jelly use sticky tentacles to ensnare copepods, larval fish, eggs and other small zooplankton, while other species have ciliated oral lobes coated in mucus to catch similar prey.
Ctenophores are near-surface dwellers, down to several hundred metres, and occasionally occur in dense aggregations in coastal waters both nearshore and in open sea habitats. As they are weak swimmers, prevailing winds and ocean currents drive them ashore. Ctenophores are hermaphrodites where each individual possesses both male and female organs. Eggs and sperm are typically released directly into the water through the mouth from the gonads which are located in the canals beneath each comb row. Fertilisation takes place externally in the water and develop into tiny ciliated free-swimming larvae. Ctenophores are prey to cnidarians jellyfish, fishes, sea turtles and the ocean sunfish.
Other common names include: Sea gooseberries
Occurrence at the Busselton Jetty:
Ctenophores don’t tend to occur at any one particular time of the year, though are mostly observed during calm seas. Water currents and prevailing winds are responsible for the occurrence of two species around the Busselton Jetty. Bolinopis sp. with a transparent, elongate, rounded body up to 8 centimetres and Beroe sp. with a more rectangular shaped body growing up to 10 centimetres long. Many fish species have been observed feeding on the ctenophores from the underwater observatory, in particular the round faced batfish (Platax teira) which tend to inhabit the surrounding waters of the Busselton Jetty in the summer months.
Image by: A. Terlick