Comb jellies (Phylum Ctenophora)
Superficially resembling jellyfish, comb jellies nevertheless form a distinct and diverse group of their own. Some species are spherical and may be lobed; more bizarre forms resemble straps, belts or the inside of a walnut floating in the water column, or are worm-like and crawl over the sea floor. All are marine, favouring warm surface waters, and typically bear eight rows or combs of tiny hairs that beat synchronously to help the animal move, and a pair of tentacles to catch food.
The defining feature of the group is the colloblast. Colloblasts are specialised cells concentrated on the tentacles that shoot out a sticky chemical to trap food. Comb jellies use their tentacles to sweep up small planktonic organisms in the water column.
The colloblasts of ctenophores and the stinging cells (cnidoblasts) of the true sea jellies, are a remarkable case of parallel evolution. Both have a spiral tether and are spring-loaded, shooting out to capture prey. But while a sea jelly cnidocyte is harpoon-like and delivers a toxin, the comb jelly colloblast ends in specialised granules that rupture after firing to form a sticky pad, capturing the prey by adhering to it. For this reason, ctenophores do not sting like sea jellies.
Well, all except one. The ctenophore Haeckelia rubra (which does occur in Australia) captures and eats sea jellies, steals their cnidocytes and deploys them on its own tentacles to capture more prey.
A ctenophore in an aquarium exhibit. Note the comb-like rows of cilia, which gives the common name comb-jelly.
Image: Maritime Aquarium at Norwalk CC-BY-ND-2.0
Almost all comb jellies produce both sperm and eggs, but they prefer not to fertilise themselves. Many comb jellies are bioluminescent and glow iridescently, especially along the eight combs.
Many ctenophores are iridescent in sunlight and bioluminescent at night.
Photo: Alexander Semenov CC-BY-SA-4.0 Source: https://commons.wikimedia.org/wiki/File:Ctenophora-Comb-jelly-Euplokamis-sp.-02.jpg
Comb jellies are a small phylum, much less diverse than sea jellies and their relatives. There are an estimated 60 species in Australian waters, of which around 35 have been named. Many comb jelly species are endemic to specific regions of the country. However, little else is known about their overall distribution or ecological status.
Thanks to Robin Hare for helping prepare this article.
Read more about ctenophores in Australia:
Gershwin, L., Zeidler, W. & Davie, P. J. F. 2010 12 30. Ctenophora of Australia. In Davie, P.J.F. & Phillips, J.A. (Eds), Proceedings of the Thirteenth International Marine Biological Workshop, the Marine Fauna and Flora of Moreton Bay, Queensland. Memoirs of the Queensland Museum 54(3): 1–45. Brisbane. ISSN 0079–8835.