Order RHIZOSTOMEAE L. Agassiz, 1862

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Order RHIZOSTOMEAE L. Agassiz, 1862

Compiler and date details

June 2012 - Lisa-ann Gershwin

DRAFT RECORD

This taxon is under review. This record is released now for public view, prior to final verification. For further information or comment email us.

Introduction

The Rhizostomes are some of the most conspicuous medusae found in Australian waters. They usually have large, fleshy and brightly coloured bodies. The oral arms are typically either stiff with thousands of tiny mouthlets along the ridges or on all surfaces, or floppy with mouthlets along the ridges. A central mouth, in the sense of other types of medusae, is not present. Rhizostomes also, like other medusae, do not have true marginal tentacles. Most species have a scalloped margin with no hint of tentacles. The unusual Lobonema may appear to have tentacles, but it may be the lappets that are drawn out into tentacle-like filaments.

Rhizostomes are very common along the Australian tropical and temperate coastlines. At least 36 species have been recorded from Australian waters. In some years the rhizostome medusae blooms are so dense along the coast of NSW or in the lower reaches of the Swan River, WA, that it would be possible to walk across the bays and harbours, if one could use them as stepping stones! In April 2000, a species of rhizostome that had not been previously identified in Australian waters, Crambione mastigophora, bloomed in massive numbers from Derby to Rottnest Island, WA (over 1200 km) for about two weeks; the causes and effects of this bloom remain unstudied, and the event itself is essentially lost to science from lack of reporting. Visitors to Glenelg (South Australia) will delight in seeing the mosaic of the rhizostome Pseudorhiza haeckelii embedded into the sidewalk on the main street near the foreshore.

Most rhizostomes give a mild to moderate sting, but are not life-threatening. The first aid treatment for rhizostome stings is to rinse well with seawater (NOT freshwater!) to get rid of any remaining stinging cells, then use ice for the pain as needed. If symptoms worsen, or if breathing becomes laboured, seek medical attention.

Further information is found in Stiasny (1921), Kramp (1970) and Kingsford et al. (2000).

Diagnosis

Scyphomedusae with umbrella margin cleft into lappets; without marginal tentacles; without a central mouth opening, but with numerous mouths upon eight adradial, fleshy, branched, arm-like appendages arising from the centre of the subumbrella; with rhopalia between margnal clefts.