Porpita porpita (Linnaeus, 1758)
Seasonally common in the Solitary Islands Marine Park. Found throughout the tropical and sub-tropical Indian, Pacific and Atlantic oceans.
Drifts on the ocean surface, normally found only when washed ashore on beaches due to onshore winds.
A passive drifter, the Porpita porpita is part of the neustonic (water surface) food web. It consists of two main parts: the float and the hydroid colony. The hard brownish float is round, firm and almost flat, and is commonly around 25mm wide, see bottom photograph. The hydroid colony radiates from the central disc, and is similar to tentacles on a jellyfish. Each strand has numerous branchlets, each of which end in knobs of nematocysts. The sting is not powerful to humans but may cause irritation. Overall diameter may reach 60 mm.
Bennett, Isobel, Australian Seashores, Angus and Robertson Publishers. 1987. p 168.