Sargassum crassifolium is my best guess for the identity of the seaweed in these photographs. This is just one of eight types of Sargassum washing ashore on the Queensland Coast in Australia. I thought at first it was something called Spiny Tops, Turbinaria ornata, but although I did find one specimen of that, most were S. crassifolium. The features that distinguish it from Spiny Tops are the arrangement of the fronds (leaves) on the stem, the relative thinness of the fronds, the shape and distribution of the spines arround the edges of the blades, and the presence of small round floats.
In Seaweeds of Queensland – A Naturalist’s Guide, The Queensland Naturalists Handbook No. 2, by A. B. Cribb, 1996, Kingswood Press, Queensland, ISBN 0 9595607 1 8, it says about this free-floating brown alga:
The up-curved leaves of this species are relatively short and rigid. In the upper part the spiny margin is double. The species is mainly a tropical one but is occasionally found in the southern part of the State.
Species of Sargassum have been used as food in various parts of the world, particularly China, Japan and Hawaii. Their habitat is the subtidal region on semi-exposed shores.
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