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Spiders are pretty adaptable creatures so it's not too surprising that some have colonised the intertidal zone around Australia's coast.
Mysterious Marine Spiders
The most common of these 'marine spiders' belong to the genus Desis. They live in the intertidal zones of rocky shores and reefs, which are underwater for hours or days at a time. They survive by building silk chambers in air-filled pockets in rock cavities, shells and seaweed holdfasts. At low tide they emerge to feed on small crustaceans or anything else they can handle with their large jaws.
Marine spiders are about one centimetre in body length, around the size of the familiar window or black house spider. They have very large, projecting jaws and compact bodies with a reddish-brown head and jaws and greyish abdomen and legs.
Although marine spiders are known from many areas of the Australian coast, it appears that people rarely notice them. Recent reports only come from the Great Barrier Reef. However, 100 years ago, marine spiders were common residents of Sydney Harbour. An 1898 report states that 'There is a very common species of spider found under stones about low water mark [at] Taylor's Bay and Watson's Bay'. The Australian Museum collections contain specimens collected in the 1930s from 'Port Jackson' and Bottle and Glass Rocks (Vaucluse Point).
Recent searching around Sydney Harbour has not located any marine spiders. This suggests that the spiders are either no longer very common or are patchily distributed. Foreshore development and pollution has no doubt had some effects. The worst scenario is that marine spiders are locally extinct in Sydney Harbour. It would be good to disprove this possibility.
If anyone has seen marine spiders in the Sydney region recently or in the past, it would be appreciated if they could contact the Australian Museum with details.