I bet I’ve looked in hundreds of rock pools and seen crinoids (feather stars), but just didn’t recognise them. They look like seaweed but are really animals. This one was waving its arms in a decidedly unplant-like manner and wouldn’t have been bigger than half my thumb length.
For the technically inclined, here are some sections showing the internal crinoid anatomy, taken from Rupert, Fox and Barnes, Invertebrate Zoolgy .
The pool it was in had water running through quite fast, so this crinoid was in a perfect position to catch floating goodies. From Edgar’s Australian Marine Life, 2nd edn:
Feather stars, or crinoids, have long branched arms with a regular arrangement of small side appendages (the pinules) producing a feather-like effect. In their normal feeding position, the arms and a centrally located mouth are held upwards. Because of a U-shaped gut, the anus is positioned on the same surface as the mouth but is offset somewhat from the centre. On the undersurface of the body is a ring of slender, jointed, calcareous appendages known as cirri, which are used for locomotion and to allow the animal to anchor to the substrate. Crinoids feed by trapping small planktonic organisms using modified tube feet that lack suckers. Food particles are transported in mucous strings down the arms along a ciliated food groove to the mouth.
Part or all of an arm can be cast off and regenerates fairly quickly, as with sea stars to which they are related.
I’m not even going to attempt ID-ing – there are over 130 crinoid species in Australia, mostly in tropical waters. It’s nice to finally recognise one of them in the wild.