swimmerets...

... word for the day and courtesy of J and a 'cray' otherwise known as the Rock lobster and even more formally Panulirus cygnus a species of spiny lobster found off the coast of Western Australia.

The four pairs of swimmerets, or pleopods, are used to make  water currents to help the beasties swim and in females used to carry the eggs or newborn.
The tail fan is used for propulsion and steering while swimming but crays are also great walkers with five pairs of legs that propel them across the ocean floor.
According to J there are 'risks' (yes the collective noun for lobsters!) of them trekking across the scallop beds (and munching as they go) to Rottnest.
Not that far above them boats are doing just the same thing...trekking that is though  a 'risk' of boat laden holiday makers may be holiday season appropriate too.
All heading for 'Rotto' for Christmas.