Monday, February 7, 2011
Stalkers of the Blue Fleet. Part 1: Glaucilla marginata
On the best day of the week for wrecked sealife, I noticed tiny striped splodges mixed with the blue tentacles and wobbly bladders of the abundant Man'O'War jellyfish Physalia phisalis. (OK, OK, in Australia they are called Bluebottles, and they are not actually jellyfish but properly a zooid Siphonophore- happy now?) I happened to have a plastic tub with me (as you do). I filled it with seawater and popped one of the droopy splooges in. On hitting the water it sprung back to life- a shining perfect little nudibranch called Glaucilla marginata. In a flurry of excitement I took dozens of photos. These are special molluscs indeed- ruthless pelagic predators which stalk the wandering 'blue fleet' of Man'O'Wars. They are not the only molluscan hunters of these drifting stingers and further along the beach I found a couple of examples, but I'll save those for future posts.
The blue fleet is a remarkable community of creatures. Normally they ply their trade far offshore but as the Physalia blindly drift where the wind blows and all the others follow the whole lot are prone to being blown ashore.
Like many pelagic sea creatures Glaucilla and its close relative Glaucus are not restricted to any one sea area- they occur almost everywhere the sea is the right temperature for them. They float upside down (the photo here is of their belly) close to the surface tension and simply bump into their prey as they drift. They can be mobile if they find themselves stuck- flexing and wriggling in that sea-slug sort of way until free. The blue colour is said to be the accumulated venom from the zooid tentacles. This obviously works as a predation deterrent- beachcombing Silver Gulls carefully avoid them.
So, a carnivorous sea slug which sails the seven seas hunting a dangerous prey- surely that has to be the weirdest thing I found on the beach that day. Not so...