Its time for another ‘Blue fleet’ post. Last time I covered the carnivorous Nudibranch Glaucilla which I encountered wrecked on a pacific coast beach after a large storm. Another predatory mollusc was among the jumble of stranded Physalia bluebottle jellyfish and sea slugs that day: violet sea snails, Janthina sp.
These stunning Gastropods secrete an adapted mucus from which they construct a bladder of bubbles. They then hang alongside this mechanical flotation device upside down and drift among the ‘blue fleet’ of Physalia, consuming one whenever the opportunity arrises. Like the Glaucilla they absorb the venom and blue colouration from the jellyfish and turn it to their advantage to deter predation. Even their shell is tainted with a blue so deep it looks violet- hence the english name Violet Sea Snail. When disturbed the live animals dumped a blob of violet coloured goop on me- presumably a dose of venom- it stained my skin for several hours.
On the day I took this photo I found two distinct forms of Janthina: one a classic form (as in the photo) with countershading (a dark upper side and a light underside- to disguise the animal from both above). Its shell was wider than it was tall. The other form was comparatively scarce and featured violet all-over and a very exaggerated whorl reminiscent of the Succinidae. The shell was significantly taller than it was wide. If anyone out there knows of any resources I can use to key these animals out please drop me line!
The ever-awesome, though now static, Sea Slug Forum has a nice photo of a live Janthina predating a Physalia in a studio aquarium.
Unlike my fellow seaside-goers, I look forward to another good bluebottle wreck next time I go to coastal New South Wales. Perhaps with the help of a wetsuit to ward off the stingers I’ll manage some photos of our blue fleet stalkers at work in the ocean? Got to be worth a try!