The white things on the snail shell are gooseneck barnacles (Lepas sp.), they like to stick to any old floating thing, such as bits of wood, plastic, styro, etc. Here is a better view of the barnacles, sticking out their cirri (feeding appendages).
Notice that the shell of Janthina is dark purple on top, and very pale lavender on the bottom (well, actually it's the bottom of the shell that's dark & the top that's light, but as you see in this snail top & bottom are reversed). The reason for the two-tone shell? The top is dark so that predators above the surface of the water cannot see the shell, and the light colored bottom means that Janthina is also well camouflaged from hungry predators coming from below it (the light color blends well with the sky above). This is a good example of countershading. Many fish employ the same trick (think about how many fish are blue on top with white bellies...).
I think Janthina looks just gorgeous against a black background...
Here's a closeup of the mouth and oral tentacles. Guess what Janthina feeds on... hint, it's a floating artist too!
Janthina feeds only on floating siphonophores, which are relatives of jellyfish. The most abundant siphonophore here on Guam is the man o'war or bluebottle, Physalia sp.! Notice its gas-filled transparent float/sail, which allows it to stay on the surface of the water.
Geeky taxonomic aside: there's some debate about whether Physalia is only 1 species or several. Right now only 1 species is formally recognized, Physalia physalis. But others argue that the Pacific form is a different species, and that it should be named Physalia utriculus. Overall the Pacific form seems to be smaller than the Atlantic form, and it only has 1 long "tentacle" (seen in the pic below), whereas the Atlantic critter has several long "tentacles". The jury is still out on this issue, but at least 1 recent study has shown more genetic diversity in Physalia than previously thought.
About those "tentacles": they aren't really tentacles. And actually Physalia isn't actually a single animal... it's a colony! Each tentacle, and also the gas-filled float, are actually separate individuals, known as zooids. Physalia has both polyp-zooids and medusalike zooids. Man o'wars start out life as just 1 polyp, that soon buds off other zooids that remain stuck together, forming a cooperating, floating colony. There are zooids specialized for catching prey (the long "fishing tentacle" in the pic above), and others specialized for defense, digestion, reproduction, etc. Of course, there's also one zooid specialized for floating. Can you guess the functions of the different zooids in the picture?
Oh, did I mention that Physalia stings are NASTY?... those beadlike structures you see on the finer zooids are batteries of stinging nematocysts. And if you're wondering what the dark granular structures are on the other zooids -- so am I. So drop me a line if you have an idea what they are. Thanks.
Since the members of the colony cannot survive if separated, Physalia is kinda in a grey area in between being a colony and being a single individual. I first learned about this strange creature/colony from Stephen Jay Gould's excellent essay "A Most Ingenious Paradox", published in the book "The Flamingo's Smile".
I picked up the Man O'War not only because it's pretty, but also because I was curious to see if my Janthina would eat it. So I put them together in a jar... here's a top view. You can see that the snail has its buccal mass (=mouth + pharynx) all extended, as if it was ready to chomp the siphonophore into little purple pieces.
Alas, it was not to be. I kept the critters alive for 2 days, but Janthina never did eat the Physalia. And sadly, the Man o'war died soon after. They're pretty much impossible to keep alive in captivity.
Couple more gratuitous underwater shots, just because these guys are so pretty. And hey, in the future I might even upload a video of the Physalia curling & uncurling its pretty zooids. Stay tuned!
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