Most sea slugs are clingy little creatures, foraging on the substrate pretending to be bits of algae or trying to pass themselves off as flakes of seaweed, sponges and cnidarians. Others are gaudy and gold, or bold and brazen, relying on biochemical defences synthesised within their bodies or co-opted from consumed but undischarged stinging nematocysts.
A few species, however, have taken off from the sea bottom to make a living in midwater. Glaucid nudibranches glide on oceanic currents in search of hapless Portuguese men-o-war. When confronted with untoward attention, the so-called Spanish dancer spreads its mantle of red and white to kick off a display of brilliant hues and unruly confusion. Not as spectacular but no less charming is the puffy flight of the moon-headed side-gilled slug (Euselenops luniceps).
At Sister's Island last week, we found a trio of these largish slugs. With their tan and blotched mantle, they are probably at home in the sandier parts of the swimming lagoon, where they would dig into the substrate to feed and hide from predators, with only their rhinophores (the two olfactory organs that look like short horns) and the pallial siphon (the single tube extending from the back and above the anus that serves as an exhaust for spent water) protruding from the grains. The broad foot provides sufficient lift when undulated to propel the slug above the sea bottom, where it bobs and banks with the earnestness of a squat and sleepy cuttlefish, overloading the minds of observers with a fatal dose of cuteness. The larger slug on the left is feeling horny; you can see it extending its penis (the pale sausagey thing poking out from its right flank) at its smaller companion, who is probably not in the mood for a session of public genital fencing.