Many marine invertebrates spend their larval life either floating or swimming in the water column before settling on the bottom to assume their adult form and habit. They may spend only a few hours, or may drift with the currents for several weeks or months and in the process achieve wide dispersal.
Despite the fact that the water column offers a such a large volume of living space, few molluscs have succeeded in adapting to a permanently pelagic existence but one such genus is Janthina, the Violet Sea Snails. They are known to exist in swarms covering as much as 200 nautical miles. They live out their lives suspended upside down at the surface of the ocean.
To achieve flotation each snail constructs an individual raft of bubbles, and it is dependent on its raft for survival. The snails cannot swim, and bubbles have a habit of popping, so each snail must continuously replenish its life support system. Only the foot of the snail is involved in that process.
The sole of the foot is extended upwards to break the surface of the water and the sides of the foot fold over the depressed central area to encase a bubble of air. Special glands on the foot coat the entrapped air with mucus, the encased air bubble then being pulled below the surface of the water and cemented to the existing raft with hardening mucus. The entire sequence has been timed and occurs in ten seconds. It may be repeated six to ten times in successions before a pause.
The raft, or float, of Janthina is firm, elastic and dry to the touch. It varies from colourless to pink or violet in colour, which may be a result of age of the float. Janthina produces eggs which hatch into young veliger larvae which swim freely in the water column. Eventually they produce a long mucous string with a ball of air bubbles at the end. This early buoyancy device brings the young snails to the surface where they can begin to construct the adult raft.
The colouration of these sea snails is a fascinating adaptation to life at the surface of the ocean. What better coloured shell could an ocean-going snail possess than the violet blue shades common to all species of Janthina? In matching the surrounding, mainly tropical, waters this is effective in concealing pelagic animals from visual predators. There are no known predators of Janthina but some fish and birds may eat them.
The potential camouflage achieved by the violet blue colour of the shell is enhanced by countershading. Those regions of the shell directed downwards in the water are paler than areas of the shell visible at the surface. This extra protective device means that potential predators approaching from below may have difficulty in distinguishing pale mauve shells against the light background of the sky, whereas predatory birds would find it less easy to spot Janthina from above because the dark violet colour blends into the dark background of the sea.
Because of their distinctive colour, unique in the realm of marine shells, it is worth keeping an eye out for a flash of violet blue when beachcombing, or walking the shores of the south and west coasts of the British Isles, particularly after intervals of stormy weather.