Michael Viney: The snail and the jellyfish went to sea

The saga of evolutionary chance has come up with some improbable pairings of predators and prey. How unlikely that a snail that lives on the seabed should float up to the surface to drift, upside down, beneath a bubble raft, feeding on little jellyfish that just happen to be blown its way.

The partnership – not quite the word, perhaps – of the violet sea snail, Janthina janthina, and the by-the-wind sailor, Velella velella, is a story almost out of Edward Lear. Not that readers finding the curious floats of Velella on western strands this winter have reported any snails in their company, and some had no idea what they'd picked up from the tideline.

The cap of the little siphonophore (not strictly a jellyfish, being a colonial animal) is more likely to survive the surf, whereas the snail’s beautiful shell is fragile and doesn’t take bouncing.

I found some together, a few decades ago, landed gently by summer wavelets on a strand on the Mullet Peninsula, in Co Mayo. And in the autumn of 2007, in a remarkable event, thousands of snails washed ashore in the northwest, some still chewing on blue-jelly shreds of Velella. But much was odd about that. Not only were the snails a species of Janthina scarcely known in Irish waters, but most of the jellyfish had their little sails, at least arguably, set quite the wrong way.

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Colony of tentacles

Let’s start with that.

Velella

’s colony of tentacles trails from a small buoyant cap of deep-blue jelly in which is set an oval, iridescent disc, feeling rather like plastic, with a triangular, vertical sail set across it. It is the disc, with its sail, that is washed up at tidelines around the world, sometimes by the million.

Like the gas-filled float of its big relative the Portuguese man-of-war, the little sail of Velella, tough and flexible, catches enough wind to keep the animal moving to encounter planktonic food. On some of its discs the sail is mounted, diagonally, northwest to southeast, so to speak, which steers them leftwards as they drift, and on others northeast to southwest, which steers them rightwards, as much as 60 degrees from the wind's direction.

Both forms occur in all the big oceans, mostly in warmer waters, where prevailing winds and currents ensure a wide dispersal. Most of those reaching our shores seem to be left-sailors. In 1992 millions of Velella arrived at Irish coasts, beginning in January and February, and their armadas were so abundant in July that, in Connemara, rock pools and gullies held dense drifts and the tidelines bore continuous glittering ribbons.

Gathering fistfuls from the shore at Thallabawn, I sat in the garden to sort them. There were 228 left-sailors and only 42 right-sailors. In the armadas that arrived in 2007 most of the jellyfish were right-sailors. And the incredibly abundant snails – the Sligo ecologist Dr Don Cotton collected more than 1,000 – were Janthina globosa, a rounder, glossier species more typical of warmer seas. Such are the novelties now sometimes served up by climate-driven changes in the ocean.

Skin of the sea

The extraordinary change in

Janthina

’s original existence, from seabed to the skin of the sea, is beginning to find explanation. At the surface it hangs upside down from a little raft of silver bubbles. It makes this by lifting its foot above the water to trap morsels of the wind in a film of mucus that hardens into something like cellophane. But how could this all have begun?

Recent research in molecular history has set the distant origins of Janthina in the family of wentletrap snails, seabed predators of corals and anemones. Their intricate spiral shells are quite different from those of Janthina, but their egg masses typically have capsules at various stages, including empty husks. In species uncovered between tides, it is suggested, these husks could trap air, "providing temporary buoyancy for both the egg mass and the attached female".

A permanent life on the ocean wave would need the creation of air-filled bubbles, but the researchers conclude that "through sequential modification of float construction and function, janthinids have become increasingly successful at exploiting neustonic resources". (You can read "Females floated first in bubble-rafting snails" in Current Biology's October 2011 issue, at iti.ms/2kK9eSL.) The three researchers include one Diarmaid Ó Foighil of the University of Michigan, in the United States, a graduate in zoology from NUI Galway.

The life of the ocean surface – neustonic resources, if you like – is often swirled together by ocean eddies, winds, fronts and upwellings, so that jellyfish, plankton and rafting snails can be gathered into something like crowds. To drift dangling and unseeing on the chance of meeting the next meal of jelly, itself a wind-blown toy yacht on the pond, does seem to ask much of success. But in nature, whatever works . . .

Michael Viney's Reflections on Another Life, a selection of columns from the past four decades, is available from irishtimes.com/irishtimesbooks