Another Life: By-the-wind sailors bring beach visitors to their knees

As jellyfish and their kin move up the research agenda, their role in the global ecosystem is becoming clearer

By-the-wind sailors: not true jellyfish at all. Illustration: Michael Viney

By-the-wind sailors: not true jellyfish at all. Illustration: Michael Viney

Sat, Jul 5, 2014, 01:00

Those remarkable calms, mirroring the sky in the sea and stroking the coasts with mere wavelets and ripples, have been setting the ocean’s drifters ashore, each on its own gentle rush of bubbles. Some, like the many barrel jellyfish delivered to the south coast, are among the biggest of their kind. But with them have come a first few of some of the smallest of drifting medusae and actually, despite appearances, not true jellyfish at all.

Velella velella, the by-the-wind sailor, arrives on our Atlantic beaches at almost any time of year and in greatly varying numbers – sometimes in glittering millions, as in 1992. Late that July they came ashore almost simultaneously along a 400km stretch, from Cork to Mayo. Tens of thousands choked rock pools in Connemara and more edged the tide in continuous ribbons. Here at Thallabawn I gathered hundreds, some the size of my palm, others just a few centimetres across.

They carried shreds of blue jelly from their float, some still with the fringe of stubby feeding tentacles. But the jelly dries out rapidly on the beach, leaving only the rainbowed, oval disc embedded in its surface and the small, transparent, triangular flap, like cellophane, set upright and diagonally across it. These are more durable structures, like bits of plastic, and, bleached by the sun, are often what the holiday beachcomber stoops for and puzzles over.

I had a reason for gathering so many from the strand. The little sail is made to catch the ocean’s surface wind and move the animal on to fresh sources of planktonic food. It is found across all the big oceans and everywhere shares a variation. On some discs, the sail is set NW-SE; on others, NE-SW. In the same wind, one animal will sail leftwards, the other to the right, either veering as much as 60 degrees away from the wind’s direction.

This engineering scatters the species widely, being mixed as larvae (or so it is hypothesised) in the middle of the ocean. In the northern Atlantic, where winds twist eastwards in rotation of Earth’s atmosphere, it is most often the left-sailing Velella that end up on our beaches. Of my discs, 228 were left-sailers and 42 were right-sailers, which seemed to bear this out.

The exact biological identity of Velella has been slow to emerge: if not a true jellyfish, then what sort of hydrozoan? Its sail suggested an affinity with the big Portuguese man-o-war, travelling beneath an inflatable crest. But that is a colonial creature, each component with a different function. Velella is an individual animal and actually, it seems, an upside-down and floating variation among the hydroids, a class of organisms more usually moored to the sea floor. Velella sinks deeply in mid-ocean to reproduce but never reaches bottom.

Carbon cycling

As jellyfish and their kin move up the research agenda, their role in the global ecosystem is becoming clearer. A recent project from University College Cork, led by Dr Tom Doyle, studied the Velella abundant in the Celtic Sea. His team found rafts of floating seaweed rounded up by currents and wind and with them lots of Velella feeding on sea life that gathers under the weed – little fish, fish eggs and zooplankton. As predators, they thus become important in carbon cycling, and, in finally drifting ashore to rot, sometimes in masses, bringing some of the open ocean’s production to nourish the edge of the land.

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