Nick Baker’s hidden Britain
The marine worm Ophelia bicornis
Worms get about. The UK’s many species occur in all manner of habitats, not just your garden soil. Among plenty of marine worms, there are a few extraverts that employ fancy fans and tendrils of sticky threads to sift and fiddle particles of edible material into their waiting mouths. But, mostly, the worm world is a relatively simple place.
By far the commonest strategy is for a worm to swallow soil or silt at one end, and, by a process of internal sorting, scrub it of the organic matter, bacteria and minerals that the animal needs. Think worm casts on your lawn: the earthworm ingests soil, removes what it needs and excretes clean soil behind it in a wiggly creation on the surface. Many marine worms do pretty much the same, so a visit to a muddy or sandy shore at low tide will reveal shapes and casts analogous to those on dry land.
But Ophelia bicornis, while superficially resembling many other coastal worms – it has a greenish-grey, segmented body up to 4cm long – is just that little bit different. What a shame worms rarely get common names, because Ophelia deserves to be called the ‘rectal sand sifter’. I should explain.
Sand isn’t just sand, and mud isn’t just mud – ask any builder, farmer or potter. They are infinitely variable in composition. Much of the character of a sediment is down to the size of the particles, and in places of tidal flow, the water sorts these into different grades. In some estuaries, where the water flow is regular and strong, fine sediments are washed away to leave a coarse grit of heavier particles. Not an easy meal if you’re a thin-skinned, delicate tube of an animal. But evolution has come up with a solution. Enter Ophelia, a specialist grit-eater at the northern edge of its distribution in Britain and quite localised here.
A life swallowing coarse sand, especially nutrient-poor gritty sand, has its challenges. For starters, you need to swallow a lot of it. Such sediments are also prone to compaction, and in a worm’s gut constipation of this sort would slow the flow and kill the creature. Yet sand particles pass through Ophelia in 15–20 minutes, thanks to a helping hand (not literally a hand, though it does bear a bit of a resemblance to one).
The ‘hand’ is a unique rectal organ called a typhlosole. It often remains hidden inside the worm’s rectum, the only clue being a fringe of paddle-shaped papilla that surround it like petals on a flower. But when the worm is actively feeding – and defecating – the strange handlike device, complete with six or so fingers, appears from within.
The typhlosole prevents constipation by means of a carpet of constantly beating hairs covering its surface. This creates a current of water to loosen the compacted sand grains, while the fingered handlike structure physically pushes out the spent sand. It all speeds up the passage of substrate through the worm’s gut.
To quote my old biology lecturer Tegwyn Harris, who worked extensively on this worm: “It is surely not inappropriate to applaud the ingenuity of a bulk-handling device that puts to novel use the rectum – a region of the digestive tract often regarded merely as a temporary store for faeces.” Quite.