The Sabellaria reefs
An eighth of a millimetre long, transparent, delicate, planktonic, the Sabellaria larva is being swirled to and fro by the currents in the sea. There may be tens of thousands of its peer group in the sea around it, all needing to touch down on solid ground. But larger animals in the plankton try to trap and eat them: mussels on the sea-bed are pumping water across their gills, ready to filter them out and wrap them in sticky mucus; there are worms with outspread crowns of tentacles, waiting to catch their prey: our larva unfurls its clusters of tiny spines to make itself prickly and unpalatable.
Some of those adult worms sitting tightly in their sandy tubes might be Sabellaria, honeycomb worms, too. But our larva is just a juvenile, and has a lot of growing to do before it can look and behave like them. So when the current brings it close to a solid surface, it swims as strongly as it can to reach that safe haven, and then it settles and crawls across the surface, searching for the best place to construct its permanent home.
Sabellaria is a worm that is related to earthworms, and even more closely to the ragworms and lugworms found on the sea-shore: like them, its body is made up of segments, through which run its gut and blood and nervous systems. Its tiny planktonic larva has to metamorphose, remodel itself, and to develop new organs and grow, before it can become a functioning adult worm. And unlike the predacious ragworms that actively hunt amongst the pebbles and weed, the adult Sabellaria is a sedentary, tube-dwelling animal that relies on food coming into contact with its tentacles that spread out around its mouth. When the tide goes out, the worm goes in - so when Dr Jane Lancaster and I were looking at worm-tubes at Tarns Bay, all we could see were pale pink snouts, deep down inside the tubes.
"They're quite boring really, they look a bit like worms," Jane said, and I had to agree. "But people come down here, or go to Dubmill Point, and they say, 'Look at the coral reefs!'."
And these - the reefs, which are built by the worms not corals - are what make Sabellaria alveolata so special to Cumbria and the south Solway coast. Surveys by Jane and by the Institute of Estuarine & Coastal Studies (IECS) at University of Hull show that the reefs are found in patches from Morecambe Bay to Beckfoot, and they are so unusual that they are designated protected areas*.
Reefs are formed by a concretion of tubes built by the worms. The worms form protective tubes around their bodies by glueing sand-grains together with a sticky secretion. Dr Larissa Naylor, now based at Exeter, showed that the worms were fussy about the size and shape of grain they used as building-blocks: when she compared the particle size in tubes on 24 randomly selected reefs with that of ten adjacent sand samples, she found that the worms tended to pick grains of a coarser size distribution than the mean particle size of the surrounding sand. They also preferred to use (as you might expect for an animal that has a soft, unprotected 'skin') grains that were flat and plate-like or elongated.
And tube-building is rapid: in the lab, Larissa found worms could build tubes up to 5cms long within two months of settling, and re-building rates of 4mm per day have been found elsewhere where worm-tubes have been damaged by trawling.
There are other worms that protect themselves by constructing sandy tubes -- the related Peacock Worm, Sabella pavonina, for example, or Lanice; Lanice's tubes are crude in comparison to the neatly abutting grains of Sabellaria's. But it is the communal nature of the tube-building, the massed apartment blocks, the high-rises and the sprawling suburbs, that makes Sabellaria so extraordinary. Lanice and Sabella are loners, but Sabellaria larvae are stimulated to settle where they "smell" the cementing secretions of others of their kind. They settle on other Sabellaria tubes, they pile in next to each other. Sometimes they construct a "disorderly network" of tubes, sometimes the tubes are straight as organ-pipes.Does one worm build more than one tube? It isn't clear. What is clear is that if danger approaches - a crab's sharp claws, a browsing fish, the worm's rapid reflexes contract it down inside its tube. Zip! - and it's gone!
On the beach by Stubb Place near Seascale, Jane Lancaster points out to me a big rock near the low-tide mark, where she usually starts her annual survey of intertidal animals and algae. Two years ago, she says, that rock was surrounded by water; this year, the pool has gone and so has the Sabellaria reef that was nearby. Sand has encroached and has buried large areas of the reef. Further along, the reef is still mostly exposed and alive - and here at Tarns Bay the worms also build flattish encrustations, referred to as "platform", that bind and consolidate the shore. It seems there are degrees of "reefiness" along the Cumbrian coast, the colonies towards the South being more reef-like while those up near Dubmill Point and Beckfoot tend towards mounds.
At Dubmill the lower shore presents an alien landscape, a scene of dark, roughly-sculpted mounds, their ragged shapes reflected in the still surface of intervening pools. The worms have altered the environment of the shore, "bringing the lower shore up to the middle", as Jane says. The mounds trap pools of water, so that lower-shore animals like shrimps, prawns, crabs, shannies, starfish and brittle stars, and algae such as pink encrusting Corallina, and red "Irish moss", Ceramium, extend their territory upshore because they can remain underwater for most of the tide's cycle.
But the changes are never permanent - the reefs and mounds change from year to year, growing or dying and breaking apart according to the weather, temperature and the worms' interactions with other animals. Jane hypothesises that there is a continuous interplay between the mussels and the worms: the Sabellaria mounds trap water, creating pools. These provide a habitat for the filamentous algae on which mussel spat like to settle (see the Mussel-beds story for photos and further information) . The mussel juveniles also settle on the Sabellaria reef because it's a rare, sought-after, hard substratum. Gradually, the mussels take over, the "mussel mud" and accumulating sediment overwhelm the worms, which die and their colonies decay. The mussel beds increase in size and block the intervening pools, the adults "inhale" the planktonic mussel spat during their feeding so that further recruitment decreases; the mussels die back - and the cycle starts again.
The worms are alive inside the tubes - see the coils of 'Sabellaria poo' (my thanks to Alan Sawyer for this valuable photo!)
Sabellaria, Lanice, mussels, cockles - they are all filter-feeders, sifting food out of the passing water. When David Dobson, Director of Operations of the North-Western Inshore Fisheries and Conservation Authorities, was talking to me about the decline in the size of mussels in the intertidal beds, he told me, "It's because there's not enough food. The Sabellaria are competing - not just the onshore ones, but there's the Sabellaria spinosa in the channels. They're very healthy and they recover very quickly [from damage by trawlers]. This always used to be the northern limit of the Sabellaria but they're expanding, possibly due to the rise in temperature of the sea - they're on the northern side of the Solway now too."
Shore-walkers at Dubmill Point (my thanks to Alan Sawyer for this photo of the September 2011 shore-walk)
The subtidal Sabellaria spinosa have a big advantage over their intertidal relations: because they are always covered by water they are always able to feed. The spinosa form reefs out in the channels of the Solway but their extent is as yet unknown: the reefs are vulnerable to damage by trawls and shrimp-boats, but research elsewhere suggests that they do indeed rebuild and recover very quickly. Perhaps, with advances in the recognition of different types of sonar patterns, it will be possible in future to map the reefs' extent.
Meanwhile, the intertidal reefs and mounds of Sabellaria alveolata, those alien landscapes of the lower shore, will continue to surprise and thrill shore-walkers who discover them.
Text copyright Ann Lingard, May 2011
* Sabellaria reef is a priority BAP (Biodiversity Action Plan) habitat; it's also considered an Annex 1 habitat (biogenic reef) under the Habitats Directive and is a Habitat of Principal Importance in England under s41 of the NERC Act. Finally Sabellaria reefs have been identified as being worthy of protection by the Marine Act by the creation of a Marine Conservation Zone (MCZ). (My thanks to Jane Lancaster for clarifying the protected status of Sabellaria reefs.)