A scallop can see you coming: a whole row of eyes glitter like tiny ballbearings among the tentacles fringing the gape of its shell. To sneak up on scallops, a scuba diver has to hold his breath and not wiggle his fins too much. Otherwise, the young ones take fright and swim away, gulping the water into their valves and squirting it out near the hinge in a jerky jet-propulsion. Even some of the bigger ones hop along the sea floor, bouncing on their hinges.
The swimmers don't go far - a couple of metres, then a rest, then another skip or two. But you can see how, in a strong tidal flow, a young scallop might end up a long way from where it started.
After centuries of blind sampling of underwater life with nets and dredges, scuba diving has transformed marine biology. When Dan Minchin, a government marine scientist, watched the swimming scallops on the underwater slopes of Killary Harbour in the 1970s, their behaviour offered new understanding about the settlement of a valuable native shellfish.
Over the following 20 years Minchin dived again and again in the sheltered bays of the Atlantic coast, from Donegal to west Cork. He was studying infant scallop spat about the size of his thumbnail - which seaweeds they settled on, how they made their hollows in the sand, how they swam around or were swept along by currents and storms.
When scallops "recess", or work themselves into the surface of the sand, they always lie on their cupped, right side, with the flat valve on top like a lid. This becomes covered in sand and makes it harder for starfish to get a grip. Scallops can tell which starfish species are out to get them and escape by swimming. But crabs are also quick to grab and smash their shells, and there's even a common Irish sea anemone, Anthopleura ballii, that seems to specialise in tearing the spat limb from limb.
Mulroy Bay in Donegal, so sheltered as to be almost landlocked, has a specially big population of scallops and the most commercially important settlement of spat anywhere in Ireland. A couple of years ago the spat-fall in August was so generous that shellfish enterprises rushed to collect it on every piece of "nursery" equipment they had.
Scallops seem, indeed, to be the star performers in an exciting drive to restore Ireland's natural beds of native shellfish. There are already enhancement and re-seeding programmes for scallops at half-a-dozen bays from Connemara to Bantry, as well as in Mulroy Bay itself. For mussels there is re-stocking of beds at Wexford and the Boyne, Carlingford and Lough Swilly. The native oyster beds of Clarenbridge and St George at the head of Galway Bay, now greatly depleted, are being revitalised by a partnership of local interests and State agencies.
All this could hardly be more timely. The beds are not only a resource in themselves and for the burgeoning mariculture industry, but a monitor on the health of our bays and estuaries - this at a time of great pressure from human settlement along the coast. As filter feeders, the bivalve molluscs are vulnerable to all kinds of pollution, from toxic metals to leaking septic tanks.
But ecologically, the news is not all so sanguine. Overfishing for the Paris market has brought our two smaller sea-urchins, Paracentrotus lividus and Psammechinus miliaris, into serious decline. They can be restocked - the trials have worked at licensed sites - and the new art of "echinoculture" may provide the market with urchins packed with juicy gonads.
Razor shells, the Ensis species, would at one time have seemed as unlikely an export as sea urchins. These are deep-burrowing shellfish which leave just a keyhole in the surface of the sand. Their subtidal beds are being fished with hydraulic dredges, notably on the coasts of Co Meath and Co Louth, but also in the Shannon Estuary and Galway Bay - this without anyone knowing much about their ecology and distribution, or about the damage that dredging could do.
The most profitable shellfish landed in Ireland is still Nephrops norvegicus, the "Dublin Bay prawn" that is neither a prawn nor lives in Dublin Bay. It is actually a small, burrowing lobster that spends most of its time in chambers dug in fine, silty mud, emerging to feed by night.
Its main population, in the western Irish Sea, is trawled for intensively off the coast between north Co Dublin and south Co Down, but there are other small grounds off the west and south coasts. The average size of the landed "prawns" has gone down markedly, but scientists who survey the Irish Sea stocks twice a year seem to think the stock can bear the present rate of fishing.
They are less sanguine about Nephrops' big brother, Homarus gammarus, and efforts to boost lobster stocks by releasing thousands of hatchery-bred infants on to the seabed (65,000 of them in 1997) are still bedevilled by one small biological mystery.
The lobster starts life as a free-swimming larva in the plankton at the surface of the sea. At about four weeks old, as a matchstick-sized miniature of its adult self, it takes a dive to the seabed and disappears into some cobbled crevice. Between that point and its later reappearance as a juvenile lobster it goes through half-a-dozen stages of development - but remains impossible to find. Irish marine biologists have appealed for specimens of the matchstick-sized Stage IV and, like their counterparts in Britain, Norway and Italy, have been "hoovering" the seabed for samples. Knowledge of the missing phase of growth and of competition will help in judging the lobster-carrying capacity of any given area.
Should we manage our shellfish-growing areas for the biggest possible crop, for a sustainable yield, or for the most diverse and stable ecosystem? And how far should we go in trying to "improve" our natural habitats with artificial reefs? These are among the questions for a big international conference on coastal shellfish restoration to be held in Cork at the end of September. For details, contact Dr Gavin Burnell, Aquaculture Development Centre, UCC (email: icsr@ucc.ie).
