Lite Under the Sand

Forest and Bird, Issue 311, 1 February 2004, Page 40

Lite Under

the Sand

ANN GRAEME

veryone loves the beach — the sun, pe: surf and especially the sand, so crisp yet yielding, squeaking under bare feet. To us, the hot sun is a pleasure and the sand is a playground. But we are just visitors to these margins of the sea. Imagine for a moment the challenges that face any animal that seeks

to live all of its life on the sandy beach, where there is no shelter from the sun or wind or crashing waves, or below the tide amongst the surging currents. From Biblical times, building a house on sand has been a metaphor for folly. The animals who inhabit the a open beach know this: they know that burrowing under the sand is much wiser. Burrowing crustaceans have made the surf beach their domain. Equipped with digging limbs and ingeniously shaped bodies, numerous species of amphipods and isopods patrol their tidal zones like an army of

refuse collectors. Under cover of darkness and when the tide is out, they eat the debris that has washed ashore. By day they burrow beneath the sand to escape the seagull’s beak, the burning sun, the drying wind and the advancing tide. Like tenants in an amphipod apartment, hundreds of sand hoppers will move in and devour a drift of seaweed so quickly that, within days, it is scarcely visible beneath the covering sand. The hopper’s sideways-flattened shape lets it cleave through the sand grains like a flea through the hairs of a dog. It makes frail burrows; temporary bolt holes in the cool, damp sand. Shaped like miniature bulldozers, the isopods burrow too. They look very like their terrestrial relations, the slaters or woodlice. The common beach isopod fossicks for food as it follows the falling tide down the beach and then unerringly retreats as the tide returns, a marathon journey for a creature the size of a woodlouse.

About the low-tide mark are the burrows of larger crustaceans, the pale ghost shrimp and the mantis shrimp. Here also lurks the formidable paddle crab. In the twinkling of an eye, its paddle-shaped back legs can stir the sand to soup and the crab will sink until only its eyes and feelers are showing.

¢ Sand hopper Talorchestia quoyana and isopod Scyphax ornatus (right).

All day it will lie hidden, ready to ambush a careless fish (or toe). At night paddle crabs roam the sand, breaking into thickshelled bivalves like the tuatua and the Venus shell. Near the low tide and beyond, on sand flats never uncovered by the sea, is the realm of the bivalve shellfish. For them, burrowing does not just create a refuge. For bivalves, burrowing is their perpetual lifestyle, a strategy so successful that many varied species have found niches at different distances from the shore and at different depths in the sand. The tuatua is typical of the bivalve way of life. In order to burrow, the tuatua makes use of the peculiar properties of wet sand. Wet sand is quite firm, but if it is shaken it loses its coherence and become runny, especially if water is added. As the tuatua

plunges its foot into the sand, water from the mantle cavity is forced around it making the sand momentarily soft. The tip of the foot dives down and anchors itself in the now firming sand and then the foot contracts, dragging the wedge-shaped shell down after it. Plunge and contract; plunge and contract; and soon the shell is

under the sand. Once buried, the tuatua extends its two siphons to open just level with the sand’s surface. They open when the tide covers them and then you can see the pairs of holes, one frilled and one smooth in outline. Sea water is drawn in the frilled siphon, which is fringed with fine tentacles to screen out the sand, and expelled out the smoothedged siphon. As it passes across the gills of the shellfish, microscopic phytoplankton, which is the tuatua’s food, is strained off and oxygen is also

"3 Ss, absorbed. Only the tuatua and morning star shell can live at the margin of the low tide and tolerate the boisterous waves that hammer the shore. Further out beyond the lowest tides live a host of other shellfish species. Common ones include the circular, heavy-shelled Dosinia with its coarse concentric sculpture and short siphons, and the trough shells of the family Mactridae, more deeply buried and with longer siphons. Deepest of all, burrowed half a metre down, is the geoduck (pronounced ‘gooey duck’), with its fused siphons looking like a miniature elephant’s trunk. Most of these buried bivalves passively filter plankton from the seawater but some have a different strategy. The beautiful sunset shell feeds on organic matter that falls on the sand. It burrows deeply and extends its long, slender siphons to the surface where its inhalant siphon roams like an animated vacuum hose, sucking up food.

Text by

¢ Allin place beneath the sand with siphons extended, from left, Tuatua Paphies subtriangulata, a trough shell Mactra discors, a geoduck Panopea zelandica, and a sunset shell Soletellina nitida

It might seem like a safe existence, buried below the sand beyond the reach of the waves, but where there is food, there are predators. Fish like snapper, stingray and gurnard cruise in from the open sea and

snuffle in the sand for worms and shellfish. Carnivorous Basset ee snails lurk below the sand, their inhalant siphons ‘sniffing’ the i ied 3 water currents. Where the bivalve uses its foot to burrow, the foot of the carnivorous snail is extended like the skirt of a hovercraft, letting the snail surge over or into the sand to find its prey. A neat hole in a morning star shell tells the story of its encounter with a necklace shell. The necklace shell has wrapped the morning star in its foot, drilled the hole with the teeth of its radula, inserted its proboscis and sucked out the meat. ee eget S

¢ At right, necklace shell Tanea zelandica, enveloping morning star shell Tawera spissa, and shell with hole

In deeper water roam our biggest and most beautiful predatory snails: the big whelks, the helmet shells, the Arabic volute and, in the north, the huge tun shell. All are sand burrowers. We have not vet mentioned the most

numerous of all the sand animals, for they sr ae a are the most inconspicuous. They make those mysterious little piles that often dot the sand. They can loosely be called ‘worms’, and include segmented and unsegmented worms, and worm-like members of other obscure phyla. Some simply swallow sand as they burrow along like an earthworm burrows through the soil. Others barge through the

sand, ambushing their prey. Yet others make permanent tunnels into which they draw and filter seawater.

Permanent burrow-owners face the same problem as the burrowing shrimps — constant housework. Sandy faeces must be ejected and so too must the sand brought in by the waves and the currents. Those little piles of sand are evidence of the unseen worms burrowing below. The seemingly empty beach is far from lifeless. Next time you walk along the sand or paddle in the surf, look out for some of the creatures which are living beneath your feet. — ANN GRAEME is national co-ordinator of Forest and Bird’s Kiwi Conservation Club. T\M GALLOWAY illustrates the KCC magazine.

FURTHER READING * The New Zealand Sea Shore by John Morton and Michael Miller * Between the Tides by Mike Bradstock These books are no longer in print but are held by most libraries.