Story: Coastal shoreline
Page 6 – Sandy beaches – lower shore
Few animals are adapted to life on the surface of beaches and sand dunes, but some shellfish and crustaceans burrow under the sand.
Four native shellfish have fed generations of New Zealanders.
Toheroa (Paphies ventricosa) and two species of tuatua (Paphies donacina, Paphies subtriangulata) favour open exposed beaches regularly pounded by heavy waves. Adult toheroa live just below the high-tide level at depths of 20–30 centimetres. Tuatua prefer deeper water, living at depths of 5–10 centimetres near the low-tide level. They seldom dwell together on the same beaches.
Pipi (Paphies australis) burrow in sandy banks of estuaries and harbour mouths, where there is some fresh water.
Toheroa were once found in their millions on the heavy surf beaches of the North Island’s west coast, and on a couple of Southland beaches. They were over-harvested in the 20th century and since the 1960s have been protected from commercial gathering.
Crabs, sandhoppers, slaters, lice and shrimps are all found on New Zealand beaches. Swimmers may be familiar with the nasty nip of the paddle crab (pāpaka, Ovalipes catharus). This swimming crab lives on sandy sea beds and is commercially harvested. Its paddle-shaped hind legs are used for digging backwards into the sand as well as for swimming. It forages by night and is a significant predator of tuatua and other shellfish. Cannibalism is quite common and the young are especially vulnerable when moulting – they shed their old shell-like covering as they grow.
Paddle crab romance
The paddle crab’s love-life is remarkable. A male needs to mate with a recently moulted female and will grab a pre-moult female and carry her around underneath him for a number of days until she moults. He will then mate with her, generally for 12–36 hours (but up to 4 days), before he releases her.
Less familiar are the mantis shrimp (Squilla armata) and ghost shrimp (Callianassa filholia). They dig tunnels in tidal sands and leave little mounds of sand and faeces at the entrance.
Kick over some dried seaweed and thousands of little sandhoppers (Talorchestia quoyana) will leap out. These creatures are decomposers – garbage eaters – and at night, away from the blazing sun and predatory gulls, they munch their way through dead bodies and seaweed cast up on the beach. During the day, if there is no food to shelter under, they burrow into the sand.
Also performing clean-up duties is the sea slater (Scyphax ornatus). It looks like the common slater, and makes burrows on the upper beach, moving down to the tidal zone at night to scavenge for food.
The drift line
One of the pleasures of the beach is checking out the flotsam along the shore. Driftwood – smooth bleached remains of forest trees – dominates river mouth beaches on the west coast, and provides habitat for small animals. Pumice, a floating volcanic rock, is common on North Island beaches. It is brought down by rivers carrying deposits from the Taupō eruption of 232 AD.
After heavy storms it is possible to find shell remains of deep-water shellfish and Janthina, the floating violet snail. Crab shells and claws from moulting animals are also common wash-ups.
Much of the drift material is human-produced rubbish. Plastic products such as polypropylene ropes and strapping bands, containers, fishing line and packing materials comprise about 70% of New Zealand’s beach litter. Rubbish that has been afloat for some time usually carries barnacles, seaweeds or shellfish.