The slope and particle size of soft beaches and flats depend both on the power of the wave attack and on the ability of rivers and coastal currents to supply new materials. Beaches can best be classified according to their topography and degree of shelter, and the burrowing animals that distinguish them accord pretty well with such a division. The above map shows an example of the occurrence of open, protected, and enclosed soft shores. Open beaches are those of medium to coarse sand, mobile, and exposed to wave attack. Protected beaches are wider flats sheltered under the lee of promontories and islands. Their sand is finer grained, less wave shifted, and often stable enough to carry seagrass or Zostera at the lower edge. Under the full protection of the land mass are the sheltered shores of harbours, generally estuaries of lagoons, having a high proportion of silt and mud and, towards low water, becoming very soft. In the furthest backwaters, built by river deposition under little wave attack, are the tidal flats fringed with mangrove in the north and Salicornia in the south, succeeded by beds of sedge and rush and ultimately salt meadow.
The great majority of burrowing animals belong to the bivalve and gastropod Mollusca, and to the Crustacea, Echinodermata, and the worms. Open beaches right through New Zealand are characterised by the fast-burrowing streamlined bivalve, Amphidesma subtriangulata, the tuatua. This may be replaced on the North Island west coast and in part of the South by the larger and faster toheroa, Amphidesma ventricosa.
At low tide is found the zigzag cockle, Tawera spissa, and just beyond it the trough shells (Mactridae) and several Tellinidae. Burrowing gastropods include the carnivorous Tonna haurakiensis and the helmet shells (Xenophalium), the deposit-feeding Zethalia, and the ciliary feeding Struthiolaria. The Crustacea of open beaches are typically the amphipodan sandhopper, Talorchestia quoyana, and the beach lice, Scyphax and Actaecia, near high tide. Active carnivorous Sphaeromidae and Cirolanidae occur at mid-level with the sandlicking or fine-deposit-feeding amphipods of the Haustoriidae and Phoxocephalidae further down. Larger burrowing crustaceans are the carnivores, the mantis shrimp, Lysiosquilla, and the swimming crab, Ovalipes, together with the frail, filter-feeding ghost shrimp, Callianassa. The chief echinoderms are the cake urchin, Arachnoides zelandicus. Worms include fast-burrowing carnivorous Nephtyidae and Glyceridae, and sand swallowers, such as Armandia.
Protected beaches carry by far the richest burrowing fauna. On the middle slope is found characteristically the third species of Amphidesma, the pipi, A. australe. Below it and freely distributed over the wide middle beach is the plump, shallow-burrowing cockle, Chione stutchburyi, and with it the long-siphoned deep-burrowing tellinid, Macomona liliana. Low tidal bivalves include Dosinia subrosea, Myadora striata, and Angulus gaimardi. The gastropods include not only Struthiolaria but also the carnivores Baryspira, Pervicacia, and Alcithoe. There are many polychaete worms, most characteristically the deposit feeders of the Ariciidae, and the makers of permanent tubes, such as Eunicidae (Onuphis and Diopatra), the long-tentacled Terebellidae, and the sand-tubed Maldanidae, Owenia fusiformis and Pectinaria australis. Scale worms include Sigalion and Lepidaesthenia, while the most active worms of all are the proboscis-shooting Glycera and Aglaophamus. Feeding by waving slender palps above the sand level are the Spionidae, and in coarser sand higher on the beach lives Platynereis australis.
Echinoderms feeding on the organic surface deposits are the brittle star, Amphiura aster, the heart urchin, Echinocardium australe, and the wormlike burrowing sea cucumber, Trochodota. On northern beaches live the spiny-armed sand star, Astropecten polyacanthus, and the large yellow hemichordate, Balanoglossuss australiensis, making continuous castings of defaecated mud. A similar diet is taken by the unsegmented worms, Urechis and Sipunculus.
The muddy sand flats of harbours are typified by the bivalves, Chione and Macomona, and by the oval trough shell, Mactra ovata, where there is more clay. In the sand the fan mussel, Atrina zelandica, lies vertically buried, attached to sand grains with its dense byssus. In countless numbers is the small bivalve, Nucula, and deep in softer muds is the thin-shelled Solenomya parkinsoni.
The Gastropoda include as roving carnivores the Cominella species, glandiformis and adspersa, and the surface-trailing deposit feeders, Zeacumantus lutolentus, and the topshell, Zediloma subrostrata. Among the sticky muds of mangroves and salt swamp is the pulmonate mudsnail, Amphibola crenata, and in salt-rush beds a second primitive pulmonate, Ophicardelus.
Low tidal Zostera flats have a rich gastropod fauna, with the bubble shells, Haminoea and Quibulla, deposit feeders casting jellylike spawn, and sluglike Philine with internal shell, a carnivore on bivalves. Cropping sea grass at the surface is sea hare, Bursatella glauca, the small comical Micrzlenchus huttoni, and tiny rissoids. At low water is often found the mobile swimming scallop, Pecten novaezelandiae.
The crabs of Zostera flat are very abundant, particularly the stalk-eyed Hemiplax hirtipes, the pillbox crab, Halicarcinus cooki, and the small Hemigrapsus cranulatus. Higher up, in salt swamps and mid-tidal flats, is the burrowing crab, Helice crassa. Abundant at low water in soft mud are not only Lysiosquilla but also the familiar snapping shrimp, Alpheus.
Typical burrowing worms found on harbour shores are the Spionidae Nereidae with several species, Cirratulidae, Glyceridae and Notomastidae. On southern flats occurs the lugworm, Arenicola affinis.
by John Edward Morton, M.SC.(N.Z.), PH.D., D.SC. (LOND.), Professor and Head of Department of Zoology, University of Auckland.
SEATO, see INTERNATIONAL RELATIONS .