On hard shores a combination of the tides and other factors produces a clearcut zonation. The living girdle is a many-stranded one, horizontally divided into separate zones or belts, characterised by quite distinctive animals and plants. Consideration of the animals must always involve the background of the plants and, for the marine ecologist, the flora and fauna are inseparably intermingled. The composition and order of the zones is under the fundamental control of the tides. But there are many other factors as well that influence the ecology of the shore. Zones are modified by the steepness and aspect of the rocks, by sun exposure or shade, by the geological character, by the sweeping of currents, the scouring of sand, and the variations in turbidity and salinity. The chief contrast we must draw is between “exposed” and “sheltered” coasts. The effects of surf pounding, splash, and spray bring about great changes in hard-shore populations. New Zealand runs through 12 degrees of latitude, so that another important variable is that of sea temperature and hydrography, responsible for large changes in shore patterns from north to south.
Ecologists recognise three major zones on hard shores, which, broadly speaking, are under tidal control. The uppermost zone or littoral fringe is that part of the shore above average high-water level. The lowermost zone or sub-littoral fringe lies between average low water and extreme low water of spring tides. The mid-littoral zone is that large part of the shore between average high and average low water. On exposed shores, however, these zones have no strict correspondence with tidal level, for under the influence of splash and spray their upper limits may be raised many feet higher. Indeed, the zones may be defined on a biological basis only by the local limits that the plants and animals, under a complex combination of environmental factors, happen to reach.
We may select an exposed shore and a sheltered shore, both from the North and from the South Island, illustrating on these the typical animals, whether fixed and zone-forming, or mobile and freely wandering. A northern shore line of moderate shelter (Diag. 1) shows the herbivorous gastropods, Melaraphe oliveri and Nerita melanotragus, scraping the rock face for lichens and algae in the littoral fringe. The upper part of the mid-littoral zone is clad with acorn barnacles, sessile Crustacea surrounded by a hard shell, and filtering off plankton with the casting nets of their feathery limbs. The highest species, much better developed under surf, is Chamaesipho brunnea. Next comes the smaller C. columna and, lowest of the three, Elminius plicatus. The barnacle zone may be cut through by a thick belt of that sessile filter-feeding bivalve, the rock oyster, Crassostrea glomerata. From the lower middle shore and downwards grow successive belts of algae.
In the barnacle zone and upon the algae, graze many species of herbivorous gastropods. Mingling high up with Nerita is the limpet, Cellana ornata. Through the oyster zone downwards will always be found the snake's skin chiton, Sypharochiton pellis-serpentis, and the topshells, Melagraphia aethiops. Further down appears a second limpet, Cellana radians, while grazing on the algae, Hormosira and Corallina, are the cat's eye, Lunella smaragda, and the long-spired cerithiid snail, Zeacumantus subcarinatus. On the paintlike coralline algae at low water graze the pauas, Haliotis, the topshell, Trochus viridis, and the large cousin of the cat's eye, Cookia sulcata. Carnivorous snails include the thaids, Lepsiella scobina, boring the shells of barnacles and oysters, and Lepsia haustrum, feeding on herbivorous gastropods as well. Several species of Cominella act as scavengers, as do the hermit crabs in pools. Echinoderms include the familiar cushion star, Patiriella regularis, and at low water the coralline grazing sea urchin, Evechinus chloroticus.
Our more exposed Auckland shore (Diag. 2) is typified by the Auckland west coast, the rocks being shown covered with sand up to the mid-littoral and cutting out the bull kelp, Durvillea, and the profusion of algae found growing under full wave exposure. In the littoral fringe are fast-running purple crabs, Leptograpsus variegatus, while the barnacle zone has the same three species as on sheltered shores, with Chamaesipho brunnea especially well developed. The little mussel, Modiolus neozelandicus, forms a jet-black band higher up, and towards low water appears the large green mussel, Mytilus (Perna) canaliculus. Just above the sand level may be found two belts of filter-feeding tubeworms, Pomatoceros cariniferus, with hard shelly tubes, and the massive sandy tubes of Sabellaria kaiparaensis. Roving animals include several limpets, Notoacmea pileopsis in the littoral fringe, then Cellana ornata, Notoacmea parviconoidea, Siphonaria zelandica, Patelloida corticata, and Radiacmea inconspicua. The large thaid gastropod, Neothais scalaris, feeds on the mussels, as does the buff or mauve starfish, Stichaster australis.
A sheltered shore in the South Island will show many of the same organisms as in the north, but with many variations and additions, as shown by the example from Otago Harbour (Diag. 3). The barnacle zone is thin, with Chamaesipho brunnea absent south of Kaikoura. In mats of Bostrychia above C. columna are the leathery slug, Onchidella nigricans, the snail, Rissellopsis varia, the minute bivalve, Lasaea, and the black-spired snail, Zeacumantus subcarinatus. The large pulmonate limpet, Benhamina obliquata, is in the south a familiar occupant of shady crevices in the barnacle zone. Where the shore is current-swept appears the barnacle, Elminius plicatus (common also in the north), with below it the encrusting bivalvess the navy mussel, Mytilus edulis aoteanus, the ribbed mussel, Aulacomya maoriana, and the Dunedin rock oyster Ostrea hefferdi.
In the sub-littoral fringe the encrusting life is vastly rich, with sponges, polyzoans, and ascidians as filter feeders and numerous anemones and hydroids as sedentary carnivores. Conspicuous along with the huge thalli of the bladder kelp, Macrocystis, are the yard-long stalks of the sea tulip, the ascidian, Pyura pachydermatina. This species and Ostrea hefferdi are chiefly denizens of the Otago coast. Otherwise the same pattern serves well for sheltered coasts from Cook Strait southwards.
Exposed shores have a general similarity through much of the New Zealand coast. Diag. 4 shows such a pattern from Banks Peninsula. It is dominated in the sub-littoral fringe by the two species of bull kelp, Durvillea antarctica and D. willana, the latter confined to the south. The chief zoning animals are the barnacles and mussels. Modiolus, Mytilus edulis, and Aulacomya often interrupt the wide extent of barnacles, but the chief zoning mussel at low water Mytilus (Perna) canaliculus, forming large byssus-attached sheets. Around it are not only Neothais and starfish, but the fast red crab, Plagusia capensis, and large colourful anemones, such as Cradactis magna. The bull kelp holdfasts have a rich eroding fauna of worms, isopods, siphonariid limpets, and chitons.
The encrusting and roving animals under movable stones (Diag. 5) represent another realm that we must briefly consider. In smooth boulder beaches at mid-tide, grapsid and other crabs are abundant, with a wide range of topshells, such as Zediloma atrovirens, Z. digna, Z. arida, and Anisodiloma lugubris. The fragile limpets, Notoacmea daedala and Atalacmea fragilis, are common. In the sub-littoral fringe the lower surface of boulders is richly encrusted with ciliary feeding invertebrates. Such are the sponges in vivid scarlets, yellows, and browns, the lacelike crusts or bushy tufts of polyzoans, the pink brachiopods (Terebratella), byssus-attached bivalves (ark shells, saddle oysters, and fan scallops), ciliary feeding gastropods, such as slipper and saucer limpets and vermetids, and the wealth of brightly coloured ascidians, both simple and compound. Roving animals sheltering under rocks include the rockfish, Acanthoclinus, blennies, and sucker fish, and a great wealth of crabs of the Cancridae, Grapsidae and Xanthidae, as well as the familiar green “half-crab” Petrolisthes. Starfish, such as Coscinasterias and Allostichaster, and brittle stars (Ophionereis and Pectinura) and the sea cucumber, Stichopus, are common. Encrusting animals include carnivores, as well as ciliary feeders: anemones, hydroids, and even cup corals, Culicia and Flabellum. Molluscan predators, such as carnivorous snails and colourful nudibranchs, are adapted for specific diets upon almost every kind of sponge, coelenterate, ascidian, and polyzoan.
Softer rocks, such as mudstones, are pitted and bored by bivalve molluscs, such as the low tidal piddocks, Pholadidea spathulata, P. tridens, and Anchomasa similis, and the date mussel, Zelithophaga truncata. At mid-tide level, the amphipod crustacean, Sphaeroma quoyanum, makes smaller pits. The disused burrows of piddocks are occupied by other species of nestling bivalves (Notirus, Notopaphia) that do not actively bore, and by terebellid and cirratulid worms.