and Bostrychia belts disappear with the onset of open coast conditions. Enteromorpha nana on Little Barrier boulders is an exception.

Despite its seasonal fluctuation in vigour, the vivid green Enteromorpha sward is too striking a feature in most seasons between M.S.L. and M.H.W.N. not to receive comment. E. procera f. minuta (or E. intestinalis where there is fresh or brackish water trickling over the rocks) is as typical of vertical wharf piles as it is of sloping or flat sandstone and basalt ledges devoid of barnacles. Possibly its most prolific growth is achieved on almost flat reefs where a certain amount of moisture is retained by the mat. In such places the community tends to be more persistent through the year. It responds quickly to continuous exposure by bleaching and dying of the tubular thalli.

Tangled, spongy clumps of Gelidium pusillum are sometimes aggregated into a belt on their own or just below the finer textured plumes and branchlets of Bostrychia arbuscula and B. mixta, but nearly always modestly retreating to a shaded cleft or overhang. If G. pusillum does survive in the open it is mostly in association with some other organism, such as Volsella, from which it may derive moisture when exposed to sun and air. Nevertheless in a series of desiccation experiments (Dellow, M.A. thesis, unpub.) this spongy red alga was found to lose water at a much slower rate than other more light tolerant algae of lower levels—e.g., Scytothamnus and Corallina.

Scarcity of Porphyra is a negative feature of the Gulf zonation. Seasonal growth of P. umbilicalis ? occurs in sheltered sandstone localities, but the community persists only from June until mid-September. Further out from land-protected shores P. columbina makes a sporadic appearance, though never in the profusion or attaining the dimensions of the thalli on the Auckland west coast (cf. Beveridge and Chapman, 1950, p. 191). No sun and shade forms were recognisable as at Piha; in fact any plants observed could be classified only as sun forms since they grew on directly insolated rocks or boulders, notably at Little Barrier, Wha Wha (east Coromandel Peninsula) and Sugar Loaf Rocks (Fig. 10).

More widespread as a basic zone former is the leathery, crustaceous red alga Apophloea sinclairii, either admixed with barnacles, chitons and limpets or the only coloniser of bare rock between M.S.L. and about M.H.W.N. in situations where there is a fairly strong tidal current and little or no pollution Fertile thalli with erect projections were observed infrequently, the most common growth form consisting of the basal crust, often decaying in the centre or disfigured by the feeding of molluscs. Cranwell and Moore's (1938, p. 387) likeness of the patches of Apophloea, when dry, to congealed blood is an apt one; for the patches are seldom close enough to merge into one another. Apophloea is interesting in its absence from Waitemata Sandstone shores. There may be an antipathy on the part of sporeling stages to establishment on this type of smooth, easily croded stratum, since from its abundance on neighbouring shores (e.g., Leigh, Rangitoto) there would presumably have been opportunity for its settlement on the many bare sandstone surfaces about its particular tide level.

The next prominent belt for the 2 feet below M.S.L. is the oyster belt, formed by Saxostrea glomerata in all situations with a certain amount of protection from direct wave action. The oyster belt is prominent on the East Coast mainland. It girdles most of the adjacent islands on their southern and western shores, and is physiognomic in sheltered harbours and bays of western Great Barrier and

Coromandel Peninsula, as well as on hard rock facies in the Upper Firth of Thames. Sensitive to excessive sedimentation and movable stones or boulders at the one extreme and to aeration and surf pounding at the other, it fades out at Te Puru, Howick, and again at Little Barrier, Sugar Loaf Rocks, and north-east Great Barrier.

Saxostrea provides in its fluted, purple-fringed shells a home for barnacles, limpets, chitons, tube-worms and algae such as Caulacanthus spinellus, Centroceras clavulatum and Gelidium caulacantheum. Lepsiella scobina, the oysterboring gastropod, and also Neothais scalaris and Lepsia haustrum, have done untold damage to the commercial harvest. Harvesting and cultivation of rock oysters constitutes a major artificial biotic factor in the Gulf and on other wave protected North Island coasts. Not only are populations of Lepsiella, Lepsia and Neothais affected, but also those of Hormosira, which occupies wide stretches otherwise available for settlement of oyster spat. Marine Department records (1949–1951) indicate the numbers of borer (Lepsiella) and pupu (Lepsia, Neothais) destroyed annually; and the area cleared of dead oyster shells and Hormosira in connection with artificial oyster cultivation. A few figures^* will suffice to illustrate the extent of this biotic interference which takes place each year in Kaipara Harbour, the Bay of Islands, Whangarei Harbour, Coromandel, Great Barrier and the Hauraki Gulf.

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In former years a number of square yards of rock were removed from high to lower levels in order to increase the space available for settlement of oyster spat. Over 100 square yards were transported during 1947–48, but no record appears after this date.

Mussel (Mytilus canaliculus) populations are likewise considerably affected by commercial harvesting. The numbers of sacks collected at Auckland and Thames for the same years are as follows:—

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Of the chief lower mid-littoral dominants there is still to be considered the distribution of Pomatoceros, Hormosira, Corallina and Xiphophora.

The occurrence of Pomatoceros coeruleus is more difficult to correlate with a given set of habitat factors, since it dominates a number of widely divergent ecological stations—e.g., Narrow Neck, the Noises Islands, Little Barrier and Fletchers Bay. However, it fluctuates markedly in density from station to station, the most notable aggregation in the vicinity of sewerage outflows (Bastion Reef, St. Leonard's Point). In this connection it is interesting to recall the following supposition of Fischer-Piette (1936, p. 227) concerning the growth

of Pomatoceros triqueter on either side of the English Channel. “J'ai posé la question de savoir s'il ne fallait pas considérer que cette prospérité de l'espèce en certaines stations qui se trouvent tout être dans des baies étaient attribuable a une influence favorisante de substances organiques …”

Sabellaria kaiparensis, the sand-tube worm frequently associating with Pomatoceros in the inner Gulf, is more restricted in distribution; and owing to its more easily eroded tubes it is less likely to be a basic zone former. Although a sheltered coast species on the east coast, it can evidently tolerate wild seas, since it has been reported as a co-dominant with Pomatoceros at Piha.

Previous writers have made us familiar with the prolific growth of the Australasian genus Hormosira banksii on low-lying rocks of the midlittoral of semi-exposed to very sheltered coasts. The under-sized var. sieberi Harv. (= var. gracilis) is characteristic of pools and of growth on the lower midlittoral coralline turf. This variety can stand up to a moderate amount of lashing by surf at low tide, but is more conspicuous in water-filled depressions on reef surfaces not exposed to direct beating of waves. There appears to be an increase in size of bladders with increased shelter, shallower and more gently shelving shores, and greater turbidity. It was seen to be most prolific at Te Haruhi Bay, in lee of Whangaparaoa Heads, and Carnahan (1952, p. 38) notes H. banksii as replacing Saxostrea and C. columna ecologically at Rangitoto. He notes as well that the “button stage” may survive up to 1 foot above the upper limit of fully developed thalli. Such an effect is often demonstrated round margins of pools populated by Hormosira at higher levels in the barnacle zone. The buttons persist in the pool shallows and drainage channels which may dry up under excessive heat. Carnahan's experiments on water loss in Hormosira indicate an inverse rate of desiccation to the volume of each water-filled bladder or receptacle.

Notheia anomala, the fucoid parasite associated with Hormosira, was not as widespread as might be imagined; in fact it was found on every host plant in only two localities, namely Motuihi Island and at Surfdale (Waiheke).

Either associated with Hormosira or on its own, the cosmopolitan Corallina officinalis is the physiognomic turf component of moderately sheltered to moderately exposed, low-lying reefs. Like other wide-ranging dominants which are peculiarly sensitive to changes in vertical level (Dellow, 1950, Fig. 11), Corallina displays no limited preference for a certain type of substrate, being equally at home on sandstone, basalt and greywacke, both in pools and on exposed rocks. Loose stones and boulders are the only rocks on which it seems unable to support erect branches, although basal crusts abound on these.

Growth may be quite normal where the water is clear, turbulent and well aerated (Station 10), but in general maximum development is achieved in situations where the Corallina-beds are subject to a certain amount of silt deposition from shallow, turbid water. The belt cuts out where estuarine conditions prevail— an analysis of its salinity tolerance would provide an interesting study.

A suitable niche for germination and growth of many small red, green and brown algae is furnished by this coralline mat; and seasonal change in presence and abundance of the subordinate species is one of its distinctive features. Codium adhaerens var. convolutum is a frequent member of the community, with spring and autumn maxima (Dellow, 1953, pp. 239–246). Under the influence of shade

or surf, however, Codium may be elevated considerably above this level among Pomatoceros (Station 5) or barnacles (Station 33). Only repeated visits to the same area over a yearly period convince one that this velvety green belt of spongy, radiating thalli could not be sufficiently long-lived to contribute to the basic zonation of a Waitemata Sandstone coast.

A light green contrast to the dingy pink of Corallina is provided by almost transparent tubes of Enteromorpha procera f. novae-zelandiae, by local cushions or reticulations of Microdictyon mutabile, and in darker crevices or overarched pools, by darker grape-like clusters of Caulerpa sedoides.

In winter the turf may be hidden from view by a pale yellow-brown assemblage —not of Hormosira alone but of saccate Colpomenia sinuosa and perhaps the more gelatinous, convoluted thalli of Leathesia difformis. Deeper pink and red tonings are provided by Laurencias (L. botrychioides and L. virgata) and in shady pools and clefts, Plocamium spp., Cladhymenia oblongifolia, Symphyocladia marchantioides, Acrosorium decumbens and Myriogramme denticulata. A conspicuous molluse of the coralline turf is Lunella smaragda, the cat's eye (Oliver, 1923, p. 252).

Once shallow, shelving shores are left behind and tidal platforms descend more abruptly into a clearer sublittoral, Corallina wanes in importance until it is completely replaced by other turf-forming red algae (the mixed algal turf community) dominated by species of Gigartina, Polysiphonia, Caulacanthus and Gelidium; by a Fucus-like band of light-brown Xiphophora; or by encrustations of whitish-pink lithothamnia. Mytilus canaliculus may also replace Corallina locally. Of these, Xiphophora takes precedence, since it is seen to dominate at almost every station where Corallina is lacking, especially in north and east sectors of the Gulf. Of course both genera can dominate in the one locality, Xiphophora flourishing on seaward slopes of rocks or boulders, Corallina taking charge on flatter reef surfaces and in shallow pools subject to gradual cover by rolling of broken waves, rather than to the direct force resulting as they break on the reef margin. Such a condition was observed at Wha Wha, an exposed station on the east coast of Coromandel Peninsula. Xiphophora itself is eliminated under conditions of maximal exposure (Needles Point).

The presence of a belt of lithothamnia, like that of Xiphophora, is generally a reliable indication of an exposed coast subject to moderate or severe buffeting by waves. At stations intermediate in this respect (e.g., Leigh, Shag Rock) the thicker calcareous growths are confined to pools, and it is not until almost oceanic conditions prevail that the belt participates in the basic zonation of the community complex, as on the northern rugged shores of Great Barrier, and farther away to the north on Moko Hinau and the Poor Knights Islands. The absence of Novastoa zelandica from the lithothamnia belt south of these islands may indicate that it has reached the southern limit of its range. Cranwell and Moore (1938, p. 389) note a similar form, that of Stephopoma roseum, in a comparable ecological niche on the largest island of the Noises.

Exposed to strong continual surge and sweeping currents, the small island in the centre of Oruawharo Beach (east coast, Great Barrier) was the only site examined where a typical West Coast Rhodophycean community occurs in the lowest midlittoral. Here Gigartina alveata and G. cranwellae are both dominants, the latter profusely intermixed with Pachymenia himantophora, Gymnogongrus nodiferus and Melanthalia abscissa.