Ecology of Southern New Zealand Sheltered Rocky Shore
[Read by title and abstract before Otago Branch on November 15, 1955; received by the Editor, January 30, 1956.]
The ecology is described of a southern New Zealand sheltered rocky shore at Aquarium Point, by the Portobello Marine Station, Otago Harbour. It is a short, current-exposed stretch in a shallow, land-locked harbour, where wave action is not continuous. Monthly average sea temperatures range from 6° C. (July) to 16° C. (January).
Supralittorally, Melarhaphe cincta forms a narrow littorinid zone, with lichen blackening. Bostrychta arbuscula dominates at high tide neap. Half-tide sun-exposed rocks are relatively bare, though 4 species of mollusc abound. Balanoids (Chamaesipho columna and Elminius plicatus) are dense only where tidal currents are strongest. Hormostra and Ulva form a band at low tide neap. From low spring tide downwards Macrosystis pyrifera and Pyura pachydermatina co-dominate. Sublittoral rock carries a dense and varied range of species, algae, ascidians, sponges and hydroids being especially in evidence.
Whereas midlittoral rocks are usually largely bare of algae, brief seasonal outbursts are shown by Porphyra? columbina, Scytosiphon lomentaria, Scytothamnus fascicularis, Adenocystis utrricularis and Leathesia difformis.
Effects of current and of aspect are considered. Filter-feeders are densest where currents are strongest. Most sessile midlittoral species ascend higher on shaded faces or under stones than where sun-exposed.
Rocky shore ecology has been intensively studied in recent decades in many parts of the world, especially by Professor T. A. Stephenson and his associates (Stephenson, T. A. and A., 1949). Earlier New Zealand investigations (Oliver, 1923; Cranwell and Moore, 1938) have recently been followed by several accounts of varied Auckland shores (e.g., Chapman, 1950, Carnahan, 1952; Dellow, 1955), and a study of an exposed shore near Christchurch (Knox, 1953). The present paper considers a more southerly shore, sheltered from continuous wave action. It will be followed by a briefer, comparative account of a more wave-exposed shore at the same latitude.
In the present instance, an intensive study has been made of a short stretch of shore This is partly because this region is rich, with varied habitats in close proximity; partly to give a basis for long-range future comparison; and partly because this particular shore, alongside the Portobello Marine Biological Station, is visited by many biologists. With the latter in view, commonest species are illustrated, and brief descriptive comments are made to enable more abundant species, in specified habitats in this region, to be recognised.
The shore has been visited at least once a month and usually much more frequently from mid-1952 to November, 1955. The writer has been living alongside it most of the time Sublittorally it has been examined from dingy (with glass-bottomed box. dip-net and hoe-net) and to a slight extent by goggle-diving. Nomenclature of molluscs follows Powell (1946), of sea-weeds Naylor (1954), of barnacles Moore (1944).
This study has been carried out while the writer has been holding Nuffield and N.Z.U Research Grants, which she gratefully acknowledges.
The work has depended extensively on identification of species by a number of workers on systematic groups. To these the writer expresses her warm thanks. They include Miss B. I. Brewin (ascidians), Dr. D. A. Brown (Bryozoa), Professor V. J. Chapman (Cyanophyceae), Mr. R. Dell, Dr. C. Fleming, and Mr. A. W. B. Powell (molluscs), Dr. H. B. Fell (echinoderms), Dr. D. E. Hurley (amphipods, isopods), Mr. G. A. Knox (polychaets), Mr. J. Moreland (fish), Dr. J. Murray (lichens), Dr. M. Naylor (sea weeds), Dr. G. A. Parry (anemones), Miss P. R. Ralph (hydroids), and Mrs. S. Rind (sponges). She also thanks the station staff (Mr. R. Stark, Mrs. R. Milne, Mrs. A. J. Black and Mr. D. McArthur) for their share of weather recording.
She is especially indebted to Dr. Naylor, Miss Ralph and Mrs. Rind for extensively making available information on their groups, often before their own work was published.
Locality, Climate, Temperatures. Aquarium Point, Otago Harbour, lies at lat. 45° 50′ S., long. 170° 39′ E., in the cold temperate zone. The convergence between subtropical and subantarctic waters is nearly always north of this. Aquarium Point, immediately south of the marine station, lies in the middle of the harbour (Text-fig. 1, arrow), on the tip of Portobello Peninsula (of Otago Peninsula) that lies nearest to Quarantine Island. The long, narrow harbour, lying between peninsula hills and.
mainland mountains, funnels prevailing southerly and northerly winds along a southwesterly or north-easterly direction. Both, more especially south-westerlies, hit Aquarium Point with gale force several times a year. But calm days are frequent, wind being absent or of Force 1 at 9 a.m. for more than ⅓ of the year. (Table 1.)
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|No. of days, 1953||94||55||52||61||42||27||20||10||3||0||0||0||0|
|No. of days, 1954||76||64||62||79||34||17||17||9||5||2||0||0||0|
From the shallow and land-locked nature of the harbour (Text-fig. 1), wave action closely follows wind force. Gale force winds and breaking waves may be succeeded in a few hours by absence of wind and dead calm sea.
Surface temperatures, taken from the end of the Station wharf nearby, range from 6° C. in July to 15–16° C. in summer (monthly averages of 9 a.m. daily readings, Text-fig. 2).
Text-fig. 2.—Otago Harbour surface temperatures, monthly averages of daily 9 a.m. readings, taken from end of main wharf at Portobello Marine Biological Station, July 1, 1952–August 31, 1955.
Individual 9 a.m. surface temperature readings show a greater range, as may be seen from Table 2.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Minimum Temp||4.8° C||5.2° C.||4.8° C||5.3° C|
|Date||13/7/52||12/7/53 and 21/7/53||28/7/54||17/7/55|
|Maximum Temp||18.6° C||17.4° C.||17.8° C|
Monthly average sea temperatures were found to differ from monthly average air temperatures usually by less than 1° C. Daily sea and air temperatures usually lie within 2° C. of each other, the greatest difference noted being 6.9°C In other words, in this shallow, land-locked harbour, lag between sea and air temperatures is only slight.
Frosts have not been observed intertidally around Aquarium Point during 1952–5, but probably sometimes occurred at night.
Hours of sunshine per month are usually between 200 and 80, with erratic summer peaks and winter troughs. Frequency of sunny days per month is not obviously correlated with season (Text-fig. 3). Frequency of rain days and amount of rain per month are likewise practically non-seasonal (Text-fig. 4).
Average annual rainfall at the marine station is 77.5 cms. The greatest fall in 24 hours during the three-year period of this study was of 4.2 cms on 13/4/54. Local humidity records have not been made, but frequent sun and frequent wind combine to subject sun-facing habitats to considerable dehydration when not covered by water. Aquarium Point lies on the sunny side of the harbour, and most of it is sun-facing for the greater part of the day.
Sea-water. Salinities have not been recorded, but moderate rainfall fairly uniformly distributed and moderate evaporation would together be expected to keep salinities.
from deviating markedly from the offshore figure of 34.5% (Sverdrup, Johnson and Fleming, 1942). The main drainage of the city of Dunedin, eight miles away, is taken to the outer coast. Septic tanks used by Otago Peninsula inhabitants presumably cause slight contamination of harbour waters. Apart from the marine station's own buildings, there are no permanently inhabited dwellings within a half-mile radius of Aquarium Point. The sea-water gives the general impression of being clean.
Region Considered. The shore studied runs from the marine station wharf, round the tip of the point, till one is stopped by a cliff some yards beyond. As many species are localised in this area, names have for convenience been given to obvious landmarks (Text-fig. 5). As one approaches at low tide from the marine station, one steps down from the inner end of the station wharf on to Slope Rock, then scrambles along the base of Wharf Cliff to Wharf Beach. The sandy part of this small beach is ignored in the present account, as being the habitat of an essentially different community. It also has scattered boulders and shingle. At its far end is massive Tide Rock, its outer vertical face ranging from mean low tide level up to extreme high spring tide. From here one passes Inner Boulders at the cliff base and Outer Boulders, partly submerged, to reach the extreme tip of the point Several stones form the Bridge (continuously exposed only at lowest springs), leading to Tip Rock and normally submerged Tip Reef beyond it. Eastwards, Long Reef separates this from stone-scattered End Beach. From here, by scrambling over or around Flat Rock, one reaches Lateral Reef, which stretches towards the beacon used as tide pole. Beyond lies stony Cave Beach.
Text-fig. 5—View of Aquarium Point during mild low spring tide, observer looking south from above. Size and direction of open-headed arrows indicate strength and direction of tidal currents. Letters indicate dense distribution of dominant lower midlittoral and sublittoral species. H. Hormosira; U, Ulva; C, cystophora torulosa; M, Macrocystis; E, Ecklonia; P, Pyura pachydermatina. Tide scale in feet shown on beacon and Tide Rock. 8ft tide level shown on cliff at left.
Substratum. The rocky shore consists of weathered trachyte, a dull orange coloured rock. In parts it is hard, forming reefs Elsewhere it merges into firm clay, with many scattered stones and boulders. No large rock pools are present.
Currents. The tip of the Point (Tip Rock, Tip Reef and the Bridge), jutting into the channel between the peninsula and Quarantine Island, is swept by strong tidal currents on both rising and falling tides; the rip being greater than one can row a light dingy against. Lateral Reef is submitted to a moderate current on falling tides, a gentler one (still in the same direction, owing to eddy effects) on rising tides. Tide Rock, Wharf Beach and Cave Beach are largely unaffected by tidal currents.
Tidal Levels. The extreme tidal range, from the lowest low tide to the highest high tide observed during the period, is 9ft. The lowest low tide, in early September, 1952, was the second lowest predicted for the year, pushed lower by an intense, stationary anticyclone. The highest high tide, on 21/5/55, was a predicted spring of 6.8ft, accompanied by a low and falling glass. From these extremes of Oft and 9ft, a scale in feet has been painted on the beacon off Lateral Reef. These are the levels in feet shown in Text-figs 6, 7 and 8 and Plate 1. From this scale, different tidal levels on various parts of the area can be read, as the tide rises and falls on a calm day, to a vertical accuracy of about 2in. This has enabled vertical ranges of main species to be plotted with reasonable precision. On this scale, only the most extreme springs come above 8ft at high tide or below 1ft 6in at low tide. Hence, 4ft 9in may be regarded as half-tide level (M.T.L), 3ft 6in is a poor low tide neap level, 6ft a poor high tide neap level. 2ft and 7ft 6in are taken as mean low spring tide and mean high spring tide levels.
Tides are semi-diurnal. They take about an hour longer to fall than to rise, rising quickly immediately after the turn of tide. Low tide usually turns about 35min before the time predicted for the port of Dunedin.
The rocky shore of Aquarium Point is mostly rather bare above tow neap tide level. From here downwards, however, the density of organisms is great, both in bulk and in number of species.
First an account is given of the main species occurring on exposed rock faces, from above downwards, at different tidal levels. Under-stone zonation described, the frequency of commoner species at different levels having been sampled. Effects of aspect and current on distribution of species is considered Seasonal changes are then described. Finally, in the discussion, an attempt is made to analyse the characteristic features of the biology of this shore in relation to the environment it presents, zoning is discussed, and comparisons with other shores are made.
I The Littorinid Zone (or Supralittoral Fringe)
At and just above high spring tide level, lichen blackening caused by two species of Verrucaria is in places conspicuous on the orange-coloured cliffs. The littorinid Melarhaphe cincta is patchily abundant over a narrow vertical band about 1 ½ft high. It aggregates in crevices, more particularly on shaded faces. (Plate 30, fig. 1). The blue-banded Melarhaphe oliveri is infrequent on Aquarium Point. But it is commoner than M. cincta on other harbour shores nearby, where there are flatter rock surfaces with tiny pools at high spring tide level. The ridged winkle Risselopsis varia and occasional green limpets (Notoacmea pileopsis sturnus) are the only other animals of any size seen in the littorinid zone by day. At night, however, the region becomes more active Hundreds of the amphipod Orchestia are scampering about, together with an occasional shore slater (Ligia novaezealandiae) and scattered insects from above.
This runs from mean high spring tide level (7ft 6in on local tide polc) down to mean low spring tide level (2ft on pole). Within this zone, clumps of Bostrychia arbuscula characterise its upper part (high neap tide level). Around mean tide level, sun-facing rocks are relatively bare. At low tide neap, assorted algae and in places lamellibranchs become dense.
The red alga Bostrychia arbuscula forms large, dark, scattered clumps on sun-facing rocks (Plate 29), a more nearly continuous zone on shaded faces Within the clumps, an amphipod and the tiny pink lamellibranch Lasea hinemoa abound. Except for seasonal outbursts, other algae of size are absent above mean tide level Black sprinklings of Lichina pygmaea (probably var. intermedia) are common among and just below Bostrychia on sun-facing rock.
Despite the sparseness of macroscopic algae above low tide neap, several species of mollusc abound, suggesting that microscopic organisms supply more food than is apparent. The four commonest molluscs are the patellid limpet Cellana ornata, the chiton Sypharochiton pelliserpentis, the periwinkle Melagraphia aethiops and the tiny black turret Zeacumantus subcarinatus (Plate 30, figs. 2, 3, 5). Cellana has the highest vertical range of these species, just reaching the littorinid zone (8ft) at times, and dropping out above low tide neap. Zeacumantus, at the other end, is more abundant below half tide than above it. Of other species, Siphonaria zelandica is patchily abundant, especially on Flat Rock and Long Reef. The larger pulmonate limpet Benhamina obliquata is in evidence in late spring on large rock masses such as Flat Rock and Slope Rock These pulmonates are distinguishable from the patellid limpets by the ridge running down one side of the shell, overlying the respiratory orifice. Other gastropods characteristic of the upper midlittoral include large and small ridged periwinkles (Anisodiloma lugubris lenior and Risselopsis varia), the predatory whelk Lepsiella scobina albomarginata and the black periwinkle Zediloma arida The small pulmonate shig Onchidella patelloides is scattered throughout the midlittoral.
The common littoral molluscs cope with the desiccation problem in a variety of wavs Specimens of Cellana ornata, the most sun-tolerant species, show shell margins tailored to fit the spots on which they normally sit when air-exposed. Individuals of Melagraphia, especially on dry days, aggregate in crevices. Sypharochiton frequently sits in tiny pools. On softer rock faces it is often in a hollow which it appears to have made. Zeacumentus and Risselopsis aggregate in crevices or on mats of Bostrychia (Plate 30, fig. 2). Whereas specimens of Benhamina living intertidally throughout the year seem tolerant of sun exposure, the more numerous individuals that come inshore to breed aggregate in overhung crevices or on south-facing rocks; so that they and their egg coils are in shade most of the day Such individuals, when air exposed, usually show a fleshy ridge of mantle between shell and rock, making them more vulnerable to desiccation than the patellid limpets.
Balanoids are for the most part sparse in the midlittoral; but they form a fairly dense covering to the upper part of Tip Rock (Plate 31, fig. 1). Here, large Elminius plicatus has the smaller Chamaesipho columna scattered freely amongst it (Plate 30. fig. 6) At low tide neap also occur occasional Balanus decorus, wriggling their pointed terga when air-exposed. C. columna occurs less densely on some other parts of this shore, chiefly where waves lap during windy weather. E. plicatus, apart from Tip Rock, is only occasional on the. Point The smaller Elminius modestus is scattered on and under stones at half-tide, where a current flows between Flat Rock and Long Reef. (Plate 30, fig. 4).
Sides of current-swept rocks in the lower midlittoral carry oysters (Ostrea hefferdi) and navy mussels (Mytilus planulatus). Ribbed mussels (Aulocomya maoriana) and fringed mussels (Modiolus areolatus) are scattered. But the large green Mytilus
canaliculus and the small black Modiolus neozelanicus, both abundant on outer, wave-exposed coast, are absent from Aquarium Point.
The serpulid Pomatoceros coeruleus, with blue-black tentacles, is locally abundant on sides of stones around mid-tide level. Leathery-looking clumps of the ascidian Pyura suteri form patches up to half tide on vertical rock faces Tufts of corallinc (probably Corallina officinalis) are common below mid-tide, and encrusting pink corallines half cover many rock faces. An orange sponge (? Haliclona) frequently engulfs coralline tufts up to half-tide level, and also occurs under stones. The white, lacy sponge Leucosolenia is found in crevices.
These are the main macroscopic species found on exposed rock above low tide neap throughout the year. Their zoning is shown diagrammatically in Text-figs. 7 and 8.
IIA. Hormosira-Ulva Sub-Zone of Low Tide Neap (Lower Midlittoral)
As one passes down the shore to low tide neap, the rocks rather suddenly assume a dense algal covering. Species here dominant are Hormosira banksii and an ulvoid (probably Ulva reticulata) Cystophora torulosa fringes parts of the shore in the lower Hormosira belt.
This zone is in many respects a transitional one. Typically mid-littoral species such as Melagraphia aethiops and Zeacumantus subcarinatus abound, but Sypharochiton is sparser than further up, and Cellana ornata drops out On the other hand, various sublittoral species ascend up into this region. On exposed rock the most conspicuously abundant ones are the pink and orange spheres of the monaxonid sponge Tethya aurantium and white encrustations of the ascidian Didemnum candidum, frequent also on Hormosira.
Species other than the dominants that are especially seen in this zone include tufted Corallina, Ostrea hefferdi, and a species of Polysiphonia Gigartina decipiens is abundant among Ulva on Tip Rock Typical molluscs here are Buccinulum littorinoides, cats-eye shell (Lunella smaragda) and Aulocomya maoriana. Various colour races of the fine-tentacled anemone Diadumene zelandica are spasmodically abundant. Where stones abut on coarse sand, the patterned anemone Anthopleura rosea is seen. Under such stones on End Beach the small pink translucent cucumber Trochodota dunedinensis abounds, together with the long-armed little brittle star, Amphiura amokurae. During their seasonal appearance, the brown algae, Colpomenia sinuosa, Leathesia difformis and the lomented, low tide form of Scytosiphon lomentaria are important in the lower midlittoral In damp, shaded positions, the serpulid Galeolaria hystrix, compound ascidians such as Hypsistozoa fasmeriana, Aplidium adamsi and Aplidium benhami, sponges such as fawn and mauve? Haliclona spp and pink Aplysilla sp. ascend into this low tide neap zone.
But the dominants of the zone below, Macrocystis pyrifera and Pyura pachydermatina, cannot thrive for any length of time above the 2ft tide level. This 2ft level, of an average low spring tide, thus acts as a well-defined boundary between the Hormosira-Ulva zone of low neap tide and the Macrocystis zone of the infralittoral.
The harder trachyte rock is not generally penetrated by organisms. But soft patches of it towards low tide, at End Beach and lower Flat Rock, are riddled with pits and tunnels carrying a characteristic grouping of species. Ubiquitous Point species that abound in this habitat include Saccoglossus otagoensis, Corella eumyota, Sigapatella novaezelandiae, Ischnochiton circumvallatus, Acanthochiton zelandicus and the sphaeromid isopod Cilicea canaliculatus. Other species occur more especially here. For instance, a large Eunicid (probably Marphysa aena) tunnels through this rock-clay in astonishing abundance. The tiny burrowing anemone Edwardsia tricolor is common in soft rock pits (as well as in coarse sand nearby) A small sipunculid occurs.
The irregular cockle, Notirus reflexus, is abundant deeply embedded in the clay; though, of more specialised lamellibranch borers, only a single specimen of piddock (Pholadidea tridens) has been found here.
III. Kelp Zone (Macrocystis-Pyura pachydermatina) (Sub- or Infralittoral Zone)
From mean low spring tide level downwards, two species are co-dominant. These are the long, slender laminarian Macrocystis pyrifera and the pink, stalked ascidian Pyura pachydermatina (Plate 31, figs. 1 and 2).
Macrosystis plants here seldom exceed 12 metres in length. Their holdfasts are attached to rock or larger stones from mean low tide spring down to the bottom of the channel. The loosely tangled haptera of the holdfasts form the habitat for numerous sponges, ascidians (especially Corella eumyota and Aplidium benhami), algae, and fans of the bryzoan Caberea zelandica; amongst which crawl an abundance of Ophiomyxa brevirima and other brittle-stars, amphipods, polychaets and Saccoglossus.
Macrocystis fronds form the habitat for a characteristic selection of sessile organisms Campanularian hydroids abound The commonest is Obelia geniculata, with its zigzag stalks; others include Silicularia bilabiata, Orthopyxis crenata and a species of Clytia. Fine, feathery tufts of Plumularia setacea var. optima, swarming with caprellids, may also occur on Macrocystis, but are more frequent on the fucoids Cystophora retroflexa and Carpophyllum flexuosum. Of encrusting bryozoa, small, untidy patches of Hippothoa hyalina are extremely abundant, and circular colonies of Membranipora membranacea, with their larger, more uniformly spaced zooids, are at times seen. The tiny white spirals of the tubeworm Spirorbis occur in thousands on some fronds. Other species are found here with lesser frequency.
Sublittorally, Macrocystis is the dominant large brown girdling the entire area considered, from Station Wharf round Tip Rock to beyond Cave Beach (Text-fig 5, M). Around Tip Rock itself, it is the only large brown the writer has seen during the study, apart from seasonal fronds of Desmaresti and very occasional Carpophyllum flexuosum. Off Outer Boulders and Lateral Reef, Macrocystis is joined by Cystophora retroflexa and occasional C. scalaris. Odd plants of Carpophyllum flexuosum, with its irregular, flattened fronds, are scattered round Lateral Reef; while off its S.E face and the small reef beyond are dense beds of Ecklonia radiata var. richardiana (broad fronds). Occasional plants of Sargassum verruculosum, like a slender Cystophora, occur off Wharf Beach and beyond Cave Beach Odd clumps of Hormosira descend to the sublittoral. These represent the total of sublittoral large brown algae seen around Aquarium Point during 1953–5.
Pyura pachydermatina, the other dominant sublittoral species, occurs in a range of habitats, from low spring tide to shallower shelf, and from sheltered to exposed shores It is endemic to Southern New Zealand, but allied, stalked species occur further north and in Australia (Brewin, 1952, p. 192) At Aquarium Point it thrives with particular vigour, forming dense pink beds (Plate 31, fig. 2) The body may here exceed 14 cms in length and stalks may exceed 1 metre Whereas Macrocystis is dense around the outer periphery of the Point, P. pachydermatina largely replaces it in the diminutive bays between reefs (Text-fig. 5, P).
The body of P. pachydermatina is usually relatively free of epibiotes; though a filamentous brown (Ectocarpus sp) gives larger specimens a dirty tinge in spring Its stalk, on the other hand, here commonly forms the habitat for an astonishing bulk and range of other organisms. The ctenostome bryozoan Elzerina blainvilli forms great brown bushy tufts on some; the stout hydroid Sertularia trispinosa encloses the stalks of others In more sheltered hollows the stalks commonly carry delicate foliose red algae, such as Myriogramme multinervis with its conspicuous parallel veining, Schizoceris davisii and Phycodrys quercifolia. Seasonal changes have not.
Tide Rock a sun-exposed vertical rock face where tidal currents are negligible Hormosira sub-zone (lower midlittoral) at base Surface above is relatively bare, lacking dense balanoid coveing Scattered oysters and Corallina at 3ft level Scattered Pomatoceros tubes at 4ft level clumps of Bostrychia at high tide neap (6ft)
Characteristic littoral species, all at natural size, Aquarium Point
Fig.1—Littorinid Zone Melarhaphe cincta aggregates in shaded crevices
Figs 2–6—Midlittoral Zone.
Fig. 2—Turrets (Zeacumentus subcarinatus) cluster on Bostrychia in upper midlittoral (Also 1 Risselopsis varia, top).
Fig. 3—Periwinkle Melagraphia aethiops and limpet Cellana ornata, abounding midlittorally
Fig. 4—Elminius modestus, locally abundant, on side of stone at half tide (also one Parvacmea helmst)
Fig 5.—Sypharochiton pelliserpentis, abundant throughout midlittoral, in tiny pool on sunexposed rock
Fig. 6—Elminius plicatus (large) and Chamaesipho columna (small, fused walls) form moderately dense cover where tidal current is strong, also eroded Cellana ornata and Mytilus planulatus
Lower littoral and sublittoral. current-exposed
Fig 1—Tip Rock, extreme low spring tide Top of rock just below half tide B, balanoids (Elminius plicatus and Chamaesipho columna). L, lamellibranchs (Mytilus planulatus and Ostrea hefferdt) A, mixed algae of lower midhttoral (Ulva, Hormostra, Gigartina, etc.). M-P. Macrocystis and Pyura pachydermatina of infralittoral exposed only at more extreme spring tides
Fig 2—Under-water bed of Pyura pachydermatina and Macrocystis
been followed closely, but there are some indications of a succession pattern in the settling of organisms on Pyura stalks. Large specimens examined in October frequently showed encrusting corallines on the basal (older) end of the stalk, Elzerina clumps in the middle, and Ulva or Enteromorpha on the younger end, near the body.
Among the swirling fronds of Macrocystis and the swaying beds of Pyura with their dense algal load, one looks down on rock faces carrying an amazingly rich and colourful range of organisms. Algae and filter-feeders abound. Enteromorphas, Ulvas and their allies flourish, ranging up to great sheets of Ulva laingii, 2 metres long or more Dark broad red sheets of Gigartina apoda contrast in texture with the delicate foliose reds on Pyura stalks Pink, bilobed fronds of Epymenia wilsonis may clothe a rocky face Abundant filamentous reds range from finest Callithamnion consanguinem to coarser Ceramium and Monospora griffithsioides, Rhodomela and Polysiphonia species Encrusting coralline algae look like dabs of pink paint on many stones.
Contrasting with the greens, browns and reds of algae are the bright oranges and purples of sponges and ascidians The large purple branching sponge Syphonochalina latituba thrives in sheltered hollows. A gray siliceous species is abundant round Tip Rock. Other siliceous sponges (as yet unidentified) are brick-red, biscuit-coloured and vivid vermilion Pink, pointed mounds of Aplysilla and an encrusting, allied yellow species abound; as do the pink and yellowish spheres of Tethya. Calcareous sponges, solitary and branching, range from cream to dirty gray.
Ascidians, the other great group of filter-feeders here, are as colourful and varied as the sponges. Twenty species have been recorded from Aquarium point (Brewin, 1946) Abundant among them are clusters of purple, pendant colonies of Hypsistozoa (Distaplia) fasmeriana and neat, orange colonies of Aplidium benhamt. Apricot clumps of Aplidium adamsi, their common cloacal apertures on raised mounds, are larger but less frequent White encrustations of Didemnum candidum abound Everywhere one looks down on the delicately vermilion-tinged siphons of Corella eumyota, and frequently on to maroon-striped siphons of Asterocarpa cerea Tall, translucent Ascidiella aspersa is more local.
Current-swept rocks around the Bridge that are not densely loaded with algae, sponges or ascidians may on closer glance show delicate athecate hydroids. Especially on calm, summer days one can look down on to expanded beds of Hydractinia? parvispina, Turritopsis nutricola, and untidy tufts of Bougainvillea ramosa In sheltered hollows rise fluffy bushes of the thecate Sertularia fasciculata, at times more than a metre high About 20 hydroid species occur around the Point, as well as the alcyonarian Clavularia novaezealandiae Anemones are poorly represented sublittorally, though an orange Sagartiid and several colour races of Diadumene zelandica are not infrequent.
Vivid vermilion touches are added by the tentacle crowns of the serpulid Galeolaria hystrix, quickly withdrawn if touched: and the thousands of tiny enteropneusts. Saccoglossus otagoensis, slowly crawling through everything.
Among the sessile organisms dart numerous cockabullies (chiefly Trypterygion varium) and spotties (Pseudolabrus celidotus) Less in evidence but not infrequent are leisurely sea horses (Hippocampus abdominalis) and the amazingly camouflaged crested weedfish (Notoclinus fenestratus). The tan brittle-star Ophiomyxa brevirima crawls about in thousands Asterina regularis and the many-armed, gray Coscinasterias calamaria, both extremely abundant on the sandy harbour bottom, commonly invade rocks and algae of the. Point An occasional brown biscuit-star (Pentagonaster pulchellus) or pink Allistochaster insignis is seen sublittorally, but no urchins have been observed round Aquarium Point Of molluscs, the bristly Acanthochiton zelandicus is seen on almost every stone The big, black prosobranch slug Scutus breviculus is usually to be found The fawn, knobbly prosobranch Lamellanria cerebroides was common in 1952–53, only occasionally seen since Numerous species of.
opisthobranchs appear intermittently, cream and yellowish dorids being most regularly present. More especially in spring and summer are seen great, yellowish, knobbly Archidoris wellingtonensis, about 15 cms long; and scattered Aplysia species sometimes equally large.
Under-stone Zoning and Frequency of Organisms
Between the reefs and rocky outcrops of Aquarium Point are numerous stones ranging from boulders downwards. Beneath these mid-littorally occurs a wider range of species than is seen on exposed rock. Text-fig 6 shows the frequency of commoner species at different tidal levels. Actual numbers of individuals of a species under a given stone are misleading without consideration of their size. For instance, several hundred specimens of the tiny worm Spirorbis doubtless have less effect on the economy of such a sub-community than one large Hemigrapsus. Hence the black blocks of varying widths for different tidal levels represent the number of stones out of six sampled at a level that showed one or more of a given species. From Text-fig. 6, the frequently occurring species under stones are seen to fall tidally into three main groups. First come high-tide species (Cyclograpsus chavauxi, Orchestia sp., Hemigrapsus edwardsii and Zediloma arida), dropping out below half tide is reached The scale-barnacle T. etraclita here is a surprise, as on the outer coast it particularly abounds in low-tide mussel beds Next comes a larger, midlittoral group, mostly extending from about high tide to about low tide, but not usually found sublittorally Of these, Elminius modestus, Zeacumantus, Melagraphia and Sypharochiton are.
Text-fig 6—Frequency at different vertical levels of commoner species under stones. At each of 15 levels (from 0 ½ft.7 ½ft), 6 stones, each about 100 sq ins under-area, were examined All macroscopic species seen beneath them, on their under surfaces, and on their lower sides were recorded. This chart shows the frequency at different levels of all species occurring 10 or more times among the 90 stones sampled. It also includes Tetraclita purpurascens which, while occurring under only 8 of the stones examined, was unexpectedly frequent in a limited zone Bar at base (line X) marks species known to occur well below lowest spring tides.
Text-fig 7.—Vertical ranges of commoner species on exposed surface of rock, white (left) blocks, on sun-facing rock, black (right) blocks, on shaded rock. Broken lines indicate scattered individuals outside main range of species
common both above and beneath stones But the other chitons, when air-exposed, are usually kept beneath stones by their taxes. A third group–Sigapatella, Saccoglossus, Caherea, Galeolaria hystrix and various sponges and ascidians—would seem to be primarily sublittoral species extending to varying extents into the littoral. The reduction in frequency of some of these at the lowest levels sampled would seem not because such levels are unsuited to them. Rather, between mean and extreme low spring tide levels, many other species join the competition for substratum; the range and density of organisms under lowest-tide stones being often extraordinarily great.
In passing from high tide downwards within limited systematic groups, one finds that with crabs the small, brown Cyclograpsus chavauxi at high spring tide gives way to the larger, purple Hemigrapsus edwardsii under upper midlittoral rocks. This overlaps with the lower midlittoral blue-green porcellanid Petrolisthes elongatus, 10 or more of which may occur under a single stone This at low spring tide level gives way to the red-brown porcellanid Petrocheles spinosus, occurring less abundantly. Of chitons, Sypharochiton pelliser pentis, ranging almost from high spring tide to low spring tide, is joined at high tide neap by the dark, smooth Amaurochiton glaucus. This becomes less common as low spring tide is approached, being progressively replaced in the lower intertidal by bristly Acanthochiton zelandicus and rufous Ischnochiton circumvallatus Both these are abundant sublittorally, Acanthochiton then occurring both on and under stones, Ischnochiton remaining underneath. Chiefly at low spring tide level may be found the large, fleshy Cryptoconchus porosus In sublittoral shell gravel off End Beach, tiny Terenochiton otagoensis abounds. Of.
serpulids, the dark-blue tentacled Pomatoceros coeruleus in the lower littoral gives way at low tide to vermilion-crowned Galeolaria hystrix. The numerous and abundant sponges also show zonation, but this will be on sounder ground when their systematics have been more fully sorted out.
Effect of Aspect
Aquarium Point, on the sunny side of the harbour, is predominantly sun-facing. But Wharf Cliff and Cave Beach, at the two ends of the area studied, are south-facing and nearly continuously shaded More locally, boulders and reefs have sun-facing and shaded aspects.
Associated with aspect are differences in the vertical zoning of several midlittoral species. These are shown in Text-fig. 7, where the white block shows the sun-facing vertical range of a species, the black block the shaded range. Various species extend higher on the shaded faces, where desiccation during air-exposure is less. For instance, the vertical range of Bostrychia and Melarhaphe is elevated (and also increased) on shaded faces. Encrusting and tufted corallines, extending only up to low tide neap where sun-exposed, largely cover shaded faces right up to Bostrychia—a marked contrast. The Hormosira band along Cave Beach and Wharf Beach is dense to about 6in above that along sun-exposed Outer Boulders. The orange sponge? Haliclona in shade ascends nearly to half tide.
Aspect also affects abundance of several species midlitorally. On the one hand, Lichina pygmaea, nearly as dense as Bostrychia on sun-facing rocks, is almost absent on shaded ones. On the other, Pyura suteri and seasonally midlittoral Benhamina specimens are practically confined to shaded sites. Notoacmea pileopsis sturnus has been found here only on shaded faces, and Pomatoceros coeruleus, Bostrychia and Melarhaphe are much more abundant in shade than where sun-exposed.
In general, aspect affects vertical zoning more towards high tide, where air exposure (hence desiccation) is more prolonged, than towards low tide. Sublittorally, effects are not obvious.
Effect of Current
On a shore such as this where wave action is not continuous, tidal currents are liable to be critical in controlling the presence and abundance of many species. They probably have two main effects. First, greater amount of water moving over filter feeders brings a greater supply of food. Secondly, strong water currents prevent silting up of sessile organisms, which may thereby become buried or have feeding filters clogged.
Around Aquarium Point (Text-fig. 5, arrows), tidal currents are strong around Tip Rock, weaker by Lateral Reef and Cave Beach, negligible by Tide Rock and Wharf Cliff. The following features would appear associated. In the midlittoral, the balanoids Elminius plicatus and Chamaesipho columna are dense on current-swept Tip Rock, sparse or absent elsewhere (cf. Plate 31, fig. 1 with Plate 29, and right and left halves of Text-fig 8). Even on Tip Rock, however, they form by no means as dense a covering as on continuously wave-exposed shores; and Elminius plicatus is of the large, wide-based, slow growing form (Plate 30, fig. 6) (Moore, 1944, p. 326). Mussels and oysters, the other important filter-feeders here with shells that permit tolerance of intertidal desiccation, likewise are dense on and around Tip Rock, sparser where currents are weaker. Modiolaria compacta, frequent on the outer coast, here is practically confined to Tip Rock.
At low neap tide, the green fringe of Ulva round more current-exposed rock gives way to Hormosira where current is less (Text-fig. 5, U, H).
Sublittorally, Macrocystis grows all round the area where there is suitable substratum. But it thrives more vigorously where there is a certain, but not excessive,
amount of water movement. Desmarestia firma, common on wave-exposed coast, has been seen here only immediately around Tip Rock. Pyura pachydermatina is both denser and larger where currents are strong. Most other ascidians and sponges likewise thrive more where current-exposed; a grey siliceous sponge having been seen here only in strong currents. Some, however, can tolerate stiller water—for instance, Didemnum candidum and Aplidium adamsi, Tethya and Siphonochalina.
Certain algae, from their distribution round Aquarium Point, might seem to thrive in quieter rather than fast-flowing water. Wider field observations suggest.
this to be the case with Codium dichotomum and Striaria attenuata, here chiefly occurring along Wharf Beach. Among larger browns, however, Ecklonia and Carpophyllum flexuosum thrive nearby in more mobile water, and Carnahan (1952) lists Cystophora torulosa as a species that dominates only in exposed places Hence their absence round Tip Rock seems due to their inability to compete with Macrocystis and Pyura there rather than because currents disagree with them.
On the usually bare sun-exposed rocks above low tide neap, brief seasonal outbursts of several algae form a transient covering Records of the more conspicuous ones, from late 1952 till late 1955, are shown in Text-fig 9. Porphyra ? columbina, absent from Aquarium Point for most of the year, shows a winter and spring burst.
In July, 1952, it predominantly covered rocks just above the Hormosira zone, giving them an oily appearance. Some individual sheets exceeded 35 cms in length. On Flat Rock, above half tide level, a typical succession has been seen for the past three years. First, the high tide form of Scytosiphon lomentaria appears at the end of June, forming a brown, grassy covering by August It is soon followed by the similar (but branched) Scytothamnus fascicularis, amongst it and extending to a higher tidal level. As these species pass their prime, small tufts of Porphyra and other algae grow on them, and Melagraphia throngs them. S. lomentaria vanishes about the beginning of December, S. fascicularis a fortnight or so later The change on Flat Rock, from a dense algal covering in September to naked rock (except for Bostrychia) in December is most striking. Adenocystis utricularis is another brown alga with a conspicuous but brief outburst. During October and November its glistening bladders are in evidence among stones at half tide level, especially on End Beach. At low tide neap, knobby brown spheres of Leathesia difformis are present from September till autumn. The large spherical bladders of Colpomenia sinuosa are to some extent seasonal (spring, summer)
Several blue-green algae are temporarily conspicuous. Bright green, irregular hemispheres of Rivularia australis are in evidence just below half tide from January till March Chiefly in autumn are seen the dark green filamentous masses of Oscillatoria corallinae and the circular, tufted patches of Lyngbya semiplena.
Sublittorally, the fernlike fronds of Desmarestia firma appear in late spring and early summer. Striaria attenuata also shows a spring outburst Foliose reds and species of Ulva and Letterstedtia are in general most conspicuous in spring.
Of midlittoral animals, the big pulmonate limpet Benhamina obliquata shows the most striking seasonal behaviour. A minority live here intertidally throughout the year, a colony of from five to one specimens having been observed on Tide Rock every month that field notes were made from October 1952 to October 1955. During each autumn and winter, only occasional other specimens were seen. In September.
and October of each year, however, dozens of others migrate up into the intertidal. Their large cream egg coils, like slices of onion, are first laid in October, continue arriving till December. After this, most adults vanish from the intertidal till the following spring.
The shore considered is “sheltered” in the sense that it lies in a shallow, landlocked harbour where wave action is not continuous But Aquarium Point is in a part of the harbour where winds are frequent and tidal currents strong. Hence “moderately sheltered” is a more precise description. The region considered mostly faces midday sun. Hot sunshine and frequent wind together cause considerable desiccation.
Characteristic features of this shore are the luxuriance of life from low tide neap downwards, its relative paucity from here up. The former would appear due to the combination of strong tidal currents with the absence of heavy surf Around current-swept Tip Rock, the sublittoral filter-feeding groups, sponges and ascidians, are both dense and greatly varied. Here, within a radius of a few feet, are 20 ascidian species and probably a wider range of sponges. The midlittoral paucity may be interpreted by considering intertidal rocks elsewhere These commonly are covered by balanoids, mussels, serpulids and fucoids. On the present shore, a mussel and two barnacles are dense only where tidal currents are maximal. This suggests that, at these temperatures, water movement is elsewhere insufficient to support a more general covering of these midlittoral filter-feeders. The barnacle Elminius modestus is in general more tolerant of calm water than the other two species here. Its sparseness on this shore is probably due to the great part of Aquarium Point with sun-facing or vertical rock, neither of which favours it (Moore, 1944, p. 332). Of serpulids, Pomatoceros coeruleus is patchily abundant over a narrow vertical zone. This species is here close to its southern limit (Knox, spoken communication). Hence probably low temperature prevents it from thriving as densely as it does near Christchurch (Knox, 1953) or Wellington. The other serpulid here, Galeolaria hystrix, is a low tide to sublittoral species Its opening is wider and its operculum less solid than that which closes tubes of P. coeruleus when exposed. It has not the habit of forming dense aggregations shown by Galeolaria caespitosa on Australian shores midlittorally (Dakin, Bennett and Pope, 1952).
A similar English shore would carry a rich fucoid covering intertidally. The only fucalean species at Aquarium Point ascending into the midlittoral is Hormosira banksii, which even on shaded faces does not come above half tide. The question arises as to how far the absence of midlittoral fucoids is due here to geographic lack of suitable species, how far to low humidity. Locally, aspect effects suggest that desiccation limits littoral growth of certain algae at Aquarium Point. For instance, Porphyra vanishes from here at about the beginning of November; whereas on the opposite (shaded) side of the harbour it is still thriving in February, and on the outer coast occurs throughout the year Around the Point itself, encrusting coralline is dense up to high tide neap on shaded faces, but on sunny rocks ascends only to low tide neap. Seasonal browns of the upper midlittoral (Adenocystis, Scytothamnus fascicularis, high tide form of Scytosiphon lomentaria) vanish from this shore before summer sun gets to its hottest. Such instances suggest that, at Aquarium Point, desiccation is critical in limiting the upward movement of at any rate some algae. Bostrychia arbuscula is the only alga of any size that flourishes high up in sun throughout the year. This species is a predominantly southern one, growing more vigorously in a wider range of habitats in Southern New Zealand than further north.
Zoning is at first sight not conspicuous on this shore, owing to the sparseness of organisms above low tide neap. However, in a general manner, Stephenson's universal zonation features can be recognised. In the present paper, to some extent his earlier scheme has been followed (Littorina zone, balanoid zone, laminarian zone), to some extent their later one (Stephenson, T. A. and A, 1949, pages 291 and 299). Along.
this shore, two littorinids and Verrucaria blackening form a narrow but often welldefined band. This appears a sufficiently clear entity to make littorinid (or supralittoral) zone seem a better description than supralittoral fringe. Midlittorally on this shore, balanoids are sparse, apparently because ecological conditions are unsuited for the thriving of any of the three potentially dominant species geographically available. Hence “midlittoral zone” is here more apt than “balanoid zone”. This obviously subdivides on this shore into upper (Bostrychia), middle (bare) and lower (Hormosira-Ulva) regions (Text-fig. 8). The writer agrees with Bennett and Pope (1953, p. 107) that terms such as “upper midlittoral” are cumbersome. If by usage “littoral” can come to be synonymous with the present “midlittoral” without loss of precision, this would thus seem desirable. The region regarded as “midlittoral” in this instance ranges, in the absence of dense balanoids, from the lower limit of Melarhaphe cincta (here mean high spring tide level) to the upper limit of attachment of the laminarian Macrocystis and Pyura pachydermatina (here mean low spring tide level). In tidal levels this agrees precisely with the littoral zone for Victorian shores of Bennett and Pope (1953, p. 107). For the region from here downwards, “infralittoral” and “sublittoral” have been used as convenient synonyms in the present paper. As both the sublittoral dominants and many associated species may occur down to a depth of several fathoms, for Aquarium Point the term sub (infra) littoral zone seems desirable to fringe. This is unlike the nearby wave-exposed outer coast, where the dominant Durvillea, confined to surf or surge, forms a fringe rather than a zone. The one species around Aquarium Point which fringes with any density at mean low spring tide is Cystophora torulosa. But it is only locally dominant (Text-fig. 5, C and Text-fig 8, left)
Critical levels for intertidal species have received much attention. On the present shore, where a tide pole and frequent calm days have enabled vertical ranges of species to be noted with some precision, such levels do not as a whole stand out. Those that perhaps deserve mention are mean low spring tide level (2ft on local tide pole), the upper limit of the infralittoral dominants and some other species; poor low neap tide level (3′–3′ 6″), above which algae are usually not dense; and mean high spring tide level (7′ 6″), separating most midlittoral molluscs from Melarhaphe. As each species tends to have its own vertical range, often markedly affected by aspect or cover, further generalities would scem unjustified.
Aquarium Point will be briefly compared with some more northern New Zealand shores. Taylor's Mistake, near Christchurch (Knox, 1953) is only partially comparable as it is continuously wave-exposed Comparisons with moderately sheltered Auckland shores are in this respect closer (e g., Trevarthen, 1953–54, p. 36; Dellow, 1955) All show the same littorinid species supralittorally. Bostrychia arbuscula is more in evidence in the south, but occurs throughout. Midlittorally, Chamaesipho columna and Elminius plicatus are usually the important balanoids throughout the country; though Chamaesipho brunnea joins them at high tide on northern, wave-exposed shores (Moore, 1946). Ostrea hefferdi in the south of the South Island replaces Crassostrea (Saxostrea) glomerata of Auckland shores Hormosira banksii thrives on sheltered shores throughout. Carpophyllum maschalocarpum, common at low spring tide in the north, has not been met as far south as Otago by the writer. Macrocystis pyrifera, on the other hand, abundant in the south, in N.Z. extends little north of Wellington (Lindauer, 1947). In this respect, South Island cold temperature shores differ from warm temperate Auckland.
Outside New Zealand, comparable Tasmanian and Victorian coasts come closest to this shore, though water temperatures there appear on the whole slightly higher (Guiler, 1950, Bennett and Pope, 1953). There, as at Aquarium Point, stalked Pyura and Macrocystis may be important sublittorally. The Macrocystis species is M. pyrifera in Tasmania, M. angustifolia in Victoria. Macrocystis pyrifera also occurs round Southern Ocean islands, the bottom and east coast of South America, and Lower.
California. The genus has a similar but slightly wider range (Womersley, 1954). Both here and across the Tasman, Hormosira banksii characterises low tide neap sheltered shores. Important midlittoral balanoids include Chamaesipho columna and Elminius modestus; though other barnacles in Australia appear to replace E. plicatus, common throughout New Zealand. Melarhaphe is the typical littorinid genus, but in Australia the species are different. South African coasts, in the warm temperate, show less close affinities with this Southern N.Z. shore (Stephenson, 1941). Comparative studies of southern South American shores would be of interest.
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E. J. Batham
Portobello Marine Biological Station, Portobello, New Zealand.