TANE 22, 1976
INTERTIDAL LIFE ON R O C K Y SHORES O F G R E A T M E R C U R Y ISLAND
by R . J . Staveley Parker
Department of Geology, University of Auckland, Private Bag, Auckland
SUMMARY
The distribution o f various zoning intertidal organisms has been studied at
several rocky shore localities around the island. The presence, absence and range
of these organisms reflects the environment o f the site.
INTRODUCTION
The intertidal marine life o f Great Mercury Island was studied during
Auckland University Field Club's scientific trip to the island in May 1975.
Offshore islands frequently display patterns o f zonation o f intertidal life
that differ considerably from those observed on the mainland. Such differences
are examined in this paper.
Red Mercury Island, lying some seven kilometres to the east o f Great
Mercury, was studied in 1972 by a similar University Field Club trip and it
would be expected that the two islands would display some degree o f similarity.
In contrast to R e d Mercury Island, however, Great Mercury Island is no longer in
its pristine state, having been modified to some extent by man's agricultural
activities.
METHODS
Transects were made at four localities around the island. These ran from
low tide level to as far above high tide level as was practical or necessary ( F i g . 1).
O n the transects with solid rock, the organisms were counted over an area
of one square metre, where possible, at regularly spaced stations. A t some places,
however, the nature o f the substrate or its aspect prevented sampling at regularly
spaced points. This was the case w i t h the boulder beach transect where the size
and close packing o f the boulders prevented regular spacing o f stations. Here,
stations were placed at points where boulders could be removed or the seaward
facing surfaces o f boulders sampled.
Algae and encrusting organisms, such as barnacles, were estimated as the
percentage cover o f one square metre. These have been represented in the kite
diagrams by stippled shading and the number o f individual mobile organisms is
represented by kite diagrams with diagonal shading.
Other organisms that were not numerous enough for the construction o f
kite diagrams are shown as bars representing the range o f the organism across the
shore.
RESULTS
Transect A ( F i g . 2)
This was situated in the harbour and had a substrate o f hard basaltic rock
with a well-developed joint pattern, offering protection to juvenile molluscs such
77
G R E A T MERCURY I.
Tamewhera Rk •
0
1
1
T1
f
The Sisters • '
N
'
\~(J?
Koru Bay
2
km
1
1
Pig. 1. Location of transects on Great Mercury Island.
as the periwinkle (Melarapha oliveri) and Nerita melanotragus. The distribution
of M. oliveri appeared to be the converse o f that frequently observed elsewhere
as it had a predominance o f juveniles towards its upper limits and large adults
towards the bottom o f its range.
The area was not subject to wave action o f great force. This is reflected by
the narrow vertical range o f the periwinkle, which reaches only a relatively short
distance up the shore. Its distribution was limited by the extent o f splash from
the waves and by Lichina pygmea which extends down the transect for a greater
distance than found i n the other transects.
In the upper portion o f its range, Lichina pygmea was restricted to the
seaward facing slopes o f the rock, whilst Xanthoria sp. prevailed on the more
horizontal upper surfaces.
A barnacle zone was absent here and the oyster (Crassostrea glomerata)
spread over that area normally occupied by barnacles. Coralline turf also
extended its range upwards into that area elsewhere occupied by barnacles.
Carnivorous boring molluscs, such as Lepsiella scobina, showed a range
very similar to that o f the oyster upon which they were feeding, and lower down
Cominella maculosa was found feeding upon Lunella smaragda. The small black
pulmonate Onchidella nigricans was present i n small groups o f six or so
78
individuals, feeding upon various algal remains amongst the oyster and upper
Corallina officinalis zones.
Immediately above l o w tide level Hormosira banksii was interspersed with
coralline turf, and below gave way to brown algae, possibly Xiphophora
chondrophylla and Carpophyllum sp.
A t l o w water level, the introduced polyzoan Dakaria subovoidea was
abundant and covered up to 90% o f the undersurface o f boulders. Its presence
again reflects the lack o f violent wave action.
Overall, this transect showed features pertaining to those described by
Morton and Miller (1968) from areas o f moderate shelter.
A Number of individuals
/2on rock surfaces
Percent coverage
of rock surfaces
Petrolisthes elongatus
Onchidella
Range of organism
along shore transect
Apophloea
pelliserpentis
= —
Li china
C
I
|
Hormosira
Sypharochiton
nigricans
•
banksii
]
I
pygmea
St1
50*/.
cover
sf5
200
individuals
st6
Fig, 2. Distribution of organisms on transect A, and key for Figs. 2-5.
79
Transect B (Fig. 3)
This was situated on the eastern side o f the island and was exposed to the
full force o f the ocean waves. The substrate was o f a tuffaceous nature which has
been rapidly eroded into numerous crevices and hollows, these offering refuge
from the pounding o f storm waves for many organisms.
Melarapha oliveri was found well up the transect reflecting the force of the
wave action. Various lichens were found with M. oliveri and these continued
down the cliff face until sea spray prevented any further colonisation.
Leptograpsus variegatus was the only crab observed here and was abundant in
the deeper crevices.
The surf barnacle Chamaesipho brunnea formed a well defined zone
continuing 1.5 metres up the cliff wall and its range was overlapped by that of
the larger acorn barnacle, Elminius plicatus. Transect B was the only transect at
which E. plicatus was observed.
Fig. 3. Distribution of organisms on transect B (see Fig. 2 for key).
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The algae Apophloea sinclairii, Hormosira banksii, Splachnidium
rugosum,
Ulva lactuca and coralline paint, formed a sequence down the lower half of the
transect.
Amongst the mobile organisms, Haliotis iris was found wedged in crevices
at low tide level, and Sypharochiton
pelliserpentis was represented by a few
juvenile individuals less than 1cm in length. Shelter was sufficient to allow
Onchidella nigricans to thrive amongst the coralline turf.
Transect C (Fig. 4)
This was located in a small gully on the western side o f Peach Grove at the
southern end o f the island, and was again an exposed area receiving the southerly
storms. The substrate was similar to that o f transect B although the tuff was not
as deeply scoured as at that locality. The gully walls, however, offered some
protection for the periwinkle which extended its range down to 2.5 metres
above l o w tide level. Nerita melanotragus was most plentiful on the gully sides
reaching m a x i m u m numbers at station 5.
Fig, 4. Distribution of organisms on transect C (see Fig. 2 for key).
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Amongst the most abundant o f the mobile organisms was the snakeskin
chiton (S. pelliserpentis) which achieved densities o f up to 150 per square metre,
both adults and juveniles being present. Other chitons were also present, mainly
about stations 3 to 4. These included Cryptoconchus porosus,
Acanthochiton
zelandicus, with its tufts o f short bristles, and the familiar green Amaurochiton
glaucus.
The red rock crab Plagusia chabrus was found towards l o w tide and the
purple crab Leptograpsus variegatus was found on the middle and upper levels o f
the transect.
O f the transects studied, this had the greatest variety and abundance o f
algae. A sequence o f algae was observed down the shore, viz.
Apophloea
sinclairii, coralline turf, Ulva lactuca, Xiphophora chondrophylla,
Pterocladia
capillacea and Carpophyllum maschalocarpum. Also present i n smaller quantities
were Melanthalia abscissa, Codium adhaerens (green button alga), Colpomenia
sinuosa (brown alga), and a red encrusting sponge, possibly Anchinoe
incrustans.
A t low tide level Evechinus chloroticus was abundant, and the c o m m o n
anemone Actinia olivacea sheltered in the water-filled hollows.
Transect D (Fig. 5)
This was the only boulder beach studied on the island and was situated on
a south-facing promontory protected from the ocean swells but exposed to
storm waves from the south and west. The beach was composed o f well-rounded
boulders, ranging from l m in diameter down to cobbles o f 5-10cm diameter.
The average size o f the boulders decreased up the beach.
Crabs were well represented here, and created a zoning pattern amongst
themselves with Cyclograpsus lavauxi occupying the uppermost reaches o f the
shore and Cyclograpsus insularum and Heterozius rotundifrons lower d o w n . The
half crab Petrolisthes elongatus succeeded Cyclograpsus insularum at about half
tide level and continued to the lowest station sampled.
Nerita melanotragus was present from the top o f the beach to station 5,
with a tendency for the larger adults to be found lower down the beach and
juveniles at the top.
Amongst the boulders, the fauna was varied. The anemones Actinia
olivacea and the deep red Actinia tenebrosa were common on the underside o f
the boulders, the former at the b o t t o m o f the beach and the latter at the top.
Amphipods were c o m m o n throughout the length o f the transect. On the lower
portion o f the beach the scale w o r m Lepidonotus
sp. and the cling fish
Diplocrepis sp. were c o m m o n nestling beneath the rocks. Near low water mark,
the shrimp Alope spinifrons and the tube worm Spirorbis sp. were c o m m o n .
The c o m m o n boulder limpet, Atalacmea fragilis, inhabited the lower
portion o f the shore (stations 1 and 2) and its range was then overlapped by
Cellana radians which continued to station 3.
The distribution o f the algae on stable boulders followed a sequence
similar to that observed in transect C .
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18
16
14
12
10
8
6
4
2
0
Fig, 5. Distribution of organisms on transect D (see Fig. 2 for key).
DISCUSSION A N D CONCLUSIONS
Great Mercury Island offers a wide range o f intertidal environments. The
degree o f exposure varied from the sheltered harbour o f transect A to the
exposed east coast which is pounded by ocean swells and storm waves, and
represented by transect B .
Shore mobility is another variable factor particularly important on the
boulder beach o f transect D . Here, storm waves could move many o f the
boulders and thus sessile organisms such as barnacles were absent. On this type
of shore, however, greater diversity o f life is found because o f the variety of
habitats available. Crabs, amphipods and cling fish inhabit the moist, shady
undersides o f boulders, a habitat not found on the hard rock transects.
O n the 3 solid rock shores studied, the species present and their
distribution was found to be very similar. Transect A lacked a barnacle zone,
reflecting its more sheltered nature i n comparison with transects B and C where
the zones were fully developed.
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Although transects B and C were exposed to continuous wave action, the
nature of the rock allowed erosion into crevices and hollows. These crevices
provided semi-sheltered microhabitats for species such as Melarapha oliveri and
Nerita melanotragus which flourished on the storm swept faces.
Other molluscs such as Haustrum haustorium and Lepsiella scobina were
dependent upon the presence o f the species on which they fed (e.g. the oyster or
the barnacle).
The mobile organisms were distributed with Melarapha oliveri at the
uppermost levels o f all the transects, continuing downwards with Nerita
melanotragus into the barnacle zone and limpet zones. Below this Onchidella
nigricans was frequently found on the mid littoral coralline turf flats w i t h
Evechinus chloroticus marking the l o w tide level.
A pattern o f zonation o f sessile organisms was also recognised. The lichen
Lichina pygmea was frequently found at the top o f the shore. A general
sequence o f algae down the shore could be seen, consisting o f
Apophloea
sinclairii,
Xiphophora
chondrophylla,
coralline turf and
Carpophyllum
angustifolium.
Although not observed on any of the shore transects, the subtropical
vermetid Novastoa lamellosa was noted at Tamewhera R o c k off the north west
coast o f the main island. Here it was seen at about half tide level on the south
side o f the rock. This rock rises sheer from about 20 metres depth to some 50
metres above the sea. A detailed study was not possible but w o u l d have proved
interesting, as it appeared to provide a similar type o f environment to that found
at the Poor Knights Islands. None o f the shore transects have shown any o f the
peculiarities o f zonation found at the Poor Knights and described by M o r t o n and
Miller (1968). Presumably, this is due to the lack o f influence from the warmer
east A u c k l a n d current at Great Mercury Island.
Comparing the Great Mercury transects w i t h those o f Red Mercury Island
(Saies et. al. 1972) little difference is to be seen except for the one occurrence o f
N. lamellosa which was not found on Red Mercury or at M o k o m o k o R o c k .
It appears that the four transects recorded here are similar to those
described by M o r t o n and Miller (1968) for the mainland, particularly the
boulder beach profile reported from Smugglers' Bay near Bream Head and the
algal zoning patterns from Ocean Beach at Whangarei Heads.
Great Mercury Island would seem to exhibit features differing little from
those observed on the mainland or those reported from Red Mercury Island.
ACKNOWLEDGEMENTS
The author wishes to acknowledge the help and assistance given by Dr R .
Grace and Mrs A . Grace; and to thank Mrs J . Mulcahy for typing the manuscript.
REFERENCES
Morton, J.E. & Miller, M.C. 1968: "The New Zealand Sea Shore". Collins, LondonAuckland. 638pp.
Saies, A.B., Beever, S.E., Hay, J.R. & Woods, R.G. 1972: "The Intertidal Life of Red
Mercury Island". Tane 18: 43-55.
Wood, D.H. 1963: "The Boulder Beach Fauna of Otata Island". Tane 9: 41-55.
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