by Dr J Floor Anthoni (2002)
www.seafriends.org.nz/issues/res/kermadec/g1eco.htm
The seascapes of the Kermadecs look barren
with their low profile, slow growing corals and extensive barren rocks,
grazed by sea urchins. Yet, here hides a variety of life that has adapted
to living in this harsh environment.
The first impression when seeing the underwater
landscape, is that of barrenness. Compared to the densely clad rocky shore
of the New Zealand main islands, the Kermadec shores appear barren, because
the organisms clinging to their rocks are smaller. Instead of large seaweeds
(macroalgae), one finds corals interspersed with matting green and red
algae. The water is too cold for corals to form extensive reefs, but it
is clear, allowing for over 30m visibility. The water moves relentlessly,
powered by large swells curving around the islands while penetrating every
sheltered nook. Sea currents are unexpectedly strong, and planktonic food
is scarce. Yet one can encounter dense schools of fish as well as large
predators. The rocks are volcanic, and often too soft to give a safe holdfast
to large organisms like corals and sponges, but glorious exceptions can
be found.
See also the gallery of corals, for identifying
the various species.
f031027: A snorkeldiver views a school of grey knife fish
close to the surface. Although knifefish swim around freely, they are not
blue sea pelagic, but stay around the bubble-impregnated wild waters close
to shore. These provide protection and also planktonic food, which is richer
close to the sruface.
f031312: Around promontories, the currents run surprisingly
fast, reason why blue maomao and demoiselles are found here, often in dense
schools. These two schools came to meet us, just out of interest for the
divers, who provide rare entertainment in these waters.
f031028: Between 2 and 6m deep, the rocks are covered in
a mosaic of corals and short algae. The water movement here is too strong
for sea urchins, and also the crown of thorns star can't survive here.
Thus the slow growing corals are allowed to grow old, almost entirely covering
all space. Notice that the gaps in between, are ocasionally grazed by urchins,
making way for rivers of red hairy algae.
f031029: Another picture of this shallow habitat zone. Notice
that there are only few species of hard coral. The dark red spots are homing
sites of sea urchins, which have died recently. Notice also the yellow-green
colouring of the coral polyps, which, although extended, do not extend
any tentacles. They just soak up the maximum amount of sunlight they can
get.
f031030: Just around the 6m boundary, hardy green seaweeds
are able to grow next to the red algae. Notice how these do not show signs
of grazing. The Kermadecs have a few resident green turtles that are able
to feed on this type of seaweed.
f031230: The most successful red and green algae grow by
sending out runners (stolons), which sprout new plants at intervals, while
also attaching to the rock at various points. This photo shows a complicated
turf of creeping green and red seaweeds, which can grow very old. Again,
no sign of grazing.
f031328: A large fruit bowl coral grows below the boundary
where most hard corals end, at 6-8m. It is surrounded by short red algae
and by soft corals. At the top of the image, a barren zone of dead coral
skeletons, grazed by urchins.
f031404: A deep and sheltered pocket at 6m depth shows a
dark grey coral (on left), capable of living in conditions of reduced light,
better suited to soft corals shown in the foreground. The fish in the background
are various species of drummer.
f031325: On the sunlit side of a pinnacle rising to 5m depth,
one sees the sharp boundary between the live hard corals above, and the
barren urchin zone below, with a pied urchin mid-left. In the foreground
the notorious crown of thorns (COT) star, which is partly responsible for
the death of the hard corals in this zone.
f031334: The shaded side of the pinnacle shows a radically
different environment of dark coloured corals (top), soft corals (below)
and encrusting sponges amongst the varied red algae. A toadstool grouper
rests near the centre, under a protruding hard coral.
f031121: This close-up of the shaded side of a shallow wall,
shows encrusting orange spongess, pink coralline algae and various other
short algae. All the white specks and blobs are composite seasquirts. Invisible
to the human eye, a variety of well camouflaged crabs and snails can be
found.
f032005: Fast growing opportunistic organisms like this mauve
encrusting sponge (or colonial seasquirt?), are not common on the Kermadecs.
Notice the large barnacles and solitary corals top left, and small barnacles
bottom centre. One must have extraordinary defences to thwart an invading
and smothering blanket like this.
f031414: A large area of hard coral has, over time, been
killed by the crown of thorns star, then grazed flat by the powerful jaws
of sea urchins. But in the process, the barren and levelled rock invites
young corals (bottom centre) to reoccupy the lost ground. It appears that
the teamwork of COT stars and urchins maintains a high level of biodiversity.
f031408: Above the zone of hard corals, in the heavy wave
wash, one finds the unique Kermadec giant limpet, maintaining its own habitat
zone, which is bounded sharply, as can be seen here. Above this limpet
zone, one finds a narrow band of large barnacles (Balanus sp.).
The heavy waves did not allow us to take photographs.
f031411: At Meyer Island, the shallows have a pleasant degree
of playfulness: grooved and rounded rocks alternated by shallow canyons.
In such an environment, the zonation patterns become blurred, and one finds
a great variety of organisms living together in apparent confusion. It
is of course, a playground for fishes. In the picture three grey drummers,
one bluefish (No 2 from left) and one green wrasse (right).
f031419: If organisms wish to grow old, they must settle
on hard rock like this ancient basalt flow, which is in short supply at
the Kermadecs. The rock here is covered in pink paint, grazed by
brown urchins and top shells. Here and there a coral colony has been able
to sprout. The top of the formation is covered in brown fleshy coral.
f031824: Two important grazers of the Kermadecs, the purple
urchin and top shell. Notice how the purple urchin folds its spines in
a special way to reduce water drag: in ten clusters all around, and pointing
downward to the rock face.
f031422: This area of hard rock has been colonised by plate
corals, possibly all at the same time (same age) . Notice how the rock
in the foreground has recently been bared, possibly by the removal of such
a plate coral.
f031425: In a sheltered gully, we found this garden of soft
corals. Notice the large area of white water above, saturated with tiny
bubbles that reflect the sunlight. As a consequence, it is much darker
here under this foam umbrella, than outside it. Hence the absence of hard
corals, which need much sunlight.
f031406: A garden of green seaweeds, brown urchins and encrusting
yellow coral. There is little sign of grazing by fishes.
f031005: A birds-eye view of the rock shows a crown of thorns
star and a trail of dead white corals. The whitest one above, must have
provided its most recent meal, because green algae have not settled on
it yet. Although the COT star does not eat voraciously, it still causes
much damage becaue the corals repair or replace themselves only very slowly.
f031237: The under side of a COT star shows its bright yellow
tube feet, and also its peculiar stomach, which it is withdrawing fast.
Rather than resembling a plastic bag as in other starfish, it has lobes
designed to engulf a coral polyp each. In this manner it can dine out on
many coral polyps in a single sitting. The COT star is a very specialised
feeder, and fortunately prefers the fastest growing corals, thus enhancing
biodiversity on the reef.
f031200: A fruit bowl coral has been damaged by the anchor
chain of a visiting ship. Fortunately, it had died long before, and is
now covered in green algae, a rich picking ground for various species of
hermit crab.
f031510: The rocky shore of the Kermadecs extends along the
abyssal slope down into the 10km deep Kermadec Trench. On elevated rocks,
a most colourful and delicate fauna of filterfeeders is found, like these
gorgonian corals. Under overhangs and at the ceilings of caves (here 27m
deep), divers can obtain a sneak preview of this most amazing and colourful
deep-sea habitat. In the photo three striped boarfish and a masked moki
(Cheilodactylus
sp.) (bottom-right).
