Poor Knights marine reserve echinoderms: the sea stars and urchins by Dr J Floor Anthoni (2007)
Echinoderms are very successful marine organisms,
adapted to all habitats and manners of feeding. Those living on and in
the sand use the sand as protection. Those living on the rocks can survive
more turbulence, grazing algae in sun-lit waters. This chapter shows the
common sea urchins, sea stars and sea cucumbers of the Poor Knights.
echinoderms of the sandy bottom The sandy habitat of the Poor Knights is small but very interesting.
One may find hat urchins sliding over the sand which is hiding the many
comb stars that live undert he surface.
f048809: the mottled sand star (Luidia varia) is a
large star with seven arms, living for long periods under the sand. This
common sand star is shown here but may not occcur at the Poor Knights.
f048810: the comb star (Astropecten polyacanthus)
lives almost its entire life burrowed under the sand where it can sometimes
be inferred from a slight five-armed depression. Finding one means that
many are around.
f012806: the hat urchin () does not burrow in the sand but
catches sand particles and other objects with its top side tube feet.
f019422: close-up of a hat urchin. Divers can distinguish
two varieties, the high hat and the low hat, but these are probably the
f012807: the underside of the hat urchin shows small spines,
all directed towards its middle such that any struggling prey will be pushed
towards the centre where its mouth is.
f019423: the cleaned top of a hat urchin shows that is both
sideways symmetric (top left to bottom right) as well as five-sided symmetric.
f009908: the mystery prize goes to the giant heart urchin
gigas), previously thought to be rare until Dr Anthoni discovered how
and where it lives . Here is a dead shell of one that died prematurely,
near the rubble of its deep burrow.
f009909: the giant heart urchin (Brissus gigas) can
grow to a size of 1.5kg, and is both two-sided as well as five-sided symmetric.
The reason it was not found is that it burrows to a depth of 30cm from
where it is difficult to detect, even though it leaves large markings on
an undisturbed sea bottom.
f032808: the top of the giant hear urchin shows all spines
combed backward, with short tubefeet in between. Living entirely underground,
this urchin causes a continuous collapse of the sand around it, to form
a funnel through which detritus falls. Tubefeet then move it further towards
f032809: the bottom shows a 'foot' of forward-combed spines
towards its mouth where a tiny commensal clam is seen. These tiny clams
move like Amazonian jungle grip-tail monkeys through the forest of spines.
The urchin also consumes sand grains which exit from its rear end in a
Echinoderms of the rocks The most numerous echinoderms of the rocky shore are the grazing sea
urchins. Their influence on the environment is decisive, as they create
and maintain barren areas, grazed further by an army of snails, limpets
f012815: sea lettuce growing just out of reach of the grazing
green sea urchins in wave-washed water that could knock the urchins off
the rock, sending them to the deep.
f022410: a male Sandager's wrasse peeping through one of
the holes in Trev's Rock, grazed by strong purple urchins.
f030621: where purple urchins survive, they are able to push
the green urchins out, as shown here.
f036909: the common green urchin (Evechinus chloroticus)
has short spines, low friction profile and a good grip, and is therefore
able to live higher up the wall.
f030424: the purple urchin (Centrostephanus rodgersi)
looks black from a distance, but in close-up is indeed purple. It has a
high friction profile and loose grip, and prefers to stay lower on the
wall, or inside hollows.
f016430: the brown urchin () has longer spines still,
and is found in even more shelter. It is an uncommon visitor from warmer
f033121: the white-spined urchin (Tripneustes gratilla)
has zillions of tubefeet, also on top, and is often found in the canopy
of the flexible weed.
f020710: a young diadema needle urchin (Diadema palmeri)
found on the colourful rock face near the sand by night. The young ones
have white sectors.
f020137: a mature diadema urchin by night. Notice how it
has lost its white sectors.
f009709: a serpent star (Ophidiaster macknighti) is
variable in colour from yellow to brown.
f025022: the knightaster (Knightaster bakeri) is always
pale, usually more flattened than the serpent star, and has tapering arms
ending in small white tips. It is also much smaller. (I don't think this
photo is the right one)
f025019: a diver finds a firebrick star (Asterodiscides
truncatus) on a vertical rock face of the western entrance to Rikoriko.
f025232: close-up of the back of a firebrick star (Asterodiscides
truncatus) shows that its spines are spherical, feeling smooth to the
f051820: the seven-armed star (Astrostole scabra)
is NZ's largest sea star. It predates on sea urchins and molluscs, but
it too is predated on by the trumpet whelk (Charonia sp.), as the star
shown here misses an arm already.
f029623: feather stars (Crinoids) are found in very protected
places, yet close to the current. Their arms are long and flexible,
branched like feathers. Their tiny tubefeet can hardly be seen by the naked
eye. They catch plankton of all kind.
f018916: the NZ sea cucumber (Stichopus mollis) looks
prickly but feels soft to rubbery. It is more common in sheltered coastal
bays polluted by sewage. It is not large, 10-20cm overall length. It feeds