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.
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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.

mottled sand star (Luidia varia)
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.
comb star (Astropecten polyacanthus)
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.
hat urchins
f012806: the hat urchin () does not burrow in the sand but catches sand particles and other objects with its top side tube feet.
hat urchin
f019422: close-up of a hat urchin. Divers can distinguish two varieties, the high hat and the low hat, but these are probably the same species.
hat urchin seen from below
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.
hat urchin dorsal pattern
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.
giant heart urchin (Brissus gigas)
f009908: the mystery prize goes to the giant heart urchin (Brissus gigas), previously thought to be rare until Dr Anthoni discovered how and where it lives [1]. Here is a dead shell of one that died prematurely, near the rubble of its deep burrow.
giant heart urchin (Brissus gigas)
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.
giant heart urchin (Brissus gigas)
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 its mouth.
giant heart urchin (Brissus gigas)
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 continuous stream.

[1] Anthoni, J F (1993): Hauraki Gulf Marine Survey an investigation of the kelp mass mortalities of 1993.

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 and chitons.

sea lettuce on Trev's Rock
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.
Sandagers wrasse at Trev's Rock
f022410: a male Sandager's wrasse peeping through one of the holes in Trev's Rock, grazed by strong purple urchins.
purple urchins Centrostephanus rodgersi
f030621: where purple urchins survive, they are able to push the green urchins out, as shown here.
common green urchin (Evechinus chloroticus)
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.
purple urchin (Centrostephanus rodgersi)
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.
brown urchin
f016430: the brown urchin () has longer spines still, and is found in even more shelter. It is an uncommon visitor from warmer seas.
white-spined urchin (Tripneustes gratilla)
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.
young diadema needle urchin (Diadema palmeri)
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.
mature diadema needle urchin (Diadema palmeri)
f020137: a mature diadema urchin by night. Notice how it has lost its white sectors.
serpent star (Ophidiaster macknighti)
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)
diver and firebrick star (Asterodiscides truncatus)
f025019: a diver finds a firebrick star (Asterodiscides truncatus) on a vertical rock face of the western entrance to Rikoriko.
firebrick star (Asterodiscides truncatus)
f025232: close-up of the back of a firebrick star (Asterodiscides truncatus) shows that its spines are spherical, feeling smooth to the touch.
seven-armed star (Astrostole scabra)
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.
feather star
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.
NZ sea cucumber (Stichopus mollis)
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 on detritus.