Starfish have always seemed to be curiously inert animals, in spite of their reputation as predators. When we pick up one on the beach, it's stiff and solid, nothing but arms covered with a bumpy skin. The only sign of life seems to be on the underside; those tube feet that cling to the rocks and refuse to let go.
So I've been surprised by the three starfish (not counting the baby) that came home on the kelp and holdfasts. In the tank, they're almost always in motion, climbing the walls and stones, investigating the pump, sliding over snails and barnacles, sometimes pausing to eat one, and even, this afternoon, trundling along a blade of eelgrass near the top of the tank.
And, underwater, they suddenly become soft and furry; the hard, spiny top is coated with a living coat, swaying in the currents; the arms are tipped and bordered with long, questing tube feet, in the shape of a land snail's eyestalk, a thin, flexible, extensible tube with a round knob on the end. But instead of two per critter, there are hundreds.
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| Part of an arm, underwater. |
Through the glass and an inch of water, the edges are blurred, but we can see three main skin structures. The pale, traslucent orange "nipples" are dermal banchiae, gill structures, absorbing oxygen from the water. These are constantly in motion while the starfish is underwater. The white, four-pointed "jaws" are
pedicellariae, little snapping pincers, which may capture small prey, or alternately, protect the other skin structures. (Could this be why we don't find starfish wearing barnacles or mussels?) Underwater, they extend enough to almost hide the spines. And, like the gills, they are always moving, snapping at anything that comes near.
Out of the water, the pedicellariae subside, and the spines are more visible. I moved the largest starfish (about 4 cm across) to a small plate, where he immediately crawled to the edge, off the white background that I'd chosen.
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| Two arms, showing pattern of spines, each one surrounded by a ring of pedicellariae, and with gills mainly along the outside edge of the arm. The white patch in the centre is the madreporite, the intake for the water circulation system of the starfish. |
Zooming in on one of these arms:
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| Spines, small and large, each surrounded by biting pedicellariae. A few gills along the bottom. |
Zooming in still more:
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| Here you can see the open "mouths" of the pedicellariae. These are two-sided; some starfish may have three-pronged "peds". The starfish is wet, but out of the water, so the "peds" are shrunken and relatively peaceful. |
And the starfish has eyes! Five ocelli, one on each arm, at the tip.
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| Each one is well protected in a ring of tube feet and pedicellariae. They don't see much other than light and shadow. |
I've been watching the three of them climb my glass walls, which gives me a good view of how the tube feet move, stretching, reaching, pulling. A starfish can move quite quickly on these, when he's motivated.
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| The medium star, longer-armed than the big one. The tube feet at the tip of each arm are extended mostly in the direction of travel. I saw one of these come up to an anemone on the wall; when the first tube foot touched an anemone tentacle, the starfish reacted, pulling the arm back and curling it away from the wall. Then the star changed direction, carefully skirting the danger zone. |
We watched a starfish eat a limpet on the wall this afternoon. It everted the stomach, pulled in the limpet, pointed shell inwards, then after a few minutes of chewing, ejected the limpet shell, this time pointed end outwards.
I don't know what this one was eating; I just caught it in time to see its everted stomach.
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| Probably eating algae or something too small for me to see. There were no shell remains. |
Zooming in further, to see the everted gut structure.
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| Here on the underside, the pedicellariae are not so obvious, and most of the structures are tube feet. They have been traditionally understood to work by suction, but recent research suggests that they expel a chemical glue, which leaves a residue on the surface. |
There are only two limpets left in the tank, but plenty of snails; mud snails and Nassas, which both seem to breed here. The big anemone eats some of them, but there will be enough to share with the starfish, so no worries. Otherwise, they will compete with the leafy hornmouth snails for what barnacles I can gather.
