Sea cucumbers, stars and urchins on Lord Howe

I was going to call this post “the pentaradials”, because sea cucumbers, sea stars, brittle stars and sea urchins all have similar body plans (fivefold symmetry), but decided I was trying too hard. It’s a good excuse to group them together, though.

Sea cucumbers

The black sea cucumber (Holothuria leucospilota) common on Lord Howe is the same one as on the coast at home. I’d often wondered what the internal structure of a sea cucumber was, and as luck would have it (although not for the sea cucumber), there was a dead one on the beach.

What’s inside a black sea cucumber

When you disturb one, it’ll sometimes spit out white stringy filaments – these are the Cuvier’s tubules, the white stuff on the left of the picture. They become very sticky on contact with water and are apparently toxic, so a defense mechanism for this animal. I didn’t realise there was so much of them inside. The sausage-looking structure is indeed the intestine. One of the other structures is the respiratory tree, the equivalent to our lungs.

[Update: you can see the internal structure of a sea cucumber and find out about its eating habits at Deep Sea News here.]

There’s also a brown and white sea cucumber (Holothuria hilla) …

Brown and white sea cucumber

… and the sandy sea cucumber (Holothuria atra) …

Sandy sea cucumber

Sea cucumbers live on the sandy floor, and hoover up sand to extract organic matter for food.

Sea urchins

The burrowing urchin (Echinometra mathaei) burrows into the soft rock and eventually has a hole for protection. According to Ruppert and Barnes, “Invertebrate Zoology”, “Boring behavior appears to be an adaptation to counteract excessive wave action, and these species [the burrowing ones] are largely found in habitats that are exposed to rough water”. Rough water is certainly the case on the east coast of Lord Howe.

Burrowing urchin

Spines and tube feet move these creatures along. You can see the “ball and socket” arrangement of how the spines attach to the main body here …

You can see the “ball and socket” joints of the spines here

The next shot shows the underside where the Aristotle’s lantern would normally be. This is the five-part pyramidal mouth structure that scrapes algae and other food organisms off the rocks. It’s what the burrowing urchin uses to dig a burrow, too.

A sea urchin moves about with this side down, so that the mouthparts can scrape off food from the rocks

There’s also the red-tipped urchin (Heliocidaris tuberculata) …

Red-tipped urchin

In deeper water, there are lots of needle-spined urchins (Diadema savignyi) in the coral patches. They need those long sharp spines to fend off the wrasses and parrot fish that would make snacks of them.

Heart urchins

Heart urchins (Breynia australasiae) live buried in the sand, moving about by means of their short spines. While snorkelling you can see the mounds in the sand where they are, or see the animals lying on the surface of the sand.

Heart urchin, top side, still with its spines

Heart urchin, underside, showing the mouth

The dead “tests” (skeletons) wash up on the beach.

The front of the animal is at the top

Heart urchins do not have an Aristotle’s lantern, but instead special tube feet move food particles along grooves in the body to the mouth (at the top of the next photo).

Underside, showing the mouth cavity at the top and the  anus at the bottom

Here are a couple of close-ups of the test structure …

The internal structure of a heart urchin

Surface of a heart urchin – the pits are where the thin spines attach

Sea stars

I also saw on the rock platform the orange sea star (Ophidiaster confertus) …

Orange sea star

… the blue sea star (Linckia laevigata) …

Blue sea star; photo by Papakuro, Wikimedia Commons

… and the seven-armed sea star (Astrostole rodolphi) …

Seven-armed sea star

You can see the tube feet it moves about with on the underside …

Tube feet of seven-armed sea star

Sea stars feed by letting their stomach out of their body through the opening in the centre of the star, and covering their food – clams, mussels, turban shells, urchins – with it. The arms are strong enough to force bivalve shells open enough to force the stomach in or just let the stomach juices in. The victim’s soft parts are then dissolved in the stomach juices. Nice!

Brittle stars

This brittle star (Ophiocoma dentata) was common under rocks on the platform at Ned’s Beach. They shelter  under rocks during the day and come out to feed at night. They are so-called because they tend to drop an arm or two when disturbed too much.

Brittle star

They are fast-moving hunters that grab  worms, molluscs and crustaceans (small crabs and related things). They can also just scoop bits and pieces they find into their mouths. They are also able to filter-feed, straining food from the current – flexible in their feeding habits.

There are a lot more critters on Lord Howe, but in a week one can see only so many. Perhaps a fourth visit to the island is in order!

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