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.]

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Between a rock and a sandy place

I flipped not my  lid, but a rock on a rock platform this morning on the coast in northern New South Wales, for International Rock-flipping Day (this year 11 September 2011). Here’s what was underneath.

Peanut worm (centre) in its usual habitat, on sand or gravel under a rock

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A gloomy day, full of life

Perhaps the heavily overcast conditions on Saturday fooled the critters at Flat Rock into thinking that it was dusk and wake-up time, but I have rarely seen so much life in the rock pools in one session.

For starters, we found five common Sydney octopuses (Octopus tetricus)! There were a couple in the same pool, but apart from each other. The white eyes are definitive of that species.

Common Sydney octopus

They all matched the colour and texture of the main seaweed in the pool, even becoming more or less ‘spiky’ as they moved fluidly across the bottom.

According to Norman and Reid’s A Guide to Squid, Cuttlefish and Octopuses of Australia, they can have an armspan of two metres, but these would have been only about one metre. This octopus ‘feeds on shellfish, which it drills and poisons, then pulls the shells open and eats the paralysed contents. This species primarily emerges at night to hunt crustaceans and gather shellfish. … Lairs can often be recognised by the bivalve shells scattered around their entrance’.

I’ve also seen the blue-lined octopus (Hapalochlaena fasciata) on this reef, but not yesterday. It was glowing blue at the time, so I left it well alone. I was slightly worried as their bites are reportedly painless, and inject a toxin that causes paralysis. If you can be kept breathing, you’ll be OK. Otherwise – not.

Another exciting find was more than 15 (I stopped counting) little ruby flatworms (Phrikoceros baibaiye). They are bright orange with small white spots and very active.  The edge of the mantle is very mobile. The head on the one below is on the left. We found one (or was it two?) appearing to chase its tail, going round and round in a swirl of skirts. Was it two mating, one laying eggs or just playing ouroboros?

Little ruby flatworm

Size of flatworm compared to Austrocochlea shell, photo by Andrew Roberts

While gazing at one flatworm, I became aware of a black nudibranch (Dendrodoris nigra) near it. Then I spotted a second black nudi nearby! You can just see the ‘naked gills’ at the back (top) of the nudi below.

Orange flatworm and black nudibranch

Another colourful inhabitant out feeding was the orange feather duster below. You can see the tube it withdraws into when it needs protection.

Orange feather duster worm

Here’s a sea urchin (Tripneustes gratilla?). It was camouflaging itself with seaweed so was barely visible in its pool.

Size comparison

When I put it back in the water, it put its feet out to move back to shelter (where I placed it in a more hidden spot after the photo).

Sidling away on a multitude of feet

Last (for this post – I’ll continue in another one as there was lots to see), I inadvertently startled a black sea cucumber, and it ejected its tubules of Cuvier in defense. These are quite sticky on the fingers. The animal regrows them.

Black sea cucumber ejecting tubules of Cuvier as a defense

Beach wanderings

At Flat Rock, the beach was very clean – smooth sand and little debris of any kind, whether plastic, seaweed or shells. The low tide was the lowest it had been for a while, so the correspondingly high high tide had reached all the way to the dunes. The rock platform itself had a lot of sand filling the shallow ponds.

Flat Rock pool. The more plants, the greater the chance of finding something interesting

Many black sea cucumbers (Holothuria leucospilota) were hoovering the sandy bottoms of their ponds for food. They are technically known as ‘detritus feeders’ – they get any organic matter they can from between the sand grains and shell grit on the bottom. You can get general information about their lives at

Black sea cucumber

You can see the mouthparts

A small octopus (possibly Octopus tetricus, the common Sydney octopus) crouched under a rock in its lair, pretending to be a piece of seaweed with one-half of a bivalve on one tentacle waving slightly in the water. Octopuses (or octopi) feed at night on shellfish, drilling into the shells and injecting a paralysing fluid. I’ve previously see the blue-ringed octopus at Flat Rock (Hapalochlaena species), and the blue rings are indeed very beautiful, but saying ‘don’t mess with me!’ I was walking carefully in a shallow pool, and took a moment to recognise it – then a loud ‘eek!’, a leap and ‘I’m outa here!’ It is venomous and fatal to humans if they are untreated, and its bite is apparently painless. If you can be kept breathing (you will be paralysed by the venom) until the venom degrades in your system, you will probably survive. You can find out more about the blue-ringed octopus at

Octopus in lair

The ‘eek and leap’ survival reflex was also shown by a friend when he encountered a moray eel on one of our outings there; he also incorporated levitation into the mix. Morays look vicious but are held to be short-sighted, so come out to give you a sniff as they are good smellers. They are not considered dangerous to humans, despite being carnivores, but if provoked will give a well-deserved bite. This one was probably the green moray (Gymnothorax prasinus). Green morays aren’t necessarily green. You can find out more about them at

Curious green moray

It was breeding season for molluscs, evidenced by the many rings of limpet egg masses.

The while circle is a limpet egg mass. A limpet is below it.

On the strand, there was evidence of storms at sea – shimmering blue things washed up in the form of small bluebottles, also called Portuguese man-o-wars (Physalia utriculus), by-the-wind sailors (Velella velella) and Porpita (Porpita pacifica) and the glaucus or blue sea slug (Glaucus atlanticus).

One weird thing about bluebottles is that they are not one animal, however much they may look like it. Each one is a conglomeration of ‘zooids’, each group of these with a different function, such as a group of stingers and a group of digesting zooids. The whole lot floats around on the surface being blown by wind and drifting on currents (there are ‘left-handed’ and ‘right-handed’ ones that get turned in different directions by the wind) and catching and eating fish, and reproducing.

You can get stung walking on them or their stingers in the sand in bare feet, so watch out if they are about. The stingers are invisible in water, and can extend a long way, so even if you are swimming and see the blue body, you may not see the tentacles until too late. The sting is not usually fatal, but is apparently very painful.

You can get pictures and information on bluebottles, at

Bluebottles get eaten by glaucus. You may be lucky to see the gorgeous glaucus, washed up on shore in a crumpled form. See photos of glaucus and more information at

You can allow the glaucus to unfold by putting it in an abandoned shell with seawater – it is worth it to see such a strange and wonderful creature.

You can find out about Velella at and and

So all these critters live more or less together on the surface of the deep ocean. The purple is a camouflage when they are viewed from underneath, so they blend in with the blue of the sky.

Another purple wonder often washed up is the violet snail Janthina. It also associates with this lot, floating about and feeding on the jellyfish-like animals. It secretes a set of bubbles that help it float on the surface near the bluebottles, on which it munches. Sea slugs and sea turtles also eat bluebottles. See for photos and info.