Five things to look for in a rockpool

A good rockpool reveals itself slowly. Walk up to the rocky platform an hour before low tide, sit on the edge, and let the water clear. Movement is the first thing the eye learns to ignore — the wave-suck and bubble-rise of the upper pool — until shapes begin to settle out of the texture. The longer the eye stays, the more there is.

This is a short guide to the five sightings that reward that patience most. Roughly in the order they emerge as the tide drops on a southern Australian rocky shore.

1. Anemones, opening as the water steadies

The first creatures to declare themselves are the anemones. When the pool is freshly cut off from the open sea and the surface is still settling, an anemone reads as a closed dome — a glossy red-brown blob clinging to the rock, looking nothing like an animal. Wait two or three minutes. As the water stills, the dome opens into a crown of short tentacles arranged in concentric rings around a central mouth disc.

The species you are almost certainly looking at on the east and south coasts is Actinia tenebrosa, the waratah anemone — named for the resemblance of its closed body to the waratah inflorescence. The tentacles, when open, are stubby and arranged in three or four rings.

In deeper pools, look for the snakelocks anemone (Anemonia), which never closes the same way — its long flowing tentacles trail outward like a Medusa head, sometimes greenish, sometimes pale brown.

The signal that you are early enough: the anemones are open. If most of the pool's anemones are still closed, sit a few minutes longer.

2. Sea stars, often half-hidden under ledges

Australian rocky-shore sea stars are not the five-armed icons of children's books. The common species along the east coast is Coscinasterias muricata, the eleven-arm sea star — though its arm count ranges from seven to fourteen, never the cartoon five. The arms are slender and slightly mismatched in length, a sign the animal regrows them after damage.

Sea stars favour the sides and undersides of rock ledges where the water stays cool. Crouch and look into the shadowed seams of the pool's edges, not the open centre. They move on hundreds of tube-feet — almost too slowly to see in real time but quickly enough that a sea star noted on the way in may have shifted by the walk out.

A separate, much smaller find: the biscuit star (Tosia australis). A flat pentagonal animal about the size of a fifty-cent piece, with stubby arms that barely project beyond the body. Easier to miss because it sits flush on the rock and looks, at first glance, like a piece of decorative coral.

3. Urchins, in the deeper crevices

Sea urchins prefer the lower zone of the pool — closer to the open sea, less likely to be exposed by a particularly low tide. The species most commonly seen in southern Australian rockpools is Heliocidaris erythrogramma, the purple sea urchin — a tight ball of densely packed radiating spines, the colour ranging from olive green to deep purple.

Further north, into central New South Wales and beyond, watch for Centrostephanus rodgersii, the long-spined urchin. The body is smaller than Heliocidaris but the spines are dramatically longer — extended needles radiating in all directions. Its range has been steadily moving south with warming water; thirty years ago you would not have found it in Tasmanian waters, and now it is well established there.

A note on touching: don't. Urchin spines break off cleanly into skin and the pieces can take days to work their way back out.

4. Shells with their occupants still inside

The empty shells of the rockpool — the ones that wash up on the surrounding sand — are the easy find. The interesting find is the same shell still occupied.

Look closely at any turban shell (Turbo) sitting in the pool: if the white shelly disc covering the opening (the operculum) is in place, the animal is alive inside. The operculum is the snail's trapdoor — pulled shut whenever the animal retracts.

A chiton is the more rewarding spot. Eight overlapping armoured plates running the length of an oval body, clinging tight to the rock surface. Chitons are easy to miss because they so perfectly match the rock — a flat slug-like shape, often crusted with the same encrusting algae as the substrate around them. Move your eye slowly across a flat-topped boulder at the pool's edge and the chitons resolve.

The limpet (Cellana) is the other near-invisible mollusc — a conical hat-shaped shell, smaller than a coin, clinging vertically to the rock face. Limpets retreat to the exact same scar each low tide; the rock face under a limpet is sometimes worn into the limpet's exact shape.

5. Crustaceans, but only if you are still

The pool's crustaceans — the small crabs and hermit crabs — only appear if the observer is still. Approach loudly or cast a shadow and they vanish into the rock crevices for the next ten minutes.

Sit, let the pool settle, and watch the open patches of sand or coralline-encrusted rock. The first movement will probably be a hermit crab — a borrowed turban shell making its way across the bottom on the crab's protruding legs. The shell looks animate in a way no other rockpool feature does; the contradiction of a shell walking is what gives the hermit crab its visual punch.

The other crustacean to watch for is the rock crab (Plagusia), a broad flattened crab the size of a matchbox, well-camouflaged against the algal mat. Rock crabs scuttle sideways across pool floors in short dashes and freeze for long periods between movements. The frozen moments are when they look like part of the rock.

What you'll probably miss but can come back for

The nudibranchs — sea slugs in extraordinary colours — are technically present in many southern Australian rockpools but they are hard to see in a single visit. Their colour pattern is camouflage against the encrusting sponges and bryozoans they eat. Coming back to the same pool five or six times improves the odds; species are highly localised.

Same with the feather star — a small flowering form like a starfish wearing a fern. Feather stars sit in dark crevices, and they only extend their feathered arms when feeding. A pool watched casually shows none; a pool watched patiently in dim light may show two or three.

The slow secret of a rockpool is that the same pool, returned to often, becomes richer each visit. The first few times you mostly see anemones and snails. By the tenth visit you are noticing brittle stars under the ledges, nudibranchs against the orange sponges, the way the algal community shifts with the season. The plates in Rockpool Folio Volume I cover this whole community — sea stars, anemones, molluscs, crustaceans, and the algal flora — at folio scale, with each plate naming its subject by common and scientific name.