The shore is organised in stripes
Stand back from a rocky COVE at low water and the ROCKS are banded, like a very slow rainbow. Grey-white BARNACLE crust at the top, then a belt of dark MUSSEL and PERIWINKLE, then brown wrack, then KELP at the bottom where the sea barely leaves.
The bands are a chart of how long each species can survive out of water. A barnacle can be dry and baking for hours; a kelp cannot manage it at all. Every creature sits as high up the shore as it can stand, because higher means fewer things able to eat it — and the line where a band stops is usually not the limit of its endurance but the point where something tougher outcompetes it.
A limpet goes home
A LIMPET is not stuck to the rock at random. Each one has a home scar — a shallow groove it has ground into the stone, matching the exact shape of its own shell — and it returns to that spot every single time the TIDE goes out, clamping down into a seal nothing can prise open.
In between it wanders off to graze, rasping algae off the rock with a tongue carrying teeth made of the strongest biological material yet measured, stronger for its weight than spider silk. Then it finds its way back, following its own mucus trail like a thread. The same limpet, the same groove, for years.
Hermit crabs queue for houses
A hermit CRAB has a soft back end and borrows an empty SHELL from a dead WHELK or PERIWINKLE. The problem is that it keeps growing and the shell does not, so it must keep moving up — and empty shells of the right size are the scarcest thing on the shore.
What happens next is startling. When a good new shell appears, hermit crabs gather and line up by size, largest first, holding on to one another. The moment the biggest moves in, the next takes the shell it vacated, and the swap runs down the whole chain in seconds. Biologists call it a vacancy chain, and it is the same arrangement people use for housing, arrived at independently by a crab.
The blob that fights
An ANEMONE looks like the most passive thing in the pool. Out of water it pulls its tentacles in and sits there as a wet red lump; in water it waves about and stings passing SHRIMP and PRAWN into submission.
It is also territorial. Beadlet anemones carry a ring of blue stinging beads under their tentacles, and where two colonies meet they fight along the border for hours, pressing the beads against each other until one gives ground. Some can clone themselves, so a whole stretch of rock may be one individual repeated, at war with the identical twins on the next boulder along.
Rules for a rock pool
The grid holds the residents: the CRAB, ANEMONE, STARFISH and spiny URCHIN, the clinging LIMPET and BARNACLE, the MUSSEL, COCKLE, WHELK and SNAIL, a SHRIMP and PRAWN in the water, a SPONGE and the SEAWEED and KELP they all shelter under, plus a smooth PEBBLE and a SHELL or two.
The etiquette matters more here than in most habitats. Turn a rock over and you must turn it back the same way up, or everything living underneath dies in the sun. Look rather than grab, keep animals in water, and put them where you found them — a ROCKPOOL is a place where a careless afternoon does real damage, and a careful one costs nothing at all.