Everything below came off the same half mile of shore in Mount's Bay. Most of it is small and most of it is brown, and four of the five are things you would step over.
Cuvier, 1803. The books call it the sea hare, which is generous. Ours run about 7 or 8 cm, though the species reaches 20. The shell is still in there, flat and transparent and amber, roughly 4 cm long, buried under the skin of the back where it holds nothing in and keeps nothing out. Two pairs of tentacles: the front pair frames the mouth, the back pair are rhinophores and do actually smell, and they are the reason for the hare. The flaps along the flanks are parapodia, joined high at the rear in this species, which is why, unlike some of its Mediterranean relatives, it cannot really swim. Lift one and it hangs in your palm with the weight and general presence of a wet slipper.
Colour follows lunch. Juveniles grazing soft red weeds like Ceramium and Polysiphonia are red. Adults that have moved on to Fucus and Ulva go brown or olive.
The ink is the good part. Disturb one and it lets go a cloud of purple, and the purple is built from r-phycoerythrin, the accessory pigment of the red seaweed it has been eating: taken apart in the gut, chemically altered, carried in the blood and concentrated in a gland for later. A sea hare raised on green algae has no purple to give. Its defence is a stored copy of its own diet.
Do not lift them out of the water to get a better photograph. Inking costs the animal something it cannot replace quickly, and you are the predator in that transaction.
Aeolidia, the grey sea slug, eats sea anemones. On this shore mostly Actinia equina, also Actinia fragacea and Anemonia viridis, and Metridium senile lower down. Sit with what that requires. An anemone is a bag of loaded harpoons. The slug bites in, swallows the harpoons without setting them off, moves the undischarged ones intact through its own gut, and parks them in sacs at the tips of the finger-like cerata along its back. While it feeds it wraps its head in thick mucus, which the firing cells discharge into harmlessly.
The tidy version of the story is that the stolen cells then serve as the slug's own armament. That is probably true and it is not settled. A 2009 paper in Marine Biology comparing cnidosac structure across the aeolids and the dendronotids put a question mark at the end of its own title, which is the correct quantity of confidence.
Common limpet. Linnaeus, 1758. It leaves its home scar to graze, wanders a few hours, and comes back to the same patch of rock by following the chemical trail of its own mucus. The scar is a socket ground into the stone by the shell until the fit is close enough to hold water when the tide drops.
It grinds with a radula longer than the shell it lives under, and the teeth on that radula are reinforced with nanofibres of goethite, an iron oxide, set in protein. Barber, Lu and Pugno measured their tensile strength in 2015 at 3.0 to 6.5 GPa, the highest recorded for any biological material, above spider silk. So the strongest material made by anything alive is used by a limpet to file a hole in a rock so that it can sit in the hole.
Cushion star. Pennant, 1777. About 5 cm across, pentagonal, arms so short and blunt they read as corners. Lives under boulders and overhangs from the lower shore down, and feeds by turning its stomach inside out onto the film of diatoms and bacteria on the stone.
It starts male and becomes female later. And it has no planktonic stage at all: eggs go down in a clutch glued under a rock and develop straight through, no drifting. So the cushion stars under one boulder field are, fairly literally, that boulder field's own.
Linnaeus, 1767. Up to 15 mm across, six chalky plates, and oddly hard to find in the far south west, because Chthamalus montagui takes the upper shore here and Semibalanus thins out towards Land's End.
nothing more than a little shrimplike creature, standing on its head in a limestone house kicking food into its mouth
That is Louis Agassiz, quoted so often that I have stopped trying to find the original, and it is exactly right. The animal is a crustacean glued down by its forehead. The six pairs of feathery cirri are thoracic limbs, and it beats them rhythmically in still water and simply holds them out in a current, which is the difference between kicking and fishing.
It is a cross-fertilising hermaphrodite with a penis substantially longer than its own body, because it has to reach a neighbour it cannot walk to. Copulation on this coast runs from November into early December, the embryos are held through the winter, and the nauplii go out between February and April to meet the spring diatom bloom. Six naupliar stages, then a cyprid, and the cyprid is the one that decides: it walks about on its antennules, tests the surface, and cements its own head to it. It chose that square centimetre once, as a larva, and lived out everything that came after without moving; I have not been able to walk carelessly across barnacled rock since I understood that.