The mechanical properties of the mesogloea of sea anemones and hydroids determine what kind of water flow the animal can withstand and hence where it can live and what shapes it can assume. Short, squat sea anemones such as Actinia equine are common in shallow rock pools on beaches, usually in small crevices or under ledges. They remain upright and open with their stubby tentacles exposed even when pounded by waves and tidal surges, because their mesogloea is relatively rigid but elastic, storing the energy of deformation and recoiling when released, like a tennis ball. Tall sea anemones such as Metridium sp. are restricted to calm waters below the regions exposed by tides, where they can project their crown of long, delicate tentacles into slow-flowing currents. Slight differences in the composition and arrangement of the macromolecules make their mesogloea more plastic, enabling them to extend slowly from a compressed to a greatly elongated shape. Being on a beach in a storm would rip them to shreds.

As well as enabling them to detect predators attacking from any direction, circular animals can withstand buffeting by water currents from many different directions.

Both the body form and the chemical composition and crystal structure of the secreted skeleton determine the corals’ ability to withstand wave action and tides. Sever storms and tidal waves, or even just he wash from passing ships, often break up coral skeletons or smother the living tissues in sand and rubble.

Many corals collect particulate food and/or prey on small animals at night and during dull weather. Non-predatory species lack tentacles, but all corals have numerous cnidocytes on their surface, some of which, as divers have found, contain toxins that can ‘sting’ people quite badly.

Like plants, sea anemones and corals (and other sessile animals, including sponges) are vulnerable to grazing animals. Several kinds of echinoderms and fish, including the parrot fish, eat the coral itself. Reefs support a rich variety of animals. Many fish that live in the open ocean as adults breed on reefs: their eggs and juveniles need the shelter and abundant food that reefs provide.

Most natural coral reefs are near coasts or surround islands and have been polluted and /or badly over-fished by the people who live there. However, underwater cables, offshore oilrigs and sunken ships and aircraft also make ideal substrata for sessile invertebrates including corals, sea anemones and sponges. Many such artificial reefs support thriving colonies of other kinds of invertebrates and fish. Harvests of many kinds of fish are noticeably better near old wrecks sunk in shallow water.

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