The bright and elegant coral shrimps (Stenopus hispidus) (STENOPODIDAe) are widely spread in all tropical waters. In the wild and in an aquarium they form stable pairs. The shrimps, as all crustacean, have the rigid chitin carapace preventing them from constant body growth, so that from time to time they moult. While the "shells" are soft, the animals are very vulnerable until it becomes hard.

The majority of sea urchins (ECHINOIDEA) are of the spherical form. Their body is enclosed with hard test. The colouring of urchins is various: white, black, red. The mouth is on the surface normally directed toward the substrate (oral) and the anus on the surface away from the substrate (aboral). Hundreds of movable spines project from the test and are used for locomotion. Tube feet and pedicellaria, jaw-like structures set on movable stalks and often containing poison glands, occur on the test's surface.

The mouth contains a masticatory apparatus, the Aristotle's lantern, containing five calcareous teeth used for grazing. Sea-urchins are nocturnal. In the daytime they are immobile and hide themselves in the reefs, exposing spines outside. Their diet consists of algae, sponges, tunicates and soft corals. Their size varies from 2-3 up to 20 sm., while length of spines – from 1-2 mm up to 30 sm. The spines protect urchins against predators.