Invertebrates are animals lacking a backbone and the group includes most of the animal species in the world. This main group is subdivided into other subgroups, among them:

Arthropods
This is a highly diverse group that includes animals from all ecological environments such as insects (bugs, butterflies, bees, ants, flies), crustaceans (, crabs, shrimp, crayfish, barnacles, Hermit crab) and arachnids (spiders, scorpions, mites, ticks).

Cnidaria
They are quite simple species of animals found exclusively in aquatic environments, predominantly in the sea, but also in shallow waters. This group includes animals such as jellyfish, sea anemones, Portuguese man o' war and corals.

Annelids or Worms
Annelids are segmented worms with a long slender body. Each segment is split into one or more rings and worms have the particularity of regenerating body segments that were amputated; they use this particularity also for reproduction purposes. Earthworms, leeches, lugworms are annelids.

Echinoderms
They are a group of marine animals that include (sea stars, brittle stars, sea urchins, sea cucumbers, crinoids) and others. Echinoderms are found at every ocean depth.

Molluscs
Molluscs have a soft unsegmented body and often an external shell. This group has highly diverse animals, not only in size and in anatomical structure, but also in behaviour and in habitat; it comprises the largest group of all the named marine organisms, but numerous molluscs also live in freshwater and terrestrial habitats. Molluscs comprise three classes: gastropods (such as the whelk), bivalves (such as mussels, oysters), and cephalopods (such as octopus, squids, cuttlefish).