Marine Invertebrates
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includes freshwater invertebrates


Over 95% of the described animal species are invertebrates. "Invertebrates" are not a taxon, but a grouping of convenience, which simply signifies that these animals are not vertebrates, i.e., they lack a skeleton of bone, either internal or external. However, many invertebrates have hard exoskeletons or outer shells (e.g., crustaceans, echinoderms, molluscs, and corals), or hydrostatic or fluid-filled skeletons (e.g., jellyfishes, leeches, etc.).

There are probably at least 1,305,000 known species of invertebrates, although it may be as many as 10,000,000. Of these, nearly 70% are terrestrial. Most diverse marine groups include Mollusks (85,000 species), Crustaceans (47,000), Cnidarians (10,000), Sponges (9,000), and Echinoderms (7,000).

On this website, invertebrates, other than Insects, are divided into "marine" (this page) and "terrestrial", but note that the former includes freshwater organisms. Although some invertebrate phyla are either aquatic or land-dwelling, others, such as Arthropoda and Mollusca, inhabit both dry land and water.


Sponges (Porifera)
Comb Jellies (Ctenophora)
Hydrozoa (Hydroid and Siphonophores)
Jellyfish (Scyphozoa)
Corals and Sea Anemones (Anthozoa)
Horseshoe Crabs (Xiphosura)
Crustaceans (Crustacea)
Mollusks (Mollusca)
Chitons (Polyplacophora)
Aquatic Snails (Gastropoda)
Octopus and Squid (Cephalopoda)
Bivalves (Bivalvia)
Segmented Worms (Annelida)
Polychaete Worms (Polychaeta)
Leeches (Hirudinea)
Echinoderms (Echinodermata)
Sea Stars (Asteroidea)
Sea Urchins (Echinoidea)
Sea Cucumbers (Holothuroidea)