Black-striped Triton, Septa hepatica


Delve into this amazing group of animals that include snails, clams, nudibranchs, squid, and various other related creatures that are found throughout Queensland waters and forests.


Molluscs are soft-bodied invertebrates usually possessing an external shell for protection and a foot for locomotion. There are some groups (e.g. land slugs, nudibranchs, octopuses) that have no external shell. Aquatic species have gills for respiration while most land snails have a primitive lung. The digestive and reproductive tissues are located together. A large fold of tissue, known as the mantle, not only produces the shell but also acts as a protective sleeve for the head and gills.  

This phylum (or major animal group) consists of 7 subdivisions known as Classes - the five main ones being:

  • Gastropods  – marine snails, nudibranchs, and land snails and slugs
  • Bivalves  – two-valved shells including clams, oysters, scallops, mussels
  • Cephalopods  – squid, cuttlefish, octopuses and nautilus
  • Chitons – molluscs with an 8-plated shell
  • Tusk Shells – tooth-like molluscs with a single curved shell.

Each Class contains many families, genera and species. The entire phylum may contain as many as 100,000-200,000 species worldwide, of which probably 10,000-15,000 occur in Australia.

Common questions

No. Although numerous species of marine snails, land snails, bivalves and chitons have hard external shells, there are other groups of molluscs such as land slugs, nudibranchs, squid and octopus that have no external shell.

The Colossal Squid, Mesonychoteuthis hamiltoni, is the largest mollusc in terms of mass between 500 kilograms and 700 kilograms. However, the Giant Squid, Architeuthis dux, can reach a greater overall length between 12 meters to 13 meters, whereas the Colossal Squid may reach 9 meters to 10 meters.

No. While most marine snails, land snails and bivalves can withdraw into their shell for protection, there are species such as marine bubble snails (e.g., Hydatina physis) and terrestrial semi-slugs (e.g., Stanisicarion virens) that have a much-reduced shell. These groups have developed other means to cope with predators and/or dehydration.

No. Although barnacles have hard calcium plates resembling a mollusc shell, they are actually crustaceans, and are related to shrimps, lobsters and crabs. Brachiopods also have hard shell-like valves resembling those of molluscan clams, but they have a completely different internal anatomy and are placed in their own distinct phylum.



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