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Mollusks - Types & Characteristics

Mollusca is a huge, incredibly diverse group of invertebrate animals commonly called mollusks, or molluscs. In fact, they are the second-largest phylum after Arthropoda. Mollusca comprises of more than 50,000 different species, but some estimate that there could be as many as 200,000.  

Even though most Mollusk species are found in marine ecosystems, some species are terrestrial. Mollusks have an almost worldwide distribution, from the most common habitats to the most extreme ones, such as the depths of the ocean. Their size ranges from 1 mm to 20 m. 

Let's take a closer look at this amazing group of animals and see their main characteristics and the different types of mollusks.

7 Characteristics of Mollusks

  1. Soft body & shell: Mollusks are invertebrates (they don’t have a backbone) and have a soft body. This soft body is protected by a calcareous shell that they produce via specialized glands. In some species, the shell is small and internal (like slugs for example).
  2. Muscular foot for locomotion: Mollusks’ body sits on a muscular foot. The foot has evolved into arms in the case of Cephalopods (octopuses).
  3. Mantle & mantle cavity: Mollusks have their body wrapped in a mass of flesh called the mantle, protecting their internal organs. It is the mantle that produces the shell. They also have a cavity between the mantle and the internal organs. The cavity contains gils for aquatic mollusks, and a lung for terrestrial mollusks.
  4. Radula for feeding: Many mollusks have a radula, which s a kind of tongue with chitinous teeth. Bivalves do not have a radula.
  5. Organs & organ systems: The mollusks’ body is organized in organs (contained in the part of their body called the visceral mass) and organ systems: nervous system, digestive system, circulatory system, respiratory system, etc.
  6. Open circulatory system: Except in Cephalopods, the mollusks’ circulatory system is open type, which means that their blood (called hemolymph) does not flow in closed blood vessels.
  7. Larva stage: Mollusks start their life as tiny free-swimming larvae called trochophores, veliger (part of zooplankton) that look different from the adults. Or sometimes, the larvae look like miniature adults.

These are just a basic list of main characteristics of mollusks, showing that they are complex, really interesting creatures that are perfectly adapted to their environment. Mollusks are very important to our world, not only with their roles in their respective ecosystems, but also as an important source of food for Humans.

Mollusks - Cuttlefish

The 7 Types of Mollusks

The phylum Mollusca is divided into 7 different classes, but most people are familiar with 3 of them: Gastropoda (such as snails), Cephalopoda (such as octopuses) and Bivalvia (such as clams). Keep reading to learn more about the 7 types of mollusks that exist today on our planet.

Frequently Asked Questions About Mollusks

Most mollusks have separated male and female sexes (and some are hermaphrodites) and generally reproduce sexually. Eggs are released into the mantle cavity, and the fertilization of the eggs can take place internally or externally. These eggs will then become larvae and evolve into adult mollusks.

The mollusks’ diets of course depend on the species and their size, but many are herbivores and feed on algae or other plants. Cephalopods are carnivores and prey on fish, other smaller cephalopods, crustaceans or bivalves, for example.

Aquatic mollusks breathe with gills, called ctenidia, enabling them to exchange gases between their blood and the water that surrounds them. Their gills are located in the mantle cavity.

Yes, all mollusks have a mantle, it is one of their shared characteristics.

Bivalves eat by filtering the water around them. Their gills are equipped with tiny hair (called cilia) that are able to capture food particles and plankton they feed on. 

Certain mollusks like snails, slugs, clams or mussels are considered decomposers because they consume organic matter.

Cephalopods possess specialized cells called chromatophores under their skin. These cells are full of pigments of different colors (they can be black, brown, orange, red or yellow). By expanding or contracting their chromatophores, cephalopods are able to change color instantly.

Learn More About Mollusk Species

Links to articles packed with surprising facts and knowledge to further learn about amazing species of mollusks, so you know what you are looking at on your next wildlife trip!