Difference Between Crustaceans and Molluscs (Mollusks)

Crustaceans vs Molluscs (Mollusks)

Crustaceans and molluscs (mollusks) are two major groups of invertebrates in Kingdom Animalia with a distinct difference between them. Crustaceans are categorized under major Phylum Arthropoda, whereas Mollusc a is considered as a major phylum. Phylum Arthropoda and Phylum Mollusca are the largest diversified groups with great number of species in animal Kingdom. This article outlines the features of each phylum, and thereby attempts to explain the difference between crustaceans and molluscs (mollusks).

What are Crustaceans?

Class Crustacea comes under the phylum Arthropoda and comprises of 35,000 species. The unique features of Arthropods are the presence of jointed appendages, tough chitinous exoskeleton, compound eyes, and endocrine system. The whole body of crustaceans are divided into two prominent parts; abdomen and cephalothorax (cephalon and thorax fused to form a cephalothorax). Shield-like carapaceen closes the cephalothorax. These creatures have three pairs of appendages as mouth parts, two pairs of antennae, and several pairs of legs. The number of leg pairs vary with the species. The most unique feature of crustaceans is the presence of two pairs of antennae that cannot be found in other arthropods. All the segmented appendages (except the first pair of antennae) are biramous and found on all the body segments. All crustaceans are largely aquatic and can be found in both marine and freshwater habitats. Marine crustaceans are crabs, shrimps, lobsters and barnacles while freshwater crustaceans include some crayfish, crabs and copepods. Few species are terrestrial (ex: pillbugs) and some species (ex: sand fleas or beach fleas) are semi terrestrial. Planktonic crustaceans like krill and larval crustaceans act as the primary food source in marine ecosystems. Some crustaceans like lobsters and crayfish are important as food source for humans. In larger crustaceans, feathery gills are used as respiratory organs, whereas in smaller crustaceans, gas exchange takes place through their cuticle. The characteristic larval form of crustacean is called ‘nauplius.’

Difference Between Crustaceans and Molluscs (Mollusks)

Antarctic Krill 

What are Mollusks (Molluscs)?

Phylum Mollusca is the second largest extremely diverse group with more than 110,000 identified species. Mollusks live in various type of environments including both aquatic and terrestrial ecosystems. The group includes snails, slugs, scallops, clams, octopuses, cuttlefish, oysters, etc. The body size of mollusks vary from microscopic to huge. The largest mollusk is the giant squid with the body size of 15 m long and weigh up to about 250 kg. The characteristic feature of all mollusks is the presence of mantle, a thick epidermis, which covers the dorsal side of the body. Some mollusks have outer calcareous shells, which is secreted by the mantle. All the mollusks except cephalopods have muscular foot as the organ of locomotion. Other than for locomotion, muscular foot is also used for attachment, food capture, digging, etc. All the organs including excretory, digestive and reproductive organs are found in a visceral mass. Trochophore and veliger larval forms are characteristics to mollusks. Mollusks like oysters, clams, scallops, muscles, octopuses, and squids are considered as important food sources of humans.

Crustaceans vs Molluscs (Mollusks)

Anatomy of a mollusk

What is the difference between Crustaceans and Molluscs (Mollusks )?

• Crustaceans belong to phylum Arthropoda, whereas mollusks are considered as a major phylum.

• Crustaceans have chitinous exoskeleton, whereas some mollusks have calcareous shells.

• Unlike mollusks, crustacean exhibits segmented biramous appendages.

• In crustaceans, the body is divided into two parts; cephalothorax and abdomen. But there is no such division found in mollusks.

• Mollusks possess muscular foot for various activities unlike crustaceans.

• Mollusca comprises more than 110,000 species, whereas Crustacea has about 35,000 identified species.

• The characteristic larval form of crustacean is called ‘nauplius’, whereas that of mollusks is Trochophore.


Images Courtesy:

  1. Antarctic Krill by Uwe Kils (CC BY-SA 3.0)
  2. Anatomy of a mollusk by KDS444 (CC BY-SA 3.0)