Difference Between Crustaceans and Molluscs (Mollusks)

The key difference between crustaceans and molluscs is that the crustaceans have segmented bodies while molluscs have unsegmented soft bodies.

Kingdom Animalia constitutes major groups of animals. All the animals in kingdom Animalia are eukaryotes and multicellular heterotrophs, and most of them can move. Moreover, kingdom Animalia also has several different phyla. Among them, phylum Arthropoda includes all insects while phylum Mollusca includes terrestrial and aquatic invertebrates which have soft bodies covered with an exoskeleton (shell). Phylum Arthropoda and Phylum Mollusca are the largest diversified groups with a great number of species in Animalia kingdom. Crustaceans are a taxon of phylum Arthropoda. Likewise, this article outlines the features of each phylum and thereby attempts to explain the difference between crustaceans and molluscs (mollusks).

CONTENTS

1. Overview and Key Difference
2. What are Crustaceans
3. What are Molluscs
4. Similarities Between Crustaceans and Molluscs
5. Side by Side Comparison – Crustaceans vs Molluscs in Tabular Form
6. Summary

What are Crustaceans?

Taxon crustacea come under phylum arthropods and comprise of 35,000 species. The unique features of arthropods are the presence of jointed appendages, tough chitinous exoskeleton, compound eyes, and endocrine system. Thus, the whole body of crustaceans has two prominent parts; abdomen and cephalothorax (Cephalon and thorax fused to form a cephalothorax). Shield-like carapace closes the cephalothorax. Moreover, these creatures have three pairs of appendages as mouth parts, two pairs of antennae, and several pairs of legs. Also, their number of leg pairs varies among the species.

The unique feature of crustaceans is the presence of two pairs of antennae that that is not present in other arthropods. Furthermore, all the segmented appendages (except the first pair of antennae) of crustaceans are biramous and present in all the body segments. Generally, 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 (e.g., pillbugs), and some species are semi-terrestrial (e.g., sand fleas or beach fleas).

Also, 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 a food source for humans. In larger crustaceans, feathery gills act as respiratory organs, whereas in smaller crustaceans, gas exchange takes place through their cuticle. The characteristic larval form of crustacean is called ‘nauplius’.

What are Molluscs (Mollusks)?

Phylum Mollusca is the second largest and extremely diverse group with more than 110,000 identified species in kingdom Animalia. Molluscs live in various types of environments including both aquatic and terrestrial ecosystems. Further, this group includes snails, slugs, scallops, clams, octopuses, cuttlefish, oysters, etc. The body size of molluscs varies from microscopic to huge. For example, the largest molluscs are the giant squid with the body size of 15 m long and weigh up to about 250 kg.

Molluscs have soft unsegmented bodies. The characteristic feature of all molluscs is the presence of mantle; a thick epidermis, which covers the dorsal side of the body. Some molluscs have outer calcareous shells, which is secreted by the mantle. All the molluscs except cephalopods have a muscular foot as the organ of locomotion. Other than for locomotion, muscular foot serves many other functions such as 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 molluscs. Some molluscs such as oysters, clams, scallops, mussels, octopuses, and squids are important food sources of humans.

What are the Similarities Between Crustaceans and Molluscs?

• Crustaceans and molluscs are animals that belong to kingdom Animalia.
• Both are important food sources of humans.
• Also, both groups have aquatic and terrestrial members.
• Furthermore, they are invertebrates.
• And, both groups include marine and freshwater
• Besides, they possess an exoskeleton.

What is the Difference Between Crustaceans and Molluscs?

Crustaceans and molluscs are two different groups of animals. Crustaceans belong to a taxon of phylum arthropods while Mollusca is a major phylum. The crustaceans are insects that have segmented bodies. On the other hand, molluscs have soft bodies which are unsegmented. Therefore, this is the key difference between crustaceans and molluscs. Furthermore, crustaceans have a chitinous exoskeleton, whereas some molluscs have calcareous shells. Hence, this is also a difference between crustaceans and molluscs.

Moreover, a further difference between crustaceans and molluscs is that the crustaceans possess segmented biramous appendages that are not present in molluscs. Additionally, a significant difference between crustaceans and molluscs is that, in crustaceans, the body is divided into two parts; cephalothorax and abdomen. But, there is no such division found in molluscs. Also, molluscs possess muscular foot for various activities, unlike crustaceans. Hence, it is a structural difference between crustaceans and molluscs.

Besides, mollusca constitutes of more than 110,000 species, whereas crustacea have about 35,000 identified species. The characteristic larval form of crustacean is called ‘nauplius’, whereas that of molluscs is trochophore. Therefore, this is one more difference between crustaceans and molluscs.

Summary – Crustaceans vs Molluscs

In summary of the difference between crustaceans and molluscs; the crustaceans are a taxon that belongs to phylum arthropods of kingdom Animalia. All crustaceans are insects with segmented bodies. On the other hand, Mollusca is another phylum of kingdom Animalia that includes aquatic and terrestrial invertebrates that possess unsegmented soft bodies. This is the key difference between crustaceans and molluscs. Furthermore, crustaceans have chitinous exoskeletons while molluscs have calcareous exoskeletons.  Moreover, crustaceans have biramous appendages while molluscs do not have.

Reference:

1. “Mollusc.” NeuroImage, Academic Press. Available here 
2. “Crustacean.” Wikipedia, Wikimedia Foundation, 16 Jan. 2019. Available here 

Image Courtesy:

1.”2699178″ by Akiroq (CC0) via pixabay
2.”Helix pomatia 89a”By Waugsberg – Own work, (CC BY-SA 3.0) via Commons Wikimedia