What Is A Crustacean? The Ultimate Guide To Crustaceans

What Is A Crustacean? Crabs are the best-known crustaceans, but this large group of animals also includes lobsters, shrimps, barnacles, woodlice and many other related animals.

On this page you’ll find out what makes a crustacean a crustacean . You’ll also meet the different types of crustacean.

Read on to find out more about these fascinating invertebrates…


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What Is A Crustacean?

New River Crayfish Crustacean
Crayfish such as this New River crayfish (Cambarus chasmodactylus) are crustaceans.

A crustacean is an invertebrate animal with a hard exoskeleton, segmented body and jointed legs.

Most crustaceans are fully aquatic, although some, including many crabs, are semi-aquatic, and others, such as woodlice, are fully terrestrial (land-dwelling).

There are around 80,000 known species of Crustacean. (Source) The best-known crustaceans include crabs, lobsters, and shrimps, but the group contains several other related types of animal including krill and barnacles.

Crustaceans are part of a much larger group of animals known as arthropods. This group includes insects and arachnids, together with several other groups of related animals.

The two main characteristics of crustaceans that separate them from other arthropods are: having two pairs of antennae; and having biramous legs (i.e. legs that split into two branches).

(You’ll find out more about the characteristics of crustaceans further down the page.)

The world’s largest living arthropod – the Japanese spider crab, and the smallest (tiny parasitic animals such as Stygotantulus stocki) – are crustaceans.

Japanese spider crab - the largest crustacean
Japanese spider crab – the world’s largest crustacean (and arthropod). Photo: Tsarli at English Wikipedia (Cropped / resized by ActiveWild.com) [CC BY-SA 3.0]
People who study crustaceans are known as carcinologists, and the study of crustaceans is known as carcinology.


Crustaceans Are Arthropods

Ghost Crab
Ghost Crab

A crustacean is an invertebrate animal in the subphylum Crustacea. Crustacea is part of the phylum Arthropoda – a large group of invertebrate animals whose members are known as arthropods.

Note: terms such as ‘phylum’, ‘subphylum’, ‘class’ and ‘family’ refer to groups of related animals. You can find out more about how animals are grouped on this page: Animal Classification.

All arthropods – be they insects, arachnids or crustaceans – have exoskeletons, jointed limbs and segmented bodies. If you go far enough back in the family trees of all living arthropods you’ll find that they all share the same ancestor.

This ancestral arthropod was probably a carnivorous marine animal with an exoskeleton and a segmented body, with each segment possessing a pair of limbs. It would probably have lived around 540 million years ago.

You can find out more about other arthropods on these pages:


As arthropods evolved, they branched off into different groups.

There are two main branches within Arthropoda. Arthropods with mouthparts known as mandibles form the group Mandibulata; those with mouthparts known as chelicerae form the group Chelicerata.

Mandibles are used for manipulating, biting, and chewing food and other objects. Crustaceans have mandibles, and therefore belong to Mandibulata.

Other arthropods with mandibles include myriapods (millipedes, centipedes and related animals), and hexapods (insects and other related, six-legged arthropods).

Millipede
Millipedes aren’t crustaceans, but both are in mandibulata.

Arthropods with chelicerae include arachnids (spiders, scorpions and related animals), sea spiders and horseshoe crabs.

Pancrustacea

wasp insect
Crustaceans, together with insects (such as this European wasp) and other hexapods, form a group known as Pancrustacea. Photo by Richard Bartz, Munich aka Makro Freak [CC BY-SA 2.5]
Crustaceans and hexapods together form a group known as Pancrustacea. Hexapods (including insects) are the most closely-related animals to crustaceans. In fact, some crustaceans are more closely-related to hexapods than they are to other crustaceans.

Whereas insects and other hexapods mastered terrestrial habitats, crustaceans are today’s arthropod rulers of the oceans and freshwater habitats.

Some biologists believe that insects branched off from crustaceans, and for this reason should be considered to be ‘terrestrial crustaceans’.


When Did Crustaceans Appear?

Crustaceans are ancient animals, having first appeared during the Cambrian Period, around 511 million years ago. The first crabs appeared during the Jurassic Period, while lobsters appeared during the early Cretaceous Period.


Characteristics of Crustaceans

Virile Crayfish
Virile Crayfish displaying its enlarged front legs, which are known as chelipeds.

Segmented Body

Like insects, the bodies of crustaceans are divided into three main parts: the head, thorax and abdomen. In some crustaceans the head and thorax are fused together to form a cephalothorax. This is often protected by a carapace (shell).

Exoskeleton

All crustaceans possess an exoskeleton. This is periodically molted to allow the crustacean to grow.

Jointed Legs

Like all arthropods, crustaceans have jointed legs, arranged in pairs. (The word arthropod means ‘jointed foot’).

Four Antennae

Unlike other arthropods, crustaceans possess two pairs of antennae. These provide the crustacean with information about its surroundings.

The antennae of some crustaceans are reduced and may not be visible.

Biramous Limbs

At least some of a crustacean’s limbs are biramous. Biramous limbs split into two branches. Crustaceans are the only arthropods to have biramous limbs.

Compound Eyes

Blue Land Crab
A blue land crab displaying its stalked compound eyes.

Most crustaceans have a pair of compound eyes, which are often mounted on stalks. Each compound eye may contain thousands of lenses. Some crustaceans have simple eyes, which contain a single lens. (Source)

Live In Aquatic Environments

Most crustaceans are fully-aquatic, living their entire lives in the oceans or in freshwater. Some crabs spend some or most of their time on land, but only a small number of species are fully terrestrial.

Woodlice are crustaceans that live on land, although most species are dependent on damp habitats to prevent them from drying out.


Crustacean Life Cycle

Crustaceans hatch from eggs and most undergo metamorphosis into their adult forms, often passing through several larval stages as they do so. Each stage is followed by a molt (moult in British English), in which the animal’s exoskeleton is replaced.

Crustacean Larva
A European lobster in its larval form. Photo © Hans Hillewaert (cropped / resized by ActiveWild.com) / [CC BY-SA 4.0]
The larvae of many crustaceans are carried by the tides, joining other small organisms to form plankton. This is an extremely important source of food for many other marine animals.

Even after progressing through the larval stage, many crustaceans have a post-larval stage, during which they resemble miniature versions of the adult stage.

Crabs, for example, go through a number of molts in a post-larval, or juvenile stage. During this time they resemble small adults, but often have tails.

Some crustaceans, most notably isopods such as woodlice, skip the larval stage altogether, and hatch in a post-larval stage.

The larvae of crustaceans often look completely different to their final adult form. For this reason it can be extremely difficult for biologists to identify crustacean larvae, and some larvae remain unidentified or ‘unlinked’ to their final adult forms.

Egg Care

Jaguar Round Crab Carrying Eggs
Jaguar round crab Xantho poressa, female at spawning time, carrying eggs under her abdomen. The Black Sea. Ukraine

While some crustaceans release their fertilized eggs directly into the water, others carry their eggs around with them until they are ready to hatch. The eggs may be stored in a special brood chamber in the body, or attached externally to the pleopods (the ‘swimming legs’, or swimmerets), or to the underside of the abdomen.

Female crabs attach their eggs to their abdomens using a sticky substance. Lobsters and several other types of crustacean attach their eggs to their pleopods.

Crustacean Migrations

Some crustaceans embark on annual migrations in order to breed. Terrestrial crabs such as the Christmas Island red crab make their way from the land into the sea. Other crustaceans, such as some spiny lobster and giant spider crab species, are known for their migrations across the sea bed.


Types of Crustaceans

There are six classes within Crustacea.

Malacostraca

Woodlouse Crustacean
Common striped woodlouse, Philoscia muscorum. Photo: Katja Schulz from Washington, D. C., USA (Cropped / Resized by ActiveWild.com) [CC BY 2.0]
Malacostraca is the largest class of crustaceans, containing around 40,000 species. Among these are well-known crustaceans such as crabs, lobsters, crayfish, shrimp, woodlice and krill.

Decapoda

Crustacean Decapod
Panama City crayfish, a decapod.

Many of these familiar crustaceans are part of the order Decapoda, whose members are known as decapods.

Decapoda contains crabs, crayfish, lobsters, prawns and shrimp. Decapoda means ‘ten feet’; the name refers to a decapod’s ten legs. The front pair of legs of many decapods (crabs, lobsters, etc.) are often modified into pincers known as chelipeds.

The bodies of most malacostracans are comprised of 20 segments divided between the main head, thorax and abdomen sections.

The head and thorax may be fused into a single unit known as a cephalothorax. The crustacean’s compound eyes are found on the head section. They are often mounted on stalks.

Many malacostracans have carapaces (shells) covering some or most of their bodies. Unlike most other crustaceans, malacostracans have appendages on their abdomens.

Crustaceans in the order Malacostraca have numerous pairs of appendages attached to each section of their bodies.

The first appendages are the two pairs of antennae, which provide sensory information.

Next are the mandibles, used to cut and chew food.

After the mandibles are several pairs of food-handling appendages called maxillae. One pair of maxillae has gills and a flat structure used to fan water over the gills.

After the maxillae are the maxillipeds, which provide sensory information. A pair of these also bear gills.

The pereiopods are the crustaceans ‘legs’. The first pair of these is larger than the others and forms the crustacean’s claws. These are used in defense and to capture food.

Mounted to the abdomen are the pleopods, or swimmerets. These are used for propulsion while swimming and may also bear gills. Female malacostracans attach their eggs to these appendages.


Branchiopoda

Branchinecta sandiegonensis is a rare species of crustacean in the family Branchinectidae and the order Anostraca, the fairy shrimp. Its common name is San Diego fairy shrimp
San Diego fairy shrimp (Branchinecta sandiegonensis), an endangered North American crustacean.

Branchiopoda is a class of crustaceans whose members include tadpole shrimps, water fleas, clam shrimps and fairy shrimps. These small crustaceans are found in freshwater habitats all around the world.


Remipedia

remipede
The remipede Speleonectes tanumekes. Photo by Joris van der Ham [CC BY 2.5]
The 29 species in the crustacean group Remipedia are blind, cave-dwelling animals found in the Caribbean, Indian Ocean, the Canary Islands and other subtropical areas.

Around one inch in length, their bodies consist of around 30 segments, each of which has a pair of biramous limbs attached.

Remipedes are the only known venomous crustaceans.


Cephalocarida

Cephalocarida are small, blind crustaceans that grow to around 3mm in length. They inhabit the sediment on the sea bed. They are also known as horseshoe shrimps.


Maxillopoda

Goose Barnacles
Goose barnacles

This group of crustaceans includes barnacles, copepods and related animals.

Barnacles are free-swimming as larvae. In their final larval stage, they find a suitable surface on which to settle. After attaching themselves to the surface by their first antennae, they undergo metamorphosis into a post-larval stage and spend the rest of their lives anchored to the same spot.

Copepods are found in marine and freshwater habitats, and are extremely abundant. Most swim using their first antennae and have a single compound eye at the center of their heads.

Many copepods are planktonic, and form an important part of the oceanic food chain.


Ostracoda

Crustacean-Seed-Shrimp
Seed shrimp. Photo: Anna33 at English Wikipedia [CC BY-SA 3.0]
Ostracods are small (1mm) crustaceans also known as seed shrimps. There are around 13,000 known living species. Most inhabit the sediment on the sea bed, but some join the plankton and others live in freshwater.


What Is A Crustacean? Conclusion

Crustaceans are a highly-diverse group of arthropods. Because they have adapted to live in most marine habitats, they may be known as the ‘insects of the sea’. Crustaceans are also found in most freshwater environments. Some crustaceans are semi-aquatic, and live their whole lives on land.

We hope that you have enjoyed finding out about these amazing invertebrates. Take a look at the following pages and find out more about our amazing planet!

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