Crayfish External Anatomy

Even though the anatomy of crayfish varies from species to species, and yet there are basic physical characteristics that are identical among all crayfish species, from the Dwarf Mexican crayfish (Cambarellus patzcuarensis) to the giant the Tasmanian giant freshwater crayfish (Astacopsis gouldi).

As a typical decapod crustacean (the name Decapoda from the Greek means “10 legs”), Crayfish’s body is divided into two main parts: cephalothorax and abdomen.

The majority of the internal organs are located in the cephalothorax of the crayfish (the head and chest area). While in the abdomen we can find mostly strong muscles, and the part of the intestine system.

In this article, I am going to talk about the external anatomy of the crayfish in detail. So, keep reading for everything there is to know about these interesting creatures.           

Crayfish Body Structure

Like all crustaceans, crayfish do not have bones (spine) or any internal skeleton. Their body is covered with an exoskeleton (heavily calcified shell), which is divided into two main parts:

1.     The cephalothorax

The cephalothorax (cephalic+ thoracic) consists of the cephalic (or head) region and the thoracic (or chest) region. In its turn, the chest also consists of 3 segments that can be seen only from the ventral side of the crayfish where each segment contains a pair of appendages that are called walking legs. 

The part of the exoskeleton that covers the cephalothorax is called the carapace. It protects their vital organs of any crayfish (brain, heart, stomach, bladder, testicular, or ovarian).

Note: If we look at the carapace from above, we can see the groove, that separates the head and chest regions. This separation is nominal because head-chest areas are basically ‘fused’ together. It means that crayfish absolutely cannot turn its head.

2.     The abdomen.

The abdomen of crayfish is located behind the cephalothorax and includes 6 abdominal segments, pleopods, and the tail. Pleopods (or the smaller appendages) are attached to the segments of the abdomen, they are often called swimmerets. 

Abdomen is the main muscle that allows crayfish to swim.

Quick Notes about Crayfish External Anatomy

Scientific name	Common name	Function
Cephalothorax consists of
Cephalic	Head	Contains a few nerve cell clusters (cerebral ganglion or ganglia).
Thoracic	Chest, Carapace or Upper body	Protection of inner organs
Rostrum	Beak or Nose	Additional protection of eyes, stabilizer
Antennae	Long whiskers	The sensor of orientation and coordination
Antennule	Short whiskers	The sensor of chemical information (food, gender discrimination, etc.)
Maxillae	Jawfoot	Help to eat and to draw water over the gills.
Maxillipeds	Jawfoot	For eating
Pereiopods	Walking legs	Movement
Chela	Claws	Holding and picking food. Defense or aggression.
Eyes	Eyes	Vision
Abdomen consists of
6 Abdomen segments	Stomach	Swimming
Tegum	–	Protection of abdominal segments
Telson	Tail	Swimming
Uropods
Pleopods	Swimmerets	In males, 1st swimmeret is used in mating. In females, swimmeretes are used to hold and fan eggs

Head

Crayfish are characterized by a joined head and chest. That is why biologists usually do not describe this part of the crayfish by itself. Nonetheless, it protects nerve cell clusters (cerebral ganglion or ganglia that can be called as a brain) and digestive gland.

 The head has 5 pairs of appendages. Such as:

• The antennules.
• Long antennae.
• The mandibles.
• 2 pairs of maxillae.

Carapace (Chest):

The carapace is the most calcified upper part of the crayfish body. It is thicker than the shell elsewhere on the crayfish. It connects to walking legs (pereiopods), maxillipeds, whiskers (antennae and antennule), and eyes of the shrimp.

Rostrum

Rostrum (from the Latin rōstrum meaning “Beak”) is a hard extension of the crayfish’s carapace that prolongs forward of the eyes. In crayfish, Rostrum doesn’t have any sensors.

Its main function is to protect the crayfish’s eyes and brain. Rostrum works as a stiffening rib layout structure. In addition, it helps to stabilize the movements of the crayfish when it swims backward.

Stalked Eyes

The eyes of the crayfish are located beneath the rostrum.  Each eye is at the end of a short, independently movable and adjustable stalk (called pedicles).

Crayfish have compound eyes that contain thousands of tiny structures, each functioning as a separate eye in multi-tiled fashion. It gives crayfish a mosaic and panoramic vision view of its world. Basically, the same as an insect.

The eye-stalks of the crayfish produce a special hormone that regulates the metabolism and molting process in the body.

Interesting fact: Crayfish can regenerate their lost limbs (claws or legs) with every molting process. However, unlike other body parts, they cannot regenerate the eyes.

Antennae and Antennule

Two pairs of Antennae and Antennule projects on either side of the tip of the rostrum.

• One set of short antennules.
• One set of long antennae.

Crayfish use long antennae as tactile or touch receptors to gather information, orientate, and coordinate their positioning in the environment. Its structure can easily catch water vibrations around, which can be very helpful in hunting, fighting, mating, or escaping. 

Even though the short antennules can also play the role of the tactile receptors, their main function is to provide chemical information (taste and “smell”) of what they are touching.

They have been reported to influence the localization of distant food odors, gender discrimination, and agonistic and social behaviors of decapod crustaceans.

Claws

Crayfish have two large claws (chelipeds or pincers) that extend forward of the carapace. The claws have 3 main functions:  fighting, feeding, and mating. Males use the claws to clamp and hold females claws during copulation.

Mandible and Maxilliped

Crayfish have 3 pairs of maxillipeds (jawfoot, and foot jaw Origin: [Maxilla + L. pes, pedis, foot.]). These are mouth appendages on the heads of the crayfish modified to rummage, hold and bring food to the mouth during eating. The second pair also helps to draw water over the gills.  

The mandible (or jaws) lies underneath the maxillipeds. Unlike humans, the jaws of crayfish open by moving from side to side.

Walking Legs

Besides the claws, crayfish have 4 pairs of walking legs. The first two pairs of walking legs are tipped with small pincers which the crayfish uses for grooming, food manipulation, and movement.

The abdomen

The abdomen of the crayfish is flexible and contains 6 segments. This is the most muscular part of the crayfish. Crayfish can swim backward by rapid and powerful contractions of the abdominal muscles.

The first segment starts right behind the carapace and the sixth segment is in front of the tail. 

The first 5 segments may also have a pair of swimmerets. The abdominal appendages are called pleopods. They create water currents and function in reproduction.

Note: Depending on the crayfish species, they are not always present in decapod males.
Note #2: When crayfish molt, they have a breaking point at the junction of the first abdominal segment and the carapace.

Swimmerets (Pleopods)

As I have already said, the first 5 abdominal segments have one pair of pleopods. The Swimmerets of crayfish are shaped like paddles and serve multiply functions:

• Swimmerets are used to create water currents, which brings oxygen to their gills.
• Ventilation of the
• Females use them to aerate their eggs.
• In males of some crayfish species, the first set of swimmerets are enlarged for grasping the female during copulation.
• Swimmerets add more control when crayfish swim forward.

Interesting fact: According to the studies, in crayfish, each swimmeret is driven by its own independent pattern.

Uropods and Telson

The last abdominal segment (the 6th segment) of the crayfish contains a modified pair of uropods. In the middle of the uropods is a triangular-shaped structure called the telson. Uropods flank telson from both sides.

Telson never has any pleopods. Uropods are paired biramous appendages. 

The uropod and telson together make up the tail fan which usually functions as locomotion when swimming backward, and functions like rudders, steering the crayfish when it swims forward or backward by forcing water forward with its tail fan.

Gender Identification

The female and male crayfish can be told apart by looking at the lower part of their abdomen.

Males have triangular-ish or two L-shaped appendages (semen transfer organs) behind their legs called claspers. They use them for internal fertilization.

The females have a circular receptacle between the bases of the last two pairs of walking legs. Instead of the triangular-ish forming appendages, there is a small nub. That indicates a female. 

In Conclusion

If you keep crayfish in your home aquarium as a pet or simply study them, it will be a good idea to have at least a basic understanding of the crayfish anatomy.

All crayfish species have segmented bodies (up of 20 body segments grouped into two main body parts the cephalothorax and the abdomen), outer shells, or exoskeletons, and paired, jointed limbs. In general,

You do not have to be a master of anatomy and physiology or know every single piece of the body but it is certainly to your advantage to learn the main body parts.

Related articles:

Crayfish Internal Anatomy
Dwarf Shrimp External Anatomy
Dwarf Shrimp Internal Anatomy
Crab External Anatomy
Crab Internal Anatomy
Introduction to the Crayfish Care – Setup, Diet, and Facts
How to Set Up a Crayfish Tank