Crayfish Internal Anatomy

Crayfish internal anatomy

Today I am going to talk about the internal structure of the crayfish. This is the second part of the article on crayfish anatomy (the first was about the external structure).

Basically, crayfish internal anatomy includes the following systems: the circulatory system, the respiratory system, the digestive system, the nervous system, and the reproductive system.

The majority of the internal organs are located in the Cephalothorax of the crayfish (the so-called head-chest area). The abdomen of crayfish is the main muscle that also contains the part of the intestine system.

Without further ado. Let’s look at each of these organs and systems in detail.

Quick Notes about Crayfish Internal Anatomy

Scientific name Common name Function

 Cephalothorax includes

Cerebral ganglion or ganglia or Encephalon Brain Contains a few nerve cell clusters.
Heart Heart Pumps blood to all organs of the crayfish
Pair maxillary and antennal glands Green glands Filtration and excretion. Remove metabolic waste and excess of water through the special pores.
Esophagus Throat Part of the digestive tract between the mouth and the stomach
Two-chamber stomach

  • cardiac stomach
  • pyloric stomach
Stomach Grinds food in the cardiac stomach and sorts it in the pyloric stomach.

The stomach is a part of the digestive tract between the esophagus and the intestine

Gastroliths  Calcium stones Stores calcium for upcoming molt.
The gastric mill   Part of the stomach with teeth.
Digestive gland Liver Secretes enzymes to break down the food and stores fats and glucose.
Cardiac muscle, and pyloric muscle Anterior muscles
Posterior muscles
Stomach muscles
Mandible muscle The mouthparts They are involved in opening and closing the mandibles.

 Abdomen includes

Ventral nerve cord Nervous system The central nervous system of crayfish. It also activates the swimmeret system.
Extensor muscle Abdomen muscles Responsible for extending the abdomen or straightening it.

They are located on top of the abdomen.

Flexor muscle Abdomen muscles Bend the abdomen segments and curls the tail of the crayfish.

They are located along the sides of the abdomen.

Rectum and Anus Excretes waste that cannot be digested.

The Circulatory System of Crayfish

Crayfish open systemAll crayfish species have an open circulatory system. It means that they almost do not have blood pressure and their entire abdominal cavity is filled with blood.

Note: Open circulatory systems function under low blood pressure and flow. It is almost ideal for simple animals with low metabolic rates. For example, very low blood pressure allows crayfish to live at great depths.

The diamond-shaped heart contracts to send the oxygenated blood into the spaces around the organs through several arteries. The heart is located at the posterior edge of the carapace.

The blood leaves the small arteries and flows around in the body cavity supplying internal organs with oxygen and nutrients. After that, when the heart relaxes, the blood passes through the gills and returns back into the heart through the two open-ended pores (ostia) in it to start the cycle again. These pores basically replace the capillary (veins) network in a crayfish body.

Crayfish have light blue blood. This is because they do not have hemoglobin. Their blood cells use hemocyanin, which gets its blue color from the copper that is part of its structure. To be precise, their so-called “Blood,” is a combination of haemocyanin and an interstitial fluid called “hemolymph”. Their blood can coagulate very rapidly.

Important: Hemolymph contains hemocyanin, which also contains two copper ions. Despite the fact that crayfish need a certain level of copper for proper blood circulation, like most invertebrates, they do not tolerate copper very well.

You can read more about “How Copper Affects Dwarf Shrimp” right here.

The Respiratory System of Crayfish

Crayfish internal anatomy - the respiratory system (gills)Crayfish do not have lungs, they use gills to gather oxygen and breath. The gills have a double set of leaf-shaped branches (in gill-chamber) and can be found on both sides of the carapace at the base of each walking leg (pereiopods). The gills can be identified as a fuzzy grey or brown organ.

Note: To be precise, the gills are located on the outside of the body cavity in a space between the body wall and the carapace. The carapace does not fit tightly against the body of the crayfish, there is a space between the body and the carapace so water can circulate through the gills.

Interesting fact: In crayfish, the gills also have a connection to the walking legs. So, as crayfish move the legs, the gills sweep through the water and that increases the contact between the water, and the gills and that increases gas exchange.

Unlike most aquatic creatures, crayfish have modified gills. They require water or high moisture to breathe. Therefore, as long as they are kept moist, the crayfish can stay on land for a long time without problems.

Crayfish Internal Anatomy 2

The Digestive System of Crayfish

Crayfish are extremely opportunistic eaters. They are omnivores who are also scavengers. It means that their digestive system is very adaptable when it comes to feeding. It can break down both plants and animals as well as whatever else the crayfish can find.

The digestive system of crayfish can be divided into three main parts:

  • Ali­mentary canal.
    It consists of a mouth, buccal cavity, stomach, rectum, and anus.
  • The gastric mill is made up of two regions: the cardiac stomach (which is near the head end) and the pyloric stomach (which is towards the posterior).
  • Digestive gland (The hepato­pancreas)
    The gland functions as a liver, pancreas, and intestine for the shrimp (all in one). Additionally, it absorbs digested nutrients and can store it for some time. Therefore, the digestive gland has two functions – digestion and storage.

The digestive system works in the following way.

  1. Crayfish use their claws to catch, crush, and tear apart the food.
  2. When, through the passage in the mouth, the food goes to the Esophagus (part of the digestive tract between the mouth and the stomach).
  3. Next, the food enters the stomach which is positioned right behind the eyes. This is a complex structure with a number of modified regions such as the ossicles which can help grind food.
  4. Food passes through the gastric mill and goes into the cardiac stomach first which is lined with teeth that will help grind up the food by using cardiac muscles.
  5. Then it will enter the pyloric stomach which serves a more assorted function, directing the digestible matter to the digestive glands whereas the larger indigestible matter will be excreted along the alimentary canal.
  6. The food that is to be digested will be diverted towards the digestive gland. The rest of it will be also excreted along the alimentary canal.
  7. From the stomach, the food passes through in the intestine.
  8. The intestine will proceed through the entire abdomen over the flexor muscles.
  9. The anus will excrete undigested material. It is located on the underside of the telson.

Interesting fact: Crayfish have teeth inside their stomachs for grinding food. This tooth-like structure is called the gastric mill.

The Green Glands

The excretory system of crayfish consists of paired antennal glands (or the green glands), which rest in the basal antennal segment of the cephalothorax.

The green glands are positioned ventrally below the stomach and near the anterior end of the body cavity. These glands have spherical shapes.

Their main function is filtration and excretion. These are sort of like kidneys. The green glands remove metabolic waste and excess of water through the special pores. The openings for the green glands are located on the outer surface near the base of each antenna.

The Nervous System of Crayfish

The Nervous System of CrayfishThe nervous system of crayfish is multifunctional.


  1. It is responsible for controlling all muscles in the body (mouthparts muscles, stomach muscles, abdomen muscles, )
  2. It also controls the production of all For example, molting in crustaceans is regulated by ecdysteroids (molting hormone). It also plays a vital role in the mating and reproduction of the crayfish.
  3. All sensory perceptions from antennae and antennule are processed in the nervous system.

Nonetheless, the nervous system of crayfish is also pretty simple compared to many other animals.

The brain is located on the back, or dorsal, side of the crayfish’s head (right behind and between the eyes). Actually, their brain can be called a brain only conditionally because it is very small and consists of only 3 nerve cell clusters (cerebral ganglion or ganglia).

These three fused nerve nodes are called:

  • protocerebrum (Responsible for the vision and for the orientation of the animal; it connects the eyes with the central nervous system).
  • deutocerebro (Responsible for locomotion and nutrition).
  • tritocerebro Responsible for food recognition and chewing functions. It connects antennal nerves with the central nervous system).

The ventral nerve cord extends along the belly of the crayfish and runs to the rear end of the body (to the telson). It also has small nerve clusters at the end of each segment (walking legs, swimmerets, etc.).

Every segment is like semi-independent and provides motor nerve to the appendages. Basically, the nervous system of the crayfish is not completely centralized. It means that even if the cerebral ganglia is disabled, some parts of the crayfish would still move and react in a purposeful way while responding to external stimuli.

You can also read the article “Do Crabs, Crayfish or Shrimp Feel Pain?”.

The Reproductive System of Crayfish

In male crayfish, testes produce spermatids that are transported to and accumulate in the vas deferens. The testes (paired and soft organs) are located near and beneath the heart.

Each of them includes numerous tiny tubes, called seminiferous tubules. Each tubule has an inner lining of a single layer of epithelial cell which trans­forms into spermatozoa.

In addition, males have different pairs of swimmerets (copulatory pleopods) that are before the walking legs. These swimmerets are significantly longer than others. Males use them to transfer spermatids during mating.

In some crayfish species, they transfer spermatids from the special ducts at the base of the fourth walking limbs during mating.

In female crayfish, there are the paired ovaries, in which the eggs are produced. The matured eggs remain near the margin and the immature eggs occupy the center. Females have oviducts at the base of the second walking legs. It is from here that eggs will be shed.

Note: There is a crayfish species that can reproduce asexually or parthenogenetically (without males). This is a Marbled crayfish (Procambarus fallax virginalis or Procambarus virginalis sp. nov.).

Crayfish and Mating Process 

When breeding is initiated, the male flips over and pins down the female and deposits spermatids near her seminal receptacle.

Insemination is external. It means that the union of reproductive cells occurs outside the body. The eggs travel down an oviduct and then they pass through a seminal receptacle where fertilization occurs.

After that female sticks the eggs on her pleopods. The brood pouch itself is formed by pleopods of the female. They hold the eggs until hatching day.

Related articles:

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

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