Crayfish Anatomy: Labeled Diagrams and Interactive 3D Model

A crayfish is a freshwater decapod crustacean: an arthropod with a hard, jointed exoskeleton, two body regions and ten legs. Its body has a cephalothorax, the head and thorax fused under one shield called the carapace, and an abdomen of six segments ending in a tail fan. It carries 19 pairs of jointed appendages, breathes with gills hidden under the sides of the carapace, has an open circulatory system with a small dorsal heart, and excretes through a pair of green glands in its head.

This page describes the red swamp crayfish, Procambarus clarkii, family Cambaridae. Its native range runs along the Gulf coastal plain from the Florida panhandle into Mexico and up the Mississippi valley to southern Illinois, adults are 5.5 to 12 cm long, and it is farmed on a large scale in Louisiana (USGS). That makes it one of the easiest crayfish to obtain for teaching. Its anatomy is typical of the North American cambarid crayfishes, so almost everything here applies equally to Faxonius (formerly Orconectes) and Cambarus, the other genera often supplied to schools.

Where other crayfish differ, it is mostly in details used to identify species: the shape of the rostrum, the spines on the carapace, the width of the areola, the shape of the male gonopods and of the female annulus ventralis, and which walking legs carry hooks in males. The classic anatomy textbook, Huxley's The Crayfish (1880), describes the European Astacus; it has the same organs but no annulus ventralis, and its gill count of 18 per side may differ slightly from that of cambarids.

Directions used below: anterior is towards the rostrum, posterior towards the telson, dorsal is the back and ventral the belly. Left and right are the crayfish's own.

The external anatomy of a crayfish includes the carapace with its rostrum and cervical groove, two compound eyes on stalks, two pairs of antennae, the mouthparts, a pair of large chelipeds, four pairs of walking legs, the six-segmented abdomen with its swimmerets, and the tail fan made of the telson and two uropods.

Seen from above, the carapace is divided by the cervical groove, which marks the boundary between the head in front and the thorax behind. Behind the groove, two branchiocardiac grooves outline a narrow strip called the areola, which lies directly over the heart. On each side, the carapace extends down as the branchiostegite, the gill cover. In the red swamp crayfish the rostrum is a triangular point without a central keel, and the spines on the sides of the carapace are small or absent.

The abdomen (often called the tail) is made of six hard rings. Each ring has a dorsal plate, the tergum, with side flaps called pleura, and a ventral plate, the sternum. Flexible joints between the rings let the abdomen curl under the body. The telson is the flat central piece at the end; it has no appendages, so it is not counted as a segment. The uropods on each side of it are the appendages of the sixth segment.

A crayfish has 19 pairs of appendages: 5 on the head, 8 on the thorax and 6 on the abdomen. From front to back they are the antennules, antennae, mandibles, first and second maxillae, three pairs of maxillipeds, the chelipeds, four pairs of walking legs, five pairs of swimmerets and the uropods. All of them are built from the same basic two-branched limb.

Lab manuals sometimes number the legs differently. Zoologists call all five pairs of thoracic legs pereiopods, so the cheliped is pereiopod 1 and the last walking leg is pereiopod 5. Many school worksheets instead call the chelipeds the first pair of legs and the four pairs behind them the walking legs 1–4. This page uses walking legs 1–4 and gives the pereiopod number in brackets. Huxley counted the eyestalks as a twentieth pair of appendages; most modern texts do not.

Two features are easy to miss. The first two pairs of walking legs end in tiny pincers, while the last two end in a single point. And the scaphognathite, the flat blade on each second maxilla, sits out of sight in the front of the gill chamber, where it beats to keep water flowing over the gills.

This is the text version of the 3D model: every labeled structure of the red swamp crayfish, grouped by body system, with what it does.

Inside the cephalothorax, the heart lies just under the carapace behind the cervical groove. The three-lobed gonad lies below it, the large digestive gland (hepatopancreas) fills both sides, the two-chambered stomach sits in the head, and the gills lie in a chamber on each side. The green glands and the brain are at the very front, and the ventral nerve cord runs along the floor of the body. The abdomen is filled mostly by muscle, with the intestine running along its top.

Viewed from the side, the organs are stacked in layers. From the top down in the thorax you find the heart in its pericardial sinus, the gonad, the intestine in the midline between the lobes of the digestive gland, and finally the sternal sinus and ventral nerve cord just above the sternites. The gills sit outside this stack, between the body wall and the branchiostegite. For a step-by-step guide to finding each organ in a real specimen, see our crayfish dissection guide.

The crayfish has an open circulatory system. A single-chambered heart pumps colourless to pale blue hemolymph (blood) out through arteries. The blood then leaves the vessels and flows through spaces called sinuses, passes through the gills, and returns to the pericardial sinus around the heart. It enters the heart through three pairs of valved openings, the ostia, because no veins lead back to it.

• Heart: Pumps hemolymph (blood) out through seven arteries to the body.
• Ostia: Let blood flow from the pericardial sinus into the heart and close when the heart contracts.
• Pericardial sinus: Collects oxygenated blood returning from the gills before it enters the heart through the ostia.
• Ophthalmic artery: Supplies the eyes, brain and front of the head.
• Antennal arteries: Supply the antennae, green glands, stomach and mouthpart muscles.
• Hepatic arteries: Supply the digestive gland (hepatopancreas).
• Dorsal abdominal artery: Supplies the abdominal muscles and the intestine.
• Sternal artery: Feeds the ventral thoracic and ventral abdominal arteries, which supply the legs, mouthparts and nerve cord.
• Ventral thoracic and abdominal arteries: Supplies the legs, mouthparts, swimmerets and nerve cord.
• Sternal sinus: Returns oxygen-poor blood to the bases of the gills.
• Branchiocardiac canals: Carry oxygenated blood from the gills back to the heart.

Follow one loop: heart → arteries → tissues → sternal sinus on the floor of the thorax → afferent channels into the gills (oxygen picked up) → branchiocardiac canals up the sides → pericardial sinus → ostia → heart. Unlike a fish heart, the crayfish heart receives blood that has already been oxygenated in the gills. The oxygen is carried by hemocyanin, a copper-containing protein dissolved in the blood, which is why oxygenated crayfish blood can look faintly blue. Huxley described it as "a clear fluid, either colourless, or of a pale neutral tint or reddish hue".

Crayfish breathe with feathery gills inside a branchial chamber on each side of the thorax, covered by the branchiostegite. Water is pulled in under the lower edge of the carapace near the legs, flows up and forward over the gills, and is pumped out at the front by the beating scaphognathite (gill bailer).

The gills are named by where they attach. Podobranchs are attached to the bases of the legs and maxillipeds, arthrobranchs to the joint membrane between leg and body, and pleurobranchs to the side wall of the thorax. Huxley counted 18 functional gills per side in Astacus: 6 podobranchs, 11 arthrobranchs and 1 pleurobranch, plus 2 rudimentary pleurobranchs. Because so many gills are fixed to the legs, pulling off a walking leg in a dissection usually brings a gill with it.

• Podobranchs: Gas exchange. Because they move with the legs, walking helps stir water over them.
• Arthrobranchs: Gas exchange.
• Pleurobranchs: Gas exchange.
• Gill filaments: Provide a large, thin surface where oxygen passes from the water into the blood and carbon dioxide passes out.
• Scaphognathite: Bails water forward out of the gill chamber. This draws fresh water in under the edges of the carapace and over the gills.

Each gill is a central axis covered in many fine filaments, which gives a large, thin surface for gas exchange. In fresh water the gills also take up salts from the water, helping the green glands keep the body fluids in balance.

Food passes from the mouthparts into the mouth, up a short oesophagus into the large cardiac stomach, where the three teeth of the gastric mill grind it. It is then sorted in the pyloric stomach: fine particles go into the digestive gland, where most digestion and absorption happen, and the rest passes along the intestine to the anus under the telson.

• Mandibles: Crush and cut food by moving side to side before it enters the mouth.
• Cardiac stomach: Stores food and grinds it with the gastric mill.
• Gastric mill: Grind food, doing the job of our teeth inside the stomach.
• Pyloric stomach: Filters the ground food: fine particles go to the digestive gland, coarse material to the intestine.
• Digestive gland: Makes digestive enzymes, absorbs digested food and stores food reserves. It does the jobs of our liver and pancreas.
• Intestine: Absorbs some nutrients and water and carries faeces to the anus.
• Gastroliths: Store calcium from the old shell so it can be used to harden the new one after the molt.

The stomach is lined with cuticle, like the outside of the body, so its lining and teeth are replaced at every molt. Huxley noted that the digestive gland, often called the liver, is made of "an immense number of short tubes, or caeca", and that its secretion is more like pancreatic juice than bile. Crayfish are omnivores and scavengers: in a dissection the stomach may hold plant fragments, detritus, insect parts or pieces of other animals.

The excretory organs of the crayfish are the two green glands, also called antennal glands, in the floor of the head. Each filters the blood, reabsorbs useful salts, stores urine in a thin bladder, and releases it through an excretory pore on the underside of the base of the antenna.

Because the crayfish lives in fresh water, water keeps flowing into its body. The main job of the green glands is therefore to get rid of that water as large amounts of dilute urine while holding on to salts. Most nitrogenous waste, in the form of ammonia, simply diffuses out across the gills.

The crayfish nervous system has a small brain (the supraoesophageal ganglion) above the oesophagus, two circumoesophageal connectives that loop around the gut, and a ventral nerve cord with a ganglion in each segment. Huxley counted thirteen sets of ganglia: the brain, six thoracic ganglionic masses and six abdominal ganglia.

• Brain: Processes signals from the eyes, antennules and antennae and coordinates behaviour.
• Optic nerves: Carry visual signals from the compound eyes to the brain.
• Circumoesophageal connectives: Connect the brain to the ventral nerve cord.
• Suboesophageal ganglion: Controls the mandibles, maxillae and maxillipeds.
• Ventral nerve cord: Carries signals between the brain and the body and controls the legs, swimmerets and tail locally.
• Abdominal ganglia: Control the swimmerets, the abdominal muscles and the tail fan.

Compare this with our own nervous system: a crayfish's nerve cord is ventral, solid and paired, while ours is dorsal, hollow and single. This is a basic difference between arthropods and vertebrates.

The main sense organs are the compound eyes, each made of many units called ommatidia; the antennules and antennae, covered with touch and chemical sensors; and the statocysts, balance organs in the base of each antennule. Huxley described the statocyst hairs as embedded in "a gelatinous mass, which contains irregular particles of sand". The lining of the statocyst is shed at each molt and the crayfish has to put new grains of sand back in.

Most of the crayfish abdomen is muscle. The large flexor muscles on the underside snap the abdomen forward under the body, and the tail fan drives the animal rapidly backward. The thin extensor muscles along the top straighten the abdomen again. This is the tail-flip, the crayfish's emergency escape.

• Abdominal flexor muscles: Contract to snap the abdomen forward under the body. The tail fan then drives the crayfish rapidly backward: the tail-flip escape.
• Abdominal extensor muscles: Straighten (extend) the abdomen again after a tail-flip, re-cocking it for the next one.
• Mandibular muscles: Close the mandibles to crush food.

The tail-flip is controlled by giant nerve fibres in the ventral nerve cord, which connect quickly to the motor neurons of the fast flexors. Wine (1984) counted about 650 neurons in each abdominal ganglion and roughly 100,000 in the whole crayfish central nervous system, which is why the escape circuit has become a classic model in neuroscience.

You can tell a male crayfish from a female by turning it over. In males, the first two pairs of swimmerets are hard, forward-pointing gonopods lying between the bases of the last walking legs. In females, the first swimmerets are small and soft, and there is a round annulus ventralis between the bases of the third and fourth walking legs. The genital pores are also in different places.

Huxley described both gonads as three-lobed ("a body of a trefoil form"), the testis having narrower and more elongated lobes than the ovary. Switch the 3D model between Female and Male to compare the gonads, ducts, pores and swimmerets.

Crayfish grow by molting (ecdysis): they shed the whole exoskeleton, swell by taking up water, and harden a new, larger shell. Young crayfish molt often; adults molt less frequently. Hormones from glands in the eyestalks hold molting back until conditions are right.

Before a molt, a freshwater crayfish moves calcium from its old shell into two gastroliths in the stomach wall, then uses it to harden the new shell. Red swamp crayfish grow quickly: the USGS notes that they can exceed 50 g within 3 to 5 months. A recently molted crayfish has a soft shell and is very vulnerable, which is why molting crayfish hide.

Crayfish and grasshoppers are both arthropods, so they share a jointed exoskeleton, segmented body, ventral nerve cord, dorsal heart and open circulation. They differ mainly because the crayfish breathes water and the grasshopper breathes air.

Because insects deliver oxygen straight to their tissues through tracheae, their blood plays little part in breathing. In the crayfish, every drop of blood must pass through the gills, so the circulation is built around them. See the grasshopper dissection pages from Biology Junction for the insect side of this comparison.

Crayfish and lobsters are close relatives with the same basic anatomy: a carapace, chelipeds, four pairs of walking legs, a six-segmented abdomen, swimmerets and a tail fan. The differences are mostly size, habitat and life history.

Lobster figures come from NOAA Fisheries; crayfish figures from the USGS. A preserved lobster can be dissected with exactly the same procedure as a crayfish.

Every crayfish appendage is a variation on one basic limb. Huxley described it as a base, the protopodite, made of two joints (the coxopodite and basipodite), which carries an inner branch, the endopodite, and an outer branch, the exopodite. Where both branches are present the limb is called biramous (two-branched). The antennules, the antennae with their antennal scale, the maxillipeds, the swimmerets and the uropods all show both branches. In the walking legs and chelipeds the exopodite has been lost, and the endopodite forms the long jointed leg. (Huxley, T. H. (1880). The Crayfish: An Introduction to the Study of Zoology. Internet Archive.)

This is why lab worksheets ask you to lay the appendages out in order. Seen side by side, the same plan turns into feelers, jaws, food handlers, a water pump, pincers, legs, egg holders and paddles. Biologists call parts that share an origin but differ in form serially homologous. When you label an appendage card, add one more column: whether the appendage is biramous or uniramous (one-branched). It turns a memory exercise into evidence for how arthropod bodies evolved by modifying a repeated segment.

A crayfish grows by molting, because its hardened exoskeleton cannot stretch. Before a molt, calcium is moved out of the old shell. Freshwater holds little dissolved calcium, so many freshwater crayfish store part of it in a pair of gastroliths, lens-shaped stones that form in the wall of the cardiac stomach. After the molt the gastroliths are broken down and their calcium is used to harden the new exoskeleton. The Western Australian Museum notes that only freshwater crustaceans form gastroliths. (Western Australian Museum: Why freshwater crayfish don't need milk for healthy bones.)

Electron microscopy of the gastroliths of the redclaw crayfish, Cherax quadricarinatus, showed that they are made mostly of amorphous (non-crystalline) calcium carbonate, laid down as nanogranules along chitin–protein fibres. Amorphous mineral dissolves more easily than crystals, which suits a store that must be emptied quickly. Huxley knew the same stones as 'crabs' eyes'. In a dissection you will only find them in a crayfish that died shortly before or after a molt, so their absence is normal. (Luquet, G. et al. (2016). Calcium deposits in the crayfish, Cherax quadricarinatus: microstructure versus elemental distribution. Microscopy and Microanalysis 22: 22–38.)

Molting replaces every cuticle-lined surface: the outer shell, the lining of the stomach with its gastric mill, the hindgut, the internal skeleton (apodemes) and the lining of the statocysts. Huxley recorded that apodemes are shed along with the other cuticular structures at ecdysis (the act of shedding). A freshly molted crayfish is soft and hides until the new shell hardens, and many crayfish eat the shed skin, recovering more of its minerals. (Huxley, T. H. (1880). The Crayfish: An Introduction to the Study of Zoology. Internet Archive.)

A crayfish's body fluids are saltier than the fresh water around it, so water constantly leaks in across the gills and other thin surfaces, and salts leak out. The green glands (antennal glands) solve the water problem. Huxley described each one as 'a discoidal body of a dull green colour' lying near the antennae, draining into a 'spacious sac', the bladder, which opens on the basal joint of the antenna. Fluid filtered from the blood is processed as it passes through the gland, salts are taken back, and the excess water leaves as dilute urine. (Huxley, T. H. (1880). The Crayfish: An Introduction to the Study of Zoology. Internet Archive.)

Compare this with a marine fish, which loses water to the sea and drinks to replace it. The crayfish has the opposite problem and the opposite solution: it makes a lot of urine and wins back salts, partly in the green gland and partly by active uptake across the gills. In decapods the antennal gland also has an unusually rich blood supply: corrosion casts showed complete capillary beds in it, one of only two places in the body where McGaw found them. (McGaw, I. J. (2005). The decapod crustacean circulatory system: a case that is neither open nor closed. Microscopy and Microanalysis 11: 18–36.)

The crayfish tail-flip is one of the best-understood behaviours in neuroscience. Wine's review describes six abdominal ganglia with about 650 neurons each, out of roughly 100,000 neurons in the whole central nervous system. A dedicated escape circuit of 90 pairs of motor neurons and 18 pairs of interneurons drives the fast flexor and extensor muscles of the abdomen. The lateral giant interneuron, a wide, fast-conducting nerve fibre in the cord, triggers the escape response; serotonin inhibits it and octopamine excites it. (Wine, J. J. (1984). The structural basis of an innate behavioural pattern. Journal of Experimental Biology 112: 283–319.)

In a dissection you can see the hardware. The thick, twisted flexor muscles fill the abdomen, while two thin extensor bands run under its roof. Fast flexors fire as powerful twitches for escape, whereas small superficial muscles hold posture. The ventral nerve cord, with a ganglion in each segment, lies under the flexors. Wine notes that an isolated abdomen can still run some of its motor programmes, although most are started by commands from the brain. (Wine, J. J. (1984). The structural basis of an innate behavioural pattern. Journal of Experimental Biology 112: 283–319.)

Textbooks call crayfish circulation open because blood leaves the arteries and flows through sinuses rather than returning in veins. The heart has no veins entering it: Huxley described six valved openings (three pairs of ostia) through which blood is drawn in from the surrounding pericardial sinus. From the heart, arteries run forward to the eyes, antennae and digestive gland, back along the abdomen, and down through the sternal artery to the ventral side. (Huxley, T. H. (1880). The Crayfish: An Introduction to the Study of Zoology. Internet Archive.)

Detailed corrosion casting in crabs showed that the arterial side branches far more finely than the word 'open' suggests. Seven arteries leave the single-chambered heart and divide into capillary-like vessels throughout the body, and the sinuses they drain into are organised, membrane-bound channels rather than one big cavity. McGaw concluded that the decapod system is best described as 'partially closed'. For a school exam, 'open' is still the expected answer; the research shows why it is a simplification. (McGaw, I. J. (2005). The decapod crustacean circulatory system: a case that is neither open nor closed. Microscopy and Microanalysis 11: 18–36.)

Crayfish families differ in how sperm reaches the female. In cambarid crayfish, the family that includes Procambarus, Faxonius and Cambarus, the male's first pleopods are tube-like gonopods with sperm channels and terminal elements, and the second pair is also modified. The female has a median spermatheca, the annulus ventralis, on her ventral thorax. In European astacid crayfish, by contrast, the female has no spermatheca and the male attaches sperm packets to the outside of her body (Bauer, 1986). (Bauer, R. T. (1986). Phylogenetic trends in sperm transfer and storage complexity in decapod crustaceans. Journal of Crustacean Biology 6: 313–325.)

Male cambarids change form with the breeding season: they molt into a breeding form with hardened gonopods (often called Form I) and later back to a non-breeding form (Form II). The Missouri Department of Conservation notes that males typically have a different body form during the autumn and winter mating period. Females carry the fertilised eggs glued to the swimmerets, and the young stay attached to the mother until they have completed two molts. (Missouri Department of Conservation: Crayfishes field guide.)

Red swamp crayfish normally reproduce sexually, but genetic testing of 120 wild individuals in China found 15 genetically identical females in four clonal groups, evidence that P. clarkii can occasionally produce offspring without mating. The authors argued that this ability, combined with the species' fast growth, makes moving it to new waters especially risky. (Yue, G. H. et al. (2008). Discovery of four natural clones in a crayfish species Procambarus clarkii. International Journal of Biological Sciences 4: 279–282.)

Use the 3D model or the diagrams above to answer these. Answers are on the dissection page.

1. What two body regions make up a crayfish, and what is the carapace?
2. Name the 19 pairs of appendages in order and give one function for each.
3. Which appendage holds the statocyst, and what does it sense?
4. Where are the gills, and which structure pumps water over them?
5. Why is the crayfish circulatory system called open?
6. What are the two chambers of the stomach and what happens in each?
7. Which organ does the job of our kidneys?
8. Give three differences between a male and a female crayfish.

Full lab questions with an answer key are in the crayfish dissection guide.