In this article, you will know more about the internal structure of the crabs. This is the second part of the article dedicated to the crab anatomy (the first one was about their external structure).
To analyze the external anatomy of crabs, I will divide their body into the following systems: the circulatory system, the respiratory system, the digestive system, the nervous system, and the reproductive system.
Without further ado. Let’s look at each of these organs and systems in detail.
The Circulatory System of Crab
As with all crustaceans, crabs have an open circulatory system. It means that all their blood (Hemolymph) is not only contained within veins and arteries. Instead, the entire abdominal cavity of the animal is filled with hemolymph that has direct contact with the crab’s internal tissues and organs.
When the crab moves, the hemolymph bathes the internal organs directly and is oxygenated when it passes through the branchial chamber housing the gills and lungs.
Hemolymph is pumped out by the heart to circulate through the tissues. When the heart relaxes, blood is drawn back toward the heart through open-ended pores called Ostia to start the cycle again. These pores basically replace the capillary (veins) network in a crab body.
Unlike the blood of vertebrates, crabs do not use hemoglobin to transport oxygen throughout their body (Hemoglobin has iron in it, which gives off a reddish color).
Instead, their ‘blood cells’ use hemocyanin, which gets its blue color from the copper that is part of its structure.
- Despite the fact that crabs 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.
- Because there are no arteries or major veins to pump the hemolymph in the body, blood pressure is very low. This is a great solution for simple animals with low metabolic rates. For example, very low blood pressure allows crabs to live at great depths in oceans and seas.
The Respiratory System of Crabs
Aquatic and terrestrial crabs have different respiratory systems. Aquatic crabs have gills to get the oxygen from water while land crabs have lungs (actually, these are modified cavities that act like lungs). Therefore, crabs absorb oxygen either through water or moisture in the air.
Interesting fact: Some air-breathing crabs have both gills and lungs.
Their gills (lungs) are located below the carapace near its claws (first pair of walking legs). They are attached near the bases of the legs and are enclosed by gill chambers on each side.
Underwater, crabs use a thin leaf-like appendage (scaphognathite) of the second maxilla that draws water through their gills.
Out of the water, crabs use special plates (articulating plates) around their gills to help seal in moisture to prevent them from drying out. As long as they are kept moist, the crab can stay on land for a long time (in some species it can be for days!).
During respiration, oxygen molecules dissolve into a layer of moisture surrounding a thin membrane of the gills. After that, the oxygen molecules cross through the membrane and go into the blood (hemolymph) of the crab. As long as they keep their gills or lungs moist, they are able to diffuse oxygen from the air or water.
Note: The gills or lungs do not fit tightly against the carapace of the crab, so water can circulate through the gills.
The Nervous System of Crab
Crabs have a condensed central nervous system consisting of several ganglia (nerve cell clusters or nerve centers). It consists of a brain (or supraesophageal ganglion) connected to a ventral nerve cord of ganglia.
The brain is located in the head at the anterior end, and the ventral nerve cord spanning from the head to the legs.
In crabs, the brain consists of 3 pairs of ganglia:
- the protocerebrum(Controls vision);
- the deutocerebrum (Tactile sensation);
- the tritocerebrum (Integrates sensory information from protocerebrum and deutocerebrum).
Note: Crabs do not have a brain like that of mammals. Instead, nerve cells concentrated in a set of ganglia. The ganglia interconnect by bundles of nerve fibers that transport the signals at high speed. This is a very rudimentary ‘brain’.
The ventral nerve cord extends along the belly of the crab 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, pleopods, etc.).
- the subesophageal ganglion (It also provides sensory information and stimulates mouthparts).
- the thoracic ganglia (Contain the neurons necessary for the movement of appendages),
- the abdominal ganglia (Innervate the pleopods, the reproductive organs, and the excretion system),
- the specialized terminal ganglion (Provide additional sensory information from the body appendages).
Every nerve center is semi-independent and provides motor nerve to the appendages. It means that even if the cerebral ganglia are disabled, some parts of the crab may still move and react for some time in a purposeful way while responding to external stimuli.
The Digestive System of Crabs
- the foregut (Functions in both mechanical and extracellular digestion),
- the midgut (This is where the main digestion takes place),
- the hindgut (This is a narrow tube which starts from the midgut and extends as far as the anus. It expels undigested food.).
Crabs have mandibles that shred food into pieces. However, in the absence of teeth in their mouths, the consumed pieces can still be too big. Therefore, their foregut has the gastric mill apparatus which functions in mastication (cutting and grinding) as a prelude to further digestion.
Note: Gastric mill consists of a series of calcified plates, that are moved against each other by powerful muscles. Once the food has been degraded into a sufficiently small size, it passes to the midgut.
Like all crustaceans, crabs have a digestive gland (the hepatopancreas). It functions as a liver, pancreas, and intestine for the crab (all in one). Additionally, it absorbs digested nutrients and can store them for some time. Therefore, the digestive gland has two functions – digestion and storage.
The digestive system works in the following way.
- Crabs use their claws to catch, crush, and tear apart the food.
- When, through the passage in the mouth, the food goes to the Esophagus (part of the digestive tract between the mouth and the stomach).
- Next, the food enters the cardiac stomach which is positioned right behind the eyes.
- Food passes through the gastric mill which can help grind food and goes into the pyloric stomach.
- The pyloric stomach serves as an assorted function, directing the digestible matter to the digestive glands whereas the larger indigestible matter will be excreted along the alimentary canal.
- From the pyloric stomach, the food passes through in the midgut.
- The intestine will proceed through the entire abdomen over the flexor muscles.
- The anus will excrete undigested material. It is located on the underside of the apron.
The Reproductive System of Crabs
In male crabs, the reproductive system s is composed of a pair of testes (they are bilaterally symmetrical creamy to whitish in color).
The testes 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 a pair 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.
Male crabs have a narrow candle-looking shape (abdomen) on the underside of their shell.
On the inner side of the abdominal flab, they have a pair of swimmerets which are used as copulatory organs.
In female crabs, there are the paired ovaries, in which the eggs are produced.
The ovaries are located dorsal to the digestive gland and extend on both sides of the anterior margin of the body cavity to the cephalothorax. The digestive gland and the ovaries are intermingled along the anterior marginal dorsal portion of the carapace.
The ovaries change from the light, soft pink of the immature ovary to the bright orange of the mature ovary.