The Underside

By looking at the underside of the crab you can tell the sex of the crab by looking at the underside of the crab. If the abdomen wide and rounded, the crab is a female. Males have a narrow abdomen. You can also see the overlapping parts of the mouth that are used for grasping and biting off chunks of food.



All decapods have ten appendages with different function based on location. The chelae or claws are the first pair of appendages used for feeding and defense. More on them below. The other pairs are legs used for walking. The three pairs closest to the mouth are called maxillipeds and are used in eating. The back pair on the abdomen are called pleopods. They are used for swimming and carrying developing eggs.




One of the first  things you will notice about the red rock crab is their large black tipped claws. Red rock crabs have very strong crushing claws in order to eat oysters and other shelled species. Crabs raised on thick-shelled species developed even stronger claws. One defense mechanism of the Red rock crab is limb sacrifice. They will give up their claws and other limbs to escape because they can be shed and regenerated.


The Backside

The large back of the crab is called the carapace. Red rock crabs have a stalked compound eyes as well as light sensitive eye spots known as ocelli and antennae on the anterior side of the carapace. The antennae are chemoreceptors that allow the crab to find food and mates as well as taste and smell.



The Inside

When you pull the back off a boiled crab, you may notice the new cuticle forming for the next molt. It has the appearance of shell, but is soft and fleshy. The inside also contains a yellow mush, sometimes called crab butter. This is the digestive gland called the hepatopancreas. It has the same function as the liver and pancreas and is used for energy storage and the secretion of digestive enzymes.


Gills and Osmoregulation

The gills are important in acquiring Oxygen for cellular respiration. The gills are found in the center of the body on both sides in a pair of cavities called the branchial chambers. The gills are made up of stacks single layered cells called lamellae surrounded by a blood sinus. The lamellae are connected to blood vessels that travel the length of the gill. Water is moved through each chamber in an ore-like motion by the scaphognathite along the bottom side of the chamber, up along the gills and between the lamellae where oxygen is extracted, then forward where it is expelled out the mouth region.

Red rock crabs need to live in a relatively constant salinity  because they are unable to control their salt concentration. If the salinity of their habitat changes, salt and water move in or out through simple diffusion or osmosis. When crabs are in an isometric medium (same concentration as their body) they neither gain nor lose water through osmosis and drinking and urination are minimal.


Cardiovascular System

Centered in the crab between the two sets of gills is the heart connected to the arteries which circulate blood throughout the body. They have a open circulatory system and their blood is colored blue due to its copper-based respiratory pigment, hemocyanin.


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