BLUE CRAB ANATOMY

The six swimming or walking legs, 3 on each side, are known as pereopods (3).

At the end of each Cheliped (1) is a claw (chela) or cutter cheliped. Each cheliped is divided into 4 main parts, listed from the closest to the main body: (a) mercus, (b) carpus, (c) propodus and (d) dactyl which is the 'moveable finger' at the side of the claw. The large part of the claw is the crusher cheliped.

Blue crabs have a cephalothorax, the head and thorax united as a single part. The mouth (2) contains two prominent triangular teeth with nine anterior lateral teeth, the hindmost tooth being the longest and directed outward.

The swimming leg (4) is divided into 7 parts: (k) coax, (j) basis and (i) ischium which are fused together to form the basischium, (h) mercus, (g) carpus, (f) propodus and (e) dactyl.

There is an opening next to the coax where it meets the main body of the crab which is the inhalant aperture which leads to the brachial chamber where the gills (7) are located.

Behind each gill chamber is a heavy endoskeletal (an internal support structure of an animal) plate called the flanc (5) that covers the swimming muscles.

The hepatopancreas (9), or mustard as many call it, is the large internal organ which secrets digestive enzymes. It also absorbs and stores digested food. Calcium and phosphate are also stored here during the intermolt period. It also functions in the capacity of cleaning the crab’s blood stream. When the crab is cooked and the carapace is removed, the hepatopancreas appears as a light brown or yellow mass giving rise to the name 'mustard'.

* 7 - The respiratory system consists of the gills located in two gill chambers. They occupy the left and right sides of the chamber. There is a thin wall over the gills which is an outer membrane covering the surface of the gill chamber. It separates the gill chamber, which is filled with seawater, from the cavity which is filled with blood. There are eight gills on each side but two of them are small and easily overlooked. Each of the eight gills consists of a long central axis to which are attached, on opposite sides, two rows of very closely spaced flat, semicircular branchial filaments. The filaments are covered by film which is molted along with the rest of the exoskeleton. The gills divide the gill chamber into water and blood regions. Water flows in an aperture to the ventral region, then across the gill filaments into the dorsal region. It then exits via another aperture. There is a special area that holds water. This system acts like a primitive lung. As long as the gills are wet, oxygen can be taken from the air in the gill chamber.

TOP OF PAGE

The thorax consists of 8 segments, or thoracomeres, of which the first 3 are part of the cephalothorax. The 5 posterior thoracomeres are not fused with the head although dorsally they appear to be because all are covered by the carapace. When examined from the bottom, however, the 5 segments can be seen to be independent of the cephlothorax. The 5 independent segments are known as the PERON; their appendages are the PEREOPODS. Each individual segment is a PEREOMERE. The pereopods are the 5 pairs of walking legs. The name Decapoda ("ten feet") alludes to these ten appendages. The anus lies at the tip of the abdomen (X). An intestine runs the length of the abdomen. The pereopods are numbered A through E and the sternites of the thoracomeres are numbers 1 through 8.

TOP OF PAGE

The first 3 segments of the thorax are fused with the 5 head segments. Together they form the maxillipeds. Together the head and the first 3 segments form the cephalothorax. Each segment has a pair of maxillipeds. The carapace covers the entire head and thorax areas.

Crossing the midline of the carapace is an H-shaped set of groves which marks the approximate dividing line between the head and the thorax. This is called the cervical groove.

The brain is found near the back of the blue crab midway between the two eye stalks beneath a layer of muscle.

More detailed blue crab anatomy is available HERE.