63
enlarged (Kaestner and Levi 1970). There are five pairs of walking legs from the
thorax section. The first pair improved and has a strong pair of claws. They use their
clamps as defence and nutritional purposes. The last walking legs of the marine species adapted to life in the pelagic areas of the sea have turned into swimming legs.
Usually the right foot is larger. Crabs often walk sideway. They move in such a way
that they can only bend their legs in one direction.
The basic crab design consists of an expanded carapace, and a strongly reduced
abdomen that is tightly tucked underneath the thorax. In addition, the first pereiopods
of brachyurans are fully chelate, and the walking legs are placed at the sides of the
body. True brachyuran crabs are often confused with hermit and porcelain crabs
belonging to the infraorder Anomura. In general, most anomuran crabs have only
three pairs of walking legs clearly visible, with the last pair being very small and
normally positioned under the abdomen and not visible externally (Jose 2015).
The true crabs (Decapoda: Eubrachyura) have a depressed carapace or cephalothorax and a much reduced, straight, and symmetrical abdomen which is closely
bent under the cephalothorax; this abdomen is never used for swimming and lacks
biramous uropods; in the female, during the spawning season, the eggs are attached
to the abdominal appendages (berried crabs). The cephalothorax has five pairs of
walking legs, the first of which is chelate (ending in pincers) and nearly always
much stronger than the other legs (Tavares 1998).
When feeding of crabs is examined, it is understood that these creatures are both
herbivorous and carnivorous. In general, crabs are fed with algae. Besides, many
things such as molluscs, worms, other crustaceans, and sediments can also be a
source of food for crabs according to their hunting abilities and species. Mixed
nutrition (plant and animal) is a necessary condition for the development and
metabolism of crabs.
Although most of them are benthic, only the larvae and some species of crabs
that show general metamorphism in their developmental stages are pelagic. They
are mostly found in rocky areas with plants and muddy and sandy grounds. Most of
them are hunted alive and attacks on fish and amphibians. Cannibalism is also common among crabs (Carroll and Winn 1989).
Crabs are found in a distinctly shaped hard shell, and at regular intervals the shell
is replaced by larger shells to allow the crabs to develop. This shell replacement
process is called moulting or ecdysis. Shell replacement period is usually between
July and October. Females begin a little later in males as they begin to change shells
in July. In this period, crabs are protected between rocks. The shell cracks in such a
way that it separates the lower and upper halves with a precise line. The soft crab
slowly emerges from the slit. It then absorbs water and swells. When the shell
replacement period is complete, the shell slowly hardens. Although the shell is not
fully set within a few months, there is no change in size until the next shell change
period. Shell replacement occurs frequently at the beginning of the life of the crab
and occurs approximately every 2 years after reaching the specific size. It is impossible to determine the exact age of the crab. There is no guide in his body. But the
average10–15 cm crab is about 4–5 years old (Edwards and Early 1978).
2.2 Crabs
enlarged (Kaestner and Levi 1970). There are five pairs of walking legs from the
thorax section. The first pair improved and has a strong pair of claws. They use their
clamps as defence and nutritional purposes. The last walking legs of the marine species adapted to life in the pelagic areas of the sea have turned into swimming legs.
Usually the right foot is larger. Crabs often walk sideway. They move in such a way
that they can only bend their legs in one direction.
The basic crab design consists of an expanded carapace, and a strongly reduced
abdomen that is tightly tucked underneath the thorax. In addition, the first pereiopods
of brachyurans are fully chelate, and the walking legs are placed at the sides of the
body. True brachyuran crabs are often confused with hermit and porcelain crabs
belonging to the infraorder Anomura. In general, most anomuran crabs have only
three pairs of walking legs clearly visible, with the last pair being very small and
normally positioned under the abdomen and not visible externally (Jose 2015).
The true crabs (Decapoda: Eubrachyura) have a depressed carapace or cephalothorax and a much reduced, straight, and symmetrical abdomen which is closely
bent under the cephalothorax; this abdomen is never used for swimming and lacks
biramous uropods; in the female, during the spawning season, the eggs are attached
to the abdominal appendages (berried crabs). The cephalothorax has five pairs of
walking legs, the first of which is chelate (ending in pincers) and nearly always
much stronger than the other legs (Tavares 1998).
When feeding of crabs is examined, it is understood that these creatures are both
herbivorous and carnivorous. In general, crabs are fed with algae. Besides, many
things such as molluscs, worms, other crustaceans, and sediments can also be a
source of food for crabs according to their hunting abilities and species. Mixed
nutrition (plant and animal) is a necessary condition for the development and
metabolism of crabs.
Although most of them are benthic, only the larvae and some species of crabs
that show general metamorphism in their developmental stages are pelagic. They
are mostly found in rocky areas with plants and muddy and sandy grounds. Most of
them are hunted alive and attacks on fish and amphibians. Cannibalism is also common among crabs (Carroll and Winn 1989).
Crabs are found in a distinctly shaped hard shell, and at regular intervals the shell
is replaced by larger shells to allow the crabs to develop. This shell replacement
process is called moulting or ecdysis. Shell replacement period is usually between
July and October. Females begin a little later in males as they begin to change shells
in July. In this period, crabs are protected between rocks. The shell cracks in such a
way that it separates the lower and upper halves with a precise line. The soft crab
slowly emerges from the slit. It then absorbs water and swells. When the shell
replacement period is complete, the shell slowly hardens. Although the shell is not
fully set within a few months, there is no change in size until the next shell change
period. Shell replacement occurs frequently at the beginning of the life of the crab
and occurs approximately every 2 years after reaching the specific size. It is impossible to determine the exact age of the crab. There is no guide in his body. But the
average10–15 cm crab is about 4–5 years old (Edwards and Early 1978).
2.2 Crabs