Study Notes on True Crabs

In this article we will discuss about True Crabs:- 1. Habit and Habitat of True Crab 2. External Structures of True Crab 3. Digestive System 4. Circulatory System 5. Respiratory System 6. Nervous System 7. Endocrine System 8. Moulting 9. Reproductive System 10. Life History.

Contents:

1. Habit and Habitat of True Crab
2. External Structures of True Crab
3. Digestive System of True Crab
4. Circulatory System of True Crab
5. Respiratory System of True Crab
6. Nervous System of True Crab
7. Endocrine System of True Crab
8. Moulting in True Crab
9. Reproductive System of True Crab
10. Life History of True Crab

1. Habit and Habitat of True Crab:

The crabs are of several types and vary in size, shape and colouration. The crabs may be marine and fresh-water. Though its body is adapted for aquatic respiration, it may live for a considerable period on land. The crabs dig burrows on the shores or banks of rivers and ponds.

These animals come out for feed­ing, but with the slightest disturbance swiftly return to the hole. On land the true crabs do lateral movement with the help of walking legs. Some crabs which live mostly in water have appendages more adapted for swimming.

2. External Structures of True Crab:

The body is more or less oval in outline and the breadth is greater than the length (Fig. 18.18). The body of true crab is divisible into cephalothorax and abdomen.

Cephalothorax:

This is the most prominent part in the true crab and is covered dorsally by a flat broad and oval carapace which is made up of fused tergites. The anterior and lateral margins of the carapace are semi-circular and spiny. The ventral side of the cephalothorax is covered by sternites and its lines of fusions are well- marked. The sternites extend inside the body as folds. Such folds are known as apodemes.

These apodemes being united with each other form a continuous ventral framework, called endophragmal skeleton. This supporting framework provides the region for the at­tachment of muscles. The carapace is strongly calcified but the sternites are un-calcified. The exoskeleton which covers the two sides of the cephalothorax are known as pleuron.

The margin of the carapace which extends later­ally is called the pleural fold. On each side the pleural fold is separated into an anterior pterygostomial region and a posterior branchiostegite.

The pterygostomial region is immovable and separated from the cara­pace by a pleural suture. The branchiostegite is continuous with the carapace and forms the outer covering of the gill chamber. The rostrum is absent in true crab.

Following struc­tures are seen in the cephalothoracic region:

(a) Mouth:

It is a square opening on the ventral side of the anterior end.

(b) Eye:

At the anterior margin of the carapace one pair of stalked compound eyes are placed within definite sockets. The stalk is two-jointed and movable.

(c) Appendages:

The appendages are jointed and are of same numbers as seen in prawn, but are structurally different.

The cephalic appendages (Fig. 18.19) include:

(i) Antennule or First antenna. These are small paired retractile appendages which usually remain folded sidewise and are placed within sockets.

(ii) Second antenna. Usually these paired appendages are small and erect. In one genus of crab (Corystes sp.), the two second antennae anastomose to form a tube through which water is sucked in for respiration. This tube filters the sand parti­cles from water. When the animal remains within the burrow, the tip of the tube pro­trudes out for the entry of water.

(iii) Man­dible. In this paired appendages, the incisor processes are not serrated like those of prawn but the palp is stout.

(iv) and (v) First and Second maxillae. Of these two pairs of maxillae, the second maxillae are more promi­nent and its exopodites are developed into scaphognathites in the exhalant passage of the gill-chamber.

The thoracic appendages are the following:

(i) Maxillipeds:

In all the three pairs of maxillipeds, the exopodites are whip­-like and drawn backward to cover the gills. In the second and third maxilli­peds, the epipodites are sword-shaped. The basal joints of third maxillipeds are flattened ventrally to cover the anteriorly placed oral appendages.

(ii) Walking legs:

The walking legs are uniramous and jointed. The notable feature in a true crab is that the bases of walking legs on each side are set apart. Among the five pairs of walking legs, the first pair is well-developed and is provided with powerful chela. The other legs are clawed but not chelated. The first leg is used for food capture and defence, while the rest of legs are meant for walking sideways.

Abdomen:

The abdomen is six segmented, flap-like and much thinner in comparison to the cephalothorax. It remains flexed in the ventral side of the cephalothorax within a groove formed by sternites. The abdominal append­ages or pleopods are rudimentary. Sixth pair of pleopods is absent in both the sexes.

Two pairs of copulatory stylets formed by 1st and 2nd pleopods are seen in the males. In fe­males, four pairs of pleopods form an egg carrying basket. Thus in a crab, well-devel­oped locomotor appendages like swimmerets of prawn are not seen in the abdomen. Anus is present at the terminal end of last abdomi­nal segment.

3. Digestive System of True Crab:

The alimentary canal of true crab begins from mouth and consists of following parts—oesophagus, gizzard and intestine (Fig. 18.20A). The oesophagus is short but the gizzard is quite conspicuous.

A pair of superior caeca arises from the junction of gizzard and intestine. The intestine runs throughout the entire length of the abdomen and just before enter­ing the abdomen it sends an elongated tube, called the inferior caecum, to the dorsal side.

The intestine finally communicates with the exterior through anus which is present in the last abdominal segment. The digestive gland, hepatopancreas, is yellowish in colour and infiltrates the fold of carapace. Food is cap­tured by the chelate legs. In case the food is large, the mandibles bite it into pieces before swallowing. The maxillulae and labrum work with mandibular palp to force the food in­side the mouth cavity.

4. Circulatory System of True Crab:

Like prawn, the circulatory system is open type. Heart is irregular in outline and placed within a pericardial sinus in the thoracic chamber. The position of heart is just be­neath the carapace and over the reproduc­tive organs which in turn are present dorsal to the intestine.

Numbers of arteries are given from the heart to the different parts of the body (Fig. 18.21). The arteries open within haemocoelomic spaces. The blood contains the pigment haemocyanin.

5. Respiratory System of True Crab:

The primary respiratory organs of true crab are called gills, which are arranged like pyramid within the two gill-chambers. Each gill-chamber is present on the side of the thoracic chamber and is completely separated from it by pleuron. Externally the gill chamber is cov­ered by the branchiostegite.

The gill-cham­ber is drawn anteriorly into a tube which opens to the exterior. The water flows within the chamber through the opening in the under-surface of branchiostegite and passes out through the anterior opening. In the crab, Corystes sp., an interesting phenom­enon is noted regarding the flow of water.

When this crab remains within the burrow, the flow of water becomes reverse, i.e., water enters from the anterior end through a tube formed by the antennae and third maxillipeds and exits through the under-surface of the branchiostegites. But when the crab comes out of its burrow, the direction of water flow again becomes same as in other crabs.

6. Nervous System of True Crab:

The nervous system of true crab illustrates extreme instance of fusion which has re­sulted due to the close approximation of segments (Fig. 18.20B). The brain is placed at the anterior end and on the dorsal side. It is formed by the fusion of a pair of cerebral ganglia. From the side of each cerebral gan­glion one oesophageal connective arises.

These two connectives encircle the gut and unite with the ventral thoracic ganglionic mass in the middle of the thorax. These two connectives are joined transversely by a post-oesophageal commissure. A ganglion is present in the oesophageal connective. The ventral thoracic ganglionic mass is formed by the fusion of suboesoyhageal, thoracic, abdominal ganglia and major part of the entire ventral nerve cord.

This mass is perforated by the sternal artery at the middle. From the brain nerves are given off to the eyes, antenna and visceral organs. The thoracic ganglionic mass inner­vates maxillae, maxillipeds and thoracic limbs. On the ventral wall of the gizzard along the path of visceral nerve, a small ganglion is present.

7. Endocrine System of True Crab:

Several endocrine organs are present in true crabs. Within each eye stalk and more to­wards the eye there is an X-organ, which contains special neurosecretory cells.

At the base of each eye stalk another gland, called sinus gland, is present which acts as reser­voir for the hormones liberated by the X-organ. Two other endocrine organs— Y-organ in the antennal segment of head and androgenic organs along the sperm duct, are epithelial and non-neural in nature.

The post-commissural organ at the back of the head and pericardial organ near the heart, work as reservoir like the sinus gland. The X- organ controls the pigment pattern in the epidermal chromatophores and also influ­ences the action of Y-organ.

The Y-organ in one hand regulates moulting and also accel­erates the growth of gonads. The Y-organ degenerates after the attainment of maturity. The products from pericardial organ increase the heart-beat.

8. Moulting in True Crab:

The true crab is well-known for its ability to shed off the old exoskeleton. Both the exter­nal and internal surfaces which are lined by cuticle throw out the old skeleton and are replaced by new covering. This phenom­enon is called moulting and it is necessary for the growth of the individual. The phe­nomenon of moulting is seen in all arthro­pods but the process is well understood in crabs.

There are four stages in the entire cycle of moulting, all of which are under hormonal control:

(1) Proecdysis or Premoult:

At this stage calcium is removed from the old exoskeleton and the calcium content of the blood in­creases. A new and soft cuticle is formed beneath the old exoskeleton.

(2) Ecdysis:

It is the stage when the old exoskeleton’ is thrown out. After throwing out the old skeleton the animal swells up by taking in water and air in its internal spaces.

(3) Metaecdysis or Postmoult:

At this phase the new exoskeleton is calcified and hardened.

(4) Intermoult:

This is the stage when the individual is with hard exoskeleton. When young the inter-moult period is short but in a full-grown crab the inter-moult is terminal or final.

The process of moulting does not arrest the activity of the individual but involves continuous change in its physiology during different stages of moulting. For example, water content, organic reserves, concentra­tion of mineral salts, etc. are seen to alter during moulting. The moulting involves behavioural changes in the crab. The animals hide out at the time of moulting to protect its soft body.

9. Reproductive System of True Crab:

Sexes are separate and the sexual dimor­phism is noted specially in the disposition of abdominal appendages. In male, the first and second pleopods are uniramous and work as intromittent organ, i.e., organ for the transfer of sperm within female. In order to do this work, the endopodites of the second pleopods act as a piston within a tube formed by the pleopods of the first.

The pleopods in female consist of unisegmented protopodite, each carrying two elongated rami. These rami with their hairy processes form a basket to carry eggs.

In both the sexes the reproduc­tive organs are united in the middle and are extended laterally within the fold of the carapace. In male, the reproductive opening is present in the last leg and on a flexible process of the coxa. In female, the oviduct of each side is provided with a chamber called spermatheca and it opens in stermum.

10. Life History of True Crab:

Fertilization is external and the fertilized eggs are carried by the female in the basket formed by rudimentary abdominal append­ages. Development passes through the ap­pearance of three larval stages one after the other. These larval stages are known as Zoea, Metazoea and Megalcrpa.

The transformation from one stage to the next is attained by successive moulting. The characters of these larval forms are discussed later together with the various other larval forms in arthropoda. It may be mentioned here that the Zoea larva of crabs has two spines—one at the anterior end and the other at the posterior end of the carapace.

The third maxilliped is flattened (excepting Homolodromia and related forms, where it is biramous and acts as oar). In Metazoea stage the thoracic limb rudiments appear beneath the carapace and abdominal segments possess pleopods. The Zoea and Metazoea are free-swimming but the, Megalopa is able to walk on the substratum.