Prawn: Palaemon malcolmsonii

Palaemon malcolmsonii: The Indian Freshwater Prawn

Palaemon malcolmsonii or Macrobrachium malcolmsonii

                               Palaemon malcolmsonii

Systematic Position:

Phylum :        Arthropoda
Subphylum:  Mandibulata
Class:              Crustacea
Subclass:        Malacostraca
Order:             Decapoda
Suborder:       Natantia
Family:            Palaemonidac 
Genus:             Palaemon
Species:           malcolmsonii

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Habits and Habitat :

Palaemon inhabits freshwater streams, rivers, ponds and lakes. 

It is a nocturnal creature, hiding at the bottom during the day and coming to the surface at night in search of food. 

It is omnivorous, feeding on small organisms, like algae, mosses, minute insects, debris, etc. 

It walks slowly at the bottom with the help of its 10 walking legs and swims actively to the surface with the help of its 10 pleopods.

 When disturbed, it suddenly springs backwards with the help of a pair of uropods, attached to the last abdominal segment.

 In a desperate attempt to escape from the enemy's grasp, it can shed off one or more of its appendages.

 This phenomenon is known as autotomy. During the breeding period (May to July) the female is seen carrying a large number of eggs between its abdominal appendages.

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External Morphology:

1. Shape and size:

 Body is elongated, more or less spindle-shaped and bilaterally symmetrical. 

It offers least resistance in swimming. Size of adult varies from species to species. 

P. malcolmsonii, now Macrobrachium malcolmsonii, found in Central India and Tamil Nadu, measures 25 to 40 cm in length. 

The giant prawn P. carcinus from Kerala is upto 90 cm long. While the dwarf prawn P. lamarrei, found almost throughout India, is 2.5 to 5 cm long.

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2. Colouration:

Young stages are translucent and white, but the adults are differently tinted  according to the species. 

Usual colour is dull pale-blue or greenish with brown orange-red patches. Preserved specimens become deep orange-red.

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3. Segmentation and body divisions:

 Body of adult prawn is distinctly divided into 19 segments or somites, all bearing jointed appendages.

 The segments are arranged into two main regions an anterior cephalothorax (fused head-thorax) and a posterior abdomen.

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(a) Cephalothorax:

 Cephalothorax is large, rigid, unjointed and more or less cylindrical in shape. 

It consists of 13 segments. The joints between segments are obliterated.

 Cephalothorax is formed by the union of two regions : 

(i) head and (ii) abdomen.

 Head consists of 5 segments, while thorax includes 8 segments, all bearing jointed appendages.

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(b) Abdomen:

 Well-developed abdomen is jointed, unlike cephalothorax. 

It is composed of 6 distinct movable segments, and a terminal conical piece, the tail-plate or telson, which is not considered a segment because of post-segmental origin. 

Abdominal segments are laterally compressed and normally bent under the cephalothorax, so that the animal looks like a dorsally rounded, comma (,) in shape. 

The abdomen looks almost circular in a cross section.

 Each abdominal segment carries a pair of jointed appendages, called pleopods or swimmerets.

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4. External apertures: 

The slit-like mouth opens mid-ventrally at the anterior end of cephalothorax.

 Anus is a longitudinal aperture lying ventrally at the base of telson. Paired renal apertures open on raised papillae on the inner surface of coxae of antennae. 

Paired female genital apertures in female open on the inner surface of coxae of the third pair of walking legs. 

Paired male genital apertures in the male are situated on the inner surface of coxae of the fifth pair of walking legs.

 There are two minute openings of statocysts, one lying in a deep depression dorsally on the basal segment (precoxa) of each antennule.

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5. Exoskeleton:

Body and appendages are covered by a hard protective calcareous shell or exoskeleton. 

It composed of chitinous cuticle which becomes variously tinted by the deposition of lime salts and sclerotin. 

The exoskeleton comprises several hardened plates, called sclerites. 

Adjacent sclerites are connected by thin, soft, uncalcified cuticle or the arthrodial membranes, making the movements feasible.

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(a) Cephalothoracic sclerites:

 All the sclerites of dorsal and lateral sides of cephalothorax unite to form a single, large and coinuous dorsal shield. 

The anterior and somewhat triangular region of dorsal shield is termed dorsal plate. 

It extends forward over the head as a laterally compressed and serrated vertical process, called rostrum. 

At the base of rostrum, on either side, is an orbital notch, which accommodates a stalked, jointed and movable compound eye. 

Just behind and below each orbital notche two spine-like outgrowths, the anterior antennal spine and the posterior hepatic spine. 

The posterior region of dorsal shield is termed carapace. 

On either side of thorax, it hangs down freely as branchiostegite or gill-cover which encloses a gill-chamber housing the gills.

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(b) Abdominal sclerites:

 The sclerite of each abdominal segment is separate, ring-like and articulates with the adjacent sclerites by thin, flexible, uncalcified arthrodial membranes providing movable joints. 

In each abdominal sclerite, its dorsal broad plate is called as tergum, the ventral narrow transverse bar-like plate as stemnum, and the two lateral flap-like plates as pleura. 

An appendage is connected with the pleuron of its side by a small plate the epimeron.

Tergum and pleura of an abdominal segment slightly cover the corresponding parts of the succeeding segment. 

This overlapping is known as the imbricate arrangement of terga and pleura. However, the pleura of second abdominal segment are much developed and overlap the pleura of both the first and third segments, thus disturbing the imbricate arrangement. 

Pleura of sixth abdominal segment are greatly reduced.

Two adjacent abdominal segments articulate with each other by means of a pair of hinge joints, one on either side. 

A hinge joint consists of a small round peg, fitting into a socket on the succeeding segment. 

However, the hinge joints are lacking between the third and fourth segments.

 Abdominal segments can move upon each other only in a vertical plane due to presence of arthrodial membranes and hinge joints between them.

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Appendages:

Each segment of body bears a pair of jointed are appendages. 

Thus, there 19 pairs of appendages in Palaemon. They considerable variations, depending show on the functions they perform. 

However, they all are of a biramous type, as they are built on the same fundamental biramous plan.

Each appendage consists of a common base or protopodite, bearing two ramii or branches, an inner or median endopodite and an outer or lateral exopodite. 

Any appendage composed of two branches is called biramous (L. bi, two + ramus, branch). Typically, the basal protopodite is composed of two segments, a proximal coxa for attachment with the body and a distal basis which bears the two ramii, both comprising several segments or podomeres.

 With the exception of antennules which are uniramous, all the appendages of Palaemon, are homologous structures, regardless of functions, because they are all biramous and have similar embryologic origin. 

As they occur in a serial sequence on body, they also illustrate an example of serial homology.

In prawn, there are 19 pairs of appendages, 13 in cephalothorax and 6 in abdomen. 

Cephalo-thoracic appendages further include 5 pairs of anterior cephalic appendages and 8 pairs of posterior thoracic appendages.

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