BSc ZOOLOGY NOTES-Prawn (Penaeus)-Morphology and anatomy

Penaeus

Phylum: Arthropoda,

Subphylum: Mandibulata,

Class: Crustacea

Penaeusis a genus of prawn under the class Crustacea. It has a worldwide distribution. The various species belonging to this genus are found both in tropical and temperate latitudes. Practically all of them are marine although some are known to spend a part of their life in the brackish water and even in freshwater.Some species represented in Indian waters are Penaeus japonicus, P.latisulcatus, P. monodon, P.semisulcatus, P. indicus, P. merguiensls and P. perucillatus.

External characters;The body of. Penaeus consists of two regions, the anterior cephalothorax and the posterior abdomen

Cephalothorax:

·         The cephalothorax is formed by the fusion of the head or cephalon and thorax.

·         The head consists of five segments and the thorax is eight-segmented.

·         The dorsal and the lateral sides of the cephalolhorax is covered by- a single chitinous exoskeleton called carapace or dorsal shield. It is formed by the fusion of five terga of the head and eight terga of the thorax.

·         A transverse cervical groove on the dorsal side seperates the anterior head from the posterior thorax.

·         The anterior region of the carapace forms in front a median serrated process known as rostrum. There are two compound eyes, which are attached to the base of the rostrum by movable stalks.

·         The free ventral flaps of the carapace on each side of the thorax is called. branchiostegite or gill cover.The space between the branchiostegite and body  wall of each side  is known as branchial chamber.

·         The respiratory organ, branchiae or gills lie  in the branchial chamber.The ventral sterna of the cephalothorax fuse and form  a plate known as sternal plate.

·         In female, the sternum of the last thoracic segment forms an outgrowth called thelycum,which encloses acavity.The male deposits its spermatophores into it. Cephalothorax bears., thirteen pairs of jointed appendages, of which the anterior five pairs are in the the cephalic region and the posterior eight pairs are in the thoracic region.

·         Infemale,a pair of genital openings lies at the base,of the sixth thoracic legs. In male, a pair of  genital aperturelies on a small papillae at the base of last pair of thoracic legs.

Abdomen:

·         The abdomen consists of six segments and a terminal piece called telson.

·         The dorsal side of each abdominal segment is covered by an exoskeleton called tergum .

·         The thin lateral downward prolongation of the tergum is known as pleuron.

·         The ventral plate-like exoskeleton of the abdominal segment is known as sternum.T

·         he part between the pleuron and ventral abdominal appendage on each side is known as epimeron.

·         The cuticle between the adjacent segments is thin. Each abdominal segment bears a pair of jointed ventral appendages. The last pair of abdominal appendages is called uropods.

·         In male, the first pair of abdominal appendages forms a copulatory apparatus calledpetasma.

·         The last piece of the abdomen, the telson, bears no appendages. Anus lies at the base of the telson ventrally. The uropods and the telson form the tail fin.

·         They are used for backward movement

Appendages:

Penaeus bears nineteen pairs of appendages.

They include cephalic, thoracic and abdominal appendages .

The appendages of prawn are many-jointed.

The segments of an appendage are known as podomeres.

 The appendages are typically biramous (= twobranched).

Each appendage has a two-jointed basal region called protopodite to which are attached two distal processes, the outer exopodite and the inner endopodite. This basic plan of the appendages is modified in different parts of the body to suit the varying functions performed by them.

Cephalic Appendages:Head region bears five pairs of appendages. These include a pair of antennules, a pair of antennae, a pair of mandibles, first pair maxillae and second pair maxillae.

Antennules:They are the anterior most appendages situated below the level of the eyestalks. They are also known as first antennae. Each antennule has a proterpodite, an exopodite and an endopodite .Protopodite has three podomeres, the proximal precoxa, middle coxa and the distal basis. The precoxa has a hollow depression on one side. The eye and the balancing sense organ/ statocyst are located in it. The opening of the statocyst lies at the base of this podomere. The basis carries short many jointed flagella-like exopodite and endopodite. They function as tactile sense organs.

Antennae:They are also known as second antennae. They lie immediately behind the antennules. Each antennule consists of protopodite, exopodite and endopodite. Protopodite has two podomeres, the proximal coxa and distal basis. The opening of the excretory duct lies in the coxa. Exopodite and endopodite are situated on the basis. Endopodite has three basal podomeres and a long narrow many-jointed filament. It functions as tactile sense organ. The exopodite is a flat broad plate-like structure and it is also known as squame. It functions as a balancing organ.

Mandibles:They lie on either side of the mouth. Each mandible has a protopodite and an endopodite. Exopodite is absent. Protopodite is a single stout calcified structure with toothed inner edge.

The teeth are used for grinding food to size. The endopodite, which is attached to the protopodite is segmented. The endopodite is also known as sensory palp.

First pair maxillae (Maxillulae):They are the smallest appendages of prawn. Each maxilla has a protopodite and an exopodite.Exopodite is absent. Each protopodite has two flattened leaf-like lobes, the coxa and basis. They protect inwards as jaws and are also called gnathobases.Endopodite is uniointed and leaf-like with narrow distal end. Both gnathobases and endopodite are provided with sharp hair like processes.

Second Pair maxillae:They lie just behind the first Pair of maxillae. Eacir maxilla is provided with a protopodite, an exoPodite and an endopodite.

Protopodite is flat and four lobed.Their edges are flattened and provided with hair-like recesses.They aid in mastication.The endopodite is small, unsegmented and leaf-iike. It is situated between the protopodite and the exopodite.The exopodite is large, expanded, flat and boat-shaped and is also known as scapho gnathite. Its movement produces

Water  currrents in the gill chamber.

Thoracic Appendages:Thorax bears eight pairs of appendages. They. include maxillipeds and peraeopodes or walking legs.They have a protopodite of two podomeres, an endopodite of five podomeres and an unsegmented exopndodite. The podomeres of the protopodite are proximal coxa and distal basis. The podomeres of the endopodite are named from the base ischium, merus,corpus, propodus and dactylus.

Maxillipeds:The first three pairs of thoracic appendages are known as maxillipeds,or foot jaws. They are directed forwards and run parallel to the middle line of the body.

First maxillipeds:The first maxilliped has a foliaceous appearance. It has a protopodite, exopodite and endopodite. Protopodite is flat and incompletely divided into two small proximal lobes and a large distal lobe.They are provided with setose processes on their edges. The exopodite is flattened and leaf-like with a broad base. Attached to the protopodite and exopodite is a proximal, triangular, flat structure called epipodite. It is respiratory in function.

Second maxillipeds:Protopodite has two podomeres, the proximal coxa and distal basis . Exopodite and endopodite are attached to the basis. Endopodite is made up of five segments. Endopodite curves distally and gives the shape ot an interrogation mark. Exopodite is slightly flattened with striations and feathery edges. A Y-shaped epipodite is attached  to the coxa of the protopodite.

Third maxillipeds:This is similar to the second maxilliped, but the endopodite is straight with the five segments . Exopodite is flat with striations and feathery edges. Y-shaped epipodite is attached to the coxa of the protopodite.

Peraeopods (walking legs):There are five pairs of peraeopods. The first three pairs of peraeopods are known as chelate legs or chelipeds and the last two pairs of peraeopods are known as non-chelate legs. In all peraeopods, the exopodite is small and fringed with many hair-like structures.

Chelate legs (Chelipeds):All chelate legs are identical in structure. But they differ slightly in size, the third one being the longest. The five jointed endopodite shows chelate articulation of the terminal two

podomeres, the propodus and the dactylus . By the hinged articulation of the dactylus to the side of the propodus, a pincer like apparatus is formed. This helps in grasping the food and passing it on to the mouth. The chelate legs are also used for walking

Non-chelate legs;In non-chelate legs, dactylus and propodus do not form chelate articulation . Epipodites are absent. Non-chelate legs are used for walking.

Abdominal appendages:There are six pairs of abdominal appendages.They are also known as pleopods or swimmerets. Each abdominal appendage consists of a protopodite and unjointed exopodite and endopodite. Protopodite is two-segmented and the exopodite is flattened and fringed with setose processes.

First pleopods:In female, exopodite is flattened, thick and fringed with setose processes. Endopodite is absent or may be present as a very small bud-like process. In male, endopodite is short and provided with hooks.The hooks of the  endopodites of the two sides interlock and form a rod-like structure called the petasma which is used for transferring sperms into the thelycum of female.

Second, to fifth pleopods:The second, third, fourth and fifth pairs of pleopods have a typical biramous structure, with a two-jointed protopodite, and unjointed exopodite and endopodite .

Sixth pleopods (Uropods):The sixth abdominal appendages are known as uropods. Each uropod has a protopodite, an exopodite and an endopodite. In protopodite the podomeres, coxa and basis fuse to form one segment. The two uropods and the telson form the tail-fin or tail-fan which acts as a balancing organ. Its sudden flexion causes backward leap.

The digestive system of Penaeus consists of the alimentary canal and the digestive gland .

Alimentary canal;The alimentary canal is a long tube, which extends from the mouth to the anus. It is divisible into three regions, stomodaeum, mesenteron and proctodaeum. The stomodaeum is the anterior region and it is lined with chitin. It consists of the mouth, the buccal cavity, the oesophagus and the stomach. The mouth lies on the ventral side of the cephalothorax, between the mandibles. It leads into a short buccal cavity. The stomach and the buccal cavity are connected by a short vertical tube called the oesophagus. The stomach consists of an anterior larger cardiac stomach and the posterior smaller pyloric stomach. The inner ventral folds of the cardiac stomach are provided with setae and spicules. The inner dorso-lateral folds of this region bear stout denticles which form the gastric armature or gastric mill for grinding the food. The pyloric stomach encloses a narrow cavity.

The pyloric stomach opens into the mesenteron.The mesenteron is the middle region, of the alimentary canal. It is not lined with chitin.The chitinous lining of the pyloric stomach forms one median dorsal,one median ventral and two lateral lippets or valvulae.They project into the mesenteron.The proctodaeum is the posterior region of the alimentary canal.It is lined

with chitin. It consists of the hindgut (or rectum) and the anus.The anus lies at the base of the telson ventrally.

Digestive gland :The digestive gland consists of a massive hepato-pancreas or liver.It lies in  the cephalothorasic region. The digestive secretions of this gland reach the mesenteron by a pair of openings.

Food and feeding:Penaeus feeds on small marine algae, other vegetation, small insects, etc.The chelate legs collect and pass the food material towards the mouth.The mandibles cut the food into small pieces. The maxillae and maxillipeds aid in swallowing the food.

Circulatory system

Circulatory system consists of heart,arteries,pericardial membrane, pericardial sinus,haemocoel , bood channels and blood or haemolymph. The heart is a triangular chamber. It lies in the pericardial space.,It is provided with paired openings called ostia. Arteries are the main tubes which arise from the anterior and posterior regions of the heart. The pericardial membrane lies below the dorsal body wall just above the alimentary canal. The space between the dorsal body wall and the pericardial membrane is the pericardial sinus.The spaces between the visceral organs form the haemocoel. It contains blood or haemolymph. From the haemocoel blood goes to the gills through the blood channels. From the gills blood goes to the heart through blood channels. The blood contains plasma, haemocytes or blood cells and the respiratory pigment haemocyanin.

Circulation of haemolymph

Haemolymph in the pericardial sinus enters the heart through the ostia. From the heart it reaches reaches the haemocoel through the arteries. Then it is collected by the afferent blood channels and returned to the pericardial sinus through the gills and efferent blood channels

Respiratory system

Penaeus takes up oxygen dissolved in sea-water. Its respiratory organs are inner lining of branchiostegites, epipodites (mastigobranchiae) and branchiae (gills) .

Branchiostegites:The ventral extension of the carapace on either side of cephalothorax is known asbranchiostegite.The space between the bodywall and branchiostegite is called gill chamber or branchial chamber. The inner lining of branchiostegite is highly vascular. It is bathed in water. Exchange of respiratory gases takes place through this inner lining.

Epipodites or mastigobranehiae;Penaeus possesses six pairs of epipodites or mastigobranchiae. They are the outgrowth of coxae of the thoracic appendages, three pairs of maxillipeds and three pairs of chelate walking legs. They lie in the branchial chamber. Epipodites of first pair of maxillipeds are flat conical plate like structures. The remaining epipodites are Y- shaped. They are bathed in water. They are richly supplied with blood. They exchange respiratory gases between water and blood.

Branchiae or Gills:Branchiae (gills) are the feather like (plumose ) outgrowth of the lateral wall of the thorax and thoracic appendages. Each gill has a stem  Three longitudinal blood channels run through the stem. They are two lateral  channels and one median channel. The two lateral channels are connected by many transverse channels. Many lateral flat gill plates arise from the stem.Marginal channels from the lateral channels penetrate into the gill plate and open into the median channel, The stem and gill plate are covered externally a thin layer of chitin. A single layer of epithelial cells lies beneath it. Epithelial layer encloses connective tissue and blood channels. This kind of gill is known as dendrobranchia.

According to the point of origin, there are three sets of branchiae. They are:

Podobranchs (podobranchiae):They are commonly known as foot gills. Penaeusl possesses one pair of podobranchs. They are the ourgrowths of the coxae of the second pair of maxillipeds.

Pleurobranchs (pleurobranchiae):They are commonly known as wall gills. They arise from the lateral wall

of thorax above the attachment of appendages. Penaeus possesses six pairs of pleurobranchs. They lie on the thoracic wall above the third pair of maxillipeds and five pairs of walking legs.

Arthrobranchs (arthrobranchiae):They are commonly known as joint gilts. They arise from the articularn membrane connecting thoracic wall and the proximal segment of the thoracic appendage. In Penneus two pairs of arthrobranchs arise from the articular  membrane connecting the thoracic wall and the proximal segment of the second maxillped,third maxilliped and first three pairs of walking legs. One pair of arthrobranchs arises from the articular membrane connecting the thoracic wall and the proximal segment of the fourth pair of walking legs.

Branchial Formula:The formula, which shows the number and arrangement of the branchiae or gills on one side of the thorax is known as branchial formula.

Mechanism of Respiration

There are two branchial or gill chambers. They enclose highly vascular respiratory organs (branchiae or gills, epipodites and inner lining of the brachiostegites). The anterior,ventral and posterior sides of each chamber are open. The exopodite (scaphognathite) of the second maxilla lies at the anteriorregion of the bianchial chamber. Its movement drives water out of the branchial chamber. So water from outside enters the chamber through the posterior side.A constant current of water flows over the respiratory organs.Oxygen dissolved in the water diffuses into the blood in the respiratory organs and carbon dioxide in the blood diffuses into the water. The setose processes along the anterior, ventral and posterior margins of the branchial or gill chambers prevent the entry of foreign particles into the chambers.

Excretory System:The excretory system consists of antennal glands and exoskeleton. There is a pair of antennal or green glands.Each lies enclosed in the proximal segment (coxa) of the antenna. Its parts are an end sac, a coiled tube and a bladder . The end sac is blind, the tubular part is glandular and the bladder is thin walled. The bladder opens to the exterior by the excretory pore. The end sac and the coiled tube are derivatives of the mesoderm and their spaces are part of the coelom. The body of Penaeus is covered by exoskeleton. It is shed during moulting. The nitrogenous waste materials are deposited on the exoskeleton and removed along with it.

Nervous system

It consists of supraoesophageal ganglion or brain, circum or perioesophageal connectives, suboesophageal ganglion, nerve cord, thoracic ganglia, abdominal ganglia and nerves .Supra-oesophageal ganglion ornbrain lies on the dorsal side of the thoracic ganglia oesophagus. Suboesophageal ganglion lies on the ventral side of theoesophagus. The brain and the suboesophageal ganglion are connected by two circum- or perioesophageal connectives. The circum-or perioesophageal connectives are connected by a visceral loop (transverse commissure). Nerve cord is double and ventral to the alimentary canal. It arises from the sub oesophageal ganglion and runs upto the end of the abdomen. Between the eleventh and twelfth segments, the two nerves of the ventral cord diverge for the passage of sternal artery. In the thoracic region and the abdominal region two nerve cords form six ganglia each. The brain innervates the eyes, antennules and antennae. The suboesophageal ganglion innervates the mandibles, two pairs of maxillae and first two pairs of maxillipeds. Thoracic and abdominal ganglia innervate the appendages of their respective segments

Sense organs;The receptor organs of Penaeus are compound eyes, statocysts, tactile organs and setae.

Compound eyes:There are two compound eyes/ one on either side of the base of the rostrum. They are at the ends of movable stalks. Each eye is made up of many optical units called simple eyes or ommatidia. The ommatidia are elongate, rod- shaped and radially placed. They lie side-by-side and are separated from each other by a sheath of black pigment cells. Each ommatidium consists of an outermost layer called cornea. It is formed by the transparent cuticle. Externally this layer is hexagonal. Each hexagonal part is called a facet. Corneagen layer lies below the cornea.It consists of two epidermal cells or lenticular cells. They secrete the outer cornea. Four cone cells lie beneath the corneagen layer and they are known as vitrellae or crystalline cone cells. The central transparent regions of. the four crystalline cone cells form the crystalline cone.The crystalline cone cells of the ommatidium are surrounded by distal pigment cells which contain dark brown or black screening pigment. Cornea, corneagen layer, cone cells, crystalline cone and distal pigment sheath form the focussing or dioptric apparatus of the ommatidium.Beneath the cone cells lie seven elongated cells retinular cells .They form the retinula. The microvilli arising from the axial surface of the retinular cell interdigitate with those of adjacent retinular cell and form a rhabdome in the central region of the retinula. The retinular cells contain proximal screening pigment. Retinular cells and rhabdome form the receptive or retinal part of the ommatidium.All the ommatidia rest on a porous basal lamina. From the basai region of each retinular cell arises an axon or nerve fibre. It passes through the pore. The axons of all the ommatidia of an eye form the optic nerve. Two types of images are formed by the compound eye of Penaeus.They are mosaic (apposition) and super-imposed (superposition) images.

Mosaic (apposition) image:Each ommatidium receives light rays from a small part of an object direct in front of it. Oblique rays are absorbed by the pigments. Straight light rays which are parallel to the axis of the ommatidium reach the rhabdome and form image of that part. The images of different parts of the object, formed by the different ommatidia together form the mosaic or apposition image of the object.This image is erect and not inverted.

Super-imposed (superposition) image:In dim light, the pigments in the distal pigment cells and the retinular cells are concentrated in their respective cells. Thus the optical separation between adjacent ommatidia is removed. Each ommatidium receives light rays from an object in front of it through the facets of the surrounding ommatidia as well as from its own and forms an image. Images formed by the ommatidum are the result of the superposition of light rays from a number of facets.The compound image thus formed is known is as superposition image.

Statocysts:There is a pair of statocysts. They are organs of orientation and equilibrium.Each statocyst lies at the basal segment of the antennule. It is a sac- like structure filled with sand particles which function as statolith. The sand particles are surrounded by elongated delicate receptor setae.

Tactile organs and setae :Antennae are the important tactile sense organs. Many sensory setae are located over the body surface especially the appendages.

Reproductive system:Male and female sexes are separate (gonochoric) and they are sexually dimorphic. The female can be distinguished from the male by. the following external characters.

Male Reproductive System:Male reproductive system consists of testes, vasa deferentia, ejaculatory bulbs, male genital openings and petasma .There are two testis. They lie in the thorax, one on either side of the middle line. They are tubular and united in front. Many caecal diverticula arise from each testis.From the Posterior region of each testis arises a tubular structure called vas deferens. It has a narrow anterior region, a swollen and convoluted middle region and a narrow posterior region.The swollen Posterior ends of the vasa deferentia are called ejaculatory bulbs. The openings of the ejaculatory bulbs to the exterior are called the male genital openings. They lie at the bases (coxae ) of the fifth walking legs. In male Penaeus the membranous endopodites of the first pair of pleopods bear hooks. The hooks interlock and form a niedian Process called petasma which is used to transfer sperms to female.

Female reproductive system:Female reproductive system consists of ovaries, oviducts, female genital openings and thelycumThere are two ovaries, one on either side of the middle line and they occupy the whole length of the thorax and abdomen. The right and left ovaries are united posteriorly and free anteriorly. Many distinct diverticula arise from the anterior region of the ovaries. From the middle region of each ovary arises an oviduct.The two oviducts open to the exterior by the female genital openings.They lie at the bases (coxa of the  third pair of walking legs.Infemale Penaeus  Sternum of the last thoracic segment forms an outgrowth called thelycum.It encloses a cavity. The male Penaeus transfers its speimatophore into this cavity

Life history of Penaeus:The female releases the eggs in the water Nauplius larva emerges from the egg. It passes the metanaupleus  protozoea ,zoea,metazoea and Mysis stages and becomes the adult.When the exoskeleton of the larvae becomes hard,further growth and increase in size is blocked. So the hard exoskeleton is shed and new soft exoskeletons formed beneath it. The periodical shedding of the old exoskeleton is termed moulting or ecdysis.