There is an increased interest in aqua cultural activities, seafood consumption, and the need for professional consideration of aquatic animal health issues. An era of fish farming (aquaculture, mariculture) has suddenly burst upon the veterinary profession presenting new problems in the form of health care unique to aquatic animals. Seafoods have found a natural niche in a growing concern for a healthier diet. As aquaculture expands, it is essential that veterinary medicine be cognizant of its new opportunities in aquatic animal medicine. This is a relatively new field, and we are still pioneering, turning to the practice of rearing aquatic animals in controlled settings. The culture of aquatic organisms under controlled conditions is receiving increased attention as a major opportunity in alternative agriculture. There is a need for more research and expertise in aquatic animal medicine. Issues related to the health of aquatic animals represent both an opportunity and challenge for veterinary medicine.

Marine shrimp aquaculture is undergoing rapid worldwide expansion. Over 40 countries have shrimp farms with China leading the world in the production of farm raised shrimp. They produced 150,000 metric tons of shrimp in 1991. Indonesia produced 140,000 metric tons of shrimp in 1991. Thailand is the third largest producer of shrimp in the world; in 1991 they produced 110,000 metric tons of shrimp. Ecuador produced 100,000 metric tons of shrimp in 1991. The rapid growth of commercial shrimp mariculture has created a need for shrimp disease research. With close confinement production, disease problems exist. To assist in developing diagnostic procedures and to assist in the monitoring of the health of pennaei shrimp, Penaeus vmmamel were used for morphologic studies.

The laterally-compressed shrimp body is grossly divided into an anterior cephalothorax and a posterior abdomen. The cephalothorax includes the head as well as eight thoracic somites. This section includes the animal's sensory and masticatory apparatus and houses most of its organs. The abdomen is made up of six segments and contains most of the musculature.

The shrimp bears 19 pairs of appendages as well as one pair of eyestalks. The appendages each consist of a basal protoiodide, a lateral exopodite, and a medial endopodite. Variations on this basic structure have allowed the shrimp to adapt different appendages for specific purposes such as propulsion, manipulation, and mastication.

The cuticle consists of four layers covering an epidermis. The composition of each layer, from outer to inner, is as follows: a) epicuticle - calcium, no chitin; b) exocuticle - calcium and chitin, also a melanin-like pigment; c) endocuticle - calcium and chitin; and d) membranous layer - chitin, no calcium.

For the shrimp, molting involves shedding not only of the external cuticle but also of the gut lining, external eye, and other structures. In the pre-ecdysial (pre-molt) stage, the new cuticle's outermost three layers are formed. Once the old cuticle has been broken down, the shrimp's body swells and expands via uptake of water. The new cuticle then hardens. The entire process is controlled through the interplay of the molting and molt-inhibiting hormones.

The cephalothorax includes the prosencephalon and the gnath-thorax. The prosencephalon includes the following:

 Eyes: There are two eyes, each mounted upon an adjustable stalk.

 Antennules: The paired antennules are three-segmented and each bears a medial and a lateral flagellum.

 Antennae: Also paired, each antenna has five segments and one very long flagellum. The exopodite has evolved into a broad antennal scale which the animal uses as a rudder.

 Labrum: Formed as a projection of the body wall projecting over the mouth, the labrum constitutes the anterior end of the body.

The gnath-thorax contains the masticatory apparatus and the thorax proper. The heavily sclerotized mandible is equipped with incisor surfaces to cut and tear food. Dorsally, the mandible extends two palp-bearing tubes which curve around the labrum. There are two pair of maxillae, both adapted for food processing. In addition, the second pair serves to pump water over the gills. The maxillipeds make up the first three pairs of thoracic appendages. They vary in shape with the second pair being paddle-shaped and the third pair pedi-form. The five pairs of pereiopods are also known as walking legs. They arise along the ventral midline, and the first three pairs are chelate. The first pair does not aid in walking and is carried pointing anteriorly.

The abdomen and its appendages are devoted to propulsion. The animal propels itself forward through the water with its five pairs of pereiopods, or swimming legs and backward by a rapid contraction of its abdominal muscles, powerfully flexing the abdomen. In the male the first pair of pereiopods is modified for sperm transfer. The telson is a single, triangular projection bearing the shrimp's anus. It extends posteriorly. The paired, paddle-shaped uropods lie ventral to the telson. They are considered evolutionarily homologous to the legs although they are considerably broader and are reduced in segmentation.

The digestive system consists of a mouth, foregut, midgut, hindgut, and hepatopancreas. The shrimp's mouth is located on the ventral surface of the head region anterior to the first maxilliped. Food entering the mouth has undergone initial mastication by the appendages adapted for this purpose. The foregut includes the esophagus and stomach. It is lined by cuticle and functions mainly to grind ingesta. A thick layer of spongy connective tissue supports the cuticle and cuticular epithelium. After passing through the esophagus, a simple channel lined with low columnar epithelium, ingesta enters the stomach. The stomach extends posteriorly approximately to the midpoint of the hepatopancreas. The stomach consists of anterior (cardiac stomach) and posterior (pyloric stomach) chambers. Tooth-like projections (ossicles) extend from the stomach wall in places and serve as grinding surfaces. The cuticle is calcified in these regions. This chamber is equipped with supra-lateral teeth for grinding. Upper and lower grooves (cardiac grooves) are believed to direct the flow of enzymes from the hepatopancreas. Surrounding the anterior chamber, a number of muscular bundles direct food against the grinding ossicles.

The posterior chamber is divided into dorsal and ventral sub-chambers. The ventral chamber houses the gastric sieve. This sieve screens masticated food for delivery to the hepatopancreas. It is made up of circular setae and grooves. If the ingesta are small enough, it will pass between the setae, through the longitudinal grooves, and into the hepatopancreatic primary ducts. The remainder of the ingesta passes into the midgut.

The midgut is the primary absorptive area of the digestive tract. It is not lined by cuticle. It passes posteriorly accompanied by the dorsal abdominal artery. The anterior midgut passes between the dorsal and ventral lobes of the hepatopancreas. In this region, the mucosal epithelium is composed of simple columnar cells. A brush, or microvillous, border extends into the lumen. Throughout its length, the midgut epithelium is supported by a basement membrane, an inner layer of circular muscle, and an outer layer of longitudinal muscle. In the central midgut, the epithelial cells are slightly shorter than those of the anterior region. In the posterior region, the epithelium ranges from cuboidal to low cuboidal and in some places is nearly squamous.

The two midgut ceca increase the absorptive surface of the midgut. The anterior midgut cecum lies at the stomach-midgut junction, and the posterior midgut cecum lies dorsally at the union of midgut and hindgut. Each consists of a blind sack with large epithelial folds projecting into the lumen. The epithelial cells are simple columnar in shape. The midgut ceca epithelium usually has a high mitotic index. The epithelium of the posterior midgut cecum connects directly to the hindgut epithelium, and the demarcation is distinct.

The deca-pod hindgut, located within the 6th abdominal segment, includes the rectal gland, rectum, and anal canal. A thin, non-calcified cuticle lines its simple, single-cell thick epithelium. As in the midgut, a thin muscular wall surrounds the tract. The extreme posterior and anterior portions of the hindgut are marked by folds which increase expendability and surface area. The middle region has no such folds. The hindgut ends with an anus, which opens between the telson and the uropods.

The hepatopancreas, the shrimp's primary digestive gland, occupies a major part of the posterior cephalothorax and surrounds the posterior stomach and anterior midgut. The hepatopancreas is composed of a large number of blind-ended tubules. Each tubule is surrounded by a network of myoepithelial cells and associated contractile fibers. The tubules are bundled into lobes by a spongy connective tissue sheath, or tunica propria.

The tubule epithelial cells exhibit a high turnover rate, suggesting holocrine secretion. A marked transition may be seen in each tubule from the new, undifferentiated end through the mature cells of the proximal region. Embryonic, or E cells, inhabit the apices of the tubules. This area is characterized by the presence of mitotic figures and binucleated cells. Proximally, the mature epithelial cells take on absorptive, secretary, and storage functions. The fibrous, or F cells exhibit a large, metachromatic cytoplasm and large nuclei. The F cells appear more basophilic in the proximal areas of the tubule. The most conspicuous cells are the B cells. These secretary cells have large nuclei and are sometimes binucleated with the nucleus or nuclei displaced toward the basal portion of the cell. These cells have a vacuolated cytoplasm as do the R cells. The R cells are long and narrow with a vacuolated cytoplasm. They often contain lipid droplets which are easily differentiated from secretary vacuoles by staining the section with an osmium tetroxide stain. With this stain, the lipid droplets appear black and the secretory vacuoles clear. A microvillous brush border extends into the tubule lumen.

The shrimp gills are found on the thorax under lateral extensions of the carapace known as branchiostergites. The primitive model for the crustacean gill involves four gills to a segment. Shrimp have dendrobranchiate gills. This term means that the gill is composed of a single central axis from which branches project at right angles. The central axis fastens to the cephalothorax wall by a tubular structure. This tubular structure is found near the ventral end of the axis. From the central axis project primary filaments. The primary filaments in turn give off secondary filaments. If any of these branches are viewed in cross section, the afferent and efferent vessels may be seen. These vessels run adjacently with the efferent vessel dorsal to the afferent.

The heart lies dorsally in the pericardial sinus in which it is suspended at various points by suspensory ligaments. The heart is composed of myocardial cells arranged in bands. The bands are in turn organized into bundles, which divide the heart into sub-chambers. The epicardium, composed of spongy connective tissue, surrounds and protects the heart. Ten ostia, six dorsal and four ventral, penetrate the epicardium and myocardium. Hemolymph enters the heart lumen from the pericardium through these ostia. The pericardium is surrounded by a pericardial septum. This septum is thick and spongy dorsally while thin and dense ventral to the heart.

Hemolymph leaves the heart through the posterior aorta and the double-branched anterior aorta. The posterior aorta exits from the heart's ventrolateral aspect. Flow back into the heart is prevented at this point by the proximal valve, which consists of a single, dorsally attached flap. Small muscle bands within this valve may add an active component to valve closure. The branches of the anterior aorta leave the heart at its dorso-anterior aspect. Each branch has an associated valve. Unlike the proximal valve, these valves have both dorsal and ventral flaps.

Hemolymph flows from the aortae through system arteries into the hemocoel lacunae, in which nutrient and gas exchange take place. From the lacunae, hemolymph flows through ventral venous sinuses to the median ventral sinus and then through the vessels of the gills. The hemolymph enters the pericardial sinus from the gills and then flows into the heart through the ostia.

Major systemic arteries of the shrimp include the median anterior artery, the paired antennal and hepatic arteries, the median posterior artery, and the sternal artery. The lumen of each major vessel is lined by an acellular intima. In large vessels, a single layer of muscle lies distal to the intima. The muscle layer is surrounded by an often multi-layered endothelium which is sometimes associated with a fibrous or lamellar connective tissue. The outermost layer is made up of elastin fibers.

We observed three distinguishable types of circulating hemocytes: Hyal-inocytes (HC), eosinophilic granulocytes (EG), and intermediate granulocytes (IG). Eosinophilic granulocytes were normally the largest of the three cell types with a small, sometimes eccentric nucleus. The cytoplasm contained an abundance of large eosinophilic granules. The intermediate granulocytes were intermediate in size and contained small granules in an abundant cytoplasm. Another cell, the cyanocyte, which has a large eosinophilic inclusion body, was abundant in the loose connective tissue but did not occur in the circulating hemolymph. The hemopoietic tissue was consistently located in three sites: 1) in the base of the second and/or first maxilliped, 2) dorsolateral to the cardiac stomach and 3) enveloping the ophthalmic artery. The tissue is arranged into lobules, each defined by a thin fibrous connective tissue layer. Hemal sinuses are lacunae interspersed between the lobules. These sinuses contain the immature, developing hemocytes. Hemopoietic tissue is often interspersed with spongy connective tissue and surrounded by striated muscle. Numerous mitotic figures indicate the tissue's high mitotic index.

Evidence is accumulating that decapods, like many invertebrates, possess a host defense mechanism which shares some characteristics with vertebrate immune response, i.e., induced production of agglutinins, bacteriolysins, complement activating mechanisms, etc. Studies in our TAMU laboratories suggest that the principal active component(s) involved in the shrimp's immune response are lectin- or lectin-like hemolymph components which participate in relatively nonspecific serologic reactions and stimulate mitogenic activities of shrimp hemocytes.

The shrimp defense system employs several hemolymph-borne factors in its fight against infection. Most of these factors are inducible, although they lack a high degree of specificity and display no apparent memory. These factors serve to immobilize and agglutinate invaders as well as to sensitize them to phagocytosis. Both hemagglutinins and bactericidins have been identified in shrimp hemolymph. The agglutinins are protein molecules which play a role similar to that of mammalian antibodies. They also serve as opsonins and display some degree of binding specificity. Studies have also identified lectins involved in the immune response. These inducible hemolymph components are absorbed onto the surface of Gram negative bacteria where they enhance phagocytosis and the production of bacteriolysins and agglutinins. The carbohydrate receptors to which they bind are common ones, explaining their nonspecific response.

In the shrimp, most excretion occurs through the antennal (or green) gland. This gland, a structure of the cephalothorax, is composed of an end sac, a bladder, and an excretory tube. The end sac lies in front of and to both sides of the esophagus. It is made up of a saccule and a labyrinth. The labyrinth is a network of tubules found throughout the cephalothorax but primarily in its anterior end. These tubules surround the supra-esophageal ganglion and the major nerve tracts. The walls of the labyrinth tubules are folded and secretary. They are made up of simple cuboidal epithelium, the cells interdigitate with the podocytes of the coelomosac epithelium. The space between the tubules and the surrounding hemolymph sinus is filled with a layer of fibrous connective tissue. The tubule epithelium occurs in both secretory and non-secretory states.

The bladder is a lobular structure which joins to the labyrinthine portion of the end sac. Its simple columnar epithelium occurs in secretory and non-secretory states, usually corresponding to the condition of the end sac epithelium. When secretary, the cells exhibit a highly vacuolated cytoplasm. The apical ends of the epithelial cells are modified into a ragged brush border. The bladder empties into a duct which runs into the basal segment of the second antenna where it empties into the external environment through a papilla.

The supra-esophageal ganglion, or brain, lies just posterior to the bases of the anterior head appendages. Various nerve tracts supply different areas of the shrimp body. The paired antennal nerves run ventral to the circum-esophageal connectives. The ventral nerve cord supplies the bulk of sensory and motor innervation. Like other crustaceans, shrimp have giant nerve fibers extending from the supra-esophageal ganglion to the last abdominal ganglion. Paired ganglia occupy each segment and organize peripheral innervation.

Innervation to the gut occurs through a sympathetic (somato-gastric) system which arises from the esophageal commissures and supplies mostly the anterior end of the digestive-tract.

The heart receives its innervation from a local system as well as from branches of the central tract. There is a cardiac ganglion in the dorsal heart wall. Both inhibitory and excitatory innervation regulates the heart. One pair of inhibitory fibers arises from the sub-esophageal ganglion. Two pairs of excitory fibers arise ventral to the inhibitory fibers and enter the heart wall.

The shrimp's compound eye consists of numerous functional units, or ommatidia. Each ommatidium is covered by a corneal layer produced by the cornea-genous cells immediately below it. Below the cornea lie the quadripartite crystalline cones, then a circle of retinular cells and their associated pigments. Seven of these cells surround each rhabdom. The rhabdom may be identified by its striated appearance. Together, these structures make up the dioptric portion of the eye. A basement membrane divides this portion from the ganglionic portion which lies proximal to it.

The eye contains three ganglia: the proximal medulla terminalis, the medial medulla internal and the distal medulla externa. Each ganglion contains a matrix of capillaries and lacunae. A sinus gland lies between the medulla interna and the medulla externa. It is composed of spongy cells and serves to store hormonal products produced by nearby neurosecretory organ. There are numerous axons associated with this gland. At the distal extremity of the ganglionic portion lies the lamina ganglionaris. This structure is composed largely of nerve and glial fibers.

Two specialized organs, the organ of Bellonci and the organ of Hanstrom (Han) occupy the ganglionic portion of the eye. The organ of Bellonci, also known as the sensory pore X organ or the pars distalis X organ lies near the dorsal surface of the eye in the region of the medulla terminalis. It consists mainly of neurosecretory cells and onion bodies. The organ of Hanstrom, also called the ganglionic X organ or the medulla terminalis X organ, lies ventrally, opposite the organ of Bellonci. Many axons enter this organ. Some connect to the sinus gland. The entire eye is attached to the head region by a proximal stalk.

Viewed grossly, the petasma is the most obvious external reproductive structure in the male. The petasma, a modified endopodite, is found on the first pleo-pod. It is made of long rods connected to a thin section of cuticle. It serves to hold the spermatophore (sperm capsule) in place on the female. The male secretes the spermatophore through the gono-pore. The gono-pore is found within the folds of cuticle between the bases of the fifth walking legs.

Spermatogenesis takes place in the testes. These lobed, paired structures are located on the dorsal surface of the hepatopancreas ventral to the heart. When a seminiferous tubule is viewed on cross section, spermatogonia are seen at one side of the tubular periphery. Nurse cells make up the remainder of the periphery. Most of the cells seen in the seminiferous tubules are primary spermatids although primary and secondary spermatocytes are also present.

The seminiferous tubules open into the vas deferens. This tube consists of two lumen separated by a double layer of epithelium and marked by a longitudinal fold. The larger primary lumen contains mature spermatozoa while the smaller secondary lumen carries the acellular primary spermatophore layer. This layer will eventually wrap around the spermatozoa, separating them into bundles ready for export. The vas deferens runs anteriorly to the glandular vesicle known as the terminal ampoule and ends in a muscular ejaculatory duct.

Shrimp spermatozoa are non-flagellated and non-motile. Each presents a protruding rigid spike with an acrosome at its base. This structure enables the sperm to penetrate the ovum at the time of fertilization.

The female shrimp releases ova through a gono-pore which opens through labiate structures between the third walking legs. Fertilization takes place on the thelycum, a structure made up of modified sternal plates on the fourth and fifth thoracic segments. The lobes of the thelycum are adapted to receive and retain the spermatophore at the time of mating.

The paired ovaries are connected by a median lobe. Each extends a long anterior projection and several shorter lateral projections. These lateral projections run ventral to the heart. In addition, each ovary projects into a dorsal lobe posterior to the heart.

The oviduct runs ventrally from the thoracic ovary to the gono-pore. It is lined by tall epithelial cells with basal nuclei and light-staining cytoplasm. Below the epithelium, there is a fibrous layer in turn surrounded by a hemal sinus. The outermost layer of the oviduct consists of muscle.

The female shrimp releases 500,000 - 1,000,000 eggs at a time. These eggs are fertilized at the time of mating. Unlike most decapods, which carry their eggs attached to their pleo-pods, Penaeid shrimp release their eggs into the water immediately after fertilization.

The shrimp egg measures approximately .22 mm in diameter. After about 14 hours it hatches to release a naupilus larva. This larval stage displays an oval, unsegmented body and three pairs of appendages. The animal utilizes these appendages for sensory reception, feeding, and, to some extent, locomotion. However, it is a poor swimmer. Its head region bears antennules, antennae, a mandible, and a single median eye. As the animal develops, its body will elongate, grow more appendages, and become segmented. New segments will develop from a formative zone anterior to the telson. The naupilus larva feeds on yolk. The shrimp remains a naupilus for 36-51 hours during which time it undergoes four molts.

After its fifth molt the shrimp becomes a protozoea (sometimes called a zoea). During this period the animal acquires four additional appendages, compound stalked eyes, and biramous uropods. The protozoea feeds on plankton. This period persists through two molts at 36-48 hour intervals. After the next molt the shrimp is considered a my-sis. This period marks the emergence of pleo-pods. Initially, the pleo-pods are unsegmented, but they acquire segmentation during the shrimp's three stages as a my-sis. Each stage lasts approximately 24 hours. The shrimp then spends 24 additional hours as a post-larva, or mastigopus, before achieving its final adult composition.

The gills, midgut, hindgut, heart, muscle, lymphoid organs, nerve cord and hepatopancreas are routinely examined when monitoring for diseases. Other structures studied are the y-organ, mandibular organ, antennal gland, tegmental glands, ovary, testis, vas deferens, and exoskeleton. The endocrine organs of the shrimp are the organ, x-organ, mandibular organ, pericardial organ, ovary, postcommissural organ and androgenic gland.

References

1.  Bell, T.A. and Lightner, D.V. A Handbook of Normal Penaeld Shrimp Histology. The World of Aquaculture Society, Baton Rouge,LA, 1988.

2.  Caceci, T. ; Neck, K. F. ; Lewis, D. H. and Sis, R. F. Ultrastructure of the hepatopancreas of the Pacific white shrimp. J. Marine Biology, Auss. UK. 68:323-37, 1988.

3.  Lewis, D.H. Response of brown shrimp to infection with Vibrio sp. Proceedings of the 4th Annual Workshop World Mariculture Society, pp. 333-338, 1973.

4.  Meglitsch, P.A. Invertebrate Zoology. Oxford Univ. Press, London, N.Y., 1967.

5.  Rigdon, R.H. and Mensik, D.J. Gastrointestinal tract of Penaeus aztecus. Crustaceana. 30(2), 1976.

6.  Sis, R. F. ; Lewis, D.H. and Caceci, T. The hemocytes and hemopoietic organs of a penaeid shrimp (Penaeus vannamei). International Association for Aquatic Animal Medicine Proceedings. 18:2-10, 1987.

7.  Tsing, A.; Arcier, J-M. and Brehelin, M. Hemocytes of Penaeid and Palaemonid shrimp: morphology, cytochemistry, and hemogram. J. of Invertebrate Pathology. 53:64-67, 1989.

8.  Young, J.H. Morphology of the White Shrimp. U.S. Printing Office, Washington D.C., 1959.