MORPHOLOGY OF THE DIGESTIVE TRACT OFTHE AFRICAN CATFISH (CLARIAS GARIEPINUS BURCHELL 1822)

ABSTRACT

Seventy five African catfish made up of 25 fingerlings, 25 juveniles and 25 adults, sourced from Yuep Farms, Umuahia, Abia State of Nigeria, were used for the study. The fish were weighed, and the standard  and total body lengths  were measured.  The fingerlings  and  juveniles  were euthanized  with chloroform,  while  the adults  were  humanely  immobilized  by stunning.  The digestive  tracts  were  dissected  out  and  the  intestinal  lengths  were  measured.  The  relative intestinal  lengths  were  also  determined.  Slices  of  oropharyngeal  wall,  tongue,  oesophagus, stomach, intestine, rectum and anus were excised and fixed in 10% neutral buffered formalin, for

16-24 hours. They were embedded in paraffin wax, sectioned and stained with haematoxylin and eosin. Adult pharyngeal pad was sliced into small pieces, decalcified and similarly  processed. The carbohydrate histochemistry was carried out using periodic acid Schiff (PAS), alcian blue (AB) at pH 2.5, and a combination of AB and PAS procedures. Sections were examined with a light microscope and photographed with a Moticam camera fixed to the microscope. Data were analysed  statistically  using  analysis  of  variance.  Duncan’s  multiple  range  test  was  used  to separate variant means, and significance was accepted at p< 0.05.

Grossly,  the oral surfaces  of the palatine  and mandibular  rostral  ends of the fish  contained roughened grinding plates, with a degree of roughness that increased with age. The tongue was attached  to  the  floor  of  the  oro-pharyngeal  cavity.  Pharyngeal  pads  were  absent  in  the fingerlings,  rudimentary  in the juveniles  and well  developed  in the  adults.  The  oesophagus connected the oropharyngeal cavity to the stomach. The stomach was J-shaped with a prominent constriction  separating  it  from  the  intestine.  The  proximal  intestinal  wall  was  thick  and continuous with coiled middle and distal parts of the intestine in all groups.  The relative intestinal length of the juveniles (1.28 ± 0.06) was significantly higher (p<0.05) than that of the fingerlings (0.87 ±0.07)  and  adults  (0.78  ±  0.04).  Histologically,  the  tunica  mucosa  of  oro-pharynx,  tongue, pharyngeal pad and oesophagus were lined by stratified mucous epithelium in all age groups. Taste  buds  were  observed  in  the  adult  oro-pharyngeal  mucosa.  The  adult  pharyngeal  pad contained teeth, mucous cells and taste buds in the stratified epithelium. The adult and juvenile oro-pharyngeal and oesophageal epithelium contained eosinophilic club cells. Lamina propria of the oro-pharynx contained dense collagen connective tissue in the adult. Muscularis mucosa was absent in all groups. The oesophageal tunica muscularis was composed of skeletal muscle fibres arranged in an inner longitudinal and outer circular layers in juveniles. In the adult oesophagus, the  circularly  oriented  muscle  fibres  predominated  and  were  interspersed  with  bundles  of longitudinal muscle fibres. An intermediate region between the adult oesophagus and stomach (oesogaster)  was  characterized  by presence of stratified mucous epithelium  and gastric gland within the lamina propria. The entire stomach surface was covered by simple columnar mucous epithelium at the base of which were intraepithelial leukocytes.   In all age groups, the cardiac and fundic regions  of the stomach  contained  glands in the lamina  propria  while  the pyloric region lacked these glands. The pyloric sphincter was comprised of thickened circular smooth

muscle coat in all age groups. The proximal intestine in all ages presented complex labyrinthine mucosal folds, the luminal parts of which bore villar extensions. The middle and distal intestinal mucosa presented simple finger-like mucosal folds in all age groups. The intestinal epithelium in all  groups  comprised  of  simple  absorptive  columnar  cells  with  brush  border,  intraepithelial leukocytes and goblet cells. The tunica muscularis contained a myenteric plexus in all groups. Carbohydrate  histochemistry  in all ages under study  revealed positive reaction to PAS in the oesophageal mucous cells, stomach simple columnar epithelium and intestinal goblet cells. At pH 2.5, the oesophageal mucous cells and intestinal goblet cells were positive to AB reaction but the stomach epithelium  was AB  negative  in all groups.  On subjecting  the mucin containing entities to combined AB and PAS reaction after diastase pre-treatment, the oesophageal and oro- pharyngeal wall mucosa  presented mixed acid and neutral mucins in all the age groups. The stomach presented  only neutral mucins while the intestine  presented mixed mucin with acid mucin dominating in the adult proximal intestine.

The  presence  of  stratified  mucous  epithelium  observed  in  the  oro-pharyngeal  wall  and oesophagus is for protection of the tract from abrasion from rough feed. The mucin  in the oesophagus is also associated with pregastric digestion of food since the teleost digestive tract lacks salivary glands. The eosinophilic club cell is for non-specific immunity. The taste buds seen in the adult group  is for selection  and rejection  of food  materials  by gustation.  The oesogaster seen in the adult group suggests a structure involved in increasing surface area for gastric digestion and production of extra mucin to reduce the effect of acidic gastric content. The  glands  in  the  stomach  lamina  propria  contain  oxyntopeptic  cells  that  produce  both hydrochloric acid and pepsinogen. The absence of gastric glands in the pyloric region may be an adaptation to reduce the quantity of gastric acid that enters the proximal intestine that needs an alkaline medium for maximal activity of pancreatic enzymes. The reticulated labyrinthine mucosal  folds  in   the  proximal  intestine  increases  surface  area  for  feed  digestion  and absorption. The increasing number of goblet cells towards the rectum is for increased mucin production  to  reduce abrasion from fecal materials. The acid mucin seen is associated with protection against mechanical injury from food fibres and pathogenic agents while the neutral mucin is involved in pre-gastric digestion in the oesophagus, buffering the effect of gastric acid in the stomach and transport of small molecules in the intestine.

CHAPTER ONE

1.0.     INTRODUCTION

1.1.     FISH: SOURCE OF FOOD FOR MAN

Food is one of the basic needs of man (Morey, 1940; Pierce, 2010). Food is produced through agriculture –which is the cultivation of plants and rearing of animals for man’s consumption. Since agriculture produces the food that provides the calories and micronutrients essential for a healthy and productive life, it is interlinked in many important ways to nutrition and health (Michael,  2011).These  nutrients  include  carbohydrates,  proteins,  fats  and  oil,  minerals, vitamins and water. Of these nutrients, it is the proteins that supply the body with amino acids necessary for growth and repair of damaged tissues. The sources of protein include plants and animals although the animal sources are preferred because of the presence of essential amino acids and higher digestibility. But the major disadvantage is higher cost. The animal sources include fish, poultry, dairy, pork, snail, and rabbit. Fishing, like other hunting activities has been a major source of food for the human race and has put an end to the unsavory outbreak of anaemia and kwashiorkor in developing countries. It accounts for one fifth of world total supply of animal protein (FAO, 1991; Olagunju et al., 2007).

In Nigeria, fisheries occupy a unique position in the agricultural sector of the economy (Kudi et al., 2008). Its contribution to Gross Domestic Product (GDP) rose from 76.76 billion in

2001 to N162.61 billion in 2005 (CBN Report, 2005). Fish is an important source of protein to a large number of Nigerians. It provides 40% of the dietary intake of animal protein of the average  Nigerian  (FDF,  1997; Sogbesan  et al., 2006). According  to  Adekoya  and Miller (2004), fish and fish products constitute more than 60% of the total protein intake in adults especially in rural areas. Amiengheme (2005) enumerated the importance of fish in Human Nutrition as follows:

•    Fish  food  has  a  nutrient  profile  superior  to  all  terrestrial  meats  (beef,  pork  and chicken)  being  an  excellent  source  of  high  quality  animal  protein  and  highly digestible energy;

•    Fish is a good source of sulphur and essential amino acids such as lysine,  leucine, valine  and  arginine.   It  is  therefore   suitable  for  supplementing   diets   of  high carbohydrate contents;

•   Fish is also a good source of thiamine as well as an extremely rich source of (Omega–

3) polysaturated fatty acids, fat soluble vitamins (A,D and E), water soluble vitamins

(B complex) and minerals (calcium, phosphorus, iron, iodine and selenium);

•    It has a high content of polysaturated (Omega III) fatty acids, which are important in lowering blood cholesterol level and high blood pressure. It reduces the risk of sudden death from heart attacks and reduces rheumatoid arthritis. It also lowers the risk of age-  related  muscular  degeneration  and  vision  impairment;  decreases  the  risk  of bowel cancer and reduces insulin resistance in skeletal muscles.

Nigerians are large consumers of fish, with an annual average demand estimate at 1.4 million metric tonnes, (Kudi et al., 2008). However a demand and supply gap of at least 0.7 million metric tonnes exists nationally with import making up the short fall at a cost of 400 billion United States dollars per year. Domestic fish production of about 0.5 million metric tonnes is supplied by artisan fishermen (85%), and fish farmers (15%)  (Adekoya and Miller, 2004; Emokaro,  2010;  Businessday,  2011).  According  to FAO  (2007),  this figure  (0.7  million metric tonnes) makes Nigeria the largest importer of fish in the developing world.

To take advantage of the large market created by this deficit, some Nigerians are increasing their participation in aquaculture, with many fish farmers focusing on African catfish, Clarias gariepinus, as they have been shown to have a potential market value of two to three times that of other cultivable species like Tilapia and Heterobranchus  (FAO, 2000; Fafioye and Oluajo, 2005; Emokaro et al., 2010; Businessday, 2011). Fish farming generates employment directly and indirectly for people involved in the value addition of processing (Olagunju et al., 2007). Aquaculture is also a ready alternative to fish supply because of great concern on fish depletion in the oceans due to over fishing; fish deaths caused by oil spillage and heavy metal pollution; natural disasters like Tsunami, flooding and excessive drought due to climate change  (Dublin-Green  et  al.,  1998;  UNEP,  2004;  Damassa,  2006;  Gabriel  et  al.,  2007; Tawari-Fufeyin  et al.,  2008).  A survey  by Addo  (2005),  revealed  that Nigerian  children below the age of 18 years, who make up about 47% of our total population are still victims of stunting, wasting and under-weight, so with the increased establishment of more aquaculture in Nigeria, it is possible to reverse this trend of malnourishment among Nigerians below the age of eighteen years.

1.4.Statement of problem

Despite  the  increasing  interest  and population  of catfish  aquaculture  in Nigeria,  there  is paucity of knowledge on the basic biology of the digestive tract of farmed African catfish (Clarias gariepinus Burchell, 1822). The knowledge obtained from this study will be useful in diagnosis  of fish pathology,  in nutritional  diseases and in fish  toxicology.  It will also provide baseline information for further investigative researches to improve growth rate, fish fillet quality and quantity and also better feeding management practices in fish farms.

1.5.Main research objective

To  study  the  morphology  and  histochemical  characteristics  of  the  digestive  tract  of  the

African catfish at different ages post-hatch, from commercial and intensively farmed fish.

Specific objectives

•   To describe the gross anatomy and morphometry of the digestive tract.

•   To describe the histological features of organs of the tract

•   To characterize the histochemistry of the mucous-producing cells of the tract.

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