We have already seen how the carp is an adaptable fish by being so productive in a varied and changing food environment. The carp’s productivity is even more remarkable when taking a closer look at its digestive system.
Generally, digestion takes place in the gut by way of mechanical and chemical processes. In the majority of vertebrates, probably the most significant organ of digestion is the stomach. An area of extreme acidity (pH 2) which breaks down the physical nature of the food and facilitates the optimum enzyme attack. However, carp do not possess a stomach missing out on this significant part of gastric digestion. So how do ‘stomachless’ carp still manage to digest such a varied diet and remain so productive?
Many areas of digestion in fish are still not clear but by studying the known facts about the digestive system we can try to deduce how the carp survives without a stomach.
On discovering food, a carp’s mouth and buccal cavity are dedicated and equipped for the seizure, control, selection and preparation of food. The buccal cavity is lined with tough ridges of a folded membrane called the mucosa. The mucosa is covered in microscopic projections called papillae and is richly provided by mucus goblet cells and taste buds.
Digestion in humans commences in the mouth with chewing to physically break up the food.
This is also true for carp where mechanical preparation of the food commences with the grinding action of pharyngeal teeth. Situated in the posterior part of the buccal cavity on the ventral surface are several pairs of pharyngeal teeth which sit directly below a tough ‘leathery’ cornified pad. Food is ground between the pad and pharyngeal teeth and this ‘grinding mill’ ensures that all food entering the gut is suitably fragmented for further digestion. The ‘chomping’ action seen in a feeding carp occurs as a result of this grinding action, often expelling the rejected fragmented material through its operculae.
The duct which carries the ground up food from the pharyngeal region of the buccal cavity to the anterior intestine and receptive sack is called the oesophagus. It is lined with tastebuds to taste the fragmented food prior to ingestion. A dense bed of cilia and goblet cells aid the passage of the selected food material through this first constriction in the gut.
Up until the early part of this century it was thought that carp were like most other teleosts in that they possessed a stomach. Even after dissection, on first viewing the carp appears to possess a stomach and it is only after microscopic investigation that shows it is merely an expanding receptive sack. Unlike in other fish which possess a true stomach such as Tilapia, this structure does not secrete acid or enzymes and does not terminate with a pyloric sphincter muscle.
Having consumed many insoluble organic nutrients in the diet such as proteins, carbohydrates and oils with complex molecular structures and high molecular weights, for the carp to utilise them it is necessary to break these complex structures into their smaller component parts. These are soluble in water and easily transported around the body to where it is needed in the fishes blood system.
In mammals, from birth the stomach is involved in the clotting of mother’s milk causing it to remain longer in the stomach for increase enzyme attack.
In contrast, newly hatched fish larvae do not posses a stomach but develop one while maturing; this is true of salmon.
But carp never do generate a stomach suggesting that carp constantly remain in their juvenile ‘stomachless’ form.
Consequently, digestion in the carp is very simplified as is clear when looking at the homogenous and apparently unspecialised intestine. As a whole, the intestine is a relatively simple and featureless structure unlike other vertebrates, has no multicellular secreting glands, valves or caecae. In fact, apart from the dilating receptive sack the intestine is visually undifferentiated along its length.
Typically the intestine is twice the length of the carp’s body and is lined with epithelial cells which are constantly being replenished as abrasive food material erodes them away. The passage of food is lubricated by secretions from goblet cells while other cells in the gut epithelium are dedicated to absorbing the broken down soluble nutrients from the gut.
The complete absence of a stomach in carp means no acid digestion takes place and no pepsin is produced in conjunction with hydrochloric acid to digest proteins. It appears that the enzymes trypsin and chymotrypsin, secreted by the pancreas are largely responsible for protein digestion.
What is not totally clear is why other fish and aquatic vertebrates require two major proteases (protein digesting enzymes) Pepsin from the stomach and Trypsin from the pancreas, while carp digest their food satisfactorily in the total absence of pepsin.
The action of acid on food in the stomachs of most vertebrates acts to breakdown the physical nature of food and to provide an optimum pH for pepsin action. The grinding action of the pharyngeal teeth may remove the necessity for action of acid on food material. Pepsin acts in the stomach optimally at a pH of 2 so as no hydrochloric acid is secreted in stomachless carp then pepsin secretion is futile.
It has also been suggested that due to the omnivorous diet of carp compared with other fish, the need for pepsin is reduced as lower levels of protein are ingested. (This is probably why protein assimilation efficiency tails off on high protein artificial diets). However this theory needs further explanation when omnivorous Tilapia do possess stomachs, exhibiting gastric digestion down to pH 1.2! In addition when comparing the intestines of omnivorous fish with carnivorous fish, those of omnivorous fish are always longer as their diets are lower in protein requiring efficient digestion and absorption to obtain sufficient nutrients.
Ducts carrying bile and enzymes from the gall bladder and pancreas enter the carp’s intestine almost immediately after the oesophagus carrying bile and enzymes respectively. The food and enzyme mixture remains in the distended receptive sack while it digests prior to entering the smaller intestine.
Bile (a salt of sulphur molecules linked to alcohol) which is produced in the liver and stored in the gallbladder (coloured dark green) is an emulsifier of lipids and oils. Working in a similar way to a detergent it breaks up larger bodies of lipid into smaller droplets producing a larger surface area for lipase attack. Lipids and oils are broken down into soluble fatty acids and glycerol.
Proteases such as trypsin and chymotrypsin secreted by the pancreas digest the proteins in the food. This breaks down proteins into amino acids or small groups of amino acids which are soluble and easily absorbed into the bloodstream.
In human digestion the action of chewing, smelling and seeing food will stimulate the mouth to ‘water’ or produce saliva. This saliva contains a starch digesting enzyme (salivary amylase) which breaks down insoluble starches and carbohydrates into soluble sugars. (Suck a piece of bread (starch) and after a while it will taste sweet). In its aquatic environment, it is impractical for carp to secrete salivary amylase so it is secreted directly into the gut from the intestinal mucosa to breakdown starch. The resultant soluble sugars are absorbed directly into the bloodstream from the gut.
It is interesting to note that in other fish and vertebrates, pancreatic and bile secretions enter the gut after the stomach. The fact that pancreatic and bile ducts enter the intestine prior to the receptive sack suggest that the sack is not some form of primitive functionless or redundant stomach.
Gut transit time (the time taken for food to pass through the fish intestine) in carp can range from 16 hours at 25oC up to 60 hours at 12oC demonstrating that rates of digestion and enzyme action increase up to an optimum with temperature. Prior to evacuation and the production of faecal strings final absorption takes place in the hind area of the intestine.
Further towards the anus the intestine is similar in structure to the oesophagus showing thicker walls of connective tissue and being densely supplied with mucous secreting goblet cells. This aids the formation, passage and ejection of faecal material, the waste produce of digestion.
In the natural carp diet, a significant percentage of faecal material is inert inorganic mineral matter of no nutritative value inadvertently ingested with food items while grubbing around on the pond bottom. Unlike carnivorous fish, the carp does not secrete collagenase, an enzyme required for the complete breakdown of the connective protein found in animal skin and tissue and this will also be a significant part of faecal matter.
In conclusion, even though the knowledge of that carp are stomachless fish is nearly 90 years old, it is still not totally understood why they do not have one and how they compensate accordingly. Needless to say, looking at the success and productivity of Cyprinids in general it makes you wonder whether having a stomach is a disadvantage or not having one is an advantage. Even so, it is comforting to know that when, as pondkeepers, we sometimes inadvertently stress our fish, we are not causing them to have stomach ulcers!