Knowing and understanding the feeding and digestion in fish and its peculiarities is essential to produce feed and additives that fit their needs and feeding behavior.

The digestive system of fish shows a huge plasticity due to the different feeding strategies that we find in the aquatic environment. The digestive tract ranges from a tubular structure without any anatomical differentiation to a developed digestive tract, with a defined muscular stomach and even specialized structures such as gill rakers.

Unlike most vertebrates, whose digestion begins in the oral cavity, in fish, the digestive process begins in the stomach, in those species where it is present or directly in the gut. Nutrient absorption occurs in the gut.

1. Feeding strategies and anatomic peculiarities

The anatomical structure of the digestive tract of fish shows a huge variability associated with both different evolutionary degrees and different types of feeding.

In general, the digestive tract is composed of a tubular structure with four regions: the oral cavity; an initial region composed of an esophagus, stomach and pylorus; a medium portion of longer length, where pyloric blind can be found; and a terminal region, which ends with the anus.

An anatomical difference between freshwater aquaculture and marine species is the presence of cardiac sphincter between the esophagus and the stomach. The presence of this anatomical structure is related to osmoregulation. Saltwater animals (hyperosmotic environment), which require continuous water ingestion to maintain their osmotic balance, lack of a cardiac sphincter. On the other hand, freshwater animals (hypoosmotic environment) have cardiac sphincter in order to minimize the entry of water into the animal.

We can classify the different fish species into the following groups based on their feeding habits: herbivorous, detritivores, planktivorous and carnivores.

• Herbivorous

They represent a small percentage and are more frequently found in warm waters, between 40ËšN and 40ËšS. They are common in tropical freshwater and coral reefs. As a result of their trophic position at the base of the food chain, they are often the most abundant type of fish in the community.

This group of animals usually has a short oral cavity, with a blunt snout and many teeth capable of crushing, scraping and even digging. The digestive tract of herbivorous fish is characterized by not having a real stomach and presenting a long gut, which allows the digestion of vegetals. These animals use a great deal of energy to obtain food, as they spend most of the day eating.

There are species of herbivores in scaridae, cichlidae and ciprinidae families. Carps (Ctenopharyngodonidellus, Hypophthalmichthys molitrix, Cyprinus carpio) and Nile tilapia (Orechromis niloticus) are the main species produced in global aquaculture.

• Detritivores

They are characterized by consuming inert organic matter that accumulates, in different degradation stages, at the bottom of lakes, ponds and certain marine habitats. They have a poorly developed digestive tract generally lacks of a stomach and has a long intestinal tract, like herbivorous species.

An example of this type of fish is the flathead grey mullet (Mugil cephalus).

• Planktivorous

Planktivorous are filter fishes that feed by sucking water and screening food particles through gill rakes present in the gills. This type of food has two advantages: very small prey can be ingested by large predators and the feeding can be continuous, day and night, since vision is not needed.

Many of this species are able to actively trap individual particles in addition to filtration. These species are known as optional filterers and can change between both modes of feeding, filtration and active capture, depending on the size of the preys, their density and the intensity of light, to optimize the amount of energy used.

This group of animals has specialized structures in the gill arches known as epibranchial organs, brush-like structures that allow the water to be siffed through the gills, capture the plankton. In addition, in some species structures known as pharyngeal pockets are observed in the oral cavity, which are capable of storing algae and plankton particles while grazing.

Clupeids (pilchard and anchovies) and scombrids (Scomber japonicus, pacific mackerel) are the main groups of planktivorous fish. Also, the rohu (Labeo rohita), one of the main species of Indian carps and one of the main aquaculture species produced worldwide.

• Carnivorous

It is the most common type of feeding within this class of vertebrates. In the trophic pyramid, they are above herbivores, as secondary consumers. Three subtypes can be destinguished.

Zooplankters

Animales able to filter enough water through their digestive tract to feed on the zooplankton. These include the catla (Catla catla), endemic to India and one of the main species used in polycultures.

Benthonic invertebrates

Fish in this group have structural and behavioral specializations that exceed those of any other group. They are, mainly, fish from coral environments and cichlids from the Great Lakes of Africa. This group includes species that await their prey hidden, mobile bento hunters (Lutjanidae, Holocentridae), crushers (Labridae) and probes capable of extracting their prey from coral-like forests (Sygnatidae).

Piscivorous

They are the most abundant group in trtmd of species. Sometimes, they swallow their entire prey and have large mouths with pointed bills to prevent their prey from escaping.
The digestive tract of piscivores fish is characterized by having a true stomach and a short intestine. Trout (Oncorhynchus mykiss) and salmon (Salmo salar) are the most relevant species within this group given their importance in industrial aquaculture.

2. Digestion in fish

Digestion is the process of food degradation that allows the animal to absorb the nutrients present in it. As in the rest of vertebrates digestion is divided into two phases: mechanical digestion, where the feed is crushed, and chemical digestion, in which the digestive enzymes degrade food to nutrients that the animal is able to absorb.

• Mechanical digestion

Unlike other vertebrates, fish lack of salivary glands in the oral cavity, so that their function is limited to capture and crush the food. In some species, the secretory glands are replaced by mucus-producing ones, which favors the transit of food.

The mouth is continued with the pharynx, which is perforated laterally by the gill arches. In some species, like most Cyprinids, dentiform structures appear in the fifth gill arch, which favor the crushing.

The esophagus is responsible for regulating the entry of water and food, sometimes it has salivary glands. It has striated muscles to regurgitate the ingested food.

The stomach is responsible for initiating chemical digestion and stores and mixes the food. In some herbivorous species, the stomach has a highly developed and circular musculature that crushes vegetal material, acting as the gizzard of birds.

• Chemical digestion

As mentioned above, fish lack of salivary glands in the oral cavity and, therefore, chemical digestion begins in the stomach, or directly in the intestine if the species lack of the former.

The stomach has cells that secrete hydrochloric acid, which hydrolyzes the food and promotes enzymatic activity by decreasing pH.

Pepsin is the main enzyme secreted in the stomach and is responsible for the degradation of proteins by hydrolysis of peptide bonds. Those species that feed on crustaceans have, in addition to pepsin, other enzymes of great importance: chitinases, enzymes responsible for the degradation of the exoskeleton of crustaceans.

In the gut, most of the digestion and absorption of nutrients is performed. Unlike other vertebrates, there is no clear differentiation between the intestinal sections and are distinguished, only, a proximal region, a middle region and a terminal region.

Although the absorption of nutrients is similar throughout the entire digestive tract, lipid digestion tends to occur in the anterior region and protein digestion in the posterior region. The terminal tract plays an important role in osmoregulation, because mineral absorption takes place there.

3. Digestibility

Unlike terrestrial animals that use carbohydrates as an energy source, aquatic animals use lipids and proteins.

The percentage of protein required by fish is very high and goes between 40-50%. One problem to mention within industrial aquaculture is the low digestibility of protein associated with the use of vegetal protein sources.

The use of sources of vegetal protein affects digestibility, on one hand, due to the presence of anti-nutritional factors (ANFs) capable of reducing the activity of digestive enzymes. An example of such anti-nutritional factors are protease inhibitors, enzymes responsible for the transition from pepsinogen to pepsin, a key stage in protein digestion.

On the other hand, sources of vegetal protein do not contain an adequate proportion of essential aminoacids and can produce imbalances. In addition, braking compounds, such as chlorogenic acid, of vegetal raw materials, affect feed palatability.

In order to improve the digestibility of vegetal protein, thermal processes have been developed. In addition, the addition of acidifiers improves the protein digestibility.

4. Conclusions

Fish feed is one of the main production costs within aquaculture farms, as it represents to up to 50% of the total production cost. Therefore, it is essential to know the different feeding habits and behaviors of aquatic animals, as well as to understand the digestive process and the digestibility of nutrients, to optimize feed utilization.

The replacement of animal protein sources with plant protein sources is one of the main challenges facing the aquaculture industry and, therefore, products capable of improving the digestibility of the protein are being developed.