Читать книгу Clinical Guide to Fish Medicine - Группа авторов - Страница 141

The gastrointestinal (GI) tract anatomy of fish differs based on evolutionary, dietary, and environmental constraints. The stomach in most species is a simple sac. There is no stomach in the jawless fish, e.g. hagfish and lamprey (Agnatha). The stomach has been secondarily lost in disparate teleost lineages, including members of the families Cyprinidae (goldfish, carp, and koi), Labridae (wrasses), and Gobiidae (gobies) (Day et al. 2011). This loss does not appear to impose dietary constraints, as stomachless fish cover the entire trophic spectrum, including carnivory, omnivory, herbivory, and detritivory (Horn et al. 2006; German et al. 2010). Intestinal length is variable in fish, but carnivorous fish generally have shorter intestines than herbivorous species. Many fish possess pyloric caeca. These are finger‐like diverticula of the intestine at the posterior end of the stomach which secrete digestive enzymes and increase the absorptive surface area. Pyloric caeca are better developed in carnivorous than herbivorous fish (Clements and Raubenheiner 2006; Ballantyne 2014). Elasmobranchs and non‐teleost ray‐finned fish (bowfin, gar, sturgeon, and lungfish) have a spiral valve. This is a distal section of intestines with multiple layers in conical, scroll, or ring shapes, which increase the absorptive surface area.

Gut passage rates in fish are dependent on species, animal size, gut morphology, temperature, food type, meal size, and feeding rate. In general, gut emptying times range from <10 hours for most herbivores to 10–158 hours for teleost carnivores (Horn 1989). Elasmobranchs have slower gut passage times than similarly sized teleost carnivores at comparable water temperatures, with reported values of one to eight days (Cortés and Gruber 1992; Wood et al. 2007). Gut passage times in carnivorous teleosts and elasmobranchs are affected by the type of prey. For example, gastric emptying in sandbar sharks (Carcharhinus plumbeus) varied by as much as 20 hours between crab and teleost prey (Medved 1985).

Assimilation efficiency assesses how efficiently animals convert ingested food into energy. Assimilation efficiency is high in adult fish (80–90%), with a wider range in larval fish (67–99%) (Govoni et al. 1986). Food type, ration size, and gut morphology likely play a role in some of these differences. For example, Atlantic herring larvae (Clupea harengus) fed high rations of brine shrimp nauplii (Artemia spp.) defecated live nauplii, suggesting that large rations may result in low assimilation efficiency, especially in larvae with straight alimentary canals (Werner and Blaxter 1980). Assimilation efficiency in elasmobranchs has been studied in a single species (lemon shark, Negaprion brevirostris) and was comparable to that measured in teleosts (73%) (Wetherbee and Gruber 1993).

As in other vertebrates, the GI tract of fish provides habitat for a diverse ecosystem of micro‐organisms that play an important role in the health and nutrition of the host. Information on gut microflora and microbial enzyme activity in fish GI tracts is lacking. It is well‐known that terrestrial vertebrate herbivores have populations of symbiotic micro‐organisms that play a key role in breaking down plant fiber (cellulose and hemicellulose) into short‐chain fatty acids (SCFAs, primarily acetate, butyrate, and propionate). Gut micro‐organism diversity has been studied in some freshwater and marine herbivorous fish. Work by Mountfort et al. (2002) identified the contribution of gut microbiota to energy metabolism and fermentation in the hindgut of three species of temperate marine herbivorous fish, finding that microbial fermentation is an important source of energy in these fish. Thus, establishing and maintaining appropriate GI tract microbial populations is integral to maintaining appropriate nutrition.