7.11 Nutritional Physiology in Larval Fish
7.11.1. Changes in Diet Assimilation During Ontogeny
7.11.1.1. Development of the Digestive System in Fish
Direct absorption of complex compounds from the yolk to the syncytium followed by intrasyncytial digestion is considered to be the primary mode of endogenous digestion during embryonic development (Heming and Buddington 1988). It is likely that both extracellular hydrolysis of complex compounds followed by assimilation of smaller by-products of hydrolysis (extracellular model including membrane linked digestion) and direct as- similation of complex compounds followed by intracellular hydrolysis (in- tracellular model) occur during the larval stage in fish (Fig. 7.21). It may
FIG. 7.24
Atlantic halibut larvae 2 days after first feeding (250 C◦days postspawn). (a) Whole larvae. (B) magnified view of the anus and feces. Photographs by Michael Rust.
FIG. 7.25
Atlantic halibut larvae from the same tank, 2 weeks later (550 C◦days postspawn).
(a) Whole larvae; (B) magnified view of the anus and feces. Photographs by Michael Rust.
FIG. 7.26
Atlantic halibut larvae from the same tank near the start of eye migration 39 days after first feeding (700 C◦days postspawn). (A) Whole larva; (B) magnified view of the anus and feces. Photographs by Michael Rust.
be that the relative contribution of each mode of digestion differs among species and changes during larval development. The quantitative impor- tance of each model for larval fish digestion has not yet been determined.
An example of ontogenetic changes in the physiology and assimilation efficiency for protein in the striped bass may be illustrative of changes tak- ing place in a large number of monogastric altricial fish species (Rooet al.
1999). Gabaudan (1984) studied the histological and histochemical devel- opment of the digestive system of striped bass larvae. He divided the larval stage into two periods after the start of exogenous feeding of what he termed
“physiological relevance.” The first period was from first feeding until about 15 to 18 days after first feeding [at 21◦C; 0 to 340–370 C◦ days after first feeding]. During the first period, the stomach is not developed and “the di- gestive processes of the larvae are probably comparable to those of stomach- less fishes such as cyprinids” (Gabaudan 1984). The second period begins about 17 days after first feeding and extends to about day 50 (at 26◦C; 350 to 1200 C◦days after first feeding). This second period is a state of transition where “the digestive processes presumably become similar to those of the adult” (Gabaudan 1984).
According to Gabaudan (1984), the stomach anlage appears about 15 days after first feeding (300 C◦ days after first feeding), along with dif- ferentiation of intestinal goblet cells. Before that time, the exocrine pan- creas is formed and contains zymogen granules; however, the density (e.g., amount of zymogen) of the zymogen granules as well as the amount and dis- tribution (after 25 days; 574 C◦days) of exocrine pancreatic acinar cells con- tinues to increase as the larva ages. Intestinal ceca and gastric glands (oxynti- copeptic cells) begin to differentiate about 19 days after first feeding (419 C◦ days) but not all of the presumptive oxynticopeptic cells contain pepsinogen granules. Not until 27 days after first feeding (626 C◦days) is zymogen seen.
Although Gabaudan did not measure changes in acid proteolytic activity in striped bass, Vu (1983) reported that measurable acid proteolytic activity began to increase 5 days after gastric gland differentiation began in sea bass larvae. The development of these glands and structures is required for ex- tracellular (luminal) digestion. In a different study, where a pH indicator solution was intubated into the stomach anlage of live larval striped bass, no color change (indicative of acid production) was evident until 24 days after first feeding [at 19◦C; 456 C◦days (Rustet al. 1993)]. Even then, the extent of color change due to the acid was limited to the region next to the epithelium and did not extend into the middle of the lumen (Fig. 7.27).
In addition to the histological evidence of developing extracellular digestive capacity, Gabaudan (1984) found evidence of pinocytosis and in- tracellular digestion from first feeding until the end of the study, after meta- morphosis. Around first feeding, the enterocytes of the anterior intestine
B
FIG. 7.27
contained lipid vacuoles and stained positive for alkaline phosphatase in the microvilli. Gabaudan (1984) postulated that the anterior intestine may be actively involved in the active transport of lipid, amino acids, and carbohy- drates. The enterocytes of the posterior intestine contained large eosino- philic supranuclear bodies and had the ability to take up intact dietary horseradish peroxidase (HRP). HRP activity was not extinguished until it was contained in supranuclear bodies, indicating that the protein was as- similated intact, and then digested intracellularly in the supranuclear body.
Similar histochemical studies have been conducted with goldfish (Gauthier and Landis 1972), common carp [Cyprinus carpio(Noaillac-Depeyre and Gas 1974)], grass carp [Ctenopharyngodon idella(Stroband 1977)], walking catfish [Clarias lazera(Stroband and Kroon 1981)], and other species, with similar results (Watanabe 1982).
In another study (Rust 1995), striped bass were fed radiolabeled amino acid-containing compounds between 19 days after first feeding (at 19◦C;
361 C◦days) to 47 days after first feeding (at 19◦C; 893 C◦days), which cor- responds to Gabaudan’s second period. The assimilation efficiency during that period for protein-bound methionine increased from about 30 to 60%.
These results agree well with the histological changes discussed previously.
If an increase in assimilation of intact protein for intracellular digestion is the only process responsible for the observed increase in assimilation efficiency, then the transport of proteins from the lumen into the ente- rocyte by pinocytosis must also increase. Increasing protein assimilation rates would likely be accompanied by increases in absorptive surface area and/or increases in pinocytotic activity within existing enterocytes. Stom- achless fish typically have long, highly folded guts and have high absorptive surface areas. The alimentary canal length in striped bass (which is corre- lated with the gut surface area) increases only from 60 to 75% of the total fish length during the larval period (Gabaudan 1984). In contrast, the al- imentary canal length in goldfish increases from 80 to more than 300%
of the total fish length over a similar period (Smith 1989). If assimilation of nutrients were due to surface area-limited processes alone (e.g., pinocy- tosis), then we would expect to see a relatively constant assimilation rate for larval striped bass. The results of the radiolabeled protein study are in conflict with this expectation (Rust 1995). Therefore, it is likely that the
FIG. 7.27
The development of acid secretion in larval striped bass (Morone saxatilis).
Yellow–green indicates a pH below 6.0; blue, pH 7.4 and above. Time is 0 min (A) and 4 min (B) after intubation with indicator solution. From Rust (1993). (a) Swimbladder, (b) esophagus, (c) foregut stomach anlage), (d) midgut, (e) hindgut, and
(f ) presumptive spleen. Photographs by Michael Rust. (Reprinted fromAquaculture, 116,Rust, M. B., Hardy, R. W., and Stickney, R. R.A new method for force-feeding larval fish.pp. 341–352. Copyright c1993, with permission from Elsevier Science.)
increase in assimilation is due to an increasing ability of the larvae to utilize extracellular and/or membrane-bound digestive processes.
Extracellular digestion is enhanced as larvae produce increasing quanti- ties of acid, develop nutrient transport mechanisms, and secrete digestive en- zymes (pancreatic and gastric) and bile into the lumen. As more hydrolytic substances are secreted into the lumen, the effective volume of material that can be digested increases regardless of increases in the gut surface area. In- creasing active transport sites and/or other membrane-bound enzymes can also increase the digestion and assimilation of nutrients without the need for an increased gut surface area.
Extracellular digestion is considered to be the more quantitatively impor- tant digestive pathway in postmetamorphic monogastric fish (Smith 1989;
Pederson 1993). Uptake of intact proteins followed by intracellular digestion also occurs in postmetamorphic monogastric fish in the posterior intestine;
however, the relative contribution to amino acid nutrition is questionable (Stroband and Van der Veen 1981; Gardner 1985; Smith 1989). Pinocytosis may function primarily as a mechanism to provide antigens for the immune system (Smith 1989) or to recover digestive enzymes (Hofer 1982).
It may be that the second period in larval striped bass development, de- scribed by Gabaudan (1984), is a change from a quantitative reliance on intracellular digestion to one on extracellular and membrane-bound diges- tion (Fig. 7.28). A model for digestive system functional development in altricial gastric larvae based upon this interpretation explains the assimi- lation data found for striped bass and other similar species. The model assumes a reliance on intracellular digestion from first feeding in those species until some time after the gastric glands begin to develop (corre- sponding to Gabaudan’s first period), followed by an increasingly efficient extracellular/membrane digestion period (corresponding to Gabaudan’s second period). The activity of intracellular digestion may not necessarily decrease in absolute terms. As more protein is processed by extracellular digestion, less is available in the posterior intestine, where intracellular di- gestion occurs. At some point, extracellular digestion reaches maximum efficiency and total assimilation efficiency reaches a plateau.