FEEDING FISH AND DIGESTION

Fish and their diets, sigh . . . it can get so confusing . . . and full of big words. It seems you can put the name of any kind of food in the front of “ivore” (vorare is Latin for “to swallow or devour”) and you will have a technical name for animals which eat that kind of food. Among fish there are herbivores which eat only plants, omnivores which eat all foods, detritivores which eat only detritus, and piscivores which eat only other fish. They can also be categorized by how wide a selection of food types they eat. A euryphagous fish eats from several types (a variety of plant and animal foods), a stenophagous fish eats from a more limited selection, and a monophagous fish eats from only one. Granted, if you walk into your local fish store and say you need a good food for a monophagous detritivore, take a camera with you to get a picture of their expression. It's just for curiosity's sake that I'm defining them – you're not likely to hear them again.

Aside from carbohydrates, proteins, and minerals, fish need a good number of vitamins. See Table #1 for the vitamins, and Table #2 for their deficiency symptoms.

To start this conversation, there are a lot of odd facts about fish feeding in general that I’d like to present. I’ll just present some of them numerically rather than organizing them in different paragraphs:

1) A fish will grow faster if surrounded by smaller fish than by larger fish or those of the same size… even if there’s lots of food around.

2) When a fish is digesting food, it uses more oxygen than when it isn’t.3) Except with sharks and rays, fish lips never have scales. I don't think this is so they can kiss more

sensually (kissing gouramis probably disagree with me (I know, don't tell me . . . they don't kiss for sensual reasons)). Sharks and rays always have scales on their lips. That's because shark scales evolved from their teeth (although there was a recent theory that their teeth evolved from their scales).

4) There are many, many factors which control how much food a fish needs, including age, gender, reproductive status, activity, temperature, and physiological stress. A one pound koi will eat much more food than a one ounce koi, but a pound of one ounce koi will eat much more food than a single one pound koi.

THE HUNT

People have a tendency to anthropomorphize (to ascribe human characteristics to things which aren't human). For instance, they think vision is as vital to a fish as it is to a person. Fish have very little focusing ability. There are several exceptions, like archerfish (who can spit a little bug into the water from a good distance away) and pufferfish, which rely mostly on their eyes. On the other hand, fish do see in color, and they do choose which bit of food to bite based to some extent on this ability. The sense of sight is extremely variable in fishes. Anglerfishes in the black depths of the ocean rely on the vision of ‘swallowable’ fishes, so these prey-fishes will strike at the glowing lure attached to the angler’s forehead. Cave fishes have made up for their lack of eyes with exceptional senses of smell and hearing. Mormyrids and knife fishes have developed electrical senses, which work like sonar, to find food for them in the very muddy (and impossible to see through) waters in which they live.

Catfish have taste buds spread across their skin and fins, rather than just on the barbels they stick down into the substrate they live on. Next time you feed some of your catfish, watch to see if a piece of food (which they aren't aware of) doesn't fall down onto their back or tail, resulting in them turning and gulping that food down without looking at it. Indeed, some of the most nocturnal catfish have the longest barbels, so they can strike at food in the dark, when they've just barely touched it with a barbel. Codfish and three-spot gouramis have taste buds on their pelvic fin extensions.

What most people think is a fish's nose isn't quite accurate. Our noses are connected to our lungs, but the little holes ahead of and below a fish's eyes aren't. It's what they smell with, but except for hagfish, chimaeras and

lungfish, fish don't breath through these nasal sacs. So how does a fish get water to flow into these “nares”, in order for them to smell what's out there? Just as a fish's tail moves from side to side as they swim, its head also moves from side to side, though less noticeably. As the head moves to one side, water is forced over its rounded skull. The nasal sac has a little flap of skin, which directs some of the flow down into the in-flowing nares and then up and out the out-flowing nares on the other side of the flap of skin (see Figure 1). Some fish actually have two holes, so that the water flows into one hole and out the other. Then when the fish's head moves back the other way, the flow through the nares is reversed. But either way the head is moving, the fish is getting fresh water into their nares. Some fish even have cilia, which whip to propel the water in the right direction. Fish frequently use their nose (er... nares) to find food. Bullheads, morays and some sharks are unable to find their food when their nares are plugged. Spiny eel nasal sacs are located one on the end of each fork of that weird snout they stick into the sand to find food. Sharks are able to find their prey by judging whether the smell is stronger in their right nares or their left. It's theorized that the purpose of a hammerhead sharks snout is to separate their nares even further, to make it easier to judge the direction of their prey. It's much like how we judge the direction of a source of sound with our ears . . . by which ear hears it louder and first. The smells which attract teleosts, sharks and rays frequently include amines and amino acids. For instance, winter flounders chase the smells of glycine and alanine. And Atlantic silversides go after alanine and methionine.

So far as the sense of hearing goes, it is important for detecting both predators, and in some cases, prey. Fish have two organs for detecting sound. They have an inner-ear very similar to ours. And they have a lateral-line system, which is shared with only a few amphibians. The inner-ear does provide hearing to fishes, but it’s not particularly strong, and it’s not directional. When the gas-bladder has a part which extends closer to the inner-ear however, its hearing is improved sharply. The gas-bladder acts like a drum with a skin on both sides to receive a shock wave on one end and transfer it out the other side almost unchanged, and the closer the end gets to the inner ear, the better their hearing is. Squirrelfish, tarpons, notopterids, deepsea cods, sea breams, herrings, and mormyrids have this kind of hearing. The only better system in fishes is when there is a physical connection between the gas-bladder and the inner-ear. Cyprinids, catfish and a few other fish have just such a mechanism, called a “Weberian apparatus”. It consists of a few bones, originally derived from ribs (it is thought), which are physically connected to the gas-bladder at one end, and the inner-ear at the other. The lateral line is a tube on both sides of fishes, which sense pulses of water pressure (sound waves), and because the points of sensitivity in the tube are measureable by length from the fishes brain, as the sound wave moves along the tube (or near the tube outside the body), the fish can measure where it came from. In other words, it gives the fish directional hearing (see Figure 2).

Several fishes have electrical generating organs to give off electrical pulses, as well as organs to receive them. Most of these fish use them for communication, but some of them can sense the texture or hardness of other objects in their vicinity. This includes other fish and life forms, especially prey. Mormyrids (elephant noses), South American knife fishes, and the African feather-fin (Gymnarchus) use this system. Sharks, for instance, have organs on their nose called Ampulae of Lorenzini to find prey. They swim along just above the bottom, searching for flatfish and/or rays, which give off a slight electrical field from muscle usage (it could be just the heart pumping). The shark senses this and bites down into the sand to catch the prey, which are buried under as much as 6 inches of sand. Other fish have electric organs so powerful that they can use it to stun and catch the prey: electric catfish (600 volts), electric eel (600 volts), and electric ray (45-60 volts) (see Figures 3, 4, and 5).

PUTTING THE BITE ON

“Putting the bite on” used to mean borrowing money from somebody. You could 'put the bite on' your boss for a ten-spot in the days before television. It was before my times, but I used to hear it in old gangster movies. In this case, I'm just referring to when the fish gets its food in its mouth. In order to catch their prey better than other fishes, they have evolved many specializations. Fish that feed off the surface (bugs which fall into the water or have adapted to life on the surface film) have a mouth which faces up. My African butterfly fish love houseflies, and my hatchetfish love wingless fruit flies. These foods are probably similar to the foods these fish eat in nature, and the nutrients in these foods are probably very similar to the nutrients they get in nature. They also have teeth which are designed to puncture the insect’s chitinous “skin”. When the sun is overhead, they will also feed on Daphnia and brine shrimp, which are attracted to the surface by the strong sunlight. Most fishes

have forward facing mouths, which are designed for eating foods in mid-water. Most cichlids and tetras are this kind of fish, and they feed on smaller fish and other mid-water creatures. Fish adapted for feeding off the bottom have down-facing mouths, and feed on worms and other creatures in the mud, as well as anything which dies and falls to the bottom, as do catfish and many cyprinids (barbs) (see Figure 6). Of course, a bottom-feeder can feed off the surface and a surface feeder can feed off the bottom; they're just not as good at it.

Most fish suck food into their mouths, and the maximum distance they can do this from is usually about ¼ the length of the head. Once inside the mouth, prey animals face a horrible array of teeth. Fish may have teeth, not only on their upper jaw (maxillary and pre-maxillary bones), and the lower jaw (mandibles), but they may have teeth on the forward side of the gill-rakers, the roof of the mouth (the vomer and palatine bones), and even the tongue (see Figure 7).

Fishes can occur with down-turned mouths, mouths facing forward, or up. Though their mouth structure might not show their plant-eating diet, their teeth will, their stomachs will, their intestines will, and the frequency they need to feed will. I'll discuss all of these but the teeth in the next chapter. It has always been very scary to stick my hand into a tank of red-bellied pacu, because they look so much like red-bellied piranha. In fact, if all the pacu in the tank have semi-circular bites taken out of their fins or bodies, I won't do it. The only way to tell for sure is to open their mouths and count the number of rows of teeth they have. Until you think about it, it's surprising that the pacu has more rows of them. The piranha is able to slice meat cleanly off their prey because they only have one row of teeth on which to concentrate the force of their bite. Add to this the fact that a piranha's teeth are interlocking (inter-digitate) and sharp as a knife and you can see why they are so scary. These teeth are designed to cut flesh from the bodies of prey which are too large to swallow whole. They are not as scary as their reputation though. Even schooling piranha don’t always attack when you expect them to. Also consider that many of them aren't schooling fish. A single piranha can take a big bite out of you, but then he swims off to digest that bite, and you aren't further molested. Tooth-wise, predatory fish which feed on other fish of good size have two tactics they attack with. Some have fewer large teeth (like pike), while others have broad bands of tiny teeth (like Nile perch). The smaller teeth don’t hold the prey as securely, but they don’t make it necessary to open the mouth as wide as would be required of a large-toothed predator to swallow a prey.

I suppose I should get back to my discussion of vegetarian teeth. In the same family as the piranha are the silver dollars of the genus Myleus. They are mostly vegetarian, and their teeth are designed to cut small pieces off of thin sheets, just as leaves are made out of. Their teeth have a cutting edge which is flat and sharp, like our incisors, with the body of the tooth being thin from front to back – chisel-shaped, as some people put it. Pufferfishes and triggerfishes have their front teeth fused into something like a beak – pufferfishes for crushing snail shells and hard coral, triggerfishes for crushing the shells of crabs, shrimp and lobsters. Algae-eaters may have teeth designed for the job (like Plecostomus) or have no teeth, but instead use horny jaws to do it (like Chinese algae-eaters and what the hobby calls sharks (Labeo). The Plecostomus teeth are like bristles (long and flexible), which terminate with an inward curve. These flexible teeth make it easier to scrape course or irregular surfaces. The pre-maxillae are the little bones at the very front of the upper jaw, and in regular catfish, these bones can’t be moved. Coincidentally, in most catfish, these bones are attached to the muscles which move their barbels around, whereas in Plecostomus they can move, and in this case serve (with the lower jaw) to open and close the mouth (not the suction disc). You can see the mouth opening and closing within the suction disc as they clean the glass. Some of the African Haplochromis cichlids have adapted to a diet of scraping algae off rocks by developing teeth with oblique, scraping edges. One of the “Oh so many” diets of African cichlids is feeding on eggs and babies of other fish. There is one Haplochromis, which is believed to have developed a way of clamping its mouth over a mouth-brooding parent’s mouth and, in some way, forcing the parent to release the babies. Some fish have pharyngeal teeth on their gill-rakers which are flat and blunt, like our molar teeth, and they use them for crushing snail shells (mollusks), for instance the saltwater plaice (a flatfish) and the Lake Victoria cichlid, Astatoreochromis. Oddly, Astatoreochromis is found in other lakes, where it doesn’t eat snails, and they have the more typical, smaller teeth. When Astatoreochromis from Lake Victoria are raised in aquaria on a diet without snails, their pharyngeal teeth turn out to be the smaller variety (see Figure 8). So in at least a few cases, the shape of their teeth is controlled by what they eat.

Freshwater fish do their best not to swallow water. After chewing up their food, or collecting little bits of food which are easily swallowed, they concentrate the food into a compact pellet. That way they can swallow the pellet without also swallowing a lot of water. This has to do with freshwater fish absorbing too much water through their skin, mouth, and gills due to osmosis. They have enough trouble trying to keep from absorbing too much water. Because they don't swallow water, it's hard to get freshwater fish to swallow medicine when they are sick.

Fish-eating predators always try to swallow their prey head first, so the fin rays and spines face away (back out of the mouth). This way the prey's rays and spines help force the prey into the mouth. If the prey fish was facing the same direction as the predator, not only would he be much harder to swallow, but flapping his tail might project him forward out of the predator's mouth. I've seen feeder goldfish escape from the mouth of an Oscar that way.

There aren’t many filter-feeding fishes kept in aquaria - lots of invertebrate filter-feeders, but only a few fish I’m familiar with. I’m sure there are way more African cichlids that do it than the few I’ve read about. The only filter-feeding fish I’ve ever kept is the “dragon goby”. He wasn’t hard to feed because dry, live, and frozen foods would all be eaten with this method. The water is sucked in the mouth and blown out the gills, but the inner side of the gills has teeth that have become long - almost like tines on a rake or teeth on a comb. These form sort of a sieve through which to sort for food (see Figure 8). There is a cichlid in Lake Nyasa (Lethrinops), that gulps up mouthfuls of sand, then blows it out his gills the same way. But he’s sorting for larger food (Chironomid larvae), so the spaces between the gill arches are quite a bit larger.

DIGESTING

Teleosts (the large majority of all fishes) usually only get to use about 80% of the food energy they eat. The rest is lost in their urine and feces. Of the energy they do absorb, a large quantity is lost in maintenance: swimming, pumping water over its gills, feeding, digestion, fighting infections, production of eggs and sperm, and possibly a few other factors. What is left goes into growth, but not in a one-to-one ratio. A population of pike only grew 0.45 grams for every gram they ate beyond what they needed for maintenance.

Just like people, fish have a stomach and intestines. They both have different purposes. When the stomach lining is stretched, (as though the fish had swallowed something), it excretes various chemicals which make the stomach contents digestible. Hydrochloric acid is one of these chemicals, producing a very acidic (2 - 4 pH) stomach environment. Other digestive enzymes, such as pepsin are produced in predatory fishes, which work in this acid environment. These enzymes break proteins in the food down into peptides and amino acids, but they only work in acid environments, so they don't break down intestinal linings. When we suffer from acid reflux, however, the acids can partially digest the lining of our throat. Fish which have no stomachs, and therefore produce no hydrochloric acid or pepsin, can't dissolve shells or bones, so I don't recommend feeding crushed snails to them (on the other hand, they aren't likely to eat them if they are offered). The amount of these chemicals which can be secreted is slowed down quite a bit in lower temperatures, so digestion is also slowed down in the cold.

When the food (now a soupy, acidic mix) passes into the intestine, it goes from an acid environment to an alkaline one. The acids now need to be gotten rid of, and enzymes compatible with an alkaline environment need to be added to the mix. While some proteins are digested better in the acid stomach, other proteins break down better in the alkaline intestine, but both kinds are only absorbed by the intestine.

The first section of the small intestine in all higher vertebrates is called the duodenum. In fish, however, divisions can't be made as easily, so this first section is sometimes referred to as the 'proximal' or 'anterior' small intestine in fishes. Because of the importance of getting rid of the acids, and producing the alkaline enzymes, a few organs and other structures are crowded into this area. These include the liver, the pylorus, the pyloric cecae, and the pancreas (or pancreatic tissue). The pancreatic tissue can all be included in one organ (the pancreas) as in sharks and rays. Or as in many other fishes, it can be scattered in little bits nearby (as in the liver

and omentum (a bit of peritoneum which suspends the intestine)). Many other enzymes are produced in or near the pancreatic tissue or the pyloric caeca. The pyloric caeca and the pancreas are attached right near the connection of the stomach and the intestine, but most fish also have pancreatic tissue attached in several other places (unlike mammals). 'Carbohydrases' is a general term for enzymes which break down carbohydrates (mostly starches and sugars). Amylase is one specific carbohydrase which breaks down starches, but there are others. Lipases (plural of lipase) are another category of enzymes from the pancreas, which break down fats and oils.

Vegetarians and detritivores (feeding on scattered bits of composting plants and animals) have small or even no stomachs. Since they eat small meals throughout the day, they don't need a stomach to store a large meal. On the other hand, the cellulose in their vegetative diet is harder to digest than meat, so the space taken up by a stomach is better put to use as more length of intestine. Many vegetarian and a few omnivorous fishes have no true stomach. Because they consume little meat or fat, vegetarians have little need of pepsin, or other protein or fat enzymes. Likewise, predators have very low levels of carbohydrate enzymes. The typical predator of large prey (1/3 their body length, for example) swallows a large, meaty animal, then doesn't hunt again till it’s stomach has made room for another meal. Predators of smaller prey (daphnia, brine shrimp, etc.) have somewhat smaller stomachs and eat more frequently, but still use protein and fat enzymes, without needing carbohydrate enzymes. Most fish aren't purely one or the other, but have physical attributes and diets that are intermediate in all respects. Fish that swallow lots of indigestible stuff (like mud or sand) also need long intestines. In a study on the stream fishes of Panama, predators had intestinal lengths a little shorter than their standard lengths (from the tip of their snouts to the end of their peduncle (not including their tail fin rays)). Fish that ate many kinds of food had intestines that ranged from equal to their standard length, to twice as long as their standard length. Herbivores had intestines that were 5 to almost 30 times their standard length (see Figure 9).

Even though teeth can poke holes in skins/shells or scrape scales away, and the stomach can squish the food around and expose it to the enzymes, the absorption of nutrients is all done in the intestine.

Some chemicals (foods in this case) are broken down more easily by acids; others are broken down easier by alkalis. For this reason, when the food passes from the acidic stomach to the intestinal system, it is going into a strong alkaline environment, in order to break down acid-resistant foods. Here, the foods encounter an entirely different batch of enzymes.

Fish don't produce saliva, but mucous is produced from inside the mouth, throughout the remainder of the system as lubrication for the passage of food. As a control, to slow the passage of food, the inner surface of the intestine is creased in a rectangular pattern (sort of like inside-out, nearly bald tire treads). So the speed of the passing food is slowed by the inner surface of the intestine and sped by lubrication and the contractions of the muscle cells wrapped around the outside of the intestine. So, although they can't willingly control the speed of the food, the fish's body controls it much like our bodies do. With mammals, production of enzymes (including salivary ones) can be stimulated by sights, sounds, tastes and smells (like Pavlov's dogs). But in fish, they are only controlled by hormones and nerves inside the peritoneum.

In order for food to cross the intestinal wall, it has to be dissolved, and no larger than a molecule. Proteins, for instance, are first broken down into their component amino acid molecules, before being transported through and into the bloodstream. In a few fish though, some of the fat can be absorbed from either the pyloric stomach (see figure 9) or the pyloric caeca into the lymph ducts surrounding these areas. They therefore can skip the entire intestinal system.

As with people, fish have symbiotic bacteria in their intestine. The fish get some benefit from the plants they eat, because their teeth and contractions of the muscles around their intestines break through the cellulose cell wall of plant cells. But fish and people would miss much of the food value from the plants they eat, if they didn't have loads of bacteria in their gut. The bacteria produce an enzyme called cellulase, which fish don't have, and the cellulase dissolves the cell walls of the plant cells, so both they and the fish can digest what's inside the plant cells.

ENZYME SOURCE DIGEST WHAT

HCl Acidic stomach Some proteinsPepsin Acidic stomach Some proteinsLactase Pyloric caeca Invertase Pyloric caeca & intestine Lipase Pyloric caeca & intestine Fats into fatty acids & glycerineBile-activated wax lipase Pyloric caeca The breakdown of waxes (copepods contain up to 50% wax) Bile Liver to gall bladder to Emulsifies fats (breaks them into little bits proximal intestine for easier digestion)Trypsin Main alkaline enzyme for breaking down proteins.Chitinase Pancreas Produced by some insect-eating fish to break down the chitins that cover the insect.Carbohydrase A general term for enzymes that break down carbohydrates.Amylase Pancreas only in carni- Specific carbohydrates, so it would be vores. Throughout the more helpful to vegetarians than meat gastrointestinal-tract in eaters vegetariansEnterokinase Intestine It has the unusual function of keeping other enzymes from digesting the fish's own intestine.Alkaline proteases Intestine, pancreas, and pyloric caecaIntestinal zymogens Collective term for enzymes Various secreted in an inactive form before being activated by contact with other enzymes

Many breeders of fish assume that the high death rates in fry are caused by disease or cannibalism. In my reading, I found out that a great deal of mortality is caused during the crossover from digesting just egg-yolk (endogenous source), to feeding on some egg-yolk, to feeding on just external sources (exogenous source). Newly hatched fry don't produce the proper digestive enzymes yet. But for a fry to survive, he obviously has to produce them all by the time he digests only external food. (www.journals.cambridge.org/article_S0990744094000215)

Predatory fishes that swallow their food in large pieces regurgitate it from their stomach frequently. Though the purpose isn't given, I assume it is to chew it further to expose fresh surfaces to the enzymes when it is re-swallowed. Although fish tongues often have teeth, their tongues don't have muscle like those of most higher animals. They are actually worked by muscles on the bones below the tongue.

Fish may have a gall bladder, which temporarily stores the secretions of the liver, and it additionally injects bile into the beginning of the small intestine. The bile emulsifies (breaks into tiny droplets) the oils and fats remaining in the small intestine. The pancreas may be an isolated organ, or it may have bits of tissue divided amongst the liver and the omentum (the tissue which suspends the intestine from the peritoneum (the sack which contains all the 'guts')).

When lipids (fats and oils) are digested, the waste products are water and carbon dioxide, which are easily disposed of by the fish. When protein is digested however, nitrogenous wastes are produced, which can be very toxic. Most aquarium fishes are teleosts, which produce about 90% ammonia and 10% urea. The ammonia is highly toxic, but useful to the fish and easily excreted through the gills. Urea is less toxic, but requires more energy to produce. Then it is released through the kidney. Fishes in highly alkaline environments typically produce more urea than fishes in more neutral or acid environments.

FOODS ON THE MARKET

I personally prefer using frozen foods in which the animals are not torn or smashed. If the shell gets torn in any way, some liquid nutrients will leak out, and liquid as well as some solid nutrients can be degraded by environmental chemicals. On the other hand, shrimp and crab meat is more digestible when their considerable shells are broken beforehand. One should probably first consider the thickness of the food's shell before deciding whether it should be crushed or solid before it is fed to the fish. It's hard to get frozen Daphnia that are still fairly whole. This is because of the expansion of water as it freezes. For some reason, the Daphnia body bursts when freezing much easier than shrimp or worms. Also consider the freshness of the food by its color: Bloodworms, brine shrimp and daphnia should be reddish if fresh. If it is gray, it has spent some time in a defrosted condition. Bloodworms can be deceptive – quite often the liquid around the worm has become red, while the worm itself is gray. You should shop around for the best store/brand of frozen food, because I can't say which store/driver/food company treats their frozen food better than the others.

As far as dry foods go, take much more into account than what is written on the container. Buy one, then test it for palatability to the fish – if they won’t eat it, it won’t matter how healthy it is. If it won’t float long enough for your surface fish to eat it, buy one that floats longer. If it doesn’t sink for your bottom fish to eat, find one that will. Flakes that are thin and flexible may not crumble easily for feeding to fry. Nor will pellets or flakes that are too thick. If you have outdoor fish, stop feeding color food when it gets cold. Color food is harder to digest when it gets cold.

REFERENCES

Functional Design in Fishes (1970) R. McN. Alexander, Hutchinson University Library, London

Functional Morphology and Classification of Teleostean Fishes (1973) Gosline, William A. The University Press of Hawaii

The Diversity Of Fishes (1999) Helfman, Collette, and Facey. Publish Blackwell Science, Inc., Malden Massachusetts

Fish Nutrition (1972) Halver, J. E., ed. in The Vitamins, pg. 29-103 by J. E. Halver, Academic Press, New York

Fishes : An Introduction to Ichthyology (2004) Peter B. Moyle & Joseph J. Cech, Jr., Prentiss-Hall, Inc. Upper Saddle River, New Jersey

Fish and Their Behavior (1988) Gunther K. H. Zupanc, Tetra Press, Melle, West Germany