Vegetables.

Even fishes we ordinarily think of as carnivores are opportunists in what they will devour. A few fish have evolved into more or less obligate vegetarians, and to suit the dietary niche they have moved into, their digestive tracts have become long and coiled, quite unlike the short gut of a carnivorous fish. Among these vegetarians are the "algae-eaters" we employ to graze algae in our aquaria. What they are grazing along with algae, some fishes less selectively than others, is the biofilm, composed of bacteria, diatoms, algae, various sessile protists, etc.

It's well known that no vertebrate vegetarian is able to digest cellulose. All vertebrate vegetarians, from dairy cows to Otocinclus, depend on a cooperative bacterial population to break down the cellulose in a vegetal diet. Ruminants, such as dairy cows, have evolved a separate stomach--— the rumen--— to house cellulose-digesting microbes and protect them from digestive acids. Plant-eating Semiprochilodus taeniatus in the mid-Amazon basin even has a special gut area filled with detritus and bacteria to aid cellulose digestion, I've been told.

In all the vegetarians, the host gets to share some of the released sugars and other bacterial products and ungraciously digests some of the bacteria too. When vegetarian fishes are starved for algae and other green feed during the long passage from home waters to your aquarium, their symbiotic gut bacteria are decimated. Later, though greens reappear, maybe in the form of a supplementary treat, there may not be a sufficient population of bacteria to process the bonanza, which passes through the fishes' digestive tract without giving up much of its cargo of nutrients. Many an Otocinclus or "Pleco" has slowly wasted away in a tank with plastic plants, on a "feast-and-starve" regimen that would suit a carnivore fine.

In your well-ordered aquarium there may not be enough algae and biofilm to keep these specialists healthy. Many fishkeepers' feeding repertory includes spirulina flakes and algae tablets, which will drop to the bottom. The tablets are full of nourishment, and they are neat enough for the tidiest aquarist-- they are manufactured to look reassuringly like pills. There is also a "control" issue here: the fishkeeper can strictly control how much his algae-eater is to get. Other fishkeepers with a more permissive approach will allow a leaf of lettuce or spinach into the aquarium, but will still pull it right out before it starts to break up and blow around. If I sound slightly testy it's because a common misunderstanding here directly affects the fishes: It is impossible to overfeed a tropical fish with vegetable matter. Ever. Any fish. Every omnivorous fish that will nibble at greens should also be given a round-the-clock chance to be grazing on vegetables, just as much as the pure vegetarians who depend on them. It is surprising how many aquarists eliminate plants from tanks where plant-nibbling fish are going to be housed, but don't think to provide substitute greens. If chopped-spinach rations have been slender lately, my Botia striata remind me by slicing small holes into the leaves of Amazon Swords.

The range of vegetables eagerly eaten by fish is broad: spinach leaves, parsley, cilantro, watercress (well, this is New York!) or lettuce, broccoli stems slit lengthwise and the untidy outer leaves of brussels sprouts, slices or chunks of zucchini and yellow squash, beet tops and beetroot skins, cooked peas and lima beans popped out of their tough skins, cucumber or sweet potato, slices of plum, pear, apple, canteloupe and winter melon, halved grapes, even the tough ends of asparagus stalks, though their indigestible white threads remain to blow around the tank.

Preparation is simple. Briefly nuke the leaves and slices first, or drop them for a minute in unsalted rapidly-boiling water, just to soften them. This "blanching" has nothing to do with nutrition or making the vegetable more digestible. Blanching eliminates some trapped inner air and lets the greens sink without being held in an artificial "veggie clip," which looks unnecessarily obtrusive I think. Blanching eliminates the surface bacteria too. Without blanching, swarming surface bacteria can quickly form a cocoonlike veil round a slice or leaf, and then fish are apt to ignore the vegetable inside.

You can freeze blanched veggies. Drain them and drop them into a bowl of cold water, just as if you were going to feed them. Parboiled spinach and other leafy veggies can be squashed into ice cube trays and frozen into convenient servings. Vegetable slices can be laid out on plastic wrap, frozen overnight and stashed in a freezer bag.

Slightly wilted, but undressed salad greens and other leafy vegetables can be rolled tight in a dishtowel and frozen. Then you can slice long coiled shreds off the end without defrosting.

When to remove the veggies from the tank? That emerald green spinach leaf is quite beautiful the first day, but the fish are likely to dig into it most enthusiastically about the third day, when it has the bedraggled olive-drab look of minestrone that's been reheated too often. Don't take it away from them just when they're getting into it! it's no worse-looking than some of those yellowing plant leaves that you haven't pruned. Plants protect their leaves with various mildly toxic chemicals, even our commercial veggies with their pumped-up sugar levels, and the fishes may wait for some of these chemicals to leach away.

"Too much feeding of spinach can cause problems because of the oxalic acid content." This was a needless fear recently aired in some web board postings. Many plants pump metabolic by-products into their leaves to deter herbivores. Oxalic acid isn't even a particularly arcane plant by-product. Besides spinach, which we eat, oxalic acid is found in higher concentrations in rhubarb leaves (which we don't eat); it provides the tang in sorrel and dandelion greens and other up-market salads. Fishes are too sensitively tuned to chemical cues to poison themselves. For example, the higher levels of oxalic acid in Cryptocoryne leaves do make them mildly toxic, and consequently fishes (and snails) don't attack them, at least not until the crypt leaf is soft and translucent and all its toxins have bled away. I'm sure you've noticed this yourself. As that spinach leaf is softening in the water, oxalic acid is leaching out, along with other plant toxins. When it's palatable, it'll get devoured. So don't fret about "oxalic acid poisoning."

About cellulose and lignin. You may not be concerned just now (and this won't be on the mid-term) but cellulose and lignin have interesting characteristics and major roles in the ecology of the aquarium.

Cellulose is built of a long chain (a polymer) of glucose molecules (polysaccharides), somewhat like starch. But cellulose is a carbohydrate with a difference: it is bonded with a different kind of hydrogen linkage, a "beta bond," indigestible by ordinary enzymes. Individual polymer chains of cellulose are arranged in parallel arrays to form microfibrils, not unlike a strong cotton thread. The "beta bonds" render the microfibrils of cellulose extremely tough and inflexible. Cellulose is very stable at ordinary temperatures, and it's not soluble in water. That renders it safe from bacteria, which have no way of ingesting specks of insoluble matter the way an amoeba can, by engulfing them. Bacteria rely on molecules, ordinarily organic molecules that are first dissolved in water, which are absorbed into the bacterial cell through proteins at the surface. A very few kinds of bacteria can excrete an enzyme, --—called cellulase, of course--— that begins to break the hydrogen bonds and cuts the long polysaccharide into sugars and simple fatty acids that can be drawn into the bacterium. Other bacteria are attracted by the localized soup, and they cluster round to scarf up stray sugar molecules. Only these bacteria, and a few fungi make cellulase. All vegetarian animals must maintain a bacterial culture in their intestinal tracts.

Soon after my barbs have gorged on zucchini, the white cellulose fibers pass right through them and emerge still quite recognizable at the nether end. So do the seeds. When I see zucchini seeds make the intestinal transit, I'm reminded that certain plants surround their seeds with tissue containing hormones that suppress germination. If such seeds don't pass through the acidic enzyme bath of an animal's digestive tract, they must be laboriously stripped of the surrounding pulp by the gardener before they can be successfully sown. Though annual flooding disperses seeds over the floodplain, the seed-bearing plants of tropical floodplains also depend on animals, including fishes, to disperse their fruits upstream.

Hemicellulose is a polymer similar to cellulose, but with a branching molecular structure. It's present in leaf structures too.

Some plants have a secondary cell wall laid over the primary, cellulose-bearing one, composed of another structural polymer, lignin. As you know, lignin is abundant in the cell walls of all woody plants; it accounts for a third to a half of the dry weight of a tree. Lignin is even tougher than cellulose, and it's even harder to digest. Wood doesn't even rot if it remains dry--— think of the medieval barns of northern Europe and the wooden Nara temples of Japan, over a thousand years old. And waterlogged but without oxygen, at the bottom of a bog for example, wood can lie undecomposed even longer than that.

Lignin owes its strength to a random, cross-linked microstructure. It is composed of a polymeric chain formed not of sugars, but of polyphenols, many of which are toxic to bacteria as uncombined molecules. Phenols in concentration are toxic in a degree to animals, too. Plants produce phenols as protection against microbial decomposition. Even plants can be negatively affected by the foreign phenols produced by certain rival plants, in the processes generally called "allelopathy."

The few organisms that can digest lignin, do it by way of their symbiotic bacteria. Termites are a well-known example; they "digest" lignin by harboring within their gut bacteria and some very primitive flagellate protozoans that have co-evolved with them. Each species of termite has its own species of protozoan, which cannot survive outside the termites' communal gut. Teredo, the specialized wood-boring clams called "shipworms," also depend on symbiotic bacteria. You would figure that the "wood-eating" Loricariid catfishes must have some similar arrangement. Sure enough, recently the first example of a lignin-digesting symbiont in the gut of a Panaque species has actually been isolated.