The Infamous Detritivore

For several years, I have been recommending and suggesting that aquarists add "detritivores" to their sand beds and to their reef systems, in general. A short time ago, I was asked, "Just what is a detritivore and why should I add some to my tank." In essence, this aquarist wanted to know why in the world that he should pay a lot of money for animals that generally are invisible. And even worse, why pay for animals that when they are not invisible, are ugly? Well, I thought the answer was obvious. Then, I thought, well, maybe it is obvious, but only if you are a marine ecologist who has spent a lot of time working with these kinds of animals. But for the average hobbyist, the questions I was asked were really pretty good ones, and definitely raise some valid points.

Figure 1. Although many different kinds of worms are referred to as Spaghetti worms, "true" spaghetti worms or terebellid polychaetes have many long tentacles arising from the head. These tentacles range out from a burrow in rock or sediment to collect the small particulate detritus that the worm feeds on.

To get to the end of this discussion, and to answer those questions, though, I need to start with the concept of detritus and discuss it. I will then discuss several different kinds of detritivores and their importance to marine aquaria.

Detritus - A Multitude of Sins

As words go, "detritus" is nice; it is short and even I can spell it correctly (at least most of the time). The word is concise and easy to say. Only its meaning is ambiguous. The term "detritus" is actually a word of Latin derivation and it means a "rubbing or wearing away." As is generally understood, detritus is fine particulate material that is eroded off of larger particles. In our systems, detritus is the small particulate material resulting from either the biological or physical processing of food, living things, or substrata. Often this is waste material from some relatively large organism. So, "detritus" means small particulate material of uncertain and variable origin.

There is a tendency for aquarists to consider that this particulate material is simply composed of fine mineral grains. And some small portion of it is. But most of it is not. Most of it is organic, and therein lies the problem. Much of this detritus is the remains of uneaten food, specifically, particulate food such as dried flake food. Another large component is fecal material from both fish and invertebrates. The fecal component is effectively the same thing as uneaten food. Even though we tend to think of feces as being waste material, in point of fact for aquarium animals, they are simply the indigestible residues of food, often with a healthy dose of digestive enzymes in it. In other words, it is uneaten, partially processed food. Other sources of detritus are the breakdown products of algae. These products include mucus, various other exudates (algae are very "leaky") and decomposing algal tissue. These materials are highly organic, and many animals find them quite nutritious.

Detritus may also be formed from animal byproducts. Corals and other cnidarians produce very significant amounts of mucus. Mucus may either dissolve in water or form small relatively insoluble aggregates. Additionally, sloughed animal cells also contribute to detrital formation. All animals recycle their covering cells (from epithelial tissue) at a rather rapid rate, and the amount of cellular debris which is shed in an aquarium may actually be quite considerable.

Additionally, detritus may be formed by some types of bacterial aggregations. These may result from the feeding activities of animals, but often they are simply a result of bacterial growth. Under favorable conditions, the marine bacteria in our tanks may grow and divide rapidly, sometimes several times an hour. Bacteria often surround themselves with mucilaginous coats. These coats tend to stick together, and result in small clumps of bacteria otherwise known as particulate organic matter or detritus.

Finally, detritus can be composed of inorganic mineral grains resulting from the actions of animals burrowing in live rock or coral skeletons, or from ingested larger mineral grains which are only partially dissolved in digestion. Most of these mineral grains are covered with some bacterial growth and hence they have a "frosting" of some edible material over the mineral itself.

An examination of all of the above ways that detritus may be formed should convince you that detritus is basically organically rich particulate material. The mineral component of detritus is simply small silt or clay-sized particles (0.063 µm or smaller particles) covered in organic material. This composition means two things: 1) detritus may be a useful as food, and 2) detrital accumulations are nutrient "time-bombs" waiting to go off.

The organic matter in detritus is loaded with carbohydrates, such as sugars and starches, nitrogen-rich proteins, and some lipids. In many ways, detritus is an ideal food source and much of it is quite rich. Left alone in a reef tank, it will become subject to chemical and biological decomposition. This, in turn, will cause a spike of dissolved nutrients. And from these spikes will come undesirable algal blooms.

Detritus Control

The problem for aquarists is that detritus amounts to condensed nutrient, and can lead to all sorts of other problems. Consequently, we need to remove it from our systems. This leaves us with several options. We can try to reduce detritus formation. Alternatively, we can accept that our systems will have detritus formation. If we do the latter, we must either remove it, or allow our systems to live with it. Let's look at these alternatives.

Reduce Detritus Formation

The most obvious way to reduce detritus formation is to reduce the food input to the system, and this was the cure for "the detritus problem" in the early days of reef keeping. However, we know that all stony corals and most soft corals need to feed, and they need to feed a lot to get the materials they need to grow and remain healthy. Basically, putting these animals as well as the fish on a starvation diet is both irresponsible and unethical. The animals become severely stressed from malnutrition, and are more prone to diseases, parasites, and other mortality factors such as attacks from nearby competitors.

Another way to reduce detritus formation is to keep the animals cool, at the lower limit of their temperature tolerance. For every Fahrenheit degree of cooling, coral reef animals will lose between three and five percent of their metabolic rate. Such animals don't need to feed much, nor do they produce much in the way of excess particulate matter. Unfortunately, they also don't grow, and don't have any metabolic excess to fight off diseases, and finding a low temperature extreme is tricky. Many corals are not found at temperatures below about 76 F, and as they near this temperature their metabolic processes slow down to where they are barely living.

Allow Detritus Formation: Remove the Detritus.

This alternative, while labor intensive, works pretty well. Probably the easiest way that this can be done is to keep a bare bottom tank, with minimal live rock and good bottom circulation. The water is continuously mechanically filtered, and the tank bottom is frequently siphoned clean. To ensure that all the detritus is removed, the aquarist must have a schedule that ensures regular cleaning and water changes. This alternative works particularly well for fish only tanks or invertebrate species tanks with a minimal number of occupants. It does require a lot of monitoring to ensure that the nutrient levels don't get out of hand.

Allow Detritus Formation: Live With It And Let the Critters Control It.

Living with detritus is, of course, Momma Nature's way of dealing with the stuff. To understand how detritus is dealt with in nature and how we may design our systems to deal with it, we need to realize that detritus contains a LOT of useful material. It contains all the necessary foods: carbohydrates, proteins, fats, and minerals. Some of these foods, such as the carbohydrates and lipids, can be converted directly into the chemical energy that organisms need to fuel their bodies' metabolism. The proteins may be broken down (or digested) into amino acids and then resynthesized into the organism's own proteins. Bacteria are perhaps the best possible foods in this regard, as they have a higher nitrogen to carbon ratio than do food derived from most multicellular organisms. Because nitrogen compounds are the building blocks of proteins, this means that bacteria are very useful foods when it comes to building or repairing tissues. In other words, in the natural world, detritus is an exceptionally valuable resource.

Figure 2. Several different, but similar appearing, species of amphipod crustaceans are found commonly in aquaria. Although different amphipods may feed in just about every way known, most of the ones in reef aquaria feed by tearing plant debris and organic detritus into "bite sized" pieces and eating it.

It should not seem surprising, then, that a wide variety of organisms have become adapted to eat or utilize detritus. One of the more interesting side effects of eating anything, including detritus, is that it creates detritus. This is due to one of the inviolate natural laws, the second law of thermodynamics, which holds that no transfer of energy or materials is one hundred percent efficient. Some energy or material is always lost back to the environment. What this means in practical terms is simple: to reduce the energy and useful materials in a given amount of detritus to insignificance will mean that that particular parcel of detritus must be eaten and digested several times. The number of these "reprocessing" events may be quite numerous. For example, it has been estimated that an average pelagic copepod fecal pellet released near the surface of the ocean is eaten and re-eaten about eight times before it makes it to the ocean bottom.

Turning the "multiple use of each detritus particle" information in the above paragraph around a bit, this means that to fully process a given quantity of detritus can take a rather large number of animals. Also, given that the "quality," the "consistency," and the food value of the detritus will change after each organism is done with it, it will also take a number of different kinds of organisms each specialized for a different type of detritus, to fully process it.

The Fate of Detritus

When detritus is eaten it is processed like any other food. First, it is digested. Some of it will be indigestible and this will be passed out the animals as feces, presumably to form more detritus. Some of the digested food, mainly the amino acids and protein fragments that made up part of the original detritus particle, will be assimilated into the body of the organism and used to build or repair tissues. Some of these will eventually be recycled and eliminated from the organism's body as ammonium ion in urine. Much of the rest of the digested foods, primarily the carbohydrates (sugars and starches) and most of the lipids (triglyceride fats) will be utilized in cellular respiration, or in other words, they will be burnt to produce energy. Eventually they get eliminated from the organism as carbon dioxide and water. Some of the lipids (steroids) will be utilized to synthesize other steroids. Many animals utilize steroids as hormones, but a lot of those animals cannot synthesize them and must get them from their food. These will eventually be broken down and used in cellular respiration. Minerals that were digested, primarily during acidic digestive processes, may be used for skeletal materials or co-factors in enzymes.

Notice in the above paragraph that the only things to actually be exported from the aquarium by detritus processing are carbon dioxide from cellular respiration, and ammonium ion from protein metabolism. The ammonium may eventually be processed by bacteria and reduced to nitrogen gas, which would exit the system at the water-air interface. Each time one of the detritivores is eaten by some other animal in the natural or artificial ecosystem, it is recycled in much the same way. If there are enough different animals in the system, with the appropriate feeding preferences, virtually all the organic carbon compounds and nitrogen containing compounds in food, be it fresh food or detritus, may be exported from the system.

Although most detritus eating animals are small creatures, the total amount of carbon and nitrogen exported from a large aquarium may be quite considerable. If one is feeding half an ounce of food per day, and this may be barely adequate amount for a moderately sized aquarium (about 100 gallons), that is a shade over 11 pounds of food added to the tank in a year. Some of this will go to build more critter flesh, but much of it will be exported as carbon dioxide or nitrogen derived ultimately from animal urine.

Reef Aquarium Detritivores

The answer to the question, "What is a detritivore?" is both simple and complex. Simply put, it is an organism that eats detritus. The question becomes more complicated in trying to specify just what organisms do this. There are alot of potential animals that may be called detritivores (Table 1).

This is a rather large array of animals, and interestingly enough, only a few of them are likely to be visible and commonly seen by aquarists. Many of the smaller animals are either over-looked, or are simply invisible to the unaided eye. Most of those animals that feed below the surface of sediments burrow through the sediments and are likely to be seen only when the aquarist disturbs or changes the sediments.

Figure 3. Nematodes, or roundworms, are common in reef tanks. This particular one, removed from my lagoonal reef tank, was about 1 cm long. Many free-living nematodes are predatory, others eat organic detrital particles.

Most reef aquaria that have been set up for more than a few months, and which have had fresh live rock or good live sand added, probably have representatives of most, if not all, of these groups. The sizes of the organisms in the groups are critical. To have a functional "guild" of detritivores, all sizes from very large to very small must be present in our systems. In a very real sense, the sizes of the animals in the detritus food webs runs the reverse of what we see elsewhere, where each succeeding level in a food web is larger. Here, large animals eat large particles, and convert the detritus into smaller particles, which in turn are eaten by smaller detritivores. These also convert the particles into yet smaller particles, which get eaten by yet smaller detritivores, and so on all the way down to microorganisms.

Conclusion

While all sizes and types of detritus feeding organisms are necessary for optimal processing of extraneous debris and organic materials in our systems, only a very few of these animals are available directly from dealers. Most of the smaller ones, particularly, must come in a rather hit-or-miss manner from our additions of live rock and live sand. It is very important for the establishment of a deep sand bed, and the concurrent maintenance of a low dissolved nutrient level in our systems, that a varied fauna of these "micro-food" processors is established and maintained.

All photos courtesy of Ronald L. Shimek, Ph. D.