If an aquarist desires to have a captive reef in their aquarium, it is essential that they try to include all levels of a normal ecosystem. Our little reefs are really small artificial ecosystems and to avoid problems, all the components and functional parts of a normal ecosystem need to present. While ecosystems may be immense, they also may be miniscule; a small puddle can be, and has been, considered to be an ecosystem. So, it makes some sense for an aquarium to be thought of in this manner.
Ecosystems consist of five components:
1. Energy. This is the light and heat entering into the aquarium from all sources.
2. Non-living Substances. These are the inorganic and organic compounds of the environment; such as air, water, dissolved minerals, and solid substrates.
3. Producers. These are organisms capable of manufacturing food from the ambient energy and simple inorganic substances and consist of plants, algae, and photosynthetic bacteria.
4. Consumers. These are organisms that consume other organisms or particulate organic matter. Consumers are animals, fungi and microorganisms that canâ€™t make their own foods.
5. Decomposers. These are organisms (animals, fungi, protists, bacteria) that break down the complex compounds of dead protoplasm and in the process release non-living substances useable by the producers.
In both natural ecosystems and our â€œecosystems in a box,â€ the input of energy and materials needs to be balanced. Neither energy nor matter can be created nor destroyed within the system. This means human intervention in aquarium systems is a necessity. Primarily due to problems of scale; we cannot provide enough energy in the form of light to keep all of the consumers in our systems alive. So, we need to supplement the production of sugars by feeding the system. We also need to take some care in providing a way of removing excess materials; our systems are simply too small to be able to do it all. Nonetheless, with a bit of foresight and planning we can enlist the aid of some natural assistants to help â€œpolice the areaâ€ and keep it clean. These assistants are sometimes called the â€œclean-upâ€ crew.
The root of all of our problems as aquarists is the small scale of our systems. The volume of water needed to produce all the food that even a small fish or a coral consumes is immense. As we need to feed the animals in our systems, we inadvertently concentrate waste materials far in excess of the ability of the small system to process them. These waste materials are somebody elseâ€™s food items, and their overabundance often dictates the necessity of having a large number of organisms that are specialized to utilize those foods.
In nature, animals are often specialized to eat only one or a few types of food. True omnivores, such as humans, are surprisingly rare in marine environments. Consequently, instead of looking for the â€œbestâ€ scavengers, we need to first consider what types of animals can even function as the clean up crew. To this end, it is worthwhile to assess the types of food that the â€œclean up crewâ€ must process.
These food items are:
1. Uneaten food
1. This is commercial processed food, such as â€œflake foods.â€
2. Feces from fish and other animals
1. In many marine animals, particularly fish, the residence time of food in the gut is short, and much of the nutrient quality of the food remains when it exits the animal.
2. This material is, in effect, â€œpartially predigested, unabsorbed, food and it is quite nutritious. Many popular aquarium fish, such as clownfish, in nature are almost wholly coprophagic; that is to say, they eat almost nothing but feces.
1. Detritus has a specific definition, that being, â€œfragmentary remains of plant and algal material.â€ So, true detritus doesnâ€™t include materials of animal origin.
4. Material of animal origin.
1. This stuff is corpses, body parts, excess gametes, molted exoskeletons, and so forth.
When aquarists speak of the clean-up crew, they probably mean â€œAnimals specialized to eat all sorts of non-living materials.â€ These are the creatures generally purchased as the clean up crew, although in most aquaria, the biomass of bacteria and fungi that are also acting to decompose non-living foods may exceed that of most of the rest of the living inhabitants.
Another name for the animal component of the clean up crew would be â€œscavengers.â€ These are animals that â€œscavengeâ€ their food from the remains left in the tank. There are two types of scavengers: â€œperfectâ€ scavengers which will not eat living organisms, and â€œimperfectâ€ scavengers which will scavenge for food, but which will eat living organisms if they are acceptable and normal prey. Generally, â€œimperfect scavengersâ€ prefer living prey.
Marine animals that will not eat other living animals, but which will eat animal remains and other non-living foods are quite rare. Probably the best all around scavengers are fireworms, which most aquarists refer to as â€œbristleâ€ worms. One fireworm species, Hermodice carunculata, must be excluded from this group as they are generally predatory. Some Eunicid worms are also good scavengers, but most Eunicids are predatory if given the opportunity. The only snails that can be considered scavengers are species of Nassarius. They will eat dead meat (or meat rich processed foods) and will also eat â€œnear-deadâ€ or â€œwoundedâ€ animals, Nassarius are NOT detritivores and will not eat anything other than meaty foods. Small crustaceans such as Sphaeromatid isopods are good scavengers. These animals are recognizable by their ability to roll up into a ball, and by the â€œtail finsâ€ on the males. The large isopod group called the Family Cirolanidae contains many scavengers, but most cirolanids tend to be parasitic or predatory on fishes, and some may even attack the aquarist. The odds of getting a bad bug here are pretty high, so I recommend that all cirolanids be avoided. A few amphipods are scavengers, but many eat detritus, and many are also predators. A few echinoderms such as the small brittle stars often sold as â€œmicro-brittle starsâ€ are good scavengers and often proliferate in a tank. Additionally, bottom â€œmoppingâ€ sea cucumbers are good detritus feeding animals even if they wonâ€™t generally eat much meaty food.
Imperfect scavengers are marine animals that will primarily eat other living animals, but which will sometimes eat animal remains and other non-living foods. These animals prefer to eat living organisms, but may scavenge food if hungry enough. Purchase or leave them in your tank at its peril. The imperfect scavengers include most large worms that are not fire worms, and they do include the fireworm, Hermodice carunculata. Fortunately, the large worms found in aquaria are almost always the beneficial fireworms such as Eurythoe. Virtually no large crustacean is an acceptable scavenger. Many, perhaps most types; all tend to be predatory if given the opportunity, virtually ALL hermit crabs are predators on small animals; they may eat some detritus, but they will eat living things first. Many shrimp species and most crabs are also either fully or mostly predatory.
Generally, snails should be avoided as scavengers. The various whelks, such as the Buccinids (or goblet whelks) which include Nassarius â€œsold-a-likesâ€ such as â€œbumble-bee snailsâ€ are predatory animals. â€œBumble beeâ€ snails, such Engina mendicaria species, for example, eat other snails and worms. Virtually no whelk is reef aquarium safe, even though they make marvelous animals to keep in a â€œspeciesâ€ tank. Echinoderms are another group that contains almost no scavengers. Virtually all regular sea stars are predatory, although some may eat alternative foods if their preferred foods are not available. The small asterinid mini-stars found in some aquaria may well be the only non-predatory exception to the general rule of predation amongst sea stars. Likewise, most large brittle stars are predatory, although they may scavenge if starved. Finally, pencil sea urchins are generalist carnivores, and although they may appear to be scavengers, in point of fact they simply eat just about everything they can catch.
Britton, J. C. and B. Morton. 1993. Are there obligate marine scavengers? In: Morton, B. Ed. The Marine Biology of the South China Sea. Hong Kong University Press. Hong Kong. pp. 357-391.
Britton, J. C. and B. Morton. 1994. Marine Carrion and Scavengers. Oceanography and Marine Biology: an Annual Review. 32:369-404.
Kohn, A. J. 1983. Feeding biology of Gastropods. In: Wilbur, K. M. Ed. Physiology (2). Academic Press. New York. pp. 1-63.
Shimek, R. L. 2004. Marine Invertebrates. 500+ Essential â€“To-Know Aquarium Species. T. F. H. Publications. Neptune City, New Jersey. 448 pp. ISBN: 1-890087-66-1
Figure 1. Most fireworms are perfect scavengers for a marine reef aquarium.
Figure 2. Nassarius snails are the only snails acceptable as scavengers.
2a. Side view. The arrow indicates a groove found only on Nassarius.
2b. Bottom view.