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Captive Ecosystem Energy Dynamics Aquarium.Net Nov 96

Dr. Ron Shimek discusses the dynamics of a reef aquarium and the necessity and type of scavengers needed. November 1996 Index for Aquarium Net, Aquarium Net has numerous articles written by the leading authors for the advanced aquarist

Captive Ecosystem Energy Dynamics and The Necessity of Scavengers in a Marine Reef Tank

By Ron Shimek

Ecology is the study of the interaction of organisms and their environment. It is a science that is over a century old, and as a mature science it has a rather well-developed data base and series of opporational rules. All reef aquarists are ecologists, whether they know it or not. Some of them have some formal training in the discipline, but most are self-taught in the expensive school of hard knocks (otherwise known as the trial and error school of "Why in blazes did THAT die?"). The first part of this article will give a short basis in some basic ecological rules as they apply to the home aquarium, and how aquarists ignore those rules at their peril. The second part will discuss some of the biological solutions to some ecological problems prevalent in some reef aquaria.

For an excellent treatment of ecosystem dynamics in a cold-water ecosystem (sorry, I couldn't find a good treatment on the net for a coral reef) check out this link:

http://gopher.wh.whoi.edu/ecosys/eco93.html

Many successful marine reef aquaria seem to be constructed to create a microcosm of a coral reef, or at least what the aquarist perceives is a reef. Often these aquaria use a sand substrate base and live rock as the biological filter, and they may or may not have a plenum or space under the sand. Generally, a great deal of care is taken to provide proper water conditions, lighting, and heat.

Some aquarists then proceed to stock their system with numerous animals to provide some aesthetic ideal, or to fulfill a "mind's eye" picture of how the captive reef should look. Probably at least as commonly, other aquarists fill their systems with organisms purchased on a "hit or miss" or "whim of the moment" or "whatever the dealer has" approach. Either way the aquarium is stocked, a captive ecosystem is created.

As in all ecosystems, our small captive ones have to have utilizable enegry provided to them. This energy enters the system in the form of the light and food for the organisms that we provide. Ideally if the system was big enough, the system could mimic the real world, and only light energy would need to be added; all the necessary other food would be generated by the plants in the system. Such captive reef ecosystems do exist, but their minimum volume appears to be on the order of several thousand gallons - a bit larger than what the average hobbyist can put in his or her living room...

Our aquaria have various food webs which pass energy through them in a way that can rather closely mimic a natural ecosystem. One basic pathway might be as follows. Light enters and provides the energy for zooxanthellae in corals to use dissolved carbon dioxide and water to create sugars. Some of this sugar leaks out of the zooxanthellae and can be utilized by the coral as food for either growth or maintenance of its body structures. Perhaps it would be utilized to make mucus, a substance produced in relatively large amounts by most corals. Mucus, technically called a glycoprotein, results from a chemical fusion of several sugars to protein molecules. Mucus is used by the coral as an epidermal protection and it often sloughs off or dissolves off the coral. That mucus is, in turn, food for other organisms such as bacteria or some microscopic animals. They are able to break this material down to its constituent molecules, basically reconverting it back to carbon dioxide and water. When they do so, they are able to harvest some of the energy of sunlight that was stored in it by the zooxanthellae. This energy is used by these organisms for their own growth and maintenance.

For a good discussion of energy in the environment and food webs, explore the various pages of the following world wide web site:

http://mcnet.marietta.edu/~mcshaffd/102/ecosystem.html#FoodChainsandWebs4

http://mcnet.marietta.edu/~mcshaffd/102/ecosystem.html#Energyflowthroughtheecosystem

In any case, the basic food chain is from light energy to a primary producer, such as a plant or an alga, to a primary consumer, such as an herbivore, then to a secondary consumer, such as a carnivore which eats the herbivore, or to a scavenger. From the scavengers, residual food with its inherent energy passes to the bacteria of the aquarium which complete the cycle by final complete breakdown. Each of these types of organism is refered to as a "trophic level" and there may be several "sub-levels" within each of them.

In all ecological systems, be they natural or man-made, these energy transfers between trophic levels are not very efficient; only about ten to fifteen percent of the energy gets transmitted from one level to the next. Hobbyists often take great care in stocking their aquaria with the organisms which are either primary producers or one of several levels of consumers. Corals with zooxanthellae, for example, can be considered to be both primary producers and consumers, as their zooxanthellae convert light energy into sugars and a few other products and the coral animals themselves consume those photosynthates which leak out of the zooxanthellae. All algae are leaky; and zooxanthellae are no exception, about forty percent of their photosynthate oozes out of their cells and can be used by the coral's tissues.

When the light goes out, the corals become total consumers, and feed. Incidently, with the lights out, the zooxanthellae are also consumers, living off the sugars they made earlier. Secondary consumers, animals such as fish or other animals that eat animals that have eaten plants, are also found in our systems, and we often provide additional food for them, in the form of supplemental feeding.

However, the scavengers, the next trophic level, are often ignored by aquarists. Such an oversight can lead to significant problems in the long-term maintenance of the captive reef ecosystem. Simply put, scavengers take uneaten food, animal debris, plant detritus, and various animal byproducts, and convert it into a form that allows the bacterial component of the tank to feed on it. Scavengers mediate and regulate this feeding so that our captive ecosystem remains more-or-less functional.

For a unique view of scavengers see: http://www.mrat.com/stamp/scavenge.html

Why should aquarists care if there are scavengers in their systems? After all, sooner-or-later, all of the stuff that scavengers can eat will be eaten by bacteria anyway. Well, simply put, the presence of a good functional scavenger trophic level provides insurance against fouling and excessive nutrient liberation. Without scavengers, for example, uneaten food can, and certainly will, be totally converted into usable bacterial food in short order. We call this process "decomposition" or "rotting." Uneaten food tends to collect in nooks and crannies or in areas where current is minimal. In these areas, relatively rapid bacterial growth will be fueled by this good food. Such bacterial metabolism can utilize rather large amounts of oxygen and create significant areas of low oxygen concentration, even in the best of tanks.

It is a peculiarity of most animal life, and particularly most marine animal life, that it cannot exist for any long period in areas of low oxygen concentration. If the excessive bacterial growth and resultant oxygen deletion occurs around immobile animals such as sponges or corals, these animals may be killed. Mobile animals may just move away, but sometimes they suffer injury as well. Additionally, these low oxygen environments favor the growth of other bacteria, especially the photosynthetic bacteria refered to as cyanobacteria. Although the name cyanobacteria is translated as "blue-green bacteria," most people refer to them as "blue-green algae;" however, these organisms are not closely related to the other algae found in a tank, and additionally they are often colored red or pink. Aquarists often refer to them as "red slime algae" (See Shimek, 1996a).

Scavengers prevent or reduce the build up of nutrients by eating the detritus or uneaten food. They metabolize the material as food, utilizing a portion of it to build their own tissues. Additionally, they release part of it as metabolic wastes over a relatively long period, several hours to a day or more. Finally, what is not utilizable by the scavengers is redeposited in the aquarium as feces, which have significantly lower quantities of material that is useful to bacteria than the original food did. In effect, they take rather high concentrations of high quality material such as uneaten food, use part of it as metabolic energy, dispose of part of it as dissolved gaseous waste, such as carbon dioxide, and redeposit the remainder as a significantly lower quality material, scavenger poop... Scavengers are nature's way of following the dictum that "The Solution to Pollution is Dilution."

What animals are good scavengers?

There are two components to the guild of scavenging organisms, based largely on size. A quick and dirty distinction of the difference between these groups, is easy. The aquarist can see the larger ones without magnification. The small organisms generally require magnification of at least thirty times to be easily visible.

The large scavengers includes those animals who seek out and find food lying on the bottom or in the upper layers of bottom sediments. Although this way of gathering nutrition is widespread, the number of acceptable large scavengers for normal reef aquaria is quite limited. Some animals that are natural scavengers cause so much disturbance in a reef tank that they are unacceptable as scavengers, many rays or skates fall into this category, as do the larger sea cucumbers. An additional major problem with large scavengers is that while most of them seem perfectly happy to eat detritus or debris, periodically they will turn around and eat some other expensive denizen of the system. This tends to limit their acceptability.

The other problems with scavenging animals relate to their habits. In nature, animals that root through sediments for food are often food themselves for other animals, and because of this, scavengers are often nocturnal. Many nocturnal animals seem to believe in the old dictum that "All cat (fish) are grey in the dark," and are drably colored. The practical consequence of this is that aquarists are loathe to spend their hard earned money to purchase animals that they seldom see, and that when they do see them, are drab and dull in color. It is always easier to purchase a brightly colored animal than a dark, dull one. Unfortunately, such expediency may not always be the best way to spend one's money...

The large animals that are acceptable scavengers come from several animal groups. Probably the most successful and diverse of these animals are arthropods. By the nature of having an exoskeleton, arthropods have a real physical limitation to the size of food they can eat, and biologists can term most of them "microphagous," meaning that they can only eat small or soft food. This has seemed to make them "preadapted" to being animals that eat small particles of soft or semi-decayed food. While a good many arthropods in nature are scavengers, the number that can live in our aquaria successfully is limited. Probably the best arthropod scavengers are small hermit crabs. The so-called "herbivorous" hermit crabs that are commonly found in reef aquaria, are in reality rather omnivorous, and will eat food particles, parts of animals, and plant debris. They do not seem to attack living animals very frequently, and as a consequence are good candidates for the scavenger array of any reef aquarium. A small herd of these crabs will do wonders in keep debris under control.

Some ophiuroids, or brittle/serpent stars are the other echinoderms that can do well as scavengers. Commonly called either brittle or serpent stars (and from the aspect of this biologist, this distinction makes absolutely NO sense whatsoever...) some of these are animals that move across ocean bottoms eating whatever they can catch. A few larger species are decidedly predatory and will live in a marine tank only at the peril of many other tank inhabitants. Because many brittle/serpent stars look alike, identification is difficult, and the common names of "red serpent star" or "black serpent star" are only of minor use. However, the small ophiuroids that are mostly gray, black, or mottled with smooth arms are generally safe for the system and are good scavengers (Shimek, 1996c).

Some of the annelids or segmented worms are also scavengers, and some of the most useful in this regard are the much-maligned "bristle worms." Although considered to be predatory and dangerous by most aquarists, bristle worms are mostly scavengers and are quite adept a gathering uneaten food. In natural environments bristle worms are common reef rock inhabitants. They are often nocturnal, and reside in small holes during the day in rocks or coral skeletons. When such material is gathered for the aquarium trade, they come along to enter our systems. I suspect that these animals are ubiquitous, and that there are very few aquarists without at least a few of them in their tanks. Unless the populations become very large or the individual worms become huge, it is unlikely they will become a nuisance in our systems, and they are useful scavengers (Shimek, 1994).

Other scavenging annelids are found in aquaria, often living in the sand substrates. These animals are seldom seen unless they happen to be burrowing near the aquarium walls. Depending on their species, they may forage on the surface after dark or just eat material worked into the sediment by other animals.

The (Mostly) Unseen Crew

If a reef aquarium is constructed with a sand substrate, that substrate gets colonized relatively rapidly by an array of small organisms. These organisms come from one of three sources, either from "live sand" purchased to provide an innoculum of living creatures or from animals that migrate into the sand from rocks, or from eggs or spores present in the water. Most animals come from the first two sources, but it is likely that most of the bacteria found in the sediments originally enter the sediments from the sand.

Clean "unlive" (zombie?) sand will become initially colonized with bacteria within a few minutes, and the bacterial populations will grow and fluctuate through the life of the system. The initial variations in bacterial populations and their various food sources cause the "cycling" seen in all aquarium systems after they are initially set up. After a few weeks, stable populations of many bacterial species are found living on the sediment particles. Their relative position in the sediments is determined by the interaction of sediment depth, availability of food sources, and availability of oxygen.

Animal inhabitants of the sediment will colonize it almost as rapidly. Most of these are microscavengers and help convert small food particles into bacterial food. Several different animal groups are specialized to live in between or on individual sand grains. As you might expect, the animals that live on sand grains are really tiny. Nevertheless, they are complex organisms with complicated behavior. As it turns out, this component of our artificial reef ecosystems actually can be more like the real reef than any other part of the system. This is because the animals are so small relative to the volume of the system that relatively normal populations can occur. Additionally, a wide variety of organisms will be introduced by the aquarist, albeit unintentionally, by the addition of live rock or live sand.

The array of animals that live either on sand grains or in the water spaces between them are called either meiofauna, interstitial fauna, or psammon. All of the names mean the same. These animals are often microscavengers eating small particles of food and debris. Additionally some of them will also scrape bacteria and diatoms off the sand grains and eat them. In our aquarium ecosystems we have a gradient of animals based on size. These animals range in size from large invertebrates such as sea anemones or corals, or large fishes down to progressively smaller animals. One way of making a well-functioning food chain in our artificial worlds is to have animals that eat every size of potential food item down to the bacterial scale. In this way, no food goes to waste and no accumulations of uneaten food occur to pollute the system (Ruppert and Barnes, 1994).

For more information on meiofauna, try this link:

http://acg60.wfunet.wfu.edu/users%2fboadarm4%2finvert.htm

The micro-scavengers of the sand are the smallest animals in our systems, and even though we generally not specifically introduce them, they are more individuals in this array and their aggregate mass is often rather substantial. The numbers of animals in this assemblage can actually cause problems for an aquarist unfortunate enough to have a water circulation failure in their system due to a power outage or pump failure. In these situations the large volume of animals living in the sand can have enough demand for oxygen in the tank that they can literally lower the oxygen concentration to near zero, resulting in significant mortalities of other more noticeable animals like fish or shrimps. The meiofaunal animals are often adapted to be able to withstand relatively long periods of oxygen deprivation. Consequently, they don't seem to suffer much during these periods.

Well, just what animals are found in this array? Small crustaceans such as harpacticoid copepods are common, particularly in the upper layers of sediments. During the night harpacticoids often will swim around the tank as demersal (or bottom-living) plankton. They may become food for some of the corals during these nightly excursions. Harpacticoids on the surface of the sediments also seem to be favorite foods of Mandarin dragonets and other substate picking fishes.

For a good large diagram of a very small bug (harpacticoid copepod) follow this link: http://inlet.geol.sc.edu/~nick/harpacticoid.html

Nematodes, or round worms, are also very common in this array. These are slender worms related to some of the hookworm and pinworm parasites of humans. The free-living nematodes are often present in large numbers and are often exceptionally important as scavengers. Not only do they eat small particles of food, but they actively move through the sediments keeping them from becoming compacted and anaerobic.

For more information on nematodes try this link:

http://www.oit.itd.umich.edu/projects/biol103/Nematoda.shtmlml

Finally, there are an array of animals with such bizarre sounding names as kinorhynchs, gastrotrichs, and acoel turbellarians that are found in this habitat (Kozloff, 1990). These are all small wormy animals that scavenge various kinds of detritus or eat other microscopic foods. Although some of them can reach lengths of a millimeter or so, they are effectively invisible without a microscope.

For some information on some of these animals try the following links:

kinorhynchs: http://acg60.wfunet.wfu.edu/users%2fboadarm4%2fkinor.htm

gastrotrichs: http://acg60.wfunet.wfu.edu/users%2fboadarm4%2fgastro.htm

In terms of absolute numbers, and probably in terms of mass of living tissue, the scavengers constitute one of the larger groups of animals in most natural marine ecosystems. To keep our captive ecosystems functioning well, they should be equally abundant in them. We can ensure that there are larger scavengers in the system, and there should be plenty of them, by purchasing them as we purchase other animals. The microscopic scavengers enter our ecosystems through purchase of live sand, and they remain and thrive in our systems providing sufficient food is supplied to the whole ecosystem. Without appropriate numbers of the clean-up crew, bacterial populations can fluctuate wildly and cause problems, sediments can go anaerobic, and the health of all other animals in the reef system is at risk.

Questions or Comments?

Ron Shimek

Literature Cited:

Kozloff, E. N. 1990. Invertebrates. Saunders College Publishing. Philadelphia. 866 pp.

Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. Saunders College Publishing. Philadelphia. 1056 pp.

Shimek, R. L. 1994. Bristleworms. Aquarium Frontiers. 1(2): 6-8.

Shimek, R. L. 1996a, Slime, Goo, and Hair - A Discussion of Algae. Aquarium.Net Cybermagazine. September 1996.

Shimek, R. L. 1996b. Gerkins or Dills - The Cucumbers of the Sea. Aquarium Frontiers. 3(3): 2-9.

Shimek, R. L. 1996c. Serpent Stars? or Brittle Stars? or What are these things anyway? Aquarium Frontiers. 3(2):2-8

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Last modified 2006-11-20 04:02
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