Aquarium: An Ecosystem in Miniature

Part of the Young Naturalist Awards Curriculum Collection.

by Charlotte, Grade 9, Virginia - 2003 YNA Winner

In urban areas where exotic wildlife seems all too distant, many people bring the natural world home in the form of an aquarium. I became a tropical fish enthusiast two years ago, when I explored the art and science of fishkeeping—the second most popular hobby in the United States. Thus, the world I explored was, in fact, one I had created—a 29-gallon tropical freshwater aquarium. After becoming an expert on water chemistry, aquarium mechanics, and the biology of various fish species, I assembled this glass-walled environment and watched its aquatic community flourish.I had maintained this miniature ecosystem, home to seven fish species, for nearly a year—but how well did I truly know it? How was each species adapted to its own needs? What happened unseen at night and on the gravel bottom? How did fish that could never meet in nature interact? What behaviors occurred, unknown to me? I planned to answer these questions this winter and, in the process, learn more about the tiny world within my own.

The Environment
The aquarium is 12 inches wide, 29 inches long, and 18 inches high; its base is covered by an inch of brown gravel. Along the back wall are the power filter and heater, which maintain clear water at a constant 75°F. The pH rarely fluctuates from 7.0, an ideal level for a community aquarium. A piece of artificial driftwood and two black-and-white variegated rocks provide refuges, while ubiquitous clusters of elodea plants (Egeria densa) oxygenate the water. Each stalk in the towering aquatic forest is supple yet brittle, frilled with countless rings of three narrow, curling leaves. The electric-green leaf tissue is almost transparent, being only several cell layers thick. This plant can grow at a rate of an inch a day; I frequently trim these specimens to 15 inches.

Columbian ramshorn snails
Columbian ramshorn snails, Marisa rotula.

At least 50 Colombian ramshorn snails, Marisa rotula, glide over glass, plants, and rocks; they are the progeny of just four original snails of a prolific species. As unpopular as they are among many fishkeepers, the snails are efficient scavengers here. Snails of one variety are golden yellow and have no pattern on their amber shells, which resemble the horns of a ram. The shells are tightly coiled, with the rims a darker honey color. The snails of the second, more abundant type have deep brown, nearly black stripes that follow the spiral of the shell in parallel lines. Each pattern is unique; there are gold shells with brown markings, some brown with gold, and several split exactly down the middle. Sizes vary from 1.25 inches in diameter to 0.5 inches, for at least four "generations" are represented.

All move by means of a flowing elastic foot. This soft, translucent base, speckled with dull brown, is triangular, with thin, slowly undulating edges. A pair of threadlike antennae writhes fluidly, and a shorter pair, half the length of the first, is attached to the constantly munching mouth. This orifice is ovular and dilates to expose intricate, mechanical scraping structures—a series of increasingly smaller mouths inside each other.

A gold mystery snail, Ampullaris, extending siphon.

A single gold mystery snail of the genus Ampullaris drifts among the ramshorns. Its nearly spherical shell is a bright lemon yellow, while the creature inside is pale peach sprinkled with neon pink dots of pigment. The antennae are longer and wispier than those of the ramshorns, and the mouth and stumpy eye stalks are fleshier. The mystery snail's brown, dime-sized operculum rests under the shell, ready to hinge like a trapdoor when the invertebrate perceives danger.

Fish Species
The vivacious clown loach, Botia macracantha, is two inches long, with a sloping snout and a double-lobed caudal fin. The fish uses three short pairs of mouth barbels in its perpetual search for food, which in the wild includes worms and crustaceans. The loach's fins and tail are bright red, and its body is marked by three black bands, one of which masks its beady eyes. All loaches lack scales, so the outlines of the spine, ribs, and visceral sac are barely visible under the opaque, creamy skin between the black bands.

Loaches, belonging to the family Cobitidae, share several unusual features, including a short spine in front of the eye. They are capable of breathing atmospheric air, absorbing the oxygen in their alimentary tracts, and expelling the air underwater. Clown loaches in particular have a peculiar manner of resting during the day—they lie limp on their sides as if injured. Alarmed, I have observed my fish in this position, but fortunately the "sleeping" behavior is completely normal for the species.

A kissing gourami, Helostoma teminckii, shimmers with rosy, iridescent scales. Behind rigid, mother-of-pearl gill covers, the narrow pectoral fins rotate freely, allowing the fish to maneuver, stop, and swim in reverse. The large silver eyes are located close behind the remarkable mouth—circular, thick-rimmed lips flex open and outward in a "kissing" gesture. Mouth-to-mouth combat between fish is used to compete for territory—not as a courtship display. I have often observed such battles between my gouramis, one at three inches long and the other half an inch smaller. The lips are studded with rows of minute teeth, which not only enable gouramis to scrape algae but also allow contenders to lock mouths when fighting. Gouramis, bettas, and other anabantids possess a labyrinth organ (a mazelike extension of the gills) that enables them to breathe air. Consequently, labyrinth fish can thrive in shallow, polluted, or oxygen-depleted waters, such as the rice paddies of their native Southeast Asia.

A red platy, Xiphophorus maculatus.
A red platy, Xiphophorus maculatus.

Three male guppies, Poecilia reticulata, dart about near the water's surface. These narrow, streamlined fish are a pearly cream that softly shimmers as they turn. The rest of the skin has well-blended strokes of vibrant orange-red, as if painted on ivory. A banner-like ruffled dorsal fin flutters horizontally and ends almost even with the caudal fin, which is circular, frilly, and a deeper red. When at rest, which is not often, the guppies let their dorsal fins droop handsomely over their backs, fanning out to resemble silk capes in hue and texture. Naturally these guppies are a fancy strain, one of hundreds of varieties. I bred these and dozens of their siblings last spring and observed a wide range of body sizes, fin lengths, and individual patterns. Guppies are cousins to the broader, stockier platy, Xiphophorus maculatus, represented by a crimson male fish that often accompanies the guppy school.

A fancy guppy, Poecilia reticulata.
A fancy guppy, Poecilia reticulata.

My personal favorite is the "pleco," a seven-inch algae-eating catfish, Hypostomus plecostomus. Affectionately named "Hoover," this individual was one of my first fish and has grown five inches in the last two years. Its torpedo-shaped body is armored with deep brown overlapping bony plates, reminding me of an ironclad submarine, while the patterns on the velvety, brown-and-ivory underside resemble leopard skin. Each of the plecostomus' broad, dark-banded fins bears a thick, pointed supporting spine; this usually dormant fish is truly magnificent when it spreads its lyre-shaped caudal fin and raises its voluminous dorsal fin to full sail. The giant's pulsing sucker mouth, fringed by two short barbels, is underneath the blunt snout. As the plecostomus is mainly vegetarian, this one avidly devours fresh lima beans, cucumbers, and zucchini but is docile toward its tankmates.The inch-long corydoras, Corydoras storbai, is a distant relative of the plecostomus, both from the Amazon. It is a humble, amusing scavenger with round stubby fins and a chunky head (hence the genus name, from the Greek for helmet). The soft gray skin and triple-lobed tail are intricately spotted with white, and the flanks are delicately stippled like a mosaic.

Hypostomus plecostomus
The rainbow shark, Epalzeorhynchus frenatus.

A colorful and belligerent member of the Cyprinid family is the rainbow shark or labeo, Epalzeorhynchus frenatus. Reminiscent of its namesake, the agile shark is sooty black with triangular red-orange fins. This territorial species is related to rasboras, danios, and, surprisingly, goldfish and carp—all of which lack teeth in the jaws. Instead, they have grinding teeth deep in the pharynx and two pairs of mouth barbels.

Feeding and Territorial Aggression
I hoped to observe the feeding behaviors of the bottom-dwellers as well as displays of aggression, which I have frequently noticed during morning feedings. I began by dropping two shrimp pellets into the tank. Within seconds, the clown loach and shark had spotted the motion and attempted to locate the landing spots. Each combed the substrate with its mouth and barbels, which contain taste buds. The shark discovered a pellet first; with short, methodical sucking motions, it grazed the pellet lengthwise like an ear of corn. Although smaller, the clown loach was equally persistent; it nibbled the second pellet until it crumbled, and then devoured the manageable particles. Also on the scent, but perhaps intimidated by the more aggressive shark and loach, the corydoras lingered out of reach of the food.

While the shark and clown loach were consuming their finds, the plecostomus lumbered into the light from behind an elodea patch. This enormous fish dwarfed the others and smoothly engulfed the loach's shrimp pellet, shuffling through the gravel with its mouth. This was not the nocturnal plecostomus' natural feeding hour—its pupils were contracted in the shape of a tight horseshoe due to the bright fluorescent light.

The shark was distracted by the three guppies, which were uninterested in the food but hovered near the left side of the tank. One received an unexpected short snap, and the trio scattered. Meanwhile, the large gourami decided to "pick on someone its own size" and pursued the smaller gourami until it took refuge in the shady rear corner. 

This Plecostomus, Hypostomus plecostomus, is nicknamed "Hoover."
This Plecostomus, Hypostomus plecostomus, is nicknamed "Hoover."

The clown loach began trailing the shark closely, as if searching for food together, but the shark was intolerant of this companionship. The two fish aligned themselves with their heads level with each other's tails. As they attempted to circle and probably nip each other, their flanks brushed—and they suddenly darted in different directions. Later, the shark repeated this behavior with the large gourami. This pugnacious fish was less severe with the corydoras; it only engaged in a short chase when the catfish was within a three-inch radius. The clown loach feigned hostility by giving the corydoras an extra nudge, acting like the sidekick of a playground bully.

I inferred that the open, well-lit tank front is prime territory for the fish due to the high availability of food. Both the bottom and surface swimmers seemed to have established a pecking order that must be continually reinforced. In the case of the gouramis, it is the more massive and aggressive fish that gains dominance and "feeding rights." At the lower level of the tank, three scavenging fish must compete for sinking food and leftovers. While the larger, faster rainbow shark uses force to assert its position, it cannot control the entire area at once and must frequently put the loach and corydoras "in line." These short altercations begin when one fish invades another one's territory; the opponents display their fins, circle each other, and usually end the dispute with a lightning-fast bite. None of the fish have sustained significant injuries, nor are they discouraged from sparring again. This form of aggression, therefore, must not be intended to disable the intruding fish; rather, it is to show the tank's inhabitants who is in control.

A kissing gourami,  Helostoma temminckii.
A kissing gourami, Helostoma temminckii.

A Kissing Gourami Battle
Less than one minute after I turned off the aquarium lights for the evening, I noticed the gouramis engaging in a kissing battle. They hovered at right angles to each other, swaying slightly and tensing their bodies in S-shapes. They then faced each other, stretched their mouths as wide as possible, and locked jaws fiercely. Each fish tried to push the other backwards in a reverse tug-of-war, though they did not struggle for more than five seconds at a time. Although the smaller gourami was less intimidating, it was an equal opponent in combat. When the fish released their grips, it was the smaller that provoked the large gourami with a "kiss" on the caudal peduncle, leading them to clash again. Shortly after, the larger gourami surfaced for air, while the smaller advanced but did not begin another bout. Probably exhausted, the fish separated to opposite ends of the tank, where the large gourami mouthed its reflection narcissistically. As if nothing had happened, they returned to night foraging.

Curiously, every time I have observed the kissing behavior it has been under dim lights, such as during a water change or at night. This may be due to the fact that kissing gouramis typically spawn at dusk. Perhaps it is their instinct to compete and interact with other fish in the twilight hours.

Brine Shrimp
The diet of my fish is chiefly dried food (i.e., flakes, pellets, and algae disks), sometimes with fresh vegetable supplements. I wondered how the fish would react to brine shrimp, Artemia salina—high-protein crustaceans that would mimic their natural fare. I used frozen shrimp, packaged in cubes, as live ones were not available from the local aquarium dealers.

Accustomed to receiving food when the aquarium lid is opened, the guppies and platy circled the surface and anticipated the usual flakes. They were noticeably excited by the shrimp odor but were hesitant to approach the thawing cube on the front glass. As dull pink brine shrimp, 0.25 inches long at most, diffused into the water one by one, the fish swallowed them and awaited more. The guppies and platy were both efficient in consuming the shrimp, for their upward-facing mouths (in aquarists' terms, superior) caught the sinking food as they swam. Several clumps sank to the mid-water level, where the shark tested a piece of this new food. It continued to devour the shrimp that the top-feeders neglected, but it seemed to adhere to its designated feeding boundaries and never ventured to the surface water.

All but the vegetarian gouramis and plecostomus shared in the shrimp feast; the guppies, especially one, did not stop eating until their stomachs were visibly distended. Even the loach, corydoras, and snails waited at the gravel and were rewarded when the cube sank. I plan to keep brine shrimp as part of my fish's weekly diet, mainly because it adds nutrition and variety, but also to simulate hunting for food in the wild. 

A corydoras, Corydoras storbai.
A corydoras, Corydoras storbai.

Through observing, sketching, and ultimately becoming more familiar with my aquarium and its piscean inhabitants, I realized how natural forces and behaviors are prevalent even in an artificial setting. Although no Central American guppy would ever flee a Sumatran rainbow shark, aquarium specimens must adapt to "alien" species as they form their own community. Similar to a wild ecosystem, the ecology of an aquarium will experience fluctuations as fish mature, reproduce, and die—and all populations will be affected by abiotic factors, such as changes in light, pH, and temperature. How will my aquarium adjust to the inevitable changes of time? What will the existing residents do in response to fish, plant, or invertebrate newcomers, or to a sudden increase in one species' population? The plecostomus and rainbow shark have grown rapidly in a year, more than doubling in size. I wonder how their growth will slow or be limited by tank volume. I have also come to revere the uniqueness of each species and its adaptations—from the flamboyant fins of the guppy to the sturdy shell of the ramshorn snail. In the year to come, I will continue to be surprised by this fascinating aquatic world.



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