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Aquarium Sea Stars

Few marine aquarium invertebrates are as crowd-pleasing as the sea stars of the phylum Echinodermata.

By Philip A. Purser

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Aquarium Fish International Magazine
Of all the invertebrates a marine hobbyist could keep in his or her tank, few are as crowd-pleasing as the sea stars. These spineless wonders come in a variety of sizes, colors and textures. Sea stars are elegant in their slow-motion treks through the tank and can be almost hypnotic to watch as their tiny suckerlike feet move them from one surface to the next. Brightly colored individuals can add just the right splash of color to an otherwise bland tank, while a few small stars might accent an already thriving and beautiful aquarium.

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Sea stars make great additions to your tank due their color, size and texture variations
Sea stars can make colorful additions to a marine reef aquarium.
Yet the variety of sea stars available to the private hobbyist also has a downside. Some stars are peaceful grazers that spend their days quietly munching on algae, while others are relentless predators, silently robbing your tank of all of its crustaceans and invertebrates. So, a little homework is in order before bringing a sea stars home. With a little know-how, the average hobbyist can add a whole new dimension to his or her marine aquarium by including one or more sea stars.

Sea Star Biology
Sea stars are invertebrates belonging to the phylum Echinodermata, which is Latin for “horn skin” or “spikey skin.” Anyone who has ever handled a sea star will attest that the name fits well with these rough-textured animals. Sea stars typically have five arms or rays extending out from a central disk, though some species have up to 20 rays. They are found throughout all the seas and oceans of the world, from the icy poles to the warm, crystalline waters of the equator.

Sea stars have no complex central nervous system: no brain, no head, no eyes. They just have a simple network of nerves that extends down each arm and over the skin. Actually, we know very little about the nervous system of sea stars. But we do know that their skin is highly sensitive to touch and changes in water conditions. This mysterious system also serves sea stars as a means of detecting food. Receptors in the skin cells may register the “scent” or pheromone trail of nearby food items.

Aside from their charming, almost “cute” appearance, the most interesting feature of sea stars is their means of locomotion. Hundreds of pairs of tubelike “feet” line the underside of each ray and are part of a vascular system of pumps and valves. A sea star takes in water through a canal in each arm and hydraulically controls each pair of feet. With plungerlike motions, each pair of feet suctions down to a surface, pulls forward, then releases and grips again. It’s an amazing system that seems almost like thousands of individual units functioning as one, rather than parts of a single organism.

At the center of these tube-clad arms is the mouth, if you can call it a mouth. This organ is more of an opening responsible for digestion, and in many species, it is extendible. It may be ejected from to digest large meals outside the body. Predatory sea stars prey on bivalves, urchins, tubeworms and even other sea stars. Not only do their tube feet allow the sea stars to seemingly glide over even the roughest surfaces, but these powerful appendages can pry apart most bivalves. Tightly gripping the shell of a clam or oyster, a sea star will begin to pull. It has only to pry the shell open a little bit, insert its stomach and digest the living oyster right out of its own shell. Urchins, tubeworms and other large prey items are subdued in the same way.

As if there is no end to the amazing adaptations of the sea stars, these animals have amazing regenerative capabilities. In the mid-1900s, commercial oyster fishermen began finding oyster-eating sea stars in their nets. In an attempt to kill the pesky sea stars, the fishermen would slice the animals in two and toss them back into the ocean. Much to their dismay, the fishermen were only compounding their woes, for the stars quickly grew lost limbs, and halves grew into whole new sea stars. If cut through the center, almost all sea stars species will regenerate a new full body — one sea star essentially becomes two.

Stars in the Home Aquarium
There are certain conditions that must be met if your sea star is to thrive. Chief among these concerns are water quality and habitat. Sea stars do best when kept in a reef tank. The size of this tank is of minimal concern, unless you plan on housing a large species, such as the crown-of-thorns (Acanthaster planci), or a colony of several smaller individuals. A single palm-sized sea star will thrive in a tank of 20 gallons, though a larger tank is recommended.

Outfit the aquarium as you would any reef tank: rock walls, outcroppings, coral, live rock, caves, nooks, and substrate of crushed coral or fine sand. Keep temperatures between 74 and 80 degrees Fahrenheit, and maintain a stable pH of 8.0 to 8.3. Sea stars are sensitive to spikes in ammonia and nitrates. They can tolerate up to 10 ppm of nitrates, but zero is preferable. Likewise, up to 0.5 ppm of phosphates is acceptable, but the lower the better. Use trickle filters with low to moderate current. Still waters encourage sea stars movement and feeding, while turbid waters tend to make sea stars “hunker down.” They cling to the rocks and move more slowly. Efficient protein skimming and a lot of activated carbon are essential, with 15-percent to 25-percent bi-weekly water changes.

Because their skin is so sensitive to biological stimuli, sea stars tend to fare poorly in tanks heavily populated with fish — the ammonia in fish waste is hazardous to most sea stars. Put only 1 inch of fish per 6 gallons of water in sea stars’ tanks. So, a 60-gallon aquarium could have 10 total inches of fish. Overstocking is perhaps the most common cause of sea star deaths in the aquarium. As long as the tank is healthy and well-filtered, it is possible to keep a good number of nonfish tankmates (shrimp, crabs, clams, snails, etc.). An ornate reef tank sporting a lot of live rock, polyps, stony corals and other such nonfish animals is perhaps the ideal environment for keeping sea stars.

Feed sea stars an initial diet of meaty fare: prawn, lancefish, cockle, mussels, shellfish, Mysis or cut squid. Most starfish (aside from the heavily predatory species, such as the crown-of-thorns) will take vegetable matter, such as boiled spinach, kale or collard greens. Feed every two to three days, or as often as your sea star will accept food. Place a morsel of food beside the sea star. If it is hungry, the sea star will scoot right over the top of the food and consume it. If housed in a tank with pestering fish that are likely to steal the food before the star can claim it, place the food on the tank bottom and place the sea star directly on top of it.

Because sea stars are naturally nocturnal, they like to hide during most of the daylight hours, tucked safely away in a cave or rocky crevasse. Daytime observing can be achieved by offering your sea stars food only during established daytime hours. Despite their lack of a brain, sea stars will quickly “learn” when feeding time is and will venture out of hiding to snag a morsel of fresh meat.

Sea stars do not suffer from many diseases. If water quality deteriorates, however, or if their dietary needs are not met, your sea stars may become afflicted by bacterial infections, which lead to open sores and lesions on the skin. Infected sea stars quickly become sessile and will neither hunt, nor flee from curious fish, such as damsels, which may bite and pick at the star’s wounds. Quarantine afflicted individuals in a hospital tank until they recover, and correct the offending water conditions. If housed in a balanced tank by a competent hobbyist, most sea stars will not suffer a minute of discomfort due to ailment or disease. If a disease or malady does break out in a sea star’s tank, avoid using copper-based medications, as these chemical agents are typically fatal to most sea star species. Next Page>>

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