I have alluded to this group several times previously. I define utilitarian invertebrate as any that the aquarist adds, deliberately or by accident, that perform some useful ecological task. Such critters may or may not also be colorful and entertaining to watch. Algae-grazers, including snails, hermit crabs, and small sea urchins, fit the category, as do brittle stars, burrowing sea cucumbers, echiurids, and peanut worms. We can also include sponges, bryozoans, hydrozoans, and many kinds of annelids in that they constantly filter the water, removing food particles. The health and diversity of this portion of the minireef ecosystem reflects the overall robustness of the entire aquarium community.
In the design guidelines presented later, I will not repeatedly mention the addition of utilitarian invertebrates, since every tank needs them. I will point out situations in which certain invertebrates are likely to be eaten by fish. Here is a rundown of commonly available species.
Star shells (Astraea sp. and Lithopoma sp.) and nerite snails (Nerita sp.) are frequently collected from shallow water in Florida and the Caribbean. Turbo snails (Turbo sp.) and top snails (Trochus sp., Tectus sp.) often come from the Pacific. Various species in all of these genera occur throughout the tropics. All feed on attached small algae that they can scrape off. They cannot eat larger types of algae and thus pose no threat to seaweeds cultivated in the tank. Literally millions of these snails are harvested for the aquarium trade each year, and, as Dr. Ron Shimek has pointed out, many die from lack of understanding on the part of the aquarist. Astraea, for example, lacks the ability to turn over if it is dislodged, a situation it almost never encounters in its natural habitat. It will simply lie there and starve if you do nothing. All these snails need full-strength seawater and react poorly to changes in salinity. Do not overstock, as many snails will starve in this situation. The usual advice to add one per gallon results in far too many than the tank can support. Finally, do not combine snails with hermit crabs, as the hermits will kill them.
Abalones (Haliotis sp.) sometimes are produced by captive cultivation. Good algae consumers, they should be maintained with the same caveats just mentioned for other snails. Another commonly cultivated grazing snail, the queen conch (Strombus gigas) reaches a large size and grows rapidly. It feeds only when grazing on sand but does eat algae greedily. Undeniably attractive and interesting, queen conchs do not fit into every aquarium design. Bubble shells include several genera, notably Bulla and Haminoea, that hide from the light and graze algae at night. Haminoea will reproduce in the aquarium.
Small hermit crabs do double duty as both algae grazer and scavengers. They are not above killing a snail for its shell, which the hermit crab uses to protect its own abdomen. Therefore, use either one or the other for algae control. Commonly imported from the tropical Atlantic region, the blue leg hermit (Clibanarius tricolor) searches constantly for food. Keep only one per ten or fifteen gallons, as it will also feed on small polyps and other encrusting invertebrates. Found not on the reef but in sandy inshore areas, C. tricolor is interchangeable with Paguristes cadenati, the scarlet hermit, and P. digueti, the red leg hermit. Since all three can potentially be destructive, they are best used for algae control and scavenging in fish-only aquariums.
Also often growing too large for the minireef, sea urchins consume algae, but many also scavenge for animal food. Among the species available, most are better choices for a large, fish-only aquarium rather than a minireef. Useful species include the longspined urchins (Diadema sp.), rock urchins (Echinometra), tuxedo urchins (Mespilia sp., Microcyphus sp.), variable urchins (Lytechinus sp.), and pencil urchins (Eucidaris sp., Heterocentrotus sp.). Longspined urchins can inflict a painful jab and should be handled with care. Tuxedo urchins and variable urchins camouflage themselves by carrying small bits of material from their surroundings. All need full-strength salinity and sufficient food to survive in the aquarium.
Understanding Invertebrates 71
Understanding Invertebrates 71
Several varieties of invertebrates qualify as scavengers because they eat detritus and leftover fish foods. Among these are several large hermit crabs that must never be added to a tank with delicate reef animals. Big hermits are, however, a hardy and reasonable choice with some fish. The striped hermit, Clibanarius vittatus, from the tropical and subtropical Atlantic, regularly appears in shops.
Among the best minireef scavengers, brittle stars and serpent stars come in many varieties. Easily identified, these echinoderms have a central disc underneath which the mouth is located. Long, snakelike arms, five in number, radiate from the disk. The flexible arms capture all sorts of foods, from detritus to small fishes. Most aquarium imports are harmless. Found all over the world in the tropics, commonly seen genera include Ophiocoma, Ophioderma, and Ophiactis. Ophiactis and Ophiocoma have spiny arms, while Ophioderma's arms are smooth. The latter may be quite colorful, whereas the other two usually come in drab shades of brown or tan. All hide during the day and feed at night. Add only one or two specimens to your tank, or there may not be enough food to go around. In a starving brittle star, the tips of the arms start to fall apart. At this stage, it may not recover even if more food becomes available.
I have previously mentioned several minor worm types that perform services by feeding on detritus particles. Certain polychaete annelids also perform this task. Spaghetti worms (family Terebellidae) lie hidden inside a burrow in live rock and have a mass of rubbery tentacles reaching a surprising distance from the entrance. The sticky tentacles efficiently gather debris and convey it to the mouth. Indigestible matter is ejected from the burrow and collects in a pile below the entrance. Often this is the only evidence of the worm's presence. The little piles are easily siphoned out. Red hair worms (family Cirratulidae) resemble spaghetti worms and perform essentially the same function. They differ in that the feeding tentacles are numerous, often red in color, and that they live in sand rather than rocks. They often enter the aquarium via live sand and can become numerous in the substrate.
With the advent of sand beds in aquariums, many burrowing organisms acquired the interest of aquarists for their role in keeping the sand aerated and mixed. Among these are sea cucumbers, including Holothuria and Actinopyga. Smaller individuals make good aquarium additions. Ceriths, Cerithium sp., are snails that burrow, or more precisely, plow, the upper layers of substrate in search of algae and edible detritus.
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