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Invertebrate Helpers For Your Reef Tank

These invertebrates help to keep your tank clean.

By Scott W. Michael |

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Keeping a reef aquarium can be a satisfying hobby, but it can also cause great frustration if things don’t go right. One way to increase your success is to be consistent about regular maintenance. Partial water changes, removing detritus, changing and/or cleaning filter media and removing algae from the tank are all important to maintaining a healthy a reef tank.

Toward this end, there are animals we can add to our tanks to help make our job a bit easier. Of the hundreds of marine invertebrates available to marine aquarists, a number can be used to help stir the substrate, ingest uneaten food particles that settle to the bottom or collect in reef crevices, and keep algae in check. These animals are the subject of this article.

Mollusks

Among the most well-known utility invertebrates are algae-eating snails. These include members of the genera Astraea, Tectus, Trochus, Turbo and Nerita. There has been much debate about which of these snails are the most effective at clearing reef tanks of pestilent algae. Trochus are favorites with many because they are very hardy, do not get too large (and thus not as likely damage coral colonies like larger Turbo species) and are also tropical, so they do well at normal reef tank temperatures. (Be aware that there are some temperate water snail species — e.g., Margarites, Norrisia, Tegula — that live abbreviated lives in tropical marine aquariums.) Astraea tend to be sought-after for similar reasons as Trochus.

One thing to consider before buying a bucketful of snails and adding them to your marine tank is that these animals (and any species we keep in our aquariums) should not be misused. Although some aquarists recommend as many as one snail per gallon of aquarium volume, I suggest a more conservative one snail per 10 gallons. You can always add more if you think you don’t have enough to get the job done. Add them slowly until you reach the current carrying capacity of your aquarium. If crabs are constantly active during the day, which means they are looking for food, or if crabs begin to die, then you’ve reached your carrying capacity. Carrying capacity will vary depending on the conditions in the tank. If you suspect your snails are starving, return some to the aquarium shop or give them away. Don’t let them die.

Smaller, tropical abalone species (e.g., Haliotis spp.) are also available to aquarists. These animals graze on detritus, diatoms and low-growing algae. They are totally dependent on these food sources and will quickly starve if insufficient algae is present in the tank. These abalones can adhere to the aquarium glass or hard substrates with incredible force. In fact, you may damage them if you try to tear them from the side of the tank.

Not all hard-working mollusks only eat algae. Some species are also used in the role of bioturbation (stirring and mixing sediments). The conchs (Strombus spp.) are frequently used in this role because they spend much of their time moving about under the sand. This activity helps mix the sand bed. Two species in the genus are being commercially raised for food and the aquarium trade: the fighting conch (S. alatus) and the queen conch (S. gigas). The former is more suitable than the latter, as it only gets one-quarter of the size; S. alatus reaches about 4 inches, whereas S. gigas gets up to 16 inches.

There are other species from the Indo-Pacific that are collected and sold in the aquarium trade. The conchs have a heavy shell and a long proboscis that is equipped with a radula. This organ is like a tooth-covered tongue that is used to rasp plant material and detritus off the substrate (often sediment surfaces). Most do all of their feeding off the sand surface, though a few species will feed off the glass, as well.

The sand snails (Nassarius spp.) also serve two roles. They are not only good for helping stir the sand bed, but they are also very effective scavengers. Many stay beneath the sediment surface with only their proboscis protruding from the sand (it sticks up like a periscope and picks up olfactory cues that flow past). You may not see any signs of their presence until food is added to the tank, at which time they erupt from the sand bed and begin to hunt for lunch. They are little armored, eating machines capable of ingesting up to 60 percent of their body weight in food in a day. Most sand snail species available in the aquarium trade grow to less than half to three-quarters of an inch, but there is at least one species in fish stores, which is often sold as the "super Tongan” Nassarius snail, that gets as long as 1 inch. In nature, there are Nassarius that can live as long as five years, though longevity for those available to hobbyists is more likely 2 to 21/2 years.

Crustaceans

The hermit crabs (super family Paguroidea) are some of the most ubiquitous of the invert "cleaners.” These crustaceans are used as scavengers as well algae controllers. It’s important to understand that not all hermits are desirable members of the reef aquarium community. Larger species (ones that are carrying shells larger than a fifty cent piece) are highly predatory and will feed on anything they can get their claws on, including fish and other invertebrates. Fortunately, there are a number of smaller species that are efficient herbivores, and though they are not totally benign, the chances that they will damage fellow tankmates is minimal.

Electric blue hermit crab
Electric blue hermit crab (Calcinus elegans). Photo by Fafner/Wikipedia

When selecting hermit crabs for your reef tank, look for species in the genera Calcinus and Clibanarius. The species most often encountered include the orangeclaw hermit crab (Calcinus tibicen), dwarf zebra hermit (C. seurati), electric blue hermit (C. elegans) and the blue-leg hermit (Clibanarius tricolor). Another less common but equally desirable species is the red-legged hermit crab (C. digueti). This species not only is a great filamentous algae-eater, but it will also avidly consume blue-green algae.

Although most members of the genus Paguristes are not recommended for a reef aquarium, there is one species in the genus that is a great community crab: the red reef hermit crab (Paguristes cadenati). All of these are small hermits (less than 1 inch in length), feeding primarily on detritus and algae. Also, the food-seeking activities of these hermits can help keep the upper layer of the sand bed stirred.

Emerald crab
Emerald crab (Mithrax sculptus). Photo by John B. Virata

None of the hermits listed are much of a threat to most invertebrate and fish tankmates. They may behave badly (nibble on a zoanthid or soft coral polyps) if underfed, but more often than not, if you see them among polyps, they are harvesting the algae and/or detritus that grows between them. Provide them with gastropod shells they can move into once they outgrow the shells they came with. If you do not provide new housing, they may oust snails from their shells and take them over. They might also fight with each other for accommodation if spare shells are not available. It is usually suggested to keep them at densities at about one crab per 5 gallons. However, I would keep even less than that if you do not feed your tank very often or if algae is in short supply.

Of course, hermits are renowned for their habit of utilizing mollusk shells as mobile homes. Although many of them will exploit empty shells when it comes time to "upsize” to a larger domicile, hermit crabs are not above stealing a home from a living gastropod. This, of course, leads to the demise of the former shell occupant. As a result, it’s possible that even the less destructive hermits will kill snails. To help avoid predation on gastropods, provide a collection of shells, so these crabs have numerous housing choices as they grow. Such empty snail shells are often available from aquarium suppliers.

Percnon gibbesi Linosa
Sally lightfoot crab (Percnon gibbesi). Photo by Tato Grasso/Wikipedia

Most of the true crabs, ones belonging to the infraorder Eubrachyura, are expert scavengers, but most are also too destructive to trust in your community aquarium, such as one omnivorous species that is used to control bubble algae: the green emerald crab (Mithrax sculptus). Another species that regularly shows up in aquarium stores is the sally lightfoot crab (Percnon gibbesi), a known hair algae grazer as well as a scavenger. Unfortunately, P. gibbesi becomes more predatory as it grows larger and has been known to capture fish and eat other invertebrates (crustaceans and cnidarians).

Echinoderms

The final group of utility inverts we will examine are the echinoderms. There are three groups that are often utilized to help maintain a reef aquarium: the sea cucumbers, the serpent (brittle) stars and the sea urchins.
Sea cucumbers are most often used in aquariums with a deep sand bed. While the dedicated detritus-eating species are not very colorful, they do perform a valuable function — they oxygenate the sand bed. These animals have oral tentacles they use to transfer detritus into their mouths. This material passes through the alimentary tract, then the nutrients are extracted. The inedible material is expelled from the anus.

In order to provide enough food for detritivorous species, it’s important to not overpopulate the tank with food competitors (including too many other sea cucumbers). It’s also a good idea to add one of these species to an established aquarium that has had time (several months) to accumulate some detritus for a sea cucumber to feed on. 
The synaptid sea cucumbers (often sold as "medusa worms”) are real oddballs. They are unique in that they lack tube feet, instead having spicules that project from their skin and help to pull them over the substrate. These animals mop up detritus with their retractable feeding tentacles. They actually make unusual and fairly durable aquarium inhabitants if they reside on their own (they are easily damaged by fish tankmates, reef cave-ins, pump impellors, overflow boxes and filter intake siphons).

I believe one of the most underutilized of the utilitarian invertebrates is the serpent or brittle stars. Members of the genera Ophiocoma, Ophiomastix and Ophioplocus usually reside in reef crevices during the day and emerge to slither over the substrate at night or when food is added to the tank. The brittle stars are some of the most effective scavengers, and will not only deal with organic debris that rests in open areas, but also food particles that collect in hidden cracks and crevices. Most feed on detritus, fecal material and uneaten food that collects on the substrate. Target feeding is usually not required if they are housed with fish that are frequently fed, but if you do not feed your fish often, you might want to squirt some food into the nooks and crannies where your serpent stars hang out. While most brittle stars are passive neighbors, there are some larger species that may cause problems if not well fed. The most notorious of these is the green brittle star (Ophiarachna incrassata). It will eat a variety of corals (e.g., Anthelia, Xenia), crustaceans and even small fish.

Sea urchins can also play a major role in algae control, but they have some definite downsides. Many move about like armored vehicles, toppling coral colonies that are not fixed to the reef structure and plowing into delicate polyps with their spiny armament. Their beaklike jaws are capable of rasping coralline algae from the substrate.

The two species that are most desirable — because they are smaller and less destructive — are the blue tuxedo urchin (Mespilia globulus) and the collector sea urchin or sea egg (Tripneustes ventricosus). They will make short work out of a crop of filamentous algae and are likely to starve if algae is in short supply. In an aquarium with little plant material to keep them nourished, supplement their diet with a sheet of freeze-dried algae several times a week. One of the downsides of keeping T. ventricosus is that it will collect debris, rubble and smaller, loose polyp-encrusted rocks — and even aquarium equipment (e.g., thermometers), which it grips with its pedicellariae (stalked appendages) and carries around. It can also deliver a mild toxin with its pedicellariae, so do not handle them with bare hands.

To acclimate echinoderms, slowly drip water from the display or quarantine tank into the echinoderm transport bag or bucket for about 40 minutes (the water should drip at a rate so the original volume increases by roughly three-fold in the 40 minutes). If possible, keep your echinoderm submerged at all times.

This ends our look at invertebrates that can help you maintain your aquarium. By adding these types of animals to your tank, you can more easily maintain pristine conditions. But make sure you know the care requirements of the utilitarian inverts you acquire, and treat them with the respect that all marine organisms deserve.

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