When it comes to the reef aquarium community, crabs have a dubious reputation. But because there are so many species of crustaceans included under the moniker of crabs, it is hard to paint with a broad brush. In fact, there are at least 7,000 species of crabs known to science, and they exhibit a diversity of feeding modes and lifestyles. Relatively few of these fascinating animals are sold in the aquarium trade, and some of these can indeed be quite destructive in the reef aquarium. But while there is no doubt that there are crabs that you want to avoid in the typical reef aquarium, there are also species that play a valuable role in keeping corals healthy on the reef and in captivity. There are several families of brachyuran crabs (order Brachyura) with members that are obligate associates of small polyped stony (SPS) corals. These crustaceans are referred to as guard or coral crabs.
In the past, these coral-associated crabs were incidentally introduced in reef aquariums in coral colonies purchased by hobbyists. In the early days, coral keepers were encouraged to rid their corals of these hitchhikers because they were viewed as problematic, not helpful. Now many hobbyists realize that these crabs are beneficial symbionts; in fact, some retailers now sell these crustaceans separately, so if your coral colony does not have a guard crab, you can get one. But before you acquire a guard crab, you should learn about the biology and husbandry requirements of these coral-associated crabs.
Classification and Characteristics
There are several families of coral-associating crabs that can be found in the aquarium trade. The most desirable coral crabs belong to the genera Trapezia (family Trapeziidae) and Tetralia (family Tetraliidae). These crabs are usually found in association with branching small-polyped stony corals in the families Pocilloporidae and Acroporidae (see the table “Some Coral Crabs and Their Host Corals”). Most of these crabs tend to be fairly host-specific and prefer members of a single genus. There are, however, some species that will inhabit other genera if their preferred hosts are not readily available.
The trapeziid and tetraliid crabs are fairly small (usually with a carapace width of less than five-eighths of an inch for the former and a bit smaller for the latter), and some are extremely colorful. For example, the rust-spotted coral crab (Trapezia rufopunctata) and the red-spotted coral crab (T. tigrina) are both pink with red spots (T. rufopunctata has many more spots than T. tigrina). These crabs will be the primary focus in this article.
There are also crabs in the family Xanthidae that inhabit SPS coral colonies, including crabs in the Cymo, Paractaea and Pseudoliomera genera. These crabs are more robust than the trapeziid and tetrallid crabs, and they are potentially more destructive to their coral host and other members of the reef aquarium community. For example, Cymo melanodactylus has been implicated in the destruction of wild colonies of Acropora in areas where population densities of these crabs are high.
While a single Cymo may not destroy a larger SPS coral colony in the aquarium, the crab may eventually cause problems in your reef tank. For this reason, many hobbyists choose to avoid them. If you want to try to keep one with your SPS corals, monitor it closely. If the polyps begin disappearing (if white patches of coral skeleton appear in a coral colony), remove that crab immediately.
Some Coral Crabs and Their Host Corals
The relationships between the trapeziid and corals, and between tetraliid crabs and corals are mutualistic in nature. One benefit the crabs provide for their coral host is defense. These little armored knights aggressively protect their coral colony from the scleractinian-feeding crown-of-thorns sea star (Acanthaster planci). The crustaceans will nip the tube feet of these echinoderms when they get too close to the coral, and the larger coral crabs have even been observed to pinch off the sea star’s integumentary spines. Studies have shown that coral colonies that contain crabs are less likely to be eaten by these coral-feeding sea stars. In fact, in one study, Pocillopora species that did not contain crab symbionts were found to be three times more likely to fall prey to the crown-of-thorns. Moreover, corals that were home to larger guard crabs (e.g., Trapezia cymodoce) were less attractive as prey to A. planci than coral species that harbor more diminutive crab species (e.g.,Tetralia spp.).
The crabs use their sense of smell to detect when their echinoderm enemy is in the neighborhood (this enables them to defend their coral host during the day or night). These crabs may also attack coral-feeding butterflyfishes that stop to pluck polyps off of their coral colony, and they may also protect coral colonies from vermetid snails, which also eat SPS corals.
Protection from predators is not the only benefit that these crabs provide. The activity of the crab between the coral branches has other positive benefits to its calcareous host. When it moves among the branches and feeds, the crab helps clean its host of sediment and also promotes irrigation of the tight spaces in the coral colony with “fresh” (oxygenated) seawater. This will increase the coral colony’s rate of growth and increase reproductive output. Studies have shown that when crabs are removed from their host Pocillopora species, the coral produces less mucus, the polyps are more likely to disintegrate, and there is often massive tissue exfoliation. Another study demonstrated that bleaching was a more common phenomenon in coral colonies that lacked crabs than those that hosted symbiotic crustaceans.
The keeper of Pocillopora and (to a lesser degree) Acropora corals should take careful note of these findings. While these coral colonies may survive in an aquarium without their crab partners, they may grow more vigorously and exhibit more natural growth forms when these crabs are present.
Acclimation and Medication
Like other crustaceans, these animals are sensitive to certain aquarium medications (e.g., copper-based medications) and should be housed at a specific gravity of 1.023 to 1.025. When acclimating them to their new home, drip water into the transport bag from the reef tank for approximately 45 minutes to an hour. If exposed to sudden changes in specific gravity and temperature, they are likely to shed limbs and may perish.
Guard Crab Feeding
The coral provides the crab with shelter and a source of nutrients. In 1967 scientists figured out through stomach-content analysis and aquarium observation that crabs in the genera Trapezia and Tetralia feed on the mucus of their coral hosts. The presence of guard crabs in pocillioporid corals has also been shown to stimulate the coral to develop special lipid-filled fat bodies that the crabs will feed on. These excess lipids are apparently not needed by the corals but help the crabs survive.
These crabs are adapted to feeding on coral slime. The end of their appendages have comblike bristles, which they use to brush and irritate the coral polyps to increase mucus production. These bristles are also used to collect the slime and associated bacteria. When the mucus is transported to the crab’s legs near the head, the crab cleans them of the mucus with its mouthparts (it’s like the crab is licking off its fingers!). While these crabs may feed mainly on mucus, recent studies have shown that they also ingest coral polyps. Rinkevich et al. (1991) found that a pair of Trapezia cymodoce could consume approximately 20 square inches of coral tissue in a month. That is a lot of coral tissue! In the wild, where conditions are optimal for growth, the crabs’ grazing does not harm the host coral. In the aquarium, however, if the host colony is too small to harbor a crab (or pair of crabs) or if the coral’s growth is stunted because it is being housed in a suboptimal environment, crabs may harm the host cnidarian. Therefore, if you only have a few SPS coral frags in your tank, don’t add a guard crab. Add them to aquariums with large, healthy colonies, and don’t add too many; shoot for a single crab per larger (6 to 8 inches in diameter) coral head.
Some aquarists suggest that these crabs will accept normal aquarium fare (e.g., mysid shrimp, frozen Cyclops) and that by feeding them introduced foods, they will be less likely to eat coral tissue. However, lab studies indicate that these crabs only feed on coral mucus and often the tissue of their host. This means that it is imperative to the survival of these crabs that are they are provided with appropriate stony coral colonies (not just for shelter but also food).
The post-larval guard crabs settle out of the plankton, and they usually inhabit smaller coral colonies rather than the more substantial coral heads that adult crabs occupy. They are thought to locate colonies by smell. It is likely that different corals have a different scent, which would enable a young crab to find its preferred scleractinian host. Adults are nocturnal, remaining in their coral sanctuary during the day. Hobbyists should be aware — these are not showy animals. They do not parade about the tank (at least during the day) and are rarely seen. If they choose to migrate to another coral head, they will do so after dark when there are fewer fish predators around that would eat them.
Adult guard and coral crabs typically occur in heterosexual pairs. A pair of these crabs may defend an area of a larger colony or an entire smaller colony from members of its own kind. Aquarists should make sure that they do not add too many guard crabs if the aquarium has a limited number of appropriate coral colonies. If there is not enough territory for all of the crabs, some may be excluded from a place to hide and the obligatory food source. Not only may these crabs exhibit intraspecific aggression (within their own species), they may also engage in heterospecific aggression (between different species). Once again, I suggest no more than one crab per medium to large colony of SPS coral in the tank.
When Crabs go Bad
If you add the wrong crab to your tank or your coral crab “goes bad,” you will probably want to remove it from your reef tank. Of course, getting it out of the tank may be easier said than done. You can set up traps for crabs that don’t specifically feed only on coral slime or tissue. A glass tumbler with a piece of food at the bottom is often enough to capture omnivorous crustaceans — but the coral crabs don’t usually fall for this trap. You are more likely going to have to take out the entire coral colony that contains the bad crab from the tank. (This will only work if the colony is not attached to the reef structure.) Place a net under the colony as you begin to lift it from the tank. If you are not careful, the crab may jump ship before the host coral is removed. Take the colony with crab and hold it over a bucket, and then chase the crab out of its shelter with a knitting needle or similar elongated object.
Because of their more cryptic lifestyle, the guard crabs are not the most showy of marine crustaceans, but they can facilitate the growth and overall health of certain SPS corals. However, if you do not provide the appropriate conditions and venue for your coral crabs, these crustaceans and their coral hosts may suffer.
Happy crabkeeping!– AFI