One of the most fascinating features of tropical reef ecosystems is the many symbiotic relationships that occur between organisms. From the corals and the unicellular algae that live in their tissues, and act as a solar-powered energy source to the hitchhiking remoras that clean sharks of parasites, the coral reef is a venue where such partnerships are ubiquitous. As the common name implies, the anemone shrimp utilize sea anemones (class Anthozoa) as a place to refuge. Somehow, they are able to live among the tentacles armed with nematocysts (stinging cells) of cnidarians without getting stung or eaten. But does the anemone benefit from the presence of these little crustaceans?
Fortunately for aquarists, a handful of species from this crustacean guild make their way into the ornamental fish trade. It therefore makes good sense for us to investigate how to best care for these amazing little creatures in our home aquariums.
Tropical anemone shrimp include members of the following popular aquarium genera: Alpheus (one known species), Lysmata (at least one species), Periclimenes (more than 10 species) and Thor (at least two species). Most anemone shrimp are totally dependent on sea anemones and are rarely found without them in the wild; that is not to say they don’t roam around on the substrate near their host, but they are always nearby. Their small size and lack of defense structures (e.g., spines) mean they are vulnerable outside of their anemone host. Some Periclimenes species are transparent with sparse markings, which helps them to disappear against their host. At least some members of this genus go even further to disappear next to the tentacles of their anemone — while resting in their host, these species (e.g., symbiotic cleaner shrimp, Periclimenes anthophilus) align their bodies parallel to the tentacles of their host anemone and gently sway from side to side.
Sea anemone shrimp are most common in tropical seas and in some warmwater regions; most sea anemones you run across are home to one or more shrimp. For example, the anemone shrimp (P. rathbunae) occurred in 83 percent of the sun anemones (Stoichactis helianthus) inspected around the island of Tobago (in the Caribbean Sea north of Venezuela).
Some shrimp have specific preferences when it comes to an anthozoan host, while others are a bit more “loosey goosey” when it comes to host specificity. Examples of some Atlantic species with “partner preferences” include: the anemone shrimp (P. rathbunae) and sun anemone (Stoichactis helianthus), the spotted cleaner shrimp (Periclimenes yucatanicus) and the giant anemone (Condylactis gigantean), and Pederson’s cleaner shrimp (P. pedersoni) and the corkscrew anemone (Bartholomea annulata). While they may have favorites, some of these species can be found in more than one host. For example, the spotted cleaner shrimp has been reported from more than eight different hosts. Other Atlantic sea anemones known to harbor shrimp include members of the genera Actinoporus, Lebrunia and Telmatactis.
In the Indo-Pacific, the most popular anemone shrimp hosts are members of the genera Actinodendron, Edwardsia, Entacmaea, Cryptodendrum, Heteractis, Macrodactyla, Phyllodiscus, Phymanthus, Stichodactyla and Thalassianthus. Some anemone shrimp will also associate with corallimorpharians or large-polyped stony corals (e.g., Euphyllia, Plerogyra), and the spotted cleaner shrimp has even been seen inhabiting upside-down jellies (Cassiopea spp.). One of the least selective shrimp is the sexy shrimp (Thor amboinensis) — this species can regularly be found with a number of different hosts, including tube anemones (cerianthid) and feather stars.
A host anemone is found soon after the young shrimp settles out of the plankton. The shrimp locate their host by sight and smell, and may use chemical cues to distinguish between different anemone species. The sea anemone body plan consists of a base, a column, a disc and tentacles. Anemone shrimp can be found on any of these anemone parts. That said, certain anemone shrimp seem to have a proclivity to hang out near the anemone’s disc edge. There may be more food that collects in this region, and the anemone’s tentacles may be easier to eat along the edges (more on this later), and cleaning species may have better access to fish clients from this position. At night, when many anemones close up, their shrimp associates may retreat into holes near the anemone’s base.
Food and Feeding Behavior
Anemone shrimp feed on tiny organisms and detritus that stick to the host’s mucus; they may also eat the slime itself. There are also many anemone shrimp that clean parasites from passing fish. Fish will visit the anemone and pose for servicing. The shrimp will jump onto the stationary fish and inspect them for (and remove) certain parasitic pests, such as juvenile isopods. Anemone shrimp available to aquarists that are known to clean include the saddled shrimp (Periclimenes holthuisi), magnificent shrimp (P. magnificus) and Pederson’s cleaner shrimp (P. pedersoni). While the sexy shrimp (Thor amboinensis) does not clean fish, it has been observed grooming large mantis shrimp.
Some anemone shrimp also eat their host. This is the case with the clown anemone shrimp (P. brevicarpalis), which has been observed to deleteriously impact host sea anemones in captivity due to tentacle feeding — but it apparently does little if any harm to its host in the wild. This is apparently because the anemone is typically in a more optimal environment and in better health in nature, or the shrimp may eat fewer tentacles in the wild where other foods are more available. Other Periclimenes species (e.g., the ornate anemone shrimp, P. ornatus) are also known to feed on the tentacles of their hosts. The sexy shrimp, too, has been indicted as a tentacle-muncher, especially when underfed in captivity. According to some aquarists, it may also eat zoanthids.
Acclimating to a Sea Anemone
One might think that the exoskeleton of a shrimp would protect it from the stinging cells of its host, but this is not the case. While the external armor of the shrimp provides some protection, it does not totally thwart anemone stings. In fact, shrimp that don’t typically associate with anemones are stung and even eaten by more potent anthozoans (e.g., carpet anemones, Stichodactyla spp.).
It turns out that anemone shrimp have to acclimate to their hosts. When a “nonacclimated” shrimp contacts an anemone, the cnidarian’s tentacles (as a result of the stinging nematocysts) will adhere to the appendages, tail and abdomen of the shrimp, causing it to rapidly jump backward and attempt to tear itself away from the anemone. An acclimating shrimp will endure the occasional sting and will gingerly make contact with the anemone’s tentacles and pick at the integument of its potential host. Once it has fully acclimated, which normally takes one to five hours, the shrimp will move about the anemone unimpeded, and the host will not respond at all to the presence of the shrimp.
What happens during this acclimation process that inhibits nematocyst discharge? Early studies suggest that the shrimp accumulates sea anemone mucus on its exoskeleton, and once it has “camouflaged” itself with the anemone’s slime, it can freely move about the anemone without being stung. But there are some problems with this theory. Shrimp regularly clean themselves of foreign material. When they engage in such grooming, wouldn’t they remove an anemone’s mucus? If it was all about getting “slimed,” wouldn’t they have to acclimate to their host after molting? They retain their ability to enter an anemone without being stung even after shedding their exoskeleton.
The other possibility is that the shrimp build up chemicals in their bodies that inhibit the discharge of the stinging cells of sea anemones during acclimation. This chemical is retained (possibly even secreted from a gland) and is not lost as a result of molting. What makes all this even more interesting is that anemone shrimp lose their nematocyst protection and have to reacclimate to a specific host if they are isolated from that cnidarian for as little as 24 hours.
As aquarists, we need to be aware of this acclimation process. Never force an anemone shrimp to enter a sea anemone. If you do, the shrimp may be stung and may, on rare occasions, be eaten. After acclimating the shrimp to its new home (preferably by dripping water into the bag it was brought home in for an hour or two), release it near its potential host, and let the process occur on its own time. Of course, this can be problematic if you are counting on the anemone to provide refuge from potentially dangerous fish tankmates. If you want to keep an anemone shrimp and host with possible predators, add the shrimp to the tank first.
Anemone shrimp may vary in their social behavior. For example, the snapping shrimp (Alpheus armatus) is highly territorial. Most other members of this guild are not territorial and exhibit a variety of different social patterns. For example, P. rathbunae is usually found singly or in pairs. Other species of Periclimenes are regularly found in groups (e.g., dozens of P. holthuisi can be observed on larger anemones), while assemblages of sexy shrimp (T. amboinensis), which can number more than 20 individuals, may also share a host. These group-forming species tend to be peaceful toward one another and usually exhibit poorly defined social behaviors (e.g., submissive behavior, aggressive behavior and individual recognition). These groups can consist of both males and females or occasionally consist of a single sex. The size (age) range of group members can also vary considerably. For those anemone shrimp that clean, the size of the group is often a function of how popular the microhabitat is for potential fish clients. If the anemone is in an area frequented by fish to clean, more shrimp will occupy the host.
At least one species, Pederson’s cleaner shrimp (P. pedersoni), has been observed to chase conspecifics away from a specific area of their host (usually the center of the oral disc). It turns out that this area tends to attract the most potential fish clients for cleaning. In this species, pecking orders have been observed, where larger individuals have more ready access to the best sites on the host and first access to fish that come to be cleaned.
Shrimp Life Spans
The natural life span of these shrimp varies. The longevity of the sexy shrimp is usually less than one year (and maybe as short as four or five months) in the wild. At least some of the Periclimenes species live longer — up to a year or two.
Keeping Anemone Shrimp
One anemone shrimp or a group of them can make a fascinating display in the home aquarium. Most of the species are not going to do well in your tank unless you keep them with an appropriate host. If you do not keep one of these crustaceans with an anemone, it will hide incessantly and is more likely to be picked off by a fish tankmate.
A nano reef is often the best home for an anemone and anemone shrimp. Of course, as with a larger tank, you will need to make sure you have enough light and water movement to keep an anemone healthy. You will also want to select the anemone you keep in such a tank very carefully, steering toward smaller, more hardy species. Some anemones that would work best for the nano tank include Condylactis gigantean, the colonial form of Entacmaea quadricolor, Lebrunia danae and some of the Phymanthus species.
Many anemone shrimp can be kept in groups. In fact, many species, such as the saddled cleaner and sexy shrimp, will do best if kept in groups of three or more. Some anemone shrimp that naturally occur singly or in pairs include the clown anemone shrimp (P. brevicarpalis), spotted cleaner shrimp (P. yucatanicus) and red snapping shrimp (Alpheus armatus). More than one of these shrimp can be kept in the same tank, but attempt to get a heterosexual pair or provide more than one appropriate host. In some species (e.g., sexy shrimp, clown anemone shrimp), females are significantly larger than males, while in others (e.g., Pederson’s cleaner shrimp) the males are larger. Hermaphroditism has been discovered in some anemone shrimp; for example, the sexy shrimp is a protandric hermaphrodite (females result from male sex change).
As mentioned, some anemone shrimp will eat their host (e.g., clown anemone shrimp, P. brevicarpalis). This is especially true if the shrimp is not getting supplemental feeding from the aquarist. While these shrimp are likely to collect food that you introduce for your fish (if you feed your fish at least once a day), you may want to target-feed your anemone shrimp by directing a stream of food at your shrimp’s host by using a pipette or turkey baster. Any meaty food should do, for example, bits of frozen mysid or finely minced seafood (take frozen shrimp or fish and use a cheese grater to produce appropriately sized morsels).
Unfortunately, because of their diminutive proportions, anemone shrimp are vulnerable to a wide array of fish tankmates. Even fish species that rarely bother ornamental crustaceans (fairy wrasses, Cirrhilabrus spp.) will pick off an anemone shrimp that strays too far from its host (see sidebar “Shrimp-Eaters”).
Here are some fish to avoid housing together with ornamental shrimp:
- All groupers (including smaller sea basses)
- Most wrasses (even those that don’t reach larger sizes or that eat zooplankton)
- Sand perches
A symbiotic relationship can be defined based on the advantages and disadvantages provided to the symbionts (individuals in the relationship). If the relationship favors both species, the relationship is known as “mutualistic.” If it benefits only one species but does not harm the other, it is known as “commensal.” If it benefits one species at the expense of the other associate, it is referred to as “parasitism.”
In the case of the shrimp-anemone association, it appears to be mutualistic (at least in those symbionts studied thus far) in most cases. It benefits the shrimp, in that these crustaceans find protection among the tentacles of the sea anemones. Many predators will avoid a shrimp that is tucked in among the stinging tentacles of an anemone. Another advantage for cleaner anemone shrimp is that the anemone serves as a recognizable focal point on the reef that fish learn to associate with cleaning behavior. The shrimp also utilize the sea anemone as a grazing surface — they ingest the slime and nanoplankton that stick to the anemone’s mucus, and they may also eat anemone waste material. In some cases, the shrimp may also nibble on the anemone’s tentacles.
But does the sea anemone enjoy any advantages? One benefit has to do with another symbiotic organism: the zooxanthellae found in the sea anemone’s tissues. When the zooxanthellae (a dinoflagellate algae) photosynthesize, they produce nutrients that are utilized by their sea anemone host. The zooxanthellae rely on light and ammonium present in the seawater to thrive. Shrimp (and other sea anemone associates, such as anemonefishes) produce ammonium that is in turn utilized by the zooxanthellae. As a result, sea anemones that host shrimp have more zooxanthellae in their tentacles than sea anemones that lack these symbionts.
At least one species of anemone-associated shrimp is also known to defend its host from potential predators. The snapping shrimp (Alpheus armatus) has been observed rebuffing fireworms (Hermodice carunculata) that approach their corkscrew anemone (Bartholomea annulata) homes. The worms regularly eat B. annulata that lack snapping shrimp partners. It is unlikely that other anemone shrimp, such as the Periclimenes species, ward off anemone predators like these snapping shrimp, which have modified claws that, when rapidly shut, produce a shock wave capable of stunning a small fish or scaring off a larger enemy.
The symbiotic relationships that occur on the reef, like that between sea anemones and certain shrimp, are truly mind-boggling. It’s amazing to think that with some care in aquarium setup and community planning that it is possible to observe this fascinating symbiosis in your own living room ocean. Happy shrimpkeeping!