There are many fascinating species of crustaceans, a number of which are available to the home aquarist. Yet none are as mystical and amazing as the snapping (pistol) shrimp (family Alpheidae). While relatively few snapping shrimp are available to aquarists, those that are available can make interesting aquarium inhabitants. Let’s examine the biology of these incredible shrimp, as well as their captive-care requirements.
Alpheidae consists of at least 46 genera (the taxa in the family are currently being revised) with more than 600 species. Most do not exceed 1 inch in length. The largest of the group reaches about 3 inches, while a number of dwarf species do not exceed half an inch in length.
This group has some amazing qualities that make them worthy candidates for home aquaria. One of their most fascinating characteristics would be the unusual, oversized chelae (the pincerlike claw of crustaceans) possessed by some family members. This structure has movable fingers known as dactylus, which can be closed very rapidly – so rapidly in fact, that it is considered to be one of the fastest known movements in the animal world (the whole process occurs in less than 300 microseconds).
When this occurs, a loud pop is produced. It was once thought that the noise was produced simply by the hammering of the moveable dactylus with the adjacent fixed finger, but more recent studies have demonstrated that something more sophisticated is at work here. It turns out that during the process of closing, a cavitation bubble is created by a jet of water that is expelled from a socket in the claw. The bubble subsequently collapses, causing a loud popping noise. The act of snapping the dactylus together not only produces this audible implosion, it also creates a flash of light (not visible to the naked eye) along with a blast of heat. Researchers suggest the temperature inside these bubbles may reach 8,540 degrees Fahrenheit for a fraction of a second (Roach, John. October 3, 2001. “Snapping Shrimp Stun Prey with Flashy Bang.” National Geographic News. /redirect.aspx?location=news.nationalgeographic.com.).
So what is the function of the snapping behavior? It is thought to serve a number of different functions, in part depending on the snapping shrimp species in question. At least some of the predatory alpheids use the snapping ability to stun their prey. They have been known to incapacitate small fish (e.g., gobies) and eat them. They will also snap when driving off intruders or when jostling with conspecifics over a mate. It has been suggested that some species may even use the driving force produced during rapid claw closing to make holes in coral rock to burrow (Werding, 1990). So don’t be surprised if you hear snapping sounds coming from an aquarium that contains one or more alpheids.
Most alpheids come from seawater environments, however there are a handful that occur in freshwater and even more that live in brackish conditions. The family is most well-represented on coral reefs. In fact, they are the dominant decapod shrimp in both number of species and number of individuals in coral reef habitats. But divers may never know this because so many alpheid species live cryptic lifestyles.
Most dwell within the reef itself, inhabiting deep cracks and crevices, but there are also many species found on adjacent rubble and sand habitats. There are also snapping shrimp that live in tide pools or small tidal puddles on intertidal sand and mud flats, as well as estuaries and mangrove swamps. Snapping shrimp can be found in water inches deep to depths in excess of 3,300 feet.
Living with a Partner
Another fascinating characteristic of the alpheids is that many live symbiotically with other organisms. There are species that live with sponges, sea anemones, corals, mollusks, other crustaceans, echinoderms and fish. For example, the snapping shrimp (Alpheus lottini) lives among the branches of Pocillopora corals and will protect its host from crown-of-thorns sea stars. They do this by snipping off the tube feet of the sea star as it moves onto the coral. In return, the shrimp finds a home among the coral’s branches, and it also feeds on the coral’s mucus. There are snapping shrimp (e.g., Athanas amazon and A. squillophilus) that share burrows with mantis shrimp and even one (Alpheus heterochaelis) known to live with a burrow-building crab (black-clawed mud crab, Panopeus herbstii).
How the snapping shrimp benefits its crustacean partner is not known, but the alpheid no doubt derives protection by living in the burrow and having a tough buddy (in the case of the mantis shrimp, which is also an alpheid) cohabitating with it. The red snapping shrimp (Alpheus armatus) lives among the tentacles of the corkscrew sea anemone (Bartholomea annulata). These snapping shrimp (usually an adult pair) live among the tentacles of this sea anemone; the tentacles provide a safe sanctuary for the crustaceans. The shrimp reciprocate by protecting their host from large fireworms (Hermodice carunculata) that eat these cnidarians. (Interested aquarists should note that A. armatus and its anemone host are often available from Atlantic ornamental fish suppliers.)
While all of these other associations are interesting, the most amazing snapping shrimp partnership occurs between at least 30 species in the genus Alpheus and about 130 different species of gobies (commonly referred to as shrimpgobies or partner gobies). In this relationship, both partners benefit. The shrimp build extensive burrows in the sand, which the gobies live in along with the crustacean. The snapping shrimp has to continually clean the burrow of debris and sand. It does this by pushing the substrate out of the entrance of the burrow in bulldozerlike fashion. It will also carry larger bits of debris from the burrow with its claws. When it engages in these activities, it is a potential target for prowling predators. It is thought to have relatively poor eyesight, which makes it even more vulnerable to its enemies. This is where the goby comes in. It acts as a sentinel and remains at the burrow entrance as the shrimp does its thing. When the alpheid leaves the burrow, it always has an antennae resting on the goby’s tail. If the fish senses danger, it will wag its tail to warn the shrimp, which immediately retreats back into its refuge.
These shrimp-goby pairs are most abundant in the Indo-West Pacific, but there are a few examples in the Atlantic Ocean, as well. Some of the Alpheus species are particular about which gobies they hang out with. Others are not that choosy. Fortunately, the most common species we encounter in the aquarium trade is the tiger snapping shrimp (Alpheus bellulus), which tends to be a generalist, hanging out with a variety of goby species. The shrimp apparently find their goby partners by smell, while the gobies rely more on visual cues to locate their crustacean buddies.
Little is known about the reproductive biology of many of the alpheids. At least some species are reported to be protandric hermaphrodites (females result from male sex change). However, this is such a large group of crustaceans that it is likely that not all species exhibit the same sexuality patterns. Separating the sexes can be difficult. In the species that regularly enter the aquarium trade (in particular, the goby-associated Alpheus), the females have a broader abdomen and broader pleopods (the swimming legs located under the abdomen). The males of at least some species (e.g., Alpheus soror) also have larger claws than females.
Adults of the goby-associated snapping shrimp are usually found in pairs and probably breed throughout the year in tropical locations. The larger the female, the more eggs she will produce. In one species of snapping shrimp, A. clypeatus, the female usually carries from six to 350 eggs. During the developmental stage, the female (which carries the eggs on her pleopods), remains in the burrow until the eggs hatch (this usually occurs in about 10 days). The female shrimp eject the larvae (zoea) out of the burrow by facing away from the entrance and rapidly beating their pleopods. The liberated zoea then enter the zooplankton where they develop.
At least some (and possibly most) of the snapping shrimp in the genus Synalpheus exhibit an unusual mating system similar to that seen in social insects, such as termites and ants (i.e., a eusocial social structure). These shrimp live in colonies within the internal channels in large sponges. In the species studied to date, colonies consist of up to 350 individuals, which are comprised of varying age classes, from recent hatchlings to mature adults. (I should point out that as many as 16,000 individual alpheids have been taken from a single Spheciospongia vesparium sponge in Florida, but this may have consisted of more than one colony.) What makes these colonies unusual is that most contain only one large breeding female (like the “queen” in a bee colony). The “queen” shrimp differs morphologically from the smaller females and males – she is bigger, has a proportionally larger abdomen, and the claws have transformed from the massive chelae that are characteristic of these snapping shrimp to small claws. Like eusocial insects, most members of the colony are closely related, and some individuals will aggressively defend the “queen.”
In the Aquarium
There are a number of snapping shrimp available to home aquarists. The most popular are those that associate with shrimpgobies. These are typically the most conspicuous species if they are provided with an appropriate fish partner. While they spend much of their time in their subterranean abodes, you will regularly see them maintaining their burrows during the day. The other snapping shrimp available in the trade tend to be quite secretive, inhabiting fissures and tunnels in the live rock or aquarium decor. In fact, you may accidentally introduce “stowaway” alpheids in live rock you add to your tank. You are more likely to hear these species snapping than see them.
Because the goby-associated species are the most-often-available and most-purchased snapping shrimp, I would like to concentrate on their husbandry in this section. When first added to the tank, the burrowing varieties will wander about and look for a place to start digging. Once they find a suitable location, they begin excavating. In the aquarium, this digging will often occur in the corners of the tank if rocks or suitable structures are not present on the sandbed.
It is a good idea to place flat pieces of live rock on the sand surface to act as a roof for a snapping shrimp burrowing chamber. If you want to give your burrowing snapping shrimp a hand, you can dig a depression in the substrate bed and then rest the flat piece of rock over it. You can then direct the shrimp into the opening of its new home with a net or a cup. The consistency of the substrate will greatly impact your alpheid’s burrow design and stability. The more heterogeneous the substrate grain size, the more complex and stable they will be. It is best to create a substrate bed that varies in grain size; you can use coral sand, pieces of shell and rubble, which will help stabilize the burrow. Once a snapping shrimp completes its new burrow, it will spend more of its day foraging, grooming and resting.
In the aquarium, many of the burrowing Alpheus will build tunnels along the glass (or acrylic) bottom of the aquarium. Because of this, you will be able to view what is going on in the burrow if your aquarium stand does not have a solid top on which the tank rests. You can stick your head under the stand and use a flashlight to watch your shrimp and its fish buddy do their thing. One thing you should remember is that burrowing snapping shrimp can cause rockwork to shift, which can sometimes result in dangerous cave-ins.
If you keep two snapping shrimp of the same sex together, they are likely to fight. Encounters begin with exploratory antennal contact (which may allow the shrimp to size up and sex their potential opponent). Then the two shrimp will push at each other with their claws. If things escalate (if one shrimp does not take off), the dominant shrimp will exhibit a threat display. It spreads out its claws, each dactylus and its abdomen, as well as lifts its abdomen high off the substrate – this is a warning that a potentially life-threatening battle is likely to ensue. If the subordinate does not relent, the aggressor will rip off appendages and fracture the carapace by “snapping” with its chelae.
Battles may result between different snapping shrimp species, as well as between conspecifics, especially if space is limited. I have seen one snapping shrimp continue to harass another until the subordinate specimen was dashing up the side of the tank trying to escape the aggressor. I have also seen individuals killed by larger heterospecifics. If fighting occurs, separate the adversaries. Beware that snapping shrimp are likely to fall prey to morays, snake eels, frogfishes, scorpionfishes, groupers, snappers, hawkfishes and triggerfishes.
Molting occurs approximately every month (it can be every two to six weeks, depending on how much the shrimp gets to eat and how fast it grows). When crustaceans molt, they are especially vulnerable to predators. When snapping shrimp molt, they remain in their subterranean refuges until the exoskeleton hardens (this usually occurs in a day or two).
The goby-associated snapping shrimp are easy to feed. They will come out of their burrows and snag pieces of meaty seafood or frozen preparations. Most will also eat some algae. They will graze on micro- and macroalgae on aquarium decor near the entrance to their burrows. If you don’t have a lot of fish and are not feeding your tank very often, you will need to target-feed your burrowing snapping shrimp. Simply direct food into the burrow entrance. Those snapping shrimp that are more cryptic will typically find enough to eat among the rocks and in reef crevices if you are regularly feeding fish.
There is one ornamental snapping shrimp that does not burrow (and thus does not associate with gobies) and that is not super secretive: the bulls-eye snapping shrimp (A. soror). This beautiful shrimp is pale orange overall, with white speckles, distinct ocelli on its abdomen, and purple legs and claws. This species often makes its home in cervices at the reef-sand interface. While it does not burrow, it will displace sand to make its home more spacious. It is a larger species, reaching about 3 inches in length.
A quick word about alpheid taxonomy. If you are like me, you want to know exactly what species of organisms inhabit your aquarium. If you have ever tried to figure out who is who when it comes to goby-associated snapping shrimp, you have probably ended up more confused after checking out field guides or identification tips on the web. There is a reason for this: the goby-associated shrimp are in need of some serious revision. For example, the species you often see in the lay guides labeled as A. djeddensis probably include four or five different species, while A. bellulus refers to three different species. So, when putting a name to our alpheids, we need to tread with much caution – that is, until scientists clean up the taxonomic mess in which the goby-associated shrimp now find themselves. AFI
A scientific explanation of how the snapping shrimp snaps its claws