Crustaceans are popular additions to home aquariums. Their biology, behavior and requirements, however, are very different from those of fish. Success with keeping these hard-shelled animals depends on knowledge of their special adaptations.
General Biology
The part of crustacean physiology that creates the most challenge for aquarists is the molt process. Failure to provide proper diet, water quality, temperature, compatible tankmates and even appropriate substrate can all result in death at the next molt. A lack of certain trace nutrients, the presence of molt-inhibiting water pollutants, and dings in the carapace from combat or inappropriate habitat are other potentially deadly pitfalls.
The actual shedding of the exoskeleton is the final stage of the molt and may take only three to 20 minutes, depending on the species. This is the most vulnerable time in a crustacean’s life, and the goal is to get it over quickly, usually in a safe place such as a cave or burrow. The molting crustacean is a tasty morsel for any predator (as anyone who has eaten a soft shell crab can testify). In an aquarium, large fish and similarly sized crustaceans will make short work of shedding tankmates. Choose your community wisely.
Crustacean behavior is fascinating, but gladiatorial combat is more common than peaceful co-existence. A “crustacean community tank” is a bit of an oxymoron, although a number of tiny crabs and shrimps commonly kept in reef tanks will tolerate each other fairly well if the aquarium is large and has a complicated substrate with a lot of hiding spots. Anyone with a serious interest in crustaceans will want to consider a number of smaller or partitioned displays that will satisfy special requirements and compatibility issues.
Larger crustaceans are best kept as individuals. American lobsters (Homarus americanus), for example, will not tolerate each other — period. Spiny lobsters (such as Panulirus spp.) and slipper lobsters (such as Scyllarides spp. and Parribacus antarcticus) can co-exist in groups, and in fact, have been shown to grow faster when kept this way; but unless ample hiding spaces and food are provided, they may be cannibalized by their roommates during a molt. Assuming that you can provide a 150-gallon or larger aquarium, they can also live with certain fish, such as medium-size moray eels.
When they feel threatened by actual or perceived physical harm, crustaceans will drop limbs. This is most commonly seen in Stenopus shrimp, which may arrive at the store with both claws lying on the bottom of the shipping bag. Such delicate species should always be handled gently, being transferred in smooth, water-filled containers rather than nets. Just the stress of being in a dry net may trigger limb dropping, but when delicate limbs and hairs become tangled, limb loss is guaranteed. While lost limbs can be replaced, they increase the incidence of disease, molt failure and death.
Water Quality
Crustaceans are more sensitive to poor water quality than fish. Water parameters should therefore be similar to those found in a reef aquarium. Crustaceans typically maintain internal salt levels similar to the salinity of their environment. They are intolerant of the unnaturally low salinities typical of some marine fish aquariums and also cannot handle rapid changes in salt levels. Water for crustaceans should be free of ammonia and nitrite, and nitrates should be kept below 20 mg/l.
Attention to certain element concentrations is crucial. Magnesium and calcium are stored and mobilized by those preparing to molt. Crustaceans kept in coral aquariums can succumb to low calcium levels and may even serve as “canaries in the coal mine” for calcium shortfalls. Heavily stocked coral aquariums with rapid growth can have calcium concentrations drop from 500 to 150 mg/l in less than two weeks if additions are forgotten. The sudden appearance of more than one dead or disoriented Lysmata shrimp in such an aquarium is a warning that calcium levels have plummeted below 200 mg/l.
Shortfalls of potassium are also critical but are an unlikely problem, unless you make your own seawater from scratch. I know of a case in which someone accidentally substituted soda ash (sodium carbonate) for potash (potassium chloride) while making up a large saltwater reservoir. When this water was used to perform a major water change on a marine invertebrate tank, all of the crabs suddenly became paralyzed. Luckily, the mistake was quickly discovered and reversed. This illustrates the importance of maintaining normal levels of major and trace elements.
Contaminants in tap or system water may be of special concern with crustaceans. Probably of greatest importance are compounds that could interfere with the molting process. Two potential threats are phthalates and heavy metals. The most commonly found phthalate, DEHP, is used to reduce the brittleness and increase the flexibility of PVC, vinyl tubing, styrene and rubber, all materials that can be used in aquarium plumbing and certain hardware. This compound is implicated in hormonal problems in mammals and has been shown to interfere with the molting process in crustaceans, which is hormonally controlled.
The largest source of heavy metals in aquariums probably comes directly from tap water. Some municipal water systems contain legal amounts of copper that are considered safe for drinking, but are nonetheless toxic to all forms of fish and invertebrate marine life. The use of a reverse osmosis unit to pre-treat tap water is always a wise precaution. Depending on the age of your house, you may have somewhat elevated lead levels, due to old pipes or faucets. Cadmium can leach out of certain types of metal pipes as they corrode. Like phthalates, a number of metals inhibit molting.
Fortunately, the EPA suggests a simple solution where either phthalates or heavy metals might be found in tap water: the use of activated carbon. I have always added high-quality activated carbon to any new system in which new plastics of any kind have been installed, particularly if new glue joints are part of the installation; residual glue solvents are toxic. Frequent large water changes and the use of protein skimmers are also good ways to keep the natural components of seawater in balance while removing accumulated toxins.
Diet
Crustaceans are known to have a high metabolic demand for calcium, cholesterol and certain lipids. Calcium is important in shell maintenance, and in the wild, can be obtained by eating echinoderms, mollusks, coralline algae or other crustaceans.
Cholesterol is used to manufacture molt-related hormones, and a deficiency can lead to molt failure. There are numerous reports, mostly anecdotal, that iodine is important to the molt process, and a number of public aquarists have seen occasional positive results after adding supplemental iodine to Japanese spider crab exhibits. Others feel it is a toxin that may force a premature molt.
The old saying “you are what you eat” applies to crustaceans, and it should perhaps be no surprise that the best diet for crustaceans is other crustaceans. Shrimp tops the list for cholesterol, followed by crab and lobster, and all of these contain natural iodine levels. Therefore, frozen shrimp are likely to be one of the best foods for all captive crustaceans.
Diets should be varied, containing other foods such as krill, fish and sinking pellets or flake food. Some dried foods appear to contain less cholesterol than fresh seafood, so these should not be the sole diet. Lipids are supplemented in some commercial feeds using fish oils and/or lecithin. Lecithin is known to improve the utilization of dietary vitamins and cholesterol.
Specially formulated crustacean foods (or foods containing crustaceans) are available through some of the major fish food companies. Specialty diets used to commercially grow crustaceans are usually only available in 50-pound bags through aquaculture supply houses. When feeding frozen crustaceans to their live kin, it is best to leave the shell in place, keeping in mind that most crustaceans will also eat their own molts to recapture nutrients lost during this process. Although the shell from a food item may often be left behind, it should be recognized as a potentially critical food item.
Vitamins are important to crustacean physiology. Vitamin C is a particularly important additive in commercial shrimp feeds. Pelleted foods fortified with vitamins are especially convenient supplements for smaller species. A couple of times a week, larger crabs and lobsters can be fed with vitamins that have been injected into their food. Water-soluble baby vitamins are ideal for this.
Crustaceans will typically stop eating a day before they molt and will resume a day or so afterward. Their feeding appendages and digestive tract are simply unusable during this interval.
Disease
The diseases of most aquarium crustaceans are poorly documented. The most obvious sign of injury or disease is the presence of black or brown spots on the shell. These are formed when an injury or pathogen breaches the shell, and blood cells coagulate and darken at the damaged site.
“Shell disease” is the most universally recognized crustacean ailment, and is typically signaled by such black spots and shell erosion. Poor diet, pathogens or injuries can contribute to this. Adhesions of old to new shell during a molt can make it impossible for the molt to be completed. This is inevitably fatal. Treatments with iodine swabs (applied locally out of the water) or antibiotics (added to the water) suitable for gram-negative bacteria such as Vibrio have been attempted, but such therapies are highly experimental and contain an element of risk.
Crustacean Ecology
The smaller crustaceans kept in reef tanks are important players in tank ecology, and many species are purchased solely for their ability to control pest algae and invertebrates. Others demonstrate fascinating symbiotic or foraging behaviors; for example, the coral crab (Lybia) carries a small sea anemone on each of its specially modified claws, using them for defense and to collect food (the food sticks to the anemones).
Stomatopods or mantis shrimp are often first encountered as a pest in coral tanks. They are commonly imported within their burrows in live rock. All stomatopods capture their prey with raptorial appendages that deliver powerful uppercuts at 30 feet/second. They drive some reefkeepers crazy because they have a nasty habit of picking off small shrimps and gobies, as well as your cleaning crew (snails and small hermits). On the positive side, they are usually brightly colored.
Those inhabiting rocky substrates are usually smaller and belong to a group known as “smashers,” which possess bladelike striking appendages. Those living in mud burrows are often “spearers,” relying on spiny appendages to impale, rather than crush, their prey. The larger species (up to a foot long) are more commonly from this group. The best way to enjoy mantis shrimp is to create a burrow that is visible through the aquarium glass. Think “ant farm.”
Cleaner shrimps are among the most sensitive species and are especially vulnerable to poor water quality, rough handling and aggression from tankmates. They are the most common species kept in reef aquariums. The Caribbean peppermint shrimp (Lysmata wurdemanni) is much appreciated for its ability to consume Aiptasia anemones, which often overgrow reef tanks. Occasionally, they develop an unwelcome appetite for disc “anemones” and large-polyped stony corals.
Lysmata grabhami and L. amboinensis are the common scarlet lady or white-striped cleaner shrimps. Small numbers generally do well together. They have occasionally been reported to damage tridacnid clams, but I have personally never observed this. When kept with moray eels, they will readily search for parasites among the eels’ teeth — and with complete impunity.
The coral-banded shrimp (Stenopus hispidus) is difficult to transport, often losing claws in the process. Animals collected as confirmed pairs co-exist nicely. Otherwise, consider keeping them singly in all but the largest aquariums. Keep other small crustaceans away from them during introductions because they can be highly aggressive, as is typical for large-clawed crustaceans.
Small numbers of all these shrimps can be kept together in a very large reef aquarium if adequate space is provided. They are compatible with the common coral crabs found in the branches of Acropora or Pocillopora corals.
Two types of hermit crabs are also commonly added to reef aquariums to control pests. The blue-legged hermit (Clibanarius tricolor) and scarlet reef hermit (Paguristes cadenati) are excellent herbivores, and they help control problem hair algae. These hermits are generally well-mannered and can co-exist with cleaning shrimps. Mithrax crabs are also good lawnmowers, but it is best to avoid them, unless you can move them to larger quarters later. Large individuals can bulldoze corals and become aggressive.
Crustaceans are always in motion, always at work, always interesting. Many of you will wish to create a living reef and “hire” a myriad of small crustacean gardeners to tend it. Others may find that a single large, rambunctious crab or lobster kept safely alone is a fascinating pet. In either case, attention to diet, environment and temperament are the keys to success with these amazing armored invertebrates.