Horseshoe crabs in captivity
08 October 2010
The various animals known as horseshoe crabs are not in fact crabs at all, and in fact are more closely related to spiders. Although only a few species are found today, the group itself is known to be a very ancient one, and horseshoe crabs will be familiar to zoologists as classic ‘living fossils’ in much the same way as lungfish and coelacanths. Two species appear in the UK aquarium trade, usually the North American Horseshoe Crab but sometimes the Mangrove Horseshoe Crab as well.
Horseshoe crabs are common laboratory animals used for both teaching and medical research. They are also widely maintained in public aquaria, including touch tanks, and wild specimens are known to be very adaptable and tolerant of pollution. But as pets, horseshoe crabs have a dismal track record. Wild horseshoe crabs can live for more than forty years, but in home aquaria their lifespan is often less than few months. As is often the case, the problem is not understanding what it is horseshoe crabs need to survive in captivity.
Taxonomy and distribution
There are four species of horseshoe crab divided into three genera: one species of Limulus, one species of Carcinoscorpius, and two species of Tachypleus. Of the four species, Limulus polyphemus is the only New World species, being found along the coast of North America, and consequently this species is known as the North American Horseshoe Crab or the Atlantic Horseshoe Crab. Its range runs from Maine as far south as the Yucatan Peninsula in the Gulf of Mexico. However, the densest populations (and the largest specimens) are found about halfway between these extremes in and around the Delaware Bay estuary, and the North American Horseshoe Crab is therefore best thought of as warm-temperate zone animal rather than a subtropical one, despite the southerly extension of its range well into the subtropics.
By contrast the Old World species come from tropical to subtropical environments from India as far north as Hong Kong, though Tachypleus tridentatus is also found in Japan. Of the three species Carcinoscorpius rotundicauda is the most strongly associated with estuaries and mangroves, and is therefore known as the Mangrove Horseshoe Crab. In fact females will travel well into freshwater when laying their eggs, and have in been reported 90 km (56 miles) up the Hooghly River. The two remaining Old World species, Tachypleus gigas and Tachypleus tridentatus, are not found in freshwater habitats, and instead tend to inhabit coastal marine environments. Neither Tachypleus gigas nor Tachypleus tridentatus are traded as aquarium animals, but they are commercial valuable as food, their eggs in particular being considered delicacies.
Horseshoe crabs are opportunistic animals that plough through sediment consuming small prey including worms and molluscs, particularly clams. They are also scavengers to some degree. Juvenile horseshoe crabs feed on small arthropods, and those species that grow up in estuarine habitats feed preferentially on insect larvae including midge and mosquito larvae.
Horseshoe crabs are sexually dimorphic. Among other traits, females tend to be bigger and darker in colour than the males. North American Horseshoe Crabs breed once a year at spring, whereas the three Asian species breed throughout the year, usually once a month, spawning being synchronised with the full moon and high tides. In all cases eggs are laid in the sediment, either freshwater streams in the case of Carcinoscorpius, or in the intertidal zone in the case of Limulus and Tachypleus. Eggs hatch after a few weeks, and the juveniles that emerge are essentially miniature versions of the adults.
As with other arthropods growth proceeds in a series of steps during which the animal moults its old exoskeleton and then quickly expands to fill out its new, larger exoskeleton. On average, a horseshoe crab will be 1.4 times bigger after it has moulted than it was before. Growth is relatively slow though, and moulting may only take place once a year. Horseshoe crabs only become sexually mature after their final moult, and they will be around 10 years old when that happens.
Of the four horseshoe crab species, only two are traded as aquarium animals, the North American Horseshoe Crab (Limulus polyphemus) and the Mangrove Horseshoe Crab (Carcinoscorpius rotundicauda). Although they both look very alike at first glance, telling the two species apart isn’t actually too hard. The North American Horseshoe Crab has a tail that is essentially triangular in cross section from base to tip, but the Mangrove Horseshoe Crab has a tail that is triangular at the base but becomes more circular in cross section along its length. Mature specimens are also very different in terms of size, adult North American Horseshoe Crabs having a body length of around 30 cm (12 inches) plus the tail, whereas the body length of Mangrove Horseshoe Crabs is only about 15 cm (6 inches) plus the tail. Juveniles of the two species can look alike, but whereas young North American Horseshoe Crabs are pale parchment in colour Mangrove Horseshoe Crabs of similar size are much darker green-brown.
In the UK trade at least, the North American Horseshoe Crab is the most commonly seen horseshoe crab and will usually be sold simply as a ‘horseshoe crab’. It is invariably sold as marine aquarium animal. By contrast the Mangrove Horseshoe Crab is more of an oddity, and is frequently sold as a ‘freshwater horseshoe crab’ or ‘brackish water horseshoe crab’.
All horseshoe crabs need a large aquarium with lots of open space. They are clumsy animals that tend to either pull down rocks or fall from them, and if the latter happens they often end up on their backs. For this reason rocks and other decorative items should be kept to a minimum, and ideally not used at all. None of the horseshoe crabs comes from reef-type environments, and they have no place at all in a reef aquarium.
Because they feed by sifting sand, a sandy substrate is essential. Without being able to feed normally horseshoe crabs will soon starve to death. The sandy area needs to be both broad and deep. Bear in mind that including its tail a North American Horseshoe Crab will get to well over 45 cm (18 inches) in length, and the sandy area needs to be at least four times the length of the adult animal and twice as wide. Depth will need to sufficient for the animal to cover its body, which will mean a substrate depth of 10 cm (4 inches) in the case of the North American Horseshoe Crab. Because the Mangrove Horseshoe Crab is much smaller, it won’t need quite as much space, the average specimen getting to about 30 cm (12 inches) when mature. Of the two species then, the Mangrove Horseshoe Crab will be the one easiest to house properly.
There is some evidence that different populations of horseshoe crabs prefer specific sets of conditions. In the case of North American Horseshoe Crabs, specimens from the Gulf of Mexico region may need to be kept a little warmer than specimens from the cooler Atlantic Coast of the United States. Similarly, Mangrove Horseshoe Crabs from Indian or Singapore may need to be kept a bit warmer than specimens from Hong Kong or China. That being the case, ask your retailer where the specimens being sold came from and act accordingly.
With that in mind, North American Horseshoe Crabs are best kept at room temperature, i.e., between 14-22 degrees C (57-72 degrees F) depending on the time of year. Even specimens from the most northerly populations are known to become inactive below 12-14 degrees C (54-57 degrees F) so that should be taken as the lowermost temperature range. On the other hand, hot summers shouldn’t cause problems, though maintenance above 25 degrees C (77 degrees F) for more than a few weeks may cause stress, particularly for specimens collected from the northern end of the species’ range. The Mangrove Horseshoe Crab needs to be kept between 20-25 degrees C (68-77 degrees F). Warmer and cooler conditions will be tolerated for extended periods, but below 18 degrees C (64 degrees F) they become inactive and in the wild at least tend to burrow into the sediment to avoid becoming too cold.
Both the North American Horseshoe Crab and the Mangrove Horseshoe Crab are able to tolerate significant variations in salinity, and are therefore considered to be euryhaline organisms. However, only the Mangrove Horseshoe Crab is ever found in completely freshwater habitats. In fact wild populations of North American Horseshoe Crabs are not found in places where the salinity is less one-quarter normal seawater salinity, or about 1.007 at 18 degrees C (64 degrees F). At that temperature normal seawater has a specific gravity of 1.027. Under lab conditions optimal growth is observed at about three-quarters normal salinity, i.e., about SG 1.020 at 18 degrees C (64 degrees F), but they also do well in fully marine conditions and are often kept that way in laboratories and public aquaria. Laboratory work indicates that North American Horseshoe Crabs switch from being osmoconformers to osmoregulators at about 66% normal salinity or SG 1.017 at 18 degrees C (64 degrees F). That’s probably the minimum salinity for long term care, and though they may tolerate lower salinities for extended periods, keeping them at lower salinities isn’t recommended. In fact as with temperature there may be some variation between populations with regard to tolerance for reduced salinity, and so aiming for high-end brackish to fully marine conditions is probably the wisest approach to take.
Given its estuarine ecology it should not be a surprise that the Mangrove Horseshoe Crab adapts to a wide salinity range and specimens have been maintained in middling brackish water conditions for extended periods without problems. Though the precise salinity probably doesn’t matter too much for this species, a good minimum would be about one-third normal marine salinity, i.e., SG 1.007 at 25 degrees C (77 degrees F).
In the wild horseshoe crabs are almost always found on muddy substrates. Needless to say this isn’t something easy to accommodate in home aquaria. The substrate of choice will usually be coral sand, possibly blended with aragonitic sand or silica sand as well. Crushed shells can be added to make the substrate more interesting. The depth of the substrate is important because the horseshoe crab should be able to cover its body completely, so in the case of an adult specimen the depth of substrate may need to be 10 cm (4 inches) or more. If they cannot dig in properly horseshoe crabs have a tendency to fall onto their backs. In the wild they flip themselves over using their tails, but if the substrate is too shallow they can’t do this because the tail can’t dig in deep enough to act as a worthwhile lever.
Horseshoe crabs are opportunistic feeders that will consume a range of different things. In captivity they can be easily fed by being removed from the tank, turnover over, and then fed by having suitable fish or seafood placed between their legs. Once the animal has gripped the food firmly it can be returned to the tank and left to feed itself. Specimens maintained in labs are often fed in a separate tank where they can defecate after feeding and then be placed back in the main aquarium. That may or may not be practical or even worthwhile under home conditions.
Small horseshoe crabs favour frozen krill, mysids, brine shrimp and bloodworms. Larger specimens will eat these foods as well as dead fish, crabs, squid, clams and mussels. Note that these are large animals that eat a lot of food, and even under lab conditions starvation is a real threat. Horseshoe crabs should be fed daily, and it is important that they are not kept with animals likely to steal any food put out for them.
Because of their very specific needs horseshoe crabs are not obvious additions to mixed species communities. With that said, there may be situations where companions such as gobies, flatfish and/or livebearers might be kept with them. Sailfin mollies for example would be good companions for Mangrove Horseshoe Crabs because they enjoy the same broad range of salinities but won’t be competing for the same types of food. Whatever tankmates you choose, be sure to check that their temperature and salinity requirements overlap those of your horseshoe crab.
Although being widely sold, the question remains whether horseshoe crabs are worth keeping in home aquaria at all. They are demanding animals in many ways, and don’t qualify as either clean-up crew or community tank oddballs. They can’t be kept in reef systems and their large size and requirement for a deep, sandy substrate means that only a very specific sort of aquarium will ever provide horseshoe crabs with a decent home. On the other hand they are fascinating animals, and given the right conditions hardy and long-lived. Definitely animals to approach with care.