Tropical Brackish Water Invertebrates (other than snails)

Although there are many invertebrates to be found in brackish waters around the world, only a very few of these are traded. The following are the ones most frequently encountered in tropical fish stores. Some, such as the blue-legged hermit crab and Asian horseshoe crab are more likely to be sold as marines, while fiddler crabs and nerites commonly get sold as freshwater animals. When adapting these organisms to brackish water, care should be taken to acclimate them to the new salinity careful. In addition, most invertebrates are intolerant of pollutants, including nitrate and copper.

Clams

Bivalve molluscs do not usually do well in aquaria. They are filter-feeders and cannot get enough to eat without regular feeding of a suitable invertebrate diet (sold to marine aquarists for feeding corals, tubeworms, etc.). The freshwater Asian clam Corbicula fluminea is one of the most commonly seen. They can reproduce in captivity, releasing larvae that are able to crawl about at once. This species will tolerate very slightly brackish water, to around SG 1.003-1.005.

Fiddler and red-claw crabs

Fiddler crabs, Uca spp are small crabs in various shades of yellow and brown, but the males have one enormously developed claw with which they display. These are semi-terrestrial animals and will not adapt to permanently being kept underwater. They need rocks, a sandbank, or some other sort of structure above the water level onto which they can climb and feed. Ideally, keep in a mudskipper style aquarium. Fiddler crabs feed on organic detritus, primarily algae and decaying plant material.

Another semi-terrestrial freshwater crab is the Asian red clawed crab Perisesarma bidens. It is found in mangrove forests and will dip into fresh, brackish and marine water. It requires similar conditions to fiddler crabs and cannot be kept permanently submerged. Maintenance of this species is not difficult, but it is an opportunistic predator and will eat small fish.

Crayfish

None of the commonly traded species of crayfish are brackish water animals, and so do not have a place in the brackish water aquarium.

Hermit crabs

Various Clibanarius species are found in tidal and mangrove habitats, and these can do well in mid to high salinity brackish water aquaria, corresponding to specific gravity levels of 1.010 upwards at 25 degrees C. Two species from North American are quite widely sold, Clibanarius vittatus (the common or green hermit crab) and Clibanarius tricolor (the Caribbean or blue-legged hermit crab). Both are hardy, easily maintained, and safe with tankmates of similar size. They will fight over shells, so it is important to provide lots of suitable snail shells that they can use as they grow.

Clibanarius longitarsus is A brackish water hermit crab from Southeast Asia that requires similar conditions to the North American species mentioned above. The so-called freshwater hermit Clibanarius africanus is an African species does best in low to middling salinity brackish water, SG 1.005-1.010 being ideal.

Terrestrial Hermit crabs

At least two terrestrial hermit crabs are sold as pets. These are the Purple Pincher hermit crab (Coenobita clypeatus) from the Caribbean and the Ecuadorian hermit crab (Coenobita compressus) from the Pacific coastline ofSouth America. Neither can be kept in an aquarium setting, since they both spend almost all their time on land. However, they might be kept alongside mudskippers and fiddler crabs assuming that these were all of similar size. Note that while Coenobita clypeatus will happily bathe in brackish water, it needs access to freshwater for drinking. Both species will drown in deep water, so it is important that they can climb out of the watery side of the tank easily, perhaps via a sand bank or sloping pieces of bogwood.

Horseshoe crabs

Horseshoe crabs (really an animal more closely related to the spiders and scorpions) are large, burrowing, animals occasionally sold as oddballs for tropical marine aquaria. The American species, Limulus polyphemus is the best known and most widely traded, and though tolerant of brackish water for extended periods, probably needs fully marine conditions over the long term. The same is probably true for its close relatives, the East Asian horseshoe crabs of the genus Tachypleus.

On the other hand, the more distantly related South Asian horseshoe crab Carcinoscorpius rotundicauda is very much an estuarine animal and has even been found in freshwater environments, such as 90km up the River Hooghly (Chatterji, 1994 in Xuhua Xia, Systematic Biology, Vol. 49, 2000). Because of this, the South Asian horseshoe crab likely makes a much better choice for the mid- to high-salinity brackish water aquarium. It is known to be very hardy, providing it is given plenty of food, and does need a sandy substrate so that it can burrow freely.

Jellyfish

The mangrove jellyfish Cassiopeia andromeda is tolerant of a range of salinities from about half-strength sea water upwards, but is very particular about substrate and lighting. A soft sand substrate is essential as these animals spend most of their time upside down settled on the sand with the tentacles upwards to catch food. They also need intense light because like corals they possess symbiotic algae. Also like corals, absolutely perfect water conditions are required, so that generous filtration and regular water changes will likely need to be supplemented with protein skimming.

Sea anemones

While there are brackish water anemones, only one is currently traded as such, the beadlet, Actinia equina. This is a hardy, coldwater species that does well even at tropical temperatures provided water quality is good. It is not photosynthetic. In the wild these anemones are not normally exposed to very low salinities for long, and in the aquarium should probably be kept at SG 1.015 upwards, and it is questionably whether even that level of salinity will do them much good over the long term.

Shrimps

The Amano shrimps have become popular as algae eaters in freshwater aquaria, although they will do well in slightly brackish water as well (and in fact only breed in brackish water). They are small and can only be kept with very small fishes of similar size, such as bumblebee gobies, that aren’t going to bother them.

A true brackish water denizen is the red-nose shrimp Caridina gracilirostris. It is similar to the Amano shrimp in 

shape and colour except that it is more slender and has a long, red rostrum (“nose”). Specimens kept in freshwater aquaria tend to be a little delicate, but in tanks with a specific gravity around 1.003-1.005, these shrimps are much more robust. They are a bit more omnivorous than Amano shrimps, and though they eat algae, meaty foods are appreciated as well.

Various species of Palaeomonetes are inexpensively sold as "ghost shrimp" or "river shrimps" and these will do well in fresh or slightly brackish water (specific gravity below 1.005) making them ideal companions for very small fishes such as bumblebee gobies, small livebearers, and other inoffensive animals. They are all scavengers to some degree, although algae in an important part of their diet and should be made available, perhaps by not scraping the walls of the aquarium.

Though not often sold as an aquarium animal per se, mention has to be made of the Hawaiian volcano shrimps sold in “self-contained ecospheres” as novelty gifts. Contrary to the advertising blurb put out with the ecospheres, these shrimps cannot survive simply by eating algae and consuming the oxygen the algae produce, but merely die very, very slowly. In fact these shrimps, known to science as Halocaridina rubra, are adapted to living in harsh environments such as coastal pools and streams where the salinity and temperature can change dramatically and quickly. It is their hardiness, rather than any clever design behind the ecospheres, that allows these shrimps to last for as long as they do. In aquaria they do very well in moderately brackish water (SG 1.010) under which conditions they will breed quite readily. At barely 1.2 cm/0.5" in size, these shrimps cannot be combined with most fish, which will simply view them as food, though possibly very small gobies such as Brachygobius aggregatus are an option.