#misunderstood crustaceans

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Procambarus, the North American swamp crays

Procambarus crayfish are among the most versatile animals, traded in aquatic retail. They were once traded as red and blue "freshwater lobsters" in the UK, technically Procambarus clarkii and P. alleni, before it became illegal to trade them there. Many were erroneously bought for ordinary community tanks, where they proceed to consume both plants and fish, and even one another. Hailing from North America, where they are known as swamp crayfishes, their wild diet has a seasonal variability. Though most of the animal protein wild swamp crayfishes consumes, probably comes from foraging insects, in winter they consume more fishes. Plants are consumed also, by these omnivores.

This information is about the species bearing the striking red coloration, P clarkii. Despite misinformation, the blue lobster P. alleni is not a morph of the same species. But much aquarium relevant information about them, is expected to be identical. Although swamp crayfishes show a strong habitat preference for slow flowing or outright stagnant freshwaters, sometimes with low dissolved oxygen levels, they are able to tolerate low end brackish conditions in estuaries, where the specific gravity may be as elevated as 1.015.

'Coldwater tropical' by their North American origin, these lobsters prefer subtropical to tropical water temperatures, but can survive as long as the water does not freeze, yet not reach 35 degrees centigrade either. Ideally, the temperature of their aquarium, should be maintained at least 18 degrees centigrade. Not surprisingly, these crayfishes have become famously invasive outside their range, where people have released them after purchasing them from vendors. Doubtless this was often after noticing their new novelty pets attacked their tankmates, or trashed carefully manicured aquascapes. On the plus side, swamp crayfish remain among the hardiest aquarium pets available, where it is still possible to obtain them.

P. clarkii become progressively herbivorous with maturity. Introduced P. clarkii living in the world famous Donana wetland of Spain, primarily consume material of vegetable origin, including detritus and Scirpus rhizomes. Animal prey included arthropods, mainly insects, and the livebearing toothcarp Gambusia, which was itself introduced to Europe. Thus these cra fishes are in fact a danger to small fishes, despite claims otherwise on the internet. Similarly, introduced P. clarkii near Sao Paolo, Brazil, consume algae and macrophytes throughout the year. Their animal foods are mainly arthropods in the spring and summer, switching more to eating vertebrate material in autumn and winter.

Compared to some other crayfishes, P. clarkii do not burrow so enthusiastically. They tend to resort to the burrow, only in the breeding season and in emergencies during dry snaps, when they can survive in self-made burrow microenvironments. Severe rains on the other hand, can prompt them to migrate overland between freshwater bodies, an ability that surely must have aided their colonisation of novel wild environments, where they have been released through human agency. The tank pH that should be provided for these tolerant lobsters, can be anything between readings of 6 and 10, or perineutral to alkaline, and the water hardness ought not be too soft.

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A lot of people are paranoid about keeping crayfishes with fish. I think the problem is based on misunderstandings about crayfish ecology and behavior. Crayfish themselves happen to confuse many people, for example,as to wether they are lobsters or not: semantically, the word 'cray' is a cognate of 'crab', and the word 'fish' was used historically (and in fisheries, up till the present) to denote many animals associated with water. Crayfish or crays are, however, not actually crabs in the modern sense of that word. Also depending on the dialect of English, words like 'crawfish' can apply also to certain marine lobsters and burrowing, estuarine mud shrimp, although they are undeniably cognates of 'crayfish'.

English as a language is very confused as to its 'cray' and 'craw-', and it is naturalists and zoologists who first attempted to standardize a certain use as particularly 'proper' across dialects. But, in any case, the root is probably from an old Indo-European root, meaning to scratch or crawl. Less possible is a relationship to the word 'scarab', from a Greek word that refers to scorpions, and of course large beetles, as well as crabs and lobsters; this word entered Greek early from a Semitic language in which it had meant 'scorpion', but the English use it today, in reference to a kind of dung beetle.

Although people will insist that crayfishes are not lobsters, crayfish are in fact s freshwater subclade of the lobsters, that evolved adaptations to consuming vegetable material as live plants and detritus; crayfish do also consume animal carrion on slow moving prey, although the exact proportions of the items in their diets varies by the species, and by such environmental situations as are determined by the season of the year. From this it may be determined that none of the crayfishes are active predators, being rather omnivorous foragers; in fact they obtain animal protein through the chemosensory cues that are specific to carrion, or by cues of touch as they go along. It is not unusual for plant eating crustaceans to consume meat opportunistically, and even preferentially when it is available But this is because the opportunity is rare, and their diets suffer deficiencies in some kinds of nutrients. Crayfish should in fact have a plant based diet in the aquarium, for the most part, and foods of animal origin should be but a supplement provided.

Now, people regularly confuse gestures of aggression by crayfish, with predatory intent. It isn't the same thing at all, though agonistic encounters can turn violent. Such hostility is related to competition for the use of available space, and bottom living fishes are the most likely to irritate a crayfish Especially those kinds of fish that hide in the same sort of locations that crayfish are disposed to utilize as a retreat. Crayfish can not only be hostile towards fish in this way, but also to one another, which some people think is related to a cannibalistic intent. But when crayfish are cannibalistic, it is a larger cray preying on a smaller individual, whilst it is molting - a process that leave a young crayfish temporarily without defence, and vulnerable to attack.

From this it can be seen that animals that do not compete with the crayfish for space, or threaten it as a predator, will be safe. Obviously this does not apply to slow moving animals such as snails, and sleeping fish are also vulnerable. Although some people cohabit small, dither-type aquarium fish with crayfish, there will always be a risk if these little fish are caught unaware. I do not condone this and I insist that all tankmates should be robust and suitably large - commonly this is expressed with reference to the length of the crayfish itself, or the size of its pincers combined. Obviously all tankmates for a crayfish must favor the same parameters in which the crayfish will thrive: so for example, the popular North American swamp crayfishes (Procambarus sp ) cohabit well with goldfishes, because they originated in similar environments, although on different continents.

It is sometimes said that the Australasian or Australo-Papuan crayfishes (Cherax sp.) are the least dangerous crayfish to fishes. Whilst it is true (on the whole) that they are more placid than the North American swamp crayfishes, they can still kill fish, and one another. The same cautions should apply when cohabiting Australasian crays with fish, as with other crays; although at least half of the common misunderstanding, is the mistaken belief that other types of crayfish are violent predators. The genus Cherax are those that also known as yabbies, especially in Australian English. As with some other colloquialisms, like 'goanna' for the local monitor lizards, it corresponds to a natural group of Australasian animals, so it is pointless to say that all crayfish are yabbies; though again, Aussies themselves also use the word 'yabby' to refer to some unrelated, saltwater crustaceans

Another kind of crayfish that is sometimes advocated for fish aquariums, is a selectively bred morph of a small, Mexican crayfish species (Cambarellus patzcuarensis). In this case it is the diminutive size of the cray that limits it's ability to damage other animals, but it still consumes snails and, at least in anecdotes, very small and unwary fishes. I am unaware of any other crayfish species currently being traded in Europe, but formerly the signal crayfish (Pacifastacus leniusculus) was widely retailed for life in outdoor ponds. This temperate climate species was considered as a friendly scavenger, but was soon realized by many that they consume underwater plants - this is the natural role of crayfish! - and less accurately, they also came to be blamed for the unobserved deaths of fishes that they scavenged, as is also their nature. Most signal crays did no harm to pond fish at all, but misunderstandings of their behavior led many people to be paranoid of keeping crayfish in ponds and aquariums.

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Our of the animals purchased for their use as biological control in reef aquariums, Mithraculus sculptus is one that has caused a great deal of worry to aquarists. This species is strongly algivorous, and is often purchased for the purpose of algal control. Subsequently the crab gets blamed for damaging corals, and even the deaths of fishes are attributed to the crab. Much confusion exists as to the riskiness versus 'reef safety' of this controversial species. To aquarists this species is known as the emerald crab or simply 'the' mithrax crab. Historically there was much debate about the distinction, or otherwise, of Mithraculus from its relative Mithrax.

Mithraculus is extremely similar to Mithrax, so close overall that observations about the general diet of one genus, also apply to the other. M. sculpta is also known as the green clinging crab in some literature Whatever we call it, this is a crab of the Caribbean and adjacent eastern Atlantic, from Florida and the Bahamas south to Brazil. It grows to about 4 centimeters, or about 1 and a 1/2 inches long.

Although ecological studies tend to stress the herbivory of Mithrax senso lato, both field and laboratory studies of these crabs in the 1970s, have demonstrated their omnivorous feedings, on macroalgae, cnidarians, and detritus. In an experiment, the captive Mithrax eagerly consumed pieces of fish and clam, as well as such marine animals as polychaetes, gastropods, and echinoderms. Mithrax consume Aiptasia anemones, so they are able to consume rather large polyps. The extent of their predation on fleshy polyps and colonies is not known.

Very motile animals such as fish, shrimp, and crabs, were consumed only by scavenging. Mithrax are clumsy foragers, certainly not hunters, and they do not detect prey well, merely consuming animal protein opportunistically. M. sculpta and similar crabs really are primarily herbivorous, although they preferentially take animal derived foodstuffs where they are available. In other words, they have evolved to exploit animal protein if it is encountered whilst feeding on algae. Mithrax crabs are able to extract live snails from their shells, but appear unable to crack hard shelled prey open.

In shallow seagrass beds, and the margins of reef flats, M. sculpta uses the branching, coralline macroalga Neogoniolithon strictum as a host, and the alga benefits from the effectiveness of M. sculpta at controlling epibionts. M. sculpta is also a facultative symbionts of Porites stony corals. The residence of M. sculpta among P. porites, reduces intrusive algal cover on the corals by more than 85%.

Such a statistic is proof of the efficiency of these craps. By day, these crabs shelter in crevices and beneath ledges. Although they are nocturnal, their algal diet restricts them to areas of sunshine. They avoid areas that are shaded during the day, which would impair the growth of their algal food. The claws of M. sculpta are spatulate, an evolutionary tendency among crabs deviating towards herbivorous lifestyles.

In the absence of appropriate algal growth, emerald crabs will consume such foods as nori. They will obviously accept defrosted meaty foods, and some dry preparations. But their natural diet, which is mostly herbivorous, ought to be replicated. Good reason exists to presume M. sculpta is a risk to slow, motile animals like gastropods and starfishes in the aquarium, as well as to large fleshy polyps. But they do not hunt fish, shrimp, or other crabs.

Aquarists should note there is some overlap between the macroalgal genera eaten by emerald crabs, and those purchased deliberately, for ornamental reasons. These crabs are also nocturnal by nature, and many aquarists never see theirs by day, sometimes fearing their pet crab is dead, only to discover the same animal alive, some period later. The species requires places to reside in the rockwork, where it will avoid bright light.

What is reef safety, anyway? A reef is a living ecosystem, and even those organisms that do not consume other organisms directly, are competing for sunlight or space. The idea of 'reef safety' implies a simplification of how organisms in an environment really interact, and is semantically meaningless. It is like saying an animal is 'forest safe' or 'prairie safe'. No one talks about any other biome, the way 'reefers' do about coral reefs.

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The gentle giant from Africa

Of all the species that are encountered in the aquarium trade, a popular freshwater shrimp, Atya gabonensis, probably has acquired the most vernacular names. To me, this species is the African or giant fan shrimp, names referencing its origins, identity, anatomy, and striking size. Bizarrely enough, the Americans appear to trade it most often, as the vampire shrimp. A. gabonensis is a gentle giant, that does not attack other animals to drink their blood, so I find this peculiar name inexplicable. The name 'monster shrimp' re fers, seemingly, to its size and neither its attitude, nor to vampires or vampirism. Far from appearing monstrous, the big, robust A. gabonensis actually look like lobsters.

Some other popular names for A. gabonensis, substitute 'filter shrimp' for 'fan shrimp', which is easy enough to understand, because the fan-like appendages of fan shrimp, actually are used in suspension feeding. This fine apparatus is employed by these fan-bearing shrimp as they strain food from the water, with a movement that looks like humans waving fans. On a geographical note, the more specific localities of Gabon and Cameroon can similarly replace the 'African' in trade names, and indeed these shrimp are imported from countries in tropical West Africa.

As might also be predicted from its species name, A. gabonensis originates in the freshwater rivers and streams, from Senegal to Gabon. There this benthic species is found on both rocky and non-rocky bottoms, and is also associated with masses of roots. This is a riverine species and it favors flowing water. Clear water favors it's algal food supply, however in some months the rivers are more turbid than in others, with implications for algal growth. Wild A. gabonensis are thought to switch their feeding behaviours by locality and month.

The name of 'fan shrimp' is common to the atyid clade of shrimp, which share the evolutionary refinement of their claws, inherited from typical shrimp ancestors, into appendages more useful for trapping food particles by filtration, and now useless for grasping, pulling, or tearing. Most of the atyid clade, including A. gabonensis, seen to obtain their food from both the water column and the substrate. A. gabonensis has a diet composed of algae and detritus, with mud, sand grains, and insect fragments, also identified as ingested by these animals.

In the rivers of West Africa, A. gabonensis are most often exposed to water temperatures of 27 to 28 degrees centigrade, and a pH of 6 to 7, sometimes approaching 8. Such a range of pH is best characterised as circumneutral, and slightly acidic to slightly base. In West African rivers, the pH may fluctuate from acidic to base on a seasonal basis. Although websites may claim otherwise, the literature on wild A. gabonensis shows the shrimp do not really inhabit hard, alkaline waters all year round, although they are not denizens of blackwaters.

It is worth noting that when Atya gabonensis is maintained in hard, alkaline water, it tends to turn bright blue. Shrimp of this colour are sometimes traded as though they are a separate species, the so-called 'blue rhino shrimp'. Sometimes, shrimp keepers have been surprised or alarmed to find their pet shrimp has changed color, to become what they had thought was a different species. The significance of this curious colour shift is unknown, and it is not thought to harm this shrimp.

Because they are such specialized filter feeders, A. gabonensis are quite inoffensive to other aquarium inhabitants. Because they neither consume not uproot plants, these crustaceans are safe in a planted aquarium. Should sufficient food not have accumulated for these shrimp, pellets or flake should be crumbled into the water. They will also consume pellets or wafers from the bottom, but only as they soften and are easy to break apart. Whatever is the feeding regimen, the aquarium water should ideally remain quite clear.

It is certainly true that the riverine A. gabonensis ought to be provided a good water flow in the aquarium, more than In most tropical community aquariums, but the high flow recommended by some authorities is not neccessary, based on common experience and wild collection records. There is a misconception online that A. gabonensis should be assumed starving, through lack of food or insufficient water flow, if they attempt to feed from the substrate. Because this behaviour is in the normal repertoire of A. gabonensis, there is no cause for alarm.

Wild A. gabonensis show a good tolerance of water chemistry that are not too extreme, and they do no not occur in estuaries, and should only be maintained in freshwater, not brackish water aquariums. A number of A. gabonensis can and should share the same aquarium, but each individual will not only need sufficient space, but also to have nooks available for their retreat. Growing A. gabonensis will also retreat into shelter whilst they molt. The decor in their aquarium should include rocks, which they will clamber on, although they will definitely utilise areas of soft substrate, and are well able to burrow in it.

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Clibanarius africanus, the not quite freshwater hermit crab

Given that hermit crabs are widely popular in coral reef aquaria, and the terrestrial species are traded also, some aquariums have asked if there are also freshwater hermit crabs. And there is a species that is mistraded as the freshwater hermit crab, and is also known as the African hermit crab, Clibanarius africanus. As its geographical moniker suggests, this is a tropical West African species. The crab itself is a small species, growing to below 4.5 centimeters, or less than 2 inches. As is well known, most hermit crabs occupy gastropod shells for their armor, and their name comes from their resemblance to human hermits peeking out of their caves. In the case of C. africanus, the shells selected belong to gastropods of the genera Tympanotonus and Pachymelania. As you might expect, C. africanus are shipped from West Africa with the shells they wear.

Despite being traded as the freshwater hermit crab, C. Africans is found in brackish environments, such as Lagos Lagoon, Nigeria, where it is found on mixed sand and mud bottoms, but displaying a strong preference for a high sand content, rather than where mud dominates the substrate composition. In this lagoon the salinity varies seasonally, being diluted in the wet season by increased rainfall and river discharge, but concentrated in the dry season by increased evaporation. C. africanus is present in the lagoon during the dry season, but not in the wet season, which is when the environmental salinity drops.

C. africanus is never truly found in freshwaters. Aquarists have reported this crab often dies, after a few months of attempted care in strictly non-brackish, freshwater aquariums. Although this hermit crab tolerates a wide range of salinity, it is not a freshwater aquarium pet, as can be proved by collection records. One paper observed that C. africanus is not found throughout the year in the Lagos lagoon, the locality where the species is most studied. The temperature there is about 28 degrees centigrade year round, and according to the paper, C. africanus is present where and when the specific gravity is around 1.02 and the pH is close to 8. The crabs noticeably inhabited water that was only marginally turbid, being absent from strongly discolored water.

A different paper identifies C. africanus as present in neighbouring creeks, shaded by salt tolerant Avicennia and Rhizophora trees, and present there mainly in the late wet and early dry season. The crabs were commonest in the creek named Abule Eledu, and less common in Abule Agege. There the water in these creeks was recorded as having a cooler temperature of 25-26 degrees, and the pH was around 7. (It should be noted that the water temperature in these creeks, can be closer to 30 degrees.) Yet another paper records the presence of these crabs where the temperature is 28-30 degrees centigrade, and the pH from 8 to 8.5. Interestingly the specific gravity of the water, according to this paper, was as low as 1.006-1.007.

Clibanarius sp. are categorised as microphagous, feeding on algae, cyanobacteria, and detritus. For this reason, Clibanarius sp. are not able to overpower gastropods with shells of a size suitable for their own occupance, as can some other hermit crab genera, and they must forage for empty shells. The genus Clibanarius is the same genus to which the popular blue leg (C. tricolor) and green (C. vittatus) hermit crabs belong. These two species are extremely popular in reef aquaria, although neither of them is really a coral reef species. A further species, the blue striped hermit crab (C. longitarsus) from Asia, is rarely imported and is regarded as estuarine, although I only remember seeing it traded as a marine species.

Clibanarius sp. are harmless to other ornamental animals in the aquarium, where they will scavenge leftovers, forage detritus, and graze on photosynthesizing biofilms. If necessary, they can be fed on sinking algae wafers. Like other hermit crabs, growing Clibanarius need newer, larger shells to be available as portable homes, until the time they are full sized. C. africanus does well in a brackish tank with a specific gravity of 1.01-1.02, and a temperature of about 28 degrees centigrade. The substrate should be of a sandy nature, in accord with their habitat preferences, and because these crabs are not rock dwellers, other decor is unimportant to them.

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The cute little red crab, from your aquarium store

The small red crab so commonly seen in aquarium stores, in the freshwater tropicals section, is Perisesarma bidens, historically known as Sesarma bidens. Historically it was often retailed as the Thai red crab, or simply the red crab. More properly this species, Which is actually quite variable in its appearancw, is the red clawed mangrove crab. Although it arrived in shipments from Thailand, P. bidens has a broad swathe of distribution, from East Africa into the West Pacific. It is a small and appealing crab, with a 5 cm or 2 inch carapace. Not surprisingly, they were often purchased for freshwater setups, when they were more commonly available in stores. Sadly, most of these crabs are too short lived in our tanks. P. bidens is in fact a herbivorous to detrivorous little crab from mangrove habitats, with a preference for estuarine forests dominated by Kandelia, but also of Avicennia and Brugueria. Such communities flourish upriver of the more salt tolerant tree, Rhizophora, but are still growing along river stretches that are subject to tidal influences. Within the mangrove food web, teeming P. bidens are important consumers of fallen leaves and algae.

Like the famous mudskippers from similar habitats, P. bidens is amphibious, so it must have access to land as well as water, and care must thus be taken that the crabs do not escape. Their water should be brackish, perhaps st a specific gravity of 1.005 to 1.015, rather than marine or freshwater, because these crabs are estuarine. Water temperture should be maintained at 24 to 26 degrees. As a burrowing crab, P. bidens also needs a reasonably deep substrate of fine sand or mud, not a gravel or a bare bottom. Their natural diet is primarily macroalgae and mangrove leaves. It consumes both fresh and decaying plant matter, and has been observed to consume some animal protein by opportunistic scavenging, such as when encountering dead fish and insects. In the aquarium, P. bidens will nibble many foods, but requires a diet that is biased towards algal and other vegetable components, such as Spirulina. Such a crab is not predicted to chase live prey around the swamp, or the home aquarium.

In the aquarium, P. bidens can show aggressive behavior towards passing fishes. This is because the species is territorial, and not because it has predatory intentions towards the fishes. Claims that P. bidens nips at sleeping fishes are unverified, and unlikely owing to the natural diet and feeding habits of this crab. Although it cannot be ruled out this species might, occasionally, seize upon a sleeping fish, it remains unproven that they ever do so, and the form and function of their grasping claws, are quite unsuited to such behavuors. Certainly it is one of the least risky crabs, to cohabit with sensibly sized fishes, that will neither annoy the crabs as a target, nor less intentionaly by their regular activities. Unfortunately, P. bidens, as a herbivore, is very likely to consume live plants, ruling out this species as a suitable member of the planted rank community. A number of P. bidens can cohabit, but only in a suitably large aquaterrarium, with hiding places, broken lines of sight, and opportunities to burrow. The decor must afford them much opportunity to retreat, as they would among mangrove roots in the wild.