The hermit crab Calcinus elegans

24.10.2022

Interesting facts about the life and breeding of the hermit Calcinus elegans. A report by Jannes Freiberg

The hermit crab Calcinus elegans
by Jannes Freiberg

General information about hermits
Hermit crabs, like other crabs and shrimps, belong to the decapods (Decapoda) and differ from them mainly in that they protect their soft abdomen with a snail shell. They are often traded because, on the one hand, they are relatively easy to keep if properly cared for and keep the aquarium free of algae, and on the other hand, their small size and bizarre appearance make them attractive aquarium inhabitants. They can easily live three years or more in the aquarium and are good residue exterminators, i.e. they clean the bottom of the aquarium of food residues and thus prevent excessive pollutant formation.
However, keeping these animals is not as easy as many people think, so I would like to explain a little about hermit crabs here, especially about one species (Calcinus elegans).

General information about Calcinus elegans
Calcinus elegans is certainly one of the most beautiful hermit species that are imported from time to time. Several colour forms of this species exist, but they are all distinguished by their particularly bright colouration. The basic colour of this crayfish is blue or black, the leg joints are blue(in black ones) and red(in blue ones), the antennae, however, are uniformly red, the pointed claws on the legs are either white or also blue. The small claws are mostly black and in some animals have an individual pattern of white dots, while the tips of the claws can be coloured dark red or white. It is interesting to note that very young crayfish up to a maximum of 5mm sometimes do not show this colouring - they are bright blue with uniform black claws. The species grows up to 6 centimetres in size compared to other hermits and is found in nature around the coast of Australia, mainly near reefs.

C. elegans is a very active species that is actually constantly on the lookout for food. It is not particularly choosy, it feeds mainly on algae, but does not disdain animal food either. Among aquarists it is considered a very good beginner animal because of its effectiveness in removing algae, but at least as many aquarists report great aggressiveness - a topic that I will address below.

The aquarium
Calcinus elegans does not make many demands on its aquarium. The water should be as clean as possible and free of pollutants, but it can tolerate a few days in poorer water, as long as this is not a permanent condition and is remedied after a maximum of four to five days. It accepts temperatures from 22°C to 28°C, although I have observed that it seems to feel most comfortable at 26°C. It should not be kept with rough fish. It should not be socialised with aggressive fish or crayfish to avoid unnecessary stress. Especially in the beginning it needs rock crevices where it can hide, but I have also observed that well acclimated animals simply retreat into their home to sleep where they are standing - whether this is in a sheltered place, in the middle of a coral or on free sand, the animals do not care.

Food
The main food source of C. elegans is algae, which means that the animals tirelessly graze the furnishings. However, as a one-sided diet is also unhealthy for crayfish in the long run, it is advisable to give the animals a little variety from time to time. Vegetable food such as iceberg lettuce, banana, carrot and cucumber slices can be used (IMPORTANT: rinse well with warm water beforehand and buy from an organic farmer, if possible, so that no pesticides get into the water) as well as small morsels of meat, such as mussel hearts and pieces of fish. This food should be placed as close to the crayfish as possible so that it gets something - usually shrimps, other crayfish and fish are just as greedy for such tidbits. Any food residues should be removed after one day at the latest so that they do not rot and pollute the water.

A tip: To prevent the food from floating on top, it is advisable to anchor it with a small stone or to use a food clip, as they are often offered in pet shops. The latter should of course be placed in such a way that the crayfish can easily reach them.

Breeding
Breeding this species is not particularly difficult either. In my case, it happened rather by chance when I discovered an adult animal in the trade and then observed it one night releasing free-swimming larvae into the water. In order to make it easier for them to survive, I started feeding them with home-grown phytoplankton and secured the filter with a very fine net, because even though the small crustaceans could not be seen very often or could not be distinguished from other crustaceans very well, I still hoped to get at least a few of them through. Obviously my method was successful, because apart from the fact that my Tridacna grew by half a centimetre - obviously due to the plankton - I noticed a few weeks later that a small snail shell was moving. I should add that at that time I was still using Coralit from the company "Dohse-Aquaristik", which also contains some tiny snail shells, which have a size of 1-3mm. When I took a closer look at one of these shells, I noticed that some blue legs were sticking out of it. In the meantime I am keeping five young C. elegans of about 2cm size, which I clearly did not put into this aquarium... The further breeding was without problems and losses, whereby I think that a large part of the free-swimming larvae did not die, but fell victim to the large tubeworm and other filter feeders.

Aggressiveness, attacks and evictions
Whoever deals a little with C. elegans will certainly also read one or the other horror story about these animals. There are often reports of crayfish that, in their search for a suitable housing, have simply "evicted" a snail, a conspecific or a tubeworm, i.e. pulled it out of its housing and killed it. Others report physical attacks on conspecifics, fellow inhabitants and soft corals, and still others tell of these animals being real Caulerpa pests. But what is the truth in all this?

Since these "forced evictions", as they are often called, seem to happen again and again, I will go into more detail here. First of all: Yes, they can happen, but only if you don't take proper precautions. To do this, you should know what situation the crab is in at the time: Hermit crabs have an extremely sensitive abdomen, which they have to hide in a snail shell for this reason. This must fulfil a number of important criteria: On the one hand, it must be large enough for the crayfish to hide completely inside, and on the other hand, it must be small enough for the crayfish to be able to transport it and hold on to it without any problems - in this sense, an enclosure is a custom-made product that must fit exactly. Once the crayfish has found a suitable home (which is the case when it is bought), there are no problems at first - until the next moult. Hermit crabs also grow by shedding their old skin. Logically, however, the enclosure in which the crayfish lives does not grow with it, and so it becomes too small for it - which is why it needs a new one that again exactly meets the criteria mentioned above. In addition, it is under massive pressure - if it does not find a new enclosure quickly, it is defenceless against all enemies. If no suitable empty enclosure can be found, it has to find another solution - it takes an already occupied house.
This is basically completely normal behaviour, but C. elegans in particular becomes conspicuous at such moments. The reason is relatively simple to explain: While other hermits are often content with somewhat smaller or larger compromises, C. elegans takes exactly what it needs - it has, at least as far as that is concerned, a higher aggressiveness.

But how can you counteract this? Quite simple: If you have a hermit crab, you should bear in mind that it increases by about 1/5-1/3 of its original size when it moults. From this you can determine the size of the future shell, which will also increase by 1/5-1/3. Now it is important to place as many different snail shells as possible in the aquarium, which differ only slightly in size and shell opening - both should, however, more or less correspond to the enlarged version of the current shell. These shells should either be placed centrally (in small tanks) or close to one of the crab's favourite places, which will then look for a suitable shell on its own.

And, before panic sets in: a crab generally searches almost the entire aquarium before it really "clears" a snail. Only if it really can't find a suitable housing (and the animals are inventive, one of my crayfish has been running around for some time with a small stone in which there happened to be a hole of a suitable size), can such attacks occur. I have been keeping my C. elegans with both tubeworms and turban snails in an aquarium for quite some time, and there has never been an attack on them - because there are some shells in the middle, from which the animals take one when needed.

As for other attacks mentioned: these don't happen either, or at least not without reason. The crayfish live together with peaceful animals, such as tubeworms and mussels, without any problems. I often see a hermit walking around on the shell of my Tridacna or on the usually sensitive tubes of my tubeworms, but neither worms nor mussel react to such disturbances in the meantime - the crabs graze the algae, but do not attack the respective animal, on the contrary, they are often very cautious and avoid touching the tentacle crown of a worm or the mantle of the mussel.
I could not record any attacks on soft corals either, only one observation that might have started this rumour: A newly acquired Xenia, which looked very healthy at first, was severely damaged by parasitic crabs, causing it to suddenly and very rapidly degrade- within a day it discoloured and died. Only afterwards did I see a crab crawling around on it and eating the dead animal- not before. I think the quick death of the coral and the typical behaviour of the scavenger might have led other people to think that the crab was to blame for all this, but I can't confirm that. Other xenia (luckily the case with the parasitic crabs happened in a quarantine aquarium), but also stony corals and crustose anemones are not bothered or even harmed by the hermits. I have not observed any real attacks on conspecifics or other animals - in aquariums that are too small and too densely stocked with C. elegans, there can often be fights over rank and territory, but these usually end peacefully and can be avoided by reducing the population. Attacks on shrimps or similar have only occurred when a young Stenopus hispidus tried to "examine" the crayfish with its claws, and the crayfish fought back fiercely, but this is also relatively normal behaviour for this species, which admittedly has a somewhat higher aggression potential when it feels threatened.

Only the last "accusation", massive feeding damage to Caulerpa algae, I can confirm without further ado. Calcinus elegans is an algae eater, Caulerpa algae are, as the name suggests, algae - it is logical that the crayfish will sooner or later come up with the idea of such a food source.

A crayfish with character
In addition to its incredible colourfulness, C. elegans has another special feature: the animals seem to have a distinct intelligence. I first noticed it when setting up my aquarium: I started a new aquarium with a run-in substrate, run-in water and live rock. I soon put my first Calcinus elegans into this aquarium, as the water values were stable. However, as there was still very little algae growing in the aquarium, I thought it was right to feed the then small crayfish (approx. 1 cm) additionally. As the aquarium was (or still is) at waist height, I knelt down in front of it and looked in, while I let a piece of vegetable or a food tablet sink in from above - always at the same spot, as it was easily accessible from this position. After only a few days, the crab reacted to me, every time it saw my face, it moved into position and was waiting for the food. I then did a test to see if it really recognised my face by having someone else kneel there and look in several times in one day - even at the times I normally did it. Although he took up exactly the same position, the crab did not pay any further attention to him. This means that the animal must be able to recognise and distinguish faces- quite a feat for such a small crayfish.(This one is now 5cm tall and still does the same ritual with me every day).

Procuring suitable shells:
For many beginners it is not exactly easy to find good snail shells for their hermit crabs- usually they do not have any deceased snails in the aquarium yet or plan to kill the existing animals. There is a very simple solution to this problem: just go to a pet shop and ask for snail shells. As a rule, every pet shop, whether sea or freshwater, has snails. There are countless species of snails that are often kept in aquariums, and accordingly there are many of these animals in shops, where they die - whether due to improper care or simply because they were fed to loaches or the like. Many pet shops are happy to give away these empty shells for free or for a few cents (subtle panning with already selected pet food often pushes the price down;-)). Interestingly, there is a wide range of sizes, colours and shapes. For very small crustaceans up to 6mm downwards, the shells of turret snails(Melanoides) and bubble snails(Physa) are particularly suitable, for somewhat larger animals you should ask for plate snails and post horn snails, the large crustaceans from 2cm upwards prefer rather apple snails and racing snails, whereby especially the latter are a good enrichment for every reef due to their lively patterning.

IMPORTANT: Be sure to explain to the pet shop what you need the shell for - otherwise it can easily happen that you get a live snail!

Once you have arrived home with the new shells, it is recommended that you first boil them to kill any bacteria. Afterwards, the enclosure should be rinsed under running water for about two minutes to remove dirt and the like. Last but not least, the house is placed in a place as free of dust as possible to dry until it can be placed in the reef as soon as it is dry. This time-consuming procedure may seem annoying, but it is nevertheless necessary, as it is the only way to ensure that nothing is introduced through the new enclosure. (If you don't understand the point of this, just remember that the decaying remains of a dead animal may have lain in this enclosure for several days, possibly mixed with the excrement of small animals.

IMPORTANT: It often happens that aquarists buy their snail shells at flea markets, on the internet or in souvenir shops. Please NEVER do this, because for these shells reefs are plundered and the snails are killed, thereby (and by the often careless divers, who often damage and knock over corals during these actions) reefs are severely damaged. Please do not resort to this method, especially as you can avoid this and save money by using the method described above.

Conclusion: Calcinus elegans is a good algae killer and can certainly be introduced into a functioning reef, provided there are enough free snail shells of a suitable size.

With salty regards
Jannes Freiberg

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