Some shrimps use the legs under the abdomen for swimming. However the true 'long tailed decapods', like all crayfish and some shrimp, can only swim backwards using the tail, not the pleopods. They are also used to fan freshwater to the gills (and may even bear gills themselves), food to the mouth, and females use these pleopods to carry their eggs.
The antennae and atennules in front of the mouth are their sensory organs (less developed in crabs), and they aid swimming and feeding in their larvae forms, the smaller pair contain fluid sacks used to orientate the animal (much like our inner ear). The large pair of antenna are very mobile and sensitive, and enable them to move safely through even the darkest waters. The antenna also hold the taste cells (aesthetasks). Their sense of touch is further enhanced by fine bristles situated on their limbs, allowing them to sense even the slightest movement.
Their eyes are usually situated on stalks, they are complex faceted eyes. There are between 7,000 and 30,000 individual eyes (onmatids) on each eyestalk. Even with these compound eyes, they principally orientate themselves by touch.
The body is covered with a solid shell primarily made up of chitin, but also contains some calcium carbonate for strength (especially in crabs). This shell is held together by an underlying layer of skin (the endoskeleton), and together forms the crustaceans exoskeleton. Its muscles are also attached to this outer shell.
In crabs, the abdomen is short and the telson is flat and folded underneath the body. The swimming legs and tail are lost altogether.
Decapods have a fast growth rate and need to moult their exoskeleton to grow.
A soft new shell is continually grown under their existing exoskeleton and as the moult draws near a hormone is released from sacks in the eye stalks. This both encourages the moulting process, and suppresses their appetite. Then they pull themselves out of the back of their old shell, unfortunately sometimes some limbs can get stuck in the old shell. Shrimp and crayfish emerge by a split in their neck, and crabs via the back of the carapace.
After they have moulted the new exoskeleton is soft and wrinkled, and they become very lethargic (due to the amount of energy used). During this time they grow and replace lost limbs (usually smaller, but sometimes completely). and the new shell is quickly 'pumped up' to allow for more growing space. They are vulnerable to attacks from tank mates when soft, and the shell can take between a few hours and a few weeks to fully harden, depending on species. They do this by absorbing calcium carbonate whilst the chitin hardens.
This whole process can be very stressful for the animal and during this time water quality needs to be kept in check.
Once it has regained some energy, the crustacean will run for cover and hide until the new shell hardens. Most species will eat their shells after they have shed, but if they leave the shell for too long, then it should be removed from the tank.
Also crab shells if treated gently can be left to dry and harden (in a variety of 'poses'), the result is a lifelike hollow crab.
Breeding crustaceans can be very easy or very difficult, depending on the type of fry produced. The sex of shrimp, crayfish and lobsters can be determined by fairly clear differences on their underside. The females will have a genital opening on the ventral part of the sixth body segment (between the walking legs), the males will have a sperm duct from the sixth to the eighth body segment.
This is detailed quite clearly in photos of Cherax destructor here,and Procambarus cubensis here.
In crabs, the females have a longer and significantly wider telson, the males is no more than a triangle, and segments three to five of the short abdomen are fused together.
This is shown in photos of Gercarcinus ruricola Here.
The parents will usually breed readily when in good condition, and will mate under a wide range of water types and temperatures. Once the pair have bred, the female will have a large number of eggs, clearly visible, held within her pleopods.
The eggs produced form in one of two ways:
The primitive form of reproduction;
Found mainly in small shrimp and crabs. The small eggs develop into tiny larvae, these larvae go through several stages of development, each stage adding a new body segment. The females produce hundreds of thousands of eggs. Unfortunately the difficulties of feeding makes it almost impossible for aquarists to breed them and successfully raise the fry. However there are far more specialised foods available for marine aquaria now, including rotifers and other tiny planktonic foods. These can be used to feed these tiny fry, and may lead to successful breeding in home aquariums.
The more specialised method of reproduction;
Usually adopted by crayfish and a few shrimp. The young are almost fully-formed in large eggs, and when born are very similar to the adults. They eat almost the same foods, and develop fast to give them a good early chance of survival. Usually the adults can develop up to 300 of these large eggs, depending on species.
These decapods come from a variety of different environments. In a home aquarium, the tank set-up should include a lot of hiding places, in the form of rock piles or roots. The size of their claws should be considered when deciding on whether to plant the aquarium. Crabs and crayfish will cause a lot of damage to plants, but the smaller shrimp need fairly dense foliage to thrive. Also the land crabs, and some aquatic crabs, need a dry place and are best suited to a palladium tank. The larger species will create a lot of waste from feeding, and water quality needs to be kept good, especially when moulting, so good filtration is a must. They will however accept a wide range of water types (fairly soft to hard, slightly acid to alkaline), but do best in moderate to hard water, with a pH of around 7.
There are a few tank set-up ideas and drawings here.
Availability within the UK aquatics trade can be a bit haphazard, although a few species of shrimp and crabs are now fairly easily found. Usually Caridina japonica, some Atyopsis sp. and some ghost shrimp are available, also Cardisoma sp, Sesarma bidens and Uca sp. of crabs are usually available to order.
Crayfish and lobsters can be a little harder to get hold of, and in some countries you may need a licence to keep certain species. It's worth having a good look at the more common shrimp species when they come in to your local fish shop. I now have 5 different species of shrimp that were being sold as ghost shrimp. Two of these 'ghost shrimp' turned out to be a nice breeding pair of new guinea Macrobrachium sp., and from one batch I obtained almost every variety of Natantia sp. shown on this site.
If you have any extra information on species covered here, (or anything on those that aren't) or if you would like to request more details on any species (not making any promises here), then please visit my FAQ Database, you can also post your questions there.
There's also a host of useful sites listed on my links page, or you can contact me by e-mail.
I will reply to all of your mail, but you will get quicker replies from within the FAQ Database.