Anatomy
These fascinating crustaceans are from the
order Decapoda. A decapods body is composed of a series of hard segments called somites.
These somites are either fused together, or linked by a flexible join forming the limbs.
They have two sets of jaws in front of the mouth, and have several appendages near the
mouth that act as feeding aids. The limbs usually start with a pair of developed arms with
claws. Behind these are 4 pairs of walking legs, and as the claws are classed as legs they
total 10, hence Decapoda.
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.
Moulting.
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.
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.
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.
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.
The more
specialized 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.
Environment.
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.
These informations from http://www.shrimpcrabsandcrayfish.co.uk.
