European crayfish or crawfish (Astacus astacus) are native to Central Europe and Southern Scandinavia.
Adult males measure 20–25 cm, females are smaller than 15 cm. They prefer clean rivers and streams of plains. They are protected in some countries, as they used to be popular as food, they are quite rare these days.
They hunt at night – they are predators eating mostly insects, snails, tadpoles and small fish. They may live for up to 20 years. Females shed their shell twice a year, while males moult only once a year. They are soft and unprotected after moulting.
Out of their five pairs of walking legs, the first pair has evolved into a pair of strong pincers. We can find further legs on their abdomen as well: males have 5+1 pairs, while females have 4+1 pairs of abdomen legs. They have two cylinder-shaped, movable eyes.
Females lay 60–150 eggs in bunches onto their own abdomen legs between October and December, in a way that the eggs are fixed by a sticky excretion material, assuring the flow of water rich in oxygen through moving the legs. Out of the many eggs usually only 20 small crabs grow up. After hatching, they stay on their mothers for two more weeks, and they only start their own life after this period.
claws - Their function is catching and crushing food.
compound eyes - They consist of thousands of units (ommatidum). They form clear images and may provide advanced spatial vision. Eyes are positioned at the tip of eyestalks. Eyestalks evolved from legs.
cephalothorax - In crustacea, the cephalon and thorax – similarly to Arachnids – fused together.
exoskeleton - An external frame providing physical protection for the body. As it cannot grow with the body, it must be moulted for the animal to grow. The exoskeleton of crustacea is hard, due to its calcium carbonate content.
two pairs of antennae - Crustacea typically have two pairs of antennae. The small antennules are fork-like. Antennae contain mechanical and chemical sensory receptors (mechanoreceptors and chemoreceptors) and position sensors. Antennae evolved from legs.
walking legs - Higher crustacea (like crayfish) have five pairs of these.
carapace - Hard, thick chitin shield protecting the cephalotorax.
cerebral ganglion - They are well-developed (similarly to other arthropods). They are responsible for – among other things – the processing of information coming from the well-developed sensory organs.
heart - Crustaceans – similarly to other arthropods – have open circulatory systems. Haemolymph fills the heart, vessels and body cavities, and transports nutrients, metabolic wastes and respiratory gases.
reproductory organ - Crayfish have separate sexes and show sexual dimorphism. They reproduce by external fertilisation.
ventral nerve cord - It starts in the brain and runs along the abdomen. The ganglia of the ventral nerve cord contain neuron cells and are joined by bundles of nerve fibres.
antennal gland - The excretory gland of crustaceans. It is also called the green gland. It is responsible for the removal of unnecessary and harmful material.
gill - Located at the base of the legs. It is responsible for the gas exchange between the water and body fluids: it extracts oxygen from water and excretes carbon dioxide.