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Malacostraca - Decapods  

The Decapods or Decapoda (lthe name means "ten footed") are a group (order) of crustaceans in the class Malacostraca They include crayfish, crabs, hermit crabs, lobsters, prawns and shrimp. It is estimated that there are early 15,000 speciesof Decapods. Nearly half of these species are crabs, with around 3000 species of shrimps and 2500 species of Anomura (including hermit crabs, porcelain crabs, squat lobsters.

Decapods live in all oceans and are found at all depths. Many others live in freshwater or estuarine waters and a few species (like the Coconut crab) are largely terrestrial, but they need to return to water to breed.

The pea crab is a tiny , soft-bodied crustacean about the size of a pea. They spend most of their lives inside the shells of other small animals, like mussels or tubeworms. There is some debate as to whether or not the relationship between the pea crab and its host is parasitic or not. Pea crabs rely on their host for protection, oxygen and food but it is not clear if this is harmful to the host.

Diversity in Decapods

All decapods have ten legs; these are five pairs of thoracic appendages on the last five thoracic segments. The front three pairs function as mouthparts and are called maxillipeds, the remainder being walking legs. In many decapods one pair of legs has enlarged pincers and are called chelipeds. Further appendages are found on the abdomen, with each segment capable of carrying a pair of swimming legs, the last pair form part of the tail fan (together with the telson) and are called uropods.

Hermit crabs have soft unprotected abdomens which they hide inside shells discarded by gastropods.

Decapods feed using a wide range of methods, some are predators using large nippers, some are filter feeders, others are herbivores, scavengers or detritus feeders.

External anatomy of a decapod

Seafood platter including prawns

Decapods as food
Prawns, lobsters, crabs (including Blue Swimmer and Mud Crabs), crayfish, Balmain and Moreton Bay bugs, shrimp and scampi are all popular seafoods and fishing for these and other marine species is important to the economy. Crustaceans should be placed in a freezer for at least 15 minutes to to allow them to be cooked humanely. They are usually cooked by boiling or steaming quickly.

Before crustaceans are cooked, they are usually green-blue in colour. This is due to proteins in the carapace. When cooked the protein wrapping uncoils and breaks down. The red-orange pigment called astaxanthin (a member of the carotene family) which is stable to heat can be now seen as it is no longer "camouflaged" by the colours of the proteins.


Strange Ocean Creatures-Decapods
The Ocean is filled with strange and wonderful creatures such
as these brightly colored crabs and other decapods.

Coconut or Robber Crab

The Coconut or Robber Crab is found on islands across the Indian and Pacific oceans and weigh up to 2.5 kg. They live on land, breathing with small gills in a chamber lined with spongy tissue rich in blood vessels. They eat coconuts and have been seen tearing the husk from a coconut then carrying it up a palm tree and dropping it to the ground several times until it breaks, allowing the crab to eat the flesh. They must return to the sea to breed.


Porcelain crab - filter feeders

Fiddler crab - despite its large pincers it is a
detritovore sifting through sand or mud for algae,
microbes, fungus, or other decaying detritus.
They use their large claw to communicate
with other Fiddler crabs.



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