The Great Barrier Reef Library

A comprehensive guide to The Great Barrier Reef

Arthropoda – Crabs, Shrimps and Spiders


By on December 26, 2017


Arthropoda are invertebrates that have an exoskeleton, segmented bodies and paired, jointed legs. In fact, Arthropoda comes from the Greek words for “jointed (Arthro) feet (Pod)”. These different joints on a leg move in different planes, allowing for extremely versatile movement.

Another noticeable feature is the exoskeleton that protects the body. Chitin, Protein and Calcium Carbonate (a feature of marine arthropods only), make up this armoured suit. It provides a place of attachment for the limbs and can differentiate into weapons, such as claws.  It even provides buoyancy in larvae with extensions that increase its surface area to make them float. The only downside, is that it limits growth and therefore, they need to periodically moult.

The moulting process is a dangerous time for an Arthropod, leaving them exposed. Whilst insects only moult up until they reach sexual maturity, crustaceans continue to moult throughout their life times.

Arthropods have incredibly complex compound eyes, often found on the end of eye stalks. With thousands of tiny, individual, visual units called Ommatidia, making up each eye, they greatly increase their ability to detect movement. Then the brain puts together the images received from each Ommatidia to create a mosaic-like image. Some arthropods even have the ability to alter the distribution of pigments, allowing them to isolate individual Ommatidia. This can be used to increase vision in low-light conditions, albeit at the expense of sharpness.

Arthropods use Haemolymph, to transport nutrients throughout the body, which is is a blood-like fluid bathing their organs. A simple heart pumps this fluid through a few arteries, returning through paired openings.

Class – Next level of classification, grouping animals with similar characteristics