CS Knowledge Base

All crustacea must replace their exoskeletons with new ones in order to grow. They replace their exoskeletons by moulting. Moulting is controlled by hormones. A new exoskeleton is then secreted to replace the old one. While they wait for their new exoskeleton to harden it can be dangerous. They may be preyed on by larger predators and not be able to defend themselves. Most crustaceans moult many times between hatching and adulthood.

The main body cavity is an open circulatory system: blood is pumped into the haemocoel by a heart.^[6] Malacostraca have haemocyanin as the oxygen-carrying pigment, while copepods, ostracods, barnacles and branchiopods have haemoglobins.^[7] This indicates something very unusual: a phylum (or subphylum) with such a fundamental difference between the physiology of its member groups.

Crustaceans have a number of larval forms. The earliest and most characteristic is the nauplius. In most groups, there are further larval stages, including the zoea (pl. zoeæ or zoeas). This name was given to it when naturalists believed it to be a separate species.^[8] It follows the nauplius stage, and often has spikes on its carapace. These may assist these small organisms in swimming.^[9] In many decapods, due to their accelerated development, the zoea is the first larval stage. In some cases, the zoea stage is followed by the mysis stage, and in others, by the megalopa stage, depending on the crustacean group involved.^[10]