Crustaceans are Arthropods whose body is covered with chitinous exoskeleton for protection. But the same exoskeleton does not allow body growth and hence must be shed in order to allow growth. The larval stages feed and grow in order to become adults and must undergo moulting or ecdysis to grow. After each moulting they change their structure and size and hence are different from the previous stage. Therefore, each species of crustaceans demonstrates several successive larval stages before it becomes adult. These larval stages are described below.
Discovered by Muller in 18th century, the Nauplius larva is the first fundamental stage in all crustaceans that sometimes hatches from the egg and sometimes passes inside the egg. Body is oval in shape and unsegmented with a large cephalothorax and rudimentary abdomen. There are three pairs of appendages, namely, antennules, antenna and the mandible; the last two pairs are biramous and are used for swimming. There is a single median eye. It has a well developed digestive system for feeding on planktons. In Branchiopoda and Copepoda, Nauplius hatched from eggs.
In some Branchiopods such as Apus, the nauplius larva transforms into metanauplius, which is slightly larger than nauplius and has cephalothorax and abdomen and a caudal furca. It also has a single median eye. Antennule is uniramous and sensory but antenna is large, biramous and locomotory in function. Mandibles reduce in size and are used for chewing food. In addition, 2 pairs of maxillae and 2 pairs of maxillipedes make their appearance in metanauplius for handling food.
In the case of marine prawns and lobsters, eggs hatch into protozoea which has a large cephalothorax and elongated unsegmented abdomen with a caudal fork and a pair of small uropods. Antennule is uniramous and segmented while antenna is biramous. There is a single median eye. Mandibles are small and masticatory in function. There are 2 pairs of maxillipedes for food gathering. Three pairs of thoracic limbs make their appearance as buds. Cephalothorax is covered by a carapace.
Zoea is the common larva of decapods and hence it has variations in its features in different species. It has a large cephalothorax that is covered with a helmet-like carapace that also sports spines and it protruded into a rostrum in front. There is one pair of compound eyes. Antennule and antenna are short and sensory in function. First and second maxillipedes are large and biramous, used for swimming. Thoracic appendages are still in bud form and non-functional. Abdomen is 6-segmented, without appendages and has a caudal furca on the tip along with a telson.
In some Malacostraca, zoea changes in to metazoea, which grows abdominal appendages for swimming.
In shrimps and some lobsters zoea transforms into mysis that resembles Mysis in general features. It has a cylindrical and elongated body bearing a cephalothorax and 6-segmented abdomen. Carapace is produced in front into a pointed rostrum. Antennule and antenna are sensory in function. There are 6 pairs of biramous thoracic appendages for locomotion and 6 pairs of abdominal appendages for swimming, out of which the last one is modified as uropod. There is a pointed telson on the tip of abdomen.
Some crustacean decapods, such as Homarus, Nephrops, have this larva which is similar to megalopa and mysis. It has cylindrical body with biramous swimming pleopods on the abdomen and biramous appendages on cephalothorax.
In spiny lobsters, the egg hatches into phyllosoma larva in which body is divisible into head, thorax and abdomen. There is a pair of stalked compound eyes and a pair each of antennules and antenna as sense organs. Body is dorsoventrally flattened and transparent. The first maxillipede is rudimentary and the second one is uniramous. The third maxillipede is large, biramous and is used for swimming. The abdomen is small, segmented and does not bear appendages. Three pairs of thoracic appendages are very long and their tips are flattened oar-like for swimming near the surface of water.
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