MEGALOPA LARVA
In crabs, zoea is transformed into megalopa, which bears a large cephalothorax and small abdomen bearing small pleopods. Telson is very small. Antennule and antenna are small and sensory in function and there is a pair of stalked eyes. The first pair of thoracic legs is large and chelate as in adults. The rest of the 4 pairs of thoracic legs are thin and long and are used for crawling. In hermit crab the megalopa larva is slightly different and is called Glaucothea. Its abdomen carries pleopods and the tip of abdomen bears a hook-like telson.
ALIMA LARVA
In some lobsters such as Squilla, the zoea is slightly different and is called Alima larva. It also has a large cephalothorax covered with carapace and a segmented abdomen bearing paired pleopods and a telson. Carapace bears spines and rostrum in front. Thoracic limbs are reduced. The antenna and antennule are uniramous and sensory in function. The second maxillipedes are prehensile for capturing food. The larva is pelagic and has a transparent body that makes it invisible to predators.
CYPRIS LARVA
In subclasses Ostracoda and Cirripedia the nauplius larva changes into Cypris larva. Body of cypris is enclosed into a bivalve shell which is secured by an adductor muscle. There is one pair of compound eyes. The antennule is large and especially modified for attachment to substratum with cement gland. The second antenna is absent. There are 6 pairs of biramous thoracic appendages for swimming. The tip of abdomen bears a caudal furca.
KENTROGEN LARVA
In Sacculina, cypris larva attaches to the host crab with its antennule and transforms into kentrogen larva by shedding its bivalve shell and appendages. Body is sac-like and elongated with undifferentiated mass of cells and a chitinous tube-like dart within the body. There are no appendages in this larva.
ERICHTHUS & ERICHTHOIDINA LARVAE
These larvae are found in stomatopods of Malacostraca such as Squilla. They have cylindrical elongated body with cephalothorax and segmented abdomen. Cephalothorax is covered with a large carapace that extends forward into a pointed rostrum. Both antennae are uniramous and sensory in function. Eyes are compound and stalked. In both these larvae the three anterior thoracic limbs are rudimentary or reduced and posterior limbs are well-developed for swimming. Pleopods are very small and hardly functional. In Erichthus maxillipedes are prehensile and used for capturing food.
CALYPTOPSIS LARVA
This is a larval stage of Euphasia and resembles Erichthoidina in general shape but does not sport a rostrum on the anterior end of carapace. Abdomen is segmented but limbless. First antenna is uniramous and the second antenna is biramous and both of them are quite well-developed. There are 4-5 pairs of biramous thoracic legs for swimming. Carapace is prominent.
EPICARIDIAN LARVA
This is a larva of parasitic isopods such as Bopyrus. Body is oval in shape with 6 pairs of thoracic limbs hook-like for clinging to the host. Pleopods are biramous and 6 pairs for swimming. First pair of thoracic appendage is prehensile. Both the first and second antenna are uniramous and sensory in function. One pair of sharp mandibles is modified for piercing the host. This larva occurs in the gill chambers of prawns and fishes.
CRYPTONISCUS LARVA
This is the second stage of the larva of Bopyrus that occurs in the gill chambers of prawns and lobsters. Body is elongated oval with 6 pairs of thoracic appendages modified for clinging on to the host. There is a single pair of pleopods for swimming. The first pair of antenna is absent while the second antenna is long and uniramous. Mouth parts are piercing and sucking type.
SIGNIFICANCE OF LARVAL STAGES
The larval stages are significant in scientific studies as they show evolutionary relationships among different groups of crustacean. Nauplius and metanauplius stages are common to all crustaceans whether they are outside or inside the eggs. They show common ancestry of all crustaceans from similar ancestors. The larvae are important for the species as they help in the dispersal of the animals to distant places.
HORMONAL CONTROL OF METAMORPHOSIS
Crustaceans possess an X-organ and a Sinus gland inside the eye stalk forming an endocrine complex. They secrete Molt Inhibiting Hormone (MIH) and Gonad Inhibiting Hormone (GIH). They also secrete Mandibular Organ Inhibiting Principle (MOIP). These hormones are released in the body and inhibit their respective organs.
The Y-organ, also called the Moulting gland, is located in the maxillary segments and releases Alpha ecdysone that later changes into beta ecdysone which effects molting of exoskeleton.
Mandibular organ is located at the base of mandibles and released the hormone Methyl Farnesoate (MF), which changes into Farnesoic Acid which is also called Molt Promoting Factor. It stimulates the Y-organ to release ecdysone, so that molting can take place.
In order to grow the larvae of crustaceans must molt several times to shed the exoskeleton. The above-mentioned glands, by secreting a complex of hormones and by feed-back mechanism, control the molting and growth process of the animal. These hormones loosen the connective tissue between the epidermis and cuticle. They also enhance the uptake of water to form and expand the new exoskeleton and quickly harden it to complete the molting process.