The following points highlight the nine important larval forms found in Crustacea. The larval forms are: 1. Nauplius Larva 2. Metanauplius Larva 3. Protozoaea Larva 4. Zoaea Larva 5. Cypris Larva 6. Mysis or Schizopod Larva 7. Megalopa Larva 8. Phyllosoma Larva 9. Alima Larva.
Larval Form # 1. Nauplius Larva:
Nauplius larva is egg-shaped and un-segmented. It has a broad anterior end with a median eye, large labrum and three paired appendages.
The median eye is characteristic of the nauplius larva and is often referred to as the nauplius eye, it is made usually of three but at times four ocelli which are pigmented cups with no lens, and are innervated by the protocerebrum. The median eye may degenerate or persist in the adult crustacean.
The appendages are uniramous antennules having two groups of sensory cells forming frontal organs, a pair of biramous antennae, and a pair of biramous mandibles for swimming, they have gnathobases directed towards the mouth, though the gnathobases of mandibles may be absent at first. A stomodaeum with mouth, proctodaeum with anus, and a midgut are also present.
A typical crustacean hatches as a free-swimming nauplius, but in Malacostraca (except in primitive forms) the nauplius is passed over as a stage within the egg membrane.
However, in certain crustaceans like Branchiopoda the nauplius metamorphoses directly into the adult but in majority of crustaceans it metamorphoses to adult through various intermediate larval stages like metanauplius, protozoaea, zoaea, cypris, mysis, megalopa, phyllosoma, alima, etc.
Larval Form # 2. Metanauplius Larva:
Metanauplius larva is like a nauplius, except that it shows some segmentation of the body, and there are four pairs of additional appendages of the thorax which shows some segmentation; these appendages are two pairs of maxillae and two pairs of maxillipedes. Some Notostraca, such as Apus, hatch as a metanauplius larva.
Larval Form # 3. Protozoaea Larva:
In marine prawns like Penaeus and some other decapods, the nauplius directly develops into protozoaea larva. The body of protozoaea is divisible into cephalothorax and abdomen. The cephalothorax is broad, segmented and covered with carapace.
The appendages that appeared in metanauplius become well developed and functional. The rudiments of other thoracic appendages also appear. The abdomen is unsegmented, without any appendage and has a forked telson.
Larval Form # 4. Zoaea Larva:
Zoaea larva has a well formed head with a long, median dorsal spine, two stalked compound eyes and one simple eye, all appendages from antennules to the last pair of maxillipedes are present, carapace is well formed and produced in front into a rostrum.
Thorax is un-segmented and rudimentary at its hinder end. Abdomen is well formed and six segmented, but it has no appendages except a forked telson. It swims by its biramous maxillipedes.
In Penaeus, protozoaea develops into zoaea. In some Anomura the egg hatches as a zoaea which passes through a metazoaea stage to become the adult. Metazoaea is, in fact, an advanced stage of zoaea but differs from it in having uniramous rudiments of thoracic appendages behind the maxillipede.
However, the third maxillipedes are biramous in hermit crab’s (Anomura) metazoaea and uniramous in that of crab (Brachyura). The abdominal appendages, i.e., pleopods also develop as buds. In some decapods, e.g., crabs, the life history starts from zoaea stage.
Larval Form # 5. Cypris Larva:
Cypris larva is covered by a bivalved shell having adductor muscle. Head has compound eyes, antennules with discs on which cement glands open, antennae are lost but remaining cephalic appendages are present, thorax has six pairs of biramous limbs, there is an abdomen of four segments. It has many adult features.
In Cirripedia, e.g., Lepas, the egg hatches as a nauplius, it changes into a cypris which gets fixed by discs of antennules with the secretion of cement glands, then it becomes a pupa which forms shell plates and rotates to assume the adult form.
Larval Form # 6. Mysis or Schizopod Larva:
Mysis or schizopod larva resembles an adult Mysis. Head and thorax have a carapace, all cephalic and thoracic appendages are present, but all thoracic appendages are alike and biramous with exopodites, abdomen has five pairs of pleopods and the sixth form uropods.
In some Decapoda, e.g., in Penaeus, a marine prawn, the egg hatches as nauplius, it passes by successive moults through zoaea stage, protozoaea stage and mysis stage which changes into an adult. In some lobsters, e.g., Homarus both nauplius and zoaea are passed within the egg, it hatches as a mysis larva which changes into an adult.
Larval Form # 7. Megalopa Larva:
Megalopa larva has a large un-segmented cephalothorax with all 13 pairs of appendages like those of a crab, abdomen is straight and in line with cephalothorax, it is like the abdomen of prawn with 6 pairs of well formed pleopods, In crabs the nauplius is passed in the egg, it hatches as a zoaea which by moulting forms the megalopa stage, the megalopa by moulting forms the adult.
In Decapoda there is a gradual abbreviation of development. Stages which are free larval forms in lower types of Crustacea are hurried through within the egg before hatching.
Larval Form # 8. Phyllosoma Larva:
In Palinurus (the rock lobster), the egg hatches directly into a delicate, transparent, extremely flattened leaf-like larva called phyllosoma or glass crab. This larva is large sized having three distinct regions in the body, the head, thorax and abdomen. An oval carapace covers the head and a part of thorax. It possesses a pair of stalked compound eyes placed anterolaterally in the head.
Thorax bears six pairs of appendages; the first thoracic or maxillipedes are rudimentary, second are uniramous, third well formed biramous, and remaining three (4th, 5th, and 6th) pairs are biramous legs which are enlarged. Abdomen shows segmentation but appendages are absent. This larva undergoes several moultings and transforms into the adult. Phyllosoma is, however, considered to be modified mysis larva.
Larval Form # 9. Alima Larva:
In some Malacostraca like Squilla, the egg directly hatches out in a young stage called alima larva. It is a pelagic form having slender body with a short but broad carapace. Its body is glassy and transparent. In addition to all cephalic appendages, only first two thoracic appendages are found.
The abdomen has distinct six segments with four or five pairs of pleopods. The alima larva is supposed to be modified zoaea stage but it differs strikingly from zoaea in having the armature of the telson and well developed large second maxillipedes.
In addition to these, the glaucothoe larva of hermit crab resembles the megalopa larva as described earlier. Likewise, the calyptosis larva of Euphausia (a malacostracan) is similar to the zoaea larva in all essential features except that it possesses sessile paired eyes in place of stalked eyes.
Importance of Larval Stages:
The importance of larval stages may be accounted as under:
1. They help in wide dispersal of the species.
2. The larval stages help in establishing relationships between various groups.
3. Occurrence of nauplius stage in all crustaceans connects the different representatives of this class together. As referred to, the nauplius establishes relationship of some obscure animals like Sacculina where adult has lost the characters of the class and even the phylum.
In fact, it is the presence of nauplius stage in its life history that connects Sacculina to class Crustacea and further the presence of cypris stage relates it to subclass Cirripedia.
4. If Haeckel’s law of recapitulation (which states that every organism during its development, i.e., ontogeny, repeats its evolutionary history, i.e., phylogeny) is considered true then it can be said that the nauplius stage represents the ancestral form of crustaceans because all crustaceans invariably pass through nauplius stage during their development.
Hence, it can be concluded that present day crustaceans have evolved through nauplius stage.