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The following points highlight the fifteen different types of Crustacean larva. The types are: 1. Nauplius Larva 2. Metanauplius Larva 3. Cypris Larva 4. Protozoea Larva 5. Zoea Larva 6. Mysis Larva 7. Metazoea Larva 8. Megalopa Larva 9. Kentrogen Larva 10. Epicaridian Larva 11. Cryptoniscus Larva 12. Erichthoidina Larva 13. Erichthus Larva 14. Alima Larva 15. Phyllosoma Larva.
Crustacean Larva: Type # 1. Nauplius Larva:
Features:
1. The free-swimming larva is oval and the body is divisible into three regions—head, trunk and anal region.
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2. A median frontal eye is present in the head region.
3. Three pairs of appendages are present, of which the first pair are un-jointed and present in the head region. It develops into the antennule of the adult.
4. The remaining two pairs are biramous and are present in the trunk region.
5. The first pair of trunk appendages which act as locomotor organs in larvae, transform into antennae and the second pair become the mandibles of the adult.
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6. Mouth is present in between the two trunk appendages and is enclosed by a prominent labrum.
7. Alimentary canal is straight and terminates at the posterior-most end of the anal segment through the anus.
8. Larva is un-segmented and devoid of ventral nerve cord and heart.
Modifications:
In all Crustaceans the nauplius is the first larval stage of development. But its appearance-varies in different forms. In Cladocera, nauplius stage appears inside the egg. In Ostracoda nauplius, pair of bivalved shells and uniramous appendages is present. In the Cirripedia, a number of spines are present in the nauplius. Alimentary canal is absent in Sacculina nauplius. In Isopoda, nauplius is maggot-like.
Crustacean Larva: Type # 2. Metanauplius Larva:
Features:
1. The oval body is divided into cephalothorax and rudiments of abdomen.
2. Presence of a fourth pair of appendages (first maxilla of the adult) in addition to the original three pairs of the Nauplius is observed.
3. Two truncated processes at the posterior end are present.
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This stage is seen in Triops (Branchiopoda) and Penaeus (Malacostraca).
Crustacean Larva: Type # 3. Cypris Larva:
Features:
1. It develops from nauplius larva.
2. Body is enclosed in a bivalve shell.
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3. A median and two compound eyes are present.
4. Anterior antennae are four jointed and each bears a characteristic disc.
5. Posterior antennae are absent.
6. Six pairs of biramous thoracic appendages are present.
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7. Short abdomen ends in a caudal fork.
8. The larva contains a mass of germ cells.
It occurs in Sacculina and barnacles (Cirripedia).
Crustacean Larva: Type # 4. Protozoea Larva:
Features:
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1. First antenna is four-jointed.
2. Second antenna is with three-jointed endopodite and four-jointed exopodite.
3. Mandibles are palpless, toothed and masticatory in function.
4. Two anterior pairs of maxillipeds are biramous.
5. Abdomen is incompletely segmented.
6. Impressions of paired eyes are visible. It is found in Penaeus.
Crustacean Larva: Type # 5. Zoea Larva:
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Features:
1. Body is divided into distinct cephalothorax and segmented abdomen.
2. The cephalothorax is covered by a carapace.
3. Compound eyes are prominent and stalked.
4. Head bears a dorsal, a frontal (rostrum) and two lateral spines.
5. It also bears antennules, antennae, mandibles, maxillae and two pairs of well-developed maxillipeds.
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6. Thoracic appendages appear as buds.
7. Limbless abdomen is with six segments including a caudal fork.
This stage appears in Penaeus and crabs (Malacostraca). In Euphausia corresponding stage is Calyptopis.
Crustacean Larva: Type # 6. Mysis Larva:
Features:
1. Body is divisible into cephalothorax and abdomen.
2. Presence of a pair of stalked compound eyes is encountered.
3. Biramous thoracic appendages are present.
4. The exopodites of thoracic appendages are reduced.
5. The abdomen is six-segmented with a telson.
It appears during development in Penaeus.
Crustacean Larva: Type # 7. Metazoea Larva:
Features:
1. Body is divisible into cephalothorax and abdomen.
2. The abdomen has a pair of telsons.
3. Thoracic limbs are well-developed.
4. Abdomen is with biramous pleopods.
This stage is distinct in the life history of corystes (Malacostraca).
Crustacean Larva: Type # 8. Megalopa Larva:
Features:
1. Cephalothorax is well-developed.
2. The cephalothorax is covered by a broad carapace.
3. The carapace is produced into a median spine anteriorly.
4. Large stalked eyes are distinctly visible.
5. Thorax is with five pairs of walking legs.
6. Functional swimmerets are present on the abdomen.
It is commonly seen in the development of crabs (Malacostraca).
Crustacean Larva: Type # 9. Kentrogen Larva:
Features:
1. Body is sac-like and elongated.
2. Undifferentiated mass of cells is present inside the body.
3. It has a chitinous tube called dart.
4. Presence of root-like processes.
5. All crustacean characteristics lost due to parasitism.
It is seen in the parasitic form like Sacculina (Cirripedia).
Crustacean Larva: Type # 10. Epicaridian Larva:
Features:
1. Two pairs of antennae are prominent.
2. A pair of frontal processes and a pair of mandibles are present.
3. Presence of six pairs of limbs in the thorax.
4. Abdomen includes six pairs of biramous pleopods.
5. This larva transforms into Cryptoniscas larva.
It occurs in parasitic Isopods.
Crustacean Larva: Type # 11. Cryptoniscus Larva:
Features:
1. Like epicardian larva cryptoniscus larva has two pairs of antennae.
2. One pair of frontal process, a pair of mandibles and six pairs of thoracic limbs are present.
3. Abdomen is six jointed.
4. The body is fusiform.
It is seen in parsitic isopod.
Crustacean Larva: Type # 12. Erichthoidina Larva:
Features:
1. The body is divisible into cephalothorax and abdomen.
2. Dorsal shield is provided with frontal spike and two lateral and a median spike.
3. Eyes are large.
4. Five pairs of thoracic appendages are present.
5. A long un-segmented tail is present. Erichthoidina larva changes into Erich thus larva.
Crustacean Larva: Type # 13. Erichthus Larva:
Features:
1. Body is divisible into cephalothorax and abdomen.
2. Anterior thoracic limbs (3 pairs) are reduced.
3. It is found in some Stomatopods (Malacostraca).
The Ericthus arises from Ericthoidina larva.
Crustacean Larva: Type # 14. Alima Larva:
Features:
1. Body is more or less like Zoea or Erichthus.
2. It has a glass-like transparency and has a slender body.
3. The body is divisible into cephalothorax and an abdomen.
4. The cephalothorax is covered by a short and broad carapace.
5. No trace of three posterior pairs of maxillipeds or of the three following pairs of thoracic appendages.
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6. The tail has the full complement of segments with the normal number of well-developed swimming feet.
7. The younger stages of Alima larva are not known.
It is seen in Squilla (Malacostraca).
Crustacean Larva: Type # 15. Phyllosoma Larva:
Features:
1. Body is more or less my sis-like.
2. It is divisible into 3 regions—an anterior cephalic, a middle thoracic and a posterior small abdomen.
3. The three-body regions are dorsoventrally compressed and glass-like transparent.
4. The head is round and leaf-like flattened thorax with biramous appendages.
5. A pair of compound eyes with long stalks is present.
6. Mouth leads into a stomach from which an anterior and lateral hepatic diverticulum springs out on each side.
7. First pair of antennae is elongated and unjoin ted.
8. The second pair of antennae is short.
9. The mandibles, anterior pair of maxillae, second pair of maxillae are present.
10. The third pair of maxillipeds is uniramous.
11. The abdomen is short and narrow. This appears in Scyllarus and Palinurus (Malacostraca).
