Phylum Arthropoda: Larval forms in Crustaceae
Posted on : 09-03-2018 Posted by : Admin

Introduction

The animals belonging to class Crustacea shows both direct and indirect development. In the direct development, the egg hatches into young one resembling adult in general structure. Progressive growth and differentiation transforms the young ones into adult.

Whereas indirect development includes larval stages which later become adults. These larval stages are different from the adult in form and structure. The larval stages achieve adulthood through the process of metamorphosis. The following is the detailed explanation of each of the larval forms of crustaceans.

Nauplius larva

• It is the first larvae hatched from egg in most of the crustaceans.
• It is free swimming larvae.
• It is minute and microscopic.
• The body has indistinct regions like a simple median eye also called as nauplius eye, three pair of jointed appendages (uniramous antennule, biramous antennae and mandible).
• Mandibles along with antennae are helpful in food collection.
• In some forms nauplius larva develops straight away into adult, but in many other crustacean forms it gives rise to other intermediate larval forms like metanauplius, protozoaea, zoaea, crypsis, mysis, megalopa, phyllosoma, alima

Metanauplius larva

• It is the larva of Apus.
• It is the second larval stage which develops from the nauplius larva.
• The body has an anterior oval cephalothorax, an elongated trunk-region and an abdomen terminating in a caudal fork provided with setae.
• The anterior end has a pair of frontal sense organs.
• Dorsal shield of the head grows back to form carapace.
• The larvae has three pair of appendages just as in nauplius, it also develops the rudiments of 4 pairs of appendages, which later become the maxillae and 2 pairs of maxillipedes of the adults.

Cypris larva

• It is the larvae of Sacculina, Balanus and Lepas.
• It develops from nauplius
• It is a free swimming larva.
• It is triangular in shape with bivalent shell.
• The larva has seven pairs of appendages, namely a pair of antennules and six pair of thoracic appendages.
• A median eye is present.
• The larva contains a mass of germ cells.
• It undergoes a remarkable series of metamorphoses to become the sessile adult form.

Protozoaea larva

• The metanauplius larva is succeeded by the protozoaea stage
• It is divisible into broad segmented cephalothorax covered with a small carapace and a slender abdomen which is unsegmented.
• Abdomen terminates in a forked telson.
• The carapace becomes enlarged and covers the dorsal surface anteriorly.
• The 7 pairs of appendages present in the metanauplius become well-developed and capable of movements.
• The rudiments of paired lateral eye begin to appear near the median eye.
• The rudiments of the remaining posterior six thoracic segments are also marked off, but the abdomen is still unsegmented and without limbs.
• The protozoaea swims by antennae.
• Marine prawns, Penaeus hatch in to protozoaea larva.

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Zoaea larva

• Zoaea is the second important larvae of the Crustacea, after the nauplius larva.
• Protozoaea stage is succeeded by the zoaea stage.
• The zoaea is characterized by a distinct cephalothorax and abdomen, 8 pair of appendages and buds of 6 more, and resembles the adult Cyclops.
• The cephalothorax is immensely developed and covered by a helmet-like carapace, which is produced into two long spines, an anterior median rostral and a posterior median dorsal.
• Two lateral spines are also present.
• The paired lateral and stalked compound eyes become well-formed and but remaining 6 pair of thoracic appendages appears in the form of bud.
• The long abdomen is distinctly made of 6 segments, and terminates in a caudal furca and still lacking in appendages.
• Zoaea swims by means of thoracic limbs.

Mysis larva

• In Penaeus, the zoaea larva, instead of converting into the megalopa stage, moults into the post larval mysis larva.
• It has 13 pairs of appendages. All the thoracic appendages are biramous. Even the 5 pairs of posterior thoracic legs are biramous with flagellar exopodites which take up the locomotory function.
• The abdomen develops similar to that of the adult form, with 5 pairs of biramous pleopods and a pair of uropods and a telson.
• The mysis larva metamorphosis in to the adult prawn by the loss of the exopodites on the thoracic legs.

Megalopa larva

• In true crabs, the zoaea larva or metazoaea larva passes through successive moults into the post larval megalopa stage.
• It has a broad and crab-like unsegmented cephalothorax.
• The carapace is produced anteriorly into a median spine.
• The eyes are large, stalked and compound.
• All the thoracic appendages are well formed of which the last 5 pairs are uniramous.
• The abdomen is also well formed, straight and bears biramous pleopods.

Phyllosoma larva

• In the rock- lobster (Palinurus), the newly hatched larva is called the phyllosoma larva or glass- crab
• It is a greatly modified mysis stage.
• It is a remarkable for its large size, extremely flattened and leaf- like delicate form and glassy transparency.
• A narrow constriction demarcates the head from thorax.
• A large oval carapace covers the head and the first two thoracic segments.
• The eyes are compound and borne by large stalks.
• Only anterior 6 pairs of thoracic appendages are present in the newly hatched larva.
• The first thoracic appendages or maxillipedes are rudimentary (Palinurus) or absent (Scyllarus) and the second are uniramous; succeed by 4 pairs of very long and biramous legs with exopodites.
• Last two pairs of thoracic appendages are usually absent.
• Abdomen, though indistinctly segmented is very small and limbless.
• Phyllosoma undergoes several moults before reaching the adult form.

Alima larva

• The so-called alima larva of Squilla hatches out from the egg directly
• It is a modified zoaea larva form.
• It is apeagic larva, having a glass-like transparency and occurring in large numbers in the plankton. It has a slender form, and a sort and broad carapace. All the head appendages are present. But only is 6-segmented, having 4 or 5 pairs of pleopods. The alima larva differs from the zoaea larva in the armature of the telson and a very large raptorial second maxillipedes.

1. Discuss the Importance of larval forms in phylum Arthropoda.
   
2. Write few points about Alima larva.
   
3. Differentiate between the Nauplius and Metanauplius larval forms.
4. Write about phyllosoma Larval form. Why is it called so?
   
5. Differentiate between the Zoea and Protozoea larval forms.

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