Arthropoda: Larval forms and parasitism in Crustacea | Zoology Optional Notes for UPSC PDF Download

Larval Stages in Crustacea: An Overview

• Crustaceans, a group of arthropods with chitinous exoskeletons, face challenges in growth due to their protective exoskeletons. They must shed their exoskeletons in a process called ecdysis to facilitate growth.
• Larval stages of crustaceans are critical for their development into adults, involving multiple molting events, each changing their structure and size.
• Different species of crustaceans exhibit various successive larval stages, leading to adult forms.
• Crustaceans can have both direct and indirect development. Direct development results in hatching eggs resembling adult forms, while indirect development involves larval stages that transform into adults through metamorphosis.

Direct vs. Indirect Development

• Direct development: Egg hatches into young resembling adult structure. Subsequent growth and differentiation lead to adult forms.
• Indirect development: Larval stages differ significantly from adults and achieve adulthood through metamorphosis.

1. Nauplius Larva

• Discovered by Muller in the 18th century, Nauplius is the initial and essential larval stage in most crustaceans.
• Characterized by an oval body with a large cephalothorax and rudimentary abdomen.
• Features three pairs of appendages: antennules, antennae, and mandibles, with the last two pairs used for swimming.
• A single median eye is present.
• Well-developed digestive system for plankton feeding.
• Some crustaceans proceed directly from nauplius to adulthood, while others go through intermediate larval forms.

2. Metanauplius Larva

• In specific Branchiopods like Apus, nauplius larvae transform into metanauplius.
• Slightly larger than nauplius, with an anterior oval cephalothorax and segmented abdomen ending in a caudal fork.
• Frontal sense organs on the anterior end.
• Development of 4 pairs of appendages that later become maxillae and 2 pairs of maxillipedes in adults.

3. Cypris Larva

• Found in Sacculina, Balanus, and Lepas.
• Develops from the nauplius larva.
• Free-swimming and triangular with a bivalved shell.
• Features seven pairs of appendages, including antennules and six pairs of thoracic appendages.
• Contains a mass of germ cells.
• Undergoes remarkable metamorphoses to become sessile adult forms.

4. Protozoea Larva

• Marine prawns and lobsters hatch into protozoea.
• Possesses a large cephalothorax and an unsegmented abdomen.
• Antennule is segmented and uniramous, while the antenna is biramous.
• Two pairs of maxillae and two pairs of maxillipedes aid in food handling.
• Protozoea undergoes development towards adult forms, growing segmented appendages.

5. Zoea Larva

• The common larva of decapods with variations among different species.
• Characterized by a large cephalothorax covered with a helmet-like carapace and spines.
• One pair of compound eyes.
• Antennule and antenna are sensory in function.
• First and second maxillipedes are biramous and used for swimming.
• Abdomen is 6-segmented with a caudal furca and telson.
• In some Malacostraca, zoea can transform into metazoea, which develops abdominal appendages for swimming.

6. Alima Larva

• Alima larva of Squilla emerges directly from the egg.
• A modified form of the zoaea larva.
• It is pelagic, transparent, and found in plankton in large numbers.
• Features a slender form and a broad carapace.
• All head appendages are present.
• A 6-segmented abdomen with 4-5 pairs of pleopods and distinct features such as an armed telson and large raptorial second maxillipedes.

Megalopa Larva: Transition to Crab Adulthood

• In true crabs, the zoaea larva or metazoaea larva undergoes successive molts to transform into the post-larval megalopa stage.
• The megalopa features a broad, unsegmented cephalothorax with a distinctive anteriorly produced median spine on the carapace.
• It boasts large, stalked, compound eyes.
• All thoracic appendages are well-formed, with the last 5 pairs being uniramous.
• The abdomen is well-developed, straight, and includes biramous pleopods.

Mysis Larva: Transformation in Shrimps and Lobsters

• In shrimps and some lobsters, the zoea metamorphoses into mysis.
• The mysis larva has a cylindrical and elongated body with a cephalothorax and a 6-segmented abdomen.
• The carapace is extended into a pointed rostrum in the front.
• Antennule and antenna serve sensory functions.
• Six pairs of biramous thoracic appendages facilitate locomotion, while the abdomen features 6 pairs of abdominal appendages for swimming, with the last pair modified as a uropod.
• A pointed telson is present at the tip of the abdomen.

Phyllosoma Larva: The Glass-Crab of Rock Lobsters

• In the rock-lobster, also known as Palinurus, the newly hatched larva is termed the phyllosoma larva or glass-crab.
• It represents a significant modification of the mysis stage.
• The phyllosoma larva stands out for its large size, extreme flatness, leaf-like delicacy, and glassy transparency.
• A narrow constriction distinguishes the head from the thorax.
• A large oval carapace covers the head and the first two thoracic segments.
• Compound eyes are supported by prominent stalks.
• The newly hatched larva initially possesses only the anterior 6 pairs of thoracic appendages.
• The first thoracic appendages or maxillipedes are rudimentary in Palinurus or absent in Scyllarus, while the second pair is uniramous.
• Four pairs of very long and biramous legs with exopodites succeed the second pair, with the last two pairs of thoracic appendages usually being absent.
• The abdomen, although indistinctly segmented, is very small and lacks limbs.
• The phyllosoma larva undergoes multiple molts before reaching the adult form.

Significance of Crustacean Larval Forms

• Larval forms are valuable in determining homologies and affinities among different groups of crustaceans.
• Species that pass through similar larval stages are considered closely related.
• Larvae play a role in the wide distribution of species and help maintain minimal food reserves in eggs.
• The development of crustaceans exemplifies Haeckel's biogenetic law or recapitulation theory, with larval stages reflecting increasing complexity before reaching the adult form.
• Nauplius is a common occurrence in the development of all crustaceans and is viewed as a representative of an ancestral form.
• Larval stages aid in identifying homologies and affinities among various groups, facilitating taxonomic classification and evolutionary studies.
• Larvae promote the dispersal of species, preventing overcrowding and conserving energy by keeping food reserves in eggs to a minimum.