larval forms of crustaceans

introduction of arthropoda and its larval forms

1. Larval forms of crustaceans By Samina HUSAIN

2. SYNOPSIS Introduction Larval forms of CRUSTACEA: Nauplius larva included in ORDER: DECAPODA Metanauplius larva included in ORDER: NOSTOSTRACA Cyris larva included in ORDER:RHIZOCEPHALA Protozoaea larva included in ORDER :EUPHAUSIACEA Zoaea larva included in ORDER :DECAPODA Alima larva included in ORDER:STOMATOPODA Megalopa larva included in ORDER :DECAPODA Mysis larva included in ORDER:DECAPODA Phyllosoma larva included in ORDER: DECAPODA Significance of larval forms of CRUSTACEA Conclusion

3. INTRODUCTION Crustaceans are Arthropods whose body is covered with chitinous exoskeleton for protection. But the same exoskeleton does not allow body growth and hence must be shed in order to allow growth. The larval stages feed and grow in order to become adults and must undergo moulting or ecdysis to grow. After each moulting they change their structure and size and hence are different from the previous stage. Crustacea was created by CUVIER & LAMARCK. Custacea show both direct & indirect development. The offspring that hatching from eggs resemble to their parent in general structure, such development is called Direct or Epimorphic development. when newly hatched young one is different from their adult, it acquire adult hood after no. of changes, such development called Indirect or Metamorphic Development

4. Different larval forms of CRUSTACEA ..

5. NAUPLIUS LARVA Discovered by Muller in 18 th  century, the Nauplius larva is the first fundamental stage in all crustaceans . Body is oval in shape and unsegmented with a large cephalothorax and rudimentary abdomen.  There are three pairs of appendages, namely, antennules, antenna and the mandible; the last two pairs are biramous and are used for swimming. There is a single median eye. It has a well developed digestive system for feeding on planktons. Posterior end has Caudal fork. The nauplius larva grow & undergo several moults & finally develop into next larva called Metanauplius.

6. NAUPLIUS LARVA

7. METANAUPLIUS LARVA In some Nostostraca such as  Apus,  the nauplius larva transforms into metanauplius, which is slightly larger than nauplius and has cephalothorax and abdomen and a caudal furca . It also has a single median eye. Antennule is uniramous and sensory but antenna is large, biramous and locomotory in function . Mandibles reduce in size and are used for chewing food.

8. METANAUPLIUS LARVA

9. 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 .

10. CYPRIS LARVA

11. P ROTOZOAEA LARVA The Metanauplius is succeeded by Protozoaea larva stage with seven pair of appendages. The dorsal shield or carapace has become enlarge & cover the dorsal surface anteriorly. Seven pair of appendages present in metanauplius become well developed & capable of movement. Protozoaea is swims by the means of their antennae.

12. PROTOZOAEA LARVA

13. ZOAEA LARVA Another important larva of crustacea is zoaea larva. example crab in which this larval stage is found. Zoaea is the common larva of decapods , It has a large cephalothorax that is covered with a helmet-like carapace that also sports spines and it protruded into a rostrum in front. There is one pair of compound eyes. Antennule and antenna are short and sensory in function. First and second maxillipedes are large and biramous, used for swimming. Thoracic appendages are still in bud form and non-functional.  Abdomen is 6-segmented, without appendages and has a caudal furca on the tip along with a telson.

14. ZOAEA LARVA

15. ALIMA LARVA The Alima larva of SQUILLA, which hatch out from egg is modified zoaea. Its is pelagic lavra having glass like transparency & long slender body with short & broad carapace. Anterior cephalothorax produced into Rostrum. The abdomen is segmented & has four pair of appendages,& end in telson.

16. A LIMA LARVA

17. MEGALOPA LARVA In crabs, zoaea 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 cephalothorax is covered by brood carapace. 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. 

18. MEGALOPA LARVA

19. MYSIS LARVA In some crustacea ,Zoaea larva develop into Mysis. In penaeus the Mysis larva are elongated, laterally compressed & transparent. Anteriorly the carapace produced into spines called Rostrum. They have five pair of cephalic appendages, eight pair of thoracic appendages, six pair of abdominal appendages Abdomen ends into Telson..

20. MYSIS LARVA

21. PHYLLOSOMA LARVA In lobsters, the egg hatches into phyllosoma larva in which body is divisible into head, thorax and abdomen. There is a pair of stalked compound eyes and a pair each of antennules and antenna as sense organs. Body is dorsoventrally flattened and transparent.  The first maxillipede is rudimentary and the second one is uniramous. The third maxillipede is large, biramous and is used for swimming.  The abdomen is small, segmented and does not bear appendages. Three pairs of thoracic appendages are very long and their tips are flattened oar-like for swimming near the surface of water.

22. PHYLLOSOMA LARVA

23. SIGNIFICANCE They help in the dispersal of species. They help to study the different group of crustacea. The larval stages are useful in finding the homologies & affinity of various group.

24. CONCLUSION Crustacea in their development under go various larval stages with increasing complexity. As the nauplius larva present in majority of crustacea life cycle. Hence it is believed that all crustacea might have evolved from the common ancestor which is resemble to nauplius larva. The other larva( metanuaplius , mysis, zoaea etc.)show the stages of evolution of higher crustacea f rom the nauplius like ancestor.

25. REFRENCES BOOKS: Author Invertebrate Zoology by Robert Debornis Hyman Series of by Hyman Arthropod

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