Crustaceans show both direct and indirect development. In direct development (e.g„ Palaemon, crayfish), the adult is attained by progressive growth and differentiation of the embryo, so that the newly hatched young resembles the parents in general structure. In indirect development, there is a larval stage which differs from the adult in many features and acquires adulthood through metamorphosis.
Many of the crustaceans undergo indirect development, involving a wide variety of larval forms. Of these 3 main larval forms are nauplius, zoaea and megalopa. Intermediate stages receive different names such as metanauplius, cypris and protozoaea. Modified and distinctive forms of zoaea are given special designation, such as mysis of lobsters, phyllosoma of spiny lobster, and allima of squilla.
LARVAE OF CRUSTACEA
- Characteristic of the class, nauplius is the simplest and commonest type or larva, found in most marine crustaceans and a few malacostracans.
- When development proceeds through many larval forms, the nauplius is the earliest and the basic larva.
- It is free swimming larvae.
- The body is minute with 3 indistinct regions, a single median eye and three pairs of jointed appendages — the uniramous antennules, mainly the balancing organ; biramous antennae, principal locomotor organs and mandibles, which along with antennae may share for food collection.
- In branchiopods the nauplius develops straight away into the adult, but in mostly other crustaceans it may give rise to other intermediate larval forms, such as metanauplius, protozoaea, zoaea, mysis, etc.
- Metanauplius is the later nauplius instar and results by the process of moulting and growth.
- Its body is divisible into a broad cephalothorax and an elongated abdomen. terminating into a pair of caudal forks.
- Besides the three pairs of nauplius appendages, it also bears the rudiments of four pairs of appendages, which are two pairs of maxillae and two pairs of maxillipedes of the adult.
- Some decapods, stomatopods and some notostracans (e.g., Apus) begin their life history with the free-swimming metanauplius larva.
- In case of marine prawns (e.g., Penaeus), and sergestid decapods, the earliest nauplius, by growth and moulting, develops into a protozoaea larva.
- Its body is divisible into a broad segmented cephalothorax covered with a small carapace and a slender abdomen which is unsegmented and bear no appendages terminating in a forked telson.
- There is a single median nauplius eye and the appendages comprise of the antennules, antennae, mouthparts and first and second maxillipedes, The protozoaea later modifies into the zoaea.
- In almost all marine decapods, except peneids and sergestids, hatching takes place at the zoaea stage (as in true crabs).
- Zoaea has a broad cephalothorax and a curved abdomen. which assists in swimming is provided with a forked telson.
- Helmet-like carapace bears two long spines, a median dorsal and a median rostrum, two lateral spines are often met with.
- A pair of large stalked movable compound eyes are present.
- In addition to protozoaeal appendages there appear rudiments of thoracic appendages.
- Biramous maxillipedes are used for swimming.
- In Cirripedia (e.g. Lepas, Sacculina), the nauplius larva passes into the Cypris Stage.
- In this form the body and appendages are enclosed within a bivalved shell provided with adductor muscle as is seen in an ostracod adult, Cypris.
- Its modified antennules have cement glands at their bases.
- All Other cephalic appendages with a compound eye only, except antennae, are present.
- Six pairs of biramous thoracic limbs are formed.
- It has abdomen with 4—5 segments.
- Mysis or Schizopod. In peneid decapods (e.g. penaeus) and lobsters zoaea is modified into Mysis; or Schizopod larva.
- It bears 13 pairs of appendages and resembles adult Mysis.
- It has 5 pairs of posterior biramous thoracic appendages.
- Abdomen is posterior similar to that of adult with 5 pairs of biramous pleopods and a pair of uropods and telson.
- In some lobsters mysis marks the begining of the life history as the naupleus and zoaea are passed within the egg but at the same time it marks the end of the life history of a prawn.
- In brachyuran decapods (true crabs), zoaea metamorphoses into the megalopa larva.
- It resembles, to some extent, the adult crab and possesses all 13 pairs of appendages.
- Abdomen bears 6 pairs of pleopods and is placed straight in line with cephalothorax.
- In crabs nauplius Stage is passed within egg which hatches as zoaea.
- It then by moulting forms megalopa to be metamorphosed into adult.
In hermit crabs, the glaucothoe corresponds to a megalopa with symmetrical abdomen and swimming pleopods.
- Larva of Palinurus, the spiny crab or rock lobster, is called Phyllosoma or glass crab.
- It is a modified mysis stage. It is remarkably large, flattened, leaf like, delicate and glassy.
- Body is distinguished into head, a transparent thorax and abdomen.
- Eyes are compound and stalked.
- Out of six pairs of thoracic appendages, the first or maxillipedes are rudimentary, second are uniramous, third well formed biramous succeeded by rest 3 (4th, 5th and 6th) pair of long biramous legs.
- A segmented but limbless abdomen is present.
- Before reaching an adult Stage, it undergoes several moultings.
- It is modified form or zoaea found in some malacostracan (e.g. Squilla) which hatches from egg.
- It is a pelagic form with glassy transparency having a slender body.
- It bears short and broad carapace.
- It has all the cephalic appendages but only first two thoracic ones.
- A six segmented abdomen with 4 or 5 pairs of pleopods, is present.
- It differs from zoaea in having well formed second maxillipedes and the armature of the telson.
Significance of larval forms of Crustacea
According to the biogenetic law or recapitulation theory of Haeckel, every organism during its development (ontogeny), repeats to some extent its evolutionary history (phylogeny). In other words, successive stages of individual development correspond with successive adult ancestors in the of evolutionary descent. Due to its occurrence in the development all Crustacea, the nauplius was previously regarded to be representing the ancestral form of Crustacea.
It was presumed that from this ancestral form the present day crustaceans evolved phylogenetically. In the other words, the other larval forms (zoaea, megalopa, etc.) show stages of evolution of the higher crustaceans from nauplius-like ancestors. But the old idea of recapitulation stands greatly modified nowadays and the crustacean larval forms are now regarded to be the larval reversions to the types much simpler than the crustacean ancestors. The larval stages arc useful for finding out the homologies and the affinities among various groups. The animals which pass through similar stages are closely related.
Larvae are helpful in the wide distribution of species and in keeping the food reserves of eggs to a minimum.