CRUSTACEAN LARVAE

• 1 CRUSTACEAN LARVAE
o 1.1 INTRODUCTION
 1.1.1 NAUPLIUS
 1.1.2 METANAUPLIUS
 1.1.3 CYPRIS
 1.1.4 'PROTOZOEA
 1.1.5 ZOEA
 1.1.6 METAZOAEA
 1.1.7 CALYPTOSIS
 1.1.8 ERICHTHUS
 1.1.9 ALIMA
 1.1.10 MEGALOPA
 1.1.11 GLAUCOTHOEA
 1.1.12 MYSIS
 1.1.13 PHYLLOSOMA
CRUSTACEAN LARVAE
INTRODUCTION
Crustaceans, are large number of arthropods, comprising almost 52,000 described species. The majority of them are aquatic living in either marine or fresh water environments, but a few groups have adapted to life on land, such as terrestrial crabs , terrestrial hermit crabs and wood lice. Crustaceans show both direct and indirect development. In most crustacea, development is accompanied by little or more metamorphosis and the various stages of development are known as larvae. Several larval forms are met within Crustacea and special terms are applied to each one of them
NAUPLIUS


It is the simplest, commonest and earliest larval form in crustacea. Nauplius is a microscopic, oval or pear shaped with an unsegmented body having a broad anterior head region, an intermediate trunk-region and a posterior bilobed anal region. It has 3 pair of unjointed appendages bearing swimming seate. The first pair is uniramous and become the antennules of the adult. Second is antennaryand third is mandibular called antennae and mandibles respectively in the adult. The head region bears a conspicuous sessile median eye. The mouths open anteriorly between the bases of antennary and mandibular feet. While the anus lies at the extremity of the caudal region. The alimentary canal is straight and made of foregut, midgut and hindgut. However mouth and alimentary canal are lacking in nauplius of Cirripedia. The larva is without a heart and a segmented ventral nerve cord.

METANAUPLIUS

It follows the nauplius, is a vaguely defined stage. It consists of an oval cephalothorax, an elongated trunk-region and an abdomen terminating in a caudal fork provided with setae. Dorsal shield of the head grows back to form carapace. In addition to the 3-origional appendages of nauplius, it also develops the rudiments of 4 pairs of appendages, which become the maxillulae the maxillae and the first 2 pairs of maxillipedes of the adult.
Branchiopoda, Cephalocardia and perhaps some Stomatopoda hatch the metanaupliu
CYPRIS


In some Cirripedia (Sacculina, Lepas), the nauplius passes into the cypris stage, in which the body and the appendages are enclosed within bivalved carapace with an adductor muscle to close it. Its modified antennules have of cement glands at their bases. It undergoes a remarkable series of metamorphoses to become the sessile adult form. In Ostracoda, the eggs typically hatch in the cypris form.
PROTOZOEA
The metanauplius larva is succeeded by the protozoaea stage with 7 pairs of appendages and the beginning of segmentation. The carapace become enlarged and covers the dorsal surface anteriorly . The 7 pairs of appendages present in the metanauplius (up to 2nd maxillipede) 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. In Penaeus hatch in the protozoaea.
ZOEA
Zoaea is the second important larvae of the Crustacea, the first being the nauplius. Protozoaea stage is succeeded by the zoaea stage. The zoaea is characterized with 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 usually also present. The paired lateral and stalked compound eyes become well formed and 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, but still lacking in appendages. Zoaea swims by means of thoracic limbs.

METAZOAEA
The older zoaea or metazoaea has well formed third maxillipedes, which are biramous and swimming organs in Anomura, but uniramous and non-swimming in Brachyura. The 6 pairs of abdominal appendages also appear in the form of buds.
CALYPTOSIS
In Euphausiacea, one of the larval stages is termed calyptopsis .It is similar in all respects to a typical zoaea except that the paired 'eyes are not stalked but sessile.
ERICHTHUS
In erichthus larva, a carapace covers the greater part of the body. Head is unsegmented, bearing median and paired eyes and all the 5 pairs of cephalic appendages. The thorax is made of segments, free from the carapace ,and bearing anterior 5 pairs of biramous swimming appendages .The broad abdomen is unsegmented and with a single pair of appendages. Such larvae are met with in Lysiosquilla.
ALIMA
The so-called alima larva of Squilla which hatch out from the egg directly, is a modified zoaea 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.

MEGALOPA


In true crabs, the zoaea larva or metazoaea passes through successive moults into the post larval megalopa stage. It has a broad and crablike 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.
GLAUCOTHOEA
In hermit crabs ,the metazoaea leads to the glaucothoe stage. It corresponds to the megalopa stage of Bracchyura with a large symmetrical abdomen and a full complement of adult appendages.
MYSIS
In Penaeus, the zoaea , instead of converting into the megalopa stage, moults into the postlarval mysis larva with 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 uptill now now fulfilled chiefly by the antennae. The abdomen develops similar to that of the adult form, with 5 pairs of biramous pleopods and a pair of uropods and telson .the mysis larva metamorphosis in to the adult prawn by the loss of the exopodites on the thoracic legs.
PHYLLOSOMA
In the rock- lobster (palinurus), the newly hatched larva, called the phyllosoma or glass- crab, 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 notatory 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.



















Larval Forms Found in Crustacea | Invertebrate Zoology
The following points highlight the nine important larval forms found in Crustacea. The larval forms are: 1. Nauplius Larva 2. Metanauplius Larva 3. Protozoaea Larva 4. Zoaea Larva 5. Cypris Larva 6. Mysis or Schizopod Larva 7. Megalopa Larva 8. Phyllosoma Larva 9. Alima Larva.
Larval Form
1. Nauplius Larva:
Nauplius larva is egg-shaped and un-segmented. It has a broad anterior end with a median eye, large labrum and three paired appendages.
The median eye is characteristic of the nauplius larva and is often referred to as the nauplius eye, it is made usually of three but at times four ocelli which are pigmented cups with no lens, and are innervated by the protocerebrum. The median eye may degenerate or persist in the adult crustacean.
The appendages are uniramous antennules having two groups of sensory cells forming frontal organs, a pair of biramous antennae, and a pair of biramous mandibles for swimming, they have gnathobases directed towards the mouth, though the gnathobases of mandibles may be absent at first. A stomodaeum with mouth, proctodaeum with anus, and a midgut are also present.
A typical crustacean hatches as a free-swimming nauplius, but in Malacostraca (except in primitive forms) the nauplius is passed over as a stage within the egg membrane.
However, in certain crustaceans like Branchiopoda the nauplius metamorphoses directly into the adult but in majority of crustaceans it metamorphoses to adult through various intermediate larval stages like metanauplius, protozoaea, zoaea, cypris, mysis, megalopa, phyllosoma, alima, etc.
Larval Form # 2. Metanauplius Larva:
Metanauplius larva is like a nauplius, except that it shows some segmentation of the body, and there are four pairs of additional appendages of the thorax which shows some segmentation; these appendages are two pairs of maxillae and two pairs of maxillipedes. Some Notostraca, such as Apus, hatch as a metanauplius larva.

Larval Form # 3. Protozoaea Larva:
In marine prawns like Penaeus and some other decapods, the nauplius directly develops into protozoaea larva. The body of protozoaea is divisible into cephalothorax and abdomen. The cephalothorax is broad, segmented and covered with carapace.
The appendages that appeared in metanauplius become well developed and functional. The rudiments of other thoracic appendages also appear. The abdomen is unsegmented, without any appendage and has a forked telson.
Larval Form # 4. Zoaea Larva:
Zoaea larva has a well formed head with a long, median dorsal spine, two stalked compound eyes and one simple eye, all appendages from antennules to the last pair of maxillipedes are present, carapace is well formed and produced in front into a rostrum.
Thorax is un-segmented and rudimentary at its hinder end. Abdomen is well formed and six segmented, but it has no appendages except a forked telson. It swims by its biramous maxillipedes.
In Penaeus, protozoaea develops into zoaea. In some Anomura the egg hatches as a zoaea which passes through a metazoaea stage to become the adult. Metazoaea is, in fact, an advanced stage of zoaea but differs from it in having uniramous rudiments of thoracic appendages behind the maxillipede.
However, the third maxillipedes are biramous in hermit crab’s (Anomura) metazoaea and uniramous in that of crab (Brachyura). The abdominal appendages, i.e., pleopods also develop as buds. In some decapods, e.g., crabs, the life history starts from zoaea stage.
Larval Form - 5. Cypris Larva:
Cypris larva is covered by a bivalved shell having adductor muscle. Head has compound eyes, antennules with discs on which cement glands open, antennae are lost but remaining cephalic appendages are present, thorax has six pairs of biramous limbs, there is an abdomen of four segments. It has many adult features.
In Cirripedia, e.g., Lepas, the egg hatches as a nauplius, it changes into a cypris which gets fixed by discs of antennules with the secretion of cement glands, then it becomes a pupa which forms shell plates and rotates to assume the adult form.
Larval Form # 6. Mysis or Schizopod Larva:
Mysis or schizopod larva resembles an adult Mysis. Head and thorax have a carapace, all cephalic and thoracic appendages are present, but all thoracic appendages are alike and biramous with exopodites, abdomen has five pairs of pleopods and the sixth form uropods.
In some Decapoda, e.g., in Penaeus, a marine prawn, the egg hatches as nauplius, it passes by successive moults through zoaea stage, protozoaea stage and mysis stage which changes into an adult. In some lobsters, e.g., Homarus both nauplius and zoaea are passed within the egg, it hatches as a mysis larva which changes into an adult.
Larval Form ---7. Megalopa Larva:
Megalopa larva has a large un-segmented cephalothorax with all 13 pairs of appendages like those of a crab, abdomen is straight and in line with cephalothorax, it is like the abdomen of prawn with 6 pairs of well formed pleopods, In crabs the nauplius is passed in the egg, it hatches as a zoaea which by moulting forms the megalopa stage, the megalopa by moulting forms the adult.

In Decapoda there is a gradual abbreviation of development. Stages which are free larval forms in lower types of Crustacea are hurried through within the egg before hatching.
Larval Form--- 8. Phyllosoma Larva:
In Palinurus (the rock lobster), the egg hatches directly into a delicate, transparent, extremely flattened leaf-like larva called phyllosoma or glass crab. This larva is large sized having three distinct regions in the body, the head, thorax and abdomen. An oval carapace covers the head and a part of thorax. It possesses a pair of stalked compound eyes placed anterolaterally in the head.
Thorax bears six pairs of appendages; the first thoracic or maxillipedes are rudimentary, second are uniramous, third well formed biramous, and remaining three (4th, 5th, and 6th) pairs are biramous legs which are enlarged. Abdomen shows segmentation but appendages are absent. This larva undergoes several moultings and transforms into the adult. Phyllosoma is, however, considered to be modified mysis larva.

Larval Form # 9. Alima Larva:
In some Malacostraca like Squilla, the egg directly hatches out in a young stage called alima larva. It is a pelagic form having slender body with a short but broad carapace. Its body is glassy and transparent. In addition to all cephalic appendages, only first two thoracic appendages are found.
The abdomen has distinct six segments with four or five pairs of pleopods. The alima larva is supposed to be modified zoaea stage but it differs strikingly from zoaea in having the armature of the telson and well developed large second maxillipedes.
In addition to these, the glaucothoe larva of hermit crab resembles the megalopa larva as described earlier. Likewise, the calyptosis larva of Euphausia (a malacostracan) is similar to the zoaea larva in all essential features except that it possesses sessile paired eyes in place of stalked eyes.
Importance of Larval Stages:
The importance of larval stages may be accounted as under:
1. They help in wide dispersal of the species.
2. The larval stages help in establishing relationships between various groups.
3. Occurrence of nauplius stage in all crustaceans connects the different representatives of this class together. As referred to, the nauplius establishes relationship of some obscure animals like Sacculina where adult has lost the characters of the class and even the phylum.
In fact, it is the presence of nauplius stage in its life history that connects Sacculina to class Crustacea and further the presence of cypris stage relates it to subclass Cirripedia.
4. If Haeckel’s law of recapitulation (which states that every organism during its development, i.e., ontogeny, repeats its evolutionary history, i.e., phylogeny) is considered true then it can be said that the nauplius stage represents the ancestral form of crustaceans because all crustaceans invariably pass through nauplius stage during their development.
Hence, it can be concluded that present day crustaceans have evolved through nauplius stage.