Crustacean larva

At its most complete, a crustacean's life cycle begins with an egg, which is usually fertilised, but may instead be produced by parthenogenesis. This egg hatches into a pre-larva or pre-zoea. Through a series of moults, the young animal then passes through various zoea stages, followed by a megalopa or post-larva. This is followed by metamorphosis into an immature form, which broadly resembles the adult, and after further moults, the adult form is finally reached. Some crustaceans continue to moult as adults, while for others, the development of gonads signals the final moult. Any organs which are absent from the adults do not generally appear in the larvae, although there are a few exceptions, such as the vestige of the fourth pereiopod in the larvae of Lucifer, and some pleopods in certain Anomura and Brachyura. In a more extreme example, the Sacculina and other Rhizocephala have a distinctive nauplius larva with its complex body structure, but the adult form lacks many organs due to extreme adaptation to its parasitic life style. ==History of the study of crustacean larva==

Antonie van Leeuwenhoek was the first person to observe the difference between larval crustaceans and the adults when he watched the eggs of Cyclops hatching in 1699. Despite this, and other observations over the following decades, there was controversy among scientists about whether or not metamorphosis occurred in crustaceans, with conflicting observations presented, based on different species, some of which went through a metamorphosis, and some of which did not. In 1828 John Vaughan Thompson published a paper "On the Metamorphoses of the Crustacea, and on Zoea, exposing their singular structure and demonstrating they are not, as has been supposed, a peculiar Genus but the Larva of Crustacea!!". His work was not believed due to crayfish not undergoing metamorphosis. This controversy persisted for several more decades, as the first descriptions of a complete series of larval forms were not published until the 1870s (Sidney Irving Smith on the American lobster in 1873; Georg Ossian Sars on the European lobster in 1875, and Walter Faxon on the shrimp Palaemonetes vulgaris in 1879). ==Larval stages==

Nauplius The genus name Nauplius was published posthumously by Otto Friedrich Müller in 1785 for animals now known to be the larvae of copepods. The nauplius stage (plural: nauplii) is characterised by consisting of only three head segments, which are covered by a single carapace. The posterior body, when present, is unsegmented. Each head segment has a pair of appendages; the antennules, antennae, and mandibles. This larval stage has various lifestyles; some are benthic while others are swimmers, some are feeding while others are non-feeders (lecithotrophic). The nauplius is also the stage at which a simple, unpaired eye is present. The eye is known for that reason as the "naupliar eye", and is often absent in later developmental stages, although it is retained into the adult form in some groups, such as the Notostraca. Some crustacean groups lack this larval type, isopods being one example. is characterised by the use of the thoracic appendages for swimming and a large dorsal spine. Post-larva The post-larva or Megalopae, also found exclusively in the Malacostraca, is characterised by the use of abdominal appendages (pleopods) for propulsion. The post-larva is usually similar to the adult form, and many names have been erected for this stage in different groups. William Elford Leach erected the genus Megalopa in 1813 for a post-larval crab; a copepod post-larva is called a copepodite; a barnacle post-larva is called a cypris; a shrimp post-larva is called a parva; a hermit crab post-larva is called a glaucothoe; a spiny lobster / furry lobsters post-larva is called a puerulus and a slipper lobster post-larva is called a nisto. ==Larvae of crustacean groups==

Branchiopoda In the Branchiopoda, the offspring hatch as a nauplius or metanauplius larva. Cephalocarida In the Mediterranean horseshoe shrimp Lightiella magdalenina, the young experience 15 stages following the nauplius, termed metanaupliar stages, and two juvenile stages, with each of the first six stages adding two trunk segments, and the last four segments being added singly. Remipedia The larvae of remipedes are lecithotrophic, consuming egg yolk rather than using external food sources. This characteristic, which is shared with malacostracan groups such as the Decapoda and Euphausiacea (krill) has been used to suggest a link between Remipedia and Malacostraca. Malacostraca Peracarida Amphipod hatchlings resemble the adults. Young isopod crustaceans hatch directly into a manca stage, which is similar in appearance to the adult. The lack of a free-swimming larval form has led to high rates of endemism in isopods, but has also allowed them to colonise the land, in the form of the woodlice. Stomatopoda The larvae of many groups of mantis shrimp are poorly known. In the superfamily Lysiosquilloidea, the larvae hatch as antizoea larvae, with five pairs of thoracic appendages, and develop into erichthus larvae, where the pleopods appear. In the Squilloidea, a pseudozoea larva develops into an alima larva, while in Gonodactyloidea, a pseudozoea develops into an erichthus. A single fossil stomatopod larva has been discovered, in the Upper Jurassic Solnhofen lithographic limestone. of Euphausia pacifica hatching, emerging backwards from the egg Krill The life cycle of krill is relatively well understood, although there are minor variations in detail from species to species. After hatching, the larvae go through several stages called nauplius, pseudometanauplius, metanauplius, calyptopsis and furcilia stages, each of which is sub-divided into several sub-stages. The pseudometanauplius stage is exclusive to the so-called "sac-spawners". Until the metanauplius stage, the larvae are reliant on the yolk reserves, but from the calyptopsis stage, they begin to feed on phytoplankton. During the furcilia stages, segments with pairs of swimmerets are added, beginning at the frontmost segments, with each new pair only becoming functional at the next moult. After the final furcilia stage, the krill resembles the adult. '' crayfish: such large eggs are often indicative of abbreviated development. Decapoda Apart from the prawns of the suborder Dendrobranchiata, all decapod crustaceans brood their eggs on the female's pleopods. This has resulted in development in decapod crustaceans being generally abbreviated. In the marine lobsters, there are three larval stages, all similar in appearance. Freshwater crayfish embryos differ from those of other crustaceans in having 40 ectoteloblast cells, rather than around 19. The larvae show abbreviated development, and hatch with a full complement of adult appendages with the exceptions of the uropods and the first pair of pleopods. Among the Anomura, there is considerable variation in the number of larval stages. In the South American freshwater genus Aegla, the young hatch from the eggs in the adult form. Copepoda Copepods have six naupliar stages, followed by a stage called the copepodid, which has the same number of body segments and appendages in all copepods. The copepodid larva has two pairs of unsegmented swimming appendages, and an unsegmented "hind-body" comprising the thorax and the abdomen. Chalimus (plural chalimi) is a stage of development of a copepod parasite of fish, such as the salmon louse (Lepeophtheirus salmonis). Chalimus Burmeister, 1834 is also a synonym for Lepeophtheirus Nordmann, 1832. Facetotecta The single genus in the Facetotecta, Hansenocaris, is only known from its larvae. They were first described by Christian Andreas Victor Hensen in 1887, and named "y-nauplia" by Hans Jacob Hansen, assuming them to be the larvae of barnacles. The adults are presumed to be parasites of other animals. ==See also==