Crustacean larva
Crustaceans may pass through a number of larval and immature stages between hatching from their eggs and reaching their adult form. Each of the stages is separated by a moult, in which the hard exoskeleton is shed to allow the animal to grow; the larvae of crustaceans bear little resemblance to the adult, there are still cases where it is not known what larvae will grow into what adults. This is true of crustaceans which live as benthic adults, more so than where the larvae are planktonic and therefore more caught. Many crustacean larvae were not recognised as larvae when they were discovered, were described as new genera and species; the names of these genera have become generalised to cover specific larval stages across wide groups of crustaceans, such as zoea and nauplius. Other terms described forms which are only found in particular groups, such as the glaucothoe of hermit crabs, or the phyllosoma of slipper lobsters and spiny lobsters. At its most complete, a crustacean's life cycle begins with an egg, fertilised, but may instead be produced by parthenogenesis.
This egg hatches into a pre-zoea. Through a series of moults, the young animal 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, after further moults, the adult form is 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 appear in the larvae, although there are a few exceptions, such as the vestige of the fourth pereiopod in the larvae of Lucifer, some pleopods in certain Anomura and crabs; 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. 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, 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, some of which did not.
This controversy persisted until the 1840s, the first descriptions of a complete series of larval forms were not published until the 1870s. 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 is characterised by the use of the appendages of the head for swimming. The nauplius is the stage at which a simple, unpaired eye first appears; the eye is known for that reason as the "naupliar eye", is absent in developmental stages, although it is retained into the adult form in some groups, such as the Notostraca. The genus Zoea was described by Louis Augustin Guillaume Bosc in 1802 for an animal now known to be the larva of a crab; the zoea stage is characterised by the use of the thoracic appendages for swimming and a large dorsal spine. The post-larva is characterised by the use of abdominal appendages for propulsion; the post-larva is similar to the adult form, so many names have been erected for the stage in different groups.
William Elford Leach erected the genus Megalopa in 1813 for a post-larval crab. In the Branchiopoda, the most basal group of crustaceans, there is no metamorphosis; every other crustacean group with free larvae shows a metamorphosis, this difference in the larvae is thought to reflect "a fundamental cleavage" of the crustaceans. In the Mediterranean horseshoe shrimp Lightiella magdalenina, the young experience 15 metanaupliar stages and 2 juvenile stages, with each of the first six stages adding 2 trunk segments, the last four segments being added singly; the larvae of remipedes are lecithotrophic, consuming egg yolk rather than using external food sources. This characteristic, shared with malacostracan groups such as the Decapoda and Euphausiacea has been used to suggest a link between Remipedia and Malacostraca. Amphipod hatchlings resemble the adults. Young isopod crustaceans hatch directly into a manca stage, 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 allowed them to colonise the land, in the form of the woodlice.
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, 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; the life cycle of krill is 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 and furcilia stages, each of which is
Decapoda
The Decapoda or decapods are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, lobsters and shrimp. Most decapods are scavengers; the order is estimated to contain nearly 15,000 species in around 2,700 genera, with around 3,300 fossil species. Nearly half of these species are crabs, with the shrimp and Anomura including hermit crabs, porcelain crabs, squat lobsters making up the bulk of the remainder; the earliest fossil decapod is the Devonian Palaeopalaemon. Decapods can have as many as 38 appendages, arranged in one pair per body segment; as the name Decapoda implies, ten of these appendages are considered legs. They are the pereiopods, found on the last five thoracic segments. In many decapods, one pair of these "legs" has enlarged pincers, called chelae, with the legs being called chelipeds. In front of the pereiopods are three pairs of maxillipeds which function as feeding appendages; the head has five pairs of appendages, including mouthparts and antennules.
There are five more pairs of appendages on the abdomen. They are called pleopods. There is one final pair called uropods, with the telson, form the tail fan. Classification within the order Decapoda depends on the structure of the gills and legs, the way in which the larvae develop, giving rise to two suborders: Dendrobranchiata and Pleocyemata; the Dendrobranchiata consist of prawns, including many species colloquially referred to as "shrimp", such as the "white shrimp", Litopenaeus setiferus. The Pleocyemata include the remaining groups, including "true shrimp"; those groups which walk rather than swim form a clade called Reptantia. This classification to the level of superfamilies follows De al.. Order Decapoda Latreille, 1802 Suborder Dendrobranchiata Bate, 1888 Penaeoidea Rafinesque, 1815 Sergestoidea Dana, 1852 Suborder Pleocyemata Burkenroad, 1963 Infraorder Stenopodidea Bate, 1888 Infraorder Caridea Dana, 1852 Procaridoidea Chace & Manning, 1972 Galatheacaridoidea Vereshchaka, 1997 Pasiphaeoidea Dana, 1852 Oplophoroidea Dana, 1852 Atyoidea De Haan, 1849 Bresilioidea Calman, 1896 Nematocarcinoidea Smith, 1884 Psalidopodoidea Wood-Mason, 1874 Stylodactyloidea Bate, 1888 Campylonotoidea Sollaud, 1913 Palaemonoidea Rafinesque, 1815 Alpheoidea Rafinesque, 1815 Processoidea Ortmann, 1896 Pandaloidea Haworth, 1825 Physetocaridoidea Chace, 1940 Crangonoidea Haworth, 1825 Infraorder Astacidea Latreille, 1802 Enoplometopoidea de Saint Laurent, 1988 Nephropoidea Dana, 1852 Astacoidea Latreille, 1802 Parastacoidea Huxley, 1879 Infraorder Glypheidea Winckler, 1882 Glypheoidea Winckler, 1882 Infraorder Axiidea de Saint Laurent, 1979b Infraorder Gebiidea de Saint Laurent, 1979 Infraorder Achelata Scholtz & Richter, 1995 Infraorder Polychelida Scholtz & Richter, 1995 Infraorder Anomura MacLeay, 1838 Aegloidea Dana, 1852 Galatheoidea Samouelle, 1819 Hippoidea Latreille, 1825a Chirostyloidea Ortmann, 1892 Lithodoidea Samouelle, 1819 Lomisoidea Bouvier, 1895 Paguroidea Latreille, 1802 Infraorder Brachyura Linnaeus, 1758 Section Dromiacea De Haan, 1833 Dromioidea De Haan, 1833 Homolodromioidea Alcock, 1900 Homoloidea De Haan, 1839 Section Raninoida De Haan, 1839 Section Cyclodorippoida Ortmann, 1892 Section Eubrachyura de Saint Laurent, 1980 Subsection Heterotremata Guinot, 1977 Aethroidea Dana, 1851 Bellioidea Dana, 1852 Bythograeoidea Williams, 1980 Calappoidea De Haan, 1833 Cancroidea Latreille, 1802 Carpilioidea Ortmann, 1893 Cheiragonoidea Ortmann, 1893 Corystoidea Samouelle, 1819 Dairoidea Serène, 1965 Dorippoidea MacLeay, 1838 Eriphioidea MacLeay, 1838 Gecarcinucoidea Rathbun, 1904 Goneplacoidea MacLeay, 1838 Hexapodoidea Miers, 1886 Leucosioidea Samouelle, 1819 Majoidea Samouelle, 1819 Orithyioidea Dana, 1852c Palicoidea Bouvier, 1898 Parthenopoidea MacLeay, Pilumnoidea Samouelle, 1819 Portunoidea Rafinesque, 1815 Potamoidea Ortmann, 1896 Pseudothelphusoidea Ortmann, 1893 Pseudozioidea Alcock, 1898 Retroplumoidea Gill, 1894 Trapezioidea Miers, 1886 Trichodactyloidea H. Milne-Edwards, 1853 Xanthoidea MacLeay, 1838 Subsection Thoracotremata Guinot, 1977 Cryptochiroidea Paul'son, 1875 Grapsoidea MacLeay, 1838 Ocypodoidea Rafinesque, 1815 Pinnotheroidea De Haan, 1833 List of Atlantic decapod species Phylogeny of Malacostraca Data related to Decapoda at Wikispecies Decapod Crustacea "Tree of Life" page at the Natural History Museum of Los Angeles County Decapoda at Curlie
Malacostraca
Malacostraca is the largest of the six classes of crustaceans, containing about 40,000 living species, divided among 16 orders. Its members, the malacostracans, display a great diversity of body forms and include crabs, crayfish, krill, amphipods, mantis shrimp and many other, less familiar animals, they have colonised freshwater and terrestrial habitats. They are segmented animals, united by a common body plan comprising 20 body segments, divided into a head and abdomen; the name Malacostraca was coined by a French zoologist Pierre André Latreille in 1802. He was curator of the arthropod collection at the National Museum of Natural History in Paris; the name comes from the Greek roots μαλακός and ὄστρακον. The name is misleading, since the shell is only soft after moulting, is hard. Malacostracans are sometimes contrasted with entomostracans, a name applied to all crustaceans outside the Malacostraca, named after the obsolete taxon Entomostraca; the class Malacostraca includes about 40,000 species, "arguably... contains a greater diversity of body forms than any other class in the animal kingdom".
Its members are characterised by the presence of three tagmata – a five-segmented head, an eight-segmented thorax and an abdomen with six segments and a telson, except in the Leptostraca, which retain the ancestral condition of seven abdominal segments. Malacostracans have abdominal appendages, a fact that differentiates them from all other major crustacean taxa except Remipedia; each body segment bears a pair of jointed appendages. The head bears two pairs of antennae, the first of, biramous and the second pair bear exopods which are flattened into antennal scales known as scaphocerites; the mouthparts consist of pairs each of mandibles and maxillae. A pair of stalked compound eyes is present, although in some taxa the eyes are unstalked, reduced or lost. Up to three thoracic segments may be fused with the head to form a cephalothorax. A carapace may be absent, present or secondarily lost, may cover the head, part or all of the thorax and some of the abdomen, it is variable in form and may be fused dorsally with some of the thoracic segments or be in two parts, hinged dorsally.
Each of the thoracic appendages is biramous and the endopods are the better developed of the branches, being used for crawling or grasping. Each endopod consist of seven articulating segments. In decapods, the claw is formed by the articulation of the dactylus against an outgrowth of the propodus. In some taxa, the exopods are lost and the appendages are uniramous. There is the six or seven-segmented abdomen. In most taxa, each abdominal segment except the last carries a pair of biramous pleopods used for swimming, gas exchange, creating a current or brooding eggs; the first and second abdominal pleopods may be modified in the male to form gonopods. The appendages of the last segment are flattened into uropods, which together with the terminal telson, make up the "tail fan", it is the sudden flexion of this tail fan that provides the thrust for the rapid escape response of these crustaceans and the tail fan is used in steering. In Leptostraca, the appendages on the telson instead form caudal rami.
The digestive tract is straight and the foregut consists of a short oesophagus and a two-chambered stomach, the first part of which contains a gizzard-like "gastric mill" for grinding food. The walls of this have chitinous ridges and calcareous ossicles; the fine particles and soluble material are moved into the midgut where chemical processing and absorption takes place in one or more pairs of large digestive caeca. The hindgut is concerned with water reclamation and the formation of faeces and the anus is situated at the base of the telson. Like other crustaceans, malacostracans have an open circulatory system in which the heart pumps blood into the hemocoel where it supplies the needs of the organs for oxygen and nutrients before diffusing back to the heart; the typical respiratory pigment in malacostracans is haemocyanin. Structures that function as kidneys are located near the base of the antennae. A brain exists in the form of ganglia close to the antennae, there are ganglia in each segment and a collection of major ganglia below the oesophagus.
Sensory organs include compound eyes, ocelli and sensory bristles. The naupliar eye is a characteristic of the nauplius larva and consists of four cup-shaped ocelli facing in different directions and able to distinguish between light and darkness. Malacostracans live in a wide range of marine and freshwater habitats, three orders have terrestrial members: Amphipoda and Decapoda, they are abundant in all marine ecosystems, most species are scavengers, although some, such as the porcelain crabs, are filter feeders, some, such as mantis shrimps, are carnivores. Most species of malacostracans have distinct sexes; the female genital openings or gonopores are located on the sixth t
Fossilworks
Fossilworks is a portal which provides query and analysis tools to facilitate access to the Paleobiology Database, a large relational database assembled by hundreds of paleontologists from around the world. Fossilworks is housed at Macquarie University, it includes many analysis and data visualization tools included in the Paleobiology Database. "Fossilworks". Retrieved 2010-04-08
Intertidal zone
The intertidal zone known as the foreshore and seashore and sometimes referred to as the littoral zone, is the area, above water at low tide and underwater at high tide. This area can include many different types of habitats, with many types of animals, such as starfish, sea urchins, numerous species of coral; the well-known area includes steep rocky cliffs, sandy beaches, or wetlands. The area can be a narrow strip, as in Pacific islands that have only a narrow tidal range, or can include many meters of shoreline where shallow beach slopes interact with high tidal excursion. Peritidal zone is similar but a somewhat wider zone, extending from above the highest tide level to below that of the lowest tide level. Organisms in the intertidal zone are adapted to an environment of harsh extremes; the intertidal zone is home to many several species from different taxa including Porifera, Coelenterates, crustaceans, etc. Water is available with the tides but varies from fresh with rain to saline and dry salt with drying between tidal inundations.
Wave splash can dislodge residents from the littoral zone. With the intertidal zone's high exposure to the sun, the temperature range can be anything from hot with full sun to near freezing in colder climates; some microclimates in the littoral zone are ameliorated by local features and larger plants such as mangroves. Adaptation in the littoral zone allows the use of nutrients supplied in high volume on a regular basis from the sea, moved to the zone by tides. Edges of habitats, in this case land and sea, are themselves significant ecologies, the littoral zone is a prime example. A typical rocky shore can be divided into a spray zone or splash zone, above the spring high-tide line and is covered by water only during storms, an intertidal zone, which lies between the high and low tidal extremes. Along most shores, the intertidal zone can be separated into the following subzones: high tide zone, middle tide zone, low tide zone; the intertidal zone is one of a number of marine biomes or habitats, including estuaries, neritic and deep zones.
Marine biologists divide the intertidal region into three zones, based on the overall average exposure of the zone. The low intertidal zone, which borders on the shallow subtidal zone, is only exposed to air at the lowest of low tides and is marine in character; the mid intertidal zone is exposed and submerged by average tides. The high intertidal zone is only covered by the highest of the high tides, spends much of its time as terrestrial habitat; the high intertidal zone borders on the splash zone. On shores exposed to heavy wave action, the intertidal zone will be influenced by waves, as the spray from breaking waves will extend the intertidal zone. Depending on the substratum and topography of the shore, additional features may be noticed. On rocky shores, tide pools form in depressions. Under certain conditions, such as those at Morecambe Bay, quicksand may form; this subregion is submerged - it is only exposed at the point of low tide and for a longer period of time during low tides. This area is teeming with life.
There is a great biodiversity. Organisms in this zone are not well adapted to periods of dryness and temperature extremes; some of the organisms in this area are abalone, sea anemones, brown seaweed, crabs, green algae, isopods, mussels, sculpin, sea cucumber, sea lettuce, sea palms, sea urchins, snails, surf grass, tube worms, whelks. Creatures in this area can grow to larger sizes because there is more available energy in the localized ecosystem. Marine vegetation can grow to much greater sizes than in the other three intertidal subregions due to the better water coverage; the water is shallow enough to allow plenty of light to reach the vegetation to allow substantial photosynthetic activity, the salinity is at normal levels. This area is protected from large predators such as fish because of the wave action and the shallow water; the intertidal region is an important model system for the study of ecology on wave-swept rocky shores. The region contains a high diversity of species, the zonation created by the tides causes species ranges to be compressed into narrow bands.
This makes it simple to study species across their entire cross-shore range, something that can be difficult in, for instance, terrestrial habitats that can stretch thousands of kilometres. Communities on wave-swept shores have high turnover due to disturbance, so it is possible to watch ecological succession over years rather than decades; the burrowing invertebrates that make up large portions of sandy beach ecosystems are known to travel great distances in cross-shore directions as beaches change on the order of days, semilunar cycles, seasons, or years. The distribution of some species has been found to correlate with geomorphic datums such as the high tide strand and the water table outcrop. Since the foreshore is alternately covered by the sea and exposed to the air, organisms living in this environment must have adaptions for both wet and dry conditions. Hazards include being smashed or carried away by rough waves, exposure to dangerously high temperatures, desiccation. Typical inhabit
Arthropod
An arthropod is an invertebrate animal having an exoskeleton, a segmented body, paired jointed appendages. Arthropods form the phylum Euarthropoda, which includes insects, arachnids and crustaceans; the term Arthropoda as proposed refers to a proposed grouping of Euarthropods and the phylum Onychophora. Arthropods are characterized by their jointed limbs and cuticle made of chitin mineralised with calcium carbonate; the arthropod body plan consists of each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. Arthopods are bilaterally symmetrical and their body possesses an external skeleton; some species have wings. Their versatility has enabled them to become the most species-rich members of all ecological guilds in most environments, they have over a million described species, making up more than 80 per cent of all described living animal species, some of which, unlike most other animals, are successful in dry environments. Arthropods range in size from the microscopic crustacean Stygotantulus up to the Japanese spider crab.
Arthropods' primary internal cavity is a haemocoel, which accommodates their internal organs, through which their haemolymph – analogue of blood – circulates. Like their exteriors, the internal organs of arthropods are built of repeated segments, their nervous system is "ladder-like", with paired ventral nerve cords running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, their brains are formed by fusion of the ganglia of these segments and encircle the esophagus; the respiratory and excretory systems of arthropods vary, depending as much on their environment as on the subphylum to which they belong. Their vision relies on various combinations of compound eyes and pigment-pit ocelli: in most species the ocelli can only detect the direction from which light is coming, the compound eyes are the main source of information, but the main eyes of spiders are ocelli that can form images and, in a few cases, can swivel to track prey.
Arthropods have a wide range of chemical and mechanical sensors based on modifications of the many setae that project through their cuticles. Arthropods' methods of reproduction and development are diverse; the evolutionary ancestry of arthropods dates back to the Cambrian period. The group is regarded as monophyletic, many analyses support the placement of arthropods with cycloneuralians in a superphylum Ecdysozoa. Overall, the basal relationships of Metazoa are not yet well resolved; the relationships between various arthropod groups are still debated. Aquatic species use either external fertilization. All arthropods lay eggs, but scorpions give birth to live young after the eggs have hatched inside the mother. Arthropod hatchlings vary from miniature adults to grubs and caterpillars that lack jointed limbs and undergo a total metamorphosis to produce the adult form; the level of maternal care for hatchlings varies from nonexistent to the prolonged care provided by scorpions. Arthropods contribute to the human food supply both directly as food, more indirectly as pollinators of crops.
Some species are known to spread severe disease to humans and crops. The word arthropod comes from the Greek ἄρθρον árthron, "joint", πούς pous, i.e. "foot" or "leg", which together mean "jointed leg". Arthropods are invertebrates with jointed limbs; the exoskeleton or cuticles consists of a polymer of glucosamine. The cuticle of many crustaceans, beetle mites, millipedes is biomineralized with calcium carbonate. Calcification of the endosternite, an internal structure used for muscle attachments occur in some opiliones. Estimates of the number of arthropod species vary between 1,170,000 and 5 to 10 million and account for over 80 per cent of all known living animal species; the number of species remains difficult to determine. This is due to the census modeling assumptions projected onto other regions in order to scale up from counts at specific locations applied to the whole world. A study in 1992 estimated that there were 500,000 species of animals and plants in Costa Rica alone, of which 365,000 were arthropods.
They are important members of marine, freshwater and air ecosystems, are one of only two major animal groups that have adapted to life in dry environments. One arthropod sub-group, insects, is the most species-rich member of all ecological guilds in land and freshwater environments; the lightest insects weigh less than 25 micrograms. Some living crustaceans are much larger; the embryos of all arthropods are segmented, built from a series of repeated modules. The last common ancestor of living arthropods consisted of a series of undifferentiated segments, each with a pair of appendages that functioned as limbs. However, all known living and fossil arthropods have grouped segments into tagmata in which segments and their limbs are specialized in various ways; the three-
