The dinoflagellates are a classification subgroup of protista. They are a large group of flagellate eukaryotes. Most are marine plankton, but they are common in freshwater habitats, their populations are distributed depending on salinity, or depth. Many dinoflagellates are known to be photosynthetic, but a large fraction of these are in fact mixotrophic, combining photosynthesis with ingestion of prey. In terms of number of species, dinoflagellates are one of the largest groups of marine eukaryotes, although this group is smaller than diatoms; some species are endosymbionts of marine animals and play an important part in the biology of coral reefs. Other dinoflagellates are unpigmented predators on other protozoa, a few forms are parasitic; some dinoflagellates produce resting stages, called dinoflagellate cysts or dinocysts, as part of their lifecycles. Dinoflagellates are considered to be protists, with Dinoflagellata. About 1,555 species of free-living marine dinoflagellates are described. Another estimate suggests about 2,000 living species, of which more than 1,700 are marine and about 220 are from fresh water.
The latest estimates suggest a total of 2,294 living dinoflagellate species, which includes marine and parasitic dinoflagellates. A bloom of certain dinoflagellates can result in a visible coloration of the water colloquially known as red tide, which can cause shellfish poisoning if humans eat contaminated shellfish; some dinoflagellates exhibit bioluminescence—primarily emitting blue-green light. In 1753, the first modern dinoflagellates were described by Henry Baker as "Animalcules which cause the Sparkling Light in Sea Water", named by Otto Friedrich Müller in 1773; the term derives from the Greek word δῖνος, meaning whirling, Latin flagellum, a diminutive term for a whip or scourge. In the 1830s, the German microscopist Christian Gottfried Ehrenberg examined many water and plankton samples and proposed several dinoflagellate genera that are still used today including Peridinium and Dinophysis; these same dinoflagellates were first defined by Otto Bütschli in 1885 as the flagellate order Dinoflagellida.
Botanists treated them as a division of algae, named Pyrrophyta or Pyrrhophyta after the bioluminescent forms, or Dinophyta. At various times, the cryptomonads and ellobiopsids have been included here, but only the last are now considered close relatives. Dinoflagellates have a known ability to transform from noncyst to cyst-forming strategies, which makes recreating their evolutionary history difficult. Dinoflagellates are unicellular and possess two dissimilar flagella arising from the ventral cell side, they have a ribbon-like transverse flagellum with multiple waves that beats to the cell's left, a more conventional one, the longitudinal flagellum, that beats posteriorly. The transverse flagellum is a wavy ribbon in which only the outer edge undulates from base to tip, due to the action of the axoneme which runs along it; the axonemal edge has simple hairs. The flagellar movement produces forward propulsion and a turning force; the longitudinal flagellum is conventional in appearance, with few or no hairs.
It beats with two periods to its wave. The flagella lie in surface grooves: the transverse one in the cingulum and the longitudinal one in the sulcus, although its distal portion projects behind the cell. In dinoflagellate species with desmokont flagellation, the two flagella are differentiated as in dinokonts, but they are not associated with grooves. Dinoflagellates have a complex cell covering called an amphiesma or cortex, composed of a series of membranes, flattened vesicles called alveolae and related structures. In armoured dinoflagellates, these support overlapping cellulose plates to create a sort of armor called the theca, as opposed to athecate dinoflagellates; these occur in various shapes and arrangements, depending on the species and sometimes on the stage of the dinoflagellate. Conventionally, the term tabulation has been used to refer to this arrangement of thecal plates; the plate configuration can be denoted with the plate tabulation formula. Fibrous extrusomes are found in many forms.
Together with various other structural and genetic details, this organization indicates a close relationship between the dinoflagellates, the Apicomplexa, ciliates, collectively referred to as the alveolates. Dinoflagellate tabulations can be grouped into six "tabulation types": gymnodinoid, gonyaulacoid–peridinioid, nannoceratopsioid and prorocentroid; the chloroplasts in most photosynthetic dinoflagellates are bound by three membranes, suggesting they were derived from some ingested algae. Most photosynthetic species contain chlorophylls a and c2, the carotenoid beta-carotene, a group of xanthophylls that appears to be unique to dinoflagellates peridinin and diadinoxanthin; these pigments give many dinoflagellates their typical golden brown color. However, the dinoflagellates Karenia brevis, Karenia mikimotoi, Karlodinium micrum have acquired other pigments through endosymbiosis, including fucoxanthin; this suggests their chloroplasts were incorporated by several endosymbiotic events involving colored or secondarily colorless forms.
The discovery of plastids in the Apicomplexa has led some to suggest they were inherited from an ancestor common to the two groups, b
The biomass is the mass of living biological organisms in a given area or ecosystem at a given time. Biomass can refer to species biomass, the mass of one or more species, or to community biomass, the mass of all species in the community, it can include plants or animals. The mass can be expressed as the total mass in the community. How biomass is measured depends on why it is being measured. Sometimes, the biomass is regarded as the natural mass of organisms in situ. For example, in a salmon fishery, the salmon biomass might be regarded as the total wet weight the salmon would have if they were taken out of the water. In other contexts, biomass can be measured in terms of the dried organic mass, so only 30% of the actual weight might count, the rest being water. For other purposes, only biological tissues count, teeth and shells are excluded. In some applications, biomass is measured as the mass of organically bound carbon, present; the total live biomass on Earth is about 550–560 billion tonnes C, the total annual primary production of biomass is just over 100 billion tonnes C/yr.
The total live biomass of bacteria may be much less. The total number of DNA base pairs on Earth, as a possible approximation of global biodiversity, is estimated at ×1037, weighs 50 billion tonnes. In comparison, the total mass of the biosphere has been estimated to be as much as 4×1012 tonnes of carbon. An ecological pyramid is a graphical representation that shows, for a given ecosystem, the relationship between biomass or biological productivity and trophic levels. A biomass pyramid shows the amount of biomass at each trophic level. A productivity pyramid shows the turn-over in biomass at each trophic level. An ecological pyramid provides a snapshot in time of an ecological community; the bottom of the pyramid represents the primary producers. The primary producers take energy from the environment in the form of sunlight or inorganic chemicals and use it to create energy-rich molecules such as carbohydrates; this mechanism is called primary production. The pyramid proceeds through the various trophic levels to the apex predators at the top.
When energy is transferred from one trophic level to the next only ten percent is used to build new biomass. The remaining ninety percent is dissipated as heat; this energy loss means that productivity pyramids are never inverted, limits food chains to about six levels. However, in oceans, biomass pyramids can be wholly or inverted, with more biomass at higher levels. Terrestrial biomass decreases markedly at each higher trophic level. Examples of terrestrial producers are grasses and shrubs; these have a much higher biomass than the animals that consume them, such as deer and insects. The level with the least biomass are the highest predators in the food chain, such as foxes and eagles. In a temperate grassland and other plants are the primary producers at the bottom of the pyramid. Come the primary consumers, such as grasshoppers and bison, followed by the secondary consumers, shrews and small cats; the tertiary consumers, large cats and wolves. The biomass pyramid decreases markedly at each higher level.
Ocean or marine biomass, in a reversal of terrestrial biomass, can increase at higher trophic levels. In the ocean, the food chain starts with phytoplankton, follows the course: Phytoplankton → zooplankton → predatory zooplankton → filter feeders → predatory fish Phytoplankton are the main primary producers at the bottom of the marine food chain. Phytoplankton use photosynthesis to convert inorganic carbon into protoplasm, they are consumed by microscopic animals called zooplankton. Zooplankton comprise the second level in the food chain, includes small crustaceans, such as copepods and krill, the larva of fish, squid and crabs. In turn, small zooplankton are consumed by both larger predatory zooplankters, such as krill, by forage fish, which are small, filter-feeding fish; this makes up the third level in the food chain. The fourth trophic level consists of predatory fish, marine mammals and seabirds that consume forage fish. Examples are swordfish and gannets. Apex predators, such as orcas, which can consume seals, shortfin mako sharks, which can consume swordfish, make up the fifth trophic level.
Baleen whales can consume zooplankton and krill directly, leading to a food chain with only three or four trophic levels. Marine environments can have inverted biomass pyramids. In particular, the biomass of consumers is larger than the biomass of primary producers; this happens because the ocean's primary producers are tiny phytoplankton that grow and reproduce so a small mass can have a fast rate of primary production. In contrast, terrestrial primary producers reproduce slowly. There is an exception with cyanobacteria. Marine cyanobacteria are the smallest known photosynthetic organisms. Prochlorococcus is the most plentiful species on Earth: a single millilitre of surface seawater may contain 100,000 cells or more. Worldwide, there are estimated to be several octillion individuals. Prochlorococcus is ubiquitous between 40°N and 40°S and dominates in the oligotrophic regions of the oceans; the bacterium accounts for an estimated 20% of the oxygen in the Earth's atmosphere, forms part of the base of the ocean food chain.
There are 50 million bacterial cells in
A microorganism, or microbe, is a microscopic organism, which may exist in its single-celled form or in a colony of cells. The possible existence of unseen microbial life was suspected from ancient times, such as in Jain scriptures from 6th century BC India and the 1st century BC book On Agriculture by Marcus Terentius Varro. Microbiology, the scientific study of microorganisms, began with their observation under the microscope in the 1670s by Antonie van Leeuwenhoek. In the 1850s, Louis Pasteur found that microorganisms caused food spoilage, debunking the theory of spontaneous generation. In the 1880s, Robert Koch discovered that microorganisms caused the diseases tuberculosis and anthrax. Microorganisms include all unicellular organisms and so are diverse. Of the three domains of life identified by Carl Woese, all of the Archaea and Bacteria are microorganisms; these were grouped together in the two domain system as Prokaryotes, the other being the eukaryotes. The third domain Eukaryota includes all multicellular organisms and many unicellular protists and protozoans.
Some protists are related to some to green plants. Many of the multicellular organisms are microscopic, namely micro-animals, some fungi and some algae, but these are not discussed here, they live in every habitat from the poles to the equator, geysers and the deep sea. Some are adapted to extremes such as hot or cold conditions, others to high pressure and a few such as Deinococcus radiodurans to high radiation environments. Microorganisms make up the microbiota found in and on all multicellular organisms. A December 2017 report stated that 3.45-billion-year-old Australian rocks once contained microorganisms, the earliest direct evidence of life on Earth. Microbes are important in human culture and health in many ways, serving to ferment foods, treat sewage, produce fuel and other bioactive compounds, they are essential tools in biology as model organisms and have been put to use in biological warfare and bioterrorism. They are a vital component of fertile soils. In the human body microorganisms make up the human microbiota including the essential gut flora.
They are the pathogens responsible for many infectious diseases and as such are the target of hygiene measures. The possible existence of microorganisms was discussed for many centuries before their discovery in the 17th century. By the fifth century BC, the Jains of present-day India postulated the existence of tiny organisms called nigodas; these nigodas are said to be born in clusters. According to the Jain leader Mahavira, the humans destroy these nigodas on a massive scale, when they eat, breathe and move. Many modern Jains assert that Mahavira's teachings presage the existence of microorganisms as discovered by modern science; the earliest known idea to indicate the possibility of diseases spreading by yet unseen organisms was that of the Roman scholar Marcus Terentius Varro in a 1st-century BC book titled On Agriculture in which he called the unseen creatures animalcules, warns against locating a homestead near a swamp: … and because there are bred certain minute creatures that cannot be seen by the eyes, which float in the air and enter the body through the mouth and nose and they cause serious diseases.
In The Canon of Medicine, Avicenna suggested that tuberculosis and other diseases might be contagious. Akshamsaddin mentioned the microbe in his work Maddat ul-Hayat about two centuries prior to Antonie Van Leeuwenhoek's discovery through experimentation: It is incorrect to assume that diseases appear one by one in humans. Disease infects by spreading from one person to another; this infection occurs through seeds that are so small they are alive. In 1546, Girolamo Fracastoro proposed that epidemic diseases were caused by transferable seedlike entities that could transmit infection by direct or indirect contact, or without contact over long distances. Antonie Van Leeuwenhoek is considered to be the father of microbiology, he was the first in 1673 to discover, describe and conduct scientific experiments with microoorganisms, using simple single-lensed microscopes of his own design. Robert Hooke, a contemporary of Leeuwenhoek used microscopy to observe microbial life in the form of the fruiting bodies of moulds.
In his 1665 book Micrographia, he made drawings of studies, he coined the term cell. Louis Pasteur exposed boiled broths to the air, in vessels that contained a filter to prevent particles from passing through to the growth medium, in vessels without a filter, but with air allowed in via a curved tube so dust particles would settle and not come in contact with the broth. By boiling the broth beforehand, Pasteur ensured that no microorganisms survived within the broths at the beginning of his experiment. Nothing grew in the broths in the course of Pasteur's experiment; this meant that the living organisms that grew in such broths came from outside, as spores on dust, rather than spontaneously generated within the broth. Thus, Pasteur supported the germ theory of disease. In 1876, Robert Koch established, he found that the blood of cattle which were infected with anthrax always had large numbers of Bacillus anthracis. Koch found that he could transmit anthrax from one animal to another by taking a small sample of blood from the infected animal and injecting it into a healthy one, this caused the healthy animal to become sick.
He found that he could grow the bacteria in a nutrient broth inject it into a heal
Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They live in compact colonies of many identical individual polyps. Corals species include the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton. A coral "group" is a colony of myriad genetically identical polyps; each polyp is a sac-like animal only a few millimeters in diameter and a few centimeters in length. A set of tentacles surround a central mouth opening. An exoskeleton is excreted near the base. Over many generations, the colony thus creates a large skeleton characteristic of the species. Individual heads grow by asexual reproduction of polyps. Corals breed sexually by spawning: polyps of the same species release gametes over a period of one to several nights around a full moon. Although some corals are able to catch small fish and plankton using stinging cells on their tentacles, most corals obtain the majority of their energy and nutrients from photosynthetic unicellular dinoflagellates in the genus Symbiodinium that live within their tissues.
These are known as zooxanthellae. Such corals require sunlight and grow in clear, shallow water at depths less than 60 metres. Corals are major contributors to the physical structure of the coral reefs that develop in tropical and subtropical waters, such as the enormous Great Barrier Reef off the coast of Queensland, Australia. Other corals do not rely on zooxanthellae and can live in much deeper water, with the cold-water genus Lophelia surviving as deep as 3,300 metres; some have been found on the Darwin Mounds, northwest of Cape Wrath and others as far north as off the coast of Washington State and the Aleutian Islands. Aristotle's pupil Theophrastus described the red coral, korallion, in his book on stones, implying it was a mineral, but he described it as a deep-sea plant in his Enquiries on Plants, where he mentions large stony plants that reveal bright flowers when under water in the Gulf of Heroes. Pliny the Elder stated boldly that several sea creatures including sea nettles and sponges "are neither animals nor plants, but are possessed of a third nature".
Petrus Gyllius copied Pliny, introducing the term zoophyta for this third group in his 1535 book On the French and Latin Names of the Fishes of the Marseilles Region. Gyllius further noted, following Aristotle, how hard it was to define what was a plant and what was an animal; the Persian polymath Al-Biruni classified sponges and corals as animals, arguing that they respond to touch. People believed corals to be plants until the eighteenth century, when William Herschel used a microscope to establish that coral had the characteristic thin cell membranes of an animal. Presently, corals are classified as certain species of animals within the sub-classes Hexacorallia and Octocorallia of the class Anthozoa in the phylum Cnidaria. Hexacorallia includes the stony corals and these groups have polyps that have a 6-fold symmetry. Octocorallia includes blue coral and soft corals and species of Octocorallia have polyps with an eightfold symmetry, each polyp having eight tentacles and eight mesenteries.
Fire corals are not true corals. Corals are sessile animals and differ from most other cnidarians in not having a medusa stage in their life cycle; the body unit of the animal is a polyp. Most corals are colonial, the initial polyp budding to produce another and the colony developing from this small start. In stony corals known as hard corals, the polyps produce a skeleton composed of calcium carbonate to strengthen and protect the organism; this is deposited by the coenosarc, the living tissue that connects them. The polyps sit in cup-shaped depressions in the skeleton known as corallites. Colonies of stony coral are variable in appearance. In soft corals, there is no stony skeleton but the tissues are toughened by the presence of tiny skeletal elements known as sclerites, which are made from calcium carbonate. Soft corals are variable in form and most are colonial. A few soft corals are stolonate. In some species this is thick and the polyps are embedded; some soft corals are form lobes. Others have a central axial skeleton embedded in the tissue matrix.
This is composed either of a fibrous protein called gorgonin or of a calcified material. In both stony and soft corals, the polyps can be retracted, with stony corals relying on their hard skeleton and cnidocytes for defence against predators, soft corals relying on chemical defences in the form of toxic substances present in the tissues known as terpenoids; the polyps of stony corals have six-fold symmetry. The mouth of each polyp is surrounded by a ring of tentacles. In stony corals these are cylindrical and taper to a point, but in soft corals they are pinnate with side branches known as pinnules. In some tropical species these are reduced to mere stubs and in some they are fused to give a paddle-like appearance. In most corals, the tentacles are retracted by day and spread out at night to catch plankton and other small organisms. Shallow water species of both stony and soft corals can be zooxanthellate, the corals supplementing their plankton diet with t
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
Crustaceans form a large, diverse arthropod taxon which includes such familiar animals as crabs, crayfish, krill and barnacles. The crustacean group is treated as a subphylum, because of recent molecular studies it is now well accepted that the crustacean group is paraphyletic, comprises all animals in the Pancrustacea clade other than hexapods; some crustaceans are more related to insects and other hexapods than they are to certain other crustaceans. The 67,000 described species range in size from Stygotantulus stocki at 0.1 mm, to the Japanese spider crab with a leg span of up to 3.8 m and a mass of 20 kg. Like other arthropods, crustaceans have an exoskeleton, they are distinguished from other groups of arthropods, such as insects and chelicerates, by the possession of biramous limbs, by their larval forms, such as the nauplius stage of branchiopods and copepods. Most crustaceans are free-living aquatic animals, but some are terrestrial, some are parasitic and some are sessile; the group has an extensive fossil record, reaching back to the Cambrian, includes living fossils such as Triops cancriformis, which has existed unchanged since the Triassic period.
More than 10 million tons of crustaceans are produced by fishery or farming for human consumption, the majority of it being shrimp and prawns. Krill and copepods are not as fished, but may be the animals with the greatest biomass on the planet, form a vital part of the food chain; the scientific study of crustaceans is known as carcinology, a scientist who works in carcinology is a carcinologist. The body of a crustacean is composed of segments, which are grouped into three regions: the cephalon or head, the pereon or thorax, the pleon or abdomen; the head and thorax may be fused together to form a cephalothorax, which may be covered by a single large carapace. The crustacean body is protected by the hard exoskeleton, which must be moulted for the animal to grow; the shell around each somite can be divided into a dorsal tergum, ventral sternum and a lateral pleuron. Various parts of the exoskeleton may be fused together; each somite, or body segment can bear a pair of appendages: on the segments of the head, these include two pairs of antennae, the mandibles and maxillae.
The abdomen bears pleopods, ends in a telson, which bears the anus, is flanked by uropods to form a tail fan. The number and variety of appendages in different crustaceans may be responsible for the group's success. Crustacean appendages are biramous, meaning they are divided into two parts, it is unclear whether the biramous condition is a derived state which evolved in crustaceans, or whether the second branch of the limb has been lost in all other groups. Trilobites, for instance possessed biramous appendages; the main body cavity is an open circulatory system, where blood is pumped into the haemocoel by a heart located near the dorsum. Malacostraca have haemocyanin as the oxygen-carrying pigment, while copepods, ostracods and branchiopods have haemoglobins; the alimentary canal consists of a straight tube that has a gizzard-like "gastric mill" for grinding food and a pair of digestive glands that absorb food. Structures that function as kidneys are located near the antennae. A brain exists in the form of ganglia close to the antennae, a collection of major ganglia is found below the gut.
In many decapods, the first pair of pleopods are specialised in the male for sperm transfer. Many terrestrial crustaceans return to the sea to release the eggs. Others, such as woodlice, lay their eggs on land, albeit in damp conditions. In most decapods, the females retain the eggs; the majority of crustaceans are aquatic, living in either marine or freshwater environments, but a few groups have adapted to life on land, such as terrestrial crabs, terrestrial hermit crabs, woodlice. Marine crustaceans are as ubiquitous in the oceans; the majority of crustaceans are motile, moving about independently, although a few taxonomic units are parasitic and live attached to their hosts, adult barnacles live a sessile life – they are attached headfirst to the substrate and cannot move independently. Some branchiurans are able to withstand rapid changes of salinity and will switch hosts from marine to non-marine species. Krill are the bottom layer and the most important part of the food chain in Antarctic animal communities.
Some crustaceans are significant invasive species, such as the Chinese mitten crab, Eriocheir sinensis, the Asian shore crab, Hemigrapsus sanguineus. The majority of crustaceans have separate sexes, reproduce sexually. A small number are hermaphrodites, including barnacles and Cephalocarida; some may change sex during the course of their life. Parthenogenesis is widespread among crustaceans, where viable eggs are produced by a female without needing fertilisation by a male; this occurs in many branchiopods, some os
Benthos is the community of organisms that live on, in, or near the seabed known as the benthic zone. This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, down to the abyssal depths. Many organisms adapted to deep-water pressure cannot survive in the upper parts of the water column; the pressure difference can be significant. Because light is absorbed before it can reach deep ocean-water, the energy source for deep benthic ecosystems is organic matter from higher up in the water column that drifts down to the depths; this dead and decaying matter sustains the benthic food chain. The term benthos, coined by Haeckel in 1891, comes from the Greek noun βένθος "depth of the sea". Benthos is used in freshwater biology to refer to organisms at the bottom of freshwater bodies of water, such as lakes and streams. There is a redundant synonym, benthon; the main food sources for the benthos are algae and organic runoff from land.
The depth of water and salinity, type of local substrate all affect what benthos is present. In coastal waters and other places where light reaches the bottom, benthic photosynthesizing diatoms can proliferate. Filter feeders, such as sponges and bivalves, dominate sandy bottoms. Deposit feeders, such as polychaetes, populate softer bottoms. Fish, such as dragonets, as well as sea stars, snails and crustaceans are important predators and scavengers. Benthic organisms, such as sea stars, clams, sea cucumbers, brittle stars and sea anemones, play an important role as a food source for fish, such as the California sheephead, humans, they are visible to the naked eye with the lower range of body size at 0.5 mm but larger than 3 mm. In the coastal water ecosystem, they include several species of organisms from different taxa including Porifera, Coelenterates, Crustaceans, Arthropods etc. Zoobenthos comprises the animals belonging to the benthos. Phytobenthos comprises the plants belonging to the benthos benthic diatoms and macroalgae.
Endobenthos lives buried, or burrowing in the sediment in the oxygenated top layer, e.g. a sea pen or a sand dollar. Epibenthos lives on e.g. like a sea cucumber or a sea snail crawling about. Hyperbenthos lives just above the sediment. Contrast the terms plankton and neuston. "Benthos". Encyclopædia Britannica. Ryan, Paddy "Benthic communities" Te Ara - the Encyclopædia of New Zealand, updated 21 September 2007. Yip and Madl, Pierre "Benthos" University of Salzburg. "Benthos"