Horseshoe crabs are marine and brackish water arthropods of the family Limulidae, suborder Xiphosurida, order Xiphosura. Their popular name is a misnomer, for they are not true crabs. Horseshoe crabs live in and around shallow coastal waters on soft sandy or muddy bottoms, they tend to spawn in the intertidal zone at spring high tides. They are eaten in Asia, used as fishing bait, in fertilizer and in science. In recent years, population declines have occurred as a consequence of coastal habitat destruction and overharvesting. Tetrodotoxin may be present in Carcinoscorpius rotundicauda; because of their origin 450 million years ago, horseshoe crabs are considered living fossils. A 2019 molecular analysis places them as the sister group of Ricinulei within Arachnida. Horseshoe crabs resemble crustaceans but belong to a separate subphylum of the arthropods and are related to arachnids. Horseshoe crabs are related to the extinct eurypterids, which include some of the largest arthropods to have existed, the two may be sister groups.
The earliest horseshoe crab fossils are found in strata from the late Ordovician period 450 million years ago. The Limulidae are the only recent family of the order Xiphosura, contains all four living species of horseshoe crabs: Carcinoscorpius rotundicauda, the mangrove horseshoe crab, found in South and Southeast Asia Limulus polyphemus, the Atlantic or American horseshoe crab, found along the American Atlantic coast and in the Gulf of Mexico Tachypleus gigas, the Indo-Pacific, Indian or southern horseshoe crab, found in South and Southeast Asia Tachypleus tridentatus, the Chinese, Japanese or tri-spine horseshoe crab, found in Southeast and East Asia The entire body of the horseshoe crab is protected by a hard carapace, it has two compound lateral eyes, each composed of about 1,000 ommatidia, plus a pair of median eyes that are able to detect both visible light and ultraviolet light, a single endoparietal eye, a pair of rudimentary lateral eyes on the top. The latter become functional.
A pair of ventral eyes is located near the mouth, as well as a cluster of photoreceptors on the telson. The horseshoe crab has five additional eyes on top of its shell. Despite having poor eyesight, the animals have the largest rods and cones of any known animal, about 100 times the size of humans', their eyes are a million times more sensitive to light at night than during the day; the mouth is located in the center of the legs, whose bases are referred to as gnathobases and have the same function as jaws and help grind up food. The horseshoe crab has five pairs of legs for walking and moving food into the mouth, each with a claw at the tip, except for the last pair. Behind its legs, the horseshoe crab has book gills, which exchange respiratory gases, are occasionally used for swimming; as in other arthropods, a true endoskeleton is absent, but the body does have an endoskeletal structure made up of cartilaginous plates that support the book gills. They are more found on the ocean floor searching for worms and molluscs, which are their main food.
They may feed on crustaceans and small fish. Females are about 20–30% larger than males; the smallest species is the largest is T. tridentatus. On average, males of C. rotundicauda are about 30 cm long, including a tail, about 15 cm, their carapace is about 15 cm wide. Some southern populations of L. polyphemus are somewhat smaller, but otherwise this species is larger. In the largest species, T. tridentatus, females can reach as much as 79.5 cm long, including their tail, up to 4 kg in weight. This is only about 10–20 cm longer than the largest females of L. polyphemus and T. gigas, but twice the weight. The juveniles grow about 33% larger with every molt until reaching adult size. During the breeding season, horseshoe crabs migrate to shallow coastal waters. A male selects a female and clings to her back. Several males surround the female and all fertilize together, which makes it easy to spot and count females as they are the large center carapace surrounded by 3-5 smaller ones; the female lays her eggs while the male fertilize them.
The female can lay between 120,000 eggs in batches of a few thousand at a time. In L. polyphemus, the eggs take about two weeks to hatch. The larvae molt six times during the first year. Natural breeding of horseshoe crabs in captivity has proven to be difficult; some evidence indicates that mating takes place only in the presence of the sand or mud in which the horseshoe crab's eggs were hatched. It is not known with certainty what is in the sand how they sense it. Artificial insemination and induced spawning have been done on a large scale in captivity, eggs and juveniles collected from the wild are raised to adulthood in captivity. Horseshoe crabs use hemocyanin to carry oxygen through their blood; because of the copper present in hemocyanin, their blood is blue. Their blood contains amebocytes, which play a similar role to the white blood cells of vertebrates in defending the organism against pathogens. Amebocytes from the blood of L. polyphemus are used to make Limulus amebocyte lysate, used for the detection of bacterial endotoxins in medical applications.
This means there is a high demand for the blood, the harvest of which involves collecting and bleeding the animal
Algae is an informal term for a large, diverse group of photosynthetic eukaryotic organisms that are not closely related, is thus polyphyletic. Including organisms ranging from unicellular microalgae genera, such as Chlorella and the diatoms, to multicellular forms, such as the giant kelp, a large brown alga which may grow up to 50 m in length. Most are aquatic and autotrophic and lack many of the distinct cell and tissue types, such as stomata and phloem, which are found in land plants; the largest and most complex marine algae are called seaweeds, while the most complex freshwater forms are the Charophyta, a division of green algae which includes, for example and the stoneworts. No definition of algae is accepted. One definition is that algae "have chlorophyll as their primary photosynthetic pigment and lack a sterile covering of cells around their reproductive cells". Although cyanobacteria are referred to as "blue-green algae", most authorities exclude all prokaryotes from the definition of algae.
Algae constitute a polyphyletic group since they do not include a common ancestor, although their plastids seem to have a single origin, from cyanobacteria, they were acquired in different ways. Green algae are examples of algae that have primary chloroplasts derived from endosymbiotic cyanobacteria. Diatoms and brown algae are examples of algae with secondary chloroplasts derived from an endosymbiotic red alga. Algae exhibit a wide range of reproductive strategies, from simple asexual cell division to complex forms of sexual reproduction. Algae lack the various structures that characterize land plants, such as the phyllids of bryophytes, rhizoids in nonvascular plants, the roots and other organs found in tracheophytes. Most are phototrophic, although some are mixotrophic, deriving energy both from photosynthesis and uptake of organic carbon either by osmotrophy, myzotrophy, or phagotrophy; some unicellular species of green algae, many golden algae, euglenids and other algae have become heterotrophs, sometimes parasitic, relying on external energy sources and have limited or no photosynthetic apparatus.
Some other heterotrophic organisms, such as the apicomplexans, are derived from cells whose ancestors possessed plastids, but are not traditionally considered as algae. Algae have photosynthetic machinery derived from cyanobacteria that produce oxygen as a by-product of photosynthesis, unlike other photosynthetic bacteria such as purple and green sulfur bacteria. Fossilized filamentous algae from the Vindhya basin have been dated back to 1.6 to 1.7 billion years ago. The singular alga retains that meaning in English; the etymology is obscure. Although some speculate that it is related to Latin algēre, "be cold", no reason is known to associate seaweed with temperature. A more source is alliga, "binding, entwining"; the Ancient Greek word for seaweed was φῦκος, which could mean either the seaweed or a red dye derived from it. The Latinization, fūcus, meant the cosmetic rouge; the etymology is uncertain, but a strong candidate has long been some word related to the Biblical פוך, "paint", a cosmetic eye-shadow used by the ancient Egyptians and other inhabitants of the eastern Mediterranean.
It could be any color: black, green, or blue. Accordingly, the modern study of marine and freshwater algae is called either phycology or algology, depending on whether the Greek or Latin root is used; the name Fucus appears in a number of taxa. The algae contain chloroplasts. Chloroplasts contain circular DNA like that in cyanobacteria and are interpreted as representing reduced endosymbiotic cyanobacteria. However, the exact origin of the chloroplasts is different among separate lineages of algae, reflecting their acquisition during different endosymbiotic events; the table below describes the composition of the three major groups of algae. Their lineage relationships are shown in the figure in the upper right. Many of these groups contain some members; some retain plastids, but not chloroplasts. Phylogeny based on plastid not nucleocytoplasmic genealogy: Linnaeus, in Species Plantarum, the starting point for modern botanical nomenclature, recognized 14 genera of algae, of which only four are considered among algae.
In Systema Naturae, Linnaeus described the genera Volvox and Corallina, a species of Acetabularia, among the animals. In 1768, Samuel Gottlieb Gmelin published the Historia Fucorum, the first work dedicated to marine algae and the first book on marine biology to use the new binomial nomenclature of Linnaeus, it included elaborate illustrations of seaweed and marine algae on folded leaves. W. H. Harvey and Lamouroux were the first to divide macroscopic algae into four divisions based on their pigmentation; this is the first use of a biochemical criterion in plant systematics. Harvey's four divisions are: red algae, brown algae, green algae, Diatomaceae. At this time, microscopic algae were discovered and reported by a different group of workers studying the Infusoria. Unlike macroalgae, which were viewed as plants, microalgae were considered animals because they are motile; the nonmotile microalgae were sometimes seen as stages of the lifecycle of plants, macroalgae, or animals. Although used as a taxonomic category in some pre-D
Sea turtles, sometimes called marine turtles, are reptiles of the order Testudines and of the suborder Cryptodira. The seven existing species of sea turtles are: the green sea turtle, loggerhead sea turtle, Kemp's ridley sea turtle, olive ridley sea turtle, hawksbill sea turtle, flatback sea turtle, leatherback sea turtle; the majority of a sea turtle's body is protected by its shell. The turtle's shell is divided into two sections: the plastron; the shell is made up of smaller plates called scutes. The leatherback is the only sea turtle. Instead, it bears a mosaic of bony plates beneath its leathery skin, it is important to note that for each of the seven types of turtles, there is no differentiation between the size of the turtles based on sex. Berger, M. Look Out for Turtles. New York: HarperCollins Publishers, 1992. Jay, L. A. Our Wild World: Sea Turtles. Minnetonka, MN: NorthWord Press, 2000. Ripple, J. Sea Turtles. Stillwater: Voyageur Press, Inc. 1996. In general, sea turtles have a more fusiform body plan than their terrestrial or freshwater counterparts.
The reduced volume of a fusiform body means sea turtles cannot retract their head and arms into their shells for protection like other turtles can. However this more stream-lined body plan reduces drag in the water and allows the turtle to swim more with less friction; the leatherback is the largest species of sea turtle. Measuring 2–3 meters in length, 1-1.5 m in width, weighing up to 700 kilograms. Other species are smaller, being 60–120 cm and proportionally narrower. Sea turtles, along with other tortoises, are part of the order Testudines. All species except the leatherback are in the family Cheloniidae; the leatherback sea turtle is the only extant member of the family Dermochelyidae. The origin of sea turtles goes back to the Late Jurassic with genera such as Plesiochelys, from Europe. In Africa, the first marine turtle is Angolachelys, from the Turonian of Angola. However, neither of these are related to extant sea turtles. A lineage of unrelated marine testudines, the pleurodire bothremydids survived well into the Cenozoic.
Other pleurodires are thought to have lived at sea, such as Araripemys. Sea turtles constitute a single radiation that became distinct from all other turtles at least 110 million years ago. Sea turtles' limbs and brains have evolved to adapt to their diets. One of the main things sea turtles consume is jellyfish and the use of their limbs to hold and forage their food has helped them eat more efficiently. Sea turtles' limbs have evolved for locomotion but now they have evolved to aid them in the ability to get food. In addition to the evolution of limbs, sea turtles' brains have evolved; the frontal cortex of a sea turtle's brain was not developed. This underdevelopment caused. Below is a cladogram showing the phylogenetic relationships of living and extinct sea turtles in the Chelonioidea based on Peer and Lee Sea turtles can be found in oceans except for the polar regions; the flatback sea turtle is found on the northern coast of Australia. The Kemp's ridley sea turtle is found in the Gulf of Mexico and along the East Coast of the United States.
Sea turtles are found in the waters over continental shelves. During the first three to five years of life, sea turtles spend most of their time in the pelagic zone floating in seaweed mats. Green sea turtles in particular are found in Sargassum mats, in which they find shelter and food. Once the sea turtle has reached adulthood it moves closer to the shore. Females will come ashore to lay their eggs on sandy beaches during the nesting season. Sea turtles migrate to reach their spawning beaches. Living in the ocean therefore means they migrate over large distances. All sea turtles have large body sizes, helpful for moving large distances. Large body sizes offer good protection against the large predators found in the ocean, it takes decades for sea turtles to reach sexual maturity. Mature turtles may migrate thousands of miles to reach breeding sites. After mating at sea, adult female sea turtles return to land to lay their eggs. Different species of sea turtles exhibit various levels of philopatry.
In the extreme case, females return to the beach. This can take place every two to four years in maturity; the mature nesting female hauls herself onto the beach, nearly always at night, finds suitable sand in which to create a nest. Using her hind flippers, she digs a circular hole 40 to 50 centimetres deep. After the hole is dug, the female starts filling the nest with her clutch of soft-shelled eggs. Depending on the species, a typical clutch may contain 50–350 eggs. After laying, she re-fills the nest with sand, re-sculpting and smoothing the surface, camouflaging the nest with vegetation until it is undetectable visually; the whole process takes thirty to sixty minutes. She returns to the ocean, leaving the eggs untended. Females may lay 1–8 clutches in a single season. Female sea turtles laying their eggs on land. Most sea turtle species nest individually, but ridley sea turtles come ashore en masse. With the Kemp's ridley sea turtles this occurs during the day. Sea turtles have temperature-dependent sex determination, meaning the developing turtle'
Placodus was a genus of marine reptiles, belonging to the order Placodontia, which swam in the shallow seas of the middle Triassic period. Fossils of Placodus have been found in Central China. Placodus had a stocky body with a long tail, reached a total length of up to 2 metres, it had a short neck, a heavy skull. They were specialized for a durophagous diet such as bivalves. Chisel-like incisors protruded from the anterior margin of the snout, were used to pluck hard-shelled benthic prey from the substrate; the back teeth were broad and flattened, would have helped to crush the prey. Before the animals' anatomy was known, they were regarded as fishes' teeth. Similar smaller teeth were present on the palatine bones. Placodus and its relatives were not as well-adapted to aquatic life as some reptile groups, like the related plesiosaurs, their flattened tails and short legs, which ended in webbed feet, would have been their main means of propulsion in the water. The parietal eye on top of the head assisted the animal with orientation, rather than its vision, its presence is regarded as a primitive characteristic.
The vertebral processes of Placodus dove-tailed into each other and were connected, so that the trunk was rigid. The abdomen was covered with a special armor formed of right-angled abdominal ribs. Equipped with dense bones, heavy belly ribs, a row of bony knobs above the backbone, Placodus was a built and negatively buoyant creature that would have had no trouble staying on the seafloor to feed; this body armour would have offered protection from predators as well, but would have hampered mobility on land, making Placodus slow and clumsy out of water. It was therefore most a terrestrial animal that ventured into the sea in search of food. Molluscs, brachiopods and other inhabitants of the seabed would have formed its staple diet
The Cambrian Period was the first geological period of the Paleozoic Era, of the Phanerozoic Eon. The Cambrian lasted 55.6 million years from the end of the preceding Ediacaran Period 541 million years ago to the beginning of the Ordovician Period 485.4 mya. Its subdivisions, its base, are somewhat in flux; the period was established by Adam Sedgwick, who named it after Cambria, the Latin name of Wales, where Britain's Cambrian rocks are best exposed. The Cambrian is unique in its unusually high proportion of lagerstätte sedimentary deposits, sites of exceptional preservation where "soft" parts of organisms are preserved as well as their more resistant shells; as a result, our understanding of the Cambrian biology surpasses that of some periods. The Cambrian marked a profound change in life on Earth. Complex, multicellular organisms became more common in the millions of years preceding the Cambrian, but it was not until this period that mineralized—hence fossilized—organisms became common; the rapid diversification of life forms in the Cambrian, known as the Cambrian explosion, produced the first representatives of all modern animal phyla.
Phylogenetic analysis has supported the view that during the Cambrian radiation, metazoa evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates. Although diverse life forms prospered in the oceans, the land is thought to have been comparatively barren—with nothing more complex than a microbial soil crust and a few molluscs that emerged to browse on the microbial biofilm. Most of the continents were dry and rocky due to a lack of vegetation. Shallow seas flanked the margins of several continents created during the breakup of the supercontinent Pannotia; the seas were warm, polar ice was absent for much of the period. Despite the long recognition of its distinction from younger Ordovician rocks and older Precambrian rocks, it was not until 1994 that the Cambrian system/period was internationally ratified; the base of the Cambrian lies atop a complex assemblage of trace fossils known as the Treptichnus pedum assemblage. The use of Treptichnus pedum, a reference ichnofossil to mark the lower boundary of the Cambrian, is difficult since the occurrence of similar trace fossils belonging to the Treptichnids group are found well below the T. pedum in Namibia and Newfoundland, in the western USA.
The stratigraphic range of T. pedum overlaps the range of the Ediacaran fossils in Namibia, in Spain. The Cambrian Period was followed by the Ordovician Period; the Cambrian is divided into ten ages. Only three series and six stages are named and have a GSSP; because the international stratigraphic subdivision is not yet complete, many local subdivisions are still used. In some of these subdivisions the Cambrian is divided into three series with locally differing names – the Early Cambrian, Middle Cambrian and Furongian. Rocks of these epochs are referred to as belonging to Upper Cambrian. Trilobite zones allow biostratigraphic correlation in the Cambrian; each of the local series is divided into several stages. The Cambrian is divided into several regional faunal stages of which the Russian-Kazakhian system is most used in international parlance: *Most Russian paleontologists define the lower boundary of the Cambrian at the base of the Tommotian Stage, characterized by diversification and global distribution of organisms with mineral skeletons and the appearance of the first Archaeocyath bioherms.
The International Commission on Stratigraphy list the Cambrian period as beginning at 541 million years ago and ending at 485.4 million years ago. The lower boundary of the Cambrian was held to represent the first appearance of complex life, represented by trilobites; the recognition of small shelly fossils before the first trilobites, Ediacara biota earlier, led to calls for a more defined base to the Cambrian period. After decades of careful consideration, a continuous sedimentary sequence at Fortune Head, Newfoundland was settled upon as a formal base of the Cambrian period, to be correlated worldwide by the earliest appearance of Treptichnus pedum. Discovery of this fossil a few metres below the GSSP led to the refinement of this statement, it is the T. pedum ichnofossil assemblage, now formally used to correlate the base of the Cambrian. This formal designation allowed radiometric dates to be obtained from samples across the globe that corresponded to the base of the Cambrian. Early dates of 570 million years ago gained favour, though the methods used to obtain this number are now considered to be unsuitable and inaccurate.
A more precise date using modern radiometric dating yield a date of 541 ± 0.3 million years ago. The ash horizon in Oman from which this date was recovered corresponds to a marked fall in the abundance of carbon-13 that correlates to equivalent excursions elsewhere in the world, to the disappearance of distinctive Ediacaran fossils. There are arguments that the dated horizon in Oman does not correspond to the Ediacaran-Cambrian boundary, but represents a facies change from marine to evaporite-dominated strata — which w
China the People's Republic of China, is a country in East Asia and the world's most populous country, with a population of around 1.404 billion. Covering 9,600,000 square kilometers, it is the third- or fourth-largest country by total area. Governed by the Communist Party of China, the state exercises jurisdiction over 22 provinces, five autonomous regions, four direct-controlled municipalities, the special administrative regions of Hong Kong and Macau. China emerged as one of the world's earliest civilizations, in the fertile basin of the Yellow River in the North China Plain. For millennia, China's political system was based on hereditary monarchies, or dynasties, beginning with the semi-legendary Xia dynasty in 21st century BCE. Since China has expanded, re-unified numerous times. In the 3rd century BCE, the Qin established the first Chinese empire; the succeeding Han dynasty, which ruled from 206 BC until 220 AD, saw some of the most advanced technology at that time, including papermaking and the compass, along with agricultural and medical improvements.
The invention of gunpowder and movable type in the Tang dynasty and Northern Song completed the Four Great Inventions. Tang culture spread in Asia, as the new Silk Route brought traders to as far as Mesopotamia and Horn of Africa. Dynastic rule ended in 1912 with the Xinhai Revolution; the Chinese Civil War resulted in a division of territory in 1949, when the Communist Party of China established the People's Republic of China, a unitary one-party sovereign state on Mainland China, while the Kuomintang-led government retreated to the island of Taiwan. The political status of Taiwan remains disputed. Since the introduction of economic reforms in 1978, China's economy has been one of the world's fastest-growing with annual growth rates above 6 percent. According to the World Bank, China's GDP grew from $150 billion in 1978 to $12.24 trillion by 2017. Since 2010, China has been the world's second-largest economy by nominal GDP and since 2014, the largest economy in the world by purchasing power parity.
China is the world's largest exporter and second-largest importer of goods. China is a recognized nuclear weapons state and has the world's largest standing army and second-largest defense budget; the PRC is a permanent member of the United Nations Security Council as it replaced the ROC in 1971, as well as an active global partner of ASEAN Plus mechanism. China is a leading member of numerous formal and informal multilateral organizations, including the Shanghai Cooperation Organization, WTO, APEC, BRICS, the BCIM, the G20. In recent times, scholars have argued that it will soon be a world superpower, rivaling the United States; the word "China" has been used in English since the 16th century. It is not a word used by the Chinese themselves, it has been traced through Portuguese and Persian back to the Sanskrit word Cīna, used in ancient India."China" appears in Richard Eden's 1555 translation of the 1516 journal of the Portuguese explorer Duarte Barbosa. Barbosa's usage was derived from Persian Chīn, in turn derived from Sanskrit Cīna.
Cīna was first used including the Mahābhārata and the Laws of Manu. In 1655, Martino Martini suggested that the word China is derived from the name of the Qin dynasty. Although this derivation is still given in various sources, it is complicated by the fact that the Sanskrit word appears in pre-Qin literature; the word may have referred to a state such as Yelang. The meaning transferred to China as a whole; the origin of the Sanskrit word is still a matter of debate, according to the Oxford English Dictionary. The official name of the modern state is the "People's Republic of China"; the shorter form is "China" Zhōngguó, from zhōng and guó, a term which developed under the Western Zhou dynasty in reference to its royal demesne. It was applied to the area around Luoyi during the Eastern Zhou and to China's Central Plain before being used as an occasional synonym for the state under the Qing, it was used as a cultural concept to distinguish the Huaxia people from perceived "barbarians". The name Zhongguo is translated as "Middle Kingdom" in English.
Archaeological evidence suggests that early hominids inhabited China between 2.24 million and 250,000 years ago. The hominid fossils of Peking Man, a Homo erectus who used fire, were discovered in a cave at Zhoukoudian near Beijing; the fossilized teeth of Homo sapiens have been discovered in Fuyan Cave in Hunan. Chinese proto-writing existed in Jiahu around 7000 BCE, Damaidi around 6000 BCE, Dadiwan from 5800–5400 BCE, Banpo dating from the 5th millennium BCE; some scholars have suggested. According to Chinese tradition, the first dynasty was the Xia, which emerged around 2100 BCE; the dynasty was considered mythical by historians until scientific excavations found early Bronze Age sites at Erlitou, Henan in 1959. It remains unclear whether these sites are the remains of the Xia dynasty or of another culture from the same period; the succeeding Shang dynasty is the earliest to be confirmed by contemporary records. The Shang ruled the plain of the Yellow River in eastern China from the 17th to the 11th century BCE.
Their oracle bone script
Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, sponges, baleen whales, many fish; some birds, such as flamingos and certain species of duck, are filter feeders. Filter feeders can play an important role in clarifying water, are therefore considered ecosystem engineers, they are important in bioaccumulation and, as a result, as indicator organisms. Most forage fish are filter feeders. For example, the Atlantic menhaden, a type of herring, lives on plankton caught in midwater. Adult menhaden can filter up to four gallons of water a minute and play an important role in clarifying ocean water, they are a natural check to the deadly red tide. In addition to these bony fish, four types of cartilaginous fishes are filter feeders; the whale shark sucks in a mouthful of water, closes its mouth and expels the water through its gills.
During the slight delay between closing the mouth and opening the gill flaps, plankton is trapped against the dermal denticles which line its gill plates and pharynx. This fine sieve-like apparatus, a unique modification of the gill rakers, prevents the passage of anything but fluid out through the gills. Any material caught in the filter between the gill bars is swallowed. Whale sharks have been observed "coughing" and it is presumed that this is a method of clearing a build up of food particles in the gill rakers; the megamouth shark has luminous organs called photophores around its mouth. It is believed they may exist to lure small fish into its mouth; the basking shark is a passive filter feeder, filtering zooplankton, small fish, invertebrates from up to 2,000 tons of water per hour. Unlike the megamouth and whale sharks, the basking shark does not appear to seek its quarry. Unlike the other large filter feeders, it relies only on the water, pushed through the gills by swimming. Manta rays can time their arrival at the spawning of large shoals of fish and feed on the free-floating eggs and sperm.
This stratagem is employed by whale sharks. Mysidacea are small crustaceans that live close to shore and hover above the sea floor collecting particles with their filter basket, they are an important food source for herring, cod and striped bass. Mysids have a high resistance to toxins in polluted areas, may contribute to high toxin levels in their predators. Antarctic krill manages to directly utilize the minute phytoplankton cells, which no other higher animal of krill size can do; this is accomplished through filter feeding, using the krill's developed front legs, providing for a efficient filtering apparatus: the six thoracopods form a effective "feeding basket" used to collect phytoplankton from the open water. In the animation at the top of this page, the krill is hovering at a 55° angle on the spot. In lower food concentrations, the feeding basket is pushed through the water for over half a meter in an opened position, the algae are combed to the mouth opening with special setae on the inner side of the thoracopods.
Porcelain crabs have feeding appendages covered with setae to filter food particles from the flowing water. Most species of barnacles are filter feeders, using their modified legs to sift plankton from the water; the baleen whales, one of two suborders of the Cetacea, are characterized by having baleen plates for filtering food from water, rather than teeth. This distinguishes them from the other suborder of the toothed whales; the suborder contains fourteen species. Baleen whales seek out a concentration of zooplakton, swim through it, either open-mouthed or gulping, filter the prey from the water using their baleens. A baleen is a row of a large number of keratin plates attached to the upper jaw with a composition similar to those in human hair or fingernails; these plates are triangular in section with the largest, inward-facing side bearing fine hairs forming a filtering mat. Right whales are slow swimmers with large mouths, their baleen plates are narrow and long — up to 4 m in bowheads — and accommodated inside the enlarged lower lip which fits onto the bowed upper jaw.
As the right whale swims, a front gap between the two rows of baleen plates lets the water in together with the prey, while the baleens filter out the water. Rorquals such as the blue whale, in contrast, have smaller heads, are fast swimmers with short and broad baleen plates. To catch prey, they open their lower jaw — 90° — swim through a swarm gulping, while lowering their tongue so that the head's ventral grooves expand and vastly increase the amount of water taken in. Baleen whales eat krill in polar or subpolar waters during summers, but can take schooling fish in the Northern Hemisphere. All baleen whales except the gray whale feed near the water surface diving deeper than 100 m or for extended periods. Gray whales live in shallow waters feeding on bottom-living organisms such as amphipods. Bivalves are aquatic molluscs. Both shells are symmetrical along the hinge line; the class has 30,000 species, including scallops, clams and mussels. Most bivalves are filter feeders, extracting organic matter