An estuary is a enclosed coastal body of brackish water with one or more rivers or streams flowing into it, with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environments, they are subject both to marine influences—such as tides and the influx of saline water—and to riverine influences—such as flows of fresh water and sediment. The mixing of sea water and fresh water provide high levels of nutrients both in the water column and in sediment, making estuaries among the most productive natural habitats in the world. Most existing estuaries formed during the Holocene epoch with the flooding of river-eroded or glacially scoured valleys when the sea level began to rise about 10,000–12,000 years ago. Estuaries are classified according to their geomorphological features or to water-circulation patterns, they can have many different names, such as bays, lagoons, inlets, or sounds, although some of these water bodies do not meet the above definition of an estuary and may be saline.
The banks of many estuaries are amongst the most populated areas of the world, with about 60% of the world's population living along estuaries and the coast. As a result, many estuaries suffer degradation from a variety of factors including: sedimentation from soil erosion from deforestation and other poor farming practices; the word "estuary" is derived from the Latin word aestuarium meaning tidal inlet of the sea, which in itself is derived from the term aestus, meaning tide. There have been many definitions proposed to describe an estuary; the most accepted definition is: "a semi-enclosed coastal body of water, which has a free connection with the open sea, within which sea water is measurably diluted with freshwater derived from land drainage". However, this definition excludes a number of coastal water bodies such as coastal lagoons and brackish seas. A more comprehensive definition of an estuary is "a semi-enclosed body of water connected to the sea as far as the tidal limit or the salt intrusion limit and receiving freshwater runoff.
This broad definition includes fjords, river mouths, tidal creeks. An estuary is a dynamic ecosystem having a connection to the open sea through which the sea water enters with the rhythm of the tides; the sea water entering the estuary streams. The pattern of dilution varies between different estuaries and depends on the volume of fresh water, the tidal range, the extent of evaporation of the water in the estuary. Drowned river valleys are known as coastal plain estuaries. In places where the sea level is rising relative to the land, sea water progressively penetrates into river valleys and the topography of the estuary remains similar to that of a river valley; this is the most common type of estuary in temperate climates. Well-studied estuaries include the Severn Estuary in the United Kingdom and the Ems Dollard along the Dutch-German border; the width-to-depth ratio of these estuaries is large, appearing wedge-shaped in the inner part and broadening and deepening seaward. Water depths exceed 30 m.
Examples of this type of estuary in the U. S. are the Hudson River, Chesapeake Bay, Delaware Bay along the Mid-Atlantic coast, Galveston Bay and Tampa Bay along the Gulf Coast. Bar-built estuaries are found in place where the deposition of sediment has kept pace with rising sea level so that the estuaries are shallow and separated from the sea by sand spits or barrier islands, they are common in tropical and subtropical locations. These estuaries are semi-isolated from ocean waters by barrier beaches. Formation of barrier beaches encloses the estuary, with only narrow inlets allowing contact with the ocean waters. Bar-built estuaries develop on sloping plains located along tectonically stable edges of continents and marginal sea coasts, they are extensive along the Atlantic and Gulf coasts of the U. S. in areas with active coastal deposition of sediments and where tidal ranges are less than 4 m. The barrier beaches that enclose bar-built estuaries have been developed in several ways: building up of offshore bars by wave action, in which sand from the sea floor is deposited in elongated bars parallel to the shoreline, reworking of sediment discharge from rivers by wave and wind action into beaches, overwash flats, dunes, engulfment of mainland beach ridges due to sea level rise and resulting in the breaching of the ridges and flooding of the coastal lowlands, forming shallow lagoons, elongation of barrier spits from the erosion of headlands due to the action of longshore currents, with the spits growing in the direction of the littoral drift.
Barrier beaches form in shallow water and are parallel to the shoreline, resulting in long, narrow estuaries. The average water depth is less than 5 m, exceeds 10 m. Examples of bar-built estuaries are Barnegat Bay, New Jersey. Fjords were formed where pleistocene glaciers deepened and widened existing river valleys so that they become U-shaped in cross s
An embryo is an early stage of development of a multicellular diploid eukaryotic organism. In general, in organisms that reproduce sexually, an embryo develops from a zygote, the single cell resulting from the fertilization of the female egg cell by the male sperm cell; the zygote possesses half the DNA from each of its two parents. In plants and some protists, the zygote will begin to divide by mitosis to produce a multicellular organism; the result of this process is an embryo. In human pregnancy, a developing fetus is considered as an embryo until the ninth week, fertilization age, or eleventh-week gestational age. After this time the embryo is referred to as a fetus. First attested in English in the mid-14c; the word embryon itself from Greek ἔμβρυον, lit. "young one", the neuter of ἔμβρυος, lit. "growing in", from ἐν, "in" and βρύω, "swell, be full". In animals, the development of the zygote into an embryo proceeds through specific recognizable stages of blastula and organogenesis; the blastula stage features a fluid-filled cavity, the blastocoel, surrounded by a sphere or sheet of cells called blastomeres.
In a placental mammal, an ovum is fertilized in a fallopian tube through which it travels into the uterus. An embryo is called a fetus at a more advanced stage of development and up until hatching. In humans, this is from the eleventh week of gestation. However, animals which develop in eggs outside the mother's body, are referred to as embryos throughout development. During gastrulation the cells of the blastula undergo coordinated processes of cell division, and/or migration to form two or three tissue layers. In triploblastic organisms, the three germ layers are called endoderm and mesoderm; the position and arrangement of the germ layers are species-specific, depending on the type of embryo produced. In vertebrates, a special population of embryonic cells called the neural crest has been proposed as a "fourth germ layer", is thought to have been an important novelty in the evolution of head structures. During organogenesis and cellular interactions between germ layers, combined with the cells' developmental potential, or competence to respond, prompt the further differentiation of organ-specific cell types.
For example, in neurogenesis, a subpopulation of ectoderm cells is set aside to become the brain, spinal cord, peripheral nerves. Modern developmental biology is extensively probing the molecular basis for every type of organogenesis, including angiogenesis, myogenesis and many others. In botany, a seed plant embryo is part of a seed, consisting of precursor tissues for the leaves and root, as well as one or more cotyledons. Once the embryo begins to germinate—grow out from the seed—it is called a seedling. Bryophytes and ferns produce an embryo, but do not produce seeds. In these plants, the embryo begins its existence attached to the inside of the archegonium on a parental gametophyte from which the egg cell was generated; the inner wall of the archegonium lies in close contact with the "foot" of the developing embryo. The structure and development of the rest of the embryo varies by group of plants; as the embryo has expanded beyond the enclosing archegonium, it is no longer termed an embryo.
Embryos are used in various fields of research and in techniques of assisted reproductive technology. An egg may be fertilized in vitro and the resulting embryo may be frozen for use; the potential of embryonic stem cell research, reproductive cloning, germline engineering are being explored. Prenatal diagnosis or preimplantation diagnosis enables testing embryos for conditions. Cryoconservation of animal genetic resources is a practice in which animal germplasms, such as embryos are collected and stored at low temperatures with the intent of conserving the genetic material; the embryos of Arabidopsis thaliana have been used as a model to understand gene activation and organogenesis of seed plants. In regards to research using human embryos, the ethics and legalities of this application continue to be debated. Researchers from MERLN Institute and the Hubrecht Institute in the Netherlands managed to grow samples of synthetic rodent embryos, combining certain types of stem cells; this method will help scientists to more study the first moments of the process of the birth of a new life, which, in turn, can lead to the emergence of new effective methods to combat infertility and other genetic diseases.
Fossilized animal embryos are known from the Precambrian, are found in great numbers during the Cambrian period. Fossilized dinosaur embryos have been discovered; some embryos do not survive to the next stage of development. When this happens it is called spontaneous abortion or miscarriage. There are many reasons; the most common natural cause of miscarriage is chromosomal abnormality in animals or genetic load in plants. In species which produce multiple embryos at the same time, miscarriage or abortion of some embryos can provide the remaining embryos with a greater share of maternal resources; this can disturb the pregnancy, causing harm to the second embryo. Genetic strains which miscarry their embryos are the source of commercial seedl
A fish fry is a meal containing battered or breaded fried fish. It also includes french fries, hushpuppies, lemon slices, tartar sauce, hot sauce, malt vinegar and dessert; some Native American versions are cooked by coating fish with egg yolk. Fish is served on Friday nights during Lent as a restaurant special or through church fundraisers. Beer is a common beverage of choice to accompany a fish fry. A fish fry may include potato pancakes and sliced caraway rye bread if served in a German restaurant or area. A Shore Lunch is traditional in Canada. For decades outdoor enthusiasts have been cooking their catch on the shores of their favorite lakes. Fish fries are common in the Midwestern and northeastern regions of the United States; this is true for predominantly Roman Catholic communities on Fridays during Lent, when regulations call for abstinence from most meat products. The tradition of Christians fasting on Fridays to recognize Jesus's crucifixion on Good Friday dates to the first century AD. Fish had been associated with religious holidays in pre-Christian times.
The first mention of fish in connection with Lent comes from Socrates of Constantinople, a church historian in the third and fourth centuries who spoke of abstaining from meat and meat products during the 40 days of Lent. The custom was mentioned by Pope Gregory I, elected in 590, was incorporated into canon law. Roman Catholic tradition has been that the flesh of warm-blooded animals is off limits on Fridays, although the 1983 Code of Canon Law provided for alternative observances of the Friday penance. McDonald's addition of the Filet-o-Fish to its menu occurred when a Cincinnati franchise in a Catholic neighborhood was struggling to sell hamburgers on Fridays during Lent. Battered or breaded haddock and cod fish fry is one of the trademarks of upstate New York cuisine and northwestern Pennsylvania Albany and Rochester, as well as Syracuse, New York, Utica, New York; the majority of restaurants in these cities serve a fish fry on Friday outside Lent, it's available throughout the week. These meals consist of a piece of fish, french fries, a dinner roll.
In the southern United States, a fish fry is a family or social gathering, held outdoors or in large halls. At a typical fish fry, quantities of fish available locally are battered and deep fried in cooking oil; the batter consists of corn meal, milk or buttermilk, seasonings. In addition to the fish and cole slaw are served; these events are potluck affairs. In Georgia and South Carolina, fish are dipped in milk into a mix of flour and seasonings before frying. Cheese grits is a side dish; the modern fish fry tradition is strong in Wisconsin, where hundreds of eateries hold fish fries on Fridays, sometimes on Wednesdays. The Friday night fish fry is a popular-year round tradition in Wisconsin among people of all religious backgrounds. Fish fries there are offered at many non-chain restaurants, taverns that serve food, some chain restaurants, at Roman Catholic churches as fundraisers. A typical Wisconsin fish fry consists of beer batter fried cod, bluegill, smelt, or in areas along the Mississippi River, catfish.
The meal comes with tartar sauce, french fries or German-style potato pancakes and rye bread, though baked beans are not uncommon. The tradition in Wisconsin began because Wisconsin was settled by Catholics of German and other backgrounds whose religion forbade eating meat on Fridays; the number of lakes in the state meant. Scandinavian settlements in northern and eastern Wisconsin favored the fish boil, a variant on the fish fry, which involves heating potatoes, white fish, salt in a large cauldron
The development of fishes is unique in some specific aspects compared to the development of other animals. Most bony fish eggs are referred to as telolecithal which means that most of the egg cell cytoplasm is yolk; the yolky end of the egg remains homogenous. Cleavage, or initial cell division, can only occur in a region called the blastodisc, a yolk free region located at the animal pole of the egg; the fish zygote is meroblastic, meaning the early cell divisions are not complete. This type of meroblastic cleavage is called discoidal because only the blastodisc becomes the embryo. In fish, waves of calcium released direct the process of cell division by coordinating the mitotic apparatus with the actin cytoskeleton, propagating cell division along the surface, assists in deepening the cleavage furrow, heals the membrane after separation of blastomeres; the fate of the first cells, called blastomeres, is determined by its location. This contrasts with the situation in some other animals, such as mammals, in which each blastomere can develop into any part of the organism.
Fish embryos go through a process called mid-blastula transition, observed around the tenth cell division in some fish species. Once zygotic gene transcription starts, slow cell division begins and cell movements are observable. During this time three cell populations become distinguished; the first population is the yolk syncytial layer. This layer forms when the cells at the vegetal pole of the blastoderm combine with the yolk cell underneath it. In development the yolk syncytial layer will be important in directing cell movements of gastrulation; the second cell population is the enveloping layer, made of superficial cells from the blastoderm that form a single epithetial cell layer. This layer functions in protection by allowing the embryo to develop in a hypotonic solution so the cell will not burst; the third set of blastomeres are the deep cells. These deep cells are located between the enveloping layer and the yolk syncytial layer and give ride to the embryo proper. Once blastoderm cells have covered half of the yolk cell, thickening throughout the margin of deep cells occurs.
The thickening is referred to as the germ ring and is made up of a superficial layer, the epiblast which will become ectoderm, an inner layer called the hypoblast which will become endoderm and mesoderm. As the blastoderm cells undergo epiboly around the yolk the internalization of cells at the blastoderm margin start to form hypoblast. Presumptive ectoderm or epiblast cells do not interalize but the deep cells do and they become the mesoderm and endoderm; as the hypoblast cells move inward future mesoderm start to move vegetally and proliferate but in development these cells alter their direction and start moving towards the animal pole. However, endodermal precursors move randomly over the yolk. Once the egg has become multicellular and positioned its germ layers with ectoderm on the outside,mesoderm in the middle, endoderm on the inside body axes have to be determined for proper development. A dorsal-ventral axis has to form and major proteins involved are BMP and Wnts. Both proteins are made in the lateral portions of the developing embryo.
BMP2B induces cells to have ventral and lateral fates while factors such as chordin can block BMPs to dorsalize the tissue. Wnt8 induces ventral and posterior regions of embryonic tissue. Wnt has inhibitors like noggin to allow for the formation of dorsal tissue. In order to aid in proper development fish have an organizer center called the Nieuwkoop center. Anterior and posterior axis formation seems to be the result of interplay of FGFs, retinoic acid. FGFs, retinoic acid, Wnts are required to turn on posterior genes. Neurulation, the formation of the central nervous system, is different in fishes than in most other chordates. Convergence and extension in the epiblast recruits presumptive neural cells from the epiblast towards the midline where they form a neural keel. A neural keel is a band of neural precursors that develops a slit like lumen to become the neural tube; the neural tube begins as a solid cord formed from the ectoderm. This cord sinks into the embryo and becomes hollow, forming the neural tube.
This process contrasts with the process in other chordates, which occurs by an infolding of the ectoderm to form a hollow tube. Throughout the years advances in research have shown that neural formation relies on interactions between extrinsic signaling factors and intrinsic transcription factors. Extrinsic signals involved are BMP, FGF and intrinsic transcription factors like SoxB1 related genes. Secreted proteins such as BMP and its antagonist Noggin and chordin act permissively to establish the fate of neural tissue in the dorsal ectoderm and enables the formation of the neural plate. Sex determination is variable in fish from environmental factors like temperature to genetic mechanisms; some fish have XX/XY chromosomes and others have ZZ/ZW. So far one gene in specific, DMRT1bY, has been described as a sex determining gene; this gene is expressed before gonads differentiate. Mutations in this gene lead to sex reversal from male to female. While this gene plays a major role in sex determination in some fish species other species have variations of this gene as well as some versions of the Sox gene as seen in zebrafish.
Many species of fishes are hermaphrodites. Some, such as the painted comber, are synchronous hermpahrodites; these fish can produce both eggs and sperm at the same time. Others are sequential hermaphrodites; these fi
Shoaling and schooling
In biology, any group of fish that stay together for social reasons are shoaling, if the group is swimming in the same direction in a coordinated manner, they are schooling. In common usage, the terms are sometimes used rather loosely. About one quarter of fish species shoal all their lives, about one half shoal for part of their lives. Fish derive many benefits from shoaling behaviour including defence against predators, enhanced foraging success, higher success in finding a mate, it is likely that fish benefit from shoal membership through increased hydrodynamic efficiency. Fish use many traits to choose shoalmates, they prefer larger shoals, shoalmates of their own species, shoalmates similar in size and appearance to themselves, healthy fish, kin. The "oddity effect" posits that any shoal member that stands out in appearance will be preferentially targeted by predators; this may explain. The oddity effect thus tends to homogenize shoals. An aggregation of fish is the general term for any collection of fish that have gathered together in some locality.
Fish aggregations can be unstructured. An unstructured aggregation might be a group of mixed species and sizes that have gathered randomly near some local resource, such as food or nesting sites. If, in addition, the aggregation comes together in an interactive, social way, they may be said to be shoaling. Although shoaling fish can relate to each other in a loose way, with each fish swimming and foraging somewhat independently, they are nonetheless aware of the other members of the group as shown by the way they adjust behaviour such as swimming, so as to remain close to the other fish in the group. Shoaling groups can include mixed-species subgroups. If the shoal becomes more organised, with the fish synchronising their swimming so they all move at the same speed and in the same direction the fish may be said to be schooling. Schooling fish are of the same species and the same age/size. Fish schools move with the individual members spaced from each other; the schools undertake complicated manoeuvres.
The intricacies of schooling are far from understood the swimming and feeding energetics. Many hypotheses to explain the function of schooling have been suggested, such as better orientation, synchronized hunting, predator confusion and reduced risk of being found. Schooling has disadvantages, such as excretion buildup in the breathing media and oxygen and food depletion; the way the fish array in the school gives energy saving advantages, though this is controversial. Fish can be facultative shoalers. Obligate shoalers, such as tunas and anchovy, spend all of their time shoaling or schooling, become agitated if separated from the group. Facultative shoalers, such as Atlantic cod and some carangids, shoal only some of the time for reproductive purposes. Shoaling fish can shift into a disciplined and coordinated school shift back to an amorphous shoal within seconds; such shifts are triggered by changes of activity from feeding, travelling or avoiding predators. When schooling fish stop to feed, they become shoals.
Shoals are more vulnerable to predator attack. The shape a shoal or school takes what the fish are doing. Schools that are travelling can form squares or ovals or amoeboid shapes. Fast moving schools form a wedge shape, while shoals that are feeding tend to become circular. Forage fish are small fish. Predators include other larger fish and marine mammals. Typical ocean forage fish are small, filter-feeding fish such as herring and menhaden. Forage fish compensate for their small size by forming schools; some swim in synchronised grids with their mouths open so they can efficiently filter feed on plankton. These schools can become huge, migrating across open oceans; the shoals are concentrated food resources for the great marine predators. These sometimes immense gatherings fuel the ocean food web. Most forage fish are pelagic fish, which means they form their schools in open water, not on or near the bottom. Forage fish are short-lived, go unnoticed by humans; the predators are keenly focused on the shoals, acutely aware of their numbers and whereabouts, make migrations themselves in schools of their own, that can span thousands of miles to connect with, or stay connected with them.
Herring are among the more spectacular schooling fish. They aggregate together in huge numbers; the largest schools are formed during migrations by merging with smaller schools. "Chains" of schools one hundred kilometres long have been observed of mullet migrating in the Caspian Sea. Radakov estimated herring schools in the North Atlantic can occupy up to 4.8 cubic kilometres with fish densities between 0.5 and 1.0 fish/cubic metre, totalling about three billion fish in a single school. These schools move along traverse the open oceans. Herring schools in general have precise arrangements which allow the school to maintain constant cruising speeds. Herrings have excellent hearing, their schools react rapidly to a predator; the herrings keep a certain distance from a moving scuba diver or a cruising predator like a killer whale, forming a vacuole which looks like a doughnut from a spotter plane. Many species of large predatory fish also
Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans and bodies of fresh water; the word zooplankton is derived from the Greek zoon, meaning "animal", planktos, meaning "wanderer" or "drifter". Individual zooplankton are microscopic, but some are larger and visible to the naked eye. Zooplankton is a categorization spanning a range of organism sizes including small protozoans and large metazoans, it includes holoplanktonic organisms whose complete life cycle lies within the plankton, as well as meroplanktonic organisms that spend part of their lives in the plankton before graduating to either the nekton or a sessile, benthic existence. Although zooplankton are transported by ambient water currents, many have locomotion, used to avoid predators or to increase prey encounter rate. Ecologically important protozoan zooplankton groups include the foraminiferans and dinoflagellates. Important metazoan zooplankton include cnidarians such as the Portuguese Man o' War; this wide phylogenetic range includes a wide range in feeding behavior: filter feeding and symbiosis with autotrophic phytoplankton as seen in corals.
Zooplankton feed on bacterioplankton, other zooplankton and nektonic organisms. As a result, zooplankton are found in surface waters where food resources are abundant. Just as any species can be limited within a geographical region, so are zooplankton. However, species of zooplankton are not dispersed uniformly or randomly within a region of the ocean; as with phytoplankton, ‘patches’ of zooplankton species exist throughout the ocean. Though few physical barriers exist above the mesopelagic, specific species of zooplankton are restricted by salinity and temperature gradients. Zooplankton patchiness can be influenced by biological factors, as well as other physical factors. Biological factors include breeding, concentration of phytoplankton, vertical migration; the physical factor that influences zooplankton distribution the most is mixing of the water column that affects nutrient availability and, in turn, phytoplankton production. Through their consumption and processing of phytoplankton and other food sources, zooplankton play a role in aquatic food webs, as a resource for consumers on higher trophic levels, as a conduit for packaging the organic material in the biological pump.
Since they are small, zooplankton can respond to increases in phytoplankton abundance, for instance, during the spring bloom. Zooplankton can act as a disease reservoir. Crustacean zooplankton have been found to house the bacterium Vibrio cholerae, which causes cholera, by allowing the cholera vibrios to attach to their chitinous exoskeletons; this symbiotic relationship enhances the bacterium's ability to survive in an aquatic environment, as the exoskeleton provides the bacterium with carbon and nitrogen. Bacterioplankton Biological pump Census of Marine Zooplankton Diel vertical migration Gelatinous zooplankton Ocean acidification Phytoplankton Plankton Primary production SAHFOS Sir Alister Hardy Foundation for Ocean Science Ocean Drifters Short film narrated by David Attenborough about the varied roles of plankton Sea Drifters BBC Audio slideshow Plankton Chronicles Short documentary films & photos COPEPOD: The global plankton database. A global coverage database of zooplankton biomass and abundance data.
Guide to the marine zooplankton of south eastern Australia, Tasmanian Aquaculture and Fisheries Institute Australian Continuous Plankton Recorder Project An Image-Based Key to Zooplankton of North America
The egg is the organic vessel containing the zygote in which an embryo develops until it can survive on its own. An egg results from fertilization of an egg cell. Most arthropods and mollusks lay eggs, although some, such as scorpions do not. Reptile eggs, bird eggs, monotreme eggs are laid out of water, are surrounded by a protective shell, either flexible or inflexible. Eggs laid on land or in nests are kept within a warm and favorable temperature range while the embryo grows; when the embryo is adequately developed it hatches, i.e. breaks out of the egg's shell. Some embryos have a temporary egg tooth they use to pip, or break the eggshell or covering; the largest recorded egg is from a whale shark, was 30 cm × 14 cm × 9 cm in size. Whale shark eggs hatch within the mother. At 1.5 kg and up to 17.8 cm × 14 cm, the ostrich egg is the largest egg of any living bird, though the extinct elephant bird and some dinosaurs laid larger eggs. The bee hummingbird produces the smallest known bird egg; some eggs laid by reptiles and most fish, amphibians and other invertebrates can be smaller.
Reproductive structures similar to the egg in other kingdoms are termed "spores," or in spermatophytes "seeds," or in gametophytes "egg cells". Several major groups of animals have distinguishable eggs; the most common reproductive strategy for fish is known as oviparity, in which the female lays undeveloped eggs that are externally fertilized by a male. Large numbers of eggs are laid at one time and the eggs are left to develop without parental care; when the larvae hatch from the egg, they carry the remains of the yolk in a yolk sac which continues to nourish the larvae for a few days as they learn how to swim. Once the yolk is consumed, there is a critical point after which they must learn how to hunt and feed or they will die. A few fish, notably the rays and most sharks use ovoviviparity in which the eggs are fertilized and develop internally; however the larvae still grow inside the egg consuming the egg's yolk and without any direct nourishment from the mother. The mother gives birth to mature young.
In certain instances, the physically most developed offspring will devour its smaller siblings for further nutrition while still within the mother's body. This is known as intrauterine cannibalism. In certain scenarios, some fish such as the hammerhead shark and reef shark are viviparous, with the egg being fertilized and developed internally, but with the mother providing direct nourishment; the eggs of fish and amphibians are jellylike. Cartilagenous fish eggs are fertilized internally and exhibit a wide variety of both internal and external embryonic development. Most fish species spawn eggs that are fertilized externally with the male inseminating the eggs after the female lays them; these eggs would dry out in the air. Air-breathing amphibians lay their eggs in water, or in protective foam as with the Coast foam-nest treefrog, Chiromantis xerampelina. Bird eggs are incubated for a time that varies according to the species. Average clutch sizes range from one to about 17; some birds lay eggs when not fertilized.
The default color of vertebrate eggs is the white of the calcium carbonate from which the shells are made, but some birds passerines, produce colored eggs. The pigment biliverdin and its zinc chelate give a green or blue ground color, protoporphyrin produces reds and browns as a ground color or as spotting. Non-passerines have white eggs, except in some ground-nesting groups such as the Charadriiformes and nightjars, where camouflage is necessary, some parasitic cuckoos which have to match the passerine host's egg. Most passerines, in contrast, lay colored eggs if there is no need of cryptic colors; however some have suggested that the protoporphyrin markings on passerine eggs act to reduce brittleness by acting as a solid state lubricant. If there is insufficient calcium available in the local soil, the egg shell may be thin in a circle around the broad end. Protoporphyrin speckling compensates for this, increases inversely to the amount of calcium in the soil. For the same reason eggs in a clutch are more spotted than early ones as the female's store of calcium is depleted.
The color of individual eggs is genetically influenced, appears to be inherited through the mother only, suggesting that the gene responsible for pigmentation is on the sex determining W chromosome. It used to be thought that color was applied to the shell before laying, but this research shows that coloration is an integral part of the development of the shell, with the same protein responsible for depositing calcium carbonate, or protoporphyrins when there is a lack of that mineral. In species such as the common guillemot, which nest in large groups, each female's eggs have different markings, making it easier for females to identify their own eggs on the crowded cliff ledges on which they breed. Bird eggshells are diverse. For example: cormorant eggs are rough and chalky tinamou eggs are shiny duck eggs are oily and waterproof cassowary eggs are pittedTiny pores in bird eggshells allow the embryo to breathe; the domestic