Ichthyoplankton (from Greek: ἰχθύς, , "fish"; and πλαγκτός, , "drifter") are the eggs and

larvae A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect developmental biology, development such as insects, some arachnids, amphibians, or cnidarians typical ...

of fish. They are mostly found in the sunlit zone of the

water column The (oceanic) water column is a concept used in oceanography to describe the physical (temperature, salinity, light penetration) and chemical ( pH, dissolved oxygen, nutrient salts) characteristics of seawater at different depths for a defined ...

, less than 200 metres deep, which is sometimes called the

epipelagic The photic zone (or euphotic zone, epipelagic zone, or sunlight zone) is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological ...

photic zone The photic zone (or euphotic zone, epipelagic zone, or sunlight zone) is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological ...

. Ichthyoplankton are

planktonic Plankton are the diverse collection of organisms that drift in water (or air) but are unable to actively propel themselves against currents (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they pro ...

, meaning they cannot swim effectively under their own power, but must drift with the ocean currents. Fish eggs cannot swim at all, and are unambiguously planktonic. Early stage larvae swim poorly, but later stage larvae swim better and cease to be planktonic as they grow into juveniles. Fish larvae are part of the

zooplankton Zooplankton are the heterotrophic component of the planktonic community (the " zoo-" prefix comes from ), having to consume other organisms to thrive. Plankton are aquatic organisms that are unable to swim effectively against currents. Consequent ...

that eat smaller plankton, while fish eggs carry their own food supply. Both eggs and larvae are themselves eaten by larger animals. Fish can produce high numbers of eggs which are often released into the open water column. Fish eggs typically have a diameter of about . The newly hatched young of oviparous fish are called

larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...

e. They are usually poorly formed, carry a large

yolk sac The yolk sac is a membranous wikt:sac, sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac' ...

(for nourishment) and are very different in appearance from juvenile and adult specimens. The larval period in oviparous fish is relatively short (usually only several weeks), and larvae rapidly grow and change appearance and structure (a process termed

metamorphosis Metamorphosis is a biological process by which an animal physically develops including birth transformation or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and different ...

) to become juveniles. During this transition larvae must switch from their yolk sac to feeding on

prey, a process which depends on typically inadequate zooplankton density, starving many larvae. Ichthyoplankton can be a useful indicator of the state and health of an

aquatic ecosystem An aquatic ecosystem is an ecosystem found in and around a body of water, in contrast to land-based terrestrial ecosystems. Aquatic ecosystems contain communities of organisms—aquatic life—that are dependent on each other and on their environ ...

. For instance, most late stage larvae in ichthyoplankton have usually been preyed on, so ichthyoplankton tends to be dominated by eggs and early stage larvae. This means that when fish, such as

anchovies An anchovy is a small, common forage fish of the family Engraulidae. Most species are found in marine waters, but several will enter brackish water, and some in South America are restricted to fresh water. More than 140 species are placed in 1 ...

and

sardine Sardine and pilchard are common names for various species of small, oily forage fish in the herring suborder Clupeoidei. The term "sardine" was first used in English during the early 15th century; a somewhat dubious etymology says it com ...

s, are

spawning Spawn is the Egg cell, eggs and Spermatozoa, sperm released or deposited into water by aquatic animals. As a verb, ''to spawn'' refers to the process of freely releasing eggs and sperm into a body of water (fresh or marine); the physical act is ...

, ichthyoplankton samples can reflect their spawning output and provide an index of relative population size for the fish. Increases or decreases in the number of adult fish stocks can be detected more rapidly and sensitively by monitoring the ichthyoplankton associated with them, compared to monitoring the adults themselves. It is also usually easier and more cost effective to sample trends in egg and larva populations than to sample trends in adult fish populations.

History

Interest in plankton originated in Britain and Germany in the nineteenth century when researchers discovered there were

microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...

s in the sea, and that they could trap them with fine-mesh nets. They started describing these microorganisms and testing different net configurations. Ichthyoplankton research started in 1864 when the Norwegian government commissioned the

marine biologist Marine biology is the scientific study of the biology of marine life, organisms that inhabit the sea. Given that in biology many phyla, families and genera have some species that live in the sea and others that live on land, marine biology clas ...

G. O. Sars to investigate

fisheries Fishery can mean either the enterprise of raising or harvesting fish and other aquatic life or, more commonly, the site where such enterprise takes place ( a.k.a., fishing grounds). Commercial fisheries include wild fisheries and fish farm ...

around the Norwegian coast. Sars found fish eggs, particularly

cod Cod (: cod) is the common name for the demersal fish genus ''Gadus'', belonging to the family (biology), family Gadidae. Cod is also used as part of the common name for a number of other fish species, and one species that belongs to genus ''Gad ...

eggs, drifting in the water. This established that fish eggs could be

pelagic The pelagic zone consists of the water column of the open ocean and can be further divided into regions by depth. The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or water column between the sur ...

, living in the open water column like other plankton.
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Around the beginning of the twentieth century, research interest in ichthyoplankton became more general when it emerged that, if ichthyoplankton was sampled

quantitatively Quantitative research is a research strategy that focuses on quantifying the collection and analysis of data. It is formed from a deductive approach where emphasis is placed on the testing of theory, shaped by empiricist and positivist philos ...

, then the samples could indicate the relative size or abundance of spawning fish stocks.

Sampling methods

File:PairoVET tow.jpg, PairoVET tow File:Bongo tow.jpg, Bongo tow File:THE "MARTIN KARLSEN," A CANADIAN SHIP, IS A FLOATING LABORATORY FROM WHICH SCIENTISTS ARE ANALYZING PHYSICAL... - NARA - 549596.tif, Retrieving a plankton sample

Research vessel A research vessel (RV or R/V) is a ship or boat designed, modified, or equipped to carry out research at sea. Research vessels carry out a number of roles. Some of these roles can be combined into a single vessel but others require a dedicated ...

s collect ichthyoplankton from the ocean using fine mesh nets. The vessels either tow the nets through the sea or pump sea water onboard and then pass it through the net.Ichthyoplankton sampling methods
Southwest Fisheries Science Center, ''

NOAA The National Oceanic and Atmospheric Administration (NOAA ) is an American scientific and regulatory agency charged with forecasting weather, monitoring oceanic and atmospheric conditions, charting the seas, conducting deep-sea exploratio ...

''. Retrieved 11 July 2020.

* There are many types of plankton tows: :*

Neuston Neuston, also called pleuston, are organisms that live at the surface of a body of water, such as an ocean, estuary, lake, river, wetland or pond. Neuston can live on top of the water surface or submersed just below the water surface. In additio ...

net tows are often made at or just below the surface using a nylon mesh net fitted to a rectangular frame :* The PairoVET tow, used for collecting fish eggs, drops a net about 70 metres into the sea from a stationary research vessel and then drags it back to the vessel. :* Ring net tows involve a nylon mesh net fitted to a circular frame. These have largely been replaced by bongo nets, which provide duplicate samples with their dual-net design. :* The bongo tow drags nets shaped like

bongo drum Bongos ( Spanish: ''bongó'') are an Afro-Cuban percussion instrument consisting of a pair of small open bottomed hand drums of different sizes. The pair consists of the larger ''hembra'' () and the smaller ''macho'' (), which are joined by a wo ...

s from a moving vessel. The net is often lowered to about 200 metres and then allowed to rise to the surface as it is towed. In this way, a sample can be collected across the whole

where most ichthyoplankton is found. :* MOCNESS tows and Tucker trawls utilize multiple nets that are mechanically opened and closed at discrete depths in order to provide insights into the vertical distribution of the plankton :* The manta trawl tows a net from a moving vessel along the surface of the water, collecting larvae, such as

grunion Grunion are two fish species of the genus ''Leuresthes'': the California grunion, ''L. tenuis'', and the Gulf grunion, ''L. sardina''. They are sardine-sized teleost fishes of the New World silverside family Atherinopsidae, found only off th ...

mahi-mahi The mahi-mahi ( ) or common dolphinfish (''Coryphaena hippurus'') is a surface-dwelling ray-finned fish found in off-shore temperate, tropical, and subtropical waters worldwide. It is also widely called dorado (not to be confused with '' Salmin ...

, and

flying fish The Exocoetidae are a family (biology), family of Saltwater fish, marine Actinopterygii, ray-finned fish in the order (biology), order Beloniformes, known colloquially as flying fish or flying cod. About 64 species are grouped in seven genus, ge ...

which live at the surface. :After the tow the plankton is flushed with a hose to the cod end (bottom) of the net for collection. The sample is then placed in preservative fluid prior to being sorted and identified in a laboratory. * Plankton pumps: Another method of collecting ichthyoplankton is to use a Continuous Underway Fish Egg Sampler (see illustration). Water from a depth of about three metres is pumped onto the vessel and filtered with a net. This method can be used while the vessel is underway.

Developmental stages

Ichthyoplankton researchers generally use the terminology and development stages introduced in 1984 by Kendall and others. This consists of three main developmental stages and two transitional stages.Kendall Jr AW, Ahlstrom EH and Moser HG (1984
"Early life history stages of fishes and their characters"
''American Society of Ichthyologists and Herpetologists'', Special publication 1: 11–22.

Skin ionocytes

Ionocytes (formerly known as mitochondrion-rich cells or as chloride cells) are responsible for maintaining optimal osmotic, ionic, and acid-base levels within the fish. Ionocytes are typically found within adult gills. However, embryonic and larval fishes often lack or have underdeveloped gills. Instead, ionocytes are found along the skin, yolk sac, and fins of the larva. As growth progresses and the gill becomes more developed, ionocytes can be found on the gill arch and gill filament. In larval fishes, the number, size, and density of ionocytes can be quantified as a relative ionocyte area, which has been proposed as a proxy for osmotic, ionic, and/or acid-base capacity of the organism. Ionocytes are also known to be plastic. Ionocyte's apical openings can widen during periods of high activity, and new ionocytes can develop along the gill lamellae during periods of environmental stress. Due to the abundant presence of Na⁺/K⁺-ATPase in the basolateral membrane, ionocytes can often be located using

immunohistochemistry Immunohistochemistry is a form of immunostaining. It involves the process of selectively identifying antigens in cells and tissue, by exploiting the principle of Antibody, antibodies binding specifically to antigens in biological tissues. Alber ...

Survival

Recruitment Recruitment is #Process, the overall process of identifying, sourcing, screening, shortlisting, and interviewing candidates for Job (role), jobs (either permanent or temporary) within an organization. Recruitment also is the process involved in ...

of fish is regulated by larval fish survival. Survival is regulated by prey abundance, predation, and

hydrology Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and drainage basin sustainability. A practitioner of hydrology is called a hydro ...

. Fish eggs and larvae are eaten by many marine organisms.Bax NJ (1998
"The significance and prediction of predation in marine fisheries"
''ICES Journal of Marine Science'', 55: 997–1030. For example, they may be fed upon by

marine invertebrate Marine invertebrates are invertebrate animals that live in marine habitats, and make up most of the macroscopic life in the oceans. It is a polyphyletic blanket term that contains all marine animals except the marine vertebrates, including the ...

s, such as

copepod Copepods (; meaning 'oar-feet') are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (living in the water column), some are benthos, benthic (living on the sedimen ...

s, arrow worms,

jellyfish Jellyfish, also known as sea jellies or simply jellies, are the #Life cycle, medusa-phase of certain gelatinous members of the subphylum Medusozoa, which is a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animal ...

amphipod Amphipoda () is an order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods () range in size from and are mostly detritivores or scavengers. There are more than 10,700 amphipod species cur ...

s, marine snails and

krill Krill ''(Euphausiids)'' (: krill) are small and exclusively marine crustaceans of the order (biology), order Euphausiacea, found in all of the world's oceans. The name "krill" comes from the Norwegian language, Norwegian word ', meaning "small ...

.Bailey, K. M., and Houde, E. D. (1989
"Predation on eggs and larvae of marine fishes and the recruitment problem"
''Advances in Marine Biology'', 25: 1–83. Because they are so abundant, marine invertebrates inflict high overall mortality rates. Adult fish also prey on fish eggs and larvae. For example,

haddock The haddock (''Melanogrammus aeglefinus'') is a saltwater ray-finned fish from the Family (biology), family Gadidae, the true cods. It is the only species in the Monotypy, monotypic genus ''Melanogrammus''. It is found in the North Atlantic Oce ...

were observed satiating themselves with

herring Herring are various species of forage fish, belonging to the Order (biology), order Clupeiformes. Herring often move in large Shoaling and schooling, schools around fishing banks and near the coast, found particularly in shallow, temperate wate ...

eggs back in 1922. Another study found

in a herring spawning area with 20,000 herring eggs in their stomachs, and concluded that they could prey on half of the total egg production. Fish also cannibalise their own eggs. For example, separate studies found northern anchovy ( ''Engraulis mordax'') were responsible for 28% of the mortality in their own egg population,Santander, H.; Alheit, J.; MacCall, A.D.; Alamo, A. (1983) tp://ftp.fao.org/docrep/fao/005/x6851b/x6851b21.pdf Egg mortality of the Peruvian anchovy (''Engraulis ringens'') caused by cannibalism and predation by sardines (''Sardinops sagax'')''FAO Fisheries Report'', 291(2-3): 443–453. Rome. while Peruvian anchoveta were responsible for 10% and South African anchovy ( ''Engraulis encrasicolus'') 70%. The most effective predators are about ten times as long as the larvae they prey on. This is true regardless of whether the predator is a crustacean, a jellyfish, or a fish.

Dispersal

Fish larvae develop first an ability to swim up and down the

for short distances. Later they develop an ability to swim horizontally for much longer distances. These swimming developments affect their dispersal.Cowen RK, CB Paris and A Srinivasan (2006) "Scaling of connectivity in marine populations". ''Science'', 311 (5760): 522–527.
PDF
/ref> In 2010, a group of scientists reported that fish larvae can drift on ocean currents and reseed fish stocks at a distant location. This finding demonstrates, for the first time, what scientists have long suspected but have never proven, that fish populations can be connected to distant populations through the process of larval drift.Christie MR, Tissot BN, Albins MA, Beets3 JP, Jia Y, Ortiz DL, Thompson SE, Hixon MA (2010
Larval Connectivity in an Effective Network of Marine Protected Areas
''PLoS ONE'', 5(12) The fish they chose to investigate was the yellow tang, because when a larva of this fish find a suitable reef it stays in the general area for the rest of its life. Thus, it is only as drifting larvae that the fish can migrate significant distances from where they are born. The tropical yellow tang is much sought after by the

aquarium An aquarium (: aquariums or aquaria) is a vivarium of any size having at least one transparent side in which aquatic plants or animals are kept and displayed. fishkeeping, Fishkeepers use aquaria to keep fish, invertebrates, amphibians, aquati ...

trade. By the late 1990s, their stocks were collapsing, so in an attempt to save them nine

marine protected areas A marine protected area (MPA) is a protected area of the world's seas, oceans, estuary, estuaries or in the US, the Great Lakes. These marine areas can come in many forms ranging from wildlife refuges to research facilities. MPAs restrict human ...

(MPAs) were established off the coast of Hawaii. Now, through the process of larval drift, fish from the MPAs are establishing themselves in different locations, and the fishery is recovering. "We've clearly shown that fish larvae that were spawned inside marine reserves can drift with currents and replenish fished areas long distances away," said one of the authors, the

Mark Hixon. "This is a direct observation, not just a model, that successful marine reserves can sustain fisheries beyond their borders."Drifting Fish Larvae Allow Marine Reserves to Rebuild Fisheries
''ScienceDaily '', 26 December 2010.

Gallery

File:C. l. maraenaeggs01.jpg, '' Coregonus maraena'' eggs about one month after fertilization File:Ragfish egg.jpg, Ragfish egg File:Salmoneggskils.jpg, Salmon eggs in different stages of development. File:Frai de poissons rouges anim.gif, Male

goldfish The goldfish (''Carassius auratus'') is a freshwater fish in the family Cyprinidae of the order Cypriniformes. It is commonly kept as a pet in indoor aquariums, and is one of the most popular aquarium fish. Goldfish released into the w ...

encourage a spawning female and discharge sperm to externally fertilize her eggs File:Goldfishfry.JPG, Within days, the vulnerable goldfish eggs hatch into

e, and rapidly develop into fry File:Pacific cod larvae.jpg,

Pacific cod The Pacific cod (''Gadus macrocephalus)'' is a species of ray-finned fish in the family Gadidae. It is a bottom-dwelling fish found in the northern Pacific Ocean, mainly on the continental shelf and upper slopes, to depths of about . It can grow ...

larva File:Walleye larva (8740460659).jpg,

Walleye The walleye (''Sander vitreus'', Synonym (taxonomy), synonym ''Stizostedion vitreum''), also called the walleyed pike, yellow pike, yellow pikeperch or yellow pickerel, is a freshwater perciform fish native to most of Canada and to the Northern ...

larva File:Larval stage of bluefin tuna.jpg,

Bluefin tuna Bluefin tuna is a common name used to refer to several species of tuna of the genus ''Thunnus''. {{Animal common name Commercial fish Thunnus Fish common names ...

larva File:Common sturgeon larva.jpg, Common sturgeon larva File:Clupeaharenguskils2.jpg,

Atlantic herring Atlantic herring (''Clupea harengus'') is a herring in the family Clupeidae. It is one of the most abundant fish species in the world. Atlantic herrings can be found on both sides of the northern Atlantic Ocean, congregating in large schools. ...

eggs, with a newly hatched larva File:Clupealarvamatchkils.jpg, Freshly hatched herring larva in a drop of water compared to a match head. File:Clupeaharenguslarvaeinsitukils.jpg, Early stage herring larvae imaged ''in situ'' with

yolk Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example bec ...

remains File:Molalavdj.jpg, A 2.7mm long larva of the

ocean sunfish The ocean sunfish (''Mola mola''), also known as the common mola, is one of the largest bony fish in the world. It is the type species of the genus ''Mola'', and one of five extant species in the family Molidae. It was once misidentified as th ...

, Mola mola, File:FMIB 47039 Ostracion hoops.jpeg,

Boxfish Ostraciidae or Ostraciontidae is a family of squared, Actinopterygii, bony fish belonging to the order Tetraodontiformes, closely related to the pufferfishes and filefishes. Fish in the family are known variously as boxfishes, cofferfishes, cowfi ...

larva

Notes

References

* Ahlstrom, Elbert H. and Moser, H. Geoffrey (1976
"Eggs and larvae of fishes and their role in systematic investigations in fisheries"
''Revue des Travaux de l'Institut des Pêches Maritimes'', 40(3-4): 379–398. * Balon, Eugene K. (1990
"Epigenesis of an epigeneticist: the development of some alternative concepts on the early ontogeny and evolution of fishes
''Guelph Ichthyology Reviews'', 1: 1–48. * Blaber, Stephen J. M. (2000
''Tropical estuarine fishes: ecology, exploitation and conservation''
John Wiley and Sons, Page 153–156. . * Browman, Howard I. and Skiftesvik, Anne Berit (2003
The Big Fish Bang: Proceedings of the 26th Annual Larval Fish Conference
Institute of Marine Research. . * Finn, Roderick Nigel and Kapoor, B. G. (2008
''Fish larval physiology''
Science Publishers.. * Cowan, J.H., Jr. and R.F. Shaw (2002
"Recruitment"
Chap. 4. pp. 88–111. In: L.A. Fuiman and R.G. Werner (eds.) ''Fishery Science: The unique contributions of early life stages'', John Wiley and Sons. .+ * Chambers RC and Trippel EA (1997
''Early life history and recruitment in fish population''
Springer. . * Houde ED (2010
"Fish Larvae"
Page 286–295. In: JH Steele, SA Thorpe and KK Turekian, ''Marine Biology'', Academic Press. . * Kendall Jr., Arthur W. (2011
''Identification of Eggs and Larvae of Marine Fishes''
東海大学出版会, 2011. . * Miller, Bruce S. and Kendall, Arthur W. (2009
''Early life history of marine fishes''
University of California Press. . * Miller TJ (2002
"Assemblages, Communities, and Species Interactions"
Pages 183–205. In: Lee A. Fuiman and Robert G. Werner, ''Fishery science: the unique contributions of early life stages'', John Wiley and Sons. .
Ichthyoplankton Information System
Alaska Fisheries Center, ''NOAA''.
Early life history section
''American Fisheries Society''.
Larval Fish Laboratory
''Colorado State University''.
''Recent advances in the study of fish eggs and larvae''
''Sci. Mar.'', 70S2: 2006.
Guides and keys to larval and early juvenile fishes
Warner College of Natural Resources, ''Colorado State University''.

External links

* Presentation by Yoshinobu Konishi, ''SEAFDEC-MFRDMD''.
Salmon Eggs Hatching at the Seymour Hatchery
''Youtube'' video. {{diversity of fish Aquatic ecology Biological oceanography Fish reproduction Ichthyology Planktology