Cornish jack
Encyclopedia
The Cornish jack, Mormyrops anguilloides, is a species
of weakly electric fish in the family Mormyridae
, native to quiet waters in much of Sub-Saharan Africa
. The largest species in its family, the Cornish jack is a nocturnal group hunter
of smaller fish
es, using electricity
to locate its prey and communicate with other members of its group. It is a commercial
game fish
valued for its size and taste.
The common name
"Cornish jack" likely originated from Europe
an settlers, who thought that this fish resembled the European pike
, whose young is known as a "jack" in some parts of England
. It is also known as "African carp", a name that is used for several other species.
, the Lake Albert drainage basin
, inland waters from Senegal
to Chad
, river
s in Cameroon
, and small coast
al basins in the Guinea
n zone. It is also widespread in the Congo River
basin, Lakes Malawi
and Tanganyika
, the Volta River
basin, the Whebi Shebeli River, and the Jubba River
. In southern Africa
, it is restricted to the middle and lower Zambezi
, and the Buzi
and Pungwe River
s.
This demersal species is found in tropical freshwater
habitat
s between 22 and 24 degrees C
. The juveniles
are found in marginal habitat
s, while the adult
s prefer deep, quiet water between boulder
s and below overhangs, away from strong currents
. They also occur beneath Salvinia
mats and in river estuaries in Lake Kariba
.
is nearly twice as long as high, smooth and depressed in front. The snout
is rounded and almost as wide as the head. The mouth
is terminal, with the upper jaw slightly longer than the lower
, both bearing a single row of small, pointed teeth. The eye
s are very small and placed in the front third of the head. The gill
openings are small and inclined at an angle. The dorsal
and anal fins are set well back on the body; the caudal fin is relatively small and forked with rounded lobes. There are 21–30 fin rays in the dorsal and 38–51 rays in the anal fin. The anal fin of the male
differs from that of the female
in having longer rays and a pronounced concavity in the anterior half.
The scales
are small, numbering 85–100 in the lateral line
. The coloration is gray above, lighter silvery white below, often with a bronze or yellow sheen. The juveniles are darker in color, being grayish blue or brown. The meristic characteristics of the body (such as the number of scales, fin rays, and vertebrae) vary by geographic location. The largest members of the family Mormyridae, Cornish jacks attain a maximum known length of 1.5 m and a weight of 15 kg.
and generate weak pulses of electricity
for navigation
, finding food, and communication
. Electroceptive
cells
allow it to detect distortions in the electric field
surrounding its body, and determine the size, distance, and properties of the causative object.
, fish in lake environments feed mainly on tilapia
(Sarotherodon
and Tilapia
) with significant season
al variation in diet
, whereas fish in river environments take both fish and crustacean
s. The juveniles feed mostly on shrimp
s and aquatic insect larva
e; larger individuals about 17 cm long also take small cichlid
s, minnow
s, and labeo
s. Historically, Cornish jacks have been thought to feed on decomposing
matter, as they were known to congregate around human
encampments where large amounts of refuse was dumped into the water.
Observations of Cornish jacks in Lake Malawi
show that they form relatively stable groups of 2 to 10 individuals. During the day, the group shelters together in cave
s, and at night they hunt for cichlid
s together over rocky
reef
s to a distance of 20 m from their shelter. Occasionally individual fish will temporarily separate from the others after capturing a cichlid. When a potential prey item is detected, the fish will approach to within 1–20 cm before stopping and producing regular pulses of electricity at 20–40 ms intervals. This "stationary probing" behavior
could allow the Cornish jack to estimate prey size, as they preferentially target smaller cichlids. After a few seconds, a strike may follow, during which the rate of electrical pulses generated may increase to once every 18–20 ms. In many cases, the targeted cichlid showed little movement prior to capture, indicating that they were unaware of the predator's presence.
Group-hunting Cornish jacks capture more prey and make more successful attacks than those hunting alone. However, it is yet unclear what exact advantages are conferred by the group, as the prey items are not shared between individuals and often individuals steal prey from each other. One possibility is that feeding efficiency is increased by multiple predators making strikes on the same prey fish within a short time of each other.
s of their electrical pulses may allow Cornish jacks to recognize each other individually, and thus maintain the identity of their groups. Cornish jacks in groups will adjust the rate of their electrical pulses so that they occur at 18–20 ms delays relative to each other; this "echo response", common in mormyrids, is especially robust in Cornish jacks and serves to minimize electrical interference between different individuals. Cornish jack hunting groups have also been recorded producing synchronized bursts of electrical pulses lasting 1–2.5 s every few minutes. These bursts have been proposed to be mutual group recognition signals.
in summer
during the rainy season. In the upper Ogun River
, Cornish jacks and other piscivorous fishes are especially abundant at the beginning and middle of the annual flood
, suggesting that they migrate
upstream to breed and retreat downstream when the water recedes. The female
s are fractional spawners and may carry 25,000 or more eggs
. In the Baoulé River, this species attains maturity at no less than 34.6 cm long. Their lifespan
may be eight or more years.
and is also taken by spearfishermen
. Due to their relatively small mouths, the bait
used can be a thin filet
of fish, worm
s, or crab
s, and they can be taken by light tackle
as they are not known for their fighting abilities. The flesh
is held in high esteem; the species name of one of its synonyms
, Mormyrops deliciosus, reflects this fact. Theodore Gill
(1902) noted that it was fished for mostly at dawn
and sunset
, and that a five-foot fish might fetch a price of 25 franc
s at Boma.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of weakly electric fish in the family Mormyridae
Mormyridae
The family Mormyridae, sometimes called "elephantfish" , are freshwater fish in the order Osteoglossiformes native to Africa. It is by far the largest family in the order with around 200 species. Members of the family are popular, if challenging, aquarium species...
, native to quiet waters in much of Sub-Saharan Africa
Sub-Saharan Africa
Sub-Saharan Africa as a geographical term refers to the area of the African continent which lies south of the Sahara. A political definition of Sub-Saharan Africa, instead, covers all African countries which are fully or partially located south of the Sahara...
. The largest species in its family, the Cornish jack is a nocturnal group hunter
Pack hunter
A pack hunter is a predator belonging to the animal kingdom, which has evolved to hunt its prey by working together with other members of its species. Normally, such animals are closely related. The most commonly known pack hunter is the Gray Wolf, the ancestor of all breeds of domesticated dogs...
of smaller fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
es, using electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
to locate its prey and communicate with other members of its group. It is a commercial
Commercial fishing
Commercial fishing is the activity of catching fish and other seafood for commercial profit, mostly from wild fisheries. It provides a large quantity of food to many countries around the world, but those who practice it as an industry must often pursue fish far into the ocean under adverse conditions...
game fish
Game fish
Game fish are fish pursued for sport by recreational anglers. They can be freshwater or marine fish. Game fish can be eaten after being caught, though increasingly anglers practise catch and release to improve fish populations. Some game fish are also targeted commercially, particularly...
valued for its size and taste.
The common name
Common name
A common name of a taxon or organism is a name in general use within a community; it is often contrasted with the scientific name for the same organism...
"Cornish jack" likely originated from Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
an settlers, who thought that this fish resembled the European pike
Esox
Esox is a genus of freshwater fish, the only living genus in the family Esocidae — the esocids which were endemic to North America, Europe and Eurasia during the Paleogene through present.The type species is E. lucius, the northern pike...
, whose young is known as a "jack" in some parts of England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
. It is also known as "African carp", a name that is used for several other species.
Distribution and habitat
The Cornish jack occurs in the White NileWhite Nile
The White Nile is a river of Africa, one of the two main tributaries of the Nile from Egypt, the other being the Blue Nile. In the strict meaning, "White Nile" refers to the river formed at Lake No at the confluence of the Bahr al Jabal and Bahr el Ghazal rivers...
, the Lake Albert drainage basin
Drainage basin
A drainage basin is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean...
, inland waters from Senegal
Senegal
Senegal , officially the Republic of Senegal , is a country in western Africa. It owes its name to the Sénégal River that borders it to the east and north...
to Chad
Chad
Chad , officially known as the Republic of Chad, is a landlocked country in Central Africa. It is bordered by Libya to the north, Sudan to the east, the Central African Republic to the south, Cameroon and Nigeria to the southwest, and Niger to the west...
, river
River
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...
s in Cameroon
Cameroon
Cameroon, officially the Republic of Cameroon , is a country in west Central Africa. It is bordered by Nigeria to the west; Chad to the northeast; the Central African Republic to the east; and Equatorial Guinea, Gabon, and the Republic of the Congo to the south. Cameroon's coastline lies on the...
, and small coast
Coast
A coastline or seashore is the area where land meets the sea or ocean. A precise line that can be called a coastline cannot be determined due to the dynamic nature of tides. The term "coastal zone" can be used instead, which is a spatial zone where interaction of the sea and land processes occurs...
al basins in the Guinea
Guinea
Guinea , officially the Republic of Guinea , is a country in West Africa. Formerly known as French Guinea , it is today sometimes called Guinea-Conakry to distinguish it from its neighbour Guinea-Bissau. Guinea is divided into eight administrative regions and subdivided into thirty-three prefectures...
n zone. It is also widespread in the Congo River
Congo River
The Congo River is a river in Africa, and is the deepest river in the world, with measured depths in excess of . It is the second largest river in the world by volume of water discharged, though it has only one-fifth the volume of the world's largest river, the Amazon...
basin, Lakes Malawi
Lake Malawi
Lake Malawi , is an African Great Lake and the southernmost lake in the Great Rift Valley system of East Africa. This lake, the third largest in Africa and the eighth largest lake in the world, is located between Malawi, Mozambique, and Tanzania...
and Tanganyika
Lake Tanganyika
Lake Tanganyika is an African Great Lake. It is estimated to be the second largest freshwater lake in the world by volume, and the second deepest, after Lake Baikal in Siberia; it is also the world's longest freshwater lake...
, the Volta River
Volta River
The Volta is a river in western Africa that drains into the Gulf of Guinea. It has three main tributaries—the Black Volta, White Volta and Red Volta...
basin, the Whebi Shebeli River, and the Jubba River
Jubba River
The Jubba River is a river in southern Somalia. It begins at the border with Ethiopia, where the Dawa and Ganale Dorya rivers meet, and flows directly south to the Indian Ocean, where it empties at the Goobweyn juncture.-History:...
. In southern Africa
Southern Africa
Southern Africa is the southernmost region of the African continent, variably defined by geography or geopolitics. Within the region are numerous territories, including the Republic of South Africa ; nowadays, the simpler term South Africa is generally reserved for the country in English.-UN...
, it is restricted to the middle and lower Zambezi
Zambezi
The Zambezi is the fourth-longest river in Africa, and the largest flowing into the Indian Ocean from Africa. The area of its basin is , slightly less than half that of the Nile...
, and the Buzi
Buzi River
Buzi River is a river in Mozambique. Buzi flows eastward through the Manica and Sofala provinces of Mozambique. It then empties to the Mozambique Channel west of Beira, forming an estuary.The Buzi River is long, with a drainage basin in size...
and Pungwe River
Pungwe River
Pungwe River is a 400 km long river in Zimbabwe and Mozambique. It rises below Mount Nyangani in the Eastern Highlands of Zimbabwe and then flows eastward through the Manica and Sofala provinces of Mozambique. It then empties to the Mozambique Channel in Beira, forming a large estuary...
s.
This demersal species is found in tropical freshwater
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
s between 22 and 24 degrees C
Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
. The juveniles
Juvenile (organism)
A juvenile is an individual organism that has not yet reached its adult form, sexual maturity or size. Juveniles sometimes look very different from the adult form, particularly in terms of their colour...
are found in marginal habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
s, while the adult
Adult
An adult is a human being or living organism that is of relatively mature age, typically associated with sexual maturity and the attainment of reproductive age....
s prefer deep, quiet water between boulder
Boulder
In geology, a boulder is a rock with grain size of usually no less than 256 mm diameter. While a boulder may be small enough to move or roll manually, others are extremely massive....
s and below overhangs, away from strong currents
Current (stream)
A current, in a river or stream, is the flow of water influenced by gravity as the water moves downhill to reduce its potential energy. The current varies spatially as well as temporally within the stream, dependent upon the flow volume of water, stream gradient, and channel geometrics...
. They also occur beneath Salvinia
Salvinia
Salvinia, a genus in the family Salviniaceae, is a floating fern named in honor of Antonio Maria Salvini, a 17th Century Italian scientist. The genus was published by Séguier, in Pl. Veron. 3: 52. 1754. About ten species exist....
mats and in river estuaries in Lake Kariba
Lake Kariba
Lake Kariba is the world's largest artificial lake and reservoir. It lies 1300 kilometers upstream from the Indian Ocean, along the border between Zambia and Zimbabwe...
.
Description
The body and head of the Cornish jack are elongated; the headHead
In anatomy, the head of an animal is the rostral part that usually comprises the brain, eyes, ears, nose and mouth . Some very simple animals may not have a head, but many bilaterally symmetric forms do....
is nearly twice as long as high, smooth and depressed in front. The snout
Snout
The snout, or muzzle, is the protruding portion of an animal's face, consisting of its nose, mouth, and jaw.-Terminology:The term "muzzle", used as a noun, can be ambiguous...
is rounded and almost as wide as the head. The mouth
Mouth
The mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth....
is terminal, with the upper jaw slightly longer than the lower
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...
, both bearing a single row of small, pointed teeth. The eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...
s are very small and placed in the front third of the head. The gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
openings are small and inclined at an angle. The dorsal
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
and anal fins are set well back on the body; the caudal fin is relatively small and forked with rounded lobes. There are 21–30 fin rays in the dorsal and 38–51 rays in the anal fin. The anal fin of the male
Male
Male refers to the biological sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization...
differs from that of the female
Female
Female is the sex of an organism, or a part of an organism, which produces non-mobile ova .- Defining characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...
in having longer rays and a pronounced concavity in the anterior half.
The scales
Scale (zoology)
In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...
are small, numbering 85–100 in the lateral line
Lateral line
The lateral line is a sense organ in aquatic organisms , used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail...
. The coloration is gray above, lighter silvery white below, often with a bronze or yellow sheen. The juveniles are darker in color, being grayish blue or brown. The meristic characteristics of the body (such as the number of scales, fin rays, and vertebrae) vary by geographic location. The largest members of the family Mormyridae, Cornish jacks attain a maximum known length of 1.5 m and a weight of 15 kg.
Biology and ecology
Like other mormyrids, Cornish jacks have an electric organElectric organ
In biology, the electric organ is an organ common to all electric fish used for the purposes of creating an electric field. The electric organ is derived from modified nerve or muscle tissue...
and generate weak pulses of electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
for navigation
Navigation
Navigation is the process of monitoring and controlling the movement of a craft or vehicle from one place to another. It is also the term of art used for the specialized knowledge used by navigators to perform navigation tasks...
, finding food, and communication
Animal communication
Animal communication is any behavior on the part of one animal that has an effect on the current or future behaviour of another animal. The study of animal communication, is sometimes called Zoosemiotics has played an important part in the...
. Electroceptive
Electroreception
Electroreception is the biological ability to perceive natural electrical stimuli. It has been observed only in aquatic or amphibious animals, since water is a much better conductor than air. Electroreception is used in electrolocation and for electrocommunication.- Overview :Electroreception is...
cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
allow it to detect distortions in the electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
surrounding its body, and determine the size, distance, and properties of the causative object.
Feeding
Adult Cornish jack are primarily piscivorous; along the Bia RiverBia River
The Bia is a river in West Africa that flows through Ghana and Côte d'Ivoire. A hydroelectric dam was built at Ayame in 1959, causing the formation of Lake Ayame ....
, fish in lake environments feed mainly on tilapia
Tilapia
Tilapia , is the common name for nearly a hundred species of cichlid fish from the tilapiine cichlid tribe. Tilapia inhabit a variety of fresh water habitats, including shallow streams, ponds, rivers and lakes. Historically, they have been of major importance in artisan fishing in Africa and the...
(Sarotherodon
Sarotherodon
Sarotherodon is a genus of tilapiine cichlids, fishes endemic to Africa and the Middle East. A few species from this genus have been introduced far outside their native range, and are important in aquaculture. They mainly inhabit fresh and brackish water, but a few can live in salt water...
and Tilapia
Tilapia (genus)
Tilapia is a large genus of cichlid fishes endemic to Africa, except for a single species that also occurs in the Middle East. The vast majority of the species are from tropical Western and Middle Africa, but there are also a few species in Northern, Eastern and Southern Africa...
) with significant season
Season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...
al variation in diet
Diet (nutrition)
In nutrition, diet is the sum of food consumed by a person or other organism. Dietary habits are the habitual decisions an individual or culture makes when choosing what foods to eat. With the word diet, it is often implied the use of specific intake of nutrition for health or weight-management...
, whereas fish in river environments take both fish and crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
s. The juveniles feed mostly on shrimp
Shrimp
Shrimp are swimming, decapod crustaceans classified in the infraorder Caridea, found widely around the world in both fresh and salt water. Adult shrimp are filter feeding benthic animals living close to the bottom. They can live in schools and can swim rapidly backwards. Shrimp are an important...
s and aquatic insect larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e; larger individuals about 17 cm long also take small cichlid
Cichlid
Cichlids are fishes from the family Cichlidae in the order Perciformes. Cichlids are members of a group known as the Labroidei along with the wrasses , damselfish , and surfperches . This family is both large and diverse. At least 1,300 species have been scientifically described, making it one of...
s, minnow
Minnow
Minnow is a general term used to refer to small freshwater and saltwater fish, especially those used as bait fish or for fishing bait. More specifically, it refers to small freshwater fish of the carp family.-True minnows:...
s, and labeo
Labeo
Labeo is a genus of carps in the family Cyprinidae. They are found mainly in the Old World tropics.It contains the typical labeos in the subfamily Labeoninae, which may not be a valid group, however, and is often included in the Cyprininae as tribe Labeonini...
s. Historically, Cornish jacks have been thought to feed on decomposing
Decomposition
Decomposition is the process by which organic material is broken down into simpler forms of matter. The process is essential for recycling the finite matter that occupies physical space in the biome. Bodies of living organisms begin to decompose shortly after death...
matter, as they were known to congregate around human
Human
Humans are the only living species in the Homo genus...
encampments where large amounts of refuse was dumped into the water.
Observations of Cornish jacks in Lake Malawi
Lake Malawi
Lake Malawi , is an African Great Lake and the southernmost lake in the Great Rift Valley system of East Africa. This lake, the third largest in Africa and the eighth largest lake in the world, is located between Malawi, Mozambique, and Tanzania...
show that they form relatively stable groups of 2 to 10 individuals. During the day, the group shelters together in cave
Cave
A cave or cavern is a natural underground space large enough for a human to enter. The term applies to natural cavities some part of which is in total darkness. The word cave also includes smaller spaces like rock shelters, sea caves, and grottos.Speleology is the science of exploration and study...
s, and at night they hunt for cichlid
Cichlid
Cichlids are fishes from the family Cichlidae in the order Perciformes. Cichlids are members of a group known as the Labroidei along with the wrasses , damselfish , and surfperches . This family is both large and diverse. At least 1,300 species have been scientifically described, making it one of...
s together over rocky
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
reef
Reef
In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....
s to a distance of 20 m from their shelter. Occasionally individual fish will temporarily separate from the others after capturing a cichlid. When a potential prey item is detected, the fish will approach to within 1–20 cm before stopping and producing regular pulses of electricity at 20–40 ms intervals. This "stationary probing" behavior
Behavior
Behavior or behaviour refers to the actions and mannerisms made by organisms, systems, or artificial entities in conjunction with its environment, which includes the other systems or organisms around as well as the physical environment...
could allow the Cornish jack to estimate prey size, as they preferentially target smaller cichlids. After a few seconds, a strike may follow, during which the rate of electrical pulses generated may increase to once every 18–20 ms. In many cases, the targeted cichlid showed little movement prior to capture, indicating that they were unaware of the predator's presence.
Group-hunting Cornish jacks capture more prey and make more successful attacks than those hunting alone. However, it is yet unclear what exact advantages are conferred by the group, as the prey items are not shared between individuals and often individuals steal prey from each other. One possibility is that feeding efficiency is increased by multiple predators making strikes on the same prey fish within a short time of each other.
Communication
Differences in the waveformWaveform
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...
s of their electrical pulses may allow Cornish jacks to recognize each other individually, and thus maintain the identity of their groups. Cornish jacks in groups will adjust the rate of their electrical pulses so that they occur at 18–20 ms delays relative to each other; this "echo response", common in mormyrids, is especially robust in Cornish jacks and serves to minimize electrical interference between different individuals. Cornish jack hunting groups have also been recorded producing synchronized bursts of electrical pulses lasting 1–2.5 s every few minutes. These bursts have been proposed to be mutual group recognition signals.
Reproduction
The Cornish jack is oviparous and spawnsSpawn (biology)
Spawn refers to the eggs and sperm released or deposited, usually into water, by aquatic animals. As a verb, spawn refers to the process of releasing the eggs and sperm, also called spawning...
in summer
Summer
Summer is the warmest of the four temperate seasons, between spring and autumn. At the summer solstice, the days are longest and the nights are shortest, with day-length decreasing as the season progresses after the solstice...
during the rainy season. In the upper Ogun River
Ogun River
The Ogun River is a waterway in Nigeria that discharges into the Lagos Lagoon.-Course and usage:The river rises in Oyo State near Shaki at coordinates and flow through Ogun State into Lagos State....
, Cornish jacks and other piscivorous fishes are especially abundant at the beginning and middle of the annual flood
Flood
A flood is an overflow of an expanse of water that submerges land. The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water...
, suggesting that they migrate
Fish migration
Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousands of kilometres...
upstream to breed and retreat downstream when the water recedes. The female
Female
Female is the sex of an organism, or a part of an organism, which produces non-mobile ova .- Defining characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...
s are fractional spawners and may carry 25,000 or more eggs
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
. In the Baoulé River, this species attains maturity at no less than 34.6 cm long. Their lifespan
Maximum life span
Maximum life span is a measure of the maximum amount of time one or more members of a population has been observed to survive between birth and death.Most living species have at least one upper limit on the number of times cells can divide...
may be eight or more years.
Relationship to humans
The Cornish jack is a popular species for anglersAngling
Angling is a method of fishing by means of an "angle" . The hook is usually attached to a fishing line and the line is often attached to a fishing rod. Fishing rods are usually fitted with a fishing reel that functions as a mechanism for storing, retrieving and paying out the line. The hook itself...
and is also taken by spearfishermen
Spearfishing
Spearfishing is an ancient method of fishing that has been used throughout the world for millennia. Early civilizations were familiar with the custom of spearing fish from rivers and streams using sharpened sticks....
. Due to their relatively small mouths, the bait
Bait (luring substance)
Bait is any substance used to attract prey, e.g. in a mousetrap.-In Australia:Baiting in Australia refers to specific campaigns to control foxes, wild dogs and dingos by poisoning in areas where they are a problem...
used can be a thin filet
Fillet (cut)
A fillet is a cut or slice of boneless meat or fish.- Meat :In the case of beef, in the USA, the term most often refers to beef tenderloin, especially filet mignon.- Chicken :...
of fish, worm
Worm
The term worm refers to an obsolete taxon used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, and stems from the Old English word wyrm. Currently it is used to describe many different distantly-related animals that typically have a long cylindrical...
s, or crab
Crab
True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...
s, and they can be taken by light tackle
Fishing tackle
Fishing tackle, is a general term that refers to the equipment used by fishermen when fishing. Almost any equipment or gear used for fishing can be called fishing tackle. Some examples are hooks, lines, sinkers, floats, rods, reels, baits, lures, spears, nets, gaffs, traps, waders and tackle...
as they are not known for their fighting abilities. The flesh
Flesh
In vertebrate animals, flesh is the colloquial for biological tissue which consists of skeletal muscles and fat as opposed to bones, viscera and integuments. Flesh may be used as food, in which case it is called meat....
is held in high esteem; the species name of one of its synonyms
Synonym (taxonomy)
In scientific nomenclature, a synonym is a scientific name that is or was used for a taxon of organisms that also goes by a different scientific name. For example, Linnaeus was the first to give a scientific name to the Norway spruce, which he called Pinus abies...
, Mormyrops deliciosus, reflects this fact. Theodore Gill
Theodore Gill
Theodore Nicholas Gill was an American ichthyologist, mammalogist, malacologist and librarian.Born and educated in New York City under private tutors, Gill early showed interest in natural history. He was associated with J...
(1902) noted that it was fished for mostly at dawn
Dawn
Dawn is the time that marks the beginning of the twilight before sunrise. It is recognized by the presence of weak sunlight, while the sun itself is still below the horizon...
and sunset
Sunset
Sunset or sundown is the daily disappearance of the Sun below the horizon in the west as a result of Earth's rotation.The time of sunset is defined in astronomy as the moment the trailing edge of the Sun's disk disappears below the horizon in the west...
, and that a five-foot fish might fetch a price of 25 franc
Franc
The franc is the name of several currency units, most notably the Swiss franc, still a major world currency today due to the prominence of Swiss financial institutions and the former currency of France, the French franc until the Euro was adopted in 1999...
s at Boma.