Otis Singleton
Electric organs in fish are derived from modified muscle or nerve tissue, called electrocytes, and have evolved at least six times among elasmobranchs and teleosts. Electric fish are divided into two types: strongly electric and weakly electric. The former can generate enough power to stun prey or zap predators, while the latter use weak electric signals to communicate with other electric fish. Electric organs are composed of stacks of specialized cells that generate electricity, and their function has been a subject of extensive study since the 1770s. The embryonic development of electric organs in fish has remained largely unknown, with recent studies providing insights into the mechanisms underlying their formation during the embryo-to-larva transition. The question of whether electric organs are functional at birth in fish remains an intriguing area of investigation.
| Characteristics | Values |
|---|---|
| Types of electric fish | Strongly electric fish, Weakly electric fish |
| Electric organ location | Tail, Head, Body |
| Electric organ shape | Cigar-shaped, Flat disk-like |
| Electric organ composition | Electrocytes, Electroplaques, Electroplaxes |
| Electric organ function | Navigation, Communication, Mating, Defence, Incapacitation of prey |
| Electric organ evolution | Convergent evolution, Six different groups |
| Electric organ development | Embryonic stages, Larval stages |
| Electric organ genetics | Small genetic changes, Sodium channel gene |
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What You'll Learn
- Electric organs are composed of specialised cells called electrocytes
- Electric organs are used for navigation, communication, mating, defence, and hunting
- Electric organs evolved from skeletal muscle or nerve tissue
- Electric organs are found in six different groups of fish
- Electric organs are controlled by the medullary command nucleus in the brain
Electric organs are composed of specialised cells called electrocytes
The configuration of electrocytes in electric organs differs between saltwater and freshwater fish due to the varying resistance of saltwater and freshwater. Saltwater fish have electrocytes arranged in a more "parallel" form, with shorter stacks and a higher number of columns, resulting in lower voltage and higher current. Conversely, freshwater fish have electrocytes stacked in a "series" form, with longer stacks and fewer columns, producing higher voltage and lower current.
The electric organ discharge (EOD) in electric fish varies with time and is used for electrolocation, communication, and, in strongly electric species, hunting or defence. The EOD is controlled by the medullary command nucleus in the brain, which releases acetylcholine to the electrocytes. Electric fish, such as the electric eel, use their electric organs to send and receive signals, aiding in species recognition and communication.
The development of electric organs in embryos and larvae of knifefish, such as Brachyhypopomus gauderio, has been studied, revealing that the electric organ primordial cells arise during embryonic stages in the ventral edge of the tail myotome. These cells then translocate into the ventral fin and develop into syncytial electrocytes at early larval stages. The mechanism behind the differentiation of specific myotomal cells into electrocytes remains unclear.
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Electric organs are used for navigation, communication, mating, defence, and hunting
Electric organs in fish are used for a variety of purposes, including navigation, communication, mating, defence, and hunting.
Navigation
Electric organ discharges (EODs) are used by electric fish for navigation, a process known as electrolocation. By generating electric fields, these fish can sense and interpret their environment, including detecting obstacles and locating prey. The frequency and pattern of EODs can vary depending on the species and the specific navigational needs.
Communication
Many electric fish species use EODs for communication. These electric signals are often simple and stereotyped, allowing fish to recognize and interpret specific patterns as territorial warnings, mating calls, or other forms of inter-species communication.
Mating
Electric organs play a crucial role in mating rituals for some electric fish species. They use EODs to attract mates, signal their readiness to mate, and synchronize their reproductive behaviours.
Defence
Strongly electric fish species use their electric organs as a defence mechanism. They can produce strong electric discharges to stun or incapacitate potential threats, including predators or competitors. This ability provides them with an effective means of protection and territorial defence.
Hunting
Strongly electric fish also utilize their electric organs for hunting. They can emit electric discharges to stun or immobilize their prey, making it easier to capture and consume. This hunting strategy increases their chances of securing a successful meal.
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Electric organs evolved from skeletal muscle or nerve tissue
Electric organs in fish are derived from either skeletal muscle or nerve tissue. Electric organs are composed of stacks of specialised cells that generate electricity, called electrocytes, electroplaques, or electroplaxes. These cells function by pumping sodium and potassium ions across their cell membranes, consuming adenosine triphosphate (ATP) in the process. Electric organ discharge (EOD) is controlled by the medullary command nucleus, a nucleus of pacemaker neurons in the brain.
In most cases, the electric organs of electric fish are derived from skeletal muscle, an electrically excitable tissue. However, there are exceptions, such as in the case of Apteronotus in Latin America, where the cells are derived from neural tissue. The electric organ of the African freshwater catfish genus Synodontis is known to have evolved from sound-producing muscles.
The evolution of electric organs in fish is a fascinating example of convergent evolution, as noted by Charles Darwin in his 1859 book "On the Origin of Species". Electric organs have evolved independently in at least six different groups of fish, including the elasmobranchs and teleosts. The two main groups of electric fish are found in Africa and South America, and both groups have developed electric organs by losing the expression of a sodium channel gene in muscle tissue. This small genetic change allowed electric fish to repurpose the tiny motors that typically make muscles contract to instead generate electric signals.
The development of electric organs in embryos and larvae of electric fish has been studied, particularly in the South American Gymnotiform knifefish Brachyhypopomus gauderio. Researchers have found that the electric organ in this species is derived from myogenic cells (myogenic electric organ/mEO) and develops during embryonic stages in the ventral edge of the tail myotome, eventually forming electrocytes at early larval stages.
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Electric organs are found in six different groups of fish
Electric organs are indeed functional at birth for some fish, such as the Gymnotiform knifefish. These electric organs are used for electro-location and electro-communication. However, the electric organ's original function has not been fully established in most cases.
The electric organs of all electric fish are derived from skeletal muscle, except in Apteronotus, where the cells are derived from neural tissue. In most electric fish, the electric organs are oriented along the length of the body, usually within the fish's musculature, as in the elephantnose fish. However, in some marine groups, such as stargazers and torpedo rays, the electric organs are oriented along the dorso-ventral axis.
The electric organ discharge (EOD) is controlled by the medullary command nucleus, a group of pacemaker neurons in the brain. The EODs need to vary with time for electrolocation, whether with pulses, as in the Mormyridae, or with waves, as in the Torpediniformes and Gymnarchus, the African knifefish. Many electric fish also use EODs for communication, and their electric signals are often simple and stereotyped.
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Electric organs are controlled by the medullary command nucleus in the brain
Electric organs in fish are composed of stacks of specialized cells called electrocytes, electroplaques, or electroplaxes. These organs are used by electric fish to create an electric field. Electric organs are derived from modified muscle or nerve tissue. The electric organ of the African freshwater catfish genus Synodontis, for example, is known to have evolved from sound-producing muscles.
Electric organ discharge (EOD) is controlled by the medullary command nucleus, a nucleus of pacemaker neurons in the brain. The medullary command nucleus is a key structure in the control pathway for electric organ discharge. It receives input from the presumed command or pacemaker nucleus and projects to the electromotoneurons in the spinal cord. The pacemaker nucleus is responsible for generating the rhythmic electrical activity that drives the electric organ discharge.
The medullary command nucleus exhibits a unique structure, with many axodentritic and axosomatic synapses of the chemical type. The pre- and postsynaptic membranes are separated by a space of about 250 Å and lack regions of tight junction, which is typically associated with the formation of electrical synapses. This absence of electrical synapses in the skate is correlated with the asynchronous nature of its electric organ discharge.
The function of the medullary command nucleus is to regulate the electric organ discharge in electric fish. Electromotor neurons release acetylcholine to the electrocytes, which then fire an action potential using their voltage-gated sodium channels. This process allows electric fish to generate electric signals for various purposes, such as navigation, communication, mating, defence, and incapacitating prey.
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Frequently asked questions
Electric organs are specialized organs that allow electric fish to create an electric field. They are composed of stacks of specialized cells called electrocytes, electroplaques, or electroplaxes.
Electric organs generate electricity by pumping sodium and potassium ions across cell membranes using transport proteins. This process consumes adenosine triphosphate (ATP). The functional asymmetry of the cells and their "in-series" arrangement allow for the summation of voltages.
It is not clear if electric organs are fully functional at birth in all electric fish species. However, studies on the development of electric organs in embryos and larvae of the knifefish, Brachyhypopomus gauderio, suggest that electric organs may become functional during early larval stages.
Electric organs can be categorized into two types: myogenic electric organs (mEO) and neurogenic electric organs (nEO). mEOs are derived from myogenic cells, while nEOs are derived from neurogenic cells.
Electric organs in fish provide valuable insights into convergent evolution, as different groups of electric fish have independently evolved similar electric capabilities. Additionally, they offer a unique model for understanding complex trait evolution and the potential application to other specialized noncontractile muscle derivatives, such as the cardiac conduction system.
