Colloquial Meeting of Chondrichthyes
Essay about the Electric Organ Discharge (EOD)
headed by: dr. A. Goldschmid
Authors: Pierre Madl & Maricela Yip
Salzburg, Jan. 2000
Electric Organ Discharge (EOD) in Electric Rays (Torpedo sp.)|
Electric rays of the families Torpedinidae, Narkidae, Hyphnidae, and an extralimital family,
are unique in having two large, kidney-shaped electric organs in the disc on either side of the head.
These organs are capable of generating strong electric shock which are administered at will.
The powerful electric organs derive from branchial muscles. The presence of electric organs require
a good conducting medium; it is thus not surprising to find such adaptations only among aqueous organisms.
Electric rays of the family Torpendinidae are cartilaginous fish renowned for their ability to produce powerful electric discharges. The kidney shaped electric organs are located on each side of the flattened disk, which is formed by greatly enlarged pectoral fins fused to the head.
Overview of electric fish (105kB)
Selection of Rays
Ultrastructure and developmental stages of the electric organ|
The adult electric organ consists of a group of vertical cellular columns, derived from sheets of lateral mesoderm present in each of the hyoid and the first four branchial arches. Their morphogenesis into electric plates can be observed step by step. The central element of each electric organ is the electrocyte, also known as electroplate. It is a highly multinucleated cell, that does not reveal myofibrils.
As early as 5mm embryo growth onwards, motor axons are found projecting from medullar regions that form, in part, the primordium of the electric lobes. These axons can soon be seen in close association with branchial arch mesoderm, a portion of which gives rise to the electric organ. Thus the branchial nerves are composed of an electromotor nerve component as well as fibers subserving other related branchial structures. The nerves keep pace with the overall development of the arch, and ultrastrstructural examination of the electromotor fibers reveals the presence of organells typically found associated with growth cones and growing processes. The distal end of the young electromotor nerve splays out considerably into progressively smaller nerve bundles which simply terminate in the interstitial space close to the developing muscle tissue of the electric organ primordium. No synaptic or junctional contacts have been found, however, at this stage of development.
EOD-lobe organ development
Integrated Development of Electrocytes
As this outpocketing enlarges with growth as cone-shaped pillars, dorsal and ventral plates of cells form between which, a series of vertical columns are generated - destined to become the future electrocyte columns. The full complement of these columns appears to be produced by about 28mm embryo growth.
Following this, a continuation to approximately 40mm is a growth phase which results in an increasing attenuation of the dorsal and ventral plates and an enlargement in length and breadth of the columns. At 40mm, a dramatic series of morphologic transformations begins within the columns signaling the termination of the myogenic phase development and the beginning of the final differentiation into the definitive electric organ.
Termination of the myogenic phase of organ development begins at 40mm embryo length
and is best seen occurring within the column myotubes where the myotubes begin rounding up ultimately to
form the horizontally flattened electrocytes characteristic of the mature electric organ. This process begins
within the most medial columns and progresses laterally first affecting the ventral pole of the myotubes.
The nuclei of myotubes become repositioned upon the equatorial plane of the rounding cell;
the myofibrils contained within this portion of the myotube become increasingly consorted and
disorganized and finally begin breaking up into isolated components. This disassembly or degradation of myofibrils
begins with a longitudinal splitting followed by the loss of the A-bands which results in the isolation of the
Z-bands with thin filaments attached.|
By the 55mm-embryo length, the transformation from myotube to electrocyte is complete and the nerves, cellular columns can now be linked to "stacks of coins" (EOD-units). It is at this time that the intercolumnar quiescent until now, generate neurites that penetrate the inter-electrocyte space and begin establishing synaptic contacts along the ventral surfaces of each electrocyte. The ingrowing neurites preferentially orient towards the ventral electrocyte surface and course along the basal lamina (the basal lamina of the ventral surface develops prior to the basal lamina of the dorsal surface). These exploring neurites are, at least initially, free of any Schwann cell coverage. As development proceeds, clusters of terminal processes are found situated at intervals along the ventral plasma membrane. Each electrocyte is ultimately innervated by a number of axons, that continue to grow in size with the overall growth of the animal indicate that synaptogenesis is an ongoing process.
Also to be noted is the temporal coexistance between degeneration seen in the fourth branchial nerve (would represent the 5th strand of nerve bundle) and the posterior lobe region. Most of the degeneration seems to be localized to the posterior poles of the electric organ lobes and appears to be related to neurons projecting into the fourth branchial arch. It is suggested that the fourth arch tissue and the caudal poles of the lobes are neuro-anatomically linked. Just after synaptogenesis in the electric organs begins neuronal, neuronal cell death in the electric lobe ends at the 80mm stage. Neuronal cell death is generally conceived to be a population control process whereby an excessively large number of neurons is reduced to the size of its target population. Cell death is occurring most prominently during a time when the target tissue is maximally expanding in terms of both numbers and mass – an apparent contradiction to the idea of population matching.
Relationship of Electric organ and Body Development - Column formation
A rays developmental stages move through several crucial phases. One of the early phases is outlined by simultaneous growth of the electric organs, pharynx, and pectroal fins; the latter extending cranically until they fuse with the rostrum at the 35mm stage.
Electtric organ primordia are already distinct at the 21mm stage, which are located in the middle parts of four pairs of branchial arches. The 24mm stage corresponds to an embryo where electric organs still retain some external metamery (division of the body into a linear series of similar segments). Although the four parts of the electric organ are still distinct, internal fusion between metameric primordia takes already place.
Tissue coalescence occurs rapidly between the 21 and 25mm stage and is linked with
the dorsal occlusion of the gill clefts, leading to the confinement of these apertures to the ventral side
of the body.|
Column genesis starts with a thickening of the mesoderm, that usually takes place at the 25mm stage. Metamorphosis of electroblasts begins when the disk of the embryo is completed; it also represents the phase of plate genesis.
As the 30mm stage is reached, the typical torpedoid shape is achieved with the shape of the dorso-ventral columns easily observable; while in the 40mm stage, focuses mainly in the increase in diameter and lengthening of the columns, with the insertion connective tissue into the inter-columnar spaces. After a series of further development characterizes synaptogenesis in that the embryo starts to emit weak EODs (small zigzag). This phase at 70mm embryo length, initiates the terminal phase of physiological maturation of electric organ, and the beginning accumulation of acethylcholin-esterase (AChe). The external gill filaments are resorbed and pigmentation develops in the skin. These visible changes parallel the final maturation of the EOD (large zigzag).
Ultimately, birth occurs around the 100mm stage and usually takes place in the months of November. Newly born and neonate Torpedo ocellata for example feed on Blenniidae (i.e. Blennius sphynx) by emitting EODs with an amplitude of approximately 4V (strength of EODs increase dramatically during the first three weeks of life, until it reaches asymptotically the maximal peaking voltage).
Morphology and physiology of a mature electric organs:
Anatomy of electric rays
The total amount of electrocytes in rays within one column varies considerably but is
embrionically quantified, and usually oscillates around 400. Thick strands of nerves (one hyoid and three
branchial nerves - as mentioned, branchial nerve IV degenerates) innervate each electrocyte contained in a
column to connect them with the Lobus electricus - situated caudally behind the cerebellum.
Electrocytes are surrounded by elastic fibers that represent the electrically conducting connective tissue. Each cell itself is again surrounded by septa-like networks of connective tissues. Several of the stapled cells are further wrapped into sheets of connective tissue forming stacks of electrocytes, the EOD-unit.
It should be mentioned that the structures outlined so far are only valid for the rays belonging to the family Torpedinidae. Electrocytes in Rajoidea do vary significantly in being either disc-shaped or cup-shaped.
Electrocyte in Torpedo galvani
Electrocyte in Rajoidea
Discharge and Regeneration of the electric Potential in mature Electrocytes|
The way in which the electrical discharges are generated, resembles the way in which muscles are controlled to produce movement. As outlined in the electrogenic phase, the electrocyte, a shortened and modified myogenic fibers, has lost the capacity to contract, but is specialized for generating an ion current flow.
Electrocytes are flattened cells with a smooth, innervated surface on one side and a highly infolded surface on the other side (surface increase). The folding increases the surface area which increases its capacitance considerably. Each electrocyte is innervated by a spinal motor nerve which, when excited, causes the electrocyte to depolarize and to initiate a brief discharge called a spike potential.
The physiological mechanism of how transmembrane potentials are used in the electric organ to produce electric potentials, is important for the understanding of the EOD.
The spike potential is caused by cations that flow inward across the smooth cell surface and outward across the roughened surface. The spike achieves a peak potential of about 100mV. As will be shown later, stacks of electrocytes that are arranged like batteries in a series, sum up these small voltages to generate the maximum peak voltage of about 50V. To achieve a powerful stroke, synchrony of discharge is required. This is produced by simultaneity of nerve impulses arriving at each electrocyte. Current flow, of course is limited in a single EOD-unit, but increased by arranging stacks in parallel (in rays up to 350 EOD-units per lobe). Thus, it is not surprising to find that such arrays of EOD-units forming an electric organ can be as heavy as 1/4 or even 1/3 of the total body weight.
When a ray discharges its electric organ, an electric field is established in the immediate vicinity.
Because of the high energy costs of maintaining a continuos DC discharge, electric rays produce
a discontinuous pulsating discharge and field. Most rays pulse at rates between 50 and 300Hz.
The conductivity of seawater is so high, that the electric field produced even by a strong discharge is essentially short-circuited and, therefore limited in range (in fresh water, where conductivity is much lower, electric fish have to generate much higher potentials - up to 600V; their electric field may extend outward from the fish for several meters and will be distorted by the presence of either conductive or highly resistant objects). On the other hand, EODs of the electric organs in their tails of Rajoidea peak between 1.5 to 4V.
The waveforms of the EOD produced by the rays during predatory events can be recorded with a storage oscilloscope in a watertight housing. The most practicable way to detect the discharge can be realized with a fixed dipole electrode; i.e. a positive and negative pole spaced 20cm apart. It is most useful to detect distant electric fields. However, to measure the maximum output voltage, it is more appropriate to use electrodes which are clamped directly on the dorsal and ventral surfaces of the electric organ
An EOD consist of a series of individual DC discharges that reveal steep rising and falling slopes with a pulse duration of 2 to 5ms. It is thus not surprising, that a ray can generate up to 100 in T.nobiliana , about 400 in T.marmorata, and up to 415 EODs per second in T.californica.
It should be mentioned though, that water temperature affects EOD pattern by decreasing amplitudes and increasing the latency period between successive pulses; i.e. T.marmorata, and Tocellata at T £ 15°C.
Typical EOD pattern emitted by T.marmorata during head-suction responses upon a prey (70kB)
Physical aspects of an EOD
(opens a new page)
Feeding and associated electrical behavior:|
Most electric rays forage exclusively on fish. Stomach contents of numerous rays (T.californica) consisted primarily of anchovies, although demersal fish are a common substitute. Unlike most rays, electric rays swim by using their well developed tails for propulsion while their pectoral fins remain immobile as their near neutral buoyancy allows them to drift motionless with minimal sinking.
Rays usually are nocturnal, in that they use electricity not only to capture prey but also for detection and localization. Such nightly swimming and drifting behavior suggest that they actively search for food by registering the relatively low external electric potentials of swimming organisms (electroreception as the 6th sense).
Elecroreceptors of rays are modified hair cells that have lost their cilia. These cell are usually lowered into the epidermis which makes synaptic contacts with axons of a sensory fiber. The cell membrane facing the exterior has a lower electrical resistance than does the basal membrane - so most of the potential drop caused by the current moving across the cell occurs across the basal membrane. Thus, these exogenous currents flowing through electroreceptor cells produce changes in the transmembrane potential by triggering the release of Ca2+ . The ions themselves modulate the release of transmitter at the base of the cell, thus determining the rate of APs in sensory fibers. Conversely, a current flowing out of the body of the fish hyperpolarizes the basal membrane of the receptor cell and decreases the release of transmitter below the spontaneous rate.
It is important to note, that during each EOD, resulting from a central nervous system command, an
inhibitory command is sent to the electroreceptors. Thus, during each period of electric discharge
the fish is insensitive to its EOD. Between pulses, electroreceptors function to sense distortion in
the electric field or the presence of a foreign electric field.
As soon as the ray pinpointed a prey, it attempts to reduce its distance by gently drifting towards it. At close distance (usually a few cm), the ray lunged forward under the prey and folds the anterior and lateral margins of the disk over it. A series of discharges appear to immobilize the prey. Trials with a photographic flashbulb showed that the energy used for paralization is sufficient to make it ignite.
Daytime predation behavior seems to vary slightly in tactics. Rays buried in sand often wait patiently until a fish passes nearby. Once at close distance, the ray's head rises quickly while the lateral portion of the disk remain in contact with the bottom. Simultaneously, a series of EOD's stuns the prey, before the ray sucks it toward the mouth in a stream of water caused by the lifting of the ray's disk.
Device to detect EOD's
Schematic illustration of prey approach (130kB)
On the basis of these observations, scientists conclude that during the day individuals
bury themselves in sand and ambush active but unwary prey, while at night they swim about in search of
prey which may be resting at that time. Scientific studies revealed that the length of an EOD sequence can
vary drastically. The duration of the entire pulse train is prolonged if the tethered prey displays some attempts
of escape. Such an attack pattern implies a feedback-loop mechanism that enables the ray to monitor the preys
state of response, itself detected by the rays electro-receptors.|
With short kicks of the tail, the folded ray either remained upright or performed barrel rolls, summersaults, or both it tries to envelop the prey in a way that it finally lands under the disk. Then, a deep undulation of the disk margin, moving in a peristalsis-like wave toward the anterior end, conveys the prey toward the mouth. Finally, the prey is swallowed head first in several gulps.
Even though the entire sequence lasts up to 20secs, only 35% of the entangled prey eventually escapes death.
References on the WWW:
Family of the elecrtric rays:
Rays - what's that?
Fish Species database: http://www.fishbase.org/search.cfm