Anatomy: Differences Worth Knowing.

Understanding anatomical and physiological variation among diverse fish groups helps the examiner make more accurate observations and diagnoses, ask appropriate and pertinent questions when taking a case history, and exercise caution when handling fish that have evolved with defense mechanisms such as poisons, sharp spines, and voltage producing organisms. This section of Anatomy will examine four areas in which fishes differ from one another.


With over 40,000 known vertebrate species, representing fish, mammals, reptiles, amphibians and birds, fish represent the largest and most anatomically diverse group of vertebrates. Having insight into the vast anatomical and physiological variation between many different fish groups, will allow you, the diagnostician, to be more adept at taking a pertinent case history, make accurate diagnoses, and exercising proper animal handling techniques.

Below is a taxonomy breakdown that shows how fishes are divided into three vertebra classes.

Kingdom Anamalia
Phylum Chordata
Subphylum Vertebrata
Class Agnathajawless fish (lamprey and hagfish)
Class Chondrichthyes cartilaginous fish (sharks, rays, skates and chimeras)
Class Osteichthyes (boney fish)

Defense Mechanisms



Family Scorpaenidae
Pterois volitans

Genera of the subfamily Pteroinae, including lionfishes and turkeyfishes, have very attractive extensions to the dorsal and pectoral fins. The dorsal spines contain toxins, which may cause moderate to severe injury to careless handlers.

Family Synanceiidae
Synanceia sp.

Well camouflaged, the stonefishes contain one of the most deadly neurotoxins known from fish. The toxin is released from the anterior dorsal spines.

Family Ictaluridae
Channel catfish
Ictalurus punctatus

The channel catfish is a popular game species in ponds and slow-moving temperate waters. This fish is also a common laboratory species. The dorsal and pectoral fins have sharp spines, which also contain a mild (but painful) toxin. With care and patience, this fish may be easily handled.


Family Electrophoridae
Electric eel
Electrophorus electricus

A variety of fishes are capable of generating electrical discharges. Some fishes, like the electric eel, electric rays (Torpedo) and the electric catfish (Malapterurus), are capable of producing strong, stunning currents from 50 up to 650 volts. Other fishes of the families Rajidae, Gymnotidae, Gymnarchidae and Mormyridae use low-voltage pulses to set up an electric field to sense prey items and obstacles.

Special Sence Organs

Family Cyprinidae
Cyprinus carpio

Observe the single pair of maxillary barbels on this Carp. Barbels on other species, such as in the catfishes, may also be located near the nares, and on the chin (mental barbels). Barbels serve as both tactile and taste sense organs.

Family Ictaluridae
Channel catfish
Ictalurus punctatus

In addition barbels, fish may refer to the water with numerous taste buds outside their mouths. Fishes of the catfish family have taste buds over their entire body surface, thus being able to taste their environment and sense a variety of chemical signals.

Family Mochokidae
Upside-down catfish
Synodontis nigriventris

When most fishes are observed swimming "belly-up," it is an indication of loss of balance or near-morbidity. This is not the case in some other catfishes in the genus Synodontis , which are occasionally seen swimming upside-down. This is an adaptation for surface feeding.

Family Anomalopidae
Flashlight fish
Anomalops katoptron

A variety of fish species maintain symbiotic luminous bacteria in specialized "light organs," the exterior surface of which, is under muscular control. Examples of fish with luminescence of bacterial origin include the flashlight fish and pinecone fish. Other bathypelagic [definable button] species (e.g.. families Stomiatidae and Batrachoididae) with light organs are self-luminous with organs known as photophores.



Like the flashlight fish, this pineapple fish with its armored scales, has luminescent light organs underneath the eyes.

Intraspecies Differences

Family Pomacanthidae
Blue angelfish
Holacanthus isabelita

Several of the marine angelfishes, particularly species of the genus Holacanthus, show striking difference in coloration between mature juveniles, bottom, and adult, top.

Family Poeciliidae
Striped mummichug (killifish)
Fundulus majalis

Observe the distinct color pattern differences between the male (upper photo)and female (lower photo) striped mummichug. In the male, the barring is transverse (vertical) throughout its life.

When the female reaches a length of about two inches, the original transverse bars are transformed (with growth) into longitudinal stripes. Note that there are subtle differences in the shapes of the dorsal and anal fins between the sexes of this species. Sexual dimorphism amongst the killifishes is common.

Family Labridae
Slippery dick
Halichoeres bivittatus

Although bisexuality is the norm for most fish species, hermaphroditism and parthenogenesis is also known. In the cases of simultaneous hermaphroditism and parthenogenesis are also known. In the case of hermaphroditism, ovotestes are common in individuals of several genera (Perca, Stizostedon, Etheostoma and some salmonid species). In other families, including Labridae and Serranidae, hermaphroditism may be sequential. An example of this is the protandrous (female first, then male) slippery dick.

Family Labridae
California Sheepshead
Semicossyphus pulcher

The male and female California sheepshead show sexual dimorphism in both head structure and body color. Compare the extreem color and demension variations in this male, above, with the female, below.

Family Cyprinidae
Fathead minnow
Pimephales promelas

Males, top,have a 'fat" head with darker pigmentation. Although not easily observed in the photograph, numerous tiny nuptial tubercles can be found on the dorsal aspect of the male's head. There is also an epithelial proliferation on the dorsal aspect of the body, anterior to the insertion of the dorsal fin, giving the spawning male a more robust appearance.

Accessory secondary sex characteristics are many and varied, and are usually sexually dimorphic. Compare the female, bottom, fathead minnow, with the sexually mature male, top.


Ornamental Variations


Family Cyprinidae
Crassius auratus

There are many ornamental varieties of fish which have been inbred for specific traits. This telescope goldfish is one of many examples of ornamental goldfish and carp (koi) which are distinctly different from the wild type. The protruding eyes in this specimen is a genetic aberration, not pathological exopthalmia

Family Cyprinidae
Crassius auratus

Another inbred variety of goldfish is the Pearlscale. The puffed-out appearance of the scales, and swollen-looking abdomen of this animal are ornamental traits. Compare this animal with the next photograph of a fantail goldfish with ascites.

Family Cyprinidae
Crassius auratus

This dorsal view of a fantail goldfish shows its swollen, fluid-filled abdomen (ascites), puffy scales, and moderate exopthalmia. The left eye is slightly more protruded than the right. Abdominal fluid was cultured and Aeromonas hydrophila was isolated.

Family Cyprinidae
Crassius auratus

The massive reddish-brown epithelial proliferation on the head, and the posterior spinal curvature on this Chocolate lionhead goldfish, make it a valuable variety in the fancy goldfish trade. The distended abdomen is normal in this variety.