A few of the perciforms are known to be harmful to humans. Swimmers have been attacked by the barracuda (Sphyraena), which is a voracious fish reaching nearly 2 metres (6 feet) in length. Perciforms possessing venom glands are also considered dangerous fishes. The dorsal spine of the weever fishes (Trachinidae) has a grooved structure containing a venom gland; in addition, there is also a stinger located on the opercular (gill cover) structure. Both the stinger and the dorsal spine can be extremely painful if stepped on in shallow waters. Similar venom-bearing structures are found in the dragonets (Callionymidae) and surgeonfishes. The venomous spines in the surgeonfish are located on either side of the caudal peduncle (the narrow stalk just in front of the tail). Especially well-armed are the electric stargazers (Astroscopus; Uranoscopidae), which are capable of discharging up to 50 volts of electricity from the modified muscle tissue just posterior to the eyes; in addition, they possess a venom spine just above the pectoral fins. The venom from uranoscopids has been known to cause death in humans.

Ciguatera fish poisoning has been attributed to some perciforms that are otherwise considered to be excellent food fishes. Among these are certain carangids, snappers, barracudas (Sphyraenidae), surgeonfishes (Acanthuridae), groupers, and porgies. A species completely edible in one area may be poisonous in an area just a few hundred miles away. This curious phenomenon has not yet been fully explained, although it has been suggested that the source of poisoning may be a toxic form of blue-green alga passed up the food chain and thus present in the food of toxic species.

Life history

Many perciforms live out their whole lives in small areas, but others, especially open-ocean (pelagic) species, perform extensive migrations, about which much remains to be learned. Some marine serranids, however, are anadromous (that is, entering fresh or brackish water to spawn); some freshwater perciforms, such as certain species of gobies, enter the sea to spawn (catadromous). Tuna (Scombridae) may travel across the entire Pacific Ocean from the California coast to Japan or the reverse. Spawning in perciforms generally takes place in shallow coastal areas or in rivers and ponds among rocks, seaweeds, and aquatic plants. Paraclinus marmoratus, a clinid blenny, is known to lay eggs at times in the lumen (cavity) of a living sponge.

Breeding behaviour among fishes of the order Perciformes is diverse. Pairing of male and female is common, although a single female may pair with more than one male, as among certain serranids, perches, and cichlids. The sexes are usually distinct, but hermaphroditism (presence of functional male and female organs in a single individual) normally occurs among certain sea basses and porgies. The young of the black sea bass (Centropristis striata) are mostly females with normal egg-laying functions; after five years, however, some of these females transform into functional males. About 11 species of sparids have been found to display hermaphroditism at one time or throughout their lifetime.

Characteristic differences usually exist, especially during the breeding season, between sexes regarding colour, size, markings, or structure. The male is generally smaller in size (some exceptions are found in sunfishes, gobies, and blennies) and has brighter coloration of the fins and body. Black, white, green, red, blue, and silver are colours characteristic of the brightly coloured males of damselfishes (Pomacentridae), wrasses (Labridae), labyrinth fishes, and cichlids. Structural differences between the sexes vary from easily observed characteristics, such as the presence of a longer dorsal fin in the male dragonets, to less-obvious characteristics, such as larger canines in the dentition of male blennies (Blenniidae) and gobies. Bands, blotches, and tinged markings may also be characteristic of the brighter-coloured males, as in some wrasses. During the spawning season, males of certain species of cichlids, parrot fishes (Scaridae), sparids, and wrasses develop a swelling on the forehead that may persist throughout life. These characteristics presumably enhance the male in its breeding and courtship activities.

Little is known of the courtship activities in most marine species of perciforms. Among those that have been studied, the male dragonet, with extended fins and gill covers, performs a display of colours while swimming repeatedly around the female. A similar courtship activity is also carried out by the fighting fishes (Betta).

Fertilization is usually external, although internal fertilization, in which the eggs are fertilized within the body of the female, is well known in such groups as surfperches (Embiotocidae); and the male in such cases generally possesses an intromittent organ, which functions in the transfer of sperm to the female. Internal fertilization also occurs among some of the gobies, clinid blennies (Clinidae), and apogonids. Perciforms that undergo internal fertilization mostly are viviparous; that is, they give birth to live young. The number of young in viviparous perciforms varies from 3 to 50 in the surfperches. The larger viviparous forms, however, have been found to produce a greater number of young.

Most perciform fishes are oviparous; that is, they lay eggs that are fertilized externally. The number of eggs laid varies from a few hundred to more than three million in a 15-kg (33-pound) yellowtail (Seriola lalandi, Carangidae). Often the eggs are released to float freely, but many species have evolved elaborate nest-building behaviour. Nest construction frequently consists merely of the clearing away of a small area under rocks, which may be on the open bottom or even inside empty sea-animal shells. The sunfishes and darters (Percidae) use their fins or body to dig a circular depression for use as a nest. Wrasses construct a nest out of stones, shells, and seaweed. Males generally undertake the task of building the nest, but in many cases both male and female share the labour.

Most labyrinth fishes build bubble nests, the procedure being similar among members of the family. The male Siamese fighting fish takes a bubble of air into his mouth, coats it with a mucous secretion and then blows the coated bubble to the surface; this process is repeated until a bubble nest is formed. After pairing, the female allows the fertilized eggs to drop to the bottom, where the male picks them up in his mouth and blows them into the bubble nest. Many marine perciforms produce pelagic eggs (that is, eggs that float on or near the surface of the open sea). Almost all the freshwater perciforms produce demersal eggs (that is, eggs that sink to the bottom). A certain amount of adhesiveness in the demersal eggs keeps the eggs together in clusters; the elongated shape of the clusters of perch eggs helps in securing the clusters to aquatic plants and rock bottoms. Not all eggs become attached to aquatic plants and other objects; the mature male humphead (Kurtidae) possesses a hooked structure on his forehead, to which the cluster of eggs is attached as soon as the female produces them. Oral incubation, in which the eggs are held in the mouth of one of the parents, is found in certain species of cardinal fishes (Apogonidae), jawfishes (Opisthognathidae), labyrinth fishes, and cichlids. The male, the female, or both may incubate the eggs orally until they hatch, after which the young may become mouthbreeders. Similarly, guarding of the nest sites may be undertaken by the male, the female, or both; however, among some sunfishes, darters (Etheostoma), and the Siamese fighting fish, only males defend their nest against intruders. In addition to guarding the nest, certain perciforms also aerate the eggs by directing a flow of water into the nest with fanning movements of the fins.

Although there is no evidence of parental care in perciforms that produce pelagic eggs, a strong protective behaviour is shown by most perciforms that build nests or carry their eggs around with them. The male dwarf cichlid may help in the care of the young, but it is the female that looks after the eggs, removing dead eggs from the clutch. In certain other cichlids (Apistogramma species, for example), the female may help free the young from the eggs by gently chewing off the eggshells. The young of many cichlids follow their mother around and quickly enter her mouth should danger threaten. The discus and the freshwater angelfishes of the cichlid family feed their young on mucous secretions of their own bodies. The male betta (Betta) guards the young until they can swim away freely on their own.

The only known case of suckling among fishes occurs in the order Perciformes. After unborn European eelpouts, which are also known as viviparous blennies (Zoarces viviparus), exhaust the yolk within their respective egg sacs, they can obtain additional nutrients from the mother’s ovarian follicles. Each young attaches its mouth to the opening of a canal inside the mother that leads to an ovarian follicle, which dispenses fats, proteins, fluid saturated with oxygen, and other nutrients.