The mail-cheeked fishes are highly variable in their mode of reproduction. Some of the methods used by them to reproduce are noted below. In the Comephoridae there is a remarkable imbalance between the numbers of each sex, the proportion of males in the total population being as low as 3 or 4 percent. The biological basis for this imbalance is unknown. Members of this family are viviparous (live-bearing). The females come near the surface to give birth to their young; the males remain at their normal depths. By contrast, the remaining cottoids are oviparous (egg-laying), including the Cottocomephoridae of Lake Baikal. The females of most of the latter family deposit their eggs in shallow coastal water, then leave the males to guard them until they hatch. This is also the general rule among the sculpins, in which the males guard the eggs. In some species the eggs are shed loosely and adhere to the bottom, but little reliable evidence is available concerning the breeding habits of most cottoid fishes. The northern Atlantic short-horned sculpin, or bullrout (Myoxocephalus scorpius), is known to build a rudimentary nest guarded by the male, as does the freshwater European miller’s thumb. The males of these and other cottids have a well-developed structure called a urogenital papilla, which some authorities have suggested is used to introduce sperm into the female. Many cottoid species develop pronounced breeding coloration with sexual differences that apparently aid in recognition between the sexes and in territorial behaviour.

Some members of the family Cyclopteridae build nests that are guarded by the male. The familiar lumpsucker, or sea hen (Cyclopterus lumpus), common on both sides of the North Atlantic, spawns along the coast in the winter. At least some of the inshore species, such as the striped sea snail (Liparis liparis), which has a distribution similar to that of the lumpsucker, deposit their spawn in clumps on hydroids (such as sea moss) and seaweeds, but there is no evidence of parental care. The females of some North Pacific cyclopterids (such as Careproctus sinensis) have a long specialized structure (ovipositor) by which they lay their eggs under the shell of the Kamchatka crab (Paralithodes camtschatica). In general, however, little is known about the breeding biology of these fish.

In contrast to the rather specialized reproductive behaviour of the sculpins and the sea snails, the sea robins produce eggs that are simply shed in batches in the open sea. So far as is known, no special breeding behaviour accompanies spawning except that these noisy fishes become increasingly loquacious during the spawning season. The members of the family Hexagrammidae (the Atka mackerel, for example) deposit one clump of eggs, often on algae in shallow water on stony bottoms; some species, however, like the lingcod (Ophiodon elongatus), care for their egg masses during incubation. The sea poachers, or pogges (Agonidae), lay relatively few eggs, often hiding them away in crevices. The eggs are relatively large, 1.5–1.9 mm (roughly 0.06 inch) in diameter in Agonus decagonus, a species found in the extreme North Atlantic. The European hook-nose (A. cataphractus) lays up to 2,400 eggs inside the hollow rhizoid (stalk) of the kelp Laminaria in a compact, membrane-covered mass. Incubation is prolonged, possibly as long as 12 months.

Scorpion fishes of the family Aploactinidae similarly shed their eggs in the open sea. Members of the scorpaenid subfamily Scorpaeninae extrude eggs in gelatinous balloon-shaped masses; those in the subfamily Sebastinae have internal fertilization and are viviparous. The three groups represent the principal evolutionary stem groups of the scorpion fishes. In the North Atlantic redfish (Sebastes marinus) fertilization is internal, and the eggs develop within the oviduct of the mother. Fertilization usually takes place during February, after which the females form shoals and migrate to spots where warm bottom currents pass. The female can be said to be a living incubator; in such fishes, which live in cold northern seas, it is clearly advantageous to carry the developing young to an area in which more favourable conditions prevail. The young at birth are very immature, nevertheless, and brood size in the redfish is relatively large (up to 360,000); the larvae must survive a lengthy planktonic life. A smaller, shallower water redfish, the Norway haddock (S. viviparus) produces much smaller broods, with brood sizes ranging from 12,000 to 30,000 young. The scorpaeniforms are distinguishable among viviparous teleosts (advanced bony fishes) by their comparatively high fecundity; comparison with many other marine fishes, single individuals of which produce millions of freely shed eggs, however, illustrates the relative advantage, at least in numbers, of bearing living young over laying eggs.

The North Pacific redfishes or rockfishes (such as Sebastodes, Sebastiscus, and Hozukius) closely resemble the North Atlantic sebastine species in their reproductive biology; all species studied have been found to have relatively large brood sizes. The sebastine rosefishes (Helicolenus), found in both the northern Atlantic and Pacific oceans, have morphological affinities with the subfamily Scorpaeninae. Studies of their reproductive biology have shown that the sebastine rosefishes have intraovarian embryos embedded in a gelatinous matrix, and they thus appear to combine sebastine viviparity with scorpaenine egg masses.

Sound production

Since the time of Aristotle, the sea robins have been known as sound-producing fishes, and their sonic performances and mechanisms are well known. They have a large swim bladder loosely attached to the dorsal wall of the body cavity; the swim bladder is vibrated by lateral muscles in which the striated fibres run at right angles to the muscles’ length. The sea robin of the North American Atlantic coast (Prionotus carolinus) produces single vibrant barks and growls, as wells as series of rapid clucks with very little provocation. Some of the sculpins (Cottidae) produce dull groans and growls; it is believed that these sounds are mechanical in origin, arising from contractions of the muscles that produce periodic movements of the pectoral girdle. Flying gurnards (Dactylopteridae) are similar to the triglids in their sonic mechanism and sound-production capacity.