A definitive courtship is rare in fish. In those fish that spawn in large groups courtship is often minimal or practically non-existant. A female will indicate her readiness to lay and this will be noticed by one or more males who will attend here, releasing their sperm (milt) as she releases her eggs. In the Common Carp for instance a female may be attended by 3 or 4 males.
In those species in which a courtship is evident it is often indicated by the existance of morphological differences between the genders. These can be in the form of special adaptations in the male for the transfer of the sperm to the female in those species where fertilization is internal; this includes all the Sharks and Rays as well as a variety of Bony Fish.
In Sharks and Rays the anal fins are modified with grooves, which, when held together, form a tube that guides the milt into the female's cloaca. In Bony fish the intromittent organ can be simply an external extension of the urogenital canal, or it can be more complicated involving modifications of the anal fin-rays to form a movable organ which may possess hooks or barbs at its end.
Apart from these functional differences males may be differentially coloured and possess a variety of enlarged and distinctly-coloured fins which serve as signalling devices to both attract females and warn away other males. Some of the better known examples of this include many of the commercially popular Cyprinodonts such as Swordfin Mollies, and the equally popular Siamese Fighting Fish.
These secondary sexual characteristics, (morphological adaptations possessed by, and defining one gender of a species, but not directly related to sex, such as a beard), may be evident all year round or they may develop at the beginning of the breeding-season and disappear after it is over, this is particularly true of the bright, vivid colours exhibited by some males.Courtship proper is normally short and usually involves the male swimming around the female showing off until he succeeds in stimulating her. In some species, such as the Siamese Fighting Fish (Betta splendens) if the female is slow to respond the male may become aggressive. In species where there is a definite pairing during the breeding season the female will signal her readiness to the male in some way such as by quivering, and or, adopting a certain ritualistic posture or position in the water. The male will then display before her, raising and lowering his fins, rubbing against her, touching her with his fins and butting her with his head until she is stimulated to release her eggs.
Most of the species that have a ritualised courtship live in fresh water. Not all species in which there is an observable courtship go in for in for any sort of post-fertilization care, although many do. A marine species with an elaborate courtship is the Common Dragonet (Callionymus lyra), it is also an example of a species in which a complex courtship does not indicate post-ferilization care, as the eggs, once released are left free to float in the sea. Territoriality often co-exists with courtship, and territoriality is most common in fresh water fish and those marine species that live in and around coral reefs and rocky shores.
While a complex and observable courtship ritual is less common that mass spawning it should be noted that even in those species that aggregate on mass to spawn it is not just a mad communal release of sperm and eggs. What normally happens is that from within the huge school pairs, or small groups of fish will suddenly separate out from the group, often rising to the surface, to spawn quickly and then return to the school. In this way a brief courtship dance may be observed as the male guides the female away from the school.
The bitterling (Rhodeus amorus) is an interesting example of a species that is only territorial during the breeding season. The species breeds by laying their eggs into the gills of the Swan Mussel where they are protected from predators until they hatch and leave the safety of the mussel's shell. Males set up territories guarding access to a Swan Mussels, and during the breeding season he develops reddish fins and an orange-yellow belly. The female however appears to be far more attracted to the swan mussel than to the male Bitterling. She will not produce the long ovipositing tube she uses to lay her eggs into the Swan Mussel's gills unless there is a living Swan Mussel present, regardless of the males beauty, and furthermore she will produce the ovipositor, and even lay her eggs when there are no males present.
Nest building occurs in a number of fish species, commonly these nests are relatively simple and created from the sand and gravel of the sea, lake or river bed, such as those made by trout and salmon. However more complex nests exist. Perhaps the best known example is the Three-spined Stickleback in which the male builds a nest of pieces of vegetation which he glues together with a secretion from his kidneys. An example of a fish that builds a much larger and more impressive nest is Heterotis (Heterotis niloticus) of Africa. The nest is built with walls of vegetation and a floor of smooth mud. It is usually about 100 cm across and the walls with walls between 20 cm and 60 cm high and 15 cm to 20 cm thick, these walls project above the water level. After spawning the adult fish leave through a hole in the wall, but the male stays and guards the hole and then, when the young emerge in five days time, he guards them. Another extensive nest builder is the Aba (Gymnarchus niloticus), also from Africa. This electric-fish builds a nest about 100 cm across, but unlike the last it is a floating nest, but again the sides rise above the water level and again the young are protected by one of the adults when they emerge.
Another interesting method of nest building is the use of bubbles to create a floating nest (the bubbles are created in the fish's mouth out of air and saliva). In The Paradise Fish (Macropodus opercularis) the male builds a bubble nest, then urges the female towards it, after spawning he collects the eggs and inserts them one by one into the bubbles of the nest, the female can lay as many as 500 eggs. Then he stays and guards the nest. At least four, and maybe more, species in the genus Microctenapoma also build nests out of bubbles.
In those species in which the males build a nest, or maintain a territory during the breeding season the males may undergo psychological changes as well as morphological ones in that they become more aggressive, particularly to con-specific males.
Territorial fighting is usually restricted to ritualized displays of fins and colored body parts and rarely results in physical contact. However ritualized bouts of physical confrontation can also occur. The males of several species of Mudskippers common to the mangroves and mudflats of Southeast Asia maintain territories and indulge in territorial battles that are easy to observe. The individuals first display fins, but if neither is willing to back down then physical interaction is inevitable. The opposing individuals line up face to face and open their mouths as wide as possible, then they push against each other in what looks like a kiss until one or the other turns side on and looks away signaling defeat.
One of the exceptions to this rule of non-destructive and ritualized combat is of course the Siamese Fighting Fish. The males of this species, at least in the artificially well lit and spacially limited conditions of a fish-tank, will inflict considerable damage on each other. Some Cichlids are also highly territorial fish and go in for territorial defense at a variety of levels of aggression. In the genus Haplochromis physical fighting will occur between two males after only a short display. Stepping down the aggression a bit we find the the genus Hemichromis in which the two males will only fight if they are obviously near equals and only after an elaborate display. Finally in the Genus Herichthys we see that there is no actual fighting, however the ritualized intimidation displays may well continue until one of the contestants is defeated through shear exhaustion.