Courtship and Spawning Displays in Fish

Nuptial colour, courtship dances in sticklebacks, bettas and killifish, bubble nests, sand bowers, mouthbrooding cues, and why courtship and aggression can look alike in fish.

Colour intensification

In many species, breeding condition is signalled by colour. Male three-spined sticklebacks (Gasterosteus aculeatus) develop blue eyes and a bright red lower head, throat and front belly in the breeding season; the red comes from dietary carotenoids, and more intensely coloured males tend to forage better and carry fewer parasites. In Astatotilapia burtoni, territorial males are brightly coloured with an eye bar, while non-territorial males look like females. Dominant male Maylandia zebra show more vivid colour and more intense egg spots on the anal fin. Colour change is not limited to males: receptive female Siamese fighting fish (Betta splendens) darken and show vertical breeding bars. In the turquoise killifish (Nothobranchius furzeri), skin colour develops from sexual maturity onward, and a laboratory welfare scoresheet notes that colour loss in old fish is age-related rather than a welfare sign by itself.

Courtship dances

• Three-spined stickleback: the male approaches a female in a zigzag of short swims left and right, returns to the nest the same way, pokes his head in and swims through the tunnel, inviting her to follow.
• Siamese fighting fish: males flare the gill covers, spread the fins and twist the body; spawning takes place in a 'nuptial embrace' in which the male wraps around the female, releasing about 10-40 eggs per embrace over 1-4 hours.
• Bluefin killifish (Lucania goodei): males circle the female, flash their fins and flick their heads near her; females visit male spawning territories daily.
• Millerichthys robustus, an annual killifish: courtship consists of alternating lateral displays with body vibration, folding and twisting of the dorsal and anal fins, and S-shaped (sigmoid) displays above the substrate.
• Haplochromine cichlids such as A. burtoni: territorial males approach females and quiver the anal fin, displaying the egg spots; in experiments, female preference was explained by courtship intensity rather than egg-spot number.

In annual killifish, territories are typically defended near the substrate where eggs are laid, while non-annual species defend territories with vegetation, matching the preferred egg-laying site.

Nest building: bubble nests, pits and bowers

Male Siamese fighting fish build bubble nests at the water surface, regularly and even without a female present; females may inspect the nest before spawning. After spawning the male alone tends the eggs, retrieving sinking ones in his mouth and placing them back in the nest; incubation takes 24-36 hours. The three spot gourami (Trichopodus trichopterus) builds a large bubble nest among floating plants and works plant fragments into it; the male guards it until the eggs hatch after about 20-30 hours.

The male three-spined stickleback digs a small pit and fills it with plant material, often filamentous algae, plus sand and debris, glued together with spiggin, a protein secreted by the kidneys. Building takes 5-6 hours but can be spread over several days. After spawning he fans the nest entrance with his pectoral fins for 7-8 days until hatching. In about 200 Lake Malawi cichlid species, males build sand bowers in the breeding season, scooping sand with the mouth into either crater-like pits or raised castles. Bowers serve both as aggressively defended territories and as courtship and spawning sites, and in Mchenga conophoros castle-building is linked with more quivering courtship and larger gonads.

Mouthbrooding cues

In mouthbrooding cichlids the female usually carries the eggs; paternal and biparental mouthbrooding are rarer. Haplochromine females collect eggs in the mouth immediately after laying, and in mbuna the male often fertilises them only once they are in her mouth. During the quivering display both partners shake the anal fin, which induces the partner to mouth at it, triggering spawning and sperm release. Egg spots were originally thought to trick females into mouthing the male's fin to ensure fertilisation, but experimental removal of egg spots did not reduce fertilisation success. Brooding fish feed less or not at all and become underweight; Maylandia zebra females carry the eggs for 3-4 weeks without eating before releasing free-swimming fry.

Courtship or aggression?

Courtship and aggression in fish often use the same signals, so the movement alone does not reveal intent. Gill flaring and fin spreading are part of betta courtship but are also the betta's first display towards territorial intruders. The red throat of a breeding stickleback attracts females and releases aggression in other males, as Niko Tinbergen showed. In bluefin killifish, fin flares precede aggression and were given more often when other males were present. Context helps: in A. burtoni, gravid females prefer dominant males while non-gravid females show no preference, and in N. furzeri females carrying eggs may show abdominal swelling near the vent. Swelling alone does not confirm readiness, since a markedly distended abdomen is also scored as a body-condition sign.

Breeding systems as a whole are covered in fish-reproductive-strategies, and per-species breeding guides give species-level detail.