The evolutionary origins of nest building behavior in vertebrates are deep rooted and go all the way back to the fish. The behavior can be very complex with stunning results such as that of the Japanese puffer fish. Here in this short observation an underwater cameraman and filmmaker Mert Gökalp captures a few distinct scenes from the Gray Triggerfish nesting behavior in the Eastern Mediterranean town of Kaş. This fish colonized the Mediterranean through the Gibraltar and reached the farthest corners of this sea.
The spawning behavior of the Gray Triggerfish, Balistes capriscus (Gmelin, 1789), has been documented particularly well on artificial reefs in the northern Gulf of Mexico. Observations included territoriality, nest building, harem spawning, and parental care. Males of the triggerfish are larger than females. During the spawning season they have a dark charcoal coloration. In Gulf of Mexico, dominant males build and maintain one to 13 nests, defending the area aggressively against other males. They also chase away other fishes. They allow one to five females to spawn in their nests. Before mating females frequently visit and inspect the nest. After fertilization occurs females stay on the nest guard and tend the eggs by fanning and blowing the eggs. Females at this stage have a contrasting black and white pattern. Males continue to defend their territories around the nest. They visit females on the nest, and continue to chase other fish away. Male aggression and sex ratios of single dominant male and up to five reproductive females indicate that gray triggerfish display harem spawning.
Here in this observation from the Mediterranean, the triggerfish are shown to be attracted to a shipwreck that serves as artificial reef but the multiple nest behavior observed in their Mexican counterparts is not displayed.
As expected because of accessibility freshwater fishes provide some of the best documented nesting and mating behaviors. In bluegill sunfishes males prepare and defend the nest. Males only allow females to approach and lay eggs and chase away other males. Unlike the triggerfish males tend the nest and fan the fertilized eggs with their fins to oxygenate so that the brood develop faster. During this period (can be up to 10 days) they can’t feed themselves and can lose 10 percent of their body weight. There are however certain small satellite males who mimic females. Genetic studies have shown that these impostors can sneak into the nest and fertilize the eggs. The video below is an outstanding work showing snapshots of nesting behavior in longear sunfish Lepomis megalotis.
Starting from 20 seconds into the video, you can see a satellite male making several attempts to fertilize the eggs. This is when the story gets interesting. Satellite males parasitize the nest and get their offspring guarded by the resident male. This is a very useful strategy with no parental investment. On the other hand resident males seem to have an equally sinister counter strategy. Once the fish fry begin to hatch resident males cannibalize on a fraction of them. Scientists were rather curious whether the resident males could differentiate hatchlings sired by satellite males. This ability has been shown in a central American fish Midas cihclids using controlled aquarium experiments. Genetic analysis on darters and sunfishes have shown that sunfishes can’t. If they could it would be an intriguing story where resident males perhaps intentionally turned a blind eye to satellites in order to consume their hatchlings. In this evolutionarily stable strategy males are cannibalistic.
We don’t know whether the nesting behavior of the triggerfish follows same lines. It is clear that risk of predation of eggs by other fish is far more stronger than nest parasitism from conspecific males since both sexes guard the nest but the behavior is a variation of the same fishy theme.
Arguably the best documented nesting and mating behavior comes from the three spined sticklebacks. Here the problem of external fertilization in high current environment is solved by evolving an intricate nest building behavior by the male. The nest constructed by sticky secretions from the kidney glands of the male allows a contained space for efficient fertilization of eggs. Nests constructed by some fishes such as the bluehead chub can attract spawning aggregations from 7 species of fish!
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