Fish believe in variety when reproducing
The Internet search phrase "fish sex" generates more hits on my Web site than any other.
I don't know what people are looking for when they type those words, but I imagine they're wondering about the ways fish get together to make baby fish.
It's a good question because there is no one way. Fish sex is as varied as the fish themselves.
In marine fish a common method of introducing sperm to eggs is called broadcast spawning. Broadcast spawners don't waste energy courting, nest building or caring for their offspring. These fish gather together and, in a swirling mass, cast their gametes to the currents.
The odds of a sperm finding an egg this way are low, which is why females of these types, which include tunas, sardines, wrasses and others, produce massive numbers of eggs. Ahi (yellowfin tuna) release 500,000 to 10 million eggs daily for weeks and do so several times a year.
Water temperature, moon cycles, food supplies and probably other factors only the fish know tell them when and where to gather for these raucous reproductive events.
An advantage of broadcast spawning is that the currents carry embryos long distances. A disadvantage is that to disperse means to float, making the embryos easy prey for fish from below and seabirds from above.
Other fish, such as salmon, leave less to chance by hiding their eggs. The female salmon digs a nest with her tail and lays her eggs in the hole. A passing male fertilizes the eggs and defends them until the female gets them buried. Then the two go off and die, their mission in life complete.
Damselfish, among others, go a step further by taking care of their embryos. Since it's the male damselfish's job to defend his territory and offspring, he chooses a spot and then courts passing females by swimming straight up and then plunging downward while singing. (It sounds like a grunt to us.)
Once a female lays her eggs in his nursery, the male fertilizes them, chases away predators, maintains oxygen levels by fanning and keeps the mass clean.
Male damselfish are stuck in one place guarding their eggs, but male pipefish and sea horses take the brood with them. After a courtship ritual, the female lays her eggs in the male's belly pouch. He fertilizes the eggs as they enter and carries them with him even after they hatch. When the youngsters are big enough to swim, the dad pops them out and they're on their own.
Male cardinalfish have a similar tactic but use their mouths rather than pouches to carry their eggs and hatchlings.
Sharks employ the mammalian technique of internal fertilization. Some female species then lay their eggs and leave; others nourish the eggs via a placenta and give birth to live pups. The pups of great white, tiger, mako and several other sharks develop inside the female's body but have no placenta. For nourishment, these pups eat each other.
Then there are the hermaphrodite fish that possess both eggs and sperm, fish that change sex to suit the occasion and variations of all of the above.
I'm not sure this is the kind of information people want when they search for "fish sex," but it should be. The ways fish have evolved to reproduce in an enormous, moving, predator-filled environment are marvels of marine biology.