I have a pair of Neon gobies, Elacatinus oceanops that spawn regularly. The male guards about fifty eggs. In the sea, hundreds of fry are produced. If they get lucky, and are swept into plankton, they live.
I understand that mammals are born with the total number of eggs they will ever produce. How do fish manage to produce thousands of eggs?
Even in mammals, there must be some process for increasing the number of egg-precursor cells, right? The statement “mammals are born with the total number of egg-precursor cells they will ever produce” just means that that process stops some time before birth, not that the process never occurs. But there’s no reason that the process couldn’t continue for longer if it needed to.
Even female humans have 1 million eggs at birth, and 300,000 at puberty. So there are far more in the ovary than are ever ovulated.
I don’t know if fish have their lifetime supply of eggs when hatched, but given the huge numbers some produce I would guess not. The ocean sunfish or mola mola can produce 300 million tiny eggs in one breeding season.
Thank you, Gentlemen.
Here’s some more information, although it doesn’t totally answer the OP’s question.
My own personal guess is that some females produce eggs the way most males produce sperm - basically, as needed.
Neon gobies are pelagic, meaning as I understand it, sea water not close to shore and not on the bottom. Royal grammas are too. The grammas have spawned twice, and none of the fry have survived. If they are swept away and live if swept into plankton, why are hundreds of them found living on shelves on reefs?
After hatching on the reef the larvae swim up into the upper waters to live and feed in the plankton and are pelagic for a time. However, once they metamorphose into the juvenile form they will swim back down and find another reef to settle on. Having pelagic larvae means that many reef fish are dispersed over large areas. The adults of course are not pelagic but reef-dwellers.
A lot of fish species create new eggs for each breeding cycle; many of them breed annually (though some do so continuously through the year). Eggs that are not expelled/used this year break down and are reabsorbed, and the cycle continues again the next year.
Many species also tend to continue growing and breeding throughout life, unlike other organisms that will stop growing and eventually stop breeding as well. So while an average walleye for example might live for 6-10 years before meeting it’s end, if it lives to 20 or 30 it will develop new eggs and breed each year.