Paper: Sibly, R. M., Kodric-brown, A., Luna, S. M., & Brown, J. H. (2018). The shark-tuna dichotomy : why tuna lay tiny eggs but sharks produce large offspring. Royal Society Open Science, green(5), 1–9. http://doi.org/10.1098/ros.180453
Why do humans typically only have one baby at a time while other animals, like fish can release millions of eggs at once? This is a relatively cumbersome evolutionary question that requires scientists to uncover how and why certain traits have evolved. As a general rule, evolution favors traits that increase an individual’s chance of reproducing and passing its genes down to the next generation (i.e. increasing the individual’s fitness).
Among the more than 30,000 species of fishes, there is a huge spectrum of reproductive tactics employed. Some broadcast spawn, sending their unfertilized gametes unprotected into the environment, while others reproduce via internal fertilization. Some species lay eggs, and others will give birth to live young. Some parents leave their babies to forage for themselves right from the start, and other babies are cared for by brooding parents. Some individuals don’t mate at all – only producing clones of themselves – and some change sex halfway through life to try and boost their reproductive output. Even with this plethora of baby-making options, fish still fall into one of two overall reproductive trends: produce lots of small eggs or produce a few larger offspring. The first strategy is seen in bony fishes (called teleosts), like tunas and Mola mola, which produce large numbers of incredibly tiny eggs that weigh only about 1 milligram each (about the weight of a small snowflake) and typically reproduce via broadcast spawning. Cartilaginous fishes (sharks, skates, rays, and chimaeras) fall into the second category, producing few, large offspring that are internally fertilized and either live-born or protected in sturdy egg cases. Some new-born sharks can weigh over 20 pounds!
Scientists wanted to know why there is such a strict dichotomy in fish reproductive strategies and why some intermediate, Goldilocks option hasn’t evolved as well. To answer this question, researchers put evolutionary hypotheses to the test, modeling just how and when these two disparate reproductive strategies would improve the fitness of species. They analyzed how reproduction would be influenced by the trade-off between offspring number and size.
The More the Merrier
Since fitness is essentially a measure of reproductive success, it would seem intuitive that a fish capable of releasing a million eggs in a single day (strategy 1), each of which has the potential to become a baby fish with its parents’ genes, would have a higher fitness than an individual that can only produce one or two babies at once (strategy 2). But here’s the flaw with that logic: although each of those little eggs has the potential to become a baby and eventually an adult fish, it is just that – potential. Fish that produce a million eggs at a single time typically broadcast spawn, meaning they spew their eggs into the big blue ocean hoping that the eggs will encounter another fish’s sperm, get fertilized, develop into an extremely tiny larval fish, and eventually grow up and reproduce. But many of the eggs or babies will be swept up by the currents, eaten by other animals, and eventually die before ever making it to adulthood – maybe not such a phenomenal idea after all.
Quality Over Quantity
Alternatively, animals that have relatively few offspring may be able to invest more in their babies. A mother only has so much energy available to give her babies. Much like sharing a pizza with your friends, the more people you invite to the party, the less food there is to go around. The cartilaginous fishes with fewer babies may be able to invest more energy into each individual, spend more protecting developing babies (by producing tough, protective egg cases or carrying developing young in-utero), and ultimately get their few offspring off to a safer start to the rest of their lives.
So there is an evolutionary trade-off at hand – produce a ton of small babies and hope that a fraction will survive to pass down your genes (even if 10% of your million babies survive you have a hundred thousand kids carrying a copy of your DNA!), or spend a lot of time and energy to ensure that at least one or two of your babies have a better chance of survival.
In the End, Death is the Great Equalizer
Based on their models, the researchers found that the fitness of each reproductive strategy is dictated not just by how much energy a mother can invest in her offspring but is also heavily influenced by the likelihood that those offspring are going to die. Bony fish and sharks experience different juvenile mortality rates. The small bony fish offspring grow fast and have a lower initial death rate. Even though the majority of these smaller fish will eventually get eaten, enough will grow rapidly and become large enough to survive into adulthood to reproduce. Sharks, on the other hand, have a higher relative mortality rate compared to their growth rates, so it is advantageous for them to produce larger offspring that are better able to compete for resources and are, therefore, more likely to survive. It is possible that these reproductive strategies are further constrained by other factors like environmental conditions (i.e. smaller eggs are more able to deal with the salty ocean than larger eggs). But for whatever reason, it appears that for marine fish there is no middle-road: it is only advantageous to have a lot of little babies or spend more energy producing a few big ones.
I received my Master’s degree from the University of Rhode Island where I studied the sensory biology of deep-sea fishes. I am fascinated by the amazing animals living in our oceans and love exploring their habitats in any way I can, whether it is by SCUBA diving in coral reefs or using a Remotely Operated Vehicle to see the deepest parts of our oceans.