Many species of fish are hermaphroditic, meaning they have both male and female reproductive organs. Some species are characterized as sequential hermaphrodites, which means they hatch as one sex and then change sex later in life, depending on the population size and availability of mates. Others are simultaneous hermaphrodites, which means the fish can fertilize its own egg with sperm.
For cases of sequential hermaphroditism, it is more common for fish to change sex from female to male. Why? Biologist (and author of "Sex in the Sea") Marah Hardt breaks down all of the advantages to that switch here.
This handy chart from University of California, Berkley, gives a great overview of hermaphroditic fish species.
To quote a fantastic article from NPR in 2009, "On the Delaware Bay shore, there’s a swinging party that’s been taking place for millions of years." The party has the same central theme every year: find a mate and make a bevy of horseshoe crab babies!
Horseshoe crabs predate the dinosaurs by almost 200 million years. Every year, between the months of May and June, these prehistoric animals flock to the Atlantic shoreline at night to spawn. Thousands of these crabs wait for high tide and a full moon (a common theme within the animal kingdom) to make their way to shore and find a partner or two to mate with.
One of the most interesting elements of this mating ritual is the visual of multiple males attached to a female in a chain of suitors vying to fertilize her eggs. They make look like impenetrable shields, but horseshoe crabs are actually built to make these long chains. Males have a small notch on the front of their heads that can easily latch them on to a female’s back. They’re also equipped with two powerful claws to ensure that the males can keep hold as the female moves.
In addition to keeping numerical tabs on the Delaware population of horseshoe crabs, our team is engaged in some very interesting research on horseshoe crab blood. Learn more about that here.