For every biologist there is a list of natural events or places that must be experienced once in their life. The contents of the list vary based on their passions and research experiences, but every biologist has a list. My passion is in wildlife ecology and disease, so many of the events on my list center around seeing events such as the bat bridge in Austen, Texas, where millions of bats emerge every night to feed. Another one of these events, which I highly recommend seeing at some point, happens every year on the east coast of the United States: horseshoe crab (Limulus polyphemus) spawning.

First of all, horseshoe crabs are pretty incredible. They are sometimes called “living fossils” because some of the fossils of their ancestors that have been discovered are about 450 million years old. That’s about 200 million years before dinosaurs! They are also more closely related to scorpions and spiders than they are to crabs and lobsters. Also, they are extremely important in the ecosystem because their eggs are a major food source for marine and terrestrial life during their spawning season. These are only a few of the reasons they are so fascinating.

This past June, my friends and I ventured to Slaughter beach on the Delaware coast for an afternoon and were able to see, firsthand, horseshoe crabs as they emerged from the surf to spawn.

This event occurs from late spring to mid-summer (usually May into mid-June) every year in Delaware, and the amount of horseshoe crabs on the shore peaks during full moons, new moons, and high tides (during full and new moons the tides are extra high).

As you view them on the beach you can tell the males from females because the females are about a third larger than males. While in the surf the males find the females from a pheromone the female is releasing. The male then attaches to the back of the female and they travel to the shore together. The female digs a small hole in the sand deposits eggs, while the male deposits sperm over the eggs, fertilizing them. Some females return to the shore multiple times and can lay tens of thousands of eggs.

There were also males that were not attached to females, but still congregated around female/male pairs. These males are referred to as satellite males, and still have success fertilizing eggs while they maneuver around the attached male.

Reading about horseshoe crab reproduction after my visit answered a lot of questions that I had about what I observed, and also opened my eyes to many of the nuances that are involved in this reproductive strategy. It made me more curious about other mating systems that have been described, and there role in evolution and ecology. Stay tuned for my next post on the types of mating systems!

Also, I would love to know what natural places are on readers’ bucket list. If you have one, let me know in a comment below!

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Sources:

https://myfwc.com/research/saltwater/crustaceans/horseshoe-crabs/facts/

https://www.nwf.org/Wildlife/Wildlife-Library/Invertebrates/Horseshoe-Crab.aspx