Nantucket Horseshoe Crab Spawning Season Going Strong

Jack Dubinsky •

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Four male horseshoe crabs fight for the opportunity to fertilize the eggs of the large female in the center of the cluster. Photo credit: Jack Dubinsky, 2022

‘Tis the season – for horseshoe crab breeding, that is. Every April, May and June, Atlantic horseshoe crabs (Limulus polyphemus) crawl into shallow waters to spawn.

Horseshoe crabs (order Xiphosura) have been around for over 440 million years and have changed very little over that time period. Despite their common name, horseshoe crabs are neither crabs nor crustaceans. They belong to subphylum Chelicerata, along with spiders, scorpions, and sea spiders, all of whom bear the characteristic fang-like chelicerae in front of their mouths. Horseshoe crabs are harmless to humans – their pincers are weak and positioned on the underside of their shell. Additionally, their long telson does not contain venom and is used as a manoeuvring tool.

Every year in Massachusetts, between 120,000 and 140,000 horseshoe crabs are collected to be used as bait to collect other marine species. Evidently, the aroma of dead horseshoe crabs is particularly attractive to snails and eels.

Atlantic horseshoe crabs are also fished for their blood. The blue horseshoe crab blood is worth around $15,000 per quart and has an important use in the medical industry. Atlantic horseshoe crab blood contain Limulus Amoebocyte Lysate (LAL), which reacts chemically with the cell membrane of many types of pathogenic bacteria. This allows medical companies to easily test whether their newly produced medical products are safe before they are shipped to hospitals and drug stores. Don’t worry - the biomedical companies are only allowed to harvest 30% of the horseshoe crab’s blood and are required to release them afterwards. Globally, there are four species of horseshoe crabs alive today, but only the Atlantic horseshoe crab contains the valuable LAL in its blood.

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"Penny" will be celebrating her 2nd birthday at the Maria Mitchell Aquarium this summer! Photo credit: Jack Dubinsky, 2023

While it can be worrying that these animals are collected for these purposes, the horseshoe crab fisheries are diligently managed by the Massachusetts Division of Marine Fisheries and the Atlantic States Marine Fisheries Commission. Thanks to regulations and population monitoring, recent data suggests that horseshoe crab populations in Massachusetts are increasing.

In Massachusetts, horseshoe crab populations are monitored with trawl net surveys conducted by the Massachusetts Division of Marine Fisheries, and by many organizations that survey horseshoe crabs on beaches during the spawning season. On Nantucket, the Maria Mitchell Association monitors Monomoy Beach, and the Nantucket Conservation Foundation monitors the beach at Warren’s Landing. We count horseshoe crabs when they are most active during the high tides around the full and new moons.

If you find a horseshoe crab on the beach, remember that just because it is out of water does not mean its life is in danger. While it is indeed unsafe for the water-breathing animals to be out of the water, moving them may actually do more harm than good. If a live horseshoe crab is flipped over on its back out of water, you may help it by flipping it over and returning it to the water. However, if the crab is buried into the sand, it may be in the process of laying eggs and moving the animal may disturb its spawning behavior. Even on hot summer days, horseshoe crabs can survive a tidal cycle out of the water, if they are buried properly, and it is best to leave them be. Remember, if you move a horseshoe crab, use both hands to support the shell. Holding a horseshoe crab by the telson will often harm it.

If you want to learn more about horseshoe crabs, you can join the Maria Mitchell Association for our Horseshoe Crab Week event from June 20th – June 23rd featuring programs and activities highlighting these special animals! The Maria Mitchell Aquarium, which opens on June 12th, also features live horseshoe crabs, including Penny, the smallest of our babies!

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