This is a quite rare footage of Atlantic horseshoe crabs (Limulus poyphemus) recorded during high tide about 10:00 am in Recompence Shore, Freeport Maine on May 26th 2010. Mating mostly happens in moonlit nights (romantic animals) on sandy beaches. Here they are mating in broad daylight on a rough pebble beach.
External fertilization is one of the ancestral ways to produce offspring however it can be rather costly. Egg production requires more resources compared to sperm. Females therefore need more time to recover after mating. Every year when the mating time comes females that are reproductively ready will always be fewer in number compared to males. Scarcity of females creates competition among males. Variation of the same theme is observable in many externally reproducing species including fish, polychete marine worms and frogs.
The economy of nature restricts female reproduction. However, this can change in other species whose sex roles have reversed such as the seahorses and pipefishes where males raise babies in their brood pouches.
Males congregate around larger-sized females and wrestle to get the best fertilization spot behind females using a special strategy. Rival males try to dislodge others by lifting them out of the water so that their gills are exposed to air. Gills must be submerged in water to exchange soluble carbon dioxide with oxygen. This reduces respiration and quickly exhausts the opponent. Becoming a father requires a lot of physical strength and endurance!
Horseshoe Crabs from Rob Lewis on Vimeo.
Horseshoe crabs have a very rich natural history. A full feature PBS documentary “Crash: A Tale of Two species” makes a very detailed description of their life cycle and interactions with other animals including the Red Knots who depend on horseshoe crabs for gaining nutrition that will enable them to migrate vast intercontinental distances every year. The annual mass reproduction event of horseshoe crabs along the Atlantic coast of United States is a spectacle enthusiastically watched by thousands of nature lovers.
Horseshoe carb blood is special for containing chemical agents used as a test for bacterial endotoxins. Pharmaceutical industry therefore widely their cerulean blood for drug testing. Blood is harvested from wild populations and now evidence suggest that the process is harming these extraordinary animals:
Horseshoe crabs are truely living fossils. Earliest fossil specimens were found in Morocco’s Fezouata formation and also in central and northern Manitoba Canada going back to 445 million years ago (Ordovician Period).
Just like insects, as horseshoe crabs grow they must shed their exoskeleton through molting. The following video shows how a baby horseshoe crab molts in an aquarium at the Maritime museum and Aquarium in Göteborg, Sweden. If you are lucky, you can come across their molted shells washed up in beaches along the Atlantic coast.
The main featured video “Horseshoe Crabs Mating in Maine” is now a part of the permanent display in “Living Fossils” exhibit of Naturama, a natural history museum based in Aargau Switzerland.
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