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Horseshoe Crab: Information, Facts and Behavior

Horseshoe crabs live primarily in and around shallow coastal waters on soft, sandy or muddy bottoms. They tend to spawn in the intertidal zone at spring high tides. They are eaten in some parts of Asia, and used as fishing bait, in fertilizer and in science (especially Limulus amebocyte lysate). In recent years, population declines have occurred as a consequence of coastal habitat destruction and overharvesting. Tetrodotoxin may be present in one horseshoe crab species, Carcinoscorpius rotundicauda.

Oldest species on earth

Horseshoe crab is the 4th oldest living species on earth. They are  “living fossils,” meaning they have existed nearly unchanged for at least 445 million years, well before even dinosaurs existed. Canadian scientists found a new horseshoe-crab fossil that dated that far back in 2008. Although the creatures were already considered one of the oldest living animals on the planet, the new fossil proved that they were a full 100 million years older than we previously knew, and yet still the same today as they were all the way back then.

Why do horseshoe crabs swim upside down?

Recently horseshoe crabs about 1-2 inches in size have been observed swimming upside around sunset in upper Charlotte Harbor and people are asking “what’s up with this?”

Well it turns out horseshoe crabs do swim upside down, or at an approximately 30 degree angle to the bottom; we just don’t typically see it. One might ask, why in the world would an animal that is already awkwardly shaped for efficient movement in the water, choose to swim upside down? How could that possibly benefit them?

Well first, let’s explore why horseshoe crabs swim at all. Scientists believe swimming my help distribute crabs to other areas. It may help them hurdle barriers that they can’t climb over or around. Swimming might also help them escape from predators or waters with oxygen levels too low for continued survival.

Swimming for horseshoe crabs actually begins before the crab hatches. Digressing for a little life history, horseshoe crabs are the only marine arthropods (includes crabs, shrimp, lobsters, and barnacles) that fertilize eggs externally without brooding the eggs. They are also the only ones that migrate from offshore deep areas to the intertidal beach, where spawning occurs on the beach. The horseshoe crabs typically come onshore with a male attached to a female and a number of “satellite” males nearby. As the female deposits her eggs in a sand nest, they are fertilized by the males.

Horseshoe crabs molt approximately 18 times before they reach sexual maturity. The first three molts occur before hatching. The fourth molt takes place at the time of hatching, about 4 weeks after the eggs were laid, and this results in the first instar or trilobite larvae. Only a couple millimeters in size, the trilobite larvae resembles the adult, but without a telson, or tail as it is commonly called. Trilobites by the way are extinct arthropods… distant relatives of modern lobsters, horseshoe crabs, and spiders. They existed for approximately 300 million years and occurred on every continent on Earth.

Back to swimming horseshoe crabs, scientists think swimming may be inherent, learned while an upside down embryo is still encased within its transparent egg. They’ve observed well-developed embryos turning somersaults, using both legs and their book gills to create movement. As such it is not surprising to see newly hatched larvae swimming awkwardly to the surface.

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