What’s up with those baby horseshoe crabs swimming upside down?

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Juvenile horseshoe crab swimming upside down in upper Charlotte Harbor – Bruce Kuechmann Photo

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.

Horseshoe Crabs Mating - FWC Photo
Horseshoe Crabs nesting on beach – FWC Photo

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 (resembling pages of a book) to create movement. As such it is not surprising to see newly hatched larvae swimming awkwardly to the surface.

Juvenile horseshoe crabs swimming upside down – Bruce Kuechmann Photo

Approximately 6-8 days after hatching horseshoe crab larvae molt into the first juvenile instar. At 5 millimeters in length the juvenile fully resembles the adult but with a shorter telson. Juveniles are also known to swim as evidenced in the upper harbor. In laboratory settings crowded conditions are known to stimulate juvenile horseshoe crabs to swim. It is thought that swimming is a way of dispersing the animals, which makes sense since horseshoe crabs are not social animals. Swimming may also be a way of avoiding cannibalism which has also been observed in controlled settings. For smaller more buoyant horseshoe crabs, swimming also appears to be an alternative to turning over in the water using the telson. In the laboratory juveniles have been observed propelling themselves off the bottom in order to right themselves.

As horseshoe crabs get larger they appear to swim less which is why we typically don’t see them at the surface. A book written by several horseshoe crab experts indicated that in all of their field observations, over the many years, that they had only observed two adult horseshoe crabs swimming at the surface in open water, and only once in the open ocean. Interestingly, the later was a horseshoe crab about one meter in length. A very big crab indeed!

This leads me to my last few tidbits about horseshoe crabs. Worldwide there are four species of horseshoe crabs. Three occur in Southeast Asia (including the big one from above) and the fourth is found on the Atlantic and Gulf coasts, from Maine to Mexico. Horseshoe crabs date back almost 450 million years, 200 million years before dinosaurs existed and over this time their body form has changed very little. It’s worth noting that horseshoe crabs are not crabs at all. They are really most closely related to spiders, scorpions, and mites. Horseshoe crabs mature around ten years of age and live about 20 years.

Horseshoe crabs benefit humans in many ways: because their blue, copper-based blood clots in the presence of bacterial toxins, it is used to test intravenous drugs, vaccines, and other medical devices. Every year thousands of crabs are harvested, bled, and then returned to the water for this purpose. Also, much of what we know about the human eye can be credited to horseshoe crabs.

The Florida Fish and Wildlife Conservation Commission is trying to document nesting sites of horseshoe crabs from around the state with the public’s help. If you would like to find out how you can be involved, please visit: http://myfwc.com/research/saltwater/crustaceans/horseshoe-crabs/documenting-beaches/


Botton, ML, RA Tankersley and RE Loveland. 2010. Developmental ecology of the American horeshoe crab Limulus Polyphemus. Current Zoology 56(5): 550-562.

FWC. 2015. Facts About Horseshoe Crabs. http://myfwc.com/researxh/saltwater/crustaxeans/horseshoe-crabs/facts/

Rudloe, A. 1981. Aspects of the biology of juvenile horseshoe crabs, Limulus Polyphemus. Bulletin of Marine Science 31(1): 125-133.

Shuster, CN and LI Anderson. 2004. in The American Horseshoe crab, Shuster, CN, Barlow, RB and Brockmann, J. Eds. Harvard University Press. Chapter 7: A History of Skeletal Structure, Pages 154 – 188.


Posted: May 27, 2015

Category: Coasts & Marine
Tags: Horseshoe Crabs, Swimming

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