April 25, 2019

A Lucky Horsehoe (Crab)

Posted by: Nate Ramey

A Lucky Horsehoe (Crab)

Everyone needs a little luck in life from time to time. This is especially true for a curious critter found here in the tidal waters of the island. This odd creature has an external skeleton, 9 eyes, and 12 legs with a jawless mouth right in the middle. This strange creature is the Atlantic Horseshoe Crab, Limulus polyphemus. Hopefully by the end of this post, you’ll be able to understand just how much luck one of these animals need in their life to succeed.

First a little background. The horseshoe crab’s name is a bit of a misnomer. They’re actual relatives are arachnids, like spiders and scorpions, rather than true crustaceans like crabs and lobster. In fact, a recent molecular study suggested that Horseshoe crabs should be put directly into the arachnid family. They are truly living fossils, with fossils found pre-dating even the dinosaurs showing them to be relatively unchanged. Most of the time we hardly see them as they tend to be benthic, feeding on worms, mollusks, and other invertebrates. A few nights of the year however, that changes.

During the spring tides, of April, May, and June here in coastal Georgia. Horseshoe crabs by the dozens emerge from the water during the full and new moons to lay their eggs on the high shore. As the new/full moon tides are the highest of the month, the eggs have the best chance at surviving without having to worry about marine predators until the next full/new moon tides approximately two weeks later. To see one of these ancient creatures crawling out of the water onto the high shore along with dozens of its kind gives a sense that you are experiencing something truly primeval. One of the first things you notice is the fact that some of the individuals are much larger than others and seem to be attracting the smaller, enthusiastic ones. These are the female horseshoe crabs who have come to lay up to 64,000 eggs in shallow depressions in the sand. The smaller, enthusiastic ones are of course the males. Once they have found a female, males will use specialized hooks on their front appendages to hang onto the female until she is ready to lay her eggs. Once that has happened the male will quickly fertilize the eggs and then both he and the female will part ways, leaving the eggs to develop on their own.

The next two weeks is a dangerous time for the eggs, which resemble little more than small green BB’s. As they wait for the next full/new moon tide to sweep them out to sea, they are exposed to many dangers. If the female has miscalculated her placement of the nest and put the nest too high on the shore, there is the threat of heat and desiccation. If she has put the nest too low, then there is the threat of predation by marine creatures. Even when the female is able to hit the “Goldilocks” zone there is still the threat of predation of the avian variety. Shorebirds of all kinds from sanderlings to whimbrels, love horseshoe crab eggs. In fact, some shorebirds like the Red Knot are highly dependent on horseshoe crab eggs when refueling during their migration from the tip of South America to the Arctic Circle. Many of the eggs will not survive these two weeks.

If by luck, the little horseshoe crab developing in the egg finally hatches out during the next full/new moon tide it will be carried back out into the water and there will spend the next decade maturing into an adult. Over this time while dodging scores of predators such as fish, crabs, and even sea turtles, they will molt and grow their shells 16-17 times until they're full size at about two feet long and weighing over 10lbs. They can then live another 2-3 decades repeating the cycle of spawning every year and creating the ancient spectacle they have been performing for eons.

^*Note larger female, vs smaller male 

^*A mix of Red Knots and Sanderlings on LSSI beach

^*A male being displayed to a curious audience at Sancho Panza Beach. You can almost see his specialized claws for holding onto the female, which are shaped like mittens.

 

^*A lone individual cruising the shallows at Sancho Panza Beach.

 

*photos by Nate Ramey*

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