Atlantic Horseshoe Crabs

December 02, 2015

Limulus polyphemus mating

Photographer: Rob Sheridan
Summary Author: Rob Sheridan

During the Ordovician Period, about 450 million years ago, after the chemistry to concentrate dissolved calcium and convert it to hard body parts evolved in warm Pre-Cambrian Seas, horseshoe crabs (shown above) first appear in the fossil record. Ever since then, their body form and life cycle haven't substantially changed. They watched as trilobites went extinct, as plants evolved from algae, colonized the land, and diversified from mosses and ferns to flowering plants. They witnessed the first amphibians evolve and become reptiles, they swam with the first fishes and watched the dinosaurs evolve and become extinct. They survived the Permian extinction and a host of other mass die-offs, rode the coasts as sea levels rose and fell and mountains rose and were eroded away. They witnessed the evolution of insects and birds and bees and eventually mammals and were there when the first primates stood erect. To horseshoe crabs, Homo sapiens are just another transient primate -- if they've noticed us at all.

Horseshoe crabs are arthropods, in the same subphylum as arachnids (including spiders). All four living species live in shallow seas with muddy or sandy bottoms where they forage for worms and mollusks. Shown here is the Atlantic horseshoe crab (Limulus Polyphemus). Multiple light sensitive organs dot its carapace and telson (rigid tail), which serves to right the animal when overturned by predators. Gills are located behind their legs. Horseshoe crab blood has hemocyanin as an oxygen carrier rather than hemoglobin (they appeared before hemoglobin evolved), explaining the blue color of their blood.

As shown in the photo, females are substantially larger than males. During breeding season, females deposit several thousand eggs at a time in soft mud or fine sand at the ocean’s edge, the following male immediately fertilizing the eggs. Most eggs and the resulting larvae are eaten by other shoreline organisms, but enough survive through 6 molts in 12 months to become adult horseshoe crabs and continue the species. At least it’s worked for the past 450 million years!

Photo Details: Camera Make, Apple; Camera Model, iPhone 5s; Exposure Time, 0.00331 second; Lens F-Number, ƒ/2.2; ISO Speed Ratings, 32; Focal Length, 4.12 mm.