Earlier this week when we were in Tarpon Springs, Florida, we saw horseshoe crabs at a beach. A couple appeared to be mating: they were traveling together one behind the other. They kept beaching themselves and then turning around and heading back to deeper waters to escape from the curious people approaching them.
Yesterday, we were on Sanibel Island further south along the west coast of Florida. We visited the J. N. "Ding" Darling National Wildlife Refuge. There we saw more mating horseshoe crabs. This time they were in murky waters. Note the mud accumulated on the backs of the crabs.
Also note that the crab in the back in both pairs, presumably the male, is slightly smaller than the female in the front. This sort of sexual size dimorphism is especially common in aquatic animals. Larger females probably cope with the demands of egg production better than do smaller ones.