Horseshoe Crabs

For summaries of our annual research studies see our Fall Newsletters.

Horseshoe crabs, Limulus polyphemus, are one of the creatures inhabiting Taunton Bay. The morphology of this group of organisms has remained largely unchanged for 445 million years. The earliest fossil evidence of horseshoe crabs is found in rocks of the Ordovician Period. The horseshoe resembles crustaceans, most notably lobsters and crabs; but it is actually more closely related to arthropods such as spiders and scorpions. Taunton Bay is the northernmost edge of this species’ breeding range, making it of critical importance to these fascinating living fossils. Friends of Taunton Bay works with scientists to monitor the population of horseshoes in Taunton Bay. Like many animals in Maine, horseshoes hibernate in the winter, burrowing into the mud, and spawn in the warmer months of late May and June at the highest daily tide. Horseshoe crabs practice mass spawning and can be seen in groups of a few dozen to a few hundred at spawning time.

Males ride along attached to one female as they travel to a spawning site where the female will lay her eggs, and males will fertilize them.

They spawn in a roughly two-week stretch, with the females laying pea-sized eggs in loosely consolidated gravel or mud and the males following them to lay sperm. While counts of mating horseshoes vary from year to year, the population observed annually on Shipyard
Point seems to be healthy and productive. After spawning, the horseshoes roam the bottom in search of small shellfish and worms until they go through a wintering phase of inactivity lasting seven months or longer. Research on the relatively isolated group in Taunton Bay indicates that there are at least two distinct populations less than 2.5 miles apart centered on Shipyard Point and in Egypt Bay.
Horseshoes reach maturity in 6 to 8 years after a series of moults in which they break out of the front of their shell (crabs and lobsters generally shed through the back) which lets them grow about 15% to 30% larger with each moult. At maturity, males and females differ in several ways (see photos). In addition to the “boxing glove” front pincer with which the males clasp the females (or other males in a train), males are slightly smaller, have a slightly higher arch on the front of their shell to accommodate fastening to other horseshoes, and
two visible sex organs beneath the first layer of their book gills.

Male horseshoe crab. Note the “boxing glove” pincers that help males hold on to the females during spawning. Photo: G. Monteux

Internationally, their unique blue blood is used by the biomedical industry to produce limulus amoebocyte lysate (LAL), an important tool for human health in the detection of contaminants in implants, drugs, vaccines, and other medical supplies. The copper-based hemocyanin in their blue blood is further used in research on human pathogens and endotoxins because it is uniquely equipped to fight such invasions. In addition, the horseshoes have an extensive visual system which serves as a model for some neurological studies.
Horseshoe crabs also play a vital ecological role in the migration of shorebirds along the entire Atlantic seaboard. The adults are prey to bald eagles, crows, and gulls. Other migratory shorebirds (e.g., Red Knot) eat their eggs during spawning season. We are also a threat to these ancient creatures. Humans use adult horseshoe crabs as bait for commercial American eel and other fisheries. The populations present in the bay are protected by Maine state regulations. A special license is required to touch or handle horseshoes for research or harvest.