Horsefoot Moon

Thu, 05/19/2011 - 1:00pm

Horsefoots are plodding out of the sea; drawn by new and full moon ebb tides, they have been doing this for 350 million years. One of earth’s oldest creatures, horseshoe crabs are truly fascinating. And now is the time to start a season of observations.

Horseshoe crabs (not a true crab, but rather an arthropod more closely related to spiders) have 10 eyes. These eyes are more like light sensors that occur on the forward part of the animal, along its tail and on the under parts near the mouth. The large lateral eyes are insect-like compound eyes that allow the crab to sense objects — such as each other in the pursuit of mates. Many of the other photo-sensing eyes on the dorsal (upper) surfaces of the body and tail are capable of “seeing” in ultraviolet light, and allow the animal to keep track of cycles of light and dark. This is a critical ability for an animal that depends on laying its eggs in sands inundated only during peak high tides, so that the developing eggs are out of reach of lesser high tide waters until they hatch (about four weeks later) at the time of another peak high tide. All of this is to say that the best time to see adult horseshoe crabs is during mating and egg laying in May and June, at the time of high tides around the full and new moons.

We are lucky on Block Island to still have horseshoe crabs along our New Harbor shores. However, the numbers are in decline, as they are throughout the Eastern Seaboard of the Atlantic. Used for conch and eel bait, these slow moving, harmless animals have fallen easy prey to over-fishing. Add loss and degradation of spawning habitat and exploitation of the crab for biomedical purposes, and the result is a species whose numbers are decreasing.

The decline of horseshoe crab numbers not only jeopardizes the species, it also threatens the life cycle of many shore birds. Over several nights, a single female horseshoe crab may lay over 100,000 eggs, more than enough to ensure the continuation of the horseshoe crab population. However, horseshoe crab eggs serve a dual purpose. When the eggs are exposed to air (through surf action or the action of a digging horseshoe crab itself in the process of laying eggs) they quickly dry out and become nonviable as future crabs — but they remain an incredibly high protein food source for millions of shore birds migrating from South America to breeding grounds in the arctic. The reduction of horseshoe crabs has largely contributed to a significant reduction in red knots and other shorebird species.

There are a myriad of fascinating facts about horseshoe crabs. Colloquially referred to as horsefoots* by old Long Island fishermen, these creatures have roamed the earth since before insects and most species existing today, and they are often referred to as living fossils. To learn more about horseshoe crabs, go to, or, or attend the Ocean View Foundation’s May 16th Night of the Living Horseshoe walk.

The following May events and Ocean View Foundations programs are sure to provide opportunities to observe the island’s horseshoe crabs and their interrelationships with other aspects of island life.

May 16 at 7:00 p.m.: Night of the Living Horseshoe Walk, Andy’s Way.

May 3 & 17, at 8:00 a.m.: Crazy-as-a-Coot Bird Walk, call 595-7055 for location.

May 17 Full Horsefoot moon.

May 28 at 8:30 p.m.: Twilight Walk & Night Sky Viewing, Hodge Preserve.


* Nomenclature can be fun!

The reference to horsefoots can be found in Carl Safina’s book, “The View From Lazy Point” (chapter entitled May). The commonly known name horseshoe crab is derived from the fact that the forward part of the animal is shaped like a horseshoe. And the genus species name Limulus polyphemus refers to oblique or askew placement of the eyes to the side of the animal (Latin limus = oblique) with polyphemus coming from Latin through Greek references (Latin, meaning much seeing; Greek myth, the Cyclops who imprisoned Odysseus).

In keeping with this series of articles I propose adding a Block Island full moon name – Horsefoot moon – to the more commonly known Planting moon of May.