Life Stages

From Egg to Trilobite Larva

Horseshoe Crab Larva Paul Ulrich © 2001

Newly laid horseshoe crab eggs are opaque, pastel-green in color, and about 1.5 mm (1/16 inch) in diameter. After fertilization, the eggs begin to develop into trilobite larvae. By day five, miniature legs are visible inside the translucent egg.

On day six, the larvae molt for the first time. If you look closely with the aid of a microscope, you won't find a tail, but you will see each larva surrounding a small sac. This is the yolk — the only source of food available before the larva hatches.

On day seven, the outer membrane of the horse-shoe crab egg ruptures, and the inner membrane swells to replace it.

Trilobite larvae and eggs.

On day seven, the outer membrane of the egg ruptures and the inner membrane swells to replace it. By the end of the second week, larvae have molted twice again in preparation for hatching. Thus, in total, the horseshoe crab undergoes four molts within the egg.

Ideally, the moisture supplied by the tides and the warmth of the sun allow the eggs to mature and hatch in the two-week period between spring tides (the higher-than-normal tides that occur at the new and full moons). In reality, however, it probably takes three or four weeks or even months for the eggs to hatch.

Upon hatching, the trilobite larvae dig their way out of the sand.They are approximately 3 mm (1/8 inch) across and look just like miniature adults, but lack a movable tail and functional compound eyes. Their digestive system is also not yet functional, and the baby crabs swim around for about a week absorbing the yolk sac as their digestive systems mature.

It Takes Eight to Ten Years for
Horseshoe Crabs to Reach Adulthood

Around day 21, the larvae settle from the water column onto the soft sediments of the Delaware Bay. As they shed their shells, their bodies expand, a telson grows, and chitin hardens the new carapace. The juvenile horseshoe crabs now look like adults, but they are less than a quarter of an inch wide!

Horseshoe crabs initially molt an average of three or four times a year. Sub-adults (horseshoe crabs that are five to seven years old) appear to molt annually. Males are sexually mature at their sixteenth molt, which is usually their eighth or ninth year. During their final molt, they develop specialized clasping claws for holding the female during reproduction. Females need at least 17 molts, or one more than the males, so they mature in their tenth year or even later and are, on the average, 30% larger than the males. A small percentage of horseshoe crabs continue to molt after reaching sexual maturity.

How big is an adult horseshoe crab? A mature male ranges from 7–9 inches across the helmet-like prosoma, with an overall length, from head to tail, of 13–16 inches long. Mature females typically are much larger than the males, ranging from 9–12 inches across the widest part of the shell and 16–20 inches long.

Scientists are not sure how long horseshoe crabs live, but some speculate that they can live for 20 years or more. Because very few horseshoe crabs live to maturity, the ones that do must have a life span that enables them to reproduce for a number of years.

The horseshoe crab's main strategy to avoid predators is to be most active at night, feeding and spawning under the cover of darkness. In fact, during spawning season, you will find 100 times more crabs on shore at night than during the day. During high tide when large aquatic predators are swimming nearby, the juvenile horseshoe crabs bury themselves in the sand for protection. At low tide, young horseshoe crabs emerge from the sediment, but now they must be cautious of predators on the shore. If the crabs are turned upside down, they will use their sword-like tail, or telson, to flip over. Shorebirds cannot penetrate the horseshoe crab's coat of armor as long as the crab is upright.

If a crab is flipped over by a wave and cannot right itself, it is vulnerable to predators. If you want to give an overturned crab a helping hand, the correct way to pick up the animal is by its helmet-like shell. Never lift the horseshoe crab by its tail because you might injure it.