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
|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
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
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.