Horseshoe crabs (Limulus polyphemus) are invertebrates that arent really crabs, despite their naming. Spiders are actually closer kin to horseshoe crabs than other crabs are, strangely enough. The “horseshoe” in their name comes from the shape of their bodies, which looks like horse hooves. These primitive creatures, fascinatingly enough, have remained the same for 360 million years.
The horseshoe crab lays its eggs on beaches in the spring, as soon as the water is 60 degrees Fahrenheit. Male horseshoe crabs are tinier than the females. They gather around the boundaries of the water in anticipation of spawning. Once the females appear, the males employ their claws to latch onto the shells of the fairer sex. After this, the females keep stopping every few feet or so to lay eggs while carrying the males. The males then fertilize the eggs by releasing sperm. It isnt uncommon for other males that arent connected to the females to simultaneously make fertilization efforts, too.
As a beachcomber and nature enthusiast I’m always keeping an eye out for interesting treasures washed up along the shore. One of my favorite finds are the small greenish-blue balls known as horseshoe crab eggs. But what exactly do horseshoe crab eggs look like? In this article, I’ll provide an in-depth guide to identifying these eggs and learning more about the incredible ancient creatures that lay them.
Before diving into the details of their eggs, let’s quickly cover some background on horseshoe crabs themselves. Horseshoe crabs are marine arthropods related to spiders and scorpions. They have survived for over 400 million years, earning them the nickname “living fossils.”
Four different species exist today. The most common along the Atlantic coast of North America is the American horseshoe crab. Others live in Asia’s coastal waters
Horseshoe crabs swarm beaches from March to July each year to mate and lay eggs in sandy nests A single female can lay around 20 clusters of 4,000 pea-sized eggs annually!
When and Where to Find Horseshoe Crab Eggs
To find horseshoe crab eggs, the best times and locations are:
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March – July: Peak spawning season along the Atlantic coast.
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Evening high tides: Horseshoe crabs come ashore to lay eggs at high tide, especially around the full or new moon.
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Sandy beaches: They prefer to bury eggs in sandy areas above the high tide line.
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Delaware Bay: One of the densest spawning areas, hosting the largest horseshoe crab population.
Other prime egg-hunting grounds include coastal beaches from Maine to Florida and along the Gulf of Mexico. Timing is key for finding fresh eggs, so check local spawning forecasts before planning a trip.
Identifying Horseshoe Crab Eggs: Size, Shape and Color
Horseshoe crab eggs are quite small, so you need to look carefully. Here are the key identifying features:
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Size: Approximately 1/4 inch diameter, similar to a pea or small marble.
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Shape: Spherical, with a slightly flattened top and rounded bottom.
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Color: Range from pale greenish to light blueish in color. Newly laid eggs look darker blue-green.
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Clustered: Eggs are laid in clusters of around 4,000, scattered through the sand.
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Smooth exterior: Each egg has a smooth, semi-translucent exterior membrane.
Horseshoe crab eggs resemble no other species’ eggs, making them relatively easy to identify. Their color, size and shape differ distinctly from fish, bird and sea turtle eggs.
What’s Inside a Horseshoe Crab Egg?
Breaking open or cutting a horseshoe crab egg reveals some fascinating internal structures:
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Yolk sac: The main yellowish yolk mass surrounded by a membrane, providing nutrition.
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Germinal disc: A concentrated white spot on the yolk containing genetic material.
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Embryo: A tiny horseshoe crab larva curled inward, often with eyespots visible.
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Perivitelline fluid: A clear liquid between the yolk and outer membrane.
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Membrane: The outer casing holding everything together.
It’s amazing to think this simple structure contains all the building blocks needed for a baby horseshoe crab to form and hatch!
From Egg to Crab: The Life Cycle
Horseshoe crab eggs take 2-4 weeks to hatch depending on conditions like temperature, moisture and oxygenation. Hatching and development unfold in several key stages:
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2 weeks: Larva develops inside the egg with eyespots and tail visible.
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Hatching: Larva emerges from egg about .5 inches long, crawling immediately to water.
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First molt: Within 24 hours, sheds exoskeleton becoming a silver-hued juvenile.
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9+ molts: Over the next 2-7 years, repeatedly sheds shell as it grows.
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Adulthood: Reaches sexual maturity around 9 years old and a size of 1-2 feet.
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Breeding: Returns to beaches like its birthplace to mate and lay new clusters of 4,000 eggs.
Horseshoe crabs can live 20+ years, continuing this ancient cycle indefinitely!
Why Study and Protect Horseshoe Crab Eggs?
Besides being intriguing objects to discover, horseshoe crab eggs also have ecological importance:
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Shorebird food source: Many migrating birds rely on abundant crab eggs for nourishment on long journeys.
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Population indicators: Monitoring where eggs are laid and how many indicates horseshoe crab health.
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Research value: Their blood is vital for medical testing, and eggs can be studied in labs.
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Conservation: Horseshoe crab numbers are declining, making egg protection vital.
When beachcombing, be sure not to disturb any intact nests. Report washed-up eggs to local wildlife officials to help conserve these living fossils.
Lots and Lots of Eggs
Female horseshoe crabs obscure parts of their bodies with sand as they lay their eggs. Most of the time, the females lay about 4,000 eggs when they dig holes in the sand for them. These miniscule eggs are deep greenish in coloration. The females can lay numerous batches of eggs throughout single nights of spawning. If they keep doing it for a string of nights in a row, they may lay at least 100,000 eggs by morning.
Hatching generally takes between two weeks and a month. Once the youngsters leave the eggs, they head straight to the water. Horseshoe crab larvae are physically similar to the adults, although they lack tails and are significantly smaller. Youthful horseshoe crabs tend to take residence on mudflats.
developing horseshoe crab eggs
What do horseshoe crab babies look like?
Horseshoe crab babies look just like adults — but with translucent shells. It takes two to four weeks for horseshoe crab eggs to hatch. Tiny crabs emerge, smaller than the eraser on a No. 2 pencil and with nearly see-through shells. The baby horseshoe crabs find shallow, sheltered waters to live in.
Do horseshoe crabs look like adults?
Juvenile horseshoe crabs look a lot like adults except that their tails are smaller. The young and adult horseshoe crabs spend most of their time on the sandy bottoms of inter-tidal flats or zones above the low tide mark and feed on various invertebrates. Why are horseshoe crabs important?
What does a horseshoe crab larva look like?
The larva looks like a tiny version of an adult horseshoe crab, but without a tail. Larval horseshoe crabs travel into the ocean water and settle on the sandy bottom of tidal flats for a year or more. As they develop, they will move into deeper waters and begin to eat more adult food.
What color is a horseshoe crab?
Other interesting Atlantic horseshoe crab facts: The color of the horseshoe crab’s blood ranges from clear to dark blue. Watch YouTube animal expert Coyote Peterson come face to face with a horseshoe crab in the video below: Do Horseshoe Crabs STING?! An Atlantic horseshoe crab makes its way back into the ocean.