The horseshoe crab is a primitive-looking arthropod with a hard, brownish-green exoskeleton and a spike-like tail. It visits the Chesapeake Bay’s sandy beaches each spring and summer to spawn.
Juveniles are found in shallow, protected waters with a sandy bottom; adults live in deeper waters
Found year-round in the middle and lower Chesapeake Bay, as far north as the Chester River. Most visible in spring and summer, when they spawn on sandy beaches.
Horseshoe crabs grow to two feet in length. They have a hard, rounded, brownish-green exoskeleton, a spike-like tail and five pairs of jointed legs. Their widely spaced eyes look like bumps on the top of their shell. Their gills have folds of membranes that look like the leaves of a book. Juveniles are sand-colored.
The horseshoe crab eats mostly worms and mollusks such as razor clams and soft shell clams. It spends most of its time rooting through bottom sediment looking for food. These crabs don’t have jaws, so they use the spiny bases of their legs to break up their food and then push it into their mouths.
Several types of shorebirds eat horseshoe crab eggs. Various fish, invertebrates and sea turtles feed on eggs and larvae. Humans catch adult horseshoe crabs to use as bait and for medical research.
Horseshoe crabs are fascinating marine animals that have been around for hundreds of millions of years. With their unique horseshoe-shaped shell and long spike-like tail, they look almost prehistoric! But what exactly do these ancient living fossils eat to survive?
In this article we’ll take a close look at the diet and feeding habits of horseshoe crabs to understand what makes them tick.
Before we dive into their diet let’s start with a quick overview of horseshoe crabs. There are only 4 existing species of horseshoe crabs found in coastal waters of Southeast Asia and the east coast of North America. The most common species is the Atlantic horseshoe crab (Limulus polyphemus) found along the Atlantic and Gulf coasts of North America.
Horseshoe crabs are more closely related to spiders and scorpions than true crabs. They belong to the family Chelicerata and can be identified by their hard semi-circular shell (called carapace), 9 eyes, 6 pairs of legs, and long spike tail (telson).
These marine arthropods are sometimes called “living fossils” since they have existed for around 450 million years with minimal change! They grow slowly, taking around 10 years to reach sexual maturity. Horseshoe crabs can live for 20-30 years.
Now let’s look at what makes up the diet of these fascinating creatures!
Horseshoe Crabs Are Bottom-Feeding Scavengers
Horseshoe crabs are primarily bottom-dwellers and scavengers. They use their legs to walk along the sandy ocean floor searching for food.
Being omnivores, horseshoe crabs feed on both plant and animal matter. Their diet includes:
- Mollusks like clams and mussels
- Marine worms
- Small crustaceans
- Algae and plant debris
Horseshoe crabs use their front legs to crush up and ingest food. They have a gizzard-like organ called a gastric mill that further grinds up the food for digestion.
Mollusks are a Dietary Staple
Mollusks like clams, mussels, and snails are a prime component of the horseshoe crab diet. Using their leg pincers, horseshoe crabs can easily crush the shells before consuming the soft mollusk inside.
Blue mussels, surf clams, and soft-shell clams are frequent menu items. On finding a clam buried in the substrate, the horseshoe crab digs it out and crushes it using its pincers before ingesting it.
They Hunt for Marine Worms
Aquatic worms like sandworms, clam worms, and tube worms are another key part of the horseshoe crab’s diet. The crabs probe the ocean floor searching for worms buried in the substrate. Once detected, the worms are dug out and consumed.
Algae and Plant Matter Also Gets Eaten
While scavenging for animal prey, horseshoe crabs also incidentally ingest softer algae and plant debris along the ocean floor. Although not a primary food target, plant matter does provide supplementary nourishment.
As omnivores, horseshoe crabs are opportunistic eaters and will consume whatever organic matter they can find. This helps keep their habitat clean and free of decaying detritus.
When and How Do Horseshoe Crabs Feed?
Now that we know what horseshoe crabs eat, let’s look at some key facts about their feeding habits:
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Nocturnal feeders – Horseshoe crabs are mostly active at night and do majority of their feeding after dusk.
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Constant foragers – They continually sift through substrate while walking, picking out food items incidentally.
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Use legs and mouth pincers – Front legs bring food to the mouth pincers which crush up the food for ingestion.
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Have a gizzard-like gastric mill – This organ further grinds up the food into digestible paste.
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Prefer softer foods – Young horseshoe crabs feed more on worms while adults opt for clams and mussels which are easier to crush and digest.
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Help clean ocean floor – By feeding on decaying plant/animal matter, horseshoe crabs help keep the benthic ecosystem clean.
Changes in Diet as they Grow
Younger horseshoe crabs have a preference for eating soft-bodied marine worms which are easier to capture and ingest. Their pincers and gastric mill are not yet strong enough to feed exclusively on hard-shelled mollusks.
As the crabs grow bigger and mature, they switch to adding more clams, mussels, and snails to their diet. Their pincers are now powerful enough to crack through tough shells of larger mollusks.
Nonetheless, adult horseshoe crabs still opportunistically feed on worms, crustaceans and plant matter as part of their omnivorous diet.
Unusual Traits Help Them Feed
Horseshoe crabs have some unique adaptations that aid their feeding:
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Multiple eyes – They have a total of 9 eyes of different types, including lateral compound eyes and median eyes. This helps them spot food sources.
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Leg pincers – Front legs have claw-like pincers for digging, grasping and crushing food.
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Tail (telson) – The long spike tail helps the crab flip itself back over if overturned while searching for food.
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Book gills – These flap-like book gills enable the horseshoe crab to breathe underwater while it feeds on the ocean floor.
Threats Facing Horseshoe Crabs
Although horseshoe crabs have survived for millions of years, their populations now face several threats:
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Habitat loss – Coastal development destroys spawning beaches and feeding grounds.
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Overharvesting – Horseshoe crabs are captured and bled for medical purposes, and also used as baitfish which is unsustainable.
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Pollution – Toxins from agricultural and industrial runoff can accumulate in their bodies.
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Climate change – Rising sea temperatures, ocean acidification, and extreme weather affects horseshoe crab development and survival.
Reproduction and Life Cycle
Eggs are laid in the spring and summer (most often in May and June), usually at high tide in the evening when the moon is full or new. Large numbers of adults crawl up onto sandy, protected beaches to mate and lay eggs.
Females lay clusters of about 4,000 greenish eggs in the sand around the high-tide mark. They return to the beach to lay more eggs during high tides throughout the season. Eggs take about one month to develop and hatch. Young swim to shallow, sandy, protected nursery areas, where they remain for up to two years.
- Horseshoe crabs are not really crabs; they are more closely related to spiders that live on land than to blue crabs.
- Horseshoe crabs have been around for more than 300 million years and are often called “living fossils.”
- Horseshoe crabs are not dangerous, even though their spikes and spines make them look that way.
- Just like other arthropods, horseshoe crabs must molt to grow. They shed their old shells and grow new ones that are bigger.
- If you keep their gills moist, horseshoe crabs can stay alive for a long time when they’re not in the water.
Horseshoe Crab Feeding
What do horseshoe crabs eat?
Just as spiders consume smaller insects, the horseshoe crab eats smaller undersea organisms such as worms, clams, crustaceans, algae, and other animals. Without teeth or mandibles, Horseshoe Crabs must crush up their food before eating.
Do horseshoe crabs eat clams?
Horseshoe crabs don’t appear picky at all about their seafood selections, so most any clam can be on the menu. Upon finding a clam, the horseshoe crab will grab the clam with its two forelegs, which are actually a pair of smaller appendages that end in pincers.
Do horseshoe crabs eat worms?
The ocean floor where the horseshoe crabs make their home also teems with a large variety of aquatic worms. Horseshoe crabs have evolved to locate these worms and dig them up out of the sand before consuming them. Next to clams, ocean-going aquatic worms are the most often found food in a horseshoe crab’s diet.
Are horseshoe crabs good pets?
Horseshoe crabs play an essential part in the ecology of the oceans they inhabit. They are the janitors, keeping the ocean floor free of carrion and algae, in addition to hunting clams and worms. But many people keep the crabs as pets in an aquarium, where they help keep the tank free of algae or other problems.