Horseshoe crabs are a fairly common sight along the Atlantic and Gulf coasts and in Southeast Asia.
In fact, these locations are where three of the four species of this particular crab reside. They are easily recognizable, with their large half-moon shell.
The unique shape makes them look more like a prehistoric battering ram than a traditional crab. The shell’s unique appearance gives the horseshoe crab its name, but these fascinating creatures are more than their shape alone.
Despite the name, the horseshoe crab is more closely related to spiders and scorpions than its more watery brethren. Yet, it has a distinct taste for certain types of seafood rather than typical land-arachnid fare.
While it may seem like they would use their long stinger-like tail for defense or hunting, horseshoe crabs are mostly docile and will not sting or bite.
The tail is mainly used as a device to flip themselves over should they get overturned by a wave or a curious animal.
That said, these crabs do have some less-than-passive tendencies, including when it comes to their diet. So, what does a horseshoe crab eat?
The Horseshoe Crab Diet
The horseshoe crab has no mandibles with which to chew and crush food. As such, their primary diet consists of things that they can easily crush up with their front legs and then ingest.
Horseshoe crabs also possess a gizzard similar to that found in many bird species, enabling them to further grind up food for digestion.
What do horseshoe crabs eat? Like most other crabs, Horseshoe crabs are bottom feeders and scavengers, searching through the floor of shallow waters for their food.
The crabs are omnivorous, so the horseshoe crab diet consists of both living prey like mollusks and aquatic worms plus algae and carrion.
Horseshoe crabs hunt and feed primarily during the nighttime hours. The primary way horseshoe crabs hunt for food is by digging and sifting through the sand in their immediate vicinity, and they do this almost constantly.
The horseshoe crab diet includes a wide variety of bi-valve and shelled mollusks commonly found on the ocean floor.
Clams factor highly in the horseshoe crab diet, including hard and soft-shelled clams like the blue mussel and the surf clam.
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.
The crab will then use those pincers to crush the outer shell of the clam as well as the inner portions, then push the crushed-up food into its mouth.
If the clam is buried, the horseshoe crab’s appendages are helpful tools for digging in the sand or mud, too.
In general, horseshoe crabs are reef-safe and contribute positively to most marine environments.
Keeping them in tanks as pets presents some complications, however. When they are small, they pose no danger to existing fish or clams.
However, as your crabs grow larger, you may wish to move them to a tank that does not contain smaller (and tasty) critters.
Pet horseshoe crabs need a similar diet to wild horseshoe crabs, but they will eat what is available should their preferred diet not be in adequate supply.
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.
Clam worms, sandworms, and tube worms are all types of aquatic worms that the horseshoe crab might have in its diet, as they are common throughout the ocean, including the crabs’ natural habitat.
Much like it does with a clam, the horseshoe crab will troll the ocean floor until it discovers the presence of a worm in the sand or mud beneath it.
It will then use its legs to dig down into the sand or mud and catch the worm, then crush it up as needed with its pincers before eating it.
Scientists often find plant material in the stomachs of horseshoe crabs. Because they are omnivores, the horseshoe crab diet also includes a wide range of algae or other plant life found on the ocean floor.
The horseshoe crab will stir up and sift through the sand or other substrate around it in its search for food and will snap up any bits of algae or other plant material dislodged by its motions.
These plants are the only portion of the horseshoe crab’s diet that they do not need to crush up with their pincers, as the plants are already soft enough for the crab to ingest.
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.
Thanks to their constant sifting of the substrate and consumption of harmful algae and molds, horseshoe crabs help keep home (and research facility) tanks clean.
The horseshoe crab is a fascinating example of biology. According to science, it is both older than the dinosaurs and largely unchanged in appearance and function in its over 500 million years of existence on Earth.
As such, their diet has largely remained unchanged as well. Because the horseshoe crab’s outer shell has sharp spines and ridges, most predators pass over the hardy crustacean because it is hard to get through.
Of course, horseshoe crabs are predators themselves when it comes to snacking on softer marine creatures.
Thanks to a steady diet of worms, mollusks, and algae, horseshoe crabs have a long and well-fed future ahead of them.