Horseshoe crabs are an ancient species dating back over 200 million years. While there were hundreds of species in the past, only five remain today. They are marine arthropods that use book-shaped gills under their abdomen to absorb oxygen from water. When they come ashore to mate, their eggs provide an important food source, and their blood is collected by pharmaceutical companies due to its unique clotting properties, which are used to treat diseases like hemophilia.

1. Phylum Arthropoda
Subphylum Chelicerata
Class Merostomata

2. Phylum Arthropoda
Subphylum Chelicerata
Class Merostomata
Common name Horseshoe Crabs

3. Phylum Arthropoda
Subphylum Chelicerata
Class Merostomata
Common name Horseshoe Crabs

4. Horseshoe crabs are
ancient animals. Only
five species remain
today but 200 million
years ago there were
hundreds of species.
This fossilized, ancient
species is very similar
to modern species.

5. Like other
Arthropods,
horseshoe crabs
have compound
eyes.

6. Horseshoe crabs
are marine and
must be able to
absorb oxygen
from the water.

7. Horseshoe crabs
are marine and
must be able to
absorb oxygen
from the water.
To do so, they have a series of seven “book gills”
under their abdomen.

8. Horseshoe crabs
are marine and
must be able to
absorb oxygen
from the water.
To do so, they have a series of seven “book gills”
under their abdomen. They’re called book gills
because they look like pages in a book.

9. Carapace
Abdomen

10. 8 legs
Telson = 5
segments of the
abdomen.
Book gills

11. Horseshoe Crabs come ashore to mate during the
highest high tide in June. They lay their eggs in the
sand.

12. Horseshoe crab eggs are an important food source for
many animals.

13. Upon hatching, Horseshoe
crab larvae look very much
like Trilobites.

14. Upon hatching, Horseshoe
crab larvae look very much
like Trilobites.
This has lead scientists to
infer that Trilobites might
be the evolutionary
ancestors of Horseshoe
Crabs.

15. Upon hatching, Horseshoe
crab larvae look very much
like Trilobites.
This has lead scientists to
infer that Trilobites might
be the evolutionary
ancestors of Horseshoe
Crabs.
Horseshoe crab larvae are referred to as “trilobite larvae”.

16. When they
come ashore,
pharmaceutical
companies
collect them
and take half of
their blood.
It has amazing
clotting ability.
.

17. When they
come ashore,
pharmaceutical
companies
collect them
and take half of
their blood.
It has amazing
clotting ability.
Proteins in it
are separated
and use to
Treat people who have Hemophilia, a
disease that prevents the blood from
clotting.
.