Past and Present
The horseshoe crab has remained virtually the
same for millions and millions of years. In virtue of that
fact, the horseshoe crab, which is estimated to have inhabited
the Earth now for over 300 million years, is often called
Courtesy UD Department of Geology
Most scientists believe that horseshoe crabs are the closest
living relative of the trilobite, a marine animal that has
long been extinct. Scientists believe that horseshoe crabs
were among the dominant creatures some 300 million years ago.
There may have been many different species of horseshoe crabs.
Today, four species of horseshoe crabs inhabit the earth's
oceans. Limulus polyphemus is found along the western
Atlantic and Gulf coasts from southern Maine to the Yucatan
Peninsula, with the Delaware Bay as the center of the population.
Tachypleus gigas and Carcinoscorpius rotundicauda are found in the Indo-Pacific region from the Bay of Bengal to Indonesia and Borneo. Tachypleus tridentatus ranges from the Philippines to the southwestern seas of Japan.
Continents on the Move
Our world looked very different 350 million years ago. The
continents were merging into a large land mass called Pangea
(click here to
see what it looked like). A tropical climate enveloped North
America, Europe, and Southeast Asia. Warm shallow seas and
coastal lagoons surrounded these areas, perfect habitat for
horseshoe crabs. Where in the world did horseshoe crabs live
at that time? Although the fossil record of horseshoe crabs
is limited because horseshoe crab shells are made of chitin
(a cellulose-like compound that breaks down quickly over time),
some fossils of horseshoe crabs and their tracks have been
found. Remember, you need to envision the
world as it looked when the fossils were being created — over
time the fossil record has moved along with the Earth's land
As the European land mass began to form, shallow seas disappeared.
The ensuing loss of habitat may have forced the horseshoe crabs
to migrate east and west to the places we find them today.
Changes in habitat initiated the movement, but regional differences
in hydrology and geology have helped to define the current
distribution of horseshoe crab species. Water temperature influences
the dispersal of horseshoe crabs along coastal regions. Neither T.
tridentatus nor L. polyphemus extends into the
northern reaches (above 45°N latitude) of the continental
shelf, where cold-water currents prevail. Seafloor geology
may also play a role in dispersal patterns. For example, the
deep water basin between the Caribbean islands and Cuba prevents L.
polyphemus from migrating farther south.
Click here to see the oldest known horseshoe crab fossil at the University of Oslo's Paleontology museum.