Living Fossils:

Horseshoe Crabs

What are living fossils?

• Informal, yet popular term used for a clade of organisms that appeared from early geologic time and survived on Earth until today with no or few changes.

Horseshoe Crabs

• First appeared on Earth around 450,000 years ago during the Ordovician Period.

• evolved in the shallow seas of the Paleozoic Era (540-248 million years ago) with other primitive arthropods called trilobites

• Characterized by a dome-shaped shell, prosoma, and a spike-shaped tail, the telson.

• have six pairs of legs, two in the front for pinching and seizing prey and five more for walking

Horseshoe crabs: Fossils

•  referred to as “living fossils” because they have changed so little from types that lived over 400 thousand years ago.

•  so perfectly evolved they lived long before the dinosaurs, and survived both the catastrophe that killed the dinosaurs and the larger Permian mass extinction

Horseshoe Crabs: Fossils

Lunataspis aurora

Extinct Relatives of Horseshoe Crabs

Location of Horseshoe Crabs in Xiphosura Phylogeny

List of Horseshoe Crab Species• Limulus

– Limulus polyphemus– Limulus coffini †– Limulus woodwardi (incertae sedis) †– Limulus henkeli (incertae sedis) †– Limulus priscus (incertae sedis) †– Limulus sandbergeri (incertae sedis)

†

• Carcinoscorpius rotundicauda• Tachypleus

– Tachypleus tridentatus– Tachypleus gigas– Tachypleus decheni (incertae sedis) †

• Casterolimulus kletti †

• Mesolimulus – Mesolimulus sibiricus †– Mesolimulus crespelli †– Mesolimulus walchi †– Mesolimulus syriacus (incertae sedis) †

• Victalimulus mcqueeni †• Limulitella †

– Limulitella bronni †– Limulitella volgensis †– Limulitella vicensis †– Limulitella liasokeuperinus †

• Tarracolimulus crespelli †• Heterolimulus gadeai †• Austrolimulus fletcheri †• Psammolimulus gottingensis †• Valloisella lievensis †

Horseshoe Crabs: Fossil Records

Earliest fossil record in Ordovician

Probable, but as yet undiscovered, Cambrian origin

NO Silurian-Devonian horseshoe crabs … so far!

Reappearance & peak diversity

Survival & recovery

Modest late Mesozoic presence

Very poor Tertiary record

Four living species

Five Phanerozoic Mass Extinction

Horseshoe Crabs: Survival through Climate Change

Horseshoe Crabs• Horseshoe crabs survived through these

“Ice Ages” and changed at little.• Decline and rise in population dictated by

climate and temperature.• Horseshoe crab fossils are frequently found

with other species fossils that indicate that they are preserved around the same time. A possible result of mass extinction.

Horseshoe Crabs and Continental Drift

Horseshoe crabs: Extant Species

• There are four extant species:– North American Species

• Limulus polyphemus

– Indo-Pacific Species• Carcinoscorpius rotundicauda• Tachypleus tridentatus• Tachypleus gigas

Distribution

Limulus polyphemus• Common name is Atlantic

horseshoe crab• Found in east coasts of North

and Central America• The name “polyphemus”

originated from the Cyclops with the same name because it is mistakenly thought to only have a single eye.

• Most famous species; frequently used for medical applications.

Carcinoscorpius rotundicauda• Known as mangrove

horseshoe crab• Found mostly on

Southeast Asian and East Indian coasts.

• Commonly found in brackish, swampy water; along mangroves.

• The telson (caudal spine) is round unlike in other species hence its name.

Tachypleus tridentatus• Also called as

Japanese horseshoe crabs

• Considered as an endangered species due to destruction of their habitat

• Found on the southern coasts of Japan and eastern tropical coasts of Asia

Tachypleus gigas

• Common names are Indonesian horseshoe crab, Indian horseshoe crab and Indo-Pacific horseshoe crab

• Largest latitudinal range of habitat among the Asian species

• Found in east coasts of India, extending to coasts of Indonesia, Malaysia and Philippines . Can be also found in east of China and south of Japan

Difference in Body TypeLimulus polyphemus Carcinoscorpius rotundicauda

Tachypleus tridentatus Tachypleus gigas

Taxonomic Classification• Phylum: Anthropoda

– Class: Merostomata• Order: Xiphosura

– Family: Limulidae» Genus: Limulus

» Limulus polyphemus» Genus: Carcinoscorpius

» Carcinoscorpius rotundicauda» Genus: Tachypleus

» Tachypleus tridentatus» Tachypleus gigas

Characteristics and FeaturesBody plan

• Three divisions of body: prosoma, opisthosoma and telson

• Prosoma- large dome-shaped shell that contain the eyes on the dorsal and the mouth and legs on the ventral

• Opisthosoma- contains spikes and book gills on the underside

• Telson- a caudal spike that is used as a rudder

Characteristics and FeaturesAppendages

• Has six pairs of jointed appendages.

• The first pair, the chelicerae, bring the food to its mouth

• The next four pairs, the ambulatory legs, have pincers that clasps the female during mating.

• The last pair, the pusher legs, is used for locomotion.

Characteristics and FeaturesCompound eye

• The Horseshoe crabs have ten eyes, two which are compound.

• The compound eye can help the horseshoe crab to locate its mate and tell whether its day or night. It cannot see color.

• The other eyes can help see ultraviolet light

Characteristics and FeaturesCirculatory System

• The blood of horseshoe crabs are copper based which make it appear blue

• The heart of this organism is tube-like

• Blood contain amebocytes that protect the crab from pathogens

Medical and Agricultural Importance

• The horseshoe crab’s blood can be used to produce LAL (Limulus amebocyte lysate), which can is used to test medicines for bacterial contamination.

• The chitin shell of these organisms can be processed as surgical thread for suturing or as wound dressing.

Fishermen often uses horseshoe crabs as bait for eels and conch.

Also they are used as fertilizers

Ecological Importance

• Horseshoe crabs plays a very important role in coastal and marine ecology

• Horseshoe crab’s eggs are important food for most migrating birds

• Horseshoe crabs larvae and eggs are also food for other organisms

• The shell of the horseshoe crabs is a habitat of other marine life. Barnacles, snails, small shrimps, sea slugs, sea sponges, sea weeds and sea anemone live on and under the shell.

Horseshoe Crab Micro-ecosystem

References

Rudkin, D. 2010. The Life and Times of the Earliest Horseshoe Crabs Retrieved from http://www.cityu.edu.hk/bch/iwscahc2011/Download/O-25%20David%20Rudkin.pdf 2011.

Anderson, LI and Selden, PA.1997. Opisthosomal fusion and phylogeny of Palaeozoic Xiphosura. Retrieved from http://homepage.mac.com/paulselden/Sites/Website/AndersonSelden.pdf 2011

The horseshoe crabs. Retrieved from http://horseshoecrab.org/ 2011

Limuloid horseshoe crabs. Retrieved from http://tolweb.org/Limuloidea/14774 2011

http://www.ceoe.udel.edu/horseshoecrab/history/pastpresent.html

Notes

Images:

http://horseshoe-crabs.com/wp-content/uploads/2009/05/horseshoe-crab-eye.jpg

http://todayilearned.co.uk/wp-content/uploads/2011/08/horseshoe-crabs-heart-blood-harvesting.jpg

http://www.horseshoecrab.org/anat

http://www.esablawg.com/esalaw/ESBlawg.nsf/images/KRII-7CT4MP/$File/HorseshoeCrabBlood.jpg

http://farm4.static.flickr.com/3659/5797661147_d08357913b.jpg

http://horseshoe-crabs.com/wp-content/uploads/2009/05/male-horseshoe-crab-claw.jpg

http://farm4.static.flickr.com/3659/5797661147_d08357913b.jpg

http://tolweb.org/tree/ToLimages/513454792_741484fc93_o.300a.jpg

http://content2.eol.org/content/2010/12/04/09/73614_large.jpg

Various images from http://wikimedia.org

http://www.biopix.eu/Temp/Limulus%20polyphemus%2000008.JPG

http://zoology.fns.uniba.sk/poznavacka/images/01_Limulus_polyphemus.jpg

http://www.ceoe.udel.edu/horseshoecrab/history/images/newworldmap.jpg