Agkistrodon

Common names: moccasins, copperheads, cantils.

Agkistrodon is a genus of venomous pitvipers found in North America from the United States south to northern Costa Rica. The name is derived from the Greek words ancistro, meaning "hook" and odon, which means "tooth", and is likely a reference to the fangs. Three species are currently recognized, all of them polytypic and closely related.

Description

Members of this genus have a number of features in common. All species have a relatively broad head with short fangs. A loreal scale is present, except in A. piscivorus. There are usually nine large symmetrical platelike scales on the crown of the head, but in all species these are often irregularly fragmented or have sutures, especially in A. bilineatus. All have a sharply defined canthus rostralis and a vertically elliptical pupil. There are 6-10 (usually 8) supralabial scales and 8-13 (usually 10-11) sublabials. The dorsal scales are mostly keeled and at midbody number 21-25 (usually 23), while A. piscivorus has 23-27 (usually 25). There are 127-157 ventral scales and 36-71 subcaudals. Of the latter, some may be divided. The anal scale is single. All have a color pattern of 10-20 dark crossbands on a lighter ground color, although sometimes the crossbands are staggered as half bands on either side of the body.

The phylogeny of the three species has long been controversial. Studies based on morphological (Gloyd & Conant, 1990) and venom characteristics (Jones, 1976) support the idea that A. bilineatus and A. contortrix are more closely related. However, an analysis of mitochondrial DNA was conducted by Knight et al. (1992), as well as more recent molecular studies (Parkinson et al., 1997, 1999) have concluded that A. bilineatus and A. piscivorus are sister taxa, with A. contortrix being a sister species to them both.

Geographic range

Found in North America from the northeastern and central USA southward through peninsular Florida and southwestern Texas. In Central America on the Atlantic versant from Tamaulipas and Nuevo León southward to the Yucatan Peninsula, Belize and Guatemala. Along the Pacific coastal plane and lower foothills from Sonora south through Guatemala, El Salvador, Honduras and Nicaragua to northwestern Costa Rica.

Behavior

All are semiaquatic to terrestrial and are often found near sources of water. However, A. contortrix and A. bilineatus are also found in dry habitats, often far from permanent streams or ponds.

Reproduction

The members of this genus are all ovoviviparous.

Venom

It is assumed that the venom of all three species is not unlike that of A. contortrix, which contains thrombinlike enzymes that act upon the coagulant activity of the blood. A study of electrophoretic patterns of proteins in venoms among and within populations of A. contortrix and A. piscivorus showed that substantial variation exists (Jones, 1976), and there is no reason to believe that these differences do not correspond with variations in toxicity.

Species

Species	Taxon author	Subsp.*	Common name	Geographic range
A. bilineatus	Günther, 1863	3	Cantil	Mexico and Central America. On the Atlantic side it is found in Mexico in Tamaulipas, Nuevo León, possibly northern Veracruz and Chiapas (in the Middle Grijalva Valley). On the Yucatan Peninsula it occurs in Campeche, Yucatán, Quintana Roo and northern Belize. On the Pacific side it is found from southern Sonora in Mexico south through Guatemala, El Salvador, Honduras and Nicaragua to southwestern Costa Rica. On the Pacific side the distribution is almost continuous, while on the Atlantic side it is disjunct.
A. contortrix^T	Linnaeus, 1766	4	Copperhead	The United States (Texas, Oklahoma, Kansas, Missouri, Arkansas, Louisiana, Mississippi, Alabama, Georgia, Florida, South Carolina, North Carolina, Tennessee, Kentucky, Virginia, West Virginia, Illinois, Indiana, Ohio, Iowa, Pennsylvania, Maryland, New Jersey, Delaware, New York, Connecticut, Massachusetts), Mexico (Chihuahua, Coahuila).
A. piscivorus	Lacépède, 1789	2	Cottonmouth	The eastern United States from extreme southeastern Virginia, south through peninsular Florida and west to Arkansas, southeastern Kansas, eastern and southern Oklahoma and eastern and central Texas. A few records exist from along the Rio Grande River in Texas, but these are thought to represent isolated populations that possibly no longer exist.