Agkistrodon piscivorus
Agkistrodon piscivorus is a species of venomous snake, a pit viper in the subfamily Crotalinae of the family Viperidae. It is one of the world's few semiaquatic vipers, and is native to the Southeastern United States. As an adult, it is large and capable of delivering a painful and potentially fatal bite. When threatened, it may respond by coiling its body and displaying its fangs. Individuals may bite when feeling threatened or being handled in any way. It tends to be found in or near water, particularly in slow-moving and shallow lakes, streams, and marshes. It is a capable swimmer, and like several species of snakes, is known to occasionally enter bays and estuaries and swim between barrier islands and the mainland.
The generic name is derived from the Greek words ἄγκιστρον agkistron "fish-hook, hook" and ὀδών odon "tooth", and the specific name comes from the Latin piscis 'fish' and voro ' eat greedily, devour'; thus, the scientific name translates to "hook-toothed fish-eater". Common names include cottonmouth, northern cottonmouth, water moccasin, swamp moccasin, black moccasin, and simply viper. Many of the common names refer to the threat display, in which this species often stands its ground and gapes at an intruder, exposing the white lining of its mouth. Many scientists dislike the use of the term water moccasin since it can lead to confusion between the venomous cottonmouth and nonvenomous water snakes.
Taxonomy and etymology
Common names
This is a list of common names for A. piscivorus, some of which also refer to other species:- aquatic moccasin
- black moccasin
- black snake
- black water viper
- blunt-tail moccasin
- Congo
- copperhead
- cottonmouth
- cotton-mouthed snake
- cottonmouth rattler
- cottonmouth water moccasin
- gaper:
- gapper
- highland moccasin
- lake moccasin
- lowland moccasin
- mangrove rattler
- moccasin
- moccasin snake
- North American cottonmouth snake
- North American water moccasin
- North American water viper
- pond moccasin
- pond rattler
- river moccasin
- river rattler
- rusty moccasin
- saltwater rattler
- short-tailed moccasin
- short-tail rattler
- small-tailed cottonmouth
- snap-jaw
- stub-tail
- stub-tail snake
- stump moccasin
- stump-tail moccasin
- stump-tail viper
- swamp lion
- swamp moccasin
- swamp rattler
- Texas moccasin
- trap jaw
- Troost's moccasin
- true horn snake
- true water moccasin
- viper
- water copperhead
- water mamba
- water moccasin
- water mokeson
- water pilot
- water pit rattler
- water pit viper
- water rattlesnake
- water viper
- white-mouth moccasin
- white-mouth rattler
- worm-tailed viper
Subspecies and taxonomic history
- Agkistrodon piscivorus, northern cottonmouth
- Agkistrodon conanti Gloyd, 1969, Florida cottonmouth
| Previous taxonomy | Current taxonomy | Geographic range |
| Eastern cottonmouth Agkistrodon piscivorus piscivorus | Northern cottonmouth Agkistrodon piscivorus | The United States in Delmarva Peninsula, the Atlantic Coastal Plain and lower Piedmont of North and South Carolina, to eastern Georgia including outer banks and offshore islands. |
| Western cottonmouth Agkistrodon piscivorus leucostoma | Northern cottonmouth Agkistrodon piscivorus | The United States, most of Alabama along coast of the Gulf of Mexico, including many offshore islands, to central Texas, and north to Oklahoma, Missouri, Illinois, and Indiana |
| Florida cottonmouth Agkistrodon piscivorus conanti Gloyd, 1969 | Florida cottonmouth Agkistrodon conanti Gloyd, 1969 | The United States, in southern Georgia and Florida peninsular, including many offshore islands |
Anatomy and description
Agkistrodon piscivorus is the largest species of the genus Agkistrodon. Adults commonly exceed in total length ; females are typically smaller than males. Total length, per one study of adults, was. Average body mass has been found to be in males and in females. Occasionally, individuals may exceed in total length, especially in the eastern part of the range, with a record specimen of 189.2 cm.Although larger ones have purportedly been seen in the wild, according to Gloyd and Conant, the largest recorded specimen of A. p. piscivorus was in total length, based on a specimen caught in the Dismal Swamp region and given to the Philadelphia Zoological Garden. This snake had apparently been injured during capture, died several days later, and was measured when straight and relaxed. Large specimens can be extremely bulky, with the mass of a specimen of about in total length known to weigh.
Many would assume that the morphology of an aquatic snake should have a small, narrow head that tapers towards the back to minimize drag in the water, especially when capturing prey. However, the pit vipers, and particularly cottonmouths, display a contradicting structure, with its bulky, triangular head, which would be assumed to be poorly suited to water, yet it is not the case.
The broad head is distinct from the neck, and the snout is blunt in profile with the rim of the top of the head extending forwards slightly further than the mouth. Substantial cranial plates are present, although the parietal plates are often fragmented, especially towards the rear. A loreal scale is absent. Six to 9 supralabials and eight to 12 infralabials are seen. At midbody, it has 23–27 rows of dorsal scales. All dorsal scale rows have keels, although those on the lowermost scale rows are weak. In males/females, the ventral scales number 130-145/128-144 and the subcaudals 38-54/36-50. Many of the latter may be divided.
Though most specimens are almost or even totally black,, the color pattern may consist of a brown, gray, tan, yellowish-olive, or blackish ground color, which is overlaid with a series of 10–17 dark brown to almost black crossbands. These crossbands, which usually have black edges, are sometimes broken along the dorsal midline to form a series of staggered halfbands on either side of the body. These crossbands are visibly lighter in the center, almost matching the ground color, often contain irregular dark markings, and extend well down onto the ventral scales. The dorsal banding pattern fades with age, so older individuals are an almost uniform olive-brown, grayish-brown, or black. The belly is white, yellowish-white, or tan, marked with dark spots, and becomes darker posteriorly. The amount of dark pigment on the belly varies from virtually none to almost completely black. The head is a more or less uniform brown color, especially in A. p. piscivorus. Subadult specimens may exhibit the same kind of dark, parietal spots characteristic of A. contortrix, but sometimes these are still visible in adults. Eastern populations have a broad, dark, postocular stripe, bordered with pale pigment above and below, that is faint or absent in western populations. The underside of the head is generally whitish, cream, or tan. Specimens between 26.8 cm and 120.3 cm snout-vent lengths displayed various dorsal colors. Those greater than 60 cm snout-vent length had less white and tan colors.
Juvenile and subadult specimens generally have a more contrasting color pattern, with dark crossbands on a lighter ground color. The ground color is then tan, brown, or reddish-brown. The tip of the tail is usually yellowish, becoming greenish-yellow or greenish in subadults, and then black in adults. On some juveniles, the banding pattern can also be seen on the tail. Young snakes wiggle the tips of their tails to lure prey animals.
This species is often confused with the copperhead, A. contortrix. This is especially true for juveniles, but differences exist. A. piscivorus has broad, dark stripes on the sides of its head that extend back from the eyes, whereas A. contortrix has only a thin, dark line that divides the pale supralabials from the somewhat darker color of the head. The watersnakes of the genus Nerodia are also similar in appearance, being thick-bodied with large heads, but they have round pupils, no loreal pit, a single anal plate, subcaudal scales that are divided throughout, and a distinctive overall color pattern.
Venom
Agkistrodon piscivorus venom is more toxic than that of A. contortrix, and is rich with powerful cytotoxic venom that destroys tissue. Although deaths are rare, the bite can leave scars, and on occasion, require amputation. Absent an anaphylactic reaction in a bitten individual, however, the venom does not cause systemic reactions in victims and does not contain neurotoxic components present in numerous rattlesnake species. Bites can be effectively treated with CroFab antivenom; this serum is derived using venom components from four species of American pit vipers.Bites from the cottonmouth are relatively frequent in the lower Mississippi River Valley and along the coast of the Gulf of Mexico, although fatalities are rare. Allen and Swindell compiled a record of A. piscivorus bites in Florida from newspaper accounts and data from the Bureau of Vital Statistics: 1934, eight bites and three fatalities ; 1935, 10; 1936, 16; 1937, 7; 1938, 6; 1939, 5; 1940, 3; 1941, 6; 1942, 3; 1943, 1; 1944, 3; 1998, 1. Wright and Wright report having encountered these snakes on countless occasions, often almost stepping on them, but never being bitten. In addition, they heard of no reports of any bites among 400 cypress cutters in the Okefenokee Swamp during the entire summer of 1921. These accounts suggest that the species is not particularly aggressive. Studies show that stressed snakes are more likely to strike. This action comes as a predator defense mechanism. Snakes with elevated hormone levels are more likely to strike. Additionally, larger snakes are more likely to strike than smaller snakes.
Brown gave an average venom yield of 125 mg, with a range of 80–237 mg, along with values of 4.0, 2.2, 2.7, 3.5, 2.0 mg/kg IV, 4.8, 5.1, 4.0, 5.5, 3.8, 6.8 mg/kg IP and 25.8 mg/kg SC for toxicity. Wolff and Githens described a specimen that yielded 3.5 ml of venom during the first extraction and 4.0 ml five weeks later. The human lethal dose is unknown, but has been estimated at 100–150 mg.
Symptoms commonly include ecchymosis and swelling. The pain is generally more severe than bites from the copperhead, but less so than those from rattlesnakes. The formation of vesicles and bullae is less common than with rattlesnake bites, although necrosis can occur. Myokymia is sometimes reported. However, the venom has strong proteolytic activity that can lead to severe tissue destruction.