Atlantic Salt Marsh Snake

Nerodia clarkii taeniata

Description

The Atlantic salt marsh snake (Nerodia clarkii taeniata ) is a small, slender snake that inhabits coastal salt marshes and mangrove swamps that vary in salinity from brackish to full strength seawater. It is unique because it is one of the few North American reptiles that lives in salt water habitats but has not developed salt glands. The Atlantic saltmarsh snake was listed as a threatened species due to habitat loss and alteration, and potential hybridization with adjacent fresh water species.

There are three subspecies of salt marsh snakes: the Gulf salt marsh snake (Nerodia clarkii clarkii ), mangrove water snake (N. c. compressicauda ), and Atlantic salt marsh snake (N. c. taeniata ). These sub-species are distinguishable as follows: their dorsal patterns are formed from a basic pattern of four rows of dark blotches running from head to tail (two lateral and two dorsolateral rows) on a lighter background. In the striped forms, the blotches fuse linearly to form stripes; in the banded forms, the blotches fuse across the back to form bands. In partially striped individuals, it is invariably the anterior portion of the body that is striped, with the pattern posteriorly consisting of bands or rows of unfused blotches. The lateral stripes have a greater tendency than do the dorsolateral stripes to break down posteriorly into rows of blotches.

The Gulf salt marsh snake has a dorsal pattern that is completely striped, or nearly so, with dark brown to black stripes on a tan background. It is not unusual for the lateral stripes in this form to break down posteriorly into rows of blotches. The man-grove water snake may be uniformly orange in color, but it more often has a pattern of dark bands on a lighter background. Individuals from throughout the range of the mangrove water snake may be partially striped; in these specimens the striping is typically limited to the neck region, but occasional specimens may be more extensively striped. Coloration in the mangrove water snake is extremely variable, with the background being gray, straw, or reddish and the bands being black, brown, or red. Populations of mangrove water snakes characteristically include at least some individuals that exhibit reddish or orange pigmentation.

The Atlantic salt marsh snake is a partially-striped salt marsh snake that reaches a maximum length of at least 32.3 in (82 cm), although it is typically less than 25.6 in (65 cm) in length. The pattern consists of a gray to pale olive background with black to dark brown stripes anteriorly, the stripes breaking up into rows of spots posteriorly. The extent of the striping is variable, but most individuals from the coastal marshes of Volusia County are striped on at least the anterior 30% of the body. The venter is black with a central row of large cream to yellowish spots. As in the case of the dorsal striping, this ventral pattern is best developed anteriorly and tends to break down posteriorly. The red pigmentation characteristic of mangrove water snakes is conspicuously lacking in Atlantic salt marsh snakes from the vicinity of Edgewater, Volusia County, and northward. There are several characters of morphology and color pattern that distinguish the salt marsh snakes from the related freshwater species of Nerodia, but one of the most reliable is the number of dorsal scale rows. The salt marsh snakes have the dorsal scales in 21 rows at midbody, whereas the freshwater banded water snake has the scales in 23 rows. Also, those populations of salt marsh snakes that are at least partially striped are easily distinguished from the freshwater form, which is completely banded.

Behavior

Most snakes adapted to life in salt water possess salt glands, through which they excrete excess salts. The salt marsh snakes apparently lack salt glands, but they nonetheless exhibit very low dehydration rates in seawater. Salt marsh snakes are apparently able to survive in seawater through their reduced rates of cutaneous water and salt exchange and their refusal to drink seawater even when they become dehydrated. By contrast, when held in seawater, their freshwater congeners quickly become dehydrated, which prompts them to drink. This merely exacerbates their dehydration and leads to death. Salt marsh snakes readily drink fresh water when it becomes available from rain or dew.

Although the Atlantic salt marsh snake is most easily observed at night, it may be active at any time of day. Its activity is influenced by tidal cycles, which strongly influence the availability of food. Although most specimens have been collected just as the tide was beginning to overflow the flats, some have been observed during low tidal stages, when they apparently feeds on small fishes that become trapped in the shallow water. The Atlantic salt marsh snake feeds primarily on small fish, but it readily takes frogs when available.

This species is ovoviviparous. Captive individuals have given birth to three to nine young from August to October. Fecundity is low relative to the adjacent freshwater species, N. fasciata, which may give birth to 50 or more young.

It is well-known that salt marsh snakes occasionally hybridize with the closely related freshwater species, especially in areas of habitat disturbance. However, despite the reproductive compatibility of the two forms, there appears to be little or no genetic introgression between them in areas of undisturbed habitat. The extent of genetic introgression associated with the local breakdown of reproductive isolation between the two species has not yet been examined.

Habitat

Atlantic salt marsh snakes are restricted to brackish, tidal marshes. They most often have been found in association with saltwort flats and salt grass-bordered tidal creeks. It is not known if they occur in the adjacent black needlerush habitat. Atlantic salt marsh snake use of marsh habitats may be limited by water level, with extreme fluctuations making the marsh too hydric or xeric. When inactive or pursued, they frequently retreat into one of the numerous fiddler crab burrows that riddle the edge of the marsh and the banks of the tidal creeks.

Distribution

The species to which the Atlantic salt marsh snake belongs, N. clarkii, is found in a narrow coastal strip from southern Texas, east along the Gulf coast, around the Florida peninsula, and up the east coast of Florida at least as far as the Halifax River, Volusia County. It is also known from the north coast of Cuba.

Recent records for populations identifiable as Atlantic salt marsh snakes are available from the barrier island a short distance north of Ponce Inlet, the mainland shoreline east of the New Smyrna Beach airport, two localities on the barrier island at New Smyrna Beach, an island in the Indian River east of Edgewater, and from a single specimen identified as N. c. taeniata captured just south of the Flagler County line. It is not known if a viable population exists in this area or to the north in Flagler County. If a population does exist, they may be isolated from populations in the northern Indian River Lagoon by the Ormond Beach-Daytona metropolitan area.

Threats

The Atlantic salt marsh snake was listed on the basis of two primary concerns: 1) loss of habitat resulting from intensive drainage and development in coastal salt marshes; and 2) the accompanying disruption of reproductive isolating mechanisms, which can lead to hybridization with the Florida banded water snake with potential swamping of the Atlantic salt marsh snake gene pool by the much larger Florida banded water snake gene pool. At the time of its listing, the Atlantic salt marsh snake was thought to include salt marsh snakes as far south as Indian River County. However, its distribution may actually be much more restricted, limited to the brackish, coastal marshes of Volusia County. If this is the case, the Atlantic salt marsh snake's vulnerability to habitat destruction and modification is even greater than previously realized.

Rising sea levels are not an immediate threat but in the long term may reduce the amount of habitat available to the Atlantic salt marsh snake. As sea levels rise, salinity in the estuaries will also rise correspondingly and possibly change the vegetation of the marsh, eventually flooding the area and making it inhospitable for the snake.

Historically, Atlantic salt marsh snake habitat probably represented a small portion of the salt marsh and changed periodically from recurring tropical storms. However, with development of Florida's Atlantic coast, habitat has been permanently lost. This development, coupled with narrow habitat flexibility and a limited range (primarily Volusia County), has resulted in reduced populations of Atlantic salt marsh snakes.

Destruction of habitat by residential and commercial construction and habitat degradation due to ditching, diking, and water level manipulation have adversely affected N. c. taeniata and their habitat. Since little was known of the population size or distribution of Atlantic salt marsh snakes during the period of rapid, unregulated, coastal development of the 1940s through the 1960s, it is impossible to quantify habitat loss or direct effects on the N. c. taeniata population. However, during review of dredge-and-fill activities from 1983 to 1992, a minimum of 36 various projects were permitted in Volusia County's salt marsh habitat. These projects included dredge-and-fill, shoreline protection projects, construction of piers and marinas, mosquito ditching, and water control structures. However, loss of salt marsh habitat appears to have slowed since 1988, indicating improved protection under existing local, state and federal conservation policies.

Conservation and Recovery

Conservation measures have consisted of limited survey work; genetic comparison with other salt marsh snakes and southern banded water snakes; and creation, restoration, and protection of suitable habitat.

Sporadic surveys conducted from 1978 to 1988 confirmed the continued presence of the Atlantic salt marsh snake at several localities in Volusia County. Localities in the vicinity of New Smyrna Beach were sampled for genetic studies. Electrophoretic analyses indicated that the salt marsh snakes are closely related to, but specifically distinct from, the southern banded water snake, and that the three subspecies of the salt marsh snake are indistinguishable from each other. Tissues were saved for possible comparison of mitochondrial DNA variation in the salt marsh snakes, but that work has not yet been performed. Additionally, to avoid risks of genetic and/or catastrophic events, self-sustaining populations are being established throughout the subspecies range. The U.S Fish and Wildlife Service recovery plan suggests that self-sustaining populations contain 100-200 adult snakes in at least ten secure, discrete sites dispersed throughout Volusia County. Management of established populations should also include development and implementation of a monitoring program for five or more years.

Contact

U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
http://southeast.fws.gov/

References

Dunson, W. A. 1979. "Occurrence of Partially Striped Forms of the Mangrove Snake Nerodia fasciata compressicauda and Comments on the Status of N. f. taeniata. " Florida Scientist 42: 102-112.

Hebrard, J. J., and R. C. Lee. 1981. "A Large Collection of Brackish Water Snakes from the Central Atlantic Coast of Florida." Copeia 1981: 886-889.

McDiarmid, R.W., ed. 1978. Rare and Endangered Biota of Florida ; Vol. 3, Amphibians and Reptiles. University Presses of Florida, Gainesville.

Woodard, D. W. 1980. "Selected Vertebrate Endangered Species of the Seacoast of the United States: The Atlantic Salt Marsh Snake." Coastal Ecosystems Project, Office of Biological Services, U.S. Fish and Wildlife Service, Atlanta.