Nerodia sipedon (Linnaeus, 1758)
Key Characters: Dark body bands anteriorly, blotches posteriorly; back scales strongly keeled; anal plate divided.
Similar Species: Broad-banded Watersnake, juvenile Plain-bellied Watersnake, Cottonmouth, Copperhead. See the Key to Snakes of Illinois for help with identification.
Subspecies: Four subspecies are currently recognized, but only two, Midland Watersnake, N. s. pleuralis (Cope, 1892) and Northern Watersnake, N. s. sipedon are known to occur in Illinois.
Description: Large (up to 120 cm TL), stout snake with highly variable dorsal coloration. Back light brown, gray, or tan with about 30 reddish brown or dark brown crossbands and blotches (northern subspecies has more than 30, midland subspecies fewer than 30). Crossbands wider on back than on side, and usually wider than intervening paler areas. Belly light yellow with many red or brown half-moons.
Habitat: Streams, lakes, ponds, and ditches. Commonly seen basking on rocks and logs or foraging in the water. Takes shelter under rocks, logs, and other debris along shore.
Natural History: Like most other water snakes, it readily bites and voids feces when handled. Mates in May and gives birth to 20-50 young in late July or August. Newborn are 15-25 cm TL. Diet consists mainly of fish and amphibians. Predators include other snakes and large shore birds. Many are killed by people who mistake them for cottonmouths (even hundreds of miles north of the range of cottonmouths) or copperheads.
Distribution Notes: Statewide, with sipedon in the northern third of the state, pleuralis in the southern fifth and intergrades in a broad band across the center.
Status: Abundant throughout Illinois in both natural and man-made bodies of water.
Etymology: Nerodia – neros (Greek) meaning flowing, liquid; or (Greek) for Nereis, a Greek sea nymph; sipedon – sepedon (Greek) meaning rottenness, decay, ‘a serpent whose bite causes mortification; pleuralis – pleura (Greek) a rib or the side and -alis (Latin) of the.
Original Description: Linnaeus, C. 1758. Systema Naturae per Regina tri Naturae secundum Classes, Ordines, Genera, Species, cum Characteribus, Differentiis, Synonymis, Locis. 10th ed. Salvii, Stockholm. 1:824 pp..
Type Specimen: Not designated. For pleuralis, Lectotype. USNM 1080
Type Locality: “America septentrionali”.
Original Name: Coluber sipedon Linnaeus, 1758. For pleuralis, Natrix fasciatus pleuralis Cope, 1892.
Nomenclatural History: Except for the confusion with N. fasciata and some generic transfers (Tropidonotus, Natrix), the nomenclatural history has been stable.