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Aquatic Garter Snake Thamnophis atratus (Thamnophis couchii atratus)


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Aquatic Garter Snake, Oregon subspecies
© Gordon Frost/

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Family: Colubridae, Colubrid Snakes view all from this family

Description 18-40" (46-102 cm). A variable garter snake, with stripes or a spotted pattern. Back stripe distinct or absent; side stripe, if present, occupies 2nd and 3rd scale rows. Usually 8 upper lip scales, 6th and 7th not enlarged. Internasal scales pointed in front. Scales keeled, in 19-21 rows. Anal plate single.

Subspecies Santa Cruz Aquatic Garter Snake (T. a. atratus), back stripe conspicuous, side stripes less apparent, throat yellow, belly bluish; San Francisco Bay south to Monterey.

Oregon Garter Snake (T. a. hydrophilus), back stripe dull yellow, back gray with prominent dark blotching; sw Oregon south along coastal northern California and Sacramento Valley.

Diablo Range Garter Snake (T. a. zaxanthus), prominent bright orange to yellow-orange back and side stripes; Contra Costa County south to San Benito County, California.

An intergrade form, formerly known as Aquatic Garter Snake (T. couchii aquaticus), occurs from San Francisco Bay area northward in Coast Ranges into Mendocino National Forest; it has conspicuous wide yellow or orange back stripe, side stripes distinct, throat yellow, belly blotched with yellow-orange.

Breeding Live-bearing; about 10-25 young approximately 7-10" (17.5-25 cm) long are born in late summer.

Habitat In or near fresh water streams, marshes, ponds, lakes.

Range Santa Barbara County (coastal California), north to southern Oregon.

Discussion The Aquatic Garter Snake was formerly considered a subspecies of Western Aquatic Garter Snake (T. couchii), which has been split into several separate species.