Aquatic Garter Snake
Thamnophis atratus
![[Aquatic Garter Snake]](/images/138ds.jpg)
Description
Physical
A medium sized slender snake, adults rarely exceed 90cm (36 inches). Scales are keeled, single anal plate. Typically has 8 upper labials, the 6th and 7th are not grossly enlarged compared to the 5th. Sixth upper labial is typically shorter than the seventh. Rear chin shields are longer than front. Internasals are longer than wide and appear pointed at the front. Typically has 19 dorsal scale rows at mid-body but occasionally has 21 or rarely, 20. Pupils are round.
Color and Pattern
Dusky olive or slate base color with two rows of alternating dark blotches on each side. On dark individuals, the blotches may not be distinct. In our area the dorsal stripe is indistinct or absent. When absent, there usually is a remainder of it near the head.
Belly may occasionally have a very small amount of black flecks but is generally plain. Throat is plain. If black pigment is present on upper labials, it generally is restricted to the very edges of the scales.
Young are patterned like adults.
Sexual Dimorphism
Males tend to have longer tails that are noticeably fat after the vent, females have slightly shorter tails that taper off quickly after the vent. Some people are quite accurate at determining sex by tail geometry, but probing is required for most of us to be certain.
While both sexes usually have 19 scale rows at midbody, females are more likely to have 21 than males (Rossman and Stewart 1987 pg. 5). Males tend to have a high vental scale count and high subcaudal scale count than females.
Distribution Range
General range map taken from CWHR, modified.
Museum records taken from following institutions: CAS-SUR, MVZ Herps, LSUMZ Herps.
Below Lake Shasta, this species is restricted the west side of the Sacramento River. Distribution in the low elevation Cottonwood Creek drainage seems to be very spotty, especially near the Sacramento River.
Fairly common in the drainages that feed Lake Shasta, where it can be found around the Sacramento River (both sides) north of the Lake all the way up into Siskiyou County, the McCloud River drainage, Squaw Creek, and then moving east along the Pit River drainage to the vicinity of Rock Creek where it coexists with the Sierra Garter Snake and Clark Creek north of Lake Britton.
Typical Elevation
We do not yet have enough records from distinct localities to calculate typical elevation. We need 3 more distinct localities.
Taxonomy
The subspecies that occurs in our area is the Oregon Gartersnake (Thamnophis atratus hydrophilus).
Pronunciation
The assumed IPA pronunciation of this species: /θɑm ˈnɑ fis/ /ɑ ˈtrɒ tʌs/
Taxonomic Etymology
Thamnophis is a concatenation of two Greek words. θάμνος means shrub or bush. όφις means snake. It is a reference to the tendency of these snakes to be found in the vicinity of bushes.
atratus literally means “mourner” in Latin and probably refers to the dark dorsal coloring (Beltz 2006).
hydrophilus is a combination of two Greek words. ὕδωρ ([H]udōr, often transliterated Hydro) means water. φίλος (Filos) means friend. It is a reference to their affinity towards water.
Taxonomy Notes
Historically all current subspecies of Thamnophis atratus were considered to be subspecies of Thamnophis couchii. It has since been demonstrated that Thamnophis atratus is a distinct species with no significant gene flow in the contact zone. Occasional hybrids are found, but they are the exception. Most specimens in the contact zone are one species or the other (Rossman and Stewart 1987).
In some literature, you may find our subpecies referred to as Thamnophis couchii hydrophilus.
Additional Information
Notes
The CWHR range map data for this species has been modified. The original range included a contact zone with the Sierra Garter Snake (Thamnophis couchii) from the Pit River drainage all the way down to Tehama County. This is incorrect. The species is not present east of the Sacramento River below Lake Shasta and in fact does not even approach the Sacramento River south of Lake Shasta until the extreme south part of the county.
Images
Unless otherwise noted, all images are of wild specimens in Shasta County.
Please be kind, do not hotlink images and do not use images without permission of the copyright owner.
![[Aquatic Garter Snake]](/images/138sm.jpg)
©Brian Hubbs
Notice the dorsal stripe is much more distinct near the head and neck and fades as it moves down the body. Shasta County, CA
Habitat Shots
Unless otherwise noted, these habitat shots are from within Shasta County where this species is known to occur.
There currently are no images to display.
Similar Species
Terrestrial Garter Snake
Terrestrial Garter Snakes (Thamnophis elegans) have a much more distinct dorsal stripe, the 6th and 7th upper labial are grossly enlarged compared to the 5th, the rear and front chin shields are about same length, and the internasals are wider than they are long and are not pointed in the front.
Sierra Garter Snake
Sierra Garter Snakes (Thamnophis couchii) in our area usually have black pigment on labials, throat, and venter. The 6th upper labial is usually longer than 7th.
Common Garter Snake
Common Garter Snakes (Thamnophis sirtalis) a much more distinct dorsal stripe, have red blotches on the side, and typically have 7 upper labials.
![[Sponsor ShastaHerps.org]](/images/SherpBanner2.png)
