Lake Erie watersnake COSEWIC assessment and status report: chapter 3

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Species Information

Name and classification

Nerodia sipedon insularum (Lake Erie Watersnake / Couleuvre d'eau du lac Érié) was designated as a distinct taxon by Conant and Clay (1937). Nerodia s. insularum is one of two subspecies of the Northern Watersnake (Nerodia sipedon) in Canada, the other being the Northern Watersnake (Nerodia sipedon sipedon; Crother et al. 2000, Crother et al.2001).

Morphological description

Dorsal patterns of the Lake Erie Watersnake are highly variable among individuals, ranging from uniformly gray, drab greenish, or brownish yellow and unpatterned or blotchless to regularly patterned with dorsal and lateral blotches (Conant and Clay 1937; Conant and Clay 1963; King 1986, 1998; King and Lawson 1995, 1997). The relative size and position of pattern elements remain fixed over the life of the snake (King 1993a as cited in USFWS 2003). The ventral surface tends to be white or yellowish white (often with dark speckling) with the bases of ventral scales the same colour as the dorsum (Conant and Clay 1937). The body scales are keeled, that is, each scale has a ridge down the middle. The snake has a single anal plate.

The Lake Erie Watersnake is non-venomous but can become aggressive if threatened. Its defensive posture is to flatten the head and body and attempt to strike. The Lake Erie Watersnake is a large snake reaching lengths at maturity of 43-125 cm snout-to-vent length (SVL) (King 1986). On Pelee and Middle Islands, mature males are between 59.1 and 71.6 cm SVL, and adult females measure between 80.2 and 88.2 cm SVL (King 1986; King 1998). Similar average sizes were found in 1999 on Pelee Island where gravid females ranged between 68 and 95 cm SVL (Bishop and Rouse unpubl. data 1999).

The Lake Erie Watersnake differs from other watersnakes by being more grayish in colouration and by having a partial or complete lack of blotches or banding (Conant and Clay 1937), whereas the Northern Watersnake has a regular, strongly-banded pattern. The Lake Erie Watersnake also lives almost exclusively on the islands of western Lake Erie (Conant and Clay 1963), whereas the Northern Watersnake is found on the mainland in Ohio and many other central and eastern states in the U.S. and in Ontario and Quebec in Canada. The habitat of Lake Erie Watersnakes is largely composed of rocky shorelines, or limestone or dolomite shelves and ledges with cracks and sparse vegetation, whereas Northern Watersnakes occur in more thickly vegetated habitats with mud, soil, or clay substrate (Conant 1951). The Lake Erie Watersnake also differs from the Northern Watersnake in diet and morphology (shorter tail and larger adult body size) (Conant 1951; Langlois 1964; King 1986, 1989, 1993a).

Genetic description

Populations on the islands of western Lake Erie are separated from the Ohio and Ontario mainland populations by 5 to 14 km of water which acts as a natural barrier (USFWS 2003). In addition to unpatterned individuals, both intermediate and regularly patterned individuals are found on the islands. This variation suggests that mainland Northern Watersnakes and the island-dwelling Lake Erie Watersnakes still intergrade. Gene flow is created by movement among and between islands and the mainland, thereby increasing genetic variation more than if the populations were completely isolated (King and Lawson 1997). However, inter-island movements appear to be rare and have only been documented twice (King 2002, D. Jacobs pers. comm. July 2005). Genetic analysis of allozyme variation suggests that 0.08% - 1% of island populations (N. s. insularum) are replaced by mainland (N. s. sipedon) individuals per generation (King and Lawson 1995, 1997). Frequency of the various colour patterns seems to have been stable from 1980 to the present suggesting that selection and gene flow have also remained stable over this period (King 2004a). Reduced and intermediate patterns appear to be favoured on islands because reduced patterning is more cryptic against the bare, rocky shorelines (King 1992, 1993b, 1993c, King and Lawson 1995; King et al. 1997).