This time of year usually means high water in the basins on Sanibel. With that, come more frequent observations of snakes by residents and visitors. This is because the wetlands, where several Sanibel snakes call home, become flooded and watersnakes or their relatives need a place to dry out and sun themselves. With no other options, those dry places become backyards, sidewalks, and parking lots. When water levels drop, most aquatic and semi-aquatic snakes retreat to dry areas near the edge of the wetland and out of the way of most people.
Unfortunately with the rise in snake observations due to high water come the reports of “water mocassins” (cottonmouths), a venomous snake in Florida. Sanibel is well within the range of the Florida cottonmouth (Agkistrodon piscivorus conanti), however they have never been documented on Sanibel, Captiva, North Captiva, Cayo Costa or Pine Island.
Watersnake, not Cottonmouth
The snake that many people are seeing, and unfortunately misidentifying, is the Florida watersnake (Nerodia fasciata pictiventris). This is a common snake of the freshwater basins of the island. In brackish and saltwater (mangrove) areas of the islands, that snake is replaced by the mangrove saltmarsh snake (Nerodia clarkia compressicauda). Both of these harmless snakes are unfairly persecuted by people because they are thought to be “mocassins” simply because they are seen in or around water.
This does not ignore the fact that we are in the range of the Florida cottonmouth and it is possible for them to arrive here by swimming across the bay or arriving in shipments of sod/mulch/plants etc. Snakes are a very important part of the many ecosystems on Sanibel. They both eat prey and get eaten by predators as part of the food web. Native snakes should not be harmed, especially on this conservation island.
A major issue is that most people cannot correctly differentiate between the two snakes. Common verbal inaccuracies that I often hear from snake novices are “it had a triangular head,” “it rattled its tail,” “it had a heavy body,” “it was swimming,” etc. The truth is that most snakes have a triangular shaped head and most exaggerate that triangular shape when they are threatened. Most snakes will rattle their tail, even though they don’t have a rattle, as a false threat. It is true that cottonmouths have a large, heavy build, but well fed watersnakes can also be massive. Finally, all local snakes can swim. There are many misnomers when it comes to snakes — and watersnakes probably experience the worst of that in regards to the cottonmouth.
Here are some key diagnostic features for the Florida watersnake (usually between 2.5- 3.5 ft) as compared to the cottonmouth:
The mangrove saltmarsh snake (usually 1 – 2.5 ft): (1)
The Florida cottonmouth (usually 3 -5 ft) as compared to the nonvenomous watersnakes of Sanibel: