Friday, August 5, 2011

Trinidad's Coastal Swamps and Snakes

Top: Liophis cobella, Middle: Helicops 
angulatus, Bottom: Hydrops triangularis. JCM
Caroni Swamp is an 8000 hectare wetland complex composed of marshes, mangrove, brackish lagoons, and tidal marshes. It is perhaps best known for its bird fauna which draws a considerable number of tourists and birders each year. However, it also supports a herpetofauna and some its members are frequently a part of the ecotours. The most obvious species are Ruschenberger's Treeboa, Corallus ruschenbergeri, and the Specteled Caiman, Caiman crocodilus. Taylor et al. (2011) conducted a survey of Ruschenberger's Treeboa over more than 115 km of transects, observed 32 individuals, and estimated the density at about 11 snakes  per km2. Treeboas are a favaorite of the tour guides, the birders and ecotourists who cruise the mangroves to see the Scarlet Ibis and the local herons. However, Caroni holds other species of snakes that are of less interest to tourists but of greater interest to science. 

Of the 3300 snakes species known, less than 80 have successfully invaded the ocean, that is about 2.4% of the snake fauna. Yet the oceans cover about 70% of the Earth's surface, so why haven't more species invaded the oceans? To add to the problem there are no mairne snakes in the Caribbean or the Atlantic. All marine snakes are in the Eastern Hemisphere. Yet, Trinidad has three aquatic species that  live in close proximity to each other, species that tolerate the brackish waters in both Caroni and Nariva swamps: Liophis cobella, Helicops angulatus, and Hydrops triangularis (Murphy, 1997)All three have been reported to feed on fishes and frogs and show a variety of adaptations to aquatic ecosystems. Caroni's brackish waters and the presence of these snakes suggests these are species are on a trajectory to adapt to life in the ocean. And, it is not difficult to imagine how aquatic snakes living in places like Caroni and Nariva swamps could transition to life in full salt water over time. 

On 20 June 2011, the Glasgow Zoological Expedition and I did a transect through Caroni, starting at the landward edge and heading west along the main channel after dark. We saw two treeboas, a few caimans, but aquatic snakes were absent. Realizing something was wrong, we took some salinity readings using a hydrometer, the first reading was 28 ppt, then it was 30, and the highest reading was 32 ppt. Full sea water is 36 ppt, and it seemed to me that while these snakes could handle brackish water, this was much to saline for the snakes I was looking for.

So, we took the hydrometer to a small villlage near Nariva Swamp on the east coast of Trinidad. The village has numerous canals and we found two of the three species (Liophis cobella and Helicops angulatus) almost immediately. We found salinity readings between 8 and 12 ppt. These three snakes are of great interest because they may hold clues as to how snakes have transitioned and adapted to marine environments.

Murphy, J. C. 1997. Amphibians and Reptiles of Trinidad and Tobago. Krieger Publishing, Malabar, Fl.

Taylor, K., H. P. Nelson, and A. Lawrence. 2011. Population Density of the Cook's tree Boa (Corallus ruschenbergerii) in the Caroni Swamp, Trinidad. Pages 8-18 In: Proceedings of the 1st Research Symposium on Biodiversity in Trinidad and Tobago (A. Lawrence and H. P. Nelson eds.). University of the West Indies Department of Life Sciences.

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