Wednesday, December 8, 2010

Snakes & Snails

Relatively few snakes feed on mollusks, but there are snakes in many lineages that have specialized to feed on gastropods. In North America the natricids in the genus Storeria feed on slugs and earthworms, in the Neotropics some of the dipsidids (Sibon, Dipsas, Tropidodipsas, and Sibynomorphus) tend to specialize in feeding on snails and slugs. In Africa members of the genus Duberria (family Pseudoxyrhophiidae) feed on slugs, However, the Asian family Pareatidae are very specialized for feeding on gastropods, and Pareas iwasakii has been well studied by Masaki Hoso and colleagues (see references below).

Above: Pareas iwasakii grasping a snail. 
Below: The skull and lower jaws of Pareas
iwasakii,  note the different number of teeth 
on each side of the jaw. From Hoso et al. (2010).
Snails with shells that coil counterclockwise have difficulty mating with snails of the same species whose shells coil clockwise because their bodies do not align properly. The snails have traded easy of mating for safety from snakes. The coil direction made mating difficult, and why a mutation causing this reversal would be favored was puzzling. Most snail shells curl clockwise. Studies of Iwasaki's Snail-eater (Pareas iwasakii) demonstrated the snake approaches the snail from behind, grasping the shell with its upper jaw and the soft body with its lower jaw. The snake then works the right and left halves of its lower jaw back and forth to extract the snail's body from the shell.

Since most snail shells turn clockwise, the snakes evolved a specialized lower jaw with more teeth on the right side than on the left. This makes it difficult for the snake to feed on snails coiled counterclockwise. When snakes try and eat snails coiled counterclockwise they frequently fail, and often drop the prey. In one study 87.5 percent of the counterclockwise snails survived the snake, suggesting the spiraling made the difference.

In a follow-up to the previous work Hoso et al. (2010) examined the snails and how genes can spread in a population. The land snails have a single gene for left–right reversal and the authors suggest that this could result in instant speciation, because dextral (shells coiled to the right) and sinistral (shells coiled to the left) snails have difficulty in mating. Hoso and colleagues show that specialized snake predation of the dextral majority drives prey speciation by reversal. Their experiments demonstrate that sinistral Satsuma snails (Stylommatophora: Camaenidae) survive predation by Pareas iwasakii. They found stylommatophoran snail speciation by reversal has been accelerated in the range of pareatid snakes, especially in snails that gain stronger anti-snake defense and reproductive isolation from dextrals by sinistrality. Molecular phylogeny of Satsuma snails further provides intriguing evidence of repetitive speciation under snake predation.

Literature
Hoso, M. 2007. Oviposition and hatchling diet of a snail-eating snake Pareas iwasakii (Colubridae: Pareatinae). Current Herpetology 26:41–43.

Hoso, M. and M. Hori. Divergent shell shape as an antipredator adaptation in tropical land snails. American Naturalist 172:726–732.

Hoso, M. and M. Hori. 2006. Identification of molluscan prey from feces of Iwasaki's slug snake, Pareas iwasakii. Herpetological Review 37:174–176.

Hoso, M., T. Asami, and M. Hori. 2007. Right-handed snakes: convergent evolution of asymmetry for functional specialization. Biology Letters, 3:169-172 DOI: 10.1098/rsbl.2006.0600

Hoso, M.,Y. Kameda, S.-P. Wu, T. Asami, M. Kato, and M. Hori. 2010. A speciation gene for left–right reversal in snails results in anti-predator adaptation. Nature, http://www.nature.com/ncomms/journal/v1/n9/full/ncomms1133.html

No comments:

Post a Comment