Adaptation in the Feeding of Predatory Snails- Eric Sanford

One of the most important mentors of my undergraduate career has been Dr. Eric Sanford, professor of Evolution and Ecology at UC Davis and invertebrate zoologist extraordinaire. This summer, I have been a research assistant for a project with Ph.D. student Emily Longman and Eric, examining the adaptation of predatory snails along the West Coast. I'm excited to share the background for that project here!

First: what is adaptation?
Evolutionary adaptation (or, just adaptation) is the process in which the environment favors organisms with traits that increase their survival or reproductive success. These are often long-term changes, occurring over multiple generations. Individuals can acclimate to changing environmental conditions during one lifetime, but these changes are not genetic and will not be passed down to their offspring. 

Local adaptation occurs when one species is spread over an environmental gradient, where individuals may need different characteristics to be successful in different areas. 

Nucella canaliculata shells
Source: gastropods.com

For example, the species used in Emily and Eric's studies of adaptation is Nucella canaliculata, the Channeled Dog-winkle. N. canaliculata is a predatory snail that feeds by "drilling" other animals with shells, such as mussels, barnacles, and other snails. 

N. canaliculata is found throughout shallow, rocky waters of the California and Oregon coasts. In this species, juveniles have very low dispersal (do not leave the area of their parents) because they crawl directly out of their egg capsules. Therefore, there is not very much crossbreeding between northern and southern populations of snails, or even between populations at adjacent headlands. Because offspring are exposed to the same environmental pressures as their parents, local adaptation is very likely to arise. 

So, what are these snails adapting to?

The two mussel species in question
Source: asnailsodyssey.com

The major food source for Oregonian populations of N. canaliculata is Mytilus trossulus, the blue mussel. In more southern populations, the major species is Mytilus californianus. M. californianus tends to be larger and thicker than M. trossulus, requiring more powerful drilling snails to penetrate their shells. 

In past experiments performed by Eric and his colleagues, snails from areas of Oregon were found to have much less success than their California counterparts when given a diet of M. californianus. Additionally, in a reciprocal transplant experiment, Californian snails moved elsewhere had greater success than northern populations transferred south. Results show that snails raised to feed on M. californianus are better at drilling them than those that are not, suggesting that different populations have adapted to varying environmental conditions. 

When regional differences exist within one species, the issue of nature vs. nurture will arise. Are these differences due to an inherent, genetic discrepancy between populations, or has growing under different environmental conditions programmed their behaviors and abilities? 

N. canaliculata Study populations in
California and Oregon.
Figure 1, Sanford and Worth 2009

To address this issue, snails from several different populations were bred in the lab, with the offspring then being raised in common conditions. The northern snails were again found with reduced capacity to drill M. californianus. These differences can be attributed only to regional, genetic differences between the snails themselves instead of environmental conditions. 

As Emily and Eric's research assistant this summer, I get to raise their next generation of N. canaliculata from across California and Oregon. I have to admit, they are extremely adorable as hatchlings. We hope to isolate the differences between populations that prevent northern snails from feeding on M. californianus. In particular, we will test whether snails from different populations show
variation in the depth of holes they can drill, which may be a response to geographic
variation in mussel shell thickness along the West Coast. We are also investigating
ways to reliably quantify mussel shell thickness, to uncover the specific selective
pressures that are causing populations to diverge. We will use archived shells to see if
the mussels change between years or are relatively constant.

We are also starting to raise snails from the same egg capsules on different diets: M. californianus, M. trossulus, and Chthamalus dalli, the little brown barnacle. Collectively, we hope to shed light on how evolution shapes the ecology of these marine invertebrates along the coastline and in changing conditions.
Thanks for reading my first official blog post, I hope it was interesting! Stay tuned for more ecological content in the future! :) 

N. canaliculata hatchling, about 2 weeks old! Please excuse my bad microscope photography!

Shoutout to my friend Sam Walkes for helping me edit this post!

Sources

Sanford, E., Roth, M.S., Johns, G.C., Wares, J.P., and Somero, G.N. 2003. Local Selection and Latitudinal Variation in a Marine Predator-Prey Interaction. Science 300, 1135-1137.

Sanford, E. and Worth, D.J. 2009. Genetic differences among populations of a marine snail drive geographic variation in predation. Ecology 90(11), 3108-3118.

Sanford, E. and Worth, D.J. 2010. Local adaptation along a continuous coastline: Prey recruitment drives differentiation in a predatory snail. Ecology 91(3), 891-901. 

-Madeline Frey