Castration or death?

Submitted by editor on 10 March 2015.

Not a choice many of us would care to face. But this is an essential risk management decision made by the snail Littorina littorea based on the micro-positioning it chooses within the intertidal zone on rocky coastlines of the North Atlantic. If a snail chooses to live high in the intertidal zone, it is exposed to copious gull feces which are the transmission mode of parasitic castrators (trematodes) to the snail. But choosing the low intertidal zone can be even worse because there the snails are subject to high mortality from crab and lobster predators. 

Gulls (Larus marinus and L. argentatus) loaf in the upper intertidal on the Isles of Shoals, Maine, USA. Larval stages of castrating trematode parasites are transmitted to the snails in the gull’s feces. Note the abundant white fecal stains on the rock. Photo: A. Malek 

Carcinus maenas, the European green crab, is a dominant predator of small snails at lower tidal elevations. Photo: S. Fierce 

What is a snail to do?! More specifically, how should a snail optimally respond to these severe, yet spatially opposing, fitness consequences? In an Early View paper “Opposing selective pressures decouple pattern and process of parasitic infection over small spatial scale”, we have studied this trade off. 

Snails seem to have crafted their behavior and life history to make the best of the situation. Basically the snails live high in the intertidal when small, since castration, although bad, is the less severe fitness consequence. (Parasitism rates are not as high as predation rates and snails can sometimes squeak out a reproductive round before the castration process is complete). However, as snails age and grow large enough, their shells are stronger and can withstand the shell-cracking pressure of predators. And so, they move to the low intertidal zone because under the water, conditions for growth are more favorable and they can live relatively free from parasite exposure. 

Although the snails’ solution sounds logical and elegant, we needed multiple years of observation and experimentation to determine this. One odd consequence of their solution is that parasitism prevalence is highest in the lowest intertidal zone even though the risk of infection there is almost nil. Snails that contract infections in the high intertidal carry them with them to the lower reaches as their increased size grants them sufficient predator protection to migrate downward. Thus, the observed pattern of high abundance of infected snails at the lowest intertidal heights belies the relative safety of that zone to infection risk. Movement of larger, older, and now more likely parasitized snails to lower tidal heights spreads the distribution of infected snails out from epicenters of infection acquisition in the high intertidal and obscures the spatial gradient in infection risk. Ultimately, the snails’ strategies that change with life stage accentuate their exposure to parasitism and subsequently decouple the process and pattern of parasite infection over a small spatial scale.

Snail enclosure cages to quantify infection rates in the high intertidal zone surrounded by many parasite-hosting gulls. Photo: A. Malek 

The authors through Jeb Byers

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