Friday, March 1, 2013

Life and death in the intertidal from a flatworm’s point of view

For the past month, I have had the pleasure of sharing thoughts and experiences with a group of interns at the Oceans Research’s Campus spanning 10 countries around the world.  While most of them came for the “big things” such as dolphins, seals, and sharks, I came with a much different plan.  I am an ecologist interested in feeding behavior of animals and the different adaptations that their prey have evolved to precisely avoid being eaten.  At first approximation, these concepts seem contradictory: if plants and animals have evolved devices to not be consumed, how come they are preyed upon anyway?  The answer is that evolution is not about perfection, but rather about “what works” (at least for a while) and allows a species to survive. Some individuals will make it, others will not.  But as a whole, a species may evolve towards less of a chance of being killed by a particular predator or groups of predators.   This, in turn, may lead to the evolution of adaptations allowing predators to breach those defenses in prey.  This reciprocal interplay results in an “arms race” between changes in characteristics that permit prey species to escape their natural enemies, and predators to counteract those adaptations.  Scientists call this phenomenon coevolution and, in many examples on land and sea, it appears that what we see now is the result of slowly-accumulating changes that are constantly yielding adaptations to exploit prey or escape predators.

I do not focus on large animals that sit at the top of the food chain, the so-called apex predators.  Instead, I take a close look at things that most persons pass by when they walk around the beach.  My work in the South African intertidal, where the land meets the sea under the influence of rising and falling tides, has been taking a fine-scale look – in fact a worm’s perspective – at the ways that a flatworm traps and consumes snails…and how snails can avoid this unfortunate fate.

The flatworm is called Planocera gilchristi (Gilchrist’s flatworm) and it lives under rocks in intertidal pools where small snails are abundant.  It is a generalist predator of snails, worms, and crustaceans.  Along with these flatworms, rock pools teem with snails such as periwinkles (Afrolittorina knysnaensis) and topshells (Oxystele species) that provide an abundant buffet of food choices.  Along with various interns in this wonderfully diverse crowd, I have been looking at the interactions between the flatworm and three of the most common snails.  Because scientific names often get hard on the tongue, we fondly nicknamed our snails the “black snails” (Oxystele tigrina has a primarily black shell), the “checkered snails” (Oxystele variegata has a checkered pattern of beige and dark brown colors), and the “pointy snails” (Afrolittorina knysnaensis is a small snail with a longer and sharper shell that is easily felt if you walk barefoot along beach rocks).  I will refer to these by their nicknames from now on.

Fig. 1.  Pictures of Gilcrist’s flatworm in normal crawling position.

Fig. 2.  Attacking a snail before consuming it.
Fig. 3.  The “pointy” snails are shown at the right around a rock crevice at low tide.
Fig. 3.  The “black” and two “checkered” snails are shown inside a rock pool.
(Photographs by E. Cruz-Rivera).  

Nature is rarely obvious about the underlying mechanisms that organize assemblages of species and our case was no exception.  We began by placing snails of these three species with flatworms in order to determine what the predators liked.  We used two snail species at a time to test which ones would be consumed more and found some pretty clear patterns.   Flatworms preferred the pointy snails and the checkered snails to the black ones, while they did not show a preference for either checkered or pointy snails when those two were the choices.  At first, this might suggest that some species of snails were yummier than others, but we decided to take a closer look.  When we videotaped the reactions of the snails to the flatworms our interpretation changed.  It turns out some of these snails are not simple victims of circumstance at the mercy of a larger predator…they fight back.  The black and checkered snails are particularly effective at recognizing attack.  These snails sense their environment by a series of antenna-like tentacles that extend in all directions from their bodies…sort of a 3600 radar.  When tentacles of the snails come in contact with the flatworms they immediately turn around and run away at noticeably higher speed than they approached…well, at least for a snail, but remember flatworms are not masters of speed either.  If snails were crawling on the side or roof of the containers we kept them in and were brushed by a flatworm, the snails immediately released their grip on the substrate and dropped to the bottom.  But perhaps the most dramatic adaptation these snails have is their ability to violently twist and shake their shell to release the grip of the flatworms.  If the snail is large enough, it will even slam the flatworm against the walls of the container or crawl out of the water carrying the flatworm to force its release.

In short, our first experiments were not measuring preferences of the predators, but rather constraints imposed by their prey.  We were able to directly test this by gluing the snails so that their escape behaviors could not be exercised.  Our suspicions were right: when the snails could not move, the black ones (previously the least eaten) became the preferred prey of the flatworms.  Interestingly, we also found that it takes the worms longer to eat the pointy snails and that they contain less meat.  So the reward per snail attacked is lower.  Again, this was reflected by comparing the two sets of experiments: free moving pointy snails were eaten more or about the same as black and checkered snails, but when the snails could not move, they were consumed much less than either one of the other two.

There are many questions remaining that we want to address in the future.  We are in the process of analyzing all the video footage we collected as well as writing up the results of all experiments.  As a scientist, my time at Oceans Research was invaluably productive.  But the credit goes to the interns who worked relentlessly and enthusiastically in my projects, and to the field specialists and directors who made sure I had access to all the equipment and resources I needed.  I am very much looking forward to my return next summer.  

Fig. 2.  Rachel Parker, an intern at Oceans Research , checks under the microscope to determine which snails - when glued by their shells to limit the snails’ escape responses - have been eaten by the flatworms (photo E. Cruz-Rivera).

By Edwin Cruz-Rivera, Ph.D.
Biology Department, The American University in Cairo
Oceans Research Research Associate

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