September 2007
by Christopher Meehan, Villanova University

Mutualisms, or mutually beneficial interactions between different species, are widespread in all of life. They offer windows into our understanding of ecology and evolution, especially when they are tightly closed systems or "obligate." One famous obligate mutualism, that of corals and their endosymbiotic algae, is easy to recognize on a dive or snorkel in Akumal Bay. However, another obligate mutualism important to biologists and common around Akumal is that between acacia plants and their resident ant-guards. Look closely and you will see ants chasing off potential predators (or clumsy humans!) as well as feeding/living off the acacias. In ant-acacia mutualisms, the ants constantly defend the plant against predators and competitors, and in exchange receive food (as nectar and leaf tips rich in fat and protein) and housing (inside hollow swollen thorns); neither ant nor plant can survive without the other. However, the mutualism does not exist in isolation and must deal with foreign species that attempt to "cheat" or exploit the system without returning any benefit.

Look even closer at some ant-acacias in Akumal and you will notice a small spider living on the plants in large numbers. This spider has developed a clever way to get around the defenses of the ants and live, breed and feed on the acacias. Called a "jumping spider," it belongs to the largest family of spiders, Salticidae, which uses great eyesight to hunt instead of building a web to catch food.

So what does this spider hunt, and why does it bother to tolerate the relentless ants? Well, it doesn't so much "hunt" as it robs—and it robs the yellow leaf tips, called "Beltian bodies," from the ants! Although it preys on baby ants (larvae) and drinks nectar, its main diet comprises the leaves that evolved to feed ants. Out of over 40,000 species of spider, it is the only one in the world that eats plants. And since spiders can't eat but only suck their prey's insides, it means that it must liquefy its salad and drink it like a milkshake! Biologists around Akumal are studying this unique spider to learn how the spider lives and fits into its surroundings as a "vegetarian." This little critter can make a big contribution to science by revealing how a single species can evolve from eating meat (carnivory) to plants (herbivory), and also how an obligate mutualism changes when a "cheating" third species joins the party. Mother Nature works in weird ways—yesterday's science fiction might turn out to be tomorrow's reality!

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