Chapter 17

The red-ridged clinging crab. In shallow-water reefs off the coast of North Carolina, red-ridged crabs remove seaweed from around corals. The corals provide the crabs with a place to live and a source of food.

Photo by Azure Computer & Photo Services/Animals Animals/Earth Scenes.

Life as a tiny crab has its challenges. There is a continuous need to find lunch and, at the same time, to avoid becoming someone else’s lunch. Consider the red-ridged clinging crab (Mithrax forceps), which reaches an adult size of only 2.5 cm. The red-ridged crab lives on the reefs off the coast of North Carolina and spends much of its time among the many branches of ivory bush coral (Oculina arbuscula) where it hides from predators. Researchers find that crab survival is much higher in areas where the coral is present than in areas without the coral.

Ivory bush coral, however, is a poor competitor on the shallow-water reefs. Large species of algae, commonly known as seaweeds, grow very fast and smother the coral. As we discussed in Chapter 2, corals are animals that have a mutualistic relationship with tiny algae, known as zooxanthellae, that live inside them. When the corals become shaded by the smothering seaweeds, the zooxanthellae cannot photosynthesize and both the corals and zooxanthellae die. So how do corals, and therefore crabs, persist on the reefs?

Fortunately for the coral, the red-ridged clinging crab mows down any seaweed that tries to establish itself near the coral. It is able to eat all species of shallow-water seaweed, even those with chemical defenses that repel many species of reef fish. To confirm the importance of the crab to the coral, researchers removed crabs from areas containing coral and found that the rate of death among corals increased fourfold. In short, crabs assist corals by reducing competition from seaweeds while corals assist crabs by providing safe places to hide from predators.

To determine whether corals offer crabs something more than a useful place to hide, researchers gave crabs a choice between living among the branches of live corals or of dead corals. The crabs overwhelmingly chose live corals because, as researchers discovered, the crabs feed on a layer of coral mucus that only exists on live corals. This mucus provides a high-energy food for crabs.

Researchers also discovered that the mutualistic relationship between crabs and corals changed in deep-water reefs where lower light and colder temperatures cause seaweed to grow more slowly. Moreover, one of the common species of seaweed on the deep-water reef is unpalatable to crabs. As a result, crabs are not able to offer any benefit to the coral, but the crabs still use coral for protection and may still consume the high-energy mucus of the coral. This means that the mutually beneficial interaction in the shallow reefs changes to a one-sided benefit in the deep reefs.

The interaction of crabs and corals illustrates how species commonly depend on one another to exist in nature and how a mutual reliance can change under different environmental conditions. In this chapter, we will see a wide variety of ways in which species rely on each other to improve their growth, survival, and reproduction.

In Chapter 1, we defined a mutualism as a positive interaction between two species in which each species receives benefits that only the other species can provide. Mutualisms are common in nature: Corals live with their symbiotic algae, crabs assist corals by removing competing seaweed, and, as we saw in Chapter 10, leaf-cutter ants farm a fungus. In this chapter, we will see how species have evolved to participate in mutualistic interactions for a wide variety of benefits. Some species benefit by obtaining resources while others benefit by obtaining a place to live, aid in defense, and assistance in pollination or seed dispersal. When we consider mutualistic relationships, it is tempting to think that each species is trying to help the other. However, as you might recall from our review of natural selection in Chapter 1, selection favors any strategy that increases the fitness of the individual. Regardless of the specific benefit, there are requirements for the evolution of mutualistic interactions and conditions under which a positive, mutualistic relationship can change to a neutral or negative interaction. Finally, we are interested in mutualisms not only because they are a common interaction in nature, but because they can affect the abundance of populations, the distribution of species, the diversity of communities, and the functioning of ecosystems.