Mutualisms and Aquatic Community Structure: The Enemy of My Enemy Is My Friend

Mark E. Hay, et. al.

Using ample aquatic anecdotes, Hay provides a context for a wide range of species interactions and how predation and competition are not the only interactions that influence community structure. Hay and friends argue that the oft-overlooked positive interactions, such as mutualisms, also play a critical role in community development and structure.

One reason that mutualisms are commonly left out is that many in the scientific community have an oversimplified definition and understanding of a mutualism. Historically, mutualisms have been limited to tightly coevolved beneficial interactions between two species. However, upon more careful examination, mutualisms are much more common and widespread than previously acknowledged. Under the right community and environmental conditions, interactions can shift and those that were previously negative, such as competition and predation, could potentially become mutualistic. This broader view of mutualism includes a multitude of interactions that play a critical role in developing and maintaining community structures and functions.

Communities are often defined by prominent foundation species. These species generally provide the structure and framework of the community by having a relatively larger influence on the community's species composition. Hay provides a few examples of mutualisms between a foundation species and other species that provide some form of benefit back.

Corals form the foundation of the complex coral reef communities along with their photosynthetic zooxanthellae symbionts, without which many coral reef systems would not survive. The zooxanthellae provide carbohydrates to the coral via photosynthesis while the corals provide the zooxanthellae with nitrogenous nutrients. Historically, corals were thought to have coevolved with a specific species of zooxanthellae. However, recent studies have shown that corals can reject and acquire new zooxanthellae symbionts in response to changing environmental conditions. For example, when moved from low-light to high-light conditions, the corals that replaced their zooxanthellae had a lower mortality rate than those that did not acquire new symbionts.

Microbes will often defend their hosts in order to increase survival of both species. One example is of shrimp embryos that are covered by a species of bacteria. The bacteria produces a chemical that protects the shrimp embryos, which act as a growth surface for the bacteria, from a pathogenic fungus. Without the bacteria, the fungus kills the embryos.

Pocilloporid corals commonly harbor a crab and shrimp to act as bodyguards and ward off attackers. The two species will shelter in the corals where they are provided nutrients and are less vulnerable to predation and in return they protect the corals from attack by starfish. The presence of pocilloporid corals protect other corals from starfish attacks despite the fact that they will compete for space and resources - exemplifying the complex interactions that can be involved with mutualisms.

Hay et al conclude the review with a number of examples of interactions that are antagonistic in relation to the two species involved, but become mutualistic when the broader community interactions are considered. For example, interspecific competition is almost always regarded as a negative interaction for both competing species. Congeneric scale-eating cichlids share the same prey resource. However, when two congeneric species of cichlids are present, attack success increased since prey fish could not be as vigilant against multiple attack strategies.