The five-star marine restaurant and spa.

By Hope Milo, Hollins University, Roanoke, Virginia ~Edited by C.E. O’Brien, Ph.D., both from The School for Field Studies, Center for Marine Resource Studies, South Caicos, Turks & Caicos Islands

Coral reefs are well known for their abundance and biodiversity, teeming as they are with multitudes of organisms hovering over heads of flower coral or ducking inside tunnels and hidey holes. In studying these bastions of diversity, researchers have discovered similarities between these marine ecosystems and our own bustling towns and cities.

This Nassau Grouper assumes a stationary, head-up position with its mouth open, a common pose at cleaning stations.

Indeed, the organisms that inhabit the reefs are quite used to living communally, as these species have co-existed for millions of years. The immense biodiversity within the rich but confined coral ecosystems has over time driven adaptive radiations of intriguing behaviors that increase organisms’ likelihood of survival and success. For instance, some fish swim together, some live together, and some even hunt together. Within species, cooperation confers benefits to individuals such as protection, coordination, and more efficient dissemination of information. However, there are also examples in which evolution results in cooperation between species. This cooperation is known as symbiosis, quite literally meaning “living together,” and it refers to a relationship in which two species live in close proximity and accrue some advantages (and sometimes, disadvantages) through their association.

There are three primary types of symbiosis, each describing the benefits—or detriments—the species involved in the relationship exchange. The first of these is parasitism, where one species benefits at the expense of another species, referred to as a host. A second type is commensalism, in which one species benefits from an association with another while the other is neither harmed nor helped. The third type, mutualism, involves two species forming an alliance that is advantageous for both. Mutualism is further subdivided into facultative and obligate forms. The former describes a beneficial relationship in which an organism could survive without the other (it is helpful, but not necessary) while the latter describes one which an organism requires for survival. 

Much like humans, reef fish are not immune to the plague of diseases or ectoparasites (“outside parasites,” think mosquitoes or ticks) that afflict us. Naturally, they lack any trained medical professionals to deal with them or, of course, any hands to swat them away. Luckily for these germ-riddled fish, an effective—and quite astounding—mechanism that “mass produces” mutualism across countless species has arisen over millions of years of evolution:  cleaning stations. It is as if aquatic entrepreneurs sensed a demand and sprang to action to meet the need.

Imagine, if you will, a drive-thru spa or even dentist, someplace you could drive up to and get a quick and convenient exfoliation or tooth-cleaning. That is essentially how these cleaning stations function on reefs. Client fish—those individuals seeking the removal of dead skin or parasites—will swim up to these stations, usually populated by gobies (in the genus Gobiosoma), juvenile or initial phase Bluehead Wrasse (Thalassoma bifasciatum), and Pederson Cleaner Shrimp (Periclimenes pedersoni). They will strike a stationary, almost trancelike pose, letting these cleaners know they have a new client in their midst. The cleaner fish, for its part, will then get to work, snatching up any ectoparasites or erstwhile skin clinging to their clients, even venturing inside potentially risky or sensitive areas such as their mouth.

This Great Barracuda, a fearsome predator, is being cleaned by several Sharknose Gobies.

Both the client and the cleaner benefit from this. The client gets cleaned, while the cleaner acquires a tasty snack. Whether this relationship is facultative or obligative (i.e. essential to survival or just a nice perk), however, is species-specific. While it’s usually facultative for the client, it can be obligative for the cleaner, as some depend entirely on cleaning for sustenance. Interestingly, some cleaner species, like the Sharknose Goby (G. evelynae) and Pederson Cleaner Shrimp (Periclimenes pedersoni) are cleaners all their lives while others, like the Bluehead Wrasse (T. bifasciatum), are only cleaners during one stage of their lives, usually when they’re young.

In order to advertise their services, cleaner fish have evolved a “uniform” of sorts—a particular color scheme which signals to potential clients that they’re “open for business.” Where many marine organisms attempt to be as inconspicuous as possible (like the flounder, who’s flat, tan body is almost undiscernible against the sandy sea floor), cleaners go the opposite route. They deck themselves out in bright colors that have been shown to provide the most distinct contrast to their surroundings:  blue, which has been shown to stand out the most when juxtaposed with a coral reef background, and yellow, which offers a stark contrast against the blue of the open water. These colors are made even more obvious by the tendency of the cleaner fish to have a black lateral stripe along their sides, making them stand out all the more.

Not every organism has to visit a cleaning station to get its spa treatment. Some larger organisms have their own personal cleaning attendants. Turtles, whales, sharks, rays, and other large fish often have one or more remora fish that travel along with them. These “groupies” attach to larger animals (or sometimes people or boats) with a modified dorsal fin that forms a suction cup. This mostly mutualistic relationship benefits remoras with free transportation, as well as food scraps and tasty skin parasites from the host organism, and benefits the host by keeping it parasite-free.

Alongside cleaning, cleaner fish have also been observed providing another service—tactile stimulations, or “touch therapy.” Research demonstrates that fish enjoy and even benefit from the sensation of touch. In fact, tactile stimulus has been shown to lower cortisol (the hormone associated with stress) levels in fish. It has even been shown that fish actively seek out these tactile sensations, as they have been observed posing for cleaners even when free of ectoparasites. 

Like your favorite spa or massage studio, cleaning stations are also peaceful retreats from the chaotic world. Studies have shown that in the presence of cleaner species, aggression from piscivores (fish-eaters) on nearby prey species decreased by 50–60%, depending on species. This fact may be attributable to the “touch therapy” cleaner fish employ as a method to avert any potential conflict from ever arising.

This Lemon Shark is enjoying a “spa treatment” from its attendant remoras.

Some cleaners do a better job than others. Within this class of cleaning individuals, some fish may occasionally “cheat,” meaning that instead of nibbling off any ectoparasites, they nip at the client fish itself, removing their much-needed scales or mucus. In order to combat this, clients have been shown to prefer cleaners that provide better—and less painful—services, refusing to revisit “cheaters,” or even spurning cleaner fish that aren’t attentive to their needs. 

These “cheating” behaviors have called into question the true nature of these cleaning stations. While they do, undoubtedly, provide a crucial service to many clients, increasing fish biodiversity and abundance, the spectrum of cleaning behavior—from mutualistic to parasitic, as when those nefarious “cheaters” nibble at their hosts—show that nature doesn’t always fit neatly into human definitions and demonstrates the sheer amount of diversity of lifestyles present in the natural world.

Indeed, cleaning stations and other instances of symbiosis on coral reefs indicate that evolution may promote higher levels of coordination among the species living in densely packed environments. If a coral reef is like an underwater city, then cleaning stations are truly akin to marine spas: a place fish may go to enjoy a time of rest, relaxation, and rejuvenation.

For detailed article references or more information about The School for Field Studies, contact Director Heidi Hertler on South Caicos at hh******@fi**********.org or visit www.fieldstudies.org.