Tuesday, April 5, 2011

Cleaning Station Alpha

You're a large wild animal. You itch. You scratch. You attract little critters that you can't reach. What do you do?

You are a small wild animal. You search. You hunt. You need to find those juicy morsels to keep your little belly full. What do you do?

Ectoparasites are any parasites that live on the exterior of another organism. They are pests and can be nuisances and detrimental effects for large animals. However, a mutualism has evolved between large pest-riddled animals and smaller organisms which feed on these pests. You are probably familiar with pictures of small birds sitting on large mammals. These birds get the benefit of an ectoparasite meal and the large mammal gets the benefit of no ectoparasites. But did you know that this type of mutualism doesn't just exist in terrestrial ecosystems?

A wide variety of small marine organisms clean external parasites (and dead skin) from other marine organisms (clients), typically other fish. These cleaners include wrasses, gobies, shrimp, cichlids, etc. Some species of cleaners will even set up a cleaning station where client fish stop in, partake of the cleaning service, and then swim away. Studies have shown that reef fish will actively visit cleaner fish to have parasites and dead or infected tissue removed. These studies typically investigate the station itself, including which species sets it up and which species partake of its services, or the behavior of the fish at the station, what keeps the cleaner fish honest and not taking little bites of healthy client tissue. There have also been several studies in various systems that have failed to show, quantitatively, any benefit to the clients, suggesting that cleaner fish are "behavioral parasites." In other words, they exploit the sensory system of the clients to obtain food, they do not increase the fitness of the client.

A new(ish) paper in the journal PLoS ONE investigates the interaction of cleaners and pelagic shark species at a seamount. Seamounts are hotspots of biodiversity in the open ocean, acting as stepping-stones for marine species to spawn and dispense their larvae. They also serve as important habitats for visiting large marine vertebrates, such as sharks, potentially acting as social refuges. This study looks at the pelagic thresher shark (Alopias pelagicus). This shark reaches 12 feet (365 cm) long, most of which is a long tail fin, and inhabits warm and temperate offshore waters circumglobally. It is known that sharks that are infected with ectoparasites suffer from a variety of health consequences including anaemia, retarded reproductive organ development, reduced respiratory efficiency, debilitating skin disease, and infections. Therefore, it is likely that visiting a cleaning station would be beneficial to the sharks.

This paper quantified the behavioral interactions between pelagic thresher sharks and cleaner wrasse to test if cleaners selectively forage on specific areas of shark clients and if shark clients modify their behavior to facilitate inspections from the cleaners. The researchers sampled fish at the Monad Shoal, in the Visayan Sea, due east from Malapascua Island, Cebu, in the Philippines. It is a seamount that rises 820 feet (250 m) from the sea floor with the top forming a plateau at 50 to 65 feet (15 to 25 m) depth. They selected five cleaning stations and set up remote video cameras to observe the behaviors of the fish. They identified the areas, or "patches," of a shark's body known to harbor high concentrations of parasites to see which patches the cleaners were inspecting the most. They categorized the behaviors in order to differentiate the behavioral patters of the sharks while they interacted with the cleaners including swim speeds and direction of locomotion and posing patterns.

The researchers found that the cleaners showed preferences for foraging on specific patches of a sharks' bodies with the highest percentages of inspections occurring on the pelvis, pectoral fins, and caudal fin. These results, particularly the high concentration of cleaning in the pelvic region, likely reflect the distribution of ectoparasites on the bodies of the sharks. There was no preference for time of day or shark sex, but there was a positive correlation between the amount of time a shark spent at a cleaning station and the number of inspections it received. The authors suggest that sharks that spent more time at a cleaning station harbored more ectoparasites. A head-stand or tail-stand posing behavior is classic to reef fish clients at cleaning stations that act as signals to solicit a cleaning interaction, but require fish to pump their gills or lie still to be cleaned. However, the thresher shark, like most oceanic sharks, is what is called an obligate ram ventilator, meaning that it must constantly swim to maintain oxygen ventilation, or to "breathe." To get around this problem and still advertise they they want the cleaning service the sharks performed a behavior called "circular-stance-swimming." The shark slows its routine swim speed while assuming a head-up swimming attitude and lowering of the caudal fin and then performs slow circular swims in/around the cleaning station.

This study suggests that cleaner wrasse play an important part in the structure of seamount communities. The cleaners show selective behavior in cleaning high parasite areas which may make them more attractive to the clients. Pelagic thresher sharks regularly visit the cleaner stations and even modify their behavior to facilitate cleaning. Overall, a very interesting interaction.

Here is the paper - and it is from PLoS ONE so it is free!
Oliver, Simon P., Nigel E. Hussey, John R. Turner, and Alison J. Beckett. (2011) Oceanic sharks clean at coastal seamount. PLoS ONE: 6(3), e14755. (DOI: 10.1371/journal.pone.0014755)

(images from reefguide.org and , respectively)
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