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)

Hammering Away

Hammerhead sharks (Family Sphyrnidae) get their name, and are easily recognizable, due to their uniquely shaped heads which are laterally expanded and dorsal-ventrally compressed (the cephalofoil) with an eye at each end. The group itself is approximately 20 million years old, and after its initial evolution it underwent divergent evolution producing species of various head and body sizes and head shapes (much of this evolution occuring within the last 6 million years). Body sizes range from as small as 3 feet (just under 1 meter) to the largest, the great hammerhead (Syphyrna mokarran), at 18 feet (about 5.5 meters). The bonnethead shark (Sphyrna tiburo, from Caribbean and tropical eastern Pacific Ocean) has the least laterally expanded head (18% of the body length) while the winghead shark (Eusphyra blochii, from Australia) has the most laterally expanded head (50% of the body length). These two species also have the most divergent cephalofoil shape as well as some of the smallest body sizes at maturity.

There are various hypotheses as to the function of the cephalofoil. One popular hypothesis is that the wing-shaped head provides lift and greater maneuverability in the water. Sharks are also known for their electroreception abilities, and it is speculated that the lateral head extensions enhance this capacity, making prey detection and capture more efficient.

A new paper published in Molecular Phylogenetics and Evolution analyzes mitrochondrial and nuclear DNA to infer the phylogeny for all species in the group. They took DNA from each of the eight hammerhead species to construct a new phylogenetic structure ("family tree" or "gene tree") for the group. The researchers used four mitochondrial genes and three nuclear genes (amounting to 6292 total base pairs in the study). Mitochondrial DNA is maternally inherited and nuclear DNA is from both parents, and these DNA types undergo differing mutation rates. By tracking the mutations in the genes, researchers can look at evolution of a species/group over time. Using this technique, they found that large (>200cm) and small (<150cm)>

Why the evolution of small body sizes? One reason may be in their development, specifically progenesis or neoteny (adults retaining juvenile characteristics). A smaller body size means that the cephalofoils may not provide as much lift in the water, but, that's ok if you develop gain other functions such as enhanced binocular vision, prey capture, and/or maneuverability. Also, all small-bodied sharks are restricted to continental shelf habitats while large-bodied sharks tend to be pelagic and circum-globally distributed. This could reflect their evolutionary origins, as increased size allows for trans-oceanic movement and colonization - either as small sharks evolving big, moving, then evolving some lineages small again or a large widely distributed shark evolved into smaller species.

So what does all this boil down to?

"The new phylogenetic hypothesis does not challenge the existing classification and taxonomy of the family Sphyrnidae. Nonetheless, we note that proposed subgenera remain paraphyletic. Continued recognition of two distinct genera (Eusphyra and Sphyrna) makes sense given the monophyly of the genus Sphyrna and the degree of divergence between Eusphyra and Sphyrna. If there is a need for subgeneric taxonomic categorization, we advocate using the inferred phylogeny as a guide for defining monophyletic subgenera."

That's just how I would have said it :o)


Here's the paper:
Kim, Douglas D., Philip Motta, Kyle Mara, Andrew P. Martin. (2010) Phylogeny of hammerhead sharks (Family Sphyrnidae) inferred from mitochondrial and nuclear genes. Molecular Phylogenetics and Evolution: 55(2), 572, (DOI: 10.1016/j.ympev.2010.01.037)

Here's a write up (but reading the above paper is actually easier and more informative):
http://www.sciencedaily.com/releases/2010/05/100518113132.htm


(images from livescience.com and arkive.com via seapics.com respecitvely)