This article originally appeared in The Shark Tagger-1998 Annual Summary
George Benz is Director of the Southeast Aquatic Research Institute and is employed by the Tennessee Aquarium. George has studied parasites of sharks for over 20 years and has often collaborated with biologists at the NMFS Narragansett Laboratory. Susan Dippenaar also studies parasites of sharks, and is a Lecturer and Ph.D. student at the University of the North in South Africa. George and Susan have provided the following overview of the general biology of copepods, the common parasites of sharks.
Most sharks are apex predators in marine ecosystems. However, this top-dog designation does not stop many hundreds of smaller organisms from relentlessly pursuing them as their home and source of food. Known as parasites, the number of these species probably far exceeds the number of sharks themselves - and this shouldn't surprise us when we consider the unsettling thought that there are probably far more parasite species on Earth than non-parasite species.
Derived from a Greek word meaning one who eats at another's table or one who lives at another's expense, a parasite is usually a smaller organism that harms a larger organism known as a host. Individual parasites usually do not kill their hosts, rather they live in close association with them and feed on or rob nutrients from them. In this way they impose an energy tax on the host that affects its ability to survive. Good parasites do not typically overtax their hosts. And, by not doing so, they ensure their own continued survival.
Many types of parasites pester sharks, including various viruses, bacteria, protozoans, worms, crustaceans, and even fishes. Among the crab-like critters, a.k.a. the crustaceans, a group known as the copepods has been exceptionally successful.
Copepods account for about 32% of all crustaceans (some 10,000 of 31,000 described species). While most of them (about 75%) are free-living, approximately 15% (1,500 known species) are parasites of fishes. A group of copepods called the siphonostomes has been most successful at parasitizing fishes, including sharks. These copepods possess a relatively long mouth tube that typically contains two sinister-looking saw-toothed mandibles that are used while feeding.
Virtually all sharks common to the western North Atlantic are tormented by copepods. The blue shark possibly holds the record for hosting the greatest variety and total number of copepods. In fact, close examination of a single blue shark from this region will probably reveal it to be infected by at least five species of copepods - including the Darth Vader look-a-like Pandarus satyrus that attaches in dense clusters on the pectoral fins, the nose inhabiting Kroeyerina elongata, the gill inhabiting species Kroyeria carchariaeglauci and Phyllothyreus cornutus, and the body pestering Echthrogaleus coleoptratus. All totaled, these copepods may number 3,000 or more per blue shark, with several hundred living on the fins and body, several hundred living in the nose, and several thousand more living on the gills.
Although the tax imposed on a shark by any individual copepod may be negligible, the overall burden inflicted by all of its parasites may be very significant. For example, research has shown that Nemesis lamna, a copepod that commonly infects the gills of the shortfin mako, causes modifications to the gill filaments that disrupt water flow. This suggests that if a mako became infected by a large number of these gill-clogging copepods, its ability to oxygenate its blood would be severely impaired and its ability to grow or even survive would be greatly compromised.
Sometimes, single copepods can radically affect the sharks they infect. For example, studies have shown that the eyes of virtually all Greenland sharks are infected by the large copepod Ommatokoita elongata. Typically, a single adult female infects the cornea of each eye. As she matures, the copepod embeds an anchoring device into the cornea and permanently tethers herself to it. There she remains throughout her life, feeding on and rubbing against the eye. Ultimately she causes such damage that the shark becomes partially blind - its eyes functioning more like photographic light meters than image-forming cameras.
How do these partially blind sharks survive? It has been reported that some Norwegian fishermen thought that the large, brightly colored copepods attached to the eyes of Greenland sharks might actually serve as lures that attract curious prey to this sluggish shark. However, as alluring as these copepods appear, no firsthand evidence supporting this notion has ever been documented.
Interestingly, a more reasonable and scientific explanation for the aforementioned Greenland shark riddle exists. If size is any indication, the Greenland shark appears to have a powerful nose. Therefore, it's possible that this shark uses smell as its primary means of detecting prey as it swims above the ocean floor sucking up small fishes and invertebrates. Recent studies suggest that the ancestors of the Greenland shark were most likely deep-water species that may not have relied heavily on eyesight. Like the deep ocean, Arctic waters are cold and dark for much of the year. Hence, the Greenland shark may have been pre-adapted to its partially blinded task of capturing prey beneath the ice in cold, dark, food-rich Arctic waters.
It's interesting to consider how successful copepods have been at colonizing specific locations on sharks. For example, Pandarus smithii typically attaches on the roof of the mouth and on the tongue of the shortfin mako. Gathering in tight clusters, these copepods are certainly well protected by the rows of dagger-like teeth only inches away. However, Anthosoma crassum really takes the top prize when one considers the routine attachment locations of copepods. Vaguely looking like a dark acorn, this large copepod excavates deep lesions between the teeth of the white shark where it routinely lives and feeds. Hence this truly is a copepod that literally puts the bite on Jaws!
Of course, where a copepod lives often determines what it will eat. While we are only beginning to learn about the diets of copepods that infect sharks, we can consider that copepods that actively move about the body surface probably feed on fish mucus and the uppermost layers of skin. More stationary copepods often form tighter associations with their shark hosts such that they excavate deeper feeding wounds while consuming deeper layers of the skin and/or blood.
Some parasitic copepods literally go to great lengths to acquire their food. For example the extremely long copepod Kroyeri caseyi, a species named in honor of retired NMFS shark specialist Jack Casey, burrows so deeply into the soft gill tissue of night sharks that only a small percentage of its body remains exposed.
Like other crustaceans, parasitic copepods produce a larva known as a nauplius. The nauplius is a free-living stage in the copepod lifecycle that swims in the water. As such, it provides a vital service to parasitic species by allowing new generations of parasites to spread from shark to shark. Therefore, the lifestyles of egg-bearing female copepods must provide the hatching nauplii an opportunity to assume a free-living existence if their species are to survive. Because of this, parasitic copepods are usually ectoparasites on their shark hosts (i.e., they remain on the surface of the shark).
Throughout 20 or so years of studying parasitic copepods of sharks one of us (GWB) has often held, poked and prodded various species in hopes of determining whether or not they can bite humans. To date these attempts have been unsuccessful, and thus these remarkable parasites are either not equipped to do so or they merely prefer to turn the other cheek. In either case this should provide some confidence for those curious readers that may wish to more closely examine the copepods that infect the sharks that they catch and, be assured, that these do not make the shark unfit for human consumption.