PARASITE BENEFITS HOST: COPEPOD ON SHARK EYES
Post # 12
Donald A. Windsor
The copepod parasite that attaches to the eyes of Greenland and Pacific sleeper sharks seems to be too harmful to persist through evolution. Surely animals with their vision impaired by this parasite would be at such a competitive disadvantage that they would not live long enough to reproduce. Yet they do. In fact, Greenland sharks (Somniosus microcephalus) are the longest lived animals (1).
The standard story is that Somniosus sharks do not depend upon their eyesight to feed and reproduce (2). But, what if they did depend on their parasitic copepod?
Perhaps the copepod parasite (Ommatokoita elongata) confers some benefit to its shark hosts. I have long wondered about parasites conferring benefits to hosts but see it at the species level (3). Maybe this grotesque case of the shark and the copepod provides an example at the individual level. In fact, also at the species level because almost all individual Greenland sharks, 99%, are parasitized by this copepod (4).
Such a high level of infestation makes me wonder if this copepod is functioning as an organ. If so, then it is benefiting its host. Berland calls it a mutualism (4). Of course, all mutualisms benefit their hosts, by definition.
Perhaps the copepod acts as a vision enhancer that enables its shark to see better. Perhaps the copepod is very sensitive to vibrations and transmits the changes in vibrations into the eye and directly to the brain.
Many aquatic animals that dwell in murky water have appendages (catfish) or whiskers (seals) that do the same thing. Some aquatic animals send out electric pulses (eels, platypus) to assist their vision. Perhaps these copepods are involved in a similar process.
Perhaps in the distant past, the Greenland sharks were losing their eyesight and only those individuals that had this copepod parasite were the ones who survived.
The best explanation (so far) seems to be the one offered by the anonymous Norwegian fishermen who told Berland (4). They claimed that the parasitic copepods lure prey to the sharks. These fishermen contended that the copepods were luminous, but Berland could find no evidence for this claim.
Greenland sharks are very sluggish and Berland wondered how they managed to catch anything to eat. So luring prey, rather than chasing it, may indeed be the answer. The chief meal in Berland’s 1961 investigation was Char. However, Neilsen in 2017 reports that seals were a common food (1).
My conclusion to this bizarre case is that perhaps what we often call parasitism is really mutualism.
1. Nielsen, Julius. Dating a Greenland shark. Natural History 2017 February; 125(2): 10-13.
2. Benz, George W. ; Borucinska, Joanna D. ; Lowry, Lloyd F. ; Whiteley, Herbert E. Ocular lesions associated with attachment of the copepod Ommatokoita elongata (Lernaeopodidae: Siphonostomatoida) to corneas of Pacific sleeper sharks Somniosus pacificus captured off Alaska in Prince William Sound. Journal of Parasitology 2002 June; 88(3): 474-481.
3. Windsor, Donald A. Parasites benefit their hosts – at the species level. parasitesdominate.blogspot.com Post #7 on 16 March 2014.
4. Berland, Bjørn. Copepod Ommatokoita elongata (Grant) in the eyes of the Greenland shark – a possible cause of mutual dependence. Nature 1961 August 19; 191(4790): 829-830.