"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

July 22, 2010

July 22 - Ommatokoita elongata

If you find the idea of having something lodged in your eye distressing (ok let's face it, who doesn't?), then today's parasite is probably your worst nightmare. Fortunately for you, it is not a human parasite. The hosts for today's parasite are Greenland sharks (Somniosus microcephalus) and Pacific sleeper sharks (Somniosus pacificus) - both large deep water sharks. Ommatokoita elongata is a parasitic copepod, approximately 5 cm in length (almost 2 inches) with a very specific and truly cringe-worthy preference about where it attaches on to the host.The adult female copepod attaches herself to the shark's eye with an anchoring structure call the bulba, and grazes on the surface of the cornea (see photo, black arrow indicates attachment point), hanging off the eyes of the shark like a grotesque tassle

There are two possible reasons for the copepod's attachment site. Shark skin is covered in microscopic, teeth-like structures call denticles which can make it difficult for parasites to attach themselves to skin (though some species of parasitic copepods
manage). Secondly the eye is considered to be a "immunologically benign environment" for parasites, thus such an attachment is less likely to illicit an immune response.

While the parasite can cause significant damage to the cornea and result in blindness for the host, most sharks seem unaffected by the presence of the parasite and many sharks have the copepod in both eyes, strangely enough. This goes to show when considering the virulence (harmfulness of a parasite to its host) of a parasite, it is worth taking into account the perspective of the host involved - what may seem debilitating to us may not necessarily be the case for the actual organism in question.

Photo source: Borucinska, J.D., Benz, G.W. and Whiteley, H.E. (1998) Ocular lesions associated with attachment of the parasitic copepod Ommatokoita elongata (Grant) to corneas of Greenland sharks, Somniosus microcephalus (Bloch & Schneider) Journal of Fish Diseases, 21:415-422

Also some good photos of live Greenland sharks with the parasite can be seen in Caloyianis, N. "Greenland Sharks." National Geographic 194, no. 3 (1998): 60–71.

Contributed by Tommy Leung.


  1. Nice blog. Creepy but nice :)
    And I have a question: is it true that about 25% of living things are parasites? (I have read in Carl Zimmerman's book, Parasite Rex)

  2. It's really hard to estimate, but I would feel confident in saying that probably at least 25% of organisms are parasites. They're often under-sampled, under-studied, and can show convergence or other forms of cryptic diversity.

  3. Actually, I'll probably go as far to say that at least 50% of life are parasites or parasitic - if by parasite you mean an organism that lives obligately in or on the body of another organism. In addition to the likes of worms and lice and other macroparasites that most people think of, you also have single-celled things like Plasmodium (malaria), Trypanosome (T. brucei for sleeping sickness, T. cruzi for Chagas disease), and the infamous Toxoplasma gondii. Apart from all those parasites of animals above, you also have parasites of plants, as well as parasitic plants themselves...then you have pathogenic bacteria and virus...a sprinkling of brood parasites here and there...

    In fact, I would feel confident to say that the vast majority of living things are parasitic - it is probably the most common way of life on this planet! We are merely the visible minority hosting the hidden majority...

  4. If you go with prokaryotes being part of "living things" - which of course, they are, then maybe the stats go down. But, out of eukaryotes, parasites have definitely got to be the majority, don't you think?

  5. Well, consider that every species of animal (I'm not even including plants here) is pretty much infested with an entire community of parasites even if you just go with the eukaryotes, and every different species has a different parasite community with various degrees of overlap, the inevitable conclusion is that free-living organisms are vastly outnumbered by parasitic ones living in or on them.

    This should be on a T-shirt: "There are two ways to live on this planet: Infecting or Infected"

    Actually...maybe there should be a third because of hyperparasites, but that also add another level to parasite biodiversity too...

  6. Is it possible to know books about sharks parasites to buy? Or resources that I can read in internet? Thanks a lot. Andrew

  7. There don't seem to be any books which are *only* about shark parasites. The marine parasitology book by Klaus Rohde cover shark parasites a little bit, but everything relating to shark parasites are found all over the place in various parasitology, marine biology, and shark biology texts.

  8. If parasites truly discuss you, do not research what lives at the base of everyone's eye lashes. Too small to see with the naked eye,(is there a pun in there). We are all, and other creatures, hosts of some kind. Even parasites are hosts to others. What I was looking for here however are parasitic cope pods, often found on shark fins, gills, mouths, etc. They are visible, 6 to 8 centimeters. This was interesting also. Thank You

  9. Hi.
    Im just trying to figure out why they choose the Greeland Sharks, and how?
    Do they ley eggs in there eyes? Do they just flowt into it?

  10. Unfortunately, not much is known about the life-cycle of this parasite at this point. Related species of parasites have planktonic larvae that drift through the water, so perhaps when they encounter a shark, they latch on and make their way to the eye.

  11. What does this parasite eat? I don't think this is a daft question because it would appear that the animal is neither absorbing fluids from the host nor in a position to feed on left overs from prey and carrion.

    1. It feeds by scraping the surface of the shark's eye. The bulb at the junction of those two "limbs" which the parasite use to attach to the shark's eye is its head. When the parasite is hungry, it simply take a nibble from the surface of the eye right in front of it.