April 9, 2012 • 2:47 pm

“Commensalism” is defined as an interaction between individuals of two species in which one individual gains something while the other neither gains nor loses. A possible example is that of the sea urchin crab, Echinoecus pentagonus, that lives in the anal pore of sea urchins in the Indian and Pacific Oceans. Here are two photos of the tiny beast (ca.1.3 cm, or 0.5 inch, across) in situ.  I can imagine that the crab gains protection (and possibly some food) from living in the hole, while the urchin suffers at best mild debilitation. (Photo by Keoki and Yuko Stender from

And here’s the reverse situation, a decorator crab (there are several species that have this common name) off Indonesia carrying a fire urchin (probably Astropyga radiata) on its back. The crab clearly gains protection by looking like an urchin, while it’s not clear what the urchin gets (perhaps a bit of extra food and freedom from competition by getting the ability to move about.

h/t: Matthew Cobb

20 thoughts on “Commensalism?

    1. In symbiosis, both parties benefit. In commensalism (as Jerry said), only one party benefits, and the other neither benefits nor suffers. In parasitism, one benefits and one suffers.

      1. I too would have provided these definitions of symbiosis, commensalism and parasitism. However, on searching around the web, it seems that some refer to `our` symbiosis` as mutualism, and use the term symbiosis more generally to refer to any long term association between different species of organism. On this reading, symbiosis would be an umbrella term encompassing commensalism, mutualism and parasitism.

        Could anyone shed any light on the relative usage of the two definitions in biology today? `

        1. You’re right. Symbiosis is the umbrella term for organisms that live in some kind of long-term interaction with each other. Commensalism, mutualism, and parasitism refer to different types of symbiotic relationships.

  1. “…while the urchin suffers at best mild debilitation.”

    I think you mean “at worst”. If mild debilitation were the best possible consequence for the urchin, then this would clearly be a case of parasitism.

    1. Along those lines, maybe Elaine Morgan would like to incorporate that into her Aquatic Ape hypothesis – that it was the crabs that drove H sapiens onto the land, to escape them.

      1. “The use of twin studies in Aquatic Ape theory on location and conformation of buttock clenching action induced by ass crab coevolution” –


  2. A beast that lives in the anal cavity of another beast? Wow. I thought it would be impossible to describe the typical fathiest and Tea Party Moran in such polite terms. But you’ve actually accomplished it.

    I’m impressed.

  3. Calling the first crab an example of “commensalism” may be a bit speciesist on your part. If I had that crab in my anal pore, it would most definitely be a parasite!

  4. The first thing I teach students in medical microbiology (after Casadevall and Pirofski) is that there is no such thing as a commensal organism, only commensal relationships. Staphylococcus aureus, Streptococcus pneumoniae, and Neisseria meningitidis all achieve reproductive success through commensal association with humans, but no one, appropriately, thinks of them as commensals.

  5. I don’t see how an urchin riding on a crab’s back is gaining much benefit. Its mouth is over a fairly small area of crab shell and there can’t be that much algae and other edible stuff in that small area. I wonder if the crab switches urchins regularly to keep them fed and alive.

    1. I expect the urchin arrived quite small, & I do not suppose urchins normally move around for a lot so assuming the urchin is eating what food particles come its way – eg from the crab’s meals, the disadvantage would be when it comes to reproduction. Assuming it cannot self fertilize it would need other urchins nearby to share sperm I imagine float in the sea producing free living young.

  6. I’m sure fans of Peter Cook and Dudley Moore are familiar with this sort of thing. In particular the worst job they ever had which involved a lobster and Jayne Mansfield.

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