Wednesday, August 8, 2012

Hitching a Ride

Phoretic mites on Mitopus morio
Being a mite, by definition, means you're going to be small. Which is useful in habitats, such as soil, where an ability to squeeze into small soil pores is an obvious advantage. It is therefore not surprising to find that of the 684 species of Acari (mites and ticks) present in Ireland, many of them are microscopic (1). Even those that are comparatively large are rarely over a couple of millimeters in length. Problems arise with this small size when a habitat becomes no longer able to sustain a mite and it needs to move on. Their minute size and soft bodies means that travel over any considerable distance is a daunting and potentially life threatening prospect.

Phoretic mites
So what to do to avoid such perilous journeys? Why, get the bus of course, in a manner of speaking. Certain species of mites, ticks and other arthropods engage in a behaviour known as phoresy, which involves a smaller organism (phoront) hitching a ride form a larger (host) for a limited amount of time (2). Not only does this facilitate movement from areas that are no longer able to sustain the phoront, but it also allows the establishment of new populations in areas that could not otherwise be reached. Often, shelter and/or food are provided indirectly as a result of such a relationship. However contravenes the strict definition of phoresy and can no longer be considered as such. Phoronts are generally smaller (usually much smaller) that their hosts and may occur in large numbers. As a method of dispersal it has existed for quite a long time, with examples existing in Baltic and Dominican amber from 40 million years ago (2).
Mites show the most impressive radiation of phoretic relationships, due mainly to selective pressures brought about by their small sizes. This is further compounded by the transient nature of many of their habitats. As phoronts, they us not only terrestrial hosts, but aquatic and aerial. The picture illustrating this post show two phoretic red mites attached to the rear leg of a harvestman, Mitopus morio.

  1. Ferriss at al., 2009. Irish Biodiversity: a Taxonomic Inventory of Fauna p. 78
  2. Houck, 2009. Encyclopedia of Insects pp. 772-774

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