Horsehair Worm

During a stop at a shallow beach in the summer of 2018 I saw what initially looked like a piece of black wire on the sand just beneath the shallow water as I walked along the shoreline.  I stopped to take a closer look and noticed that it appeared to move which seemed a bit odd because there wasn’t a current and it was not a wavy day.  As I stood there and watched the thin, “strand-like” creature about 30 centimeters long began to wriggle which propelled it through the water.  It turns out that I stumbled upon an adult horsehair worm.  Also referred to as string snakes or Gordian worms these animals are placed within the Phylum Nematomorpha.

According to Walter Dodds and Matt Whiles (in Freshwater Ecology, second edition, 2010), these multicellular parasitic animals are about 3 mm wide and can several centimeters or even up to 1 meter long.  The adult is reproductive but does not ingest food as it lacks a functional digestive tract.  Instead, the successful sexually reproductive adults lay eggs that hatch and become larvae.  The larvae then attaches (as a hardened cyst) onto vegetation and is available to be ingested by a host animal (typically an invertebrate such as grasshoppers and crickets).  In order for the parasitic adult horsehair worm to emerge from the body cavity of the host, the host species (even if not aquatic) must be within a marine environment.  Research (Thomas et al. 2002 “Do hairworms (Nematomorpha) manipulate the water seeking behaviours of their terrestrial host?” in Journal of Evolutionary Biology. 15(3):356-361) indicates that the parasitic horsehair worm may affect the host’s behaviour causing it to seek out water.  This ensures that the developed adult worm can emerge within the required aquatic environment.

On the bright side, only “some” of the horsehair species are parasitic on humans.  Hmmm, but which ones?  Just when you thought it was safe to go into the water.  However, from a totally different perspective, it probably makes a leech or bloodsucker “latch-on” event seem like just a walk in the park.  Just saying……