Bugs in the Snow


Jason Shields sent in this photo and granted permission to share. Taken on a snowy hike near Wescott Bay, San Juan Island, WA 2/13 or 2/14 2021. 

The bugs in the snow in this photo are isopods, commonly known as woodlice. Jason sent the photo to me after finding them on top of the snow, scattered about, alive, but moving slowly. He asked if I might know why they were out in such cold weather like this. At the time, I could only speculate, but did some reading and came up with a possibility that could answer this curiosity. 

The literature I sourced suggests that isopods like these can be infected with a helminth parasite capable of altering behavior. Several published scientific papers specifically investigated the Acanthocephalan parasite (Plagiorhynchus cylindraceus), also known as the Spiny or Thorny-head worm parasitizing isopods.  For the curious, the name Ancanthcephalan comes from Greek akantha, and kephale; head, referring to the rows of hooks or spines encircling the head of the worm.  

In this relationship, the acanthocephalan parasitic worm (P. cylindraceus) is what is referred to as an indirect or two-host parasite, meaning it uses two animals to complete its developmental life cycle.  It is also an endoparasite, developing internally in an animal (as opposed to ectoparasite – developing on the outside of the animal). 

The isopod is the initial host in this parasitic cycle. In feeding on terrestrial detritus, the acanthocephalan parasite’s eggs are ingested.  The eggs exist in the environment after being excreted in the feces of songbirds, especially the American Robin (Turdus migratorius), and European Starling (Sturnus vulgaris). 

Within hours of ingestion by the isopod, P.cylindraceus eggs hatch into larvae.  These tiny larvae with spines soon mature into a form called a cystacanth, burrowing through the intestinal cavity of the isopod where they lodge into the body cavity, remaining attached to the intestine by a stalk. This site of attachment is important in that the worms lack their own digestive system and must gain nutrients via the host’s dietary intake.  Over approximately 60-65 days, the cystocanths continue to grow and develop sexual organs.   At the end of this period they are now mature enough to infest the next host (a bird).  

While the exact mechanism remains unknown, the worms alter the behavior of the isopod making it “suicidal” or more susceptible to becoming prey and eaten by a bird.  It is suspected that biochemical cues are responsible for inducing these behavioral changes.  

Scientists have recorded infected individuals of these isopods leaving sheltered leaf litter where they are concealed, and moving into open areas where they are more visible to predators and more likely to be eaten.  Inside the bird host, the worm resides in the small intestine, where it continues to grow, often becoming as long as 15 mm.  At the end of development in the bird host, reproductively mature, fertilized female worms produce eggs that are excreted in the bird’s feces, repeating the cycle.  

Are the isopods Jason found in the snow parasitized?  Possibly.  It may remain a mystery, but an intriguing one indeed.  

References

Amato, José F. R., Amato, Suzana B., Araujo, Paula B., & Quadros, Aline F. (2003). First report of pigmentation dystrophy in terrestrial isopods, Atlantoscia floridana (van Name) (Isopoda, Oniscidea), induced by larval acanthocephalans. Revista Brasileira de Zoologia20(4), 711-716. https://dx.doi.org/10.1590/S0101-81752003000400026

Frank, Kenneth. (2015). PILLBUGS (Isopods; Armadillidium).

https://www.researchgate.net/publication/309680327_PILLBUGS_Isopods_Armadillidium

Moore, J. (1983). Responses of an Avian Predator and Its Isopod Prey to an Acanthocephalan Parasite. Ecology, 64(5), 1000-1015. doi:10.2307/1937807

https://www.jstor.org/stable/1937807?seq=1

Moore, J. (1984). Parasites That Change the Behavior of Their Host. Scientific American. 250(5). 108-115. https://www.jstor.org/stable/10.2307/24969371

Nickol, B., and G. E. Dappen (1982).  Armadillidum vulgare (Isopoda) as an intermediate host of Plagiorhynchus cylindraceus (Acanthocephala) and isopod response to infection. Journal of Parasitology 68(4) 570-575.  https://www.jstor.org/stable/328091

Phillips, Anna. 2010.  March 21 – Plagiorhynchus cylindraceusParasite of the Day.http://dailyparasite.blogspot.com/2010/03/march-21-plagiorhynchus-cylindraceus.html

Wehr, E.E., J.T. Lucker. 1952. Insects and Helminths, in The Year Book of Agriculture. naldc.nal.usda.gov https://naldc.nal.usda.gov/download/IND43894194/PDF

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