EPOW - Ecology Picture of the Week

Explanation:   What are these odd organisms that I found one hot August day growing on submerged rocks in a small, slow-moving tributary to the Middle Fork John Day River in eastern Oregon?  They are each the size of the nail on my little finger.  They are ear-shaped.  Many have bulges.

They are the blue-green alga or Cyanophyceae called Nostoc, which forms leathery or rubbery discs that adhere to the surface of rocks in flowing water.  Blue-green alga are photoautotrophic -- they photosynthesize sunlight into energy.  

And the bulges are midges -- tiny members of the order of flies, Diptera -- whose larval forms live as commensals (symbiotes) within the tiny Nostoc colonies.  These midges are likely Cricotopus sp. (family Chironomidae).  

Nostoc is also home to other commensal organisms, such as some  caddisflies of family Hydropsychidae that are more free-living and that can exhibit legs or gills when inhabiting their Nostoc homes.   Obviously, there is advantage given to insects to inhabiting Nostoc, but does Nostoc receive any benefit in return, such as nutrients from the insects' waste products?  If the benefit is one way only, it is called commensalism.  If it is two way, it is called mutualism.  I have not found studies demonstrating true mutualism, although such research may be wanting.

Nostoc does not always require streams.  Some species of Nostoc can form part of biological soil crusts in arid environments.  

Acknowledgment:  Many thanks to aquatic entomologist Peggy Wilzbach for the identification.

Information:
references on symbiosis between nostic and midges:
     Brock, E.M. (1960) Mutualism between the midge Cricotopus and the alga Nostoc. Ecology 41:474-483.
     Dodds, W.K. and Marra, J.L. (1989) Behaviors of the midge, Cricotopus (Diptera: Chironomidae) related to the mutualism with Nostoc parmelioides (Cyanobacteria). Aquatic Insects 11:201-208.
     Ward, A. K., C. N. Dahm, and K. W. Cummins. 1985. Nostoc (Cyanophyta) productivity in Oregon stream ecosystems: invertebrate influences and differences between morphological types. J. Phycol. 21:223-227. 
     Wirth, W.W. (1975) The species of Cricotopus midges living in the blue-green alga Nostoc in California (Diptera: Tendipedidae). Pan-Pacific Entomology 33:121-126.

general refs on Nostoc:
     Bergman, B., and B. Osborne. 2002. The Gunnera-Nostoc symbiosis. Biology and Environment: Proceedings of the Royal Irish Academy 102B(1):35-39. 
     Parsons, R. 2002. Nodule function and regulation in the Gunnera-Nostoc symbioses. Biology and Environment: Proceedings of the Royal Irish Academy 102B(1):41-43. 
     Thiel, T., J. C. Meeks, J. Elhai, M. Potts, F. Larimer, J. Lamerdin, P. Predki, and R. Atlas. n.d. Nitrogen fixation: analysis of the genome of the cynanobacterium Nostoc punctiforme. Department of Biology, University of Missouri, St Louis MO. 5 pp.