Placoneis gastrum

Placoneis gastrum is generally epipelic in meso- to eutrophic and brackish freshwater lakes and streams (Cox 2003). Placoneis gastrum was a significant indicator of low total phosphorus and marginal total nitrogen in the Awash, Blue Nile, and Omo-Gibe river basins in Ethiopia (Beyene et al. 2013). It was originally described among marine algae from Vera Cruz, Mexico, but its presence there was not confirmed in a later study (Cox 2003). Populations of P. gastrum were found in the plankton in Lazy Lagoon and Beck’s Canal in West Lake Okoboji, Dickinson County, Iowa. It has also been found in Douglas Lake, Michigan, and many other sites around the world (Cox 2003).

Chloroplasts are single, usually positioned on one side of the cell opposite the nucleus, and consist of a lobed plate on each valve connected by a central bridge which contains a flat pyrenoid (Mann and Stickle 1995). The chloroplast and the pyrenoid are split during cytokinesis rather than dividing on their own before cell division, and these, along with the nucleus, are stationary during reproduction (Mann and Stickle 1995). The chloroplast structure was a defining part of Mereschkowsky’s original Placoneis description (Cox 2003), where he also stated that Placoneis was likely more closely related to Cymbella than to the naviculoids (Mann and Stickle 1995). Placoneis gastrum has been observed to occasionally form triplets for sexual reproduction connected by a single mucilage capsule (Mann and Stickle 1995).