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Name derivation:

Gr. helicto- twisted or curving,     dictyot - netted or reticulate

Classification:

Helicodictyon  L.A.Whitford and  1996;  the single species description is currently accepted taxonomically (Guiry and Guiry 2013)

Order Chaetophorales;  Family Chaetophoraceae

Morphology:

Twisted filaments of short cells, embedded in an irregularly-shaped mucilage matrix.

Plants of branching filaments. Growth indeterminate by typical division of cells, forming a spiral net with a dense gelatinous sheath. Floating unattached. Plastids 1-3, each a short yellow-green plate with a pyrenoid and containing chlorophyll-b. Reserve food starch. The single known species (H. planctonicum) is characterized by an axial filament which grows at both ends in a circular manner. Branches arise on opposite sides and these also grow in a curved or spiralling manner so as to form an irregular microscopic net. The filaments are surrounded by a dense pectic sheath which frequently entraps a gas bubble, causing the unattached colony to float. Reproduction by fragmentation of the colony and by bi-flagellate zoospores.  Old colonies lose their buble and sink (Whitford and Schumacher 1966).

 

How the entrapped gas bubble is formed is apparently unknown, nor is the gas content known. Speculating, it could be respiratory (CO2) or photosynthetic (O2) metabolism.  It seems unlikely to be entrapment of a pre-existing bubble captured by a young colony.

Similar genera:

 

Habitat:

Mostly planktonic in small lakes in the USA.  Considered 'rare' because it is rarely collected or reported, yet forms blooms where found (Whitford 1983).

Citing Phycokey:

Baker, A.L. et al.  .  Phycokey -- an image based key to Algae (PS Protista), Cyanobacteria, and other aquatic objects. University of New Hampshire Center for Freshwater Biology. http://cfb.unh.edu/phycokey/phycokey.htm

 

References:

Guiry, M.D. and G.M. Guiry  2013.  AlgaeBase. World-wide electronic publication, National Universityof Ireland, Galway.  http://www.algaebase.org; searched on 22 February 2013.

Whitford, L.A.  1983 On Rare Fresh-water Algae.  Transactions of the American Microscopical Society 102(4):401-403.

Whitford, L.A. and G.J. Schumacher  1966.  Helicodictyon nom. nov. pro Heterodictyon and Helicodictyaceae fam. nov. (Ulotrichales). Phycologia 5:274-276.