Identify that alga:
Web-based key to genera -11: planktonic colonies with chloroplasts + sheath

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Planktonic colony cells enclosed in a mucilage sheath:


Mucilage sheath:

sheath diffuse, without a definite outline Click on the thumbnail for a larger image  
  Mucilage sheath may only be visible with the aid of Indian Ink: cells crescent-shaped or twisted


  Kirchneriella: forms coenobia of 2,4,8..32 cells

 

 

 

  Sheath may be indistinct, but cells are clearly linked in pairs by dichotomous branching threads radiating from the centre; chloroplast a parietal cup + 1 pyrenoid..


  Dictyosphaerium

 

 

 

Sheath of very dense mucilage, almost obscuring cells

 
 Botryococcus: cells rounded or elongate; colony contains yellow oil droplets which can be squeezed out under a coverslip, sometimes forms green to yellow-orange blooms.

 

 

 


Sheath with a firm outline    
  layered (lamellate);    
 

cells spherical/elliptical, chloroplast parietal cup + pyrenoid

   
  Gloeocystis:sheath may be concentrically lamellate

 

 

 

 

with concentric lamellations; cells spherical, chloroplast stellate (see second image)

 
  Asterococcus: concentric lamellations not always present, chloroplast form may be difficult to see also

 

 
  Sheath plain, not layered:    
 

sheath sac-like, cells with pseudocilia extending through the sheath

 
  [Pseudocilium = flexible, but not motile, projecting fibre]

 

 
 

(a) colony normally attached at 1 end

 
  Apiocystis

 

 
 

(b) colony planktonic

 
  Tetraspora?

 

 
 

no pseudocilia, spherical cells

   
 

 

 

in groups of 4, 8, 16 within a spherical sheath

 

 ?
 Sphaerocystis: (coenobium)

 

 
 

 

 

in groups of 4 or 8 in tight tetrads or in a wreath-shape

Coenochloris or Coenococcus: no illustration available; considerable scope for confusion between these (and Sphaerocystis), and some doubt whether they are different.

 

 
 

 

in groups of 2-4 cells in mucilage sheath, with semicircular mother-cell wall remnants lying between; chloroplast a parietal cup, 1 pyrenoid


  Schizochlamydella:

 
   

Similar genus: Westella - cells in 4's, sometimes arranged quadrately, linked by mother-cell wall remnants

 

 
 

no pseudocilia, oval cells

   
 

 

Cells apparently oval or kidney-shaped, in pairs, linked together: actually desmid semicells

 
  Cosmocladium: a colonial desmid; the cells reminiscent of Cosmarium

 

 
    Cells ranging from spherical to oval or spindle-shaped (depending on species), in coenobia of 2,4,8,...64, within a mucilage sheath composed of gelatinised mother-cell wall

 
  Oocystis : chloroplast(s) parietal plate or trough, with or without a pyrenoid

 

 
    Cells solitary or in coenobia enclosed by mother-cell wall; cells somewhat kidney-shaped, with one side curved and the opposite nearly straight


  Oonephris: chloroplast stellate, but often shape not readily distinguishable

 

 
    Coenobia of 4 cells in a flat, quadrate arrangement; cells various shapes

 Crucigenia

 

 
    Coenobia in a flat, quadrate arrangement; cells oval, or tapering to one end, often with a space in the centre. Cells in groups of 4 enclosed by mucilage sheath, sometimes connected to 3 other coenobia by mother cell-wall remains, all lying in same plane


 Crucigeniella

 

 
 

cells elongated, cigar-shaped

   
    cells pointed, banana-shaped in groups of 4 which all lie parallel and aligned.


  Quadrigula

 

 
    cells as Quadrigula, with bundles of cells parallel but not aligned.

  Pseudoquadrigula

 

 
 

cells elongated, spindle-shaped or tapering

   
    Grouped in 2's or 4's with alternate cells pointing in opposite directions, linked by mother-cell wall remnants, in a diffuse mucilage sheath


 Dichotomococcus

 

 
    Similar genera:
Quadricoccus
(cells more oval)
Forming coenobia linked in a wreath-like formation:
Komarekia
Crucigenia (some species)
Coronastrum
 
       
 

Genera not included:
Botryosphaerella, Ecballocystis,Elaktothrix, Granulocystopsis, Hormotila, Juranyella, Palmodictyon, Paulschulzia, Schizochlamys, Sphaerobotrys

   

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John Kinross