The choanoflagellates (Class Craspedophyceae/Choanoflagellida) are a well-defined group of unicellular protists. The protoplast is usually colourless and bears a single flagellum surrounded by a ring or collar of tentacles used to trap food particles. In Antarctic waters, two families have been identified. In the family Codonosigidae, the protoplast can be either naked or have a fine, often indistinct sheath. Species of the family Acanthoecidae have a protoplast that is surrounded by a lorica, or basket, constructed of costal strips (siliceous cylindrical or flattened rods). The costal strips can be longitudinally, transversely, irregularly or spirally arranged, and they often have anterior spines or projections or a posterior pedicel. The taxonomy of the loricate choanoflagellates (Acanthoecidae) is based almost entirely on the structure of the lorica determined by the arrangement of costae and the differentiation of costal strips. Within a single species there can be considerable variation in cell dimensions.
Loricate choanoflagelllates have been recorded from many Antarctic localities and they can be an abundant component of the Antarctic marine plankton. They trap and ingest food, such as bacteria, cyanobacteria and other particulate matter, and can occur in numbers in the order of 107 cells per litre, thus suggesting they play an important role in Antarctic marine food webs.
32 species are included in our Lucid® interactive key. Some species can be identifed solely by light microscopy (LM) but others may require the use of either a scanning electon microscope (SEM) or a transmission electron microscope (TEM). Primarily we have used common or obvious characteristics such as ‘size’ and ‘lorica composition’of cells to separate species. A thumbnail image of each species is included as a “first impression” and in some cases this can lead directly to a successful identification. Further images can be viewed on factsheets that are linked to the key. Images are sourced from Antarctic Marine Protists (eds. Scott & Marchant, 2005) unless otherwise credited.
Taxonomic names are those used in ‘AlgaeBase’ (accessed April 2013). The following references and weblinks may be useful to the user:
- Throndsen, J. (1997), The Planktonic Marine Flagellates. In Tomas, C.R. (ed.), Identifying Marine Phytoplankton. (Academic Press, San Diego). pp. 591–833.
- Scott,F.J. & Marchant, H.M. (eds) (2005). Antarctic Marine Protists (ABRS & AAD, Canberra).
- Leadbeater, B.S.C. (2008). Choanoflagellate evolution: the morphological perspective. Protistology 5(4), 256-267.
- Leakey, R.J.G., Leadbeater, S.C., Mitchell, E., McCready, Murray W.A. (2002). The abundance and biomass of choanoflagellates and other nanoflagellates in waters of contrasting temperature to the north-west of South Georgia in the Southern Ocean. Europ. J. Protistol. 38, 333-350.