Zygnema Agardh   

Kingdom: Plantae
Phylum: Chlorophyta – green algae
Class: Zygnematophyceae
Order: Zygnematales

British distribution (genus): Widespread.
World distribution (genus): Throughout the world.

Zygnema is a genus of about 140 species, all freshwater. They form unbranched filaments, with each cell containing a pair of stellate chloroplasts and each chloroplast possessing a single, conspicuous pyrenoid. Sexual reproduction takes place by conjugation, with the fusion of two amoeboid gametes in a conjugation canal formed between adjacent filaments, forming a thick-walled zygote. There are no flagellated reproductive cells.

14 species are recognised in the British Isles by Johnson (in John et al., 2002), their identification depending on reproductive features. The related genus, Zygnemopsis, with two British species, both rare, also cannot be distinguished from Zygnema when only vegetative. Many of the species have broad ecological requirements, occuring in ponds, streams, ditches and similar waterbodies, often forming extensive 'blanket weed' growth. Z. stellinum is given as a fairly common species in ditches and in moorland pools.

Photograph of material in culture, 1998, species unnamed.

Related genera include Spirogyra and Mougeotia. The systematic position of the conjugating green algae has been questioned, with some authorities moving them out of the phylum Chlorophyta.

•   John, D.M., Whitton, B.A., & Brook, A.J., ed. (2002). The freshwater algal flora of the British Isles, Cambridge University Press, Cambridge.

© A.J. Silverside
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