Here are my conclusions regarding the ID of your mystery duckweed: Landoltia punctata (G.F.W. Meyer) Les & Crawford.

The plants have 5-7 roots and 3-5 veins. This rules out duckweeds of the genus Lemna which have 1-3 veins and a single root.

Duckweeds with multiple roots belong to sections Spirodela and Oligorrhizae within the genus Spirodela. Section Spirodela can be ruled out because it has 7-21 roots and 7-16 or more veins. In addition, the plant body is only 1 to 1.5 times as long as wide, and only a few (1-5) roots penetrate the ventral scale or prophyllum.

The mystery duckweed must be in section Oligorrhizae because it has only 5-7 roots and 3-5 veins. The plant body is 2 (3) or more times as long as wide, and all of the roots pass through the ventral prophyllum. The latter characteristic is difficult to see.

According to the late Elias Landolt there is only one species in section Oligorrhizae: Spirodela punctata (G.F.W. Meyer) Thompson which has been changed to Landoltia punctata (G.F.W. Meyer) Les & Crawford. There is some controversy over the precise scientific name for this species, but I stand by the latter binomial. The late Dr. Landolt also agreed with this name. See my on-line explanation at the following link:

I must admit that the majority of plants in the sample sent to me have plant bodies that are a little longer, thinner and less noticeably punctate than the Landoltia punctata I am familiar with. In addition, most of them have a green ventral side compared with the reddish anthocyanin on ventral side of typical L. punctata in California. According to Landolt this species is polymorphic and may appear different under different growing conditions. These plants were growing in an aquaculture tank under presumably ideal growing conditions. Perhaps this explains their difference from L. punctata populations I have collected in the field (mostly southern California). According to the references I have consulted, I don't know any other species name to give them.

I will grow them outdoors in buckets at my home in San Diego County and see if their size, shape and pigmentation changes.

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