Dotted duckweed, or Landoltia punctata, is probably unknown to the majority of aquarists, however, it is a rather common plant in our aquaria. It is often mistaken for Lemna minor. Indeed it does look like our well-known duckweed with its oval fronds.
However, whereas Lemna species only have one root per frond, Landoltia punctata has a tuft of several roots (2 to 7) that grow on each frond. The underside of the fronds of Landoltia is often reddish, that of Lemna species is green in most cases.
Spirodela species like greater duckweed (Spirodela polyrhiza) also have tufts of several roots and a reddish underside, however, other than Landoltia, the number of roots is larger (7-21), and the fronds are larger and round.
Until 1999, Landoltia punctata was thought to be a Spirodela species, however, then it was placed in the newly created genus Landoltia, as it is more closely related to the other duckweeds (Lemna, Wolffia, Wolffiella) than with true Spirodela species, according to recent research.
Originally, L. punctata was native to waterbodies on the southern hemisphere (South America, South Asia, Australia and probably also South Africa). However, today L. punctata had been introduced into warm regions of other continents, too, by human activity, and it now grows wild in New Zealand, the USA, Egypt, Israel and Northern Italy, amongst others. Like other duckweeds, it prefers nutrient-rich stagnant to slow-flowing water. In arid regions e.g. in Australia it is known to survive dryness in the form of seeds; there it is found in many periodic waters. L. punctata is susceptible against heavy frost and does not grow in regions with absolute minimum temperatures of under -20°C (-4 °F). It is probably often introduced in aquaria with aquatic plant deliveries from nurseries from where it also gets released into nature.
Its use, demands and vexatiousness as aquarium weed are comparable to those of Lemna. However, as the fronds of Landoltia punctata can grow rather large (up to 8 mm long), this duckweed species might be a little easier to control than its relatives. Like those, L. punctata has proven a useful surface cover in rearing tanks for fish fry. Its red underside and its root tufts give it an interesting look.
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