Crassula aquatica (L.) Schoenl.
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Basis for Listing
This tiny cryptic plant is a summer flowering annual of aquatic habitats. It is very inconspicuous and seldom seen. Crassula aquatica (water pygmyweed) was first found in Minnesota in 1945 at the muddy margin of a prairie pool in extreme southwestern Minnesota (Rock County), but it has not been observed at the site since, and it is believed that the habitat has been destroyed. Until the mid-1990s, there were only three other known collections of this species in Minnesota, all from the extreme north in Namakan Lake in Voyageurs National Park. These collections represented an unusual deepwater form with elongate internodes. In the late 1990s, six additional collections of the deepwater form were made, four in Namakan Lake and two in nearby Kabetogama Lake.
An obscure plant, the dwarf form of C. aquatica has succulent, linear, opposite leaves, no more than 6 mm (0.24 in.) long. Leaves lack petioles. The tiny flowers, which are whitish in the deepwater form and pink in the dwarf form, have 4 petals and are solitary in the leaf axil (Hitchcock and Cronquist 1973; Chadde 1998).
Crassula aquatica is found in aquatic habitats including mud flats, the muddy margins of vernal pools, and rooted in muck in shallow water up to 0.9 m (3 ft.) of bays, channels, and creeks. The deepwater plants grow in association with Sagittaria latifolia (broad-leaved arrowhead), Elodea canadensis (Canadian elodea), Callitriche palustris (spring water starwort), Nymphaea odorata (American white waterlily), Vallisneria americana (eelgrass), and various species of Potamogeton (pondweeds). On outcrops in southwestern Minnesota, the dwarf form grows in association with Elatine triandra (three-stamened waterwort), Isoetes melanopoda (black-footed quillwort), and Limosella aquatica (mudwort)
Biology / Life History
Crassula aquatica is an annual plant. The fruit is an erect purplish follicle or a dry pod that opens along one slit. It contains 6-12 seeds. Each year's population is dependent upon successful seed production in the preceding year, a well-stocked seed bank, and suitable environmental conditions.
Conservation / Management
Degradation of water quality and unnatural changes in water levels could impact C. aquatica where it grows in lakes. All the known northern populations in Minnesota occur in Voyageurs National Park where they are presumably protected from those threats. Two of the three populations known in southwestern Minnesota are protected in a State Park or National Wildlife Refuge. The population growing on private land is vulnerable to herbicide overspray for weed control and vehicle traffic.
Best Time to Search
The best time to search for C. aquatica is when it is flowering, which is usually in July or August but sometimes as early as June.
Conservation Efforts in Minnesota
No specific conservation efforts have been undertaken on behalf of this species, however most known populations are located on public lands.
References and Additional Information
Chadde, S. W. 1998. A Great Lakes wetland flora. Pocketflora Press, Calumet, Michigan. 402 pp.
Hitchcock, C. L., and A. Cronquist. 1973. Flora of the Pacific Northwest: an illustrated manual. University of Washington Press, Seattle, Washington. 730 pp.