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 Crassula aquatica    (L.) Schoenl.

Pigmyweed 


MN Status:

threatened
Federal Status:
none
CITES:
none
USFS:
none


Group:

vascular plant
Class:
Dicotyledoneae
Order:
Rosales
Family:
Crassulaceae
Life Form:
forb
Longevity:
annual
Leaf Duration:
deciduous
Water Regime:
aquatic
Light:
full sun
Habitats:

(Mouse over a habitat for definition)


Best time to see:

  Foliage   Flower   Fruit  
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Crassula aquatica Crassula aquatica Crassula aquatica Crassula aquatica Crassula aquatica Crassula aquatica

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Map Interpretation

Map Interpretation

  Synonyms

Tillaea aquatica

  Basis for Listing

This tiny, cryptic plant is a summer flowering annual of aquatic habitats. It is very inconspicuous and seldom seen. Crassula aquatica 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.

In 2006-2008, the Minnesota Biological Survey discovered three new C. aquatica populations of the more typical dwarf, succulent form in Rock County. These tiny plants grow in mud at the bottom of ephemeral rainwater pools and are most easily seen growing upright on the moist mud that remains for a few weeks after the pools evaporate. The plants disappear when the mud dries out, typically by late June. Two of these populations are in Touch the Sky National Wildlife Refuge and Blue Mounds State Park, where they consist of 1,000s of plants confined to just a few ephemeral rainwater pools. Crassula aquatica was listed as a threatened species in Minnesota in 1996.

  Description

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).

  Habitat

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.

The best time to search for C. aquatica is when it is flowering in July-August.

  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.

  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

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.