Rare Species Guide

  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 (water pygmyweed) was first found in Minnesota in 1945 at the muddy margin of a prairie pool in extreme southwestern Minnesota and is still known to occur at a small number of sites in Rock County (Inner Coteau Subsection). It was then found in Namakan Lake and nearby Kabetogama Lake in the extreme northern part of the state in St. Louis County (Border Lakes Subsection). The northern plants represent an unusual deep-water form with elongate internodes. Although the species is widespread in North America, the  Minnesota populations appear to the only ones in the Midwest. The populations fluctuate in size year-to-year depending on variations in precipitation and water levels, but the habitats are very limited and susceptible to degradation. 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. The flowers are tiny, no more than 2 mm (0.08 in.) long and found singly in the axils of the leaves. They have 4 sepals and 4 petals, which are whitish in the deep-water form and pink in the dwarf form. The seeds are less than 0.5 mm (0.02 in.) (Bywater and Wickens 1984).

  Habitat

Crassula aquatica is found in aquatic habitats, including mud flats, the muddy margins of vernal pools, and rooted in sediments 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 outcrop pools in southwestern Minnesota, the dwarf form grows in association with Elatine triandra (three-stamened waterwort), Isoetes melanopoda (prairie quillwort), and Limosella aquatica (mudwort).

  Biology / Life History

Crassula aquatica is an annual plant that roots in bottom sediments of shallow water. The flowers are pollinated by small flying insects when the stems are above water. 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 preceding years, a well-stocked seed bank, and suitable environmental conditions. Seeds can survive in the soil seed bank for many years, though precise data on longevity are lacking. One study in Sweden estimated seed density of 51,000 per square meter (Milberg and Stridh 1994), indicating a great potential to survive periods of drought or other environmental stress. Germination is rapid when moisture conditions become ideal, usually within 2-3 days (Bliss and Zedler 1998).

Crassula aquatica can grow completely submerged underwater or stranded on mud after the water has receded or evaporated. In fact, a viable population probably needs both conditions in order to maintain a viable, long-term seed bank. It survives with no loss in vigor when the water level decreases and exposes plants above water level but does not do well if it is re-flooded (Mork et al. 2018). It is easily crowded out by taller perennial vegetation, which may be favored by conditions where the water level is stable and does not fluctuate. Stranded plants can survive mild frost but are killed by a hard frost. Monitoring on Lake Namakan indicated it was favored by frequent winter drawdowns and disappeared when drawdowns were suspended (Meeker et al. 2018).

  Conservation / Management

Degradation of water quality and unnatural changes in water levels could impact C. aquatica populations wherever they occur. All the known northern populations in Minnesota occur in Voyageurs National Park, where they are presumably protected from many threats, yet the water levels in the park are regulated by a plan that does not take into account the perpetuation of the C. aquatica populations that occur there. Two of the six populations known in southwestern Minnesota occur in a State Park or National Wildlife Refuge. However, management involves livestock grazing, prescribed burns, and herbicide application, which have untested effects on the populations of C. aquatica. Well established populations of C. aquatica, those which poses a substantial seed bank, may be able to survive such management under certain conditions, but monitoring should be incorporated into any management activity.

  Best Time to Search

The best time to search for Crassula aquatica is when it is flowering, which is usually in July or August but sometimes as early as June.

  Conservation Efforts in Minnesota

Although most known populations are located on public lands, no specific conservation efforts have been undertaken on behalf of this species.

  Authors/Revisions

Welby R. Smith (MNDNR), 2023

(Note: all content ©MNDNR)

  References and Additional Information

Bliss, S. A., and P. H. Zedler. 1998. The germination process in vernal pools: sensitivity to environmental conditions and effects on community structure. Oecologia 113:67-73.

Bywater, M., and G. E. Wickens. 1984. New World species of the genus Crassula. Kew Bulletin 39(4):699-728.

Harris, F. 2009. Rock pools on the prairie: ephemeral pools on the prairie form amazing mini-ecosystems. Minnesota Conservation Volunteer 72(423):30-39.

Meeker, J. E., D. A. Wilcox, and A. G. Harris. 2018. Changes in wetland vegetation in regulated lakes in northern Minnesota, USA ten years after a new regulation plan was implemented. Wetlands 38:437-449.

Mork, O., T. Kragh, E. Kristensen, and K. Sand-Jensen. 2018. Adaptation and growth performance of four endangered amphibious freshwater species. Aquatic Botany 150:16-22.