Mudmat (Glossostigma cleistanthum)

Synonyms: Glossostigma diandrum

Description:

Mudmat (Glossostigma cleistanthum) is a tiny aquatic plant in the lopseed family, native to Australia, New Zealand, India, and East Africa [2]. It was brought to North America through the aquarium trade and has escaped into lakes and ponds in the Northeast, where its creeping stems form dense green mats on the shallow bottom [1][2]. MISIN tracks it because those mats displace native water plants ? and, unusually for an aquatic weed, it can invade clean, undisturbed lakes and now grows alongside rare native species [1][2].

Habit:

Mudmat is easy to overlook: it is a very small aquatic plant that hugs the bottom of shallow water, spreading by creeping stems that root at the joints into a dense, low, bright-green mat [1]. Its tiny leaves ? only about 1/2 to 3/4 inch (1 to 2 cm) long ? are spoon-shaped, narrow at the base and rounded at the tip (some people say "bunny ears"), and grow in opposite pairs along the stems [1]. It usually grows fully underwater but can also grow on wet mud at the water's edge [1].

Because it is so small and grows below the surface, it is hard to spot from shore ? it is most often found by wading or snorkeling the shallows of clear lakes and ponds in summer, where it looks like a fine green carpet on sandy bottoms [1][2]. Underwater plants make tiny self-pollinating flowers on very short stalks; plants on exposed mud make small, pale, bell-shaped flowers [1].

At-a-glance cues:

1. A dense, low, bright-green mat carpeting the sandy bottom of shallow, clear water [1]
2. Tiny spoon-shaped ("bunny-ear") leaves about 1/2 to 3/4 inch long, in opposite pairs [1]
3. Creeping stems that root at the joints [1]
4. Grows in clear, low-nutrient lakes and ponds, on sandy bottoms [2]

Leaves:

1. Arrangement: opposite (in pairs) along the creeping stems [1]
2. Shape: spoon- or paddle-shaped ? narrow at the base, broadened and rounded at the tip [1]
3. Surface: smooth and bright green [1]
4. Size: tiny, about 1/2 to 3/4 inch (1 to 2 cm) long [1]

Stems:

1. Habit: a small, herbaceous (non-woody) aquatic plant ? a perennial when underwater, sometimes an annual when growing on exposed mud [1]
2. Form: slender creeping stems that run along the bottom and root at the joints (nodes), forming dense mats [1]
3. Roots: a shallow, fine root system [1]

Flowers:

1. Two forms: underwater plants make tiny self-pollinating (closed) flowers on very short stalks; plants on exposed mud make small, pale, bell-shaped open flowers on stalks about as long as the leaves [1]
2. Color: pale ? whitish to pastel ? on the open flowers [1]
3. Bloom time: summer [1]
4. Sex: flowers are perfect ? each has both male and female parts [1]

Fruit and seeds:

1. Fruit: a small capsule [1]
2. Seeds: many tiny seeds; a dense stand can release thousands of seeds per square meter [1]
3. Spread: mainly by broken stem fragments that root where they settle, plus seeds [1]

Habitat:

Mudmat lives in the shallow water (the littoral zone) of lakes and ponds, and sometimes on wet mud at the shoreline [1][2]. It is native to Australia, New Zealand, India, and East Africa, and was introduced to North America through the aquarium trade [2].

It grows best on sandy bottoms in clear, low-nutrient water with low acidity (low pH) ? the kind of clean, undisturbed lakes where few other plants grow thickly [2]. It tolerates a wide range of conditions, including cold northern winters, which lets it persist and spread once it arrives [1].

Reproduction:

Mudmat spreads mostly by pieces of itself [1]. Its brittle creeping stems break easily, and the broken fragments root wherever they settle on the bottom ? so a single plant, or a bit carried on a boat, trailer, anchor, or fishing gear, can start a whole new mat [1]. Waterfowl also move fragments and seeds from lake to lake [1]. Underwater plants make tiny self-pollinating flowers that set seed without needing a partner, and a dense stand can drop thousands of seeds per square meter into the sediment [1].

This is how it reaches a new lake: as fragments or seeds hitchhiking on wet boats, gear, or birds ? or, at the start, as a dumped aquarium plant [1][2]. Because every fragment can grow into a new patch, control means removing every piece and never moving wet plants or water between lakes [1].

Impact and Damage:

Mudmat is not a threat to people or pets, but it is a serious problem for lakes [2]. Its creeping stems knit into dense mats that carpet the bottom and crowd out native water plants [1]. What makes it especially worrying is that, unlike most aquatic invaders ? which need disturbed, murky, or nutrient-rich water ? mudmat can move into clean, clear, undisturbed lakes, the very places that shelter rare native plants [2].

In the Northeast it now grows alongside plants that are on state endangered-species lists in Connecticut and New Jersey, so it directly threatens some of the region's rarest aquatic wildflowers [2]. For a lakefront homeowner, it is mostly a below-the-surface weed that spreads on boats and gear rather than a nuisance you would notice from the dock ? which is exactly why it moves between lakes unseen [1]. No dollar figure for its damage in North America has been established.

Similar species:

Mudmat is a tiny bottom-carpeting plant, and a few small aquatic plants can look similar underwater. These comparisons come from published sources, since MISIN does not yet track look-alikes for it.

Mudworts (Limosella species) are small native aquatic plants that also grow in low mats on wet mud and shallow bottoms, but their leaves are narrow and grass-like or on long stalks, without mudmat's paired, spoon-shaped "bunny-ear" leaves running along creeping stems [1].

Bladderworts (Utricularia species) are native aquatic plants found in the same shallow, clear, low-nutrient water, but they have thread-like, often branching underwater leaves ? sometimes bearing tiny bladders ? rather than mudmat's small, solid, paddle-shaped leaves [1].

Monitoring and rapid response:

Signs to watch for:

1. A dense, low, bright-green carpet of tiny paired leaves on the sandy bottom of shallow, clear water ? best found by wading or snorkeling in summer [1][2].

What you can do:

1. If you find a small patch, remove it by hand, carefully collecting every fragment ? stems, roots, and loose pieces ? because any piece left behind can root and start a new mat [1]. Bag it and throw it in the trash on land, away from the water [1].
2. Clean, drain, and dry your boat, trailer, anchor, waders, and fishing gear before moving to another lake, and never dump aquarium plants or water into any waterway [1].
3. Don't rake or churn a mat without collecting the pieces ? breaking it up without removing the fragments just spreads it [1].
4. For crews and land managers: survey the clear-water shallows where it hides, target small, new patches for hand or mechanical removal with full fragment capture, and coordinate boat-cleaning and monitoring across connected lakes, since it moves on watercraft and waterfowl [1][2].

When to call a professional:

1. Established mats over larger areas of a lake bottom, or any removal in deeper or sensitive aquatic habitat, where full fragment capture and permits are needed [1].
2. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. Wisconsin Department of Natural Resources. Mudmat (Glossostigma cleistanthum). dnr.wisconsin.gov
2. Les DH, Capers RS, Tippery NP (2006). "Introduction of Glossostigma (Phrymaceae) to North America: a taxonomic and ecological overview." American Journal of Botany 93(6):927?939. doi.org/10.3732/ajb.93.6.927

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