Growth & Development

An unexpected killer lurks deep in Italian lakes

You would expect to see flowers of Utricularia australis on display just above the surface of a pond or lake. Now botanists have found open U. australis flowers on a lakebed over four metres below the surface.
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Lake Bolsena and Lake Bracciano are two large volcanic lakes in central Italy. In ponds around the area, you can find Utricularia australis, southern Bladderwort, a carnivorous plant with showy yellow flowers. Beneath the water’s surface, stems with bladders, suction traps lie in wait to trap passing prey. Simona Ceschin and colleagues report in Aquatic Botany that they have found U. australis flowering on the lakebed.

Utricularia australis. Photo: Stefan.lefnaer / Wikimedia Commons.

The flowers are of particular interest to scientists studying Utricularia. The plant doesn’t have roots and can be highly flexible in how it grows. But while it can have a variable body plan, the flowers usually stick to similar patterns. Chasmogamous flowers are open to attract insects as pollinators, showy and often fragrant. The scapes, flower stems from the main stem, lift the flowers a little away from the water’s surface. Utricularia can also have cleistogamous flowers. These are closed flowers that self-pollinate. These tend to lie on the water’s surface or can even be pushed below into the water.

Surprisingly, the lakebed flowers are chasmogamous, open for pollinators. One of the obvious differences the team found was that the scapes were over three times longer than usual. Ceschin and colleagues found other differences between emerged and submerged flowers: “The main qualitative characters differentiating the two flower types were the chromatic nectar signalling on the corolla, petal consistency, pollen morphology and anther colour. The emerged flowers had stronger nectar signalling (brownish-red streaks) on the lower lip; in the underwater flowers these markings were less colourful. Petal consistency also differed; the emerged flowers of U. australis had more resistant petals than the underwater flowers, which were thinner to the point of being semi-transparent. Underwater pollen was different from that of the subaerial flowers; underwater pollen was always held within a soft mucilaginous mass, which tended to burst when mechanically stimulated, while subaerial pollen was dry and grainy.”

The report is the first of Utricularia flowering at this depth, though the authors note that there have been reports of Utricularia in a vegetative state at lower depths. It is a puzzle as to how these populations of bladderworts are doing so well, but the team have some suggestions.

First, the plant is growing among stands of Chara, algae growing on the lakebed. These neighbours shelter the Utricularia plants in relatively still water. The environment may also make the lakebed a better hunting ground than the surface. The stability may offset the loss of light from living up to six metres below the lake surface. Why hasn’t U. australis been in other similar habitats if this is the case? The answer the botanists give is simple: have people searched for them?

Ceschin and colleagues write: “That these populations are far removed from their usual habitat, their occurrence elsewhere may have been simply overlooked, and growth at depth may be a more common occurrence. It is conceivable that this can represent a newly revealed ecological trait of the species since it is not an isolated occurrence, as this study describes populations in two different lakes that are around 50 km apart, and shows that each population is well established, occurring for the last 11 years at least.”

If lakebeds are more amenable to Utricularia than previously thought, then it could have implications for the conservation of lake habitats.

RESEARCH ARTICLE

Ceschin, S., Pelella, E., Azzella, M.M., Bellini, A. and Ellwood, N.T.W. (2022) “Unusual underwater flowering of Utricularia australis populations: a botanical enigma?,” Aquatic Botany. Elsevier BV. doi:10.1016/j.aquabot.2021.103487.

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