Hidden beneath the water’s surface is a botanical world that is among nature’s most innovative and ecologically important.
As I highlighted in a recent paper , an extraordinary range of adaptations have evolved in aquatic plants for life beneath the water’s surface. Some flower underwater, others capture animals in ingenious traps. Here are seven facts that show how these remarkable organisms challenge our assumptions about what plants are and how they survive.
Many people think of plants as nice-looking greens. Essential for clean air, yes, but simple organisms. A step change in research is shaking up the way scientists think about plants: they are far more complex and more like us than you might imagine. This blossoming field of science is too delightful to do it justice in one or two stories. This article is part of a series, Plant Curious , exploring scientific studies that challenge the way you view plantlife.
1. Plants can’t stop returning to water
When we think of plants, we often picture life on land: forests, grasslands and meadows. Yet throughout their evolutionary history, plants have repeatedly returned to the water, where they first evolved. Around 500 million years ago , plants moved from water onto land. Since then, many have moved back. Scientists estimate that the aquatic lifestyle has evolved independently more than 100 times across different plant groups.
Water lilies float their leaves on the surface, duckweeds drift freely and seagrasses live entirely submerged in the ocean. Some of these groups made their return over 100 million years ago . The repeated evolution of aquatic plants is one of nature’s most striking examples of convergent evolution.
2. The plants that aren’t plants
Some of the most conspicuous organisms beneath the waves are seaweeds. They photosynthesise and often resemble underwater plants. Yet despite their appearance, seaweeds are not true plants .
Seaweeds belong to several distinct lineages of algae found across the evolutionary tree of life. The giant kelps that form underwater forests are brown algae. Nori and dulse are red algae, while sea lettuce belongs to the green algae.
Unlike plants, they lack true roots, stems and leaves, and they do not produce flowers or seeds. However, their plant-like appearance is a reminder that evolution can produce similar forms in very different groups of organisms when they face similar environmental challenges.
3. Great depths
Plants need sunlight to photosynthesise, which usually limits them to land or shallow water. Yet some aquatic mosses survive at astonishing depths. Knieff’s hook-moss (Drepanocladus aduncus) has been recorded at 140 metres below the surface in the exceptionally clear waters of Crater Lake, Oregon, US. This makes it the deepest known underwater plant which also grows on land, surviving at a depth greater than the height of the London Eye.
Deep-water mosses have also been recorded in lakes in New Zealand , Antarctica and elsewhere, so deep these environments are effectively pitch black and few animals can survive.
4. Rootless
Roots are one of the defining features of plants, anchoring them in the ground and absorbing water and nutrients from the soil. Yet many aquatic plants have reduced their roots dramatically – and some have seemingly lost them altogether .
Life underwater changes the rules. Water and dissolved nutrients surround the plant, making extensive root systems less important than they are on land. Instead, many aquatic species absorb nutrients directly through their leaves and stems.
Duckweeds provide one of the most extreme examples. Some species have only
a single root compared to multi-rooted relatives like giant duckweed. Members of the genus Wolffia – the world’s smallest flowering plants – have no roots at all , floating freely on the water’s surface. Individual Wolffia can be a millimetre in length, with flowers at 0.3 millimetres.
5. Underwater carnivores
Not all aquatic plants rely on sunlight and dissolved nutrients. Some supplement their diet by capturing and digesting animals.
The most spectacular examples are the bladderworts (Utricularia), a group of rootless aquatic plants found in freshwater habitats around the world. Their leaves are modified into tiny bladder-like traps that create a vacuum by pumping water out of the chamber.
When a small animal brushes against trigger hairs near the trap entrance, a door springs open and the prey is sucked inside in less than a millisecond. This makes bladderwort traps among the fastest movements in the plant kingdom. While they usually capture tiny aquatic invertebrates, fish larvae and tadpoles have occasionally been recorded as prey.
The carnivore lifestyle allows bladderworts to thrive in nutrient-poor waters where other plants struggle.
6. Pollination by current
When we think of plant pollination, people often pictures bees buzzing between colourful petals on a sunny afternoon. However, if your flower is underwater, pollination becomes tricky. Instead of relying on insects or wind , many aquatic plants such as the seagrasses have turned water itself into their personal matchmaker, allowing the water’s currents to drift pollen directly to its destination.
To attract pollinators, land plants routinely release airborne perfumes. But beneath the water’s surface, these traditional gas signals do not work effectively. This limitation has led to an evolutionary shift: underwater pioneers like seagrasses have lost the genes responsible for producing these airborne scents. These no longer provided an advantage so they were lost over time.
7. Seagrasses and mangroves are major carbon sinks
Seagrasses and mangroves capture and store carbon in their tissues and the
sediments beneath them, making them some of the planet’s most effective natural carbon sinks . Together with seagrass meadows, they store what scientists call “blue carbon” – carbon trapped in coastal ecosystems that can remain locked away for centuries or even millennia.
These ecosystems, including seagrass meadows and mangroves, store 11.5 billion tons of carbon worldwide. Of the 11.5 billion tons of carbon stored, mangroves represents the largest blue carbon pool at 6.5 billion tons .
Whether they are trapping prey in milliseconds, growing in near darkness or storing carbon for centuries, aquatic plants demonstrate the remarkable versatility of life.
