It wasn’t all that long ago that the scientific community believed pollination occurred almost entirely on land. The primary pollinators were flying insects, birds and bats on one side and the wind on the other. Very little occurred underwater. They believed water movement carried male gametes to female plant and algae parts*. Therefore, no animals participated in waterborne pollination.
*In this article, we’ve largely lumped aquatic plants and algae together to simplify things a bit. But algae are not plants.
For example, many algae act much like corals and launch male and female gametes into the water at specific periods. Or launch male gametes while females remain on the alga. When two compatible gametes run into each other, a new alga can form, then settle down and start a new colony.
So, that was basically water-based pollination. And even that was mostly thought to occur on a very small scale. These algae, and indeed even higher aquatic plants, tended, we believed, to mostly reproduce vegetatively.
True enough, entomologists did know that insects helped pollinate aquatic plants… but not underwater. This is the case with several otherwise aquatic higher plants (species of bladderworts [Utricularia spp.], for example). They live all their life underwater, except when they bloom. They send their flowers up above the water so insects can pollinate them. Generally, the pollinated flowers are then pulled back underwater to mature. Later, the seeds break free to float with currents and waves to new environments. There they settle to form new aquatic plants.
New Studies Show Underwater Pollination More Common Than Thought
However, conceptions are now changing.
For a few decades now, scientists have noticed the presence of tiny sea creatures (usually sowbug like crustaceans called isopods and also worms, plus more microscopic organisms), often many of them, clustering on aquatic plants long thought to be water pollinated. And certain of them began to wonder what they were doing there.
And it now looks like some of them, at least, are aquatic pollinators. “Sea bees,” scientists have been jokingly calling them, although none so far has turned out to be an actual insect. And certainly, none looks anything like a bee!
But it turns out there are isopods, worms and others that visit underwater flowers or sex organs specifically when the male gametes are fertile, usually early in the evening. They feed on them, true enough. But also, gametes get stuck on the invertebrate’s body. Then they switch to female parts as they become fertile during the night… and as they visit the female parts, the gametes are pulled free. In that way, pollination can occur.
This type of exchange is already well known in terrestrial ecosystems and it makes perfect sense in aquatic ones as well. What’s odd is to see crustaceans carrying it out rather than bees and butterflies!
Which Came First?
It would seem obvious that animal pollination must have started on land, where free-flying insects can readily visit the flowers of their choice. And where the vast majority of pollinators have been discovered. Indeed, most higher aquatic plants, like sea grass (Thalassia spp.), evolved on land first before returning to the sea, bringing with them insect-pollinated flowers.
Once in the water, though, the flowers had to adapt. They needed new strategies for pollination, including offering rewards of sugar and other services to something other than insects. And that included using little aquatic creatures of various kinds. That’s what the team of Vivianne Solis-Weiss, a marine biologist at Universidad Nacional Autónoma de México, announced in a report in the journal Inter-Research Science Publisher in 2012.
And a recently published study* shows a link between a small marine isopod (crustacean) called Idotea balthica and red alga (Gracilaria gracilis). Red algae lack motile sperm and it seemed rather surprising that they had to uniquely rely on water currents to transport their gametes to the female organs. But when idoteas crawled over them, clumps of gametes stuck to their carapace. In this study, when virgin female red algae were placed in aquariums with idoteas, their rate of pollinization went up 20 times, proving that animal pollination is clearly possible and probably the norm.
*Study carried out by an international research team led by Myriam Valero based at the Station Biologique de Roscoff (CNRS, Sorbonne Université, Pontificia Catolica Universidad de Chile and Universidad Austral de Chile), France.
Which Really Came First?
This was especially exciting in that red algae evolved about 1 billion years ago, while the earliest known pollination with the help of animals is apparently only 140 million years old. That could mean that it was likely fertilization by underwater animals that came first, long, long before plants moved onto land!
This has led to an entirely new term: zoobenthophilous pollination, which means the transfer of pollen by invertebrate animals in the benthic zone, which is the community of fauna and flora of the seabed.
If ever your gardening adventures lead to growing aquatic plants, you may have to adapt to making sea bees as happy in their aquatic environment as we now try to make native bees and other pollinators in our gardens!
Here are a few articles you can go to in order to read more about this fascinating subject: