Crustaceans Discovered the First Scientifically 'Pollinating' Seaweed

Pollination is a hallmark of flowering plants, with animal pollinators such as bees and birds sustaining the world’s food supply – not to mention our cravings for coffee, honey and macadamia nuts. But new research raises the possibility that animal-assisted pollination may have appeared in the ocean long before plants moved ashore.

The study, carried out by a research group based in France and Chile, is the first to document a species of seaweed that relies on tiny marine crustaceans speckled in pollen-like spores to reproduce.

Since red algae Gracilaria gracilis evolved long before land plants appeared, the researchers say their research suggests animal-assisted pollination could have occurred about 650 million years ago in the oceans once a suitable pollinator appeared.

On land in flowering plants and gymnosperms that have seeds, the male reproductive cells, or gametes, fly in the form of pollen grains, which are carried by the wind, through the water, or by surprise insects, to hopefully land on a female mate somewhere far away. .

Scientists later discovered that mosses (a rootless, non-flowering plant classified as a lichen) and some fungi also use animals and insects to facilitate reproduction, increasing what they know about animal-mediated pollination.

While it’s often debated, researchers think it originated in concert with land plants about 140 million years ago — or at least during the Mesozoic, which stretches back about 252 million years.

Just a few years ago, scientists discovered foraging marine invertebrates carrying seagrass sperm, throwing into the ocean the long-held theory that the oceans lack pollinators.

Now, a new study from Emma Lavaut, an evolutionary biology graduate student at the Sorbonne University in Paris, and colleagues, explains how tiny crustaceans called isopods, Idotea balthicahelp fertilize a kind of red seaweed, G. graciliswhich evolved about 1 billion years ago, long before 500 million years ago when land plants appeared.

“Studies by Lavaut et al. have expanded both the variety and history of animal-mediated male gamete transfer, taking the concept of pollination from [land] plants into algae and potentially push them back to the earliest evolution of marine invertebrates,” wrote Jeff Ollerton and Zong-Xin Ren, two ecologists at the Kunming Institute of Botany of the Chinese Academy of Sciences, in perspective accompanying the paper in Science.

A type of photosynthetic algae, seaweeds are only very distantly related to so-called true plants.

G. gracilis also differ from most other seaweeds in that male gametes do not have flagella to propel them through the water, remaining in the oceans – unless they can catch the ridges of passing creatures, as this new work often does.

In a series of laboratory experiments, Lavaut and colleagues demonstrated how small marine isopods, which forage along male strands G. gracilisinadvertently collect male gametes (spermatia) of seaweed as they do, transferring them to female plants.

You can see in the image below, an idotea decorated with fluorescence-stained spermatia, which suggests that crustaceans can function as pollinators.

An idotea appendage covered with spermatia. (Sebastien Colin, Max Planck Institute of Biology/CNRS/SU).

“Our results show for the first time that biotic interactions dramatically increase the likelihood of fertilization in seaweeds,” write Lavaut and colleagues.

Fertilization success is about 20 times higher in the presence of I. baltic than without a creature, the team found.

But they haven’t compared the pollination of these crustaceans to the dispersal of pollen along water currents to see which plays a bigger role.

The origins of plants using animal pollinators also remain wide open, as the researchers only concluded this based on the evolutionary history of the animals involved.

Lavaut and colleagues think that seaweeds provide habitat, shelter, and abundant food for grazing idoteas. In return, the little crustaceans don’t just help G. gracilis reproduce, but their appetite for plants is like a colonizing parasite G. gracilis The leaves actually increase the growth rate of seaweed, the researchers found.

Idotea balthica, perched on a red seaweed leaf. (Wilfried Thomas, CNRS/SU).

However, in a world of rapidly changing human-caused climate, these intricate mutualistic relationships between plants or algae and animals are threatened as are the ecosystems they maintain.

Seaweed like G. gracilis rely on calm coastal waters for breeding, when coastlines are hit by storms and sea levels slowly rise inland. Meanwhile, ocean acidification can weaken crustacean exoskeletons – although this needs to be studied in isopods.

While the threat of global warming is very clear, evolutionary-minded ecologists are still confused about what G. gracilis do before I. baltic appeared on the scene, because isopods are not as old as algae, evolving ‘only’ 300 million years ago.

Although they most likely relied solely on ocean currents, “how these seaweeds reproduced before this is a mystery,” explain Ollerton and Ren.

If science has taught us anything, we should always be prepared for more surprises. Recent estimates from Ollerton suggest that only one-tenth of the more than 300,000 species of flowering plants that animals pollinate have documented pollinators.

So which species do their magic? “Undoubtedly more revelations await the careful observer of species interactions,” concluded Ollerton and Ren.

This study was published in Science.

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