Crustaceans Discovered 'Pollinating' Seaweeds in Scientific First

Flowering plants are characterized by pollination, which is carried out by animals like bees and birds, who also satisfy human needs for coffee, honey, and macadamia nuts. However, recent findings suggest that animal-assisted pollination may have begun in the water before plants migrated to land.

The study, carried out by research teams headquartered in France and Chile, is the first to provide information on a type of seaweed that relies on tiny marine crustaceans covered in spores that resemble pollen to reproduce.

The researchers claim their research demonstrates animal-assisted pollination might have developed about 650 million years ago in the waters once a suitable pollinator arrived. This is because the red algae Gracilaria gracilis originated long before terrestrial plants existed.

Male reproductive cells, or gametes, take flight as pollen grains from seed-bearing flowering plants and gymnosperms on land. These grains are transported by wind, water, or flying insects in the hopes that they will eventually touch down on a nearby female counterpart.

Scientists later learned that some fungi and mosses also employ animals and insects to aid in reproduction, upending their understanding of animal-mediated pollination. Mosses are a form of rootless, non-flowering plant classed as a bryophyte.

Despite frequent disagreement, scientists believed it to have begun together with terrestrial plants approximately 140 million years ago, or at least during the Mesozoic, which lasted for around 252 million years.

Recent discoveries of foraging marine invertebrates carrying seagrass sperm have disproven the conventional wisdom that there are no pollinators in the waters.

Now, this new study by Emma Lavaut, a graduate student in evolutionary biology at the Sorbonne University in Paris, and coworkers describes how tiny crustaceans called isopods, Idotea balthica, aid in fertilizing a species of red seaweed, G. gracilis, which evolved approximately 1 billion years ago, long before the appearance of land plants 500 million years ago.

According to Jeff Ollerton and Zong-Xin Ren, two ecologists at the Kunming Institute of Botany of the Chinese Academy of Sciences, "the study by Lavaut et al. has broadened both the variety and the history of animal-mediated male gamete transfer, taking the concept of pollination from [land] plants to algae and potentially pushing it back to the early evolution of marine invertebrates."

Seaweeds are a form of photosynthesizing algae that are only tangentially connected to what is considered to be genuine plants.

The male gametes of G. gracilis are likewise different from those of most other seaweeds in that they lack a flagellum, leaving them floating in the ocean unless they can catch a ridge on a passing animal, as this new research reveals they frequently do.

Lavaut and colleagues demonstrated in a series of laboratory tests how the tiny marine isopods, which browse along strands of male G. gracilis, unintentionally capture the male gametes (spermatia) of the seaweed as they do, transporting them to female plants.

An idotea decorated with fluorescently coloured spermatia can be seen in the image below, which shows that crabs may act as pollinators.

According to Lavaut and colleagues, these findings "show for the first time that biotic interactions greatly boost the chance of fertilization in a seaweed."

The scientists discovered that the presence of I. balthica increased fertilization success by around 20 times compared to the absence of the rodents.

The spreading of pollen via water currents has not yet been compared to this crustacean pollination to determine which has a bigger impact.

Given that the researchers only deduced this based on the evolutionary history of the species concerned, the origins of plants requiring animal pollinators likewise remain a mystery.

According to Lavaut and colleagues, grazing idoteas can find a home, protection, and enough of food in the seaweed. The researchers discovered that in exchange, the tiny crustaceans not only assist G. gracilis in reproducing, but their appetite for parasite-like plants that inhabit G. gracilis fronds actually accelerates the seaweed's growth rates.

However, these complex mutualistic ties between plants or algae and animals are threatened just as much as the ecosystems they support in a world of rapid human-caused climate change.

When storms batter coastlines and sea levels are progressively rising inland, seaweeds like G. gracilis must reproduce in calm coastal waters. Although isopods need to be investigated, ocean acidification can damage the exoskeletons of crustaceans.

Ecologists who are interested in evolution are baffled as to what G. gracilis accomplished before I. balthica emerged on the scene, despite the fact that the threat posed by global warming is crystal obvious. This is because the isopods are just 300 million years older than the algae.

Ollerton and Ren add that "how these seaweeds were reproducing before this is a mystery," despite the fact that they most likely just relied on ocean currents.

If science has taught us anything, it is to continually be on the lookout for new surprises. Only 10% of the more than 300,000 known species of animal-pollinated flowering plants have had their pollinators identified, according to recent estimates from Ollerton.

Which species are therefore doing their magic? Ollerton and Ren conclude, "No doubt many more findings awaiting the diligent viewer of species interactions."