How small invertebrates help certain algae reproduce

• Like terrestrial pollinators – which “help” flowering plants to reproduce – marine animals facilitate the meeting of gametes in algae, according to our partner The Conversation.

• This is the case of the marine invertebrate

  Idotea balthica

  , which participates in the dispersal of spermatia from the red alga

  Gracilaria gracilis

  during its feeding activity, just as the bee transports pollen.

• This analysis was conducted by Emma Lavaut, PhD student in evolutionary biology and algae ecology, and Antoine Faure, PhD student in water sciences.

Sexual reproduction is a fundamental biological process in the evolution of living things.

In the terrestrial environment, pollination is a perfect example of co-evolution developed between plants and animals over time, making it possible to increase the efficiency of reproduction in plants, while animals benefit from a source of food. .

Thus, pollinators, by helping more than 90% of flowering plant species to reproduce, occupy an essential place in our ecosystems.

If many scientists have been interested in the vulnerability of its interactions in the face of global changes and its repercussions on the maintenance of ecosystems, no one, or almost, has been interested in the place of these animal-plant relationships in the marine environment. .

However, the marine environment is subject to increasing anthropogenic pressure, and is also the victim of a dynamic of biodiversity collapse, which is difficult to stem without knowing the complexity of the relationships between the organisms that make up this ecosystem.

Our team has been interested in animal-plant relationships in the context of algae reproduction, and we have shown in a study recently published in Science that animals facilitate the encounter of gametes in algae.

Sexual reproduction at sea

In the marine environment, many animal and plant species release their male and female reproductive cells (gametes) directly into seawater. allowing them to swim the final distance to their partner.

But, to travel most of the distance, the gametes are dependent on the movement of water, long considered the main vector involved in their encounter.

This dogma was recently lifted in 2016 in the flowering plant

, which constitutes the first known case of pollination in the marine environment.

We sought to find out whether animals also play a role in the reproduction of another emblematic plant species of the sea: algae?

​Male gametes without flagellum

This question is particularly relevant for the red alga

, the males of which release flagellum-free gametes, called “spermatia”, into the seawater, while the females keep their gametes which are not released into the environment.

Like the pollen grain, the spermatia is completely incapable of moving on its own and is dependent on sea currents to reach the female, on which fertilization takes place, as in plants.

By referring to the flagellate gametes of brown or green algae, for example, which can move on their own, we can think that the probability of the spermatia reaching the female is low.

However, reproduction in

is not uncommon, as has been shown in previous studies on a population monitored for more than twenty years, located at Cap Gris-Nez in northern France, which makes it a model of choice.

One study even shows that the majority of fertilizations in this population take place at low tide when individuals are submerged in puddles with very little water movement.

In the field, the marine invertebrate

– which belongs to the same order as woodlice (

) – is often associated with the red algae

.

This animal, commonly called “idota”, feeds on the “epiphytes” of

, that is to say small parasitic algae which grow on the red algae.

In plants, it is the feeding activity of animals – when they feed on pollen – which is the involuntary origin of the pollination process.

Our team therefore explored whether the honey bee could participate in the dispersal of spermatia during its feeding activity, as the bee transports pollen.

​Invertebrates facilitate fertilization in the red alga

Gracilaria gracilis

We have set up laboratory experiments to demonstrate that idotae promote the dispersal and encounter of gametes in

.

We placed male and female algae in aquaria, either in the presence or absence of idotae.

When fertilization takes place, a structure visible to the naked eye, the "cystocarp", forms on the female algae.

Thus, at the end of the experiment, the number of fertilizations could be estimated by counting the cystocarps formed on the female algae.

And the result is clear: there are twenty times more fertilization when the idotées are present.

But at this stage of the experiment, there was nothing to confirm that the Idotea acted as a pollinator by transporting the male gametes to the females.

Indeed, the idotees could simply participate in the dispersion of the spermatia by the water movements created when they swim in the aquarium.

In a second experiment, female algae alone (without male algae) were placed in an aquarium in the presence of idotas previously "incubated" with male algae.

Here again, multiple fertilizations were observed, making it possible to assert that these fertilizations could only have been carried out with spermatia transported by the idotae.

These results were confirmed thanks to microscopy images showing the spermatia hanging on the body and the legs of the idote.

​Ecological and evolutionary implications

This study demonstrates for the first time the role of an animal in fertilization in red algae, and this relationship could be much stronger than we think, since both parties derive reciprocal benefits from their association.

Idotes obtain a food source by grazing on the epiphytes on the surface of the algae.

They could also use it as shelter against predators thanks to their pigmentation allowing them to hide perfectly in the algae.

On the other hand, the elimination of epiphytes by isopods allows a better growth of the algae and our present study shows that idotae increase the reproductive success of algae.

This result shows that the associations between plants and animals are much more complex than we think in the marine environment, and questions the specificity of this relationship: could other red algae benefit from this gamete dispersal system? ?

Are other invertebrates involved?

It is important to answer these questions and to discover the place that these associations occupy in the functioning of marine ecosystems, especially in a context where anthropization and the resulting global changes threaten the dynamics of these ecosystems.

These results also raise questions about the evolutionary history of the relationships between plants and animals.

Indeed, animal pollination would have appeared 140 million years ago, with the first flowering plants.

However, our results show that similar relationships exist in the marine environment.

The first red algae differentiated around 900 million years ago and the first multicellular animals around 650 million years ago, well before the differentiation of flowering plants.

The appearance of associations between plants and animals could thus be much older than we currently think and have taken place well before the colonization of the terrestrial environment by plants.

Our "REPRODUCTION" file

Of course, it is also possible that the role of animals in the fertilization of plants evolved more recently in parallel in terrestrial and marine environments.

One way to address this question is to study the frequency of animal pollination in different species of red algae.

If it is widespread, it suggests that the role of animals is probably very old.

This analysis was written by Emma Lavaut, doctoral student in the international unit "Evolutionary Biology and Ecology of Algae" (EBEA, evolutionary biology and ecology of algae) / IRL3614, CNRS, SU, Roscoff Biological Station - Sorbonne University, and Antoine Faure, PhD student in water sciences at the National Institute for Scientific Research (INRS).

The original article was published on The Conversation website.

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