ABOVE: A small crustacean (Idotea balthica) moving on a branch of the red alga Gracilaria gracilis Wilfried Thomas @Station Biologique de Roscoff, CNRS, SU, Roscoff, France

Once considered to be uniquely for the benefit of terrestrial plants, the matchmaking abilities of insects, birds, and bats were found to be shared by marine animals a few years ago, when scientists reported that nocturnal invertebrates pollinate the flowers of a seagrass species. A study published online today (July 28) in Science shatters yet another pollination paradigm by showing that a small crustacean species facilitates the fertilization of the red alga Gracilaria gracilis. This is the first evidence of animal-mediated “pollination” in a photosynthetic organism belonging to a group that is much more ancient than land plants. 

The leader of the study, Myriam Valero, a population geneticist at Roscoff Marine Station in France, explains that the reproductive cycle of red algae shares some similarities with that of land plants. For instance, the male gametes—called spermatia—are not flagellated, and the female gametes are not released into the water. Thus, the nonmotile spermatia “have to find their way to the female plant,” she explains, and until now it was believed that they did so by relying solely on abiotic factors such as water currents.

But while collecting the algae for study, Valero and her colleagues always found little crustaceans of one to a few centimeters long around it, she recounts. At first glance, these animals (Idotea balthica) are difficult to see in the field, she says, since they share the algae’s color “and they are quite mimetic.” But once you bring the seaweed into the lab, you can see hundreds of them. The animals find shelter in the algae and food in the form of diatoms. The researchers began to wonder whether the continuous presence of these creatures could be helping to transfer the algae’s male gametes to the female organ. 

They first tested their hypothesis by placing male and female G. gracilis individuals 15 centimeters apart in aquaria with calm seawater, minimizing the possibility that water movement could intervene in fertilization. In one set of aquaria, they added I. balthica, and in the other they did not. After one hour, they quantified the number of fertilized zygotes, called cystocarps, which are visible to the naked eye. The difference was striking: the aquarium containing crustaceans had an average of about 20 times more cystocarps per centimeter of female body than the control.

Then, to test whether the increased fertilization in the presence of the animals could be due to the flow of water generated by their movement, or whether they directly carry spermatia, Valero and her colleagues conducted a second experiment. They preincubated the crustaceans with male seaweed in its sexual phase for one hour, then moved them to an aquarium containing only G. gracilis females. The fertilization success after one hour in the presence of I. balthica was again significantly higher than in a control aquarium without them.

Three-dimensional reconstruction of a crustacean carrying on its body stained spermatia, observed as small cyan dots.<br><br>
Young crustacean (Idotea balthica) carrying stained spermatia of Gracilaria gracilis on its body. Three-dimensional reconstruction from confocal laser scanning microscopy.
Sebastien Colin; Max Planck Institute for Biology, TÜbingen, Germany; Station Biologique de Roscoff, CNRS, SU, Roscoff, France

Valero acknowledges, however, that they did not test for the relative importance of animal transport versus that of water flow, and hypothesizes that in the wild this might depend on environmental conditions. For instance, the crustaceans’ help might be more relevant when the algae experience low tides and the water is relatively calm, she says. 

“We really have no idea” about how important animal-mediated marine pollination is, concurs Brigitta van Tussenbroek, an ecologist at the Institute of Marine Sciences and Limnology at the National Autonomous University of Mexico whose team first described underwater pollination of a seagrass species. Both scientists agree that this is an important question yet to be answered. 

Van Tussenbroek, who was not involved in the new study, adds that it’s “really great” that the authors have found evidence of animal pollinators in another aquatic photosynthetic organism “completely unrelated in evolutionary terms . . . to seagrasses.” Algae and seagrass may not be evolutionarily related, but they are in the same setting and share “the same environmental pressures,” she notes. It was only a matter of time before another instance of underwater pollination by animals was found, she says, adding that this phenomenon is likely “not unique to these two.”

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