The crucial role of insects in the pollination of flowering plants is well known, but the fertilization of algae by marine animals has until now been thought to be non-existent. A team led by a CNRS researcher from the Franco-Chilean Research Unit of Evolutionary Biology and Ecology of Algae at the Roscoff Marine Station (CNRS / Sorbonne University / Pontifical Catholic University of Chile / Australian University of Chile) discovered that small crustaceans known as idothei contribute to the reproductive cycle of red algae Gracilaria gracilis. The scientists’ findings are published in Science (July 29, 2022). They suggest that animal fertilization is much older than previously thought.
Do marine animals participate in the reproductive cycle of algae like pollinating insects on land? The distribution of the male gametes or spermatozoa of red algae usually depends on the movement of water, and until now scientists did not recognize the role of animals.
However, an international group led by Miriam Valera, a CNRS scientist, associated with the Research Unit of Evolutionary Biology and Ecology of Algae (CNRS / Pontifical Catholic University of Chile / Sorbonne University / Australian University of Chile) and Roscoff Marine Station (CNRS / Sorbonne University)1showed that tiny sea creatures called idothei act as ‘sea bees’ for red algae Gracilaria gracilisnaya.
Idothei promote fertilization G. gracilis as they float among these algae. The surfaces of male algae are dotted with reproductive structures that produce sperm, covered with mucus, a sticky substance. As the idethea passes, the spermatia adhere to the cuticle and are then deposited on the thalli of any female algae with which the crustacean comes in contact, thus helping G. gracilis reproduction
But the idoteas also benefit from this arrangement. Seaweeds provide them with space and plank: Idothei cling to the algae as protection against strong currents, and they chew on the small organisms that grow on their thallus. This is an example of a mutualistic interaction – a win-win situation for both plants and animals – and the first time an interaction of this kind has been observed between an algae and an animal.
Although these initial findings do not indicate the extent to which gamete transfer by animals contributes to algal fertilization compared to water movement, which was previously thought to be the only mode of gamete dispersal, they provide surprising insight into the origins of animal-mediated plant fertilization. Before this discovery, it was believed that the latter appeared among land plants 140 million years ago. Red algae originated more than 800 million years ago, and their fertilization through animal mediators may have occurred long before pollination on land began. Valero’s team now wants to focus on a few other questions: Do ideates cause spermatogenesis? Are they able to distinguish the male G. gracilis algae from female individuals? And most importantly, is there a similar interaction between other marine species?
1 Other participating scientists come from the Instituto de Ciencias Ambientales y Evolutivas (Facultad de Ciencias, Universidad Austral de Chile), the Laboratory of Integrative Biology of Marine Models (CNRS / Sorbonne University) and the Max Planck Institute for Biology in Tübingen (BioOptics facility) .