One type of red alga can reproduce with the help of its friends: tiny sea crustaceans. These small creatures transport sex cells between males and females, like pollen-laden honey bees buzzing among flowers. Researchers report that this is the first sample of animal-driven cross-pollination in algae. The red algae and crustaceans are both part of a much older group than land plants. This raises the possibility of a new form of pollination that evolved in the ocean hundreds of millions of years earlier than previously thought. The transfer of male sexual cells, or pollen, to a female flower is known as pollination.
This usually takes place on land. In 2016, researchers discovered that marine invertebrates could “pollinate” seagrass flowers by moving between gelatinous pollen-masses from seagrasses. These are descended from land plants. However, similar behavior was not yet documented in algae. Gracilaria gratis, like other red algae, doesn’t have male sex cells that can be freely swum. It is also known as spermatia. The flow of water was thought to disperse its sex cells to female algae, much like wind can blow out pollen to fertilize land plants. Myriam Valero, the population geneticist at Paris‘s Sorbonne University, was part of the new study. She and her colleagues were studying G. gracilis’ genetics and mating. After taking seaweed samples and placing them in tanks for laboratory testing, the team noticed hundreds of tiny, oblong crustaceans within the tanks. The team was intrigued by the similarities between the algae’s spermatia and pollen and began to wonder if these crustaceans could help “pollinate” the alga.
The lab experiment involved placing male and female alga 15 cm apart in tanks that had no water movement. Some tanks contained the centimeter-long Idotea balthica (a type of isopod crustacean), while others did not. Cystocarps are formed when a female red alga fertilizes successfully. The team counted cystocarps to determine how many spermatia were fertilizing the female alga. Fertilization success rates were 20 times higher when isopods were present than when they were absent. They also created tanks that contained only female algae and wood that were exposed to male algae. Cytocarps were found in some female algae, which confirmed that crustaceans — relatives to land-living pill bugs — use the algal stalks as sex cells. Further confirmation of the role of isopods was found when the team examined the crustaceans with a powerful microscope. The creatures looked like bumblebees coated in pollen.
This discovery suggests that algae could have been one of the first organisms to reproduce by using animals to spread sexual cells. Already evidence was available that animals-driven fertilization and pollination-like service evolved before land-based pollination. Scorpion flies could have pollinated conifers tens to millions of years before flowering plants developed approximately 130 million years ago. Tiny arthropods can fertilize mosses, which look similar to the first land plants. They evolved around 300 million years ago. Red algae could be more than 800 million years old, and complex animal life goes back over half a century.
This means that animal-focused fertilization may have been possible even before scientists thought. Conrad Labandeira, a paleobiologist from the Smithsonian National Museum of Natural History, Washington, D.C., says that such a system could have extended to the Precambrian when red alga was present. He says the pollinators would not have been isopods but rather a very early arthropod group. G. gracilis may still be helped by water movement to spread its spermatia. Valero states that most of the algae’s fertilization happens in low tide rocky pools, which are calm. “We believe that the Idotea’s influence might be very important under these conditions.” The isopods might be compensated by the bushy alga and food they provide. Now, the team wants to find out if other red algae use the animal “pollinators” or if multiple animal partners are involved in their reproduction.