When it comes to reproduction, one type of red algae gets by with a little help from its friends: tiny marine crustaceans that transport gametes between male and female algae like pollen-laden bees buzzing between flowers.
The discovery is the first known example of animal-driven “pollination” of algae, researchers reported July 29 Science. Both red algae and crustaceans belong to much more ancient groups than land plants, raising the possibility that a form of pollination first evolved in the ocean, hundreds of millions earlier than previously thought.
Pollination generally describes the transfer of male gametes – pollen – to a female flower, usually on land. Then in 2016, researchers discovered that various marine invertebrates “pollinate” seagrass flowers by feeding on and moving between the gelatinous pollen masses of seagrasses that originate from land plants. But nothing similar has yet been documented in algae.
Like other red algae, Gracilaria gracilis no free-swimming male gametes. Called spermatozoa, its gametes were generally thought to be dispersed into female algae by the flow of water, similar to how wind might spread pollen to fertilize some land plants.
In the new study, Miriam Valero, a population geneticist at the Sorbonne University in Paris, and her colleagues studied the genetics and mating of G. gracilis. After gathering taking samples of the seaweed and storing them in laboratory tanks, the team kept spotting hundreds of small, elongated crustaceans in the tanks. This discovery, and the similarity of algal sperm to pollen, led the team to wonder if the crustaceans help “pollinate” the algae.
In the lab, the researchers placed male and female algae 15 centimeters apart in tanks with no water movement. Some tanks also include a centimeter length Idotea balthica, a type of isopod crustacean, while others do not. When successful fertilization occurs on the body of a female red algae, it creates a bladder-like structure called a cystocarp. By counting the cystocarps, the team quantified how many sperm reached and fertilized the female algae. When the isopods were present, fertilization success was about 20 times higher than in their absence.
The team also created tanks with only female algae and isopods exposed to male algae earlier. Some of the female algae then gave birth to cystocarps, offering more evidence that the crustaceans — relatives of land bugs — transfer gametes between algae stems. The team further confirmed the role of isopods when they looked at the crustaceans under a powerful microscope – like bumblebees dusted with pollen, the creatures had sperm stuck all over their bodies.
The discovery suggests that algae may have been among the first organisms to reproduce using animals to spread gametes.
There was already evidence that animal-driven fertilization and pollination-like services evolved before terrestrial flower pollination. Scorpion flies may have pollinated conifers tens of millions of years before flowering plants evolved about 130 million years ago (SN: 11/5/09). Mosses, which are very similar to the first land plants that evolved about 300 million years before flowering plants, can be fertilized by small arthropods. Red algae are probably more than 800 million years old, and complex animal life dates back more than half a billion years. This means that animal-powered fertilization may have occurred even earlier than scientists realized.
“Such a system could have extended into the Precambrian, when red algae were present,” said Conrad Labandeira, a paleobiologist at the Smithsonian National Museum of Natural History in Washington, DC. The pollinators would not have been isopods, he says, but very early arthropod groups.
Water movement can still help G. gracilis to spread his sperm. But much of the algae’s fertilization occurs in rocky pools at low tide, when the water is calm, Valero says. “We believe that the influence of idiot can be very important in these conditions.
In exchange for their services, the isopods can receive shelter from the scrubby algae and access to food stuck to their surface.
The team now wants to know if other red algae also use animal “pollinators” and if more than one animal partner is involved in the reproduction of the algae.