This is the first known case of an animal fertilizing seaweed, and was deemed significant enough to be published in Science. Click on the screenshot to see it; the pdf is here, and the full reference at the bottom.

There was one case reported previously; as the authors note, “foraging marine invertebrates were shown to carry and transfer pollen grains from male to female flowers in the seagrass Thalassia testudinum K. D. Koenig.” The pollinators were crustaceans, as they are in this case, and today’s results were apparently deemed Science-worthy because they involve seaweeds, long thought to have been fertilized only by water currents, and because the authors not only did experiments supporting fertilization by isopod crustaceans, but also showed that  male gametes of the seaweed are affixed to the body of the isopods.

The players are the marine isopod Idotea balthica, and the seaweed is the red alga Gracilaria gracilis. Here they are:

The fertilizer, I. balthica:

The fertilizee, G. gracilis, which grows in a bushlike fashion:

The isopod crustacean is often found tightly affixed to the seaweed, eating epiphytes that cover it, and the isopod gets food. The authors suggest that this association is a mutualism, perhaps evolved:

Although I. balthica grazes on other seaweed, it does not feed directly on Gracilaria but rather eliminates epiphytes at the surface of the thallus while protecting itself from predators. We suggest that the relationship could be mutually beneficial. For I. balthica, the seaweed provides shelter, and epiphytic diatoms found adhering to the surface of the thallus, whose frustules (or thecae) are found in the feces of idoteas, appear to be an important food source. In return, G. gracilis benefits through increased growth rate owing to reduced fouling and improved reproductive success.

The seaweed is dioecious, meaning that entire plants are either male or female. This poses the problem of getting the male gametes, which are immotile cells called spermatia, to the female receptive structure, called the trichogyne. Water flow is an inefficient way to do this, especially if opposite-sex plants are far apart and the spermatia don’t live that long. But Isopods can help by moving between the plants.

Fertilization, by the way, occurs when spermatia encounter the trichogyne, forming a diploid structure on the female plant called a cystocarp. If you see one, you know that the female has been fertilized. Below a cystocarp with the caption, both taken from the paper’s supplementary materials. The cystocarp itself produces spores that are released into the environment, and these undergo a complex life cycle including meiosis, so that the resulting seaweed plants are actually haploid, having only one set of chromosomes. Red algae have remarkably complex life cycles (see here).

Male isopods of the species noted above are often found gripping the male reproductive structures of red algae, and here’s where they pick up the spermatia from the male (in the photo below, the scale bar is 1 mm long:

The authors did two tests to see if isopods effect fertilization between male and female seaweeds, and also inspected the isopods to see if they actually carried spermatia. All the data show that yes, the isopods are pollinators. The experiments are simple.

Experiment 1: Virgin female algae were placed in saltwater aquaria along with males producing spermatia. There were also isopods, as well as a control with the two sexes of algae but with no isopods. Fertilization of the females was then measured.

A diagram of the experiment:  “A” is the experimental setup with isopods, “B” the isopod-less control

Fertilization success was measured by the number of cystocarps per centimeter of stem. As the data show, fertilization was 20 times higher when isopods were around than when they weren’t. The small sample size resulted in pretty big error bars, but the result is a statistically significant difference, though not “highly” significant.

Experiment 2: Virgin female seaweed plants were placed in saltwater aquaria along with isopods that had either contacted reproductive male thalli along with a control that had been handled identically but without isopods.  Fertilization ws then measured. I show the design and then the results, which were also significant, though fertilization was far less effective—perhaps because in the experiment above the isopods could go back and forth repeatedly between males and females.

A significant difference: more fertilization when isopods were there:

Finally, the authors examined the isopods themselves that had been associated with the male seaweeds, using staining and confocal laser scanning microscopy. You can see the spermatia (stained green dots) adhering to the crustaceans in the photos below:

There are a lot of them, and they tend to stick to the junctions between body segments of the isopod.

The upshot: In the laboratory, there’s no doubt that isopods can act as “pollinators” of this species of seaweed, and we can see the male gametes attached to the isopod’s body. Since this was all done in the lab, it would be nice to have confirmatory evidence in the wild.  It would be easy to capture male isopods on either male or female seaweed and determine if they, too, carry the male gametes. Experiments in the wild would be harder; you can think of some yourselves.

And, of course, given the inefficiency of fertilization in seaweeds, which when dioecious depend on water currents to get male gametes to distant female reproductive structures—a wasteful and inefficient process. Perhaps there is far more pollination of marine plants by animals than we’ve ever suspected. Right now, we know of only two cases, but that’s because experiments like these are hard to do, and perhaps because researchers hadn’t considered the possibility of animal pollination.

h/t: Athayde

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Reference:   Lavaut, E. et al. 2022. Pollinators of the sea: A discovery of animal-mediated fertilization in seaweed. Science 377:528-530