This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Last summer I was hiking in the tundra near Gray's and Torrey's Peaks when I came upon a moss that looked strange. It had little flattened discs that looked something like this:
What were these things?
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Then I happened to pick up a book called "Gathering Moss", and suddenly I knew what it was I had seen.
To understand, you must first appreciate this salient fact: moss, unlike most other land plants, still make swimming sperm*. (Wait, moss make sperm? Yes, yes they do. Why they do and how it fits into the Great Arc of Moss Life is a story for another day) And these sperm are surprisingly bad swimmers, and kinda dumb, to boot.
This is all the more remarkable considering that they have one of the most herculean fertilization tasks on the planet. They must swim, unaided, across an ephemeral film of water coating the nooks and crannies of mosses in search of an egg concealed within a protective structure called an archegonium. As Gathering Moss author Robin Wall Kimmerer puts it, "it's a race between sperm and evaporation."
Kimmerer further explains their depressing lot in life:
Moss sperm are produced in great numbers, but each tiny cell has a vanishingly small chance of ever finding an egg. Unlike the peepers who call so strenuously to their mates, moss sperm have no signal to guide them to their destination and so swim at random in the water film. Most are simply lost in the labyrinth of leaves. The small sperm are weak swimmers and carry limited energy for their travels. Once released from the antheridium, the clock begins to count down their survival probability. Within an hour, all will be dead, having exhausted their resources. The eggs continue to wait.
Wow. Life is tough for all sperm when you think about it, and among them, moss sperm seem to have drawn the short stick.
Moss sperm also happen to have a rather bizarre shape that may have something else to do with their dilemma. You might describe it as two coiled tails attached to one of those Cleopatra-style snake armbands. Here's our handy-dandy chart o' plant sperm, with a rough sketch of one at the bottom.
And here's a link to a detailed diagram of a sperm cell from a liverwort, a close relative of mosses. These things look like they have all the handling of a Kinetics Sculpture Race entry. If I had to drive one around on my way to find an egg, I'd end up in the ditch too.
Fortunately, the mosses have not left everything up to the navigational prowess of their male gametes. Through natural selection, they have found a better way. At least two better ways, actually. The first was the solution to my alpine moss mystery.
Here's Robin Wall Kimmerer again:
The sperm need all the help they can get in moving toward the egg, seldom travelling more than four inches from the structure called an antheridium that produced them. Some species have devised other means to increase the distance, harnessing the power of a splash to spread the sperm. In species like Polytrichum, the antherridia are surrounded by a flat disk made of leaves, radiating around them like the petals of a sunflower. A raindrop plummeting onto this disk can splash the sperm as far as ten inches, more than doubling the distance they can travel.
So what I saw were splash cups -- bowl-shaped structures designed to harness the power of a falling rain drop to the parabolic shape of a cup to passively launch propagules into the world. Let's call this the "Evel Knievel Strategy". Splash cups are widely employed among plants and fungi for spore or propagule dispersal. The bird's nest fungi and moss-relatives called liverworts both have similar structures for dispersing packets of their spores or tissue.
The second way could be called the "Douglas Adams Strategy". That is: hitchhike. In a paper published in Nature this past week, scientists followed up on findings that tiny arthropods called springtails and oribatid mites (little moss-dwelling arachnids with Beyonce-esque hindquarters) may be responsible for giving moss sperm their big break by unwittingly picking up moss sperm as they lumber through the microforest of a moss.
What is a springtail? Here's David Attenborough to explain:
WARNING: Severe Cuteness Threat Level Red
On second thought, perhaps *this* is the strategy that should be known as the "Evel Knievel". Did you notice how many of them were hanging out on moss fronds? This is their preferred habitat, among the mosses and mulch of the world, and they are one of the most abundant micro-animals on the planet.
In any case, it had been known for some time that moss sperm were hitching rides on springtails at least some of the time. What was less well known was whether hitchin' a ride on a springtail was a significant contributor to what biologists like to politely term "reproductive success". Nor did anyone know whether or not the mosses might be further manipulating the sperm-loaded springtails with the olfactory equivalent of "come hither".
Scientists at Portland State University decided to find out. And the answer to both questions was a rousing "Yes".
Into some experimental chambers they placed nothing but moss. In others, they sprayed the mosses with water. And in a third broup, I can only hope that someone in the lab got to utter "Release the Springtails!" while cackling evilly as they deposited them into their new mossy homes.
As it turned out, left to their own devices, moss sperm were just as miserable at reaching moss eggs as their reputations implied. Only about 2-8% of unsplashed and un-springtailed mosses managed to hook up even one egg with one sperm. Splashed mosses managed to fertilize an egg about 10-20% of the time, while springtails managed to boost the chance of at least one instance of reproductive success to about 15-30%. And when scientists put the two together? Moss nooky overdrive. Almost half of the mosses in these samples managed to deliver sperm to egg at least once.
You may have noticed that the splash cups in the photo at the top of this page are red and somewhat floral. And also that Robin Kimmerer compared the disc-like structures to sunflowers. Well, it turns out the flower comparison may be even more apt. Because when the Portland State scientists set out to measure what scents might be coming from the plants, they found that although both female and male plants produced odors, the females were making a much wider variety of strong scents**. Furthermore, many of the compounds they were making had been previously identified in flower perfumes. And, when given a choice of odors produced by male or female moss plants, but no actual access to or sight of the plants, springtails preferred the scent of a female.
Shat we have here may be a case of convergent evolution -- when a similar trait is acquired by two distantly related groups -- of a plant-pollinator relationship involving arthropods. In fact, considering that the bryophytes, the ancestors of mosses, made their appearance on land in the neighborhood of 450 million years ago, and flowering plants didn't evolve until about 125 million years ago, I'd say the odds are that the flowering plants are the second ones to this party.
It also turns out that male sperm, although still demonstrably wretched swimmers, might not be so hapless as it has seemed.The authors of the Nature paper noted that scientists recently found that moss sperm may be more "long-lived and stress tolerant" than previously thought, which may enable them to travel fairly long distances as long as they're not the ones having to do the driving. In a sense, moss sperm may be perfectly well adapted for reaching moss eggs in their preferred environment. Happy mosses are never at a loss for splashing water, or eager springtails.
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*Swimming sperm are a holdover from mosses' ancestors, which were green algae that lived in the sea. When you live in water, swimming sperm are de rigeur. When mosses sprouted onto land, they retained this feature, while the lineages that led to most other land plants ultimately did not.
**In this article in Science News, Cornell University scientist Robert Raguso points out that the scent compounds produced by female plants in the study varied greatly, implying that they either weren't important for reproduction, or that some sort of deception might be going on. If the latter is the case, I've read about that sort of thing going on here in Colorado, where dainty and incredibly cute fairy slipper orchids vary greatly in their odor and its strength.
Why? Because the orchids contain no actual reward (i.e. nectar -- so they're cheating the bees) for the pollinator, different aromas are more likely to make the same poor insect fall for the same scam twice, increasing their odds of pollination. Whether that's what's going on in mosses is still anyone's guess, but if true, it certainly adds an interesting twist to what the mosses are doing with the springtails.