Last year I wrote about two groups of plants that emit foul odors when they bloom: corpse flowers and carrion flowers. Their scent is akin to the smell of rotting flesh, hence their common names. The purpose of this repugnant act is to attract a specific group of pollinators: flies, carrion beetles, and other insects that are attracted to gross things. Though this particular strategy is rare, these aren’t the only plants that have evolved to produce stinky smells in order to recruit such insects to aid in their reproductive processes. For one, there is a very unique group of mosses that do this, commonly known as dung mosses. Judging from the name, you can probably imagine what their smell must be like. However, their common name doesn’t just describe their scent, but also where they live.
At least three genera (Splachnum, Tetraplodon, and Tayloria) in the family Splachnaceae include species that go by the common name, dung moss. All Splachnum and Tetraplodon species and many species in the genus Tayloria are entomophilous. Entomophily is a “pollination syndrome”, a subject we will explore more thoroughly in future posts, in which pollen or spores are distributed by insects. Compare this to anemophily, or wind pollination, which is the more common way that moss spores are distributed. In fact, dung mosses are the only mosses known to exhibit entomophily.
Before we go too much further, it’s probably important to have a basic understanding of how mosses differ from other plants. Mosses are in a group of non-vascular and non-flowering plants called bryophytes. Vascular tissues are the means by which water and nutrients are transported to and from different plant parts. Lacking vascular tissues, water and nutrients are simply absorbed by the leaves of bryophytes (although some species have structures akin to vascular tissue), which is why they typically grow low to the ground and in moist environments. Bryophytes also lack true roots and instead have rhizoids, threadlike structures that anchor the plants to the ground or to some other substrate (such as dung).
Another major distinction between bryophytes and other plants is that bryophytes spend most of their life cycle as a haploid gametophyte rather than a diploid sporophyte (haploid meaning that it only has one set of chromosomes; diploid meaning that there are two sets of chromosomes, one from the father and one from the mother). In most plants, the haploid gametophyte is a sperm (pollen) or an egg. In bryophytes, the familiar green, leafy structure is actually the gametophyte. The gametophyte houses sperm and egg cells, and when the egg is fertilized by sperm it forms a zygote that develops into the sporophyte structure which extends above the leafy gametophyte. A capsule at the top of the sporophyte contains spores which are eventually released and, upon finding themselves on a suitable substrate in a hospitable environment, germinate to produce new plants. The spore then is comparable to a seed in vascular, seed-bearing plants.
As stated earlier, the spores of most mosses are distributed by wind. Dung mosses, on the other hand, employ flies in the distribution of their spores. They attract the flies by emitting scents that only flies can love from an area on the capsule of the sporophyte called the apophysis. This area is often enlarged and brightly colored in yellow, magenta, or red, giving it a flower-like appearance which acts as a visual attractant. The smells emitted vary depending on the type of substrate a particular species of dung moss has become adapted to living on. Some dung mosses grow on the dung of herbivores and others on the dung of carnivores. Some even prefer the dung of a particular group of animals; for example, a population of Tetraplodon fuegiensis was found to be restricted to the feces and remains of foxes. However, dung is not the only material that dung mosses call home. Certain species grow on carrion, skeletal remains, or antlers. The smells these species produce attract flies that prefer dead flesh and bone in various states of decay.
The spores of dung mosses are small and sticky. When a fly visits these plants, the spores adhere to its body in clumps. The fly then moves on to its substrate of choice to lay its eggs, and the spores are deposited where they will then germinate and grow into new moss plants. Flies that visit dung mosses receive nothing in return for doing so, but instead are simply “tricked” into disseminating the propagules. The story is similar with corpse flowers and carrion flowers; flies are drawn in by the smells and recruited to transmit pollen but receive no nectar reward for their work.
There are 73 species in the Splachnaceae family, and nearly half of these species are dung mosses. These mosses are mostly found in temperate habitats in both the northern and southern hemispheres, with a few species occurring in the mountains of subtropical regions. They can be found in both wet and relatively dry habitats. Dung mosses are generally fast growing but short lived, with some lasting only about 2 years. It isn’t entirely clear how and why mosses in this family evolved to become entomophilous, but one major benefit of being this way is that their spores are reliably deposited on suitable habitat. Because of this directed dispersal, they can produce fewer and smaller spores, which is an economical use of resources.
Koponen, A. 2009. Entomophily in the Splachnaceae. Botanical Journal of the Linnean Society 104: 115-127.
Marino, P., R. Raguso, and B. Goffinet. 2009. The ecology and evolution of fly dispersed dung mosses (Family Splachnaceae): Manipulating insect behavior through odour and visual cues. Symbiosis 47: 61-76.