Mossy Millipedes: Meet the Platyrhacids

by Joseph DeSisto

I’ve been wanting to write an article about bryophytes — mosses and related plants — for some time now, ever since I was able to take a course on bryology here at UConn. Conveniently, bryophytes and invertebrates have formed some amazing relationships, giving me the perfect excuse to write about them! Today’s story comes from one such relationship, between several different bryophytes and an unusual millipede in the family Platyrhacidae.

A platyrhacid millipede, Nyssodesmus python, from Costa Rica. Photo by D. Gordon E. Robertson, licensed under CC BY-SA 3.0.

A platyrhacid millipede, Nyssodesmus python, from Costa Rica. Photo by D. Gordon E. Robertson, licensed under CC BY-SA 3.0.

The platyrhacids are just one of many “flat-backed” millipede families in the order Polydesmida, which itself accounts for roughly a third of the known millipede diversity. What makes the platyrhacids a bit odd-looking among other flat-backs is that their back plates (tergites) often have backwards-pointing triangular projections, like the enlarged scales on crocodile backs.

While other millipedes have such projections, and some platyrhacids do not, I’ll still take this opportunity to give platyrhacids a common name: the crocodile millipedes. The name works on a second level: just as crocodiles often have algae growing on their bodies, so does one crocodile millipede have mosses and liveworts growing on its tergites.

First, a primer on mosses and other bryophytes. Just as millipedes are one of the oldest terrestrial invertebrate lineages, so bryophytes are the oldest living land plants. Because of this, they are often regarded as primitive, but in fact bryophytes are both complex and incredibly diverse.

The resilience and diversity of mosses have allowed them to colonize an immense variety of habitats. Photo by Thomas Bresson, licensed under CC BY 3.0.

The resilience and diversity of mosses have allowed them to colonize an immense variety of habitats. Photo by Thomas Bresson, licensed under CC BY 3.0.

Mosses and liverworts grow all over the place, from streams and on the surfaces of tree leaves to bare rock and the sides of houses. Their ability to colonize new habitats is aided by their modes of reproduction: in addition to producing microscopic spores, bryophytes can develop from broken-off fragments. A study by Lewis et al. (2014) suggests that some moss populations owe their existence to tiny moss fragments spread by birds migrating from Alaska to sub-Antarctic Chile!

Now let’s get back to crocodile millipedes. The species in question is Psammodesmus bryophorus, which I’m going to call the mossy crocodile millipede. The mossy crocodile millipede was discovered in a mountain rainforest, high in the Andes of Colombia (Hoffman et al. 2011 — incidentally this was one of the last new millipedes described by Hoffman, one of the greatest millipede taxonomists of all time).

The mossy crocodile millipede, , complete with its bryophyte camouflage. Photo from Martinez-Torres et al. (2011), licensed under CC BY 4.0.

The mossy crocodile millipede, Psammodesmus bryophorus, complete with its bryophyte camouflage. Photo from Martínez-Torres et al. (2011), licensed under CC BY 4.0.

These millipedes are not common — only 22 specimens could be found in the site where they were discovered, the Río Nambi Natural Reserve. Of those, 15 specimens carried at least one, and typically many, moss or liverwort species on their backs (Martínez-Torres et al. 2011). Although moss spores are often spread by insects, for bryophytes to actually be growing on an animal is unusual. Among arthropods, only a few kinds of weevil and one harvestman have been reported carrying just a few moss species.

And yet, Martínez-Torres et al. (2011) report on their specimens “complex mosaics of bryophyte species.” Not only were the millipedes carrying bryophytes, they were carrying ten different species, in five different families! Many of these had never been reported as living on arthropod exoskeletons before, making this a big discovery both for diplopodologists (millipede scientists) and bryologists.

A liverwort growing on one of the tergites of the mossy crocodile millipede. Photo from Martinez-Torres et al. (2011), licensed under CC BY 4.0.

A liverwort growing on one of the tergites of the mossy crocodile millipede. Photo from Martínez-Torres et al. (2011), licensed under CC BY 4.0.

This may not be a totally symbiotic relationship — it’s entirely possible that the mosses and liverworts benefit only by having a growing site, while the millipedes get a nice suit of camouflage. All ten of the bryophytes here are also found either on soil or on the surfaces of other plants. Even so, it’s tempting to think there might be a more complex relationship.

Perhaps the millipedes help the bryophytes disperse their offspring. Or maybe the millipedes deter tiny animals that might eat the mosses, since millipedes are essentially walking cyanide bombs. We can only hope more mossy crocodile millipedes will be found — perhaps then we can get a better idea of what’s really going on in the mountain forests of Colombia.

I know I said only Tuesday and Thursday, but I’m on a bit of a writing binge right now. If you’re someone who gets an e-mail notification every time I post a new article … sorry about that (only a little). When I come down from whatever this is, things will normalize, I promise.

For an excellent blog devoted to bryophytes, be sure to check out Moss Plants and More by Jessica Budke.

Cited:

Hoffman R., D. Martínez, and A.E.F. Daza 2011. A new Colombian species in the millipede genus Psammodesmus, symbiotic host for bryophytes (Polydesmida: Platyrhacidae). Zootaxa 3015: 52-60.

Lewis L.R., E. Behling, H. Gousse, E. Qian, C.S. Elphick, J.F. Lamarre, J. Bêty, J. Liebezeit, R. Rozzi, and B. Goffinet. 2014. First evidence of bryophyte diaspores in the plumage of transequatorial migrant birds. PeerJ 2:e424 doi: 10.7717/peerj.424

Martínez-Torres S.D., A.E.F. Daza, and E.L. Linares-Castillo. 2011. Meeting between kingdoms: Discovery of a close association between Diplopoda and Bryophyta in a transitional Andean-Pacific forest in Colombia. International Journal of Myriapodology 6: 29-36.

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