Apr 082016
 

An elusive giant has risen from the muck at the bottom of a Florida canal. It is the ciliate Loxodes rex, a large and shapely thing once thought to be endemic to equatorial Africa. Hunter Hines, a Ph.D. student affiliated with the Harbor Branch Oceanographic Institution, has published the first  record of the species on the North American continent, and has also posted video of the creature on YouTube:

It is not the first time the species has turned up far from its supposed home. Specimens of the big guy have previously been reported from an artificial pond Thailand.  It also turns up on a diversity checklist from São Paulo (though identifications in lists of that kind can’t be independently corroborated). The new study provides better images and metrics, although silver staining and gene sequencing were not done.

Its presence in Florida is interesting, because Loxodes rex is one of the 52 ciliate “flagship species” listed by Wilhelm Foissner as good candidates for testing the hypothesis that some species of ciliate have a restricted geographical range (Foissner, 2008). The ciliates on Foissner’s list are all eye-catching creatures easily seen in the light microscope, difficult to misidentify and unlikely to be overlooked in diversity studies. All of them seem to show some evidence of endemism: that is, they have not been found everywhere in the world, but have turned up only in certain regions. This makes them ideal for proving or disproving the hypothesis that ciliates, like orchids and elephants, have biogeography. As Foissner and his collaborators put it: “[T]he hypothesis of restricted distribution of certain ciliate species must be refused when a considerable number of them is found in all or most biogeographic regions.” (Foissner et al, 2008)

Loxodes rex, from Hines et al, 2016

Loxodes rex, from Hines et al, 2016

To some readers, it might seem odd that there is any controversy about this. Cave snakes and capybaras have biogeography. Why not ciliates? That question takes us back to a hundred-year-old conjecture by two Dutch biologists, Martinus Beijerinck and Baas Becking, concerning the apparent ubiquity of microbial species. Their speculation was that microorganisms, by virtue of their small size and certain special talents (such as the ability to form resting cysts), were not confined by geographical barriers. Every species could exist anywhere in the world, as long as its preferred environmental conditions are available. If an organism is small enough to waft about in the wind, or travel from place to place in the soggy tailfeathers of a bird, its distribution cannot be limited to any particular region. Baas Becking bundled this idea into a phrase so concise and quotable that it has become almost obligatory to repeat it every time the subject of microbial endemism is raised: “Everything is everywhere, but the environment selects.”

This is sometimes dignified as the “Baas Becking hypothesis,” but Foissner argues that is not a proper hypothesis at all, because it is unfalsifiable (Foissner, 2006). After all, if an organism in one pond fails to appear in another, the difference can always be explained by some small dissimilarity in the local conditions. No two ponds provide identical environmental conditions–indeed, there can be an impressive diversity of micro-environments within a single pond, and local conditions can change from one hour to the next. To test the “ubiquity hypothesis” as expressed in Becking’s memorable phrase, you’d need to have two truly identical bodies of water in two distinct biogeographical regions. It is an impractical experiment, because there are simply too many variables.

The main competitor to “everything is everywhere” is the “moderate endemicity model” advanced by Foissner and others, which holds that while as many as 2/3 of microbial species may be cosmopolitan, the rest are found in only certain places. In other words: Some things are not everywhere. This hypothesis is testable, thanks to the terms Foissner himself has set: if a “considerable number” of the flagship species turn out to be cosmopolitan, moderate endemicity is falsified.

By that standard, it does seem that the newly expanded range of Loxodes rex slightly improves the larger case against ciliate endemism–although, it should be said, Hines does not address Foissner’s stipulation that the flagship species must be found in a site that is not “prone to be contaminated by invaders” (Foissner, 2006).  Obviously, the loss of one purported endemic does not constitute a “considerable number,” but the authors of the Loxodes paper also claim to have found “other large ciliates with alleged restricted distributions,” and have already posted video of one, the fairly cool-looking Frontonia vesiculosa. It will be interesting to see what else they turn up.

Dragesco's drawing of Loxodes rex (A) next to Loxodes magnus (B). Magnus is big, but rex is the king.

Dragesco’s drawing of Loxodes rex (A) next to Loxodes magnus (B). Magnus is big, but rex is the King.

While Hines et al. do not show stained specimens, the imaging is fairly clear, and the data provided do seem to show that members of the Florida population of Loxodes rex are morphologically close to the ones Jean Dragesco found in Africa. This this can be explained by dispersal, either through natural means (everything is everywhere) or by recent human activity.

Or, it could also be (and what’s a blog for, if not wild speculation?) that both populations are remnants from a single group that split when the Gondwanan parts of the supercontinent Pangaea broke up some 180 million years ago (Florida, unlike most of North America, was largely attached to Gondwana). If that is true, then phylogenetic analysis of the Florida and Africa populations–if anyone ever gets around to it–would show a lot of genetic divergence (a few hundred millions years’ worth), even though morphology of the species has been conserved. Morphological stasis of a species, even over long stretches of geological time, is unexceptional, as Stephen Jay Gould argued at exhausting length in The Structure of Evolutionary Theory, and recent work by Heger et al. on the testate amoeba Hyalosphenia papilio has shown how much genetic diversity can be concealed behind a highly conserved morphology.

If the Florida population of Loxodes rex turns out to be a stable, genetically distinct subgroup, like the cryptic species Hyalosphenia papilio studied by Heger et al., the notion that ciliates have biogeography remains somewhat intact, while the practice of defining species by small numbers of morphological characters suffers another small blow.

Of course, that moves the goal posts for testing the “ubiquity hypothesis,” but I do feel we need to ask whether the ongoing dispute about whether “everything is everywhere” is, in part, an artifact created by simplistic species concepts.

The paper by Hines et al. is Open Access, and can be read here: The First Record for the Americas of Loxodes rex, a Flagship Ciliate with an Alleged Restricted Biogeography.

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  3 Responses to “Out of Africa? A Ciliate Turns Up on the Wrong Side of the Atlantic”

  1. Nice post! It makes me wonder how speciation occurs with the apparent lack of geographic barriers.

    Luke

    • Hi Luke, thanks for the comment! There’s a lot of potential debate packed into it. First, I have to stress that “lack of geographic barriers” of the flagship morphospecies has yet to be established. And then, there is the species problem itself. In principle, two populations can be morphologically indistinguishable, yet belong to completely separate biological species (cryptic species, like Paramecium aurelia, might prove to be more common than not, among ciliates). And then, there is the vast subject of the speciation process to think about. If everything turns out to be everywhere, allopatric speciation would be impossible, but other modes — sympatric, peripatric — are still in play.

      Some of the EiE debate might be generated by loose definitions of terms. What do we really mean by “geographic”, as opposed to “environmental” barriers? Consider the old notion that size of the organism is sufficient to determine its potential ubiquity. Orchid seeds are typically protistan in size, and readily float in the air. Some are very small indeed (seeds of Anoectochilus imitans are just 50 micrometres long). Nonetheless, orchids (a notably species-rich group) do have biogeography. Presumably, dispersal is limited not by crude physical barriers but by their highly specialized needs, such as methods of pollination that tie them to particular species of insects. One could say that the presence of a needed pollinator is an environmental limit, not a geographical one–but at that point the argument begins to seem narrowly philosophical, a disagreement about terms. The fact remains that orchid species are indeed endemic to certain areas. And it could be that similar quasi-environmental barriers (presence of important symbionts or trace compounds, for instance) might confine some protists to a small range.

      • I agree that there are likely many protists with ranges defined by quasi-environmental barriers, but it seems like these are an exception to a general trend of cosmopolitanism. The only ciliates that seem to have any geographic distribution, other than Foissner’s flagship species, are obligate symbionts.
        Of course, these findings are biased by the fact that most well-studied species are common. It would be hard to say that a rare species is endemic to a certain region just because it hasn’t been discovered elsewhere. Adding to this, very few studies differentiate between cryptic species when assessing species distribution. I think the future of biogeography studies will be purely genetic to accommodate the biological species concept and to target rarer species.

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