Field of Science

The Camisiids: Cryptic Inhabitants of Soil and Wood

Various views of Camisia biverrucata, copyright Pierre Bornand.


The animal in the above pictures is a typical representative of the Camisiidae, a widely distributed family of oribatid mites. Members of this family can be found in soil, on the trunks of trees, or hidden among mosses and lichens. They are slow-moving animals and are often concealed from potential predators by an encrusting layer of dirt and organic debris. Carrying this encrusting layer may be related to a reduction in the offensive chemical-producing glands that are used by many other oribatids for defense (Raspotnig et al. 2008). In members of the genus Camisia, the openings of these glands are completely covered by dirt, but in the genera Platynothrus and Heminothrus the openings still protrude above the encrustation. The recently described Paracamisia osornensis, which does not carry an encrusting layer, retains a large offensive gland (Olszanowski & Norton 2002).

Close to 100 species have been assigned to this family; though found in most parts of the world, camisiids are most diverse in the Northern Hemisphere. One species in particular, Platynothrus peltifer, is almost global in distribution and the range of habitats in which it has been found includes soil, litter, peat and even aquatic habitats (Norton & Behan-Pelletier 2009) When one is as small and metabolically undemanding as these animals are, there may be surprisingly little difference between being out in the air or immersed in water, and even primarily terrestrial oribatids may survive submersion almost indefinitely. Genetic studies of P. peltifer have identified a high level of within-species divergence and it has been calculated on this basis that this species may have survived almost unchanged in external appearance for some 100 million years (Heethoff et al. 2007).

The ubiquitous Platynothrus peltifer, copyright Centre for Biodiversity Genomics.


The Camisiidae are closely related to another oribatid family, the Crotoniidae, that is found in South America and Australasia. One of the more significant differences between the two families is that whereas the camisiids appear to be entirely parthenogenetic, crotoniids reproduce sexually. Recent analyses, both molecular and morphological, indicate that the 'camisiids' are paraphyletic with regard to the crotoniids, leading Colloff & Cameron (2009) to treat the latter as a subfamily, Crotoniinae, of the former. This re-classification has been accepted by other authors though the law of priority requires that the combined family should be known as the Crotoniidae, not Camisiidae. The nested position of the sexual crotoniines within the asexual 'camisiids', with other related oribatid families also being asexual, has led to the suggestion that the crotoniines have somehow re-evolved sexuality. This would be fascinating if true, seemingly violating the usual principle that complex features can't be re-evolved once lost. Personally, I tend to be sceptical of claims like this (see this old post, for instance). I would like to see evidence beyond simple phylogenetic position to indicate if this is a true re-evolution rather than an historical bias towards loss of sexuality giving a misleading image.

REFERENCES

Colloff, M. J., & S. L. Cameron. 2009. Revision of the oribatid mite genus Austronothrus Hammer (Acari: Oribatida): sexual dimorphism and a re-evaluation of the phylogenetic relationships of the family Crotoniidae. Invertebrate Systematics 23: 87–110.

Heethoff, M., K. Domes, M. Laumann, M. Maraun, R. A. Norton & S. Scheu. 2007. High genetic divergences indicate ancient separation of parthenogenetic lineages of the oribatid mite Platynothrus peltifer (Acari, Oribatida). Journal of Evolutionary Biology 20: 392–402.

Norton, R. A., & V. M. Behan-Pelletier. 2009. Suborder Oribatida. In: Krantz, G. W., & D. E. Walter (eds) A Manual of Acarology 3rd ed. pp. 430–564. Texas Tech University Press.

Olszanowski, Z., & R. A. Norton. 2002. Paracamisia osornensis gen. n., sp. n. (Acari: oribatida) from Valdivian forest soil in Chile. Zootaxa 25: 1–15.

Raspotnig, G., E. Stabentheiner, P. Föttinger, M. Schaider, G. Krisper, G. Rechberger & H. J. Leis. 2008. Opisthonotal glands in the Camisiidae (Acari, Oribatida): evidence for a regressive evolutionary trend. Journal of Zoological Systematics and Evolutionary Research 47 (1): 77–87.

Fishing Mice

In a 1950 discussion of the origins of the fauna of South America, the great American palaeontologist G. G. Simpson dismissed the enormous radiation of muroid rodents in that continent as mere "field mice" exhibiting little regional differentiation. George Gaylord Simpson may have been one of the leading thinkers in mid-20th Century evolutionary theory, but in this respect he was just plain wrong. The South American mice and rats include a wide variety of divergent forms, some of them specialised in surprising ways. Consider, for instance, the fishing mice of the Ichthyomyini.

Illustration of Stolzmann's crab-eating mouse Ichthyomys stolzmanni by Joseph Smit.


The Ichthyomyini are a small assemblage of less than twenty species of mice found between Mexico and the north of South America from Peru to French Guiana (Voss 1988). Though some species are known from lower altitudes, the majority are found in alpine habitats in association with fast-flowing mountain streams (albeit in no location are they known to be common). Ichthyomyins seem to show a particular preference for hanging around waterfalls (Barnett 1997) and are not found in association with standing water such as swamps or ponds. They are moderate in size, ranging from ten to twenty centimetres in length excluding the tail. They show a number of adaptations for foraging underwater: the hind feet are partially webbed and have a more or less elongate fringe of stiff hairs that aid in swimming, the tail is furry rather than scaly, the eyes are small, the external ears are reduced in size (in a couple of species they are completely hidden by the fur and in one, Anotomys leander, the external pinnae are missing entirely), and the whiskers are long, strong, and arranged in such a way that they almost look more like the whiskers of a sea lion than of a mouse. The nerves associated with these whiskers are also enlarged and they evidently provide the main means of finding food.

Peruvian fish-eating rat Neusticomys peruviensis, copyright Carlos Boada.


Or, as I should say, finding prey. As far as we know, these mice seem to be entirely carnivorous. Only a couple of examples are known of specimens with plant matter in their stomachs and the significance of those finds remains uncertain. The primary source of food for most species is small invertebrates such as aquatic insects. Where freshwater crabs are available, a number of species show preferences for those. In larger species, the diet may be supplemented to a greater or lesser degree by small vertebrates such as fish or tadpoles. In line with their carnivorous diet, ichthyomyins are also characterised by a shorter, less complicated gut than other mice. Little is known about breeding and nesting habits in ichthyomyins. A specimen of Chibchanomys kept in captivity made tunnels in mossy vegetation (Barnett 1997). The few known specimens of gravid females indicate that litters are small with no more than two foetuses being carried at a time.

Voss (1988) recognised five genera of Ichthyomyini. The largest of these, Neusticomys, includes about half a dozen species that may more closely resemble the ancestral morphology for the group. Their hind feet are narrower than those of other ichthyomyins and the fringe of swimming hairs is shorter (Packer & Lee 2007). Where one species found in Colombia and Ecuador, Neusticomys monticolus, overlaps in range with Anotomys leander, it shows a preference for more sheltered sections of stream banks whereas A. leander is found in more exposed rapids.

Undescribed species of Chibchanomys, copyright Alexander Pari.


In most ichthyomyins, the coat consists of a layer of dense, woolly underfur covered by an overcoat of long guard hairs mixed with glossy, often distally flattened awn hairs. In Anotomys leander, Chibchanomys trichotis, and Neusticomys monticolus, the awn hairs are missing so these species have a dull grayish black appearance overall rather than than the glossy coat of other species. Chibchanomys trichotis retains minute external ear flaps albeit not ones that are visible past the coat; Anotomys leander, as noted above, lacks external ear flaps but does have the positions of the ear openings marked by a prominent white spot. Both these last two species were placed in monotypic genera by Voss (1988), but Barnett (1997) refers to an at-that-point undescribed species of Chibchanomys.

The remaining two genera were recognised by Voss (1988) as including four species apiece. Species of Rheomys, found in the mountains of Central America, have the most extensively webbed hind feet among the ichthyomyins. This is the only genus of fishing mice found in Central America; the other genera are all restricted to South America. The genus Ichthyomys includes the largest species of the group and also the species that feed on the highest proportion of vertebrates. This difference in diet is reflected in their dentition: Ichthyomys species have proportionately larger incisors and smaller molars than other ichthyomyins, with greater emphasis on using the incisors to grasp and slice struggling prey.

Rheomys raptor, from Villalobos-Chaves et al. (2016).


All told, the ichthyomyins are a remarkable radiation. Ecologically, they are close parallels to forms found elsewhere such as water shrews or desmans, but most other semi-aquatic mammals are distinctly larger in size. Even with less than twenty species, the ichthyomyins represent more species than there are of similarly sized semi-aquatic mammals anywhere else in the world. However, as noted above, ichthyomyins are not common anywhere they occur, and factors such as deforestation and climate change could represent a significant threat to their survival. It would be unfortunate if this remarkable radiation was to fade away.

REFERENCES

Barnett, A. A. 1997. The ecology and natural history of a fishing mouse Chibchanomys spec. nov. (Ichthyomyini: Muridae) from the Andes of southern Ecuador. Zeitschrift für Säugetierkunde 62: 43–52.

Packer, J. B., & T. E. Lee Jr. 2007. Neusticomys monticolus. Mammalian Species 805: 1–3.

Voss, R. S. 1988. Systematics and ecology of ichthyomyine rodents (Muroidea): patterns of morphological evolution in a small adaptive radiation. Bulletin of the American Museum of Natural History 188 (2): 259–493.

Seriously, What Is This Thing?

So there weren't too many people speculating about the identity of that mysterious figure (hi, Adam!) As it happens, there was a reason I'd put it out there: the reason being, I really don't have any idea what it is either.

Spinita spp., from Kordè in Koren' (2003). 1: S. sanashticgolica, 2: S. cryptosa, 3: S. spinoglobosa.


The figure comes from a Russian book, Атлас ископаемой фауны и флоры палеозоя Республики Бурятия ('Atlas of the Palaeozoic fossil fauna and flora of the Republic of Buryatia'), edited by T. N. Koren' and published in 2003 in Ulan-Udè. Buryatia is a Russian republic in south-eastern Siberia, wrapping around the eastern and southern coasts of Lake Baikal. The fossils shown above come from the Lower Cambrian (the Botomian stage in the Russian system) of the Eastern Sayan Mountains. Going by the appearance of the figures, I presume they're being examined as thin sections, a commonly used method for studying Palaeozoic microfossils. Though as microfossils go, these are definitely on the large side: the specimen figured as 1a is a centimetre long and three millimetres wide. The other specimens are smaller, about half a centimetre in length.

When I saw these figures, I was just mystified. Their describer, K. B. Kordè, regarded them as a new class of 'Nemathelminthes', claiming that 'the first impression that is created from the described material is that they are representatives of the Kinorhyncha or Gastrotricha'. I'm not sure that I would agree with that. I found myself wondering if they were even animals, though I was hard pressed to think what else they might be. Not being familiar with the interpretation of thin sections, the thought did cross my mind to ask how certain can we be that these are even fossils, but I think that may be a bit uncharitable. Kordè also suggested that a break in the apparent cuticle of the S. sanashticgolica specimen about halfway along the flattened side (interpreted as the venter) might be the mouth. If so, that would be very unlike any kinorhynch or gastrotrich I've heard of. Could be a flatworm, I suppose, though Kordè then goes on to read the cluster of spines at one end (as magnified in figure 2b) as marking the anus which would seem to put paid to that! Said spines, or papillae, or whatever, are also supposed to have medial channels that Kordè interprets as nephridia.

All in all, I can't express anything other than confusion about this one. Certainly I haven't been able to find any further commentary on these enigmas; a Google search for Spinita sanashticgolica brings up just one result, an offhand mention in this book which seems to be just referring to it as found in the same formation as another fossil. Confusingly enough, that mention seems to date from 1986, a good seventeen years before Koren' (2003) was even printed: whether that indicates that the latter publication was not actually the first time the description of Spinita saw print, or whether this genus saw time floating around in unpublished communications, I have no idea.

Name the Bug Revived

It's been a very long time since I last did one of these, and I'm not sure if I still have the readership for it, but I'm genuinely interested to know what anyone can make of this (attribution to follow):


Some necessary context: they're fossils, Cambrian in age, I presume being examined in thin section. The specimens numbered 1, 2 and 3 were described as three different species of a single genus. Even if you don't know exactly what it is, let me know what you think it might be...

Edit: Forgot to give an indicator of size. They're about the one-centimetre range in maximum breadth.

The Origins of a Closed Bolete

Boletes are a distinctive group of mushrooms in which the underside of the fruiting body is covered by tubular pores instead of gills. Though boletes are classified in the fungal order Boletales, not all members of this order produce bolete-type fruiting bodies (as exemplified in an earlier post). Consider, for example, the case of Gastrosuillus.

'Gastrosuillus' sp., copyright Danny Miller.


Gastrosuillus was recognised in 1989 for a small group of species found in North America that closely resembled members of the more typical bolete genus Suillus (the slippery jacks) except for their production of secotioid fruiting bodies, in which the pores are distorted and do not form a flattened plane, and may remain covered by an external membrane (secotioid fruiting bodies may be considered an intermediate form between typical mushrooms and the gastroid fruiting bodies of fungi such as puffballs). All Gastrosuillus species were extremely rare, known only from single locations or even single collections. Gastrosuillus suilloides and G. amaranthii were found in California, G. imbellus in Oregon, and G. laricinus in New York State. All four were found on the ground in conifer forest; fruiting bodies of G. suilloides could be buried (Bessette et al. 2000).

From its inception, a close relationship with and possibly even derivation from members of the genus Suillus seems to have been on the cards for Gastrosuillus. It should be noted that Suillus was not the only bolete genus with a secotioid satellite: as Gastrosuillus was to Suillus, so Gastroboletus was to Boletus, and Gastroleccinum was to Leccinum. So it should have come as little surprise when a molecular analysis of Gastrosuillus species by Kretzer & Bruns (1997) found them to be nested within Suillus, nor forming a single clade within that genus. Instead, the western species were well separated from the New York G. laricinus. As a result, Kretzer & Bruns advocated the synonymisation of the two genera.

Typical form of larch bolete Suillus grevillei, copyright Luridiformis.


But the demotions didn't stop there. Not only was Gastrosuillus laricinus nested molecularly within Suillus, it appeared to be nested within a particular species, S. grevillei (conversely, the California species form a distinct lineage that is, so far as we know, entirely secotioid; the Oregon G. imbellus has not been examined molecularly owing to difficulties in extracting DNA from the single known specimen). The sole known location for G. laricinus lies within the range of S. grevillei, with the two species having been found in close proximity, and the indications were that G. laricinus was a very recent derivative of S. grevillei or possibly even a mere growth variant. Again, this is not entirely without precedent. Secotioid variants have been recorded of other mushroom species, and secotioid-like forms of the agaricoid mushroom Lentinus tigrinus have even been shown to be the result of a recessive allele of a single gene. Kretzer & Bruns (1997) therefore suggested that G. laricinus be synonymised entirely with S. grevillei. This action does not appear to have gained universal acceptance (for instance, the two are provisionally treated as distinct by Bessette et al., 2000) but is certainly worthy of consideration.

REFERENCES

Bessette, A. E., W. C. Roody & A. R. Bessette. 2000. North American Boletes: A color guide to the fleshy pored mushrooms. Syracuse University Press.

Kretzer, A., & T. D. Bruns. 1997. Molecular revisitation of the genus Gastrosuillus. Mycologia 89 (4): 586–589.

Cliff Ferns

Historically, the higher classification of ferns has tended to be a bit wobbly. Compared to flowering plants, ferns often offer fewer readily observable features that may offer clues to relationships. As a result, the position of many fern taxa has long been uncertain. One such group is the cliff ferns of the genus Woodsia.

Woodsia scopulina, copyright Jim Morefield.


Cliff ferns, as their name suggests, are commonly found growing on rocks. There are a few dozen species, mostly found in cooler regions of the Northern Hemisphere. A single species, Woodsia montevidensis, extends into South America and southern Africa (Rothfels et al. 2012). They have short creeping rhizomes with a covering of scales and leaves bearing a mixture of scales and hairs. The most distinctive feature of the cliff ferns can only be seen on fertile fronds: the sori (spore packets) are covered by an indusium that is attached to the leaf basally relative to the sori. These indusia are commonly composed of an array of scales or filamentous sections, in contrast to the solid indusia of other ferns.

Underside of pinnule of Woodsia plummerae, showing the filamentous indusia, from here.


Historically, Woodsia has been placed in a family Woodsia with a number of superficially similar fern genera such as the bladder ferns of the genus Cystopteris. However, molecular phylogenetic analyses have disputed the monophyly of such a group. Rothfels et al. (2012) divided the 'woodsioid' ferns between no less than six different families with Woodsiaceae in the strict sense limited to the cliff ferns alone. Though some authors have divided the cliff ferns between multiple genera, an analysis of the group by Shao et al. (2015) found it difficult to reliably distinguish such subgroups and recommended recognition of only a single genus. They did, however, recognise three major clades within Woodsia identified by molecular phylogenetic analysis as distinct subgenera. The type subgenus Woodsia is distinctive among ferns in possessing articulated stems; species of this subgenus are widespread in the Palaearctic region. The subgenus Physematium is mostly found in the Americas and is characterised by bicolored scales on the rhizome. The third subgenus, Cheilanthopsis, is found in eastern Asia with the centre of diversity in the Himalayan region. The rhizome scales are concolorous, and the indusia are solid and globose rather than being composed of individual segments. In some cases in this subgenus, the sori are covered by 'false indusia', indusium-like structures that are formed from inrolled leaf margins rather than being independent membranes.

REFERENCES

Rothfels, C. J., M. A. Sundue, L.-Y. Kuo, A. Larsson, M. Kato, E. Schuettpelz & K. M. Pryer. 2012. A revised family-level classification for eupolypod II ferns (Polypodiidae: Polypodiales). Taxon 61 (3): 515–533.

Shao, Y., R. Wei, X. Zhang & Q. Xiang. 2015. Molecular phylogeny of the cliff ferns (Woodsiaceae: Polypodiales) with a proposed infrageneric classification. PLoS One 10 (9): e0136318.

The Solemyoida: A Taste for Sulphur

Atlantic awning clam Solemya velum, copyright Guus Roeselers.


The small bivalves that make up the Solemyoida were long a mystery, ecology-wise. Though they have a long history, potentially going back as far as the Ordovician (Cope 2000), they are not known to have ever been diverse, and only just over fifty species are known from the modern fauna. Living solemyoids are divided between two very distinct families that probably diverged near the origin of the group. The Solemyidae, awning clams, have relatively long shells that gape at each end, no teeth in the dorsal hinge, and tend to have an unusually thick periostracum (the overlying layer of horny proteinaceous matter that covers the outside of the mineral shell). They generally live in burrows buried deep in sediment. The Nucinellidae are a group of minute clams with an average length of about half a centimetre that are mostly found in deep waters, generally not buried quite so deep in the mud as the awning clams. They have a less elongate shell than the Solemyidae that does not gape and simple peg-like teeth in the hinge. What the two families do share is a markedly reduced gut and feeding appendages that initially caused much speculation about what exactly they were feeding on.

Nucinella sp. with foot extended, from Taylor & Glover (2010). Scale bar equals 1 mm.


The answer, as it turns out, was that they were not exactly 'feeding' on much, if anything. Solemyoids have relatively large gills that provide a comfortable living place for sulphur-oxidising bacteria, sheltered from the outside world while the host clam keeps up a continuous flow of water through its burrow from above the sediment surface. In return, the bacteria fix hydrogen sulphide rising from the underlying mud to provide both themselves and their host with nutrients. In this way, solemyoids have largely been able to get by without actively eating for close to 450 million years, achieving something the likes of Jasmuheen can only dream of.

REFERENCE

Cope, J. C. W. 2000. A new look at early bivalve phylogeny. In: Harper, E. M., J. D. Taylor & J. A. Crame (eds) The Evolutionary Biology of the Bivalvia pp. 81–95. The Geological Society: London.