Field of Science

The Lipinia Skinks

For a long time in the history of herpetology, many of the smaller skinks in the Asia-Pacific region were combined in a single sprawling genus Lygosoma. For an equally long time, this state of affairs had been considered unsatisfactory, with not much defining Lygosoma other than "it ain't anything else". However, it was until the mid-20th Century that researchers began reliably identifying recognisable subgroups within the lygosomine mass that they could carve off into separate genera. One of these ex-Lygosoma isolates is now recognised as the genus Lipinia.

Yellow-striped tree skink Lipinia vittigera, copyright Sergey Yeliseev.

Lipinia is a genus of about thirty known species of small skinks (reaching at most a snout-vent length of nearly six centimetres) found in south-east Asia and the islands of the tropical Pacific. Even now, the genus remains difficult to clearly define with its species having a fairly unspecialised habitus. It belongs to a group of genera in which the lower eyelid usually has a transparent window, presumably allowing the skink to retain some modicum of vision even with its eyes closed. Within this cluster of genera, distinguishing features of Lipinia include smooth scales, slightly to strongly expanded subdigital lamellae, and the loss of the postorbital bone. They often show a strongly striped dorsal pattern with a pale mid-dorsal stripe anteriorly (Shea & Greer 2002; Poyarkov et al. 2019). Species are diurnal and may be arboreal, semi-arboreal or terrestrial in habits (terrestrial species are quite reclusive). The clutch of eggs laid by females is usually quite small, two at most (one for each oviduct). In a number of species, one of the oviducts is vestigial and only a single egg is laid.

Moth skink Lipinia noctua, from Ecology Asia.

The most widespread species in the genus is the moth skink Lipinia noctua, found over a range extending from eastern Indonesia to the Pitcairn Islands (I have no idea why it is called a 'moth skink'). It is believed to have originally been native to New Guinea before spreading over its current range in association with humans, an inadvertent stowaway in ocean-crossing boats and canoes*. It was doubtless assisted in this spread by its ovoviviparous habit: that is, rather than laying eggs in the manner of related species, eggs are retained in the mother's body until young can be born free-living. So thorough was the transmission of this little skink by humans that its genetics have been investigated in relation to the settlement of the islands (Austin 1999). They support a picture of rapid eastward expansion; when Polynesian explorers discovered the islands that would eventually become their people's home, the skinks were there discovering them too.

*A note on terminology: though the manned craft used by the initial settlers of the Pacific islands are commonly referred to in English as 'canoes', these were not just the small craft many people associated with the term. Polynesian waka/vaka/etc. (the exact term, of course, varies linguistically) can be sizable ships, twenty metres or more in length, with commensurately sizable crews. Those used for long ocean crossings would have double hulls or outriggers and would largely be propelled by sail rather than oars.


Austin, C. C. 1999. Lizards took express train to Polynesia. Nature 397: 113–114.

Poyarkov, N. A., Jr, P. Geissler, V. A. Gorin, E. A. Dunayev, T. Hartmann & C. Suwannapoom. 2019. Counting stripes: revision of the Lipinia vittigera complex (Reptilia, Squamata, Scincidae) with description of two new species from Indochina. Zoological Research 40 (5): 358–393.

Shea, G. M., & A. E. Greer. 2002. From Sphenomorphus to Lipinia: generic reassignment of two poorly known New Guinea skinks. Journal of Herpetology 36 (2): 148–156.

In the Arms of Pseudisograptus

From the Ordovician to the early Devonian, the graptoloids were a major component of the oceanic fauna. These colonial floaters are among the characteristic fossils of the early Palaeozoic and have received a lot of attention due to their use in biostratigraphy. The evolution of graptoloids has been presented as a process of increasing simplification, of progressive reductions in colonial complexity and density. Like all illustrations of evolutionary trends, this is an overly simplistic representation of how things actually occurred but it's not entirely incorrect. The history of graptoloids was indeed marked by a number of significant transitions were particular growth forms overran their predecessors. One genus that may have played a significant role in the lead-up to one of these turnovers was Pseudisograptus.

Pseudisograptus manubriatus koi, entire fossil and close-up diagram of initial thecae, from Cooper & Ni (1986).

The graptoloid genus Pseudisograptus has been collected from rocks in Australia, North America and eastern Asia dating to the latter part of the Floian stage of the early Ordovician, a bit over 470 million years ago (Cooper & Ni 1986). It is characterised by colonies growing in two branches (stipes) with the stipes spreading outwards and upwards like a pair of wings (indeed, one species of this genus luxuriates in the name of Pseudisograptus angel). In large specimens, the stipes reach about two centimetres in length and about three millimetres wide (from inner margin to the outer apex of the individual thecae). Pseudisograptus species are very similar to, and until 1972 where classified with, species of the related genus Isograptus. They differ, however, in the arrangement and growth of the earliest thecae in the colony. Whereas Isograptus stipes grow outwards immediately from the oldest theca, Pseudisograptus have the first few thecae on each stipes elongated and growing downwards before the stipes makes a later sharp turn upwards. As a result, between the two 'wings' of the stipes there is a more or less distinct triangle (referred to as the manubrium) formed from the bases of the early thecae. At the top of the manubrium is an upright thread, the nema. In a number of graptoloid fossils, an inflated structure has been identified at the top of the nema that probably functioned as a float. I don't know if such a structure has ever been identified in a Pseudisograptus fossil but I imagine it would quite easily be lost in the course of preservation.

Basal section of Cardiograptus amplus, from Fortey et al. (2005). Not a true biserial graptoloid but illustrative of the way biserial forms may have evolved from biramous ancestors.

Pseudisograptus' disappearance from the fossil record coincides with one of the aforementioned turnovers in graptoloid diversity, the appearance of the biserial graptoloids. These forms, which had two rows of thecae arising from a single central line, rapidly replaced most of the earlier branched forms. The rapid appearance of the biserial graptoloids has made their origins difficult to work out but current thinking is that they arose from a form similar to Pseudisograptus, in which the upward growth of the stipes became steep enough that they met in the middle along the nema. One interesting detail is that two lineages appear to have achieved biseriality at about the same time from closely related but separate ancestors. In the glossograptids, the conjoined stipes met each other side-by-side; in the diplograptids, they met back to back. Pseudisograptus has, at different times, been implicated in the ancestry of both of these groups. Cooper & Ni (1986) regarded Pseudisograptus as paraphyletic and including the direct ancestors of the glossograptids. In contrast, more recent studies by Fortey et al. (2005) and Maletz et al. (2009) have placed Pseudisograptus closer to the diplograptids. These studies have been more agnostic as to whether Pseudisograptus was a direct ancestor or a close relative. If the former is the case then, while the exact Pseudisograptus morphotype would disappear at the end of the Floian, their genetic lineage would continue strong for nearly ninety million more years.


Cooper, R. A., & Ni Y. 1986. Taxonomy, phylogeny, and variability of Pseudisograptus Beavis. Palaeontology 29 (2): 313–363.

Fortey, R. A., Y. Zhang & C. Mellish. 2005. The relationships of biserial graptolites. Palaeontology 48 (6): 1241–1272.

Maletz, J., J. Carlucci & C. E. Mitchell. 2009. Graptoloid cladistics, taxonomy and phylogeny. Bulletin of Geosciences 84 (1): 7–19.

Woolly Orchids

The orchids of the Orchidaceae are widely recognised as one of the most diverse families of plants in the modern world, both in number of species and morphologically. They are readily distinguished from other flowering plants by a unique combination of features including the fusion of the male and female organs of the flower into a central column. Rather than being released as individual grains, pollen is aggregated into compact masses called pollinia that are attached to pollinators as whole units. Most orchid species also have the lower of the flower's three petals enlarged and differentiated into a distinctive lip that may present a bewildering array of shapes and colours. Because of their striking and colourful appearance, many orchids have long attracted attention from humans and many are popular ornamentals. But there are also some major groups of orchids that have been more neglected and one such group is members of the subtribe Eriinae.

Dendrolirium tomentosum, copyright Orchi.

The Eriinae comprise about a thousand known species of orchid found mostly in the tropics of Asia and the west Pacific, with a handful of species described from Africa. Most are epiphytes and lithophytes (growing on rocks); a smaller number are terrestrial. Because the flowers of eriines tend to be fairly small and simple, they have attracted less notice than other members of the family, but in some parts of their range they are among the most abundant epiphytic orchids (Ng et al. 2018). Within the Orchidaceae, eriines are a subgroup of the subfamily Epidendroideae, characterised by compact, laterally compressed pollinia, and the tribe Podochileae, with duplicate leaves, a short and massive column, and often spherical silica cells in the stems (Szlachetko 1995). The features distinguishing Eriinae from other subtribes of Podochileae are more vague and there are reasons to believe the Eriinae ultimately represent the paraphyletic residue of the tribe once the more specialised subgroups are removed (Ng et al. 2018). One recent classification of the orchids recommended abandoning subtribes within the Podochileae altogether (Chase et al. 2015). Nevertheless, features characteristic of most eriines include a terminal or upper lateral inflorescence, eight pollinia per flower, and sticky caudicles on the pollinia composed of apical pollen grains. The lip is commonly divided into three lobes. Another common feature of the group (and the inspiration for the name of the type genus Eria, meaning 'woolly') is a covering of hairs on the flower and sometimes the inflorescence. In one genus, Trichotosia, the leaves are also hairy.

Ascidieria grandis, copyright Dick Culbert.

Historically, the majority of eriines have been included in a broad genus Eria. However, as with the subtribe as a whole, recent studies have indicated that this sense of Eria is not monophyletic and hence its species should probably be divided between several genera. Ng et al. (2018) recognised 21 genera among the eriines. The African species, previously placed in their own genus Stolzia, were united with the closely related Asian genus Porpax.

The pollination biology of eriines is, for the most part, not well known. Some have speculated that they were pollinated by beetles; one website I found showed pollinia attached to a gnat. The two species of the genus Callostylis have flowers whose appearance suggests pollination by pseudocopulation (tricking male insects into attempting to mate with them by mimicking females) but such flowers are unique within the Podochileae (Ng et al. 2018). At least some eriines have flowers producing 'pseudopollen' from broken-off hairs (Pansarin & Maciel 2017). This pseudopollen is collected and eaten by pollinators. Thus, though the most common means of attracting pollinators among orchids is via deception, at least some eriines are willing to pay their way in life.


Ng, Y. P., A. Schuiteman, H. A. Pedersen, G. Petersen, S. Watthana, O. Seberg, A. M. Pridgeon, P. J. Cribb & M. W. Chase. 2018. Phylogenetics and systematics of Eria and related genera (Orchidaceae: Podochileae). Botanical Journal of the Linnean Society 186: 179–201.

Pansarin, E. R., & A. A. Maciel. 2017. Evolution of pollination systems involving edible trichomes in orchids. AoB Plants 9: plx033.

Szlachetko, D. L. 1995. Systema Orchidalium. Fragmenta Floristica et Geobotanica Supplementum 3: 1–152.

The Shells of Ducks and Swans

The freshwater environment has been a challenging one for bivalves. Though there is a reasonable diversity of freshwater bivalves around the world, they tend to be dominated by members of a select few lineages. One of the most successful groups of freshwater bivalves is the family Unionidae, and among the more widespread unionids are the freshwater mussels of the genus Anodonta.

Swan mussel Anodonta cygnea, copyright Gail Hampshire.

Anodonta species are found widely across northern Eurasia and North America, commonly referred to as 'mussels' in Eurasia and 'floaters' in North America. They are relatively large bivalves (one of the largest, the swan mussel Anodonta cygnea of Eurasia, can be up to about twenty centimetres across) with an irregularly elliptical shape and a relatively thin shell. One of their distinguishing features compared to other freshwater bivalves is the teeth of the hinge connecting the shell valves have been lost. Instead, the valves are primarily held together by the dorsal ligament (Moore 1969). Freshwater mussels are most commonly found in mud at the bottom of slow-moving or still waters, such as lakes or slow rivers.

One of the main hurdles to bivalve colonisation of fresh water has been the question of dispersal. In most marine bivalves, populations mostly disperse via their planktonic larvae. But because of the directed flow of water in rivers and the like, passive plankton fare less well in freshwater environments. If you just float along a stream, eventually you'll be washed out to sea. Anodonta species, like other unionids, solve the problem of getting back upstream through parasitic larvae called glochidia. Female Anodonta have the rear part of the gills modified into a pouch (or marsupium) in which the developing larvae are initially incubated. When they are released by their mother, the glochidia already possess a bivalved, sharp-edged shell. Released glochidia swim towards a suitable host in the form of a passing fish and use the valves of the shell to clamp onto a narrow appendage of the host's body such as its fins or gills. Eventually, a cyst forms around the attached glochid within with it develops until it is ready to emerge and attain maturity.

Winged floater Anodonta nuttalliana, a North American species, copyright Eric Wagner.

Freshwater molluscs have a history of being subject to taxonomic chicanery, through the Nouvelle École of late nineteenth-century France and other excesses of typological enthusiasm. Anodonta is no exception. The shells of freshwater mussels tend to be very plastic in morphology, their size, shape and appearance being strongly affected by their developmental environment. As a result, they include what were labelled by Riccardi et al. (2020) as "some of the most over-described species on the planet". The swan mussel A. cygnea alone has had somewhere in the region of 550 different species-group names applied to it at one time or another. Modern estimates of Anodonta diversity are considerably more conservative. Just four species are currently recognised from Eurasia (Riccardi et al. 2020) with the swan mussel and the duck mussel A. anatina being the most widespread (offhand, I don't know whether the mussels get their vernacular names because they're eaten by swans and ducks or because the shape of the shell is supposed to look like a swan or duck). Considering the travails of shell-based taxonomy, it is noteworthy that these species often cannot be distinguished with certainty without checking the soft tissue. North America is home to six or seven recognised species with diversity being higher to the west of the continent.

Nevertheless, there are still grounds for questioning the current taxonomy of Anodonta. Molecular studies of the genus by Chong et al. (2008), Bolotov et al. (2020) and Riccardi et al. (2020) have all suggested that Anodonta as currently recognised may be paraphyletic to closely related genera. In particular, there may be a divide between the Eurasian and North American lineages with the North American species closer to taxa found in eastern Asia. Anodonta has been a problem genus in the past and it sees no reason why it should allow itself to be reformed.


Bolotov, I. N., A. V. Kondakov, E. S. Konopleva, I. V. Vikhrev, O. V. Aksenova, A. S. Aksenov, Y. V. Bespalaya, A. V. Borovskoy, P. P. Danilov, G. A. Dvoryankin, M. Y. Gofarov, M. B. Kabakov, O. K. Klishko, Y. S. Kolosova, A. A. Lyubas, A. P. Novoselov, D. M. Palatov, G. N. Savvinov, N. M. Solomonov, V. M. Spitsyn, S. E. Sokolova, A. A. Tomilova, E. Froufe, A. E. Bogan, M. Lopes-Lima, A. A. Makhrov & M. V. Vinarski. 2020. Integrative taxonomy, biogeography and conservation of freshwater mussels (Unionidae) in Russia. Scientific Reports 10: 3072.

Chong, J. P., J. C. B. Box, J. K. Howard, D. Wolf, T. L. Myers & K. E. Mock. 2008. Three deeply divided lineages of the freshwater mussel genus Anodonta in western North America. Conserv. Genet. 9: 1303–1309.

Moore, R. C. (ed.) 1969. Treatise on Invertebrate Paleontology pt N. Mollusca 6. Bivalvia vol. 1. The Geological Society of America, Inc. and The University of Kansas.

Riccardi, N., E. Froufe, A. E. Bogan, A. Zieritz, A. Teixeira, I. Vanetti, S. Varandas, S. Zaccara, K.-O. Nagel & M. Lopes-Lima. 2020. Phylogeny of European Anodontini (Bivalvia: Unionidae) with a redescription of Anodonta exulcerata. Zoological Journal of the Linnean Society 189: 745–761.

Gastrotrichs and their Tacky Little Tubes

When I was a student, I was taught that known animal diversity could be divided between somewhere in the region of a couple of dozen 'phyla'. These were the fundamental units of animal classification, the basic archetypes of animal morphology. Many of these were the major assemblages with which we all were familiar: chordates, arthropods, molluscs and the like. But many were the so-called 'lesser phyla', those taxonomic orphans that, whether small in size or small in number or both, tended to escape observation and study by the majority of people. One such 'minor phylum' was the collection of small worm-like animals known as the Gastrotricha.

Polymerurus nodicaudus, a paucitubulate gastrotrich, from Balsamo et al. (2015). Scale bar equals 100 µm.

Gastrotrichs are, in general, minute (Todaro et al. 2019). The largest reach about three-and-a-half millimetres in length, the smallest are about sixty microns, and there are probably many more at the lower range than the higher. They are dorsoventrally flattened with numerous cilia, and their cuticle may often be differentiated into a covering of scales or spines. Gastrotrichs are aquatic and are often referred to as part of the meiofauna, the assemblage of animals specialised for living within and crawling through the spaces between sand grains. That is indeed the preferred habitat for many species and gastrotrichs may be among the most abundant inhabitants of this milieu, edged out only by the nematodes and copepods. However, other species live above the sediment surface, crawling over the surface of aquatic vegetation or even floating among the plankton. Over 850 species are known to date, of which are a bit over 500 are marine (with all marine species being meiofaunal) and the remainder are found in fresh water. They feed on micro-organisms such as bacteria and algae, swallowing them by means of a muscular pharynx.

Gastrotrichs differ from other animals in a number of significant features. Among these is the differentiation of the outer cuticle into two distinct layers. The outermost of these layers, the epicuticle, covers the entire outer surface of the body, including coating the cilia. Gastrotrichs also possess characteristic tubular outgrowths ending in adhesive glands. Their relationships to other animals remain uncertain. Most authors now agree that they represent an early-diverging branch of the Lophotrochozoa, the animal superclade including such creatures as molluscs and annelids. It is possible that they are more closely related to flatworms than anything else but even then the relationship would hardly be close.

Pseudostomella etrusca, a macrodasyidan gastrotrich, from Todaro et al. (2011). Scale bar = 50 µm.

Historically, gastrotrichs have been divided between two orders, the Macrodasyida and Chaetonotida. This division was supported by structural features of the pharynx and the body wall but is also reflected in the distribution of the adhesive tubes. The Macrodasyida, which are usually vermiform, possess adhesive tubules at both the anterior and posterior ends of the body, as well as laterally. Macrodasyidans are always interstitial in habits and usually marine. The Chaetonotida, on the other hand, lack anterior tubules. Chaetonotidans were further divided between two major taxa. One of these was the isolated genus Neodasys which is vermiform and interstitial like a macrodasyidan, and possesses both lateral and posterior tubules. The remaining Chaetonotida were recognised as the suborder Paucitubulatina. As indicated by their name (meaning 'few tubules'), members of this suborder are characterised by the reduction in number of adhesive tubules, usually to a single pair at the end of the body (a few species have two pairs of tubules, others lack distinct tubules and have the adhesive glands opening directly on the main body). They are short, generally shaped more or less like a bowling pin, and are the most ecologically diverse major gastrotrich group, including both marine and freshwater forms.

A phylogenetic analysis of gastrotrichs by Kieneke et al. (2008), however, questioned the established classification of the group. Rather than affirming a basal division between Chaetonotida and Macrodasyida, their results placed Neodasys as the sister group of all other gastrotrichs. Such a division may be reflected in the nature of their adhesive tubules: Neodasys has tubules containing a single gland but Macrodasyida and Paucitubulatina have two glands per tubule (unfortunately, because of the lack of close outgroups, it's hard to know which tubule type was ancestral). Within the Macrodasyida + Paucitubulatina clade, the macrodasyidans were then paraphyletic to the paucitubulates. Interestingly, the sister group to the Paucitubulatina was a clade of the only two known freshwater macrodasyidans, Marinellina and Redudasys. The implication was that gastrotrichs may have made the move to fresh water on just one occasion (followed by a number of returns to the sea among paucitubulates). This is not an isolated case: a number of phylogenetic studies of micro-organisms have found deep divides between marine and freshwater lineages. It seems it's hard to adjust to a life less salty.


Kieneke, A., O. Riemann & W. H. Ahlrichs. 2008. Novel implications for the basal internal relationships of Gastrotricha revealed by an analysis of morphological characters. Zoologica Scripta 37 (4): 429–460.

Todaro, M. A., J. A. Sibaja-Cordero, O. A. Segura-Bermúdez, G. Coto-Delgado, N. Goebel-Otárola, J. D. Barquero, M. Cullell-Delgado & M. Dal Zotto. 2019. An introduction to the study of Gastrotricha, with a taxonomic key to families and genera of the group. Diversity 11: 117.

To Dung and Beyond

When most people think of a fly, odds are that they imagine one of the group of flies known as calyptrates. This is the clade that includes, among others, such animals as house flies, blow flies and flesh flies. Calyptrates are often reasonably large as flies go and they often have life styles (such as larvae feeding on decaying matter) that bring them close to humans and their homes. One of the most recognisable features of this clade, and the inspiration for its name, is enlargement of the lower calypter, a lobe at the base of the wing. This lower calypter can be moved semi-independently of the rest of the wing which is how calyptrate flies are able to fly acrobatically and avoid being swatted. Nevertheless, there is one significant subgroup of 'calyptrate' flies that has foregone the advantages of an enlarged calypter, commonly recognised as the family Scathophagidae.

Yellow dung fly Scathophaga stercoraria, copyright Derek Parker.

The Scathophagidae are a modestly diverse family of flies with about 250 known species, the great majority of which are found in the Holarctic region (Vockeroth 1987). Only a handful of species are found in more southerly regions, mostly at higher altitudes. They are medium-sized flies, ranging between three and eleven millimetres in length, fairly similar to a house fly in overall appearance but generally more slender and bristly. They are commonly referred to as 'dung flies', in reference to the larval diet of one of the most widespread and best known species, Scathophaga stercoraria (whose scientific name broadly and appropriately translates as 'shit-eater, thing of shit'). However, despite the unremarkable number of species, scathophagids are actually more diverse in their larval habits. As far as we know, adult scathophagids are all predators on other insects.

Scathophagids are divided between two subfamilies, distinguished by features of the male terminalia. In Scathophaginae, the sixth abdominal tergite (dorsal plate) of the male is hairy and usually separate from the following fused syntergosternite 7 + 8 (with dorsal and ventral plates of the segments fused to form a ring). In Delininae, the sixth tergite lacks hairs and is always fused to the following syntergosternite. The subfamilies also differ in life history. Larvae of Delininae are leaf-miners on monocots, hatching from eggs laid on the leaf surface. The Scathophaginae are more diverse. As already indicated, some are saprobes. As well as the dung-feeding S. stercoraria, the genus Scathophaga also includes species which specialise on rotting seaweed on the sea-shore (a milieu which, offhand, supports a range of fly species belonging to numerous families). Other species are, like the Delininae, miners in plant tissue though they are found in a wider range of hosts (both monocots and dicots) and their eggs are inserted by the female directly into the plant tissue. A handful are aquatic or semi-aquatic predators, feeding on small invertebrates along lake shores or in sewage, or on the eggs of caddisflies in fast-running streams.

Cordilura pubera, a plant-feeding scathophagid, copyright Aleksandrs Balodis.

Considering the more derived character of the male terminalia in the Delininae, and the more disparate life habits of the Scathophaginae, some authors have suggested that the latter may be paraphyletic to the former. It has also been presumed that the Scathophagidae as a whole is ancestrally saprobic, considering that saprobic habits are also the norm in related fly families such as the Muscidae (house flies). However, a molecular phylogenetic analysis of the Scathophagidae by Kutty et al. (2007) supported monophyly of both subfamilies. Their results indicated that the original scathophagids were plant-feeders with saprobic lineages arising within the family on two separate occasions. Predatory larvae also evolved twice, once as a further development from saprobes and once direct from plant-feeding ancestors. The diet of this family started out fresh but, somewhere along the line, some species decided they'd rather eat muck.


Kutty, S. N., M. V. Bernasconi, F. Šifner & R. Meier. 2007. Sensitivity analysis, molecular systematics and natural history evolution of Scathophagidae (Diptera: Cyclorrhapha: Calyptratae). Cladistics 23: 64–83.

Vockeroth, J. R. 1987. Scatophagidae. In: McAlpine, J. F. (ed.) Manual of Nearctic Diptera vol. 2 pp. 1085–1097. Biosystematics Research Centre: Ottawa.