Field of Science

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.

REFERENCES

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.

REFERENCES

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.