Gastrotricha

Cephalodasys littoralis, from Renaud-Mornant (1986).

Belongs within: Eumetazoa.
Contains: Paucitubulatina, Dactylopodolidae, Turbanellidae, Macrodasyidae, Thaumastodermatidae.

Gastrotrichs and their tacky little tubes
Published 10 April 2021

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.

Systematics of Gastrotricha
<==Gastrotricha
| i. s.: UndulaHL00
| Diuronotus aspetosSRT18
|--Chaetonotida [Chaetonotoidea]MV04
| |--PaucitubulatinaHL01
| `--Neodasys Remane 1927 [Multitubulata, Multitubulatina, Neodasyidae]HL01
| |--N. chaetonotoideus Remane 1927R-M86
| `--N. uchidai Remane 1961R-M86
`--Macrodasyida [Lepidodasyidae, Macrodasyoidea]MV04
|--DactylopodolidaeHL01
`--+--+--Pleurodasys Remane 1927HL01
| | |--P. helgolandicus Remane 1927R-M86
| | `--P. megasoma Boaden 1963R-M86
| `--TurbanellidaeHL01
|--+--PlanodasyidaeHL01
| | |--Planodasys Rao in Rao & Clausen 1970R-M86
| | | `--P. marginalis Rao in Rao & Clausen 1970R-M86
| | |--Thiodasys Boaden 1974R-M86
| | | `--T. sterreri Boaden 1974R-M86
| | `--Crasiella Clausen 1966R-M86, HL01
| | |--C. diplura Clausen 1968R-M86
| | |--C. indica Rao 1981R-M86
| | `--C. pacifica Schmidt 1974R-M86
| `--+--Dolichodasys Gagne 1976HL01
| | |--D. carolinensis Ruppert & Shaw 1977R-M86
| | |--D. delicatus Ruppert & Shaw 1977R-M86
| | `--D. elongatus Gagne 1977R-M86
| |--Paradasys Remane 1934HL01
| | |--P. hexadactylus Karling 1954R-M86
| | |--P. lineatus Rao 1980R-M86
| | |--P. littoralis Rao & Ganapati 1968R-M86
| | |--P. pacificus Schmidt 1974R-M86
| | `--P. subterraneus Remane 1935TFB99
| `--Cephalodasys Remane 1926HL01
| |--C. cambriensis (Boaden 1963)R-M86
| |--C. caudatus Rao 1981R-M86
| |--C. littoralis Renaud-Debyser 1964R-M86
| |--C. maximus Remane 1926R-M86
| |--C. miniceraus Hummon 1974R-M86
| |--C. pacificus Schmidt 1974R-M86
| |--C. palavensis Fize 1963R-M86
| `--C. turbanelloides (Boaden 1960)TBT02
`--+--Mesodasys Remane 1951HL01
| |--M. adenotubulatus Hummon, Todaro & Tongiorgi 1993TBT02
| |--M. hexapodus (Rao & Ganapati 1968)R-M86
| |--M. laticaudatus Remane 1951TBT02
| |--M. littoralis Remane 1951R-M86
| `--M. lobocercus (Boaden 1960)R-M86
|--Megadasys Schmidt 1974HL01
| `--M. pacificus Schmidt 1974R-M86
|--MacrodasyidaeHL01
`--+--Lepidodasys Remane 1926HL01
| |--L. martini Remane 1926TBT02
| `--L. platyurus Remane 1927R-M86
`--ThaumastodermatidaeHL01

*Type species of generic name indicated

References

[HL00] Hochberg, R., & M. K. Litvaitis. 2000. Phylogeny of Gastrotricha: a morphology-based framework of gastrotrich relationships. Biological Bulletin 198 (2): 299–305.

[HL01] Hochberg, R., & M. K. Litvaitis. 2001. Macrodasyida (Gastrotricha): a cladistic analysis of morphology. Invertebrate Biology 120 (2): 124–135.

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.

[MV04] Manylov, O. G., N. S. Vladychenskaya, I. A. Milyutina, O. S. Kedrova, N. P. Korokhov, G. A. Dvoryanchikov, V. V. Aleshin & N. B. Petrov. 2004. Analysis of 18S rRNA gene sequences suggests significant molecular differences between Macrodasyida and Chaetonotida (Gastrotricha). Molecular Phylogenetics and Evolution 30: 850–854.

[R-M86] Renaud-Mornant, J. 1986. Gastrotricha. In: Botosaneanu, L. (ed.) Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) pp. 86–109. E. J. Brill/Dr W. Backhuys: Leiden.

[SRT18] Schiffer, P. H., H. E. Robertson & M. J. Telford. 2018. Orthonectids are highly degenerate annelid worms. Current Biology 28: 1970–1974.

[TBT02] Todaro, M. A., M. Balsamo & P. Tongiorgi. 2002. Marine gastrotrich fauna in Corsica (France), with a description of a new species of the genus Tetranchyroderma (Macrodasyida, Thaumastodermatidae). Sarsia 87: 248–257.

[TFB99] Tongiorgi, P., E. Fregni & M. Balsamo. 1999. Gastrotricha from Italian brackish water environments with the description of a new species of Chaetonotus. Journal of the Marine Biological Association of the United Kingdom 79: 585–592.

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.

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