biology / Phylogeny / science

Defining the Water Bears: Phylogeny of the Tardigrade


The tardigrade, or ‘Tardigrada‘ which translates to ‘the slow stepper’ has established itself amongst scientific literature as one of the most resilient organisms on earth. In short, these delightful little things are the ‘Scrappy Doo’ of the natural world. Small, but undeniably tough.Their remarkable ability to cope under extreme conditions has been the subject of several publications, ranging from physiological and molecular studies of cold tolerance, to their ecological impacts in various different environments. And arguably, the secret motivation behind Destiny’s Child song ‘Survivor’.



Several studies have aimed to uncover evolutionary ambiguities surrounding the origins of Phylum Tardigrada, through both morphological and molecular methods. Traditional morphological studies place Tardigrada in a monophyletic group, collectively named the ‘Panarthropoda‘, which is comprised of other invertebrates such as the velvet worms (Onychophora) and spiders and insects (Arthropoda) (Campbell et al., 2011).

Alternative theories based on molecular data however, place Tardigrada in closer phylogenetic proximity to roundworms (Nematoda).

With this in mind, one recent study by Bertolani et al., 2014, found that that using a combination of both molecular and morphological data is necessary to better elucidate evolutionary relationships of tardigrades. The study examined the largest class of Tardigrada, ‘Eutardigrada’, and uncovered a new subfamily Pilatobiinae (Hypsibiidae) with the new genus Pilatobius.

Molecular Research

With open access databases now available online, such as NCBI (National Center for Biotechnology Information) we now have the luxury of having access to a vast library of biomedical and genomic information. Searching ‘tardigrade’ alone on the NCBI database brings up results with 12,443 expressed sequence tag sequences, 1063 genome survey sequences, 812 DNA and RNA sequences, and 288 protein sequences. It clearly would be of great disadvantage not to utilise this information, however morphological studies still stand their ground.

Molecular analysis of this species can be used in a number of different scientific areas, for example one study used partial mitochondrial cytochrome c oxidase subunit 1 (COI) gene sequences in order to access the distribution of different tardigrade taxa over the Antarctic continent (Velasco-Castrillion et al., 2015). The results of this study were particularly useful in understanding the biodiversity of tardigrades, and recognition of OTU’s (operational taxonomic units).

Another use is in the analysis of biochemical mechanisms such as cryptobiosis, which research has revealed to be vital for tardigrades tolerances to extreme desiccation. One study by Schokraie et al., (2010) used proteomic analysis to quantify differences in protein expression, which are ‘characteristic of the active and anhydrobiotic states of tardigrades’.

Cool eh? The list goes on! With increasing access to proteomic and genomic data online, the applications of molecular based techniques will only increase and in turn become more fine tuned with current advances.

Morphological Analysis

As well as molecular approaches, numerous studies have analysed tardigrades from a morphological point of view. This is often using biological imaging techniques such as light microscopy and scanning electron microscopy, as seen in work by Vecchi et al., 2016.

tardigrade electron microscope

(Scanning electron microscope image of tardigrade. Photo credit: Bob Goldstein and Vicky Madden).

Another example turning away from molecular techniques is the use of selective staining, as seen in one study by Mayer et al., (2013) in which performed retrograde staining of nerves in the onychophoran Euperipatoides rowelli in order to compare the locations of motor neurons within the nervous systems of arthropods, tardigrades and onychophorans. Through this staining, they were able to identify shifts in the positions of the motor neurons in different locations. This helped uncover three major correspondences between the segmental ganglia of tardigrades and arthropods, and the evolutionary history of these parts.

Similiarly epifluoresence has been used to view and interpret characteristic physical features of tardigrades. Perry, Miller and Lindsay (2014) used epifluorescence microscopy coupled with ultraviolet (UV) autofluorescence to image ‘endogenous fluorochromes’, which are a known component of tardigrade cuticles, claws and bucco-pharyngeal apparatus. Yes that’s right – even tardigrades have cuticles, although you won’t necessarily see them down the local nail bar getting a French manicure.

Staining has also shown to be an incredibly useful technique in the study of parasites such as Toxoplasma gondii, with immunoflorescent staining (Ferguson et al., 2005). If you get the time, look up Toxoplasma in the news at the moment, the research is absolutely mad, but highly interesting. Anyway guys…

Current research and the future

So as scientific research moves forward in the next few years, what can we expect? Should we disregard one method and focus on the other? This very premise is discussed by Halanych (2016), who states that the reason we’ve finally come to some conclusions in regards to the phylogeny of Tardigrades is because we’ve stopped limiting ourselves with the assumptions made by morphological analyses. However we don’t have to disregard morphological data, as it allows us to understand evolutionary patterns. Halanych instead proposes that tree-building be based on molecular data, and then separately compared to morphological data in light of the tree.

This is an interesting concept, which further emphasises the biological ‘umming and ahhhing’ we’ve been doing over the last decade. In this instance, it’s not about shifting the focus from one to the other, but altering the way in which we use and combine the data which is available to us. For example, your cup of tea will turn out differently depending on what order you add the ingredients, but there’ll always be a divide between the milk adders and the water brewers, arguing what method is ‘correct’. The same is true of science, and also tardigrade phylogeny.

Incidentally as a devoted connoisseur of tea, if you add the milk first…YOU ARE WRONG. *cough, snigger, cough*.

– H.



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