Evolution and Phylogeny
The oldest tardigrade fossil found dates to 550 million years ago, the fossil shows a juvenile tardigrade that looks similar to modern day tardigrades₁. Due to the small size of tardigrades, the fossil record is not a reliable source for tardigrade evolution. While there are about 1,200 species in the tardigrade family, there has not been enough selective pressure to trigger major evolution in millions of years₂. Researchers have mainly relied on morphological evidence to determine phylogenetic position. Traditionally, tardigrades have been divided into three classes, Heterotardigrada, Eutaardigrada, and Mesotargradia. However, like most things in the tardigrade phylogeny, the Mesotardigradia class has been repeatedly questioned₃.
The tardigrade phylogeny is a major source of conflict. While researchers agree that Arthrotardigrada and Echiniscoidea are sister groups based on shared characteristics, there are claims that these similarities are a result of convergent evolution₄. This has caused confusion about the correct phylogenetic position of tardigrades. This confusion has not resolved after many molecular studies, which place them as either basal to both arthropods and coelomate protostomes or on a moderately supported clade between Priapulida and Arthropoda₄. Due to this uncertainty, a large study broke down key characteristics to try to explain the relationship to arthropods. Tardigrades are composed of a total of seven or eight somewhat indistinct segments, and only larval or parasitic arthropods have a similar number of segments₄ (Figure 1). Dozens of studies spanning decades all claim different classifications for within the tardigrade family. The current accepted clades are mainly based on morphological features such as size, shape, claw size and shape, morphology of eggs and spermatozoa. However, other studies are more focused on molecular data showing the conflict between trees. For instance, Heterotarigradia classification is supported by molecular data, but is contested based on morphological data₆.
Figure 1. Clear example of segmentation found on Tardigradia. Arthropods are the only relative that share this extensive segmentation, leading some researchers to believe they are very closely related. However, some other researchers claim this is due to convergent evolution. Photo credit - Ian Woods
Other studies based on morphology have claimed annelid-arthropod lineage, while others have them placed by onychophoran–arthropod complex or the aschelminthes group₅. One large study from 2017 based on morphological and molecular data claimed that a new class be created within the tardigrade family₃. This study included a large tardigrade diversity, as it covered over 80% of tardigrade families and subfamilies and 53% of genera. Based on their results, the researchers proposed that at least four biodiversity groups should be added on the Metazoan and Ecdysozoa phylogenies. The heterotardigrades from the marine order Arthrotardigrada, heterotardigrades from the terrestrial Echiniscus (formally order Echiniscoidea), one apotardigrade from the newly created class Apotardigrada, (formerly order Apochela), and an eutardigrade from the new created order Macrobiotoidea₃. By studying HOX genes, another recent study claims that tardigrades are segmented worms and are more closely related to nematodes than arthropods and belong in the Ecdysozoa family₂ (Figure 2).
Figure 2. These are simplified versions of the proposed phylogenic trees based off phylogenic or molecular data. With some even suggesting the bias of researchers has lead to Tardigrada being forced in the tree
Tardigrade ancestors share derived characteristics with arthropods, although it is up for debate if this is due to a shared ancestor or convergent evolution₄. The general morphology and physical abilities remain constant throughout tardigrade species, however changes to features such as body size, leg structure and external coloration have occured to best fit their niche. When looking into habitat preferences and gene trees, the data suggest tardigrades have adapted to marine environments twice, to freshwater environments at least three times, and to terrestrial environments twice. The placement of the terrestrial Oreellidae as the basal heterotardigrade suggests that tardigrades were ancestrally terrestrial₆. The most studied feature of the tardigrade is the brain. Results of brain laser scanning suggests that general brain morphology is conserved across Tardigrada. Based on the results, they found direct parallels between the tardigrade brain and the segmental trunk ganglia of the tardigrade ventral nervous system₇. They suggested that the tardigrade brain retains aspects of an ancestral cycloneuralian brain, while exhibiting ganglionic structure characteristic of euarthropods and onychophorans₇.
The conflict surrounding the tardigrade phylogeny will not be resolved quickly due to the confusion surrounding the relationship with arthorpods. While morphological evidence presents a clear relationship, molecular studies do not show the same level of support. Additional large studies that are not biased towards arthropods need to be conducted to fully understand the structure of the tardigrade phylogeny.
Works Cited
₁ Bertolani R, Grimaldi. 2000. A new Eutardigrade in amber from the upper cretaceous of New Jersey.
₂ Yoshida Y, Koutsovoulos G, Laetsch DR, Stevens L, Kumar S, Horikawa DD, Ishino K, Komine S, Kunieda T, Tomita M, et al. 2017. Comparative genomics of the tardigrades Hypsibius dujardini and Ramazzottius varieornatus. Tyler-Smith C, editor. PLOS Biol. 15(7):e2002266. doi:10.1371/journal.pbio.2002266.
₃ Guil N, Jørgensen A, Kristensen R. 2019. An upgraded comprehensive multilocus phylogeny of the Tardigrada tree of life. Zool Scr. 48(1):120–137. doi:10.1111/zsc.12321.
₄ Dewel RA, Dewel WC. 1998. The place of tardigrades in arthropod evolution. In: Arthopod Relationships. Vol. 55. (The Systematics Association Special Volume Series). p. 109–123.
₅Dewel RA, Clark WH. 1973. Studies on the tardigrades. I. Fine structure of the anterior foregut of Milnesium tardigradum Doyère. Tissue Cell. 5(1):133–146. doi:10.1016/s0040-8166(73)80011-4.
₆ Nichols PB, Nelson DR, Garey JR. 2006. A Family Level Analysis of Tardigrade Phylogeny. Hydrobiol Dordr. 558(1):53–60. doi:http://dx.doi.org.ezproxy.ithaca.edu:2048/10.1007/s10750-005-1414-8.
₇Smith FW, Bartels PJ, Goldstein B. 2017. A Hypothesis for the Composition of the Tardigrade Brain and its Implications for Panarthropod Brain Evolution. Integr Comp Biol. 57(3):546–559. doi:10.1093/icb/icx081.