The Rise of Clawless Water Bears

 By: Giana Krisha N. Baturiano | Resistome

When we think of the grand story of evolution, the dramatic chapters of colossal mammals and soaring birds usually come to mind. At the margins of this story, the small creatures remain as footnotes in evolutionary narratives. But we all know that footnotes often reveal the greatest truths. Modern science invites us to narrow down our attention on resilient, nearly invisible champions of survival: the tardigrades.

Have you ever imagined stubby little bears wading through ponds or stout piglets nestled in a rich bed of green moss? Now shrink that image until it is no larger than the period at the end of this sentence. That is the scale of tardigrades, microanimals with bodies encased not in fur or skin, but in an exoskeleton. Commonly termed as water bears or moss piglets, these microinvertebrates tardigrades are far from the vertebral world of bears and piglets. Instead, tardigrades lodge in thin films of freshwater, marine sediments, and cushions of moss and lichen. Beyond their adorable appearance, tardigrades are astonishingly tough organisms, known to withstand extreme conditions such as desiccation, freezing, ionizing radiation, high pressure, and even the vacuum of outer space. They survive such harsh environments by entering a state of cryptobiosis, suspending metabolic processes until favorable conditions return. Their resilience and morphological diversity open new horizons for understanding evolution and biodiversity.

Named after the Latin tardigradus, meaning “slow walker,” tardigrades are characterized by four pairs of stout legs ending in claws, a flexible segmented body, and a buccal-pharyngeal feeding apparatus equipped with stylets for piercing food sources. Their stubby legs propel them through water films as they inhabit a remarkable range of environments, from deep oceans to mountain moss. Tardigrades stand as their own distinct lineage under Phylum Tardigrada, which consists of two major classes: Eutardigrada and Heterotardigrada. Most tardigrades possess claws that facilitate anchoring and locomotion across rough substrates. However, recent research has revealed that even tardigrades without claws can survive just as effectively, challenging assumptions about what structures are essential for tardigrade survival.

In October 2025, a new genus of tardigrade was established, Sinunguibius gen. nov., the second known genus within Class Eutardigrada characterized by the complete absence of claws. The key morphological character that differentiates this new genus from the also clawless genus Apodibius is the number of placoids in the pharynx. Diverging from the usual pharyngeal arrangement in its relatives, the new Sinunguibius genus possesses distinct gibbosities, specialized peribuccal structures, and three placoids, deviating from the more typical two-placoid configuration. The complete loss of claws is surmised to be a case of convergent evolution, occurring independently in at least two families, Doryphoribiidae and Hexapodibiidae.

Using molecular markers 18S rRNA and 28S rRNA, the researchers reconstructed the molecular phylogeny of tardigrades collected from mosses in the Kalyango Hills of Uganda. With these markers, they were able to place the new genus Sinunguibius within the family Hexapodibiidae, which is distinct from the other clawless genus Apodibius, supporting the hypothesis that the loss of claws in Eutardigrades is a case of convergent evolution. 18S rRNA and 28S rRNA are commonly used in tardigrade systematics due to their high conservation, slow evolutionary rate, and reliability in resolving deep phylogenetic relationships.

This new knowledge sheds light on how tardigrades, even without their characteristic appendages, continue to endure extreme environments. Whether clawed or clawless, tardigrades illustrate the subtle and  inventive strategies that life employs to persist. In doing so, tardigrades complete the wider story of evolution, demonstrating that biodiversity is not only driven by the giants of life, but also by the tiny survivors that quietly shape biodiversity.