Work

Daily locomotor rhythms and ecdysial stage-dependent behaviors in the tardigrade Hypsibius exemplaris

Year: 2026

Type: preprint

Abstract: Tardigrades, key ecdysozoan organisms, have poorly characterized daily behavioral rhythms linked to their molting (ecdysial) cycle. We continuously tracked the locomotor activity of the model tardigrade Hypsibius exemplaris using infrared videography and deep learning-based automated tracking under three photic regimes: a light–dark cycle, constant darkness following entrainment, and constant darkness without prior entrainment. We identified a robust endogenous circadian-like rhythm in locomotor activity that persisted in constant darkness, independent of photic input. Although the light–dark cycle enhanced the rhythm amplitude and the overall animal activity, it was not required for rhythmicity. Individual phases varied widely, indicating a lack of population-level synchronization. Furthermore, we developed a behavioral classification system based on activity level, which enables categorizing distinct behaviors. Behavioral patterns were modulated by distinct ecdysial stages, with quiescence gating premolt and time-of-day dependent patterns in feeding and wandering. These findings provide evidence consistent with an endogenous, circadian-like timing system in tardigrades that integrates daily activity with molting cycles and establish H. exemplaris as a promising model for studying ecdysozoan chronobiology. Summary statement Tardigrade locomotor activity is shaped by an endogenous circadian-like timing system that persists without light input and is embedded within the ecdysial cycle. This work establishes Hypsibius exemplaris as a tractable model for studying how biological clocks interact with molting programs.

Source: bioRxiv (Cold Spring Harbor Laboratory)

DOI: 10.64898/2026.09.23.753707

Authors: Baris Can Ulku, Lars Hering, Natalia Filimonova, Elfriede Friedmann, Georg Mayer

Institutions: University of Kassel, Taras Shevchenko National University of Kyiv

Language: English

Topic: Circadian rhythm and melatonin

Subfield: Endocrine and Autonomic Systems

Field: Neuroscience

Domain: Life Sciences

Open Access status: green

Funder: Deutsche Forschungsgemeinschaft

Awards: G2510115630, Identifikation der Hauptkomponenten der EcdysteroidBiosynthese und Detektion der Ecdysteroidsynthetisierenden Gewebe bei Tardigraden, Onychophoren und Cheliceraten: Einblicke in die Evolution der Häutung bei Panarthropoden, G4457758041, G7065877029, GRK 2749: Biologische Uhren auf multiplen Zeitskalen