Worm blob dynamics under thermal stress

Aggregate formation and clustering are common behaviors observed from bacteria to humans, and can facilitate the survival of the collective [Allee, 1978]. Here we discuss aggregation behavior in aquatic worms and its potential biological function. We first show how aquatic worms can be induced to aggregate into ensembles of thousands of worms that knot together, forming an active viscoelastic ‘blob’. This worm blob can be modulated from an elastic solid-like state to a viscous liquid-like state by changing the surrounding temperature. To establish how this complex transition occurs, we measure both the dynamics of individual worms and the collective. In single worms, we find that activity of an individual worm increases with temperature and reaches a peak value (0.05 cm/s) around 32°C. Thus, in groups at lower temperature (10°C), individual worms are less active and entangle together, while at higher temperatures (35°C), individual worms are more active and disentangle. More specifically, the steady-state density of the cluster decreases from 75% to 33% as temperature increases from 15 to 35°C, respectively. Our results suggest that worm’s aggregates may serve as a robust survival strategy of the collective against environmental thermal stresses.