Meeting Abstract
Aggregate formation and clustering are common behaviors observed from bacteria to humans, and can facilitate the survival of the collective [Allee, 1978]. Worms have soft bodies with moist-skin and can thus individually suffer due to environmental stresses. Aquatic worms (Lumbriculus variegatus , ~3 cm long) naturally aggregate into multi-cm diameter ensembles of thousands of worms knotted together, forming an active viscoelastic ‘blob’; these are presumed to confer benefits, but systematic studies are lacking. We are interested to subject the collectives to evaporative, thermal and mechanical stresses and measure mortality and damage of individuals. Here, we focus on thermal stresses, measuring the mortality rate as a function of cluster size (N=1, 5, 10, 20 worms, 10 replicates per condition) under controlled laboratory conditions. When single worms were placed on a dry plate at room temperature and humidity (24 °C, 48%), they died after 56 ±16 min while in a cluster with 20 worms, animals perished after 342 ±24 min; the time to death scaled approximately linearly with N. To gain insight into the dynamics of the blob and the worms, we performed time-lapse imaging, tracking the projected area (A) of the aggregate. Worms moved continuously throughout all clusters, presumably to keep their surfaces moist. For N<20, the aggregate monotonically reduced total surface area by shrinking into a circularly symmetric cluster. For N=20, the blob expanded for ~50 min until it achieved nearly double its projected area, after which it decreased in size. Final A before death increased with increasing N. We hypothesize that larger clusters first search for more favorable conditions before beginning a stereotyped shrinking to reduce surface area to volume to avoid evaporative losses.
