Worm Buoys: Emergent Collective Interfacial Latching of Aquatic Worms

California blackworms (L. variegatus) are aquatic worms that naturally live in the benthic zones of freshwater habitats. In the absence of granular substrate such as detritus or fine sand, blackworms thigmotactically form collective worm “blobs” with each other for protection. Consequently, the resulting high density of entangled worms drives anoxia (dissolved oxygen (DO) <1 mg/L) within the blob and in its proximate regions. While blackworms can breathe through their skin, they supplement respiration by anchoring their heads in granular materials (or with conspecifics) and subsequently raising their tails upwards to access higher DO. If the water is shallow enough, blackworms position their tails on the surface, which bend at a right angle and lay parallel along the air-water interface. Due to its material property which we hypothesize to contain a pattern of hydrophobic and hydrophilic regions, portions of their tail break surface tension to access oxygen directly from air. These hydrophobic portions provide an upward force that allow worms to “latch” and hang freely using only the interface. After about 70% of the worms in a blob has latched onto the surface, the entire collective lifts off from the ground, forming a “worm buoy”. We estimate that one segment from a single worm weighing 4 mg (in water) exert enough upward force to support itself on the interface. We hypothesize this floating structure promotes collective survival and dispersal by allowing worm blobs to float (rather than climb) over obstacles.