Meeting Abstract
Cephalopod eggs are exceptionally large compared to other marine invertebrates. Larger eggs produce larger hatchlings that experience a higher Reynold’s number environment, where fluid momentum and turbulent flow increasingly dominate over viscous forces. We hypothesize that larger cephalopod egg sizes evolved in response to functional constraints inherent to their muscular jet and fin propulsion systems when operating at low Reynold’s numbers. To test the influence of viscosity on swimming at the size limit of cephalopods, the “world’s smallest” swimming cephalopod, hatchlings of the pygmy squid Idiosepius paradoxus, were used. Polyvinylpyrrolidone (PVP) can increase seawater viscosity without adverse affects on animals. Using a PVP concentration series, we tested the effect of viscosity on swimming rates in pygmy squid hatchlings. Average velocities per viscosity were calculated by video analysis. We found swimming became increasingly inefficient with increasing viscosity, and jet propulsion broke at dynamic viscosities greater than 9 cP. Hatchling sizes in normal seawater were calculated for PVP-based viscosities and velocities, and egg sizes were extrapolated. Our results indicate that at egg sizes typical in non-cephalopod molluscs, a cephalopod hatchling would swim poorly or not at all in normal seawater. Thus, the evolution of exceptionally large eggs in cephalopods may reflect a functional constraint of their muscular jet propulsion system when operating at low Reynold’s numbers. Future studies may include testing the functional limits of jet and fin propulsion in other species, PIV characterization of swimming at increasing viscosities, and analysis of neural activity during swimming.
