Meeting Abstract
Squid are known for their use of jet propulsion, and paralarvae rely on this locomotor strategy immediately after hatching. Unlike their adult counterparts operating at high Reynolds number (Re) where inertia dominates, paralarvae must contend with both inertial and viscous forces in intermediary Re regimes. Observations of jet wake structure in squid paralarvae reveal that vortex rings, a key feature in potential thrust and efficiency enhancement, appear distorted when compared to observations in adults at high Re. It is possible that viscosity plays a role in changing jet wake structure, with consequences for the effectiveness of jet propulsion as Re decreases. Numerical simulations on how Re affects jet propulsion have proposed a theorized limit at Re = 10, below which swimming speeds quickly decay and jetting is no longer effective. By using micro particle tracking velocimetry to visualize jet hydrodynamics of tethered and free-swimming squid paralarvae in seawater and in fluids with twice and four times the viscosity of seawater, we find experimental evidence supporting the idea of a critical limit for jet propulsion. When Re < 10, significantly less thrust is produced and vorticity in the wake is disorganized with no evidence of coherent ring structures. Moreover, paralarvae are unable to swim under such conditions even though the magnitude of mantle contractions increases. For a dynamically similar scenario in seawater, a paralarva would have to be smaller than that for any known species of squid, suggesting that viscous forces at low Re may play a role in hydrodynamically constraining size at hatch for these small, jet-propelled organisms.
