Squid paralarvae turn with high agility using jets near the Reynolds number threshold for viscous domination

Turning is critical for survival in the ocean, as marine animals need to maneuver to capture prey, elude predators and navigate complex environments. While prior research has focused on turning performance of adult swimmers, less is known about early ontogenetic stages that locomote within lower Reynolds number (Re) regimes, especially young jetters. To evaluate squid paralarval turning proficiency and the role of the pulsed jet in maneuvers, recently hatched longfin inshore squid, Doryteuthis pealeii, swimming in a viewing chamber were studied using digital particle image velocimetry and kinematic motion analyses. Paralarvae exhibited a wide repertoire of turning behaviors, including those performed arms-first and tail-first. Paralarval turns were broader [higher mean length-specific turning radii (R/Lmean)] and faster [higher mean angular velocity (Ωmean)] than those of older squids, with some turns (∼8%) involving peak angular velocities (Ωmax)>2000 deg s-1. Relative to cuttlefish hatchlings, squid paralarvae exhibited lower R/Lmean and higher Ωmean and Ωmax. Higher angular jet impulse produced turns of greater Ωmean and total angular displacement, and R/Lmean and Ωmean increased with higher Resquid. Paralarval jets ranged from isolated vortex rings (short pulses), some of which occurred near the viscous dominated condition of Re<1, to elongated vorticity structures with and without leading edge vortex ring formation (long pulses). Despite the range of jet flows produced, strong relationships between jet length-to-diameter ratios and kinematic properties were not observed. The ability of paralarvae to produce a diversity of directed jets at low/intermediate Re is integral to their turning versatility and ultimately survival.