Hypoxia tolerance of escape jets
California market squid (Doryteuthis opalescens) encounter highly variable oxygen availability in the water as they swim around their Monterey Bay habitat. Depending on the depth and season, they could be totally fine with near 100% oxygen saturation or they can experience <20%, all while needing to maintain essential behaviors like escaping predators, catching prey, and reproducing.
By measuring simultaneous nerve and muscle activity during escape jets at different oxygen levels, I found low oxygen, or hypoxia, to slow and weaken jets only when it was severe (5%) but not when it was moderate (20%-40%). Plus, many squid were able to recover fully after seawater was reoxygenated. What's most exciting is that the changes I saw in the jet appear to be driven by similar changes at the nerve level. Suppressing nerve and muscle activity during severe hypoxia could be a strategy to tolerate the lack of oxygen until more becomes available. The changes in escape jet characteristics, though recoverable, could have large impacts on predator-prey interactions involving squid. Just a few milliseconds too late, and you could be someone's next meal!
Neural inputs and hydrodynamic output
At the basis of any movement is a signal sent by nerves to muscles telling those muscles to contract. In squid, signals from the giant and non-giant axon systems lead to jet propulsion, sending a stream of seawater out of the animal to push it the other way. Hydrodynamic features of the jet, like its shape and structure, allow us to calculate how much force is produced when the squid swims. I wanted to understand how these many levels of output interact - can we predict the force generated in the jet not only by the amount of muscle contraction but even further upstream, by the type of neural activity that starts off the chain of events?
In collaboration with Dr. Ian Bartol at Old Dominion University, I characterized jets using 3D particle tracking velocimetry (PTV) while simultaneously measuring neural activity and muscular contraction in brief squid (Lolliguncula brevis). The seawater surrounding each squid was seeded with particles illuminated by a pulsed laser to reveal shapes of jets shooting through the water. It turns out that jets initiated by the giant axon system, while on average stronger, are more limited in the range of forces they can generate. In contrast, those associated with non-giant axon activity are much more flexible in the hydrodynamic output achieved, sometime exceeding the force of giant axon jets, though they are on average less strong. By differentially recruiting the use of these two neural systems, squid can elicit a diverse set of hydrodynamic output in their jets.
Thrust production in squid hatchlings
Like adult squid but unlike almost any other marine larvae, squid hatchlings use jet propulsion to move as soon as they hatch out of their eggs. What's interesting for them and their 1-2 mm long bodies is that the seawater feels more like honey than water to them because they operate at intermediate Reynolds numbers, a regime with both inertial and viscous forces. Although their jet hydrodynamics are generally similar to those in adult squid, previous work shows that the jet structures are a bit more smeared and spread out in hatchlings, presumably due to the influence of viscosity that is absent for adult jets. However, it is not clear how the thrust generated by the jet is impacted by the smeariness of the wake structure.
To test how thrust production changes due to the influence of viscosity at intermediate Reynolds number, I collaborated with Dr. Kakani Katija at Monterey Bay Aquarium Research Institute to compare wake structure and thrust production of hatchlings jetting in seawater of different viscosities. With the help of two undergraduates from CSU Monterey Bay, we used PTV like with the brief squid project to visualize wake structures and calculate thrust. So far, we've found the smeariness of the jet to increase with viscosity, confirming the difference in wake structure between hatchling and adult is influenced by viscous forces.