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First published online May 29, 2009
Journal of Experimental Biology 212, 1889-1903 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.027771
Hydrodynamics of pulsed jetting in juvenile and adult brief squid Lolliguncula brevis: evidence of multiple jet `modes' and their implications for propulsive efficiency
1 Department of Biological Sciences, Old Dominion University, Norfolk, VA 23529,
USA
2 Department of Mechanical Engineering, Southern Methodist University, Dallas,
TX 75275, USA
3 Department of Biology, Franklin and Marshall College, Lancaster, PA 17604,
USA
* Author for correspondence (e-mail: ibartol{at}odu.edu)
Accepted 5 March 2009
The dynamics of pulsed jetting in squids throughout ontogeny is not well
understood, especially with regard to the development of vortex rings, which
are common features of mechanically generated jet pulses (also known as
starting jets). Studies of mechanically generated starting jets have revealed
a limiting principle for vortex ring formation characterized in terms of a
`formation number' (F), which delineates the transition between the
formation of isolated vortex rings and vortex rings that have `pinched off'
from the generating jet. Near F, there exists an optimum in
pulse-averaged thrust with (potentially) low energetic cost, raising the
question: do squids produce vortex rings and if so, do they fall near
F, where propulsive benefits presumably occur? To better understand
vortex ring dynamics and propulsive jet efficiency throughout ontogeny, brief
squid Lolliguncula brevis ranging from 3.3 to 9.1 cm dorsal mantle
length (DML) and swimming at speeds of 2.43–22.2
cms–1 (0.54–3.50 DMLs–1) were
studied using digital particle image velocimetry (DPIV). A range of jet
structures were observed but most structures could be classified as variations
of two principal jet modes: (1) jet mode I, where the ejected fluid rolled up
into an isolated vortex ring; and (2) jet mode II, where the ejected fluid
developed into a leading vortex ring that separated or `pinched off' from a
long trailing jet. The ratio of jet length [based on the vorticity extent
(L
)] to jet diameter [based on peak vorticity
locations (D)] was <3.0 for jet mode I and
>3.0 for jet mode II, placing the transition between modes in rough
agreement with F determined in mechanical jet studies. Jet mode II
produced greater time-averaged thrust and lift forces and was the jet mode
most heavily used whereas jet mode I had higher propulsive efficiency, lower
slip, shorter jet periods and a higher frequency of fin activity associated
with it. No relationship between
L/D and speed was
detected and there was no apparent speed preference for the jet modes within
the speed range considered in this study; however, propulsive efficiency did
increase with speed partly because of a reduction in slip and jet angle with
speed. Trends in higher slip, lower propulsive efficiency and higher relative
lift production were observed for squid <5.0 cm DML compared with
squid 
5.0 cm DML. While these trends were observed when jet mode
I and II were equally represented among the size classes, there was also
greater relative dependence on jet mode I than jet mode II for squid <5.0
cm DML when all of the available jet sequences were examined.
Collectively, these results indicate that 
5.0 cm DML is an
important ontogenetic transition for the hydrodynamics of pulsed jetting in
squids. The significance of our findings is that from early juvenile through
to adult life stages, L. brevis is capable of producing a diversity
of vortex ring-based jet structures, ranging from efficient short pulses to
high-force longer duration pulses. Given that some of these structures had
L/Ds near F,
and F represented the delineation between the two primary jet modes
observed, fluid dynamics probably played an integral role in the evolution of
squid locomotive systems. When this flexibility in jet dynamics is coupled
with the highly versatile fins, which are capable of producing multiple
hydrodynamic modes as well, it is clear that squid have a locomotive
repertoire far more complex than originally thought.
Key words: hydrodynamics, ontogeny, squid, swimming, vortex rings
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