  Copepods participate in large-scale diel vertical migrations (DVM) as primary
consumers in marine ecosystems. Given that they are negatively buoyant, gravity
facilitates their downward cruising but impedes their upward relocation. In
principle, overcoming this retarding force using drag-based propulsion during
upward swimming incurs extra energy costs that neutrally buoyant or
gravity-assisted downward swimmers fundamentally avoid. This added power
requirement seemingly renders upward swimming disproportionally less efficient
than downward swimming. However, we found a compensatory mechanism enhancing
thrust in upward-swimming copepods. Using experimentally derived velocity and
pressure fields, we observed that copepods pull water to the anterior,
generating sub-ambient pressure gradients when cruising upwards, thereby
inducing an upstream suction force to complement the thrust produced by the
legs. Contrary to expectations that drag forces always dominate the leg
recovery phase, the results show that the upstream suction generates net thrust
for about a third of the recovery stroke. Such a suction-thrust mechanism
promotes cost-effective swimming in larger animals like fish and jellyfish.
However, it has never been reported at small scales. We compare these results
with downward-swimming copepods that push rather than pull on water and
demonstrate they do not benefit from thrust-enhancing suction effects during
the recovery stroke. Instead, downward cruisers leverage gravitational effects
to swim faster. `Puller' and `pusher' behaviors are driven by alterations of
the leg kinematics, indicating a response to the body orientation rather than a
fixed organism trait. These results offer insights into an essential swimming
mechanism that can inform us about the role of mesozooplankton in driving
biogenic hydrodynamic transport and its effects on marine biogeochemistry.
