Measurement and hydrodynamic modelling quantifies the mechanical cost of cyprid locomotion.

Barnacle cyprids must locate a settlement site using finite lipid reserves acquired during earlier feeding stages, yet the mechanical cost of their locomotion remains uncertain. Previous estimates of cyprid swimming costs are limited and do not resolve how stroke-scale mechanics, intermittency and temperature shape overall expenditure. Here, we combine high-speed kinematic measurements with an unsteady hydrodynamic model based on the Basset-Boussinesq-Oseen equation to quantify the mechanical cost of swimming in the acorn barnacle Amphibalanus amphitrite. Measured stroke-timing distributions inform power estimates from drag, added mass, inertia and the Basset history term. We find that drag dominates mechanical power, with the Basset term providing the next largest contribution. Temperature exerts competing effects: reduced viscosity tends to lower mechanical power, but the observed shortening of power-stroke duration offsets this effect, such that predicted power approximately doubles for every 5∘C increase. Power expenditure during individual strokes ranges from hundreds to more than a thousand picowatts, but locomotion is strongly intermittent, yielding a cycle-averaged mechanical power of only about 30-120 pW, or a few microjoules of mechanical energy per day. Although these estimates are relatively small compared with the cyprid's finite lipid reserves, they could be significant if muscle power conversion is sufficiently inefficient.