The impact of the introduced ctenophore Beroe ovata on its prey Mnemiopsis leidyi, another invader ctenophore voraciously feeding on mesozooplankton, and consequently on the mesozooplankton community, was evaluated by undertaking both laboratory and field studies in the northern Black Sea. Ingestion and growth rates as well as the gross growth efficiency of B. ovata were estimated from laboratory experiments. The daily ration of ctenophores was related to food abundance within a wide range of prey concentration and never reached saturation. Beroe ovata required high food rations (not less than 20% of body weight per day) for growth. The abundances, biomasses and population structures of these two introduced ctenophore species were also monitored, along with mesozooplankton, in inshore waters of the northern Black Sea (i.e. Sevastopol Bay and adjacent regions) over a period of 3 years (1999–2001) which is after B. ovata’s arrival. The annual dynamics of the M. leidyi population were similar for the last 3 years: very low abundances and biomass values were observed during most of the year (unlike the previous years), with a sudden increase in summer–early autumn, but only for about a 2 month period. The B. ovata bloom during the peak M. leidyi biomass resulted in the M. leidyi biomass falling sharply to extremely low values. The predatory impact of M. leidyi on prey zooplankton was found to be reduced during the period of study compared with before.

Calanoid copepods have two swimming gaits, namely cruise swimming that is propelled by the beating of the cephalic feeding appendages and short-lasting jumps that are propelled by the power strokes of the four or five pairs of thoracal swimming legs. The latter may be 100 times faster than the former, and the required forces and power production are consequently much larger. Here, we estimated the magnitude and size scaling of swimming speed, leg beat frequency, forces, power requirements, and energetics of these two propulsion modes. We used data from the literature together with new data to estimate forces by two different approaches in 37 species of calanoid copepods: the direct measurement of forces produced by copepods attached to a tensiometer and the indirect estimation of forces from swimming speed or acceleration in combination with experimentally estimated drag coefficients. Depending on the approach, we found that the propulsive forces, both for cruise swimming and escape jumps, scaled with prosome length (L) to a power between 2 and 3. We further found that power requirements scales for both type of swimming as L³. Finally, we found that the cost of transportation (i.e., calories per unit body mass and distance transported) was higher for swimming-by-jumping than for cruise swimming by a factor of 7 for large copepods but only a factor of 3 for small ones. This may explain why only small cyclopoid copepods can afford this hydrodynamically stealthy transportation mode as their routine, while large copepods are cruise swimmers.

The effect of locomotor activity on respiration rate was studied in the food-deprived copepod Calanus euxinus tethered to a force sensor. The power generated by mouth appendages during cruising locomotion, with a frequency of 40 Hz, accounted for 0.026 and 0.0031 W for metabolic and mechanical processes, respectively. To overcome total hydrodynamic drag during foraging with a mean swimming speed of 3.2 cm s⁻¹, the copepods need 0.4 × 10⁻³ W, equating to 1.3% of total metabolism. The losses of mechanical energy for body propulsion amounted to 1.3 × 10⁻³ W, whilst the cost of feeding current generation run up to 1.8 × 10⁻³ W, or 58% of the total. Changing of locomotor activity and respiration rate during feeding was examined separately in tethered and free-swimming copepods. At algal concentration of ∼300 μg C L⁻¹, the magnitude of specific dynamic action (SDA) averaged 1.2 ± 0.44 nL O₂ μg C⁻¹ h⁻¹ in copepodites V and females, with similar moving activity before and during feeding. The contribution of SDA into total metabolism varied from 23 to 85% in C. euxinus with low activity level and constituted only 10% in active animals. In starved copepods, with low locomotor activity, feeding events stimulated the increase in frequency and total duration of locomotion which resulted in elevated energy expenditure enhancing the ‘apparent SDA’.