From nauplii to copepodites: behavioral and kinematic transitions in the cruise-feeding calanoid copepod Clausocalanus furcatus

Developmental transitions from nauplii to copepodites are critical in ocean ecology, yet behavioral and kinematic changes across stages remain understudied, especially in cruise-feeding copepods. Using high-speed microscale imaging, this study quantified morphology, swimming and appendage beating in nauplii and copepodites of the copepod Clausocalanus furcatus. Nauplii swam in a looping swim-and-sink pattern at prosome-length-specific speeds of 38.4 ± 12.4 L s−1 with appendage beat frequencies of 99.4 ± 21.7 Hz, whereas copepodites mainly cruised at 14.8 ± 4.2 L s−1 and 103.1 ± 12.1 Hz. Despite major differences in morphology, size and kinematics, both stages achieved similar body-volume-specific maximal clearance rates (~ 106 body volumes d−1), sufficient for survival in nutritionally dilute oceans. Although beat frequencies varied little across stages, the nondimensional beat number decreased from > 1 in nauplii to < 1 in copepodites, suggesting a shift toward energetically inexpensive, unsteady viscous vortical flows that may also reduce hydrodynamic predation risk. Copepodites captured prey through direct interception aided by coordinated appendage movements and through active maneuvering, with short mean reaction distance (~ 0.5 body widths) and reaction-to-capture time (~ 30 ms), indicating non-contact detection. These results demonstrate how C. furcatus maintains clearance rates through ontogenetic shifts in kinematics, highlighting behavioral and sensory adaptations that enable cruise-feeding copepods to thrive in oligotrophic oceans.