Metachronal Swimming in Pacific krill, Euphausia pacifica

Euphausia pacifica, Pacific krill, is a key link in the trophic cascade in northern Pacific temperate waters and is subject to strong diel vertical migration. The krill is negatively buoyant, requiring the animal to constantly beat its pleopods in order to maintain its depth. Metachronal motion, where consecutive appendages beat in sequence, is an effective means of both counteracting negative buoyancy and for forward propulsion. In other crustaceans, swimming modes are modified by pleopod beat frequency and each pleopod’s range of motion. Here, we seek to further understand the specific nuances of metachronal propulsion in E. pacifica, how it varies with different swimming modes, and how it differs among decapods including the closely related Antarctic krill, E. superba. E. pacifica were recorded in the dark under two orthogonally-positioned high-speed cameras in order to allow for three-dimensional analysis. Body angle, telson angle, velocity, and acceleration were measured for each recording. Pleopod motion was quantified by phase duration, phase lag between each pleopod, the angle between the protopodite and the body throughout one cycle, and the angle between the endopodites and the protopodites throughout one cycle. A cycle comprises the time between the initiations of consecutive downward power strokes on the most posterior pleopod. Preliminary observations indicate two main modes of swimming based on the distance covered by the krill: fast forward swimming and hovering. Fast forward swimming has a greater velocity and a higher net-to-gross-displacement ratio compared to hovering. This is accomplished by increasing the angle range of the protopodite relative to the body and increasing the angle range between the endopodites and the protopodite. This increases the stroke amplitude of the pleopod, allowing for more thrust in the power stroke of each pleopod.