An Invited Reply to: A Comment on: “The swim-and-sink behaviour of copepods: A revisit to mechanical power requirement and a new hypothesis on function” (2023), by Jiang

Abstract

Weihs [1] mentioned that the swim-and-sink behaviour of copepods was better known as the hop-and-sink behaviour. The biological reality, however, is that the two behaviours are different. Bainbridge [3] referred to the swimming of the calanoid copepod Calanus finmarchicus as ‘hop and sink’ but provided no further description of the behaviour. Later on, Strickler [4] used ‘hop and sink’ to describe the jerky swimming pattern displayed by cyclopoid copepods and provided a detail description and kinematic analysis of the behaviour. Here, we present one of his high-speed Schlieren videos of a cyclopoid copepod in hop-and-sink together with its path speed as a function of time (electronic supplementary material, video S1; figure 1). Each hop starts with the cyclopoid beating its two antennules (A1) and culminates with the cyclopoid sequentially beating its four pairs of swimming legs along with its urosome [5]. Each hop was brief and highly unsteady, with a power stroke duration of 16.9 ± 0.8 ms (mean ± s.d., n = 6) and attaining a maximum speed of 27.0 ± 4.1 mm s−1 (mean ± s.d., n = 6). To characterize the briefness or impulsiveness of a hop, we calculate the non-dimensional jump number Jn [6] defined as the ratio of the power stroke duration τ to the viscous timescale 𝐿2p/(4⁢𝜈), where Lp is the prosome length of the cyclopoid and ν is the kinematic viscosity of water. For τ = 16.9 ms, Lp = 0.77 mm, and ν = 1.0 × 10−6 m2 s−1, Jn = 0.11, indicating that the power stroke duration is much shorter than the viscous timescale. By contrast, the swim-and-sink behaviour of the calanoid copepod Centropages sp. involves only the calanoid's cephalic appendages that vibrate continuously during each swimming phase, operating at a much longer upward-swimming duration and attaining a much slower speed (table 2 in [2]). If we neglect the small oscillations due to vibrating of the cephalic appendages and the feeding activities of the calanoid, we can consider swim-and-sink as a piecewise quasi-steady behaviour (electronic supplementary material, video S2; figure 2). Additionally, we should say that the ‘hop and sink’ of the calanoid C. finmarchicus seen in the diving observations with the naked eye should actually be ‘swim and sink’.