Growth Patterns of Calanus finmarchicus

The general growth pattern of C. finmarchicus is shown in Fig. 2. The length measure used throughout this paper is the volumetric length, i.e., the cubic root of estimated body volume from the measured N and C content. Even though this measure is rather abstract, it has the advantage that nauplii and copepodites can be compared in the same graphs, that C and N growth can be compared, and that the growth patterns can easily be compared to the von Bertalanffy curve as expected for most animals in a DEB context (Nisbet et al. 2000 (link)). The volumetric length based on N content represents structural biomass plus the egg buffer for the embryo (including the non-feeding naupliar stages). The length based on C content includes all biomass (egg buffer, structure and lipid storage). As long the C/N ratio is 4 ( $d_{\mathrm{C}}/d_{\mathrm{N}}$ in Table 3), volumetric length based on C and N will be the same; a higher value for C-based length in Fig. 2 indicates lipid storage. The conversion from the three state variable (Table 2, all in dry weight) to volumetric length measures is explained in more detail later.Fig. 2

Volumetric body length of C. finmarchicus, calculated from N content (representing structural biomass plus egg buffer) and C content (including lipid storage as well) from egg to adulthood (left panel) at 8 °C (data from Campbell et al. 2001 (link)). Life stages indicated are Embryo, Juvenile, Sub-adult and Adult (Fig. 1). Straight lines drawn by eye to indicate linearity. Right panel shows a standard (solid line) and truncated (broken line) von Bertalanffy curve on length basis for reference

As with the congeneric C. sinicus (Jager et al. 2015 (link)), the embryo only decreases in size over time (Fig. 2). Egg buffer is converted into structural biomass, whereby mass is lost on maturation, maintenance and conversions. In contrast to C. sinicus, however, the post-embryonic growth of C. finmarchicus does not resemble a truncated von Bertalanffy curve (observation 2 in Table 1). Rather, the growth curve for juveniles and sub-adults consists of two more-or-less linear phases (Fig. 2). Such a pattern has also been observed for other calanoid species, such as C. marshallae (Peterson 1986 (link)) and the genus Acartia
(Miller et al. 1977 (link)). Deviations from the von Bertalanffy curve in the early life stages are quite common, especially among animal species sporting larval development, and several modifications in a DEB context have been proposed (Kooijman 2014 (link)). Here we assume that the copepods go through a ‘type A acceleration’, where the specific assimilation rate makes a step-up in one of the late naupliar or early copepodite stages.