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KEYWORDS: Copepods; UVB; Visible light; Life cycle; Feeding - page 9 / 16

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D. M. MARTYNOVA AND A. V. GORDEEVA

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LIGHT-DEPENDENT BEHAVIOR OF ABUNDANT ZOOPLANKTON SPECIES

Arctic species Mature O. borealis did not display any significant responses to the RY during spring and autumn equinox and polar day period (early June). This was true for copepods from both photic and aphotic layers (Table I).

Naupliar stages of a large Arctic copepod species C. glacialis inhabiting the upper water layer in May exhibited strong positive response to light (Fig. 3e), which increased after the animals were kept in darkness for 48 h. In contrast, CIV copepodites, inhabiting the aphotic layer in August, avoided light during the exper- iment, even after 48 h acclimation in light. The same developmental stages of another Arctic species M. longa, inhabiting the aphotic layer, had almost identical light behavior in October (Fig. 3f and Table I).

Fig. 3. (Continued).

animals in darkness increased the positive light response, but this tendency was much less pronounced after a 48 h acclimation period. In October, different conspecifics either preferred or avoided light, while few individuals were left in the middle chamber (Fig. 3d). Significant differences in light responses were also found between mature and immature specimens (Table I). However, effects of various acclimation factors were insignificant.

Pseudocalanus minutus was characterized by the most pronounced variability of light-dependent behavior. Their naupliar stages, inhabiting the photic layer in Ma , showed strong positive phototaxis (Fig. 3g), which increased after animals were kept in darkness. The same was observed for CI and CII copepodites that dominated in the upper water layer in June (Fig. 3h). CIII copepodites of the studied species, sampled from the aphotic layer, formed two distinct groups by their photobehavior (Fig. 3i). The most numerous group was characterized by positive phototaxis, the other individ- uals displayed negative phototaxis. The same pattern was observed in August (Table I). CIV copepodites inhabiting the deep (aphotic) water layer in early June also divided into two groups, but the 48 h acclimation period without food supply increased the number of individuals with positive phototaxis. In October, this developmental stage showed negative light-dependent behavior, and no significant effect of the acclimation environment was found. The CV copepodites from the aphotic water layer had the same pattern of photobeha- vior as CIII and CIV, both in early June and August, divided into two groups. Additionall , as in CIV cope- podites, the acclimation environment did not signifi- cantly affect their responses to light for the group of negative phototaxis. In the spring equinox, CV copepo- dites and adults were equally distributed in through the water column. They exhibited positive phototaxis, which was more pronounced after 24 h of starvation (Table I).

Combining all the data, we tried to find a regular pattern in the relationships between trophic and biogeo- graphical status, vertical distribution and light response of the studied species. As can be seen in Fig. 4a, more than a half of all the studied species were quite sensitive to the RY light. Moreover, most of them are herbivorous

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