D. M. MARTYNOVA AND A. V. GORDEEVA
LIGHT-DEPENDENT BEHAVIOR OF ABUNDANT ZOOPLANKTON SPECIES
pronounced ontogenetic vertical migrations. This species tended to diapause as CIII-CV copepodites in deep water layers through summer, autumn and winter periods, when it has stored significant amount of lipids for wintering during the spring time and then became inactive (Prygunkova, 1974). We assume that diapausing copepodites may avoid the light, trying to ’migrate in deeper layers’, while those, which still have to store lipids, may exhibit the opposite response by ’migrating to food-rich layer’.
Obviousl , the absence of a light response is more likely in such species as O. borealis, whose life cycle occurs almost completely in darkness (Prygunkova, 1974).
The light responses of polychaete larvae agree with their life cycle. The vertical distribution of meroplank- tonic polychaete larvae differs between age groups (Shuvalov, 1978). Younger stages presumably feed on phytoplankton and obviously inhabit the upper near- surface water layer with optimal feeding conditions. When these animals reach a certain stage, they settle to the bottom substrate and the sign of phototaxis prob- ably reverses.
The positive response of the eurybiont O. similis to light may also be accounted for by their feeding pattern. Oithona is known to actively destroy faecal pellets of calanoids (Svensen and Nejstgaard, 2003). The peak concentration of suspended organic materials (including pellets) in the White Sea is observed in the upper water layers (Lukashin et al., 2003; Martynova, 2003). The maximum of the vertical distribution of O. similis is also usually observed in the upper 25 m (Prygunkova, 1974; original data).
The relationships between zooplankton vertical distri- bution and visible light differ across species. The most surprising results were obtained for the phototactic response to UV radiation, which in some cases was posi- tive. As it was stated by many of authors, UV light deters most of hydrobionts due to its harmful effect (Frank and Widder, 1994; Martin et al., 2000; Adams, 2001; Johnsen and Widder, 2001; Rhode et al., 2001). However, we tend to explain such unexpected behavior in the White Sea species by the low UV dose the animals are typically exposed to. We hypothesize that low-intensity UV light may signal the upper food-rich layer for herbivores (Acartia and Pseudocalanus). This is consistent with the increasing positive effect of UV on starved herbivores (Table II). As it was shown by Forward and Cronin (1979), intertidal plankton species had different sensi- tivity to UV and green/blue light with respect to their preferable depth, whereas deep-dwelling animals had no reaction to UV light. We suppose that low UV intensity may not be harmful for the animals, but can provide certain biologically relevant signal.
The role of visual predators in diel vertical migration behavior has been widely discussed (Gliwicz and Pijanowska, 1988; Bollens and Frost, 1989; Neill, 1990; Bollens et al., 1993; Loose and Dawidowicz, 1994). The role of predator pressure as a factor inducing DVM overlaps with the focus of the present research; however, some assumptions may be made. The herring Clupea pallasi maris-albi is the most abundant planktivorous fish, which may represent up to 95% of all visual predator abundance in the White Sea (Berger et al., 2001). The adult specimens are the most active in May during reproduction period, consuming mostly the large cala- noid copepod C. glacialis. In summer and autumn period, herring larvae, and various coelenterate and
chaetognath species many small plankton Centropages and some
(’non-visual predators’) feed on
species (e.g. Oithona, Acartia, others). Young copepodite
emora, stages and
Kosobokova, 2003; Kosobokova et al., 2005). Data chemical cues inducing migrations in the White Sea totally absent.
Analyzing phototactic behavior in different life history stages of zooplankton species, we found no evi- dence that young stages are more vulnerable than elder copepodites, even in ecologically similar species. Thus, we hypothesize a relationship between light responses of crustaceans and such ecological features as trophic characteristics and the inhabited water layer. Light is considered one of the most important factors inducing diurnal vertical migrations of zooplankton in upper water layer (Williamson et al., 1994; Ringelberg, 1999; Hansson et al., 2007). However, biological processes, which determine behavioral responses, are still far from being understood.
To summarize, we documented significant differences in responses of the species studied to light of different wavelengths. Positive phototaxis to red and yellow light is associated with the tendency of herbivorous crus- taceans to keep within the layer with the highest food availability (photic layer, where 95% of all phytoplank- ton is aggregated). Animals may also avoid this layer. Avoidance of the photic layer may be linked with the ontogenetic diapause, which in Arctic species occurs in the aphotic layer. Positive responses to light in predomi- nantly herbivorous P. minutus and omnivorous Acartia spp. to extreme, but low dose UVB, especially after star- vation, may also be accounted for by their tendency to migrate to upper water layers with higher food avail- ability. In addition, we speculate that migrations of older copepodite stages of predatory crustaceans Metridia are associated with prey search. Thus, we hypothesize a relationship between light sensitivit , on the one hand, and trophic characteristics, migration
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