Exposure to bloom-like concentrations of two marine Synechococcus cyanobacteria (strains CC9311 and CC9902) differentially alters fish behaviour

Abstract

Coastal California experiences large-scale blooms of Synechococcus cyanobacteria, which are predicted to become more prevalent by the end of the 21st century as a result of global climate change. This study investigated whether exposure to bloom-like concentrations of two Synechococcus strains, CC9311 and CC9902, alters fish behaviour. Black perch (Embiotoca jacksoni) were exposed to Synechococcus strain CC9311 or CC9902 (1.5 × 10(6) cells ml(-1)) or to control seawater in experimental aquaria for 3 days. Fish movement inside a testing arena was then recorded and analysed using video camera-based motion-tracking software. Compared with control fish, fish exposed to CC9311 demonstrated a significant preference for the dark zone of the tank in the light-dark test, which is an indication of increased anxiety. Furthermore, fish exposed to CC9311 also had a statistically significant decrease in velocity and increase in immobility and they meandered more in comparison to control fish. There was a similar trend in velocity, immobility and meandering in fish exposed to CC9902, but there were no significant differences in behaviour or locomotion between this group and control fish. Identical results were obtained with a second batch of fish. Additionally, in this second trial we also investigated whether fish would recover after a 3 day period in seawater without cyanobacteria. Indeed, there were no longer any significant differences in behaviour among treatments, demonstrating that the sp. CC9311-induced alteration of behaviour is reversible. These results demonstrate that blooms of specific marine Synechococcus strains can induce differential sublethal effects in fish, namely alterations light-dark preference behaviour and motility.

Keywords: Fish physiology; harmful algal bloom; scototaxis.

Figure 1:

Exposure to Synechococcus for 3 days. (a) Fish from the control, Synechococcus sp. CC9902 and Synechococcus sp. CC9311 groups were individually placed in the light–dark preference test arena, and their location was recorded for 900 s. The upper trace illustrates the movement of one representative fish from each treatment over the duration of the trial. The heatmap (below) represents the movement of the same fish throughout the trial and is proportional to the time the fish spent in each pixel. (b) Control (CTL) and Synechococcus sp. CC9902 (9902) fish did not have a zone preference, whereas Synechococcus sp. CC9311 (9311) fish spent significantly more time in the dark zone, indicating increased anxiety. *Statistically significant difference from 300 s, P < 0.05. (c) Average velocity was significantly lower in the CC9311 group, but not in the CC9902 group, compared with the control group, nor in the CC9311 group compared with the CC9902 group. (d) Meandering was significantly greater in the CC9311 group, but not in the CC9902 group, compared with the control group, nor in the CC9311 group compared with the CC9902 group. (e) Immobility was significantly higher in the CC9311 group, but not in the CC9902 group, compared with the control group, nor in the CC9311 group compared with the CC9902 group. (f and g) There were no significant differences among groups in time spent in the middle zone or number of zone transitions. For b–g, control (n = 15), CC9902 (n = 13) and CC9311 (n = 16). Letters indicate statistically significant differences between the treatments (the same letter indicates lack of statistically significant differences).

Figure 2:

Recovery after Synechococcus exposure. After the 3 day exposure to Synechococcus sp. CC9902, Synechococcus sp. CC9311 or control conditions, all groups were placed back into normal, flowing seawater and retested after 3 days. There were no longer significant differences in time spent in the dark zone (a), average velocity (b) and meandering (c) in the CC9311 group compared with the control group. There were also no significant differences among groups in immobility (d), time spent in the middle zone (e) and number of zone transitions (f).