RNA:DNA ratio during the critical period and early larval growth of the red drum Sciaenops ocellatus

Eggs from two separate spawning stocks of the red drum Sciaenops ocellatus (Linnaeus) were hatched, and the larvae were reared in the laboratory for 2 wk under closely controlled conditions. Total RNA, DNA, and soluble protein were measured in each population daily in triplicate pooled samples of larvae from each of three tanks. Growth rate in mm d^-1 was determined for each population at 2 d intervals. Growth rate explained 72 and 95% of the variation in the RNA:DNA ratios of the two populations individually, and 86% of the variation in the RNA:DNA ratio when data from the two populations were combined. The RNA:DNA ratio appeared to be most effective as an indicator of growth in rapidly growing larvae, and to lose some resolution when growth was intermittent. The rates of deposition of RNA, DNA, and protein into tissue were all highly correlated with growth rate and with each other. Mean population RNA:DNA ratios of red drum yolk-sac larvae decreased from Day 1 post-hatch until larvae initiated successful feeding behavior, and then increased steadily throghout the remainder of the experimental period. This pattern of change in the RNA:DNA ratios during the yolk-sac stage appears to be an intrinsic developmental pattern of red drum ontogeny. The lowest values for the RNA:DNA ratio were observed just prior to the initiation of feeding or during the critical period, indicating that red drum larvae experience a decrease in capacity for protein synthesis as they initiate feeding. Intrinsic variation in the RNA:DNA ratio during development suggests that caution be used when comparing the RNA:DNA ratios of yolk-sac larvae to a critical ratio calculated from Buckley's general model.     

Metal contamination increases the sensitivity of larvae but not gametes to ocean acidification in the polychaete Pomatoceros lamarckii (Quatrefages)

Ocean acidification is not happening in isolation but against a background of chronic low-level pollution for most coastal marine environments. The reproductive and larval stages of marine invertebrates can be highly sensitive to the impacts of both environmental pollutants and ocean acidification, but very little is currently known regarding the potential impacts of combined contaminant and high CO₂ exposures on the health of marine organisms. Ocean acidification research to date has focused heavily on the responses of calcifying marine invertebrate larvae and algae, and as such the polychaetes as a group, despite their ecological importance, remain understudied. Here, we investigate the effects of elevated seawater CO₂ (pH range 8.1–7.4, plus an extreme pH of 7.2 in the sperm motility experiments), in combination with the environmental pollutant copper (0.002 μM), on the early life history stages of the intertidal polychaete Pomatoceros lamarckii from two populations. P. lamarckii sperm appear to be robust to elevated seawater CO₂. Whilst all three of the sperm motility end points measured showed a response to elevated CO₂, these responses were small and not linear. The percentage of motile sperm and sperm curvilinear velocity were significantly reduced in the lower pH treatments of 7.4 and 7.2, whereas sperm straight-line velocity (VSL) was mostly unaffected except for an increased VSL at pH 8.0. Fertilisation success was investigated using two populations from the South West (UK), one from Torquay and one from Plymouth Sound. Fertilisation success was slightly but significantly reduced at the 7.6 and 7.4 pH treatments for both populations (a 9.0 % reduction in fertilisation success from pH 8.1 to 7.4 for Torquay), but with a greater effect observed in the population from Plymouth Sound (a 13.33 % reduction in fertilisation success). No additional impact of 0.002 μM copper exposure on fertilisation success was found. Larval survival was found to be much more sensitive to elevated CO₂ than sperm function or fertilisation, and a significant interaction with copper exposure was observed. These results demonstrate the potential for polychaete larvae to be affected by predicted ocean acidification conditions and that chronic coastal pollutants, such as copper, have the potential to alter larval susceptibility to ocean acidification conditions.     

A novel method for investigating the collective behaviour of fish: introducing ‘Robofish’

Collective animal behaviour has attracted much attention recently, but cause-and-effect within interaction sequences has often been difficult to establish. To tackle this problem, we constructed a robotic fish (‘Robofish’) with which three-spined sticklebacks (Gasterosteus aculeatus L.) interact. Robofish is a computer-controlled replica stickleback that can be programmed to move around a tank. First, we demonstrated the functioning of the method: that the sticklebacks interacted with Robofish. We examined two types of interaction: recruitment and leadership. We found that Robofish could recruit a single fish from a refuge and could initiate a turn in singletons and in groups of ten, i.e. act as a leader. We also showed that the influence of Robofish diminished after the first 30 min that fish spent in a new environment. Second, using this method, we investigated the effects of metric and topological inter-individual distance on the influence that Robofish had on the orientation of fish in a shoal of ten. We found that inter-individual interactions during this turn were predominantly mediated by topological, rather than metric, distance. Finally, we discussed the potential of this novel method and the importance of our findings for the study of collective animal behaviour.