Sammendrag
The benefits of bioluminescence for bacteria in symbiotic association with a host have been well-characterized. However, the ecological importance of this natural phenomenon for free-living marine bacteria remains unclear. Previous explanations, having proposed that bioluminescence enhances some physiological and biochemical functions, are currently questioned. One promising explanation is that non-symbiotic bacteria, after being ingested, colonize the digestive tract of transparent zooplankton which is a natural prey for many visual predators, such as fish. Thus, bacteria profit from the nutritious gut of different consumers.
The hypothesis of this thesis was that lumpfish larva as a pelagic visual predator is able to detect and selectively consumes bioluminescent zooplankton. Artemia franciscana was chosen as a model organism due to the favorable features of the species, such as the body transparency, for the bioluminescence detection and its regular use as living prey for farmed fish. The ability of zooplankton to become luminescent after ingestion of bioluminescent Vibrio harveyi in the density of 10^7 cells/mL was firstly verified. The pattern of bioluminescence from bacteria and luminescent prey was characterized at 28°C and the stability of bioluminescence after the temperature downshift from 28 to 15°C was observed. Bioluminescence was unstable due to the extended incubation (6 hours) of bacteria and luminescent zooplankton at 15°C.
Differences in the swimming speed of zooplankton after bacterial infection were observed at 28°C but were not significant. The effects of bacterial infection on the swimming speed of A. franciscana were of higher importance at 15°C, as differences in prey’s behavior due to the cold seawater may alter predators-prey interactions. Our experiments revealed that V. harveyi did not affect the swimming speed of the prey.
Overall, the main hypothesis of this study was not verified. The prey selection by lumpfish was measured as α-index. Lumpfish’s selectivity was neutral as the fish did not reveal any significant preference (0.507 ± 0.05, p =0.847) between the luminescent and the non-luminescent zooplankton. However, the bioluminescence background due to the presence of luminescent bacteria in the water of the tray and the poor performance of lumpfish larvae, render our outcome questionable.