Effects of UV-B radiation on phototaxis and settlement of barnacle larvae (balanus amphitrite) in relation to cytological changes in larval eyes

Abstract

Environmental impact of UV-B enhancement resulting from ozone depletion has become a global concern. Barnacles play an important role in the trophodynamics of coastal systems worldwide. UV-B may damage eyes of barnacle larvae, thereby affecting their phototactic and settlement behaviors. This in turn, may affect individual's fitness and threaten sustainability of natural populations. Using the barnacle Balanus amphitrite as a model, this thesis aims to study the effects of UV-B on: (1) phototaxis of barnacle stage II nauplii, (2) settlement success of cyprids, (3) cytological changes in larval eyes, and relate these to the observed behavioral impairments. Significant reduction in positive phototaxis was found when stage II nauplii were exposed to low (72 ±3 µWcmˉ²) and high (199 ± 10 µWcmˉ² levels of UV-B for 3 and 1 hours, respectively. Significant decreases in settlement success of cyprids occurred after exposure to the low and high UV-B irradiances for 12 and 4 hours, respectively. Impairments of these two behaviors depend on dose rather than irradiance of UV-6. LOEDs for phototactic and settlement impairments were 7.7 kJmˉ²and 28.6 kJmˉ², respectively, whereas the respective EDS0s were 2.6 KJmˉ² and 12.33 KJmˉ². Thus, phototaxis of stage II nauplii was some 4 to 5 times more sensitive to UV-B than settlement of cyprid. Concomitant disruption of rhabdomere organization and integrity was found in eyes of nauplii, which may impair photoreception and reduce phototransduction efficacy. Mitochondria swelling with membranous disruption were also clearly evident, suggested that ATP supply for phototransduction might be affected. Photoreceptors of cyprids, however, were less sensitive to UV-B. The cypris compound eyes were not inflicted by UV-B, and only mild rhabdomere disorganization was observed in the naupliar eyes of cyprids at a dose higher than that required inhibiting cypris settlement. Our results suggest that rhabdomeric alteration alone could not fully explain the reduction of settlement success caused by UV-B.