Abstract
Sea urchin larvae are particularly vulnerable to enhanced ultraviolet radiation (UVR) exposure as they are small, highly transparent, undergo rapid cell division and occupy the upper regions of the water column. Anthropogenically induced ozone depletion is responsible for elevated UVR levels globally, particularly over Polar regions. UVR has the potential to induce a range of deleterious effects on planktonic larvae including increased metabolic costs, DNA damage and mortality. Respiration efficiency rates are a good measure of an organism’s metabolic costs of exposure to UVR. Larvae may mitigate UVR induced damage via a number of strategies including the upregulation of metabolic repair pathways and the dietary uptake of sunscreen compounds such as mycosporine-like amino acids (MAAs).
This study investigates the link between UVR exposure, larval metabolism and mortality, and MAA protection in sea urchin larvae from a range of latitudes, including a tropical (Tripneustes gratilla) two temperate (Evechinus chloroticus, Pseudechinus huttoni) and an Antarctic (Sterechinus neumayeri) species. Larvae were exposed to standardised levels of UVR in the laboratory as well as varying levels of UVR in the field using in situ techniques that were standardised to allow direct comparisons among species.
In the laboratory, larvae were exposed to a UVR dose equivalent to that experienced at equatorial latitudes to simulate extreme UVR enhancement. UVR was partitioned into different wavelength components using long-pass light filters (UVB: 280 nm, 305 nm, UVA: 320 nm, 375 nm, 400 nm). Larvae were also fed a range of alga feeds to induce varying MAA levels and exposed to a strict UVR light regime. In the field, larvae were poured into 125 ml UVR transparent bags (10 larvae per ml), situated under two light filters (UVR transparent filter, UVR opaque filter) and moored at two depths (0.5 m, 3 m) for up to eight hours. Larval respiration and mortality were quantified following UVR exposure for each experiment.
Larval respiration was found to change when exposed to UVR, but was species and wavelength specific. Metabolic depression and elevated mortality were observed in temperate and particularly in Antarctic S. nuemayeri species, exhibiting as much as a 77% reduction in respiration rate (relative to control samples) and a 22% mortality rate following laboratory UVR exposure. These results may indicate a level of UVR induced damage beyond the organisms metabolic repair capacity. In contrast, tropical T. gratilla larvae displayed metabolic enhancement (up to 135% increase in respiration rate in the field), potentially interpreted as upregulation of repair mechanisms, and negligible mortality following both laboratory and field UVR exposures. Changes in larval respiration and mortality rates were primarily influenced by UVB radiation, as confirmed using wavelength specific exposures.
MAA accumulation varied among sea urchin species and with feed treatment. Total MAA concentrations (nmol mg-1 dry wt) following a 21 day feeding trial ranged from 0 to 7.6 in E. chloroticus, 0 to 6.4 in P. huttoni and 0 to 2.7 in S. neumayeri larvae. Three specific MAAs were identified in both algae feed and larvae and are listed in order of highest concentration: mycosporine-glycine, shinorine and porphyra-334.
Correlations between MAA concentration and larval respiration / mortality can be drawn for certain species. Elevated MAA concentrations were associated with a reduction in the level of metabolic depression and increase in survival rates in both temperate species following standardised UVR exposure. No correlation was observed for Antarctic S. neumayeri larvae which suffered greatest metabolic depression and mortality during UVR exposure, suggesting insufficient accumulation of MAAs to provide protection.
Sea urchins are ecologically important organisms, and often greatly affect marine communities. The ability to withstand elevated UVR levels brought about by ozone depletion is particularly important during the vulnerable larval stages. The results of this study broadly imply that a correlation exists between larval vulnerability to UVB radiation and latitude, with tropical T. gratilla larvae being most resilient and Antarctic S. neumayeri larvae most sensitive. The combination of high ozone depletion and limited MAA concentrations in Antarctic waters could see S. neumayeri larvae more severely impacted by enhanced UVR over the coming century.