The impacts of acute environmental changes on the physiology of critically endangered school sharks (Galeorhinus galeus) in southeast Tasmania

Due to strengthening wind systems that have driven the East Australian Current farther south, Tasmania is a climate change hotspot and is one of the world’s fastest warming marine regions. In turn, this is causing more extreme weather events on acute scales, such as increased storm events, precipitation, cold snaps, heat waves, and dry spells. Estuarine habitats are highly susceptible to temperature and salinity variability due to their shallow depths and increasing challenges from the changing climate. The Pittwater Estuary, located in Southeast Tasmania, is a key pupping ground and nursery for a variety of chondrichthyan species including the critically endangered school shark (Galeorhinus galeus). Mature female G. galeus move into the Pittwater Estuary to give birth in the late austral spring/early summer where the neonates (< 1 year old) grow and develop during summer and autumn before leaving at the beginning of winter. Juveniles (1-2 years old) come back into southeast Tasmanian inshore waters adjacent to the estuary to spend summer/autumn in subsequent years, but not back into the estuary itself, suggesting ontogenetic differences in the physiological capabilities of G. galeus. Given that the estuary experiences regular acute environmental variations and the frequency and severity of extreme weather events are predicted to increase under climate change scenarios, this research aims to understand and compare how these acute changes in temperature and salinity will impact the physiology, metabolism, and behaviour of neonatal and juvenile G. galeus.
The first experiment involved isolating the abiotic factors to determine how temperature alone affects G. galeus physiology, as studies had previously been conducted investigating only the effects of salinity. We sought to determine the critical thermal maximum (CT_max) and the effects of acute (48 hour) ecologically relevant thermal stress conditions to evaluate and compare the effects for neonate and juvenile G. galeus. The simulated conditions included acute (48 hour) cool (14°C) and hot (22°C) periods in summer/autumn, followed by a 24 hour recovery period, to determine how extreme environmental events affect the physiology of neonatal and juvenile G. galeus. Blood samples were collected every 24 hours and haematocrit, haemoglobin, lactate, glucose, and plasma osmolality were measured. Oxygen consumption was also measured prior to experimentation, after 48 hours of acute exposure, and post-exposure after recovery.
Both life stages had the same CT_max of approximately 31.4°C, but neonatal G. galeus had far less of a stress response compared to the juveniles, as demonstrated by lower glucose and lactate levels. This is indicative of neonates in the estuary frequently encountering rapidly fluctuating conditions due to the nature of the habitat, making them more capable of handling rapid environmental changes compared to juveniles. During the acute thermal stress experiments, several metrics including plasma osmolality levels and key physiological stress indicators, glucose and lactate, were significantly impacted by treatment conditions. The elevated levels of lactate during the 22°C trial indicated that both neonates and juveniles activated anaerobic metabolism under warm conditions, but this was not observed in the cooler treatment. Both life stages were able to sufficiently recover from all treatments, with the exception of juveniles not returning to pre-exposure mean corpuscular haemoglobin concentration (MCHC) levels after the 22°C treatment and neonates having lower blood glucose and lactate levels compared to pre-treatment levels after the 14°C treatment. Neonates and juveniles had consistent haemoglobin levels, allowing for more oxygen carrying and distribution capacity, during all treatments but haematocrit increased in neonates under cooler conditions which yielded lower MCHC. Conversely, neonates were able to maintain plasma osmolality levels across all treatments, but juveniles had much lower levels at 14°C compared to the other trials.
Scaled, routine metabolic rates during the exposure period show that neonates have much more pronounced changes in oxygen consumption rates (ṀO₂) with an acute increase in temperature when compared to juveniles, indicating that neonates may be more energetically challenged by acute thermal changes than juveniles. This is further exemplified by the calculated temperature scaling quotient (Q₁₀) values, which describes the correlation between temperature and metabolic rate as higher values mean more oxygen consumption changes per degree change, and neonates had a higher Q₁₀ value of 2.01 compared to 1.42 for juveniles, although these values for both life stages indicate some temperature insensitivity.
Given that temperature and salinity changes in an estuary are intrinsically linked, the next experiment focused on how G. galeus would be affected by the combination of stressors by simulating both a hot/dry (22°C and 40‰) and a cold/wet (14°C and 25‰) summer/autumn. Blood samples and ṀO₂ were measured as per the acute temperature change experiment, with the addition of the analysis of shark swimming behaviour which was measured prior to experimentation, after 48 hours of acute exposure, and post-exposure after recovery.
ṀO₂ was significantly impacted by age as well as the interaction of the treatments over time. However, all animals returned to pre-exposure ṀO₂ rates after recovery, with the exception of juveniles after the cold, hyposaline treatment with ṀO2 remaining lower than at the start of the experiment. Neonates had higher ṀO2 rates compared to juveniles, but both life stages had decreased oxygen consumption in cold, hyposaline water. Behaviourally, both life stages were impacted in the same way, where the amount of time spent resting increased with temperature and salinity, which may have significant repercussions as G. galeus is a ram ventilating species, i.e., it needs to swim to respire efficiently. By looking at the combination of the sharks’ movement patterns with the ṀO2 findings, both warmer, hypersaline and cooler, hyposaline conditions could have serious metabolic consequences for both neonate and juvenile G. galeus. During the cooler, hyposaline treatment, neonates and juveniles did not spend significant time resting compared to warmer temperatures and instead depressed their metabolism, as indicated by a significant decrease in ṀO2. However, during the warm, hypersaline treatment, G. galeus spent significantly more time resting and it is possible that ṀO2 did not significantly increase as this change in behaviour allowed them to maintain ṀO2 levels. Therefore, both conditions will inhibit the sharks’ abilities to expend energy on necessary activities such as escaping predators or adequate growth for higher chances of survival in the open ocean.
Both simulated acute changes to hot/dry and cold/wet weather conditions elicited significant changes in plasma osmolality and physiological responses for both life stages, with neonates generally more tolerant to the changes than juveniles, including having lower levels of physiological stress indicators (glucose and lactate) and higher MCHC, allowing for more oxygen carrying and distribution capacity. Furthermore, all groups were able to recover from these exposures for all metrics except that, compared to pre-exposure levels, plasma osmolality was higher in both neonates and juveniles after the warm, hypersaline treatment.
In the final experiment, we wanted to expand on the combined acute temperature and salinity change experiment to determine what physiological mechanisms may be associated with the observed changes. Neonate and juvenile G. galeus were exposed for 48 hours to the same simulated hot/dry and cold/wet conditions as described above, and samples were subsequently collected. Gill surface area was examined using scanning electron microscopy, to determine whether gill remodelling, which changes the amount of surface area available for oxygen or ion diffusion, had occurred. Using a stable isotope tracer, the fractional rates of protein synthesis in gill, heart, liver, and muscle were measured to determine how much energy was being put towards protein synthesis in each tissue type, as this process can account for 20-40% of resting oxygen consumption rates in fish, which will affect overall metabolism. This is the first known study to investigate rates of protein synthesis in chondrichthyans.
While there were no changes observed in the cold, hyposaline treatment, neonatal and juvenile G. galeus increased the fractional rates of muscle protein synthesis as temperature and salinity increased, likely because of associated higher metabolic rates. This indicates that neonates may reside in the estuary to prioritize growth so that they are large enough to leave the safety of the nursery in winter and will have the energetic reserves for dispersal. Juveniles can also exploit these temperatures to increase muscle mass to maximise survival and fitness. We also found that these increases in metabolic demands were reflected in the gill surface area as the percent lamellae is higher in warm, hypersaline waters, compared to the cold, hyposaline treatment, to support higher rates of oxygen diffusion. While higher temperatures and hypersalinity may have short-term benefits for shark development, the higher energy demands associated with these environmental conditions could have longer term impacts on neonatal growth rates, especially if conditions reach the pejus temperature; a physiological tipping point where growth will decline despite rising (or cooling) temperatures.
In the first experiment, where only temperature was chan ...