Abstract
As the earth’s oceans warm due to a changing climate, extreme temperature events such as
marine heatwaves (MHW) are set to increase in frequency, duration and intensity. MHWs pose
serious threats to the world’s marine ecosystems, with mass mortalities and biodiversity
changes already being seen throughout the ocean. Benthic invertebrates are particularly
challenged by these extreme events as they are generally stenothermal and relatively immobile.
MHWs can cause metabolic shifts within organisms, leading to energetic trade-offs that can
result in reduced growth, reproductive failure and mortality.
This thesis focusses on two species of asteroid: the common rocky shore sea star Patiriella
regularis, native to NZ, and the Antarctic keystone sea star Odontaster validus. These species
play key roles within their respective ecosystems, and a greater knowledge of how the effects
of how MHWs may influence them and their populations is imperative to understanding what
marine ecosystems may look like in the future. Three key objectives in this study are presented:
(1) investigate the effects of MHW exposure on the physiological function and fitness of P.
regularis, (2) investigate the effects of MHW exposure on the physiological function and
fitness of O. validus, and (3) assess whether the acclimation of O. validus to far future
temperatures results in increased resilience to MHW events. To examine this, MHWs were
simulated using the MHW parameters occurring in MHWs from Portobello, Otago (for P.
regularis) and Rothera Research station (for O. validus). Three key physiological rates were
measured to investigate the effects of MHWs on these two asteroids: oxygen consumption (mg
O2 h-1 g-1), ammonia excretion (mg NH3 h-1 g-1) and activity rate (sec). To assess mass-
scaling relationships, wet weight (g) and radial length (cm) were also recorded. Oxygen
consumption was measured by using a fibre optic measurement cable on an oxygen sensor dot
placed within experimental containers every third day. Water samples were then processed in
an auto-analyser to obtain the ammonia excretion rates. For activity rates, sea stars were flipped
3 times, and their fastest time taken to right themselves was recorded (if they did not right
themselves it was marked as a failure).
P. regularis experienced at its peak a category 3 simulated MHW that had a maximum intensity
of 5.8℃ and lasted 34 days. The simulated MHW was found to significantly increase the
respiration rates and ammonia excretion of P. regularis and reduce the species mobility. Largeii
individuals of P. regularis coped better than smaller individuals, potentially changing
intraspecific dynamics. Overall frequent and prolong MHWs in the future could divert energy
away from key processes such as growth and reproduction in the future, thus reducing the
species function, threatening the long-term viability of the population. O. validus acclimated
to 0℃ tanks experienced at its peak a category 4 simulated MHW that had a maximum intensity
of 4.44℃ and lasted 2 days. Individuals acclimated to 4℃ also experienced at its peak a
category 4 simulated MHW, that had a maximum intensity of 4.98℃ and lasted 25 days. For
both treatments, respiration rates, ammonia excretion rates and activity rates increased
significantly, indicating reduced fitness and function. Between the 2 treatments there were
marked differences, with 4℃ O. validus showing evidence of hardening through reduced
response variability in the face of MHW conditions. Although the large difference in
temperatures between the two MHW treatments ammonia excretion rates were not significantly
different at peak MHW conditions. 4℃ O. validus also had much faster righting times than 0℃
individuals. This provides evidence that O. validus acclimated to higher temperatures may have
increased resilience to future MHWs. However, O. validus acclimated to 4℃ showed evidence
of reduced recovery in respiration rate and an increase in righting time once temperatures
started to decline. This may be due to the significantly steeper decline in wet weight than 0℃
individuals during the experiment, indicating a high usage of stored energy reserves. This
suggest that although 4℃ O. validus may initially cope well in MHWs, the long term viability
of the species when experiencing repeat and intense MHWs could be severely impacted.
In conclusion, both species of sea stars displayed reduced function while enduring an MHW,
potentially disrupting ecosystem functioning and dynamics. The acclimation of O. validus to
4°C did result in some increased resilience to MHWs. However, the cumulative adverse effects
from elevated temperatures may hinder the long-term viability of populations due to energy
being diverted away from growth and reproduction. Overall, these changes in physiological
and behavioural responses reduce the prospect of these organisms thriving in the face of MHWs
increasing in duration, intensity, and frequency over the coming century. Although immediate
survival did not seem to be in danger, at future temperatures, enduring frequent, long, and
intense MHWs will pose significant risks to the health of these two species. Further research
into repeated MHWs, ocean acidification, and reproduction is needed to provide a more robust
understanding of the future success of P. regularis and O. validus.