$68,040 – $92,343 pa (plus 9.5% employer superannuation)
Full-time, starting early 2020
One year, fixed term with the possibility of extension to a second year
Monash University Clayton campus
Professor Dustin Marshall is seeking an experienced ecologist / evolutionary biologist, who specialises in microalgal biology with a strong empirical background, to explore the ways in which size affects the structure and function of marine phytoplankton. This position will be with the Centre for Geometric Biology within the School of Biological Sciences at Monash University.
As the successful candidate, you will be expected to maintain the Centre’s evolved lines of the microalgae Dunaliella and use these evolved microalgae to undertake experiments that test ecological and evolutionary theories. You will also have a strong quantitative background and have a demonstrated track record in producing high-quality publications.
Key selection criteria
A doctoral qualification in empirical ecology / evolutionary biology using microalgae as a model species.
Demonstrated analytical and manuscript preparation skills; including an excellent track record of refereed research publications in high impact journals.
Demonstrated experience in empirical research using cutting-edge quantitative approaches.
Strong leadership, organisational and project management skills.
$66,706 to $90,532 pa + 9.5% employer superannuation
Full-time, starting late 2018
Monash University Clayton campus
Professor Dustin Marshall is seeking a marine larval biologist, with strong quantitative skills, to explore the ways in which temperature affects the energetics of development in marine invertebrates. This position will be with the Centre for Geometric Biology within the School of Biological Sciences at Monash University.
As the successful candidate, you will be expected to undertake experiments to determine the relative performance of different larval types across every stage of the life history, but more importantly demonstrate a strong conceptual understanding of relevant life history theory and have a demonstrated track record in producing high quality publications.
Key selection criteria
A doctoral qualification in larval biology
Demonstrated analytical and manuscript preparation skills; including an excellent track record of refereed research publications in high impact journals
Demonstrated experience in empirical research using cutting-edge quantitative approaches
Ability to solve complex problems by using discretion, innovation and the exercise of diagnostic skills and/or expertise
Well-developed planning and organisational skills, with the ability to prioritise multiple tasks and set and meet deadlines
Excellent written communication and verbal communication skills with proven ability to produce clear, succinct reports and documents
A demonstrated awareness of the principles of confidentiality, privacy and information handling
A demonstrated capacity to work in a collegiate manner with other staff in the workplace
Demonstrated computer literacy and proficiency in the production of high level work using software such as Microsoft Office applications and specified University software programs, with the capability and willingness to learn new packages as appropriate.
Enquiries to Professor Dustin Marshall on +61 3 9902 4449
Apart from mammals and birds, most animals develop as eggs exposed to the vagaries of the outside world. This development is energetically “costly”. Going from a tiny egg to a fully functioning organism can deplete up to 60% of the energy reserves provided by a parent.
In cold-blooded animals such as marine invertebrates (including sea stars and corals), fish and reptiles, and even insects, embryonic development is very sensitive to changes in the temperature of the environment.
Thus, in a warming world, many cold-blooded species face a new challenge: developing successfully despite rising temperatures.
For our research, published today in Nature Ecology and Evolution, we mined existing literature for data on how temperature impacts the metabolic and development rates of 71 different species, ranging from tropical crocodiles to Antarctic krill.
We found over time, species tend to fine-tune their physiology so that the temperature of the place they inhabit is the temperature needed to minimise the “costs” of their embryonic development.
Temperature increases associated with global warming could substantially impact many of these species.
The perfect weather to grow an embryo
The energy costs of embryonic development are determined by two key rates. The “metabolic” rate refers to the rate at which energy is used by the embryo, and the “development” rate determines how long it takes the embryo to fully develop, and become an independent organism.
Both of these rates are heavily impacted by environmental temperature. Any change in temperature affecting them is therefore costly to an embryo’s development.
Generally, a 10°C increase in temperature will cause an embryo’s development and metabolic rate to more than triple.
For any species, there is one temperature that achieves the perfect energetic balance between relatively rapid development and low metabolism. This optimal temperature, also called the “Goldilocks” temperature, is neither too hot, nor too cold.
When the temperature is too cold for a certain species, development takes a long time. When it’s too hot, development time decreases while the metabolic rate continues to rise. An imbalance on either side can negatively impact a natural population’s resilience and ability to replenish.
As an embryo’s developmental costs increase past the optimum, mothers must invest more resources into each offspring to offset these costs.
When offspring become more costly to make, mothers make fewer, larger offspring. These offspring start life with fewer energy reserves, reducing their chances of successfully reproducing as adults themselves.
Thus, when it comes to embryonic development, higher-than ideal temperatures pack a nasty punch for natural populations.
For each species in our study, we found a narrow band of temperatures that minimised developmental cost. Temperatures that were too high or too low caused massive blow-outs in the energy budget of developing embryos.
In particular, aquatic species (fish and invertebrates) in cool temperate waters seem likely to experience lower costs in the near future. In contrast, certain tropical aquatic species (including coral reef organisms) are already experiencing temperatures that exceed their optimum. This is likely to get worse.
It’s important to note that for all species, increasing environmental temperature will eventually come with costs.
Even if a slight temperature increase reduces costs for one species, too much of an increase will still have a negative impact. This is true for all the organisms we studied.
A key question now is: how quickly can species evolve to adapt to our warming climate?
Authors: Dustin J Marshall, Amanda K Pettersen, Michael Bode, and Craig R White
Published in:Nature Ecology & Evolution
Metazoans must develop from zygotes to feeding organisms. In doing so, developing offspring consume up to 60% of the energy provided by their parent.
The cost of development depends on two rates: metabolic rate, which determines the rate that energy is used; and developmental rate, which determines the length of the developmental period. Both development and metabolism are highly temperature-dependent such that developmental costs should be sensitive to the local thermal environment.
Here, we develop, parameterize and test developmental cost theory, a physiologically explicit theory that reveals that ectotherms have narrow thermal windows in which developmental costs are minimized (Topt).
Our developmental cost theory-derived estimates of Topt predict the natural thermal environment of 71 species across seven phyla remarkably well (R2⁓0.83).
Developmental cost theory predicts that costs of development are much more sensitive to small changes in temperature than classic measures such as survival. Warming-driven changes to developmental costs are predicted to strongly affect population replenishment and developmental cost theory provides a mechanistic foundation for determining which species are most at risk. Developmental cost theory predicts that tropical aquatic species and most non-nesting terrestrial species are likely to incur the greatest increase in developmental costs from future warming.
Marshall DJ, Pettersen AK, Bode M, White CR (2020) Developmental cost theory predicts thermal environment and vulnerability to global warming. Nature Ecology & EvolutionDOIEPDF
Authors: Giulia Ghedini, Michel Loreau, and Dustin J Marshall
Robert MacArthur’s niche theory makes explicit predictions on how community function should change over time in a competitive community. A key prediction is that succession progressively minimizes the energy wasted by a community, but this minimization is a trade‐off between energy losses from unutilised resources and costs of maintenance. By predicting how competition determines community efficiency over time MacArthur’s theory may inform on the impacts of disturbance on community function and invasion risk.
We provide a rare test of this theory using phytoplankton communities, and find that older communities wasted less energy than younger ones but that the reduction in energy wastage was not monotonic over time. While community structure followed consistent and clear trajectories, community function was more idiosyncratic among adjoining successional stages and driven by total community biomass rather than species composition.
Our results suggest that subtle shifts in successional sequence can alter community efficiency and these effects determine community function independently of individual species membership.
We conclude that, at least in phytoplankton communities, general trends in community function are predictable over time accordingly to MacArthur’s theory. Tests of MacArthur’s minimization principle across very different systems should be a priority given the potential of this theory to inform on the functional properties of communities.
Ghedini G, Loreau M, Marshall DJ (2020) Community efficiency during succession: a test of MacArthur’s minimization principle in phytoplankton communities. Ecology DOI
Biologists have been familiar with a pattern of smaller body sizes with increasing temperatures for a long time, in fact, so familiar that Bergmann dubbed a “Temperature-Size Rule” in 1847.
Like many things to do with size, it is difficult to separate the effects of temperature on size from other traits that co-vary with size; metabolism for example. It may be that higher temperatures cause the evolution of faster metabolic rates and metabolic rate is genetically correlated with size. So that it is, in fact, metabolic rate that is the target of selection, not size.
Martino Malerba and Dustin Marshall were again able to take advantage of the evolved large and small algal cells to see if they could unambiguously assign any effects of temperature on size, to size alone. They wanted to find out if (and how) temperature affected fitness for different sized organisms.
To do this they used algal cells that had experienced 290 generations of artificial selection and where large selected cells were 13 times bigger than small selected cells. They then exposed these different lines (including the control lines) to three temperatures 18 °C, 22 °C and 26 °C and measured cell size, population density and cell production rates after three and six days.
They found that the smaller cells did better at higher temperatures; that is, the fitness proxies of cell production rate and population densities were both greater for small cells at higher temperatures. This means that Martino and Dustin have shown that size on its own can affect performance across different temperatures.
They then wanted to know why are cells smaller at higher temperatures; what is the advantage? It has long been thought that smaller cells do better in warmer temperatures because they have a greater surface-area to volume ratio. This would make them better able to take up resources such as nutrients, CO2 and light at the same time as increasing temperatures increase a cell’s demand for resources through increased enzyme activity and protein synthesis.
If this was the case, Martino and Dustin expected the large and small cells to show differences in performance at higher temperatures when resources were abundant (days 0 to 3) compared to when resources were depleted (days 3 to 6). But they found no difference in the fitness of large and small cells that related to resources suggesting that advantages of smaller cells at higher temperatures was not related to a greater surface-area to volume ratio.
Instead they measured the concentrations of reactive oxygen species in their selected lines of large and small cells. Reactive oxygen species are known to increase oxidative stress, damage DNA and so reduce the performance of a cell and also accumulate at higher temperatures. Martino found that the larger cells had almost five times more reactive oxygen species than smaller cells. And the larger cells had relatively smaller nuclei, meaning that there was twice the reactive oxygen species loading around the nuclei in large selected cells.
Martino and Dustin think that it is likely that small cells do better at higher temperatures, not because they are able to access more resources per unit volume, but because they are less prone to toxicity from reactive oxygen species.
Body size often declines with increasing temperature. Although there is ample evidence for this effect to be adaptive, it remains unclear whether size shrinking at warmer temperatures is driven by specific properties of being smaller (e.g., surface to volume ratio) or by traits that are correlated with size (e.g., metabolism, growth).
We used 290 generations (22 months) of artificial selection on a unicellular phytoplankton species to evolve a 13‐fold difference in volume between small‐selected and large‐selected cells and tested their performance at 22 °C (usual temperature), 18 °C (−4), and 26 °C (+4).
Warmer temperatures increased fitness in small‐selected individuals and reduced fitness in large‐selected ones, indicating changes in size alone are sufficient to mediate temperature‐dependent performance.
Our results are incompatible with the often‐cited geometric argument of warmer temperature intensifying resource limitation. Instead, we find evidence that is consistent with larger cells being more vulnerable to reactive oxygen species. By engineering cells of different sizes, our results suggest that smaller‐celled species are pre‐adapted for higher temperatures.
We discuss the potential repercussions for global carbon cycles and the biological pump under climate warming.
Malerba ME, Marshall DJ (2019) Testing the drivers of the temperature-size covariance using artificial selection. EvolutionPDFDOI
Global warming will increase ocean temperatures at the same time as it reduces seawater viscosity and Evatt Chirgwin wanted to know how this combination of physiological and physical change would affect male fertility in a small tubeworm. He found that both these factors independently reduced male fertility, and together altered selection pressures on sperm morphology.
Most marine species release gametes into the water column and successful fertilisation depends on a sperm locating and fusing with an egg. This high-risk strategy is in stark contrast with many terrestrial species where sperm and eggs interact in the controlled environment of a female reproductive tract, making marine species more vulnerable to global warming.
Projected ocean temperature increases are expected to reduce male fertility because exposure to temperatures outside the usual range can disrupt physiological processes and cell function. But the viscosity or ‘thickness’ of the seawater will also change with increasing temperatures, and Evatt was interested in understanding how the fact that sperm are able to move more easily through the water would affect male fertility.
Because these two things tend to change together, up until now no one has considered how decreases in viscosity at higher temperatures might alter fertility as well as selection pressure on sperm structure. Sperm with larger heads have increased ‘drag’ while a long tail can increase swimming speeds – these might not matter so much when seawater is easier to pass through.
So how do temperature and viscosity affect male fertility and the selection forces acting on the size and shape of sperm? Evatt and his supervisors (Keyne Monro and Dustin Marshall) measured fertilisation success at three temperatures and used a hydrophilic polymer that allows warmer water to be adjusted to the same viscosity as cooler water (but not the other way around).
Evatt measured head size, midpiece size and tail length in the sperm of 157 males that had access to eggs from a variety of females in five different fertilisation environments.
The team found that the isolated effects of temperature and viscosity each caused fertility to decline by around 5% from current to moderate warming and by another 5% from moderate to extreme warming. But temperature and viscosity acted together to alter selection on sperm morphology. The ‘midpiece’ that houses the mitochondria, was a target for selection at the projected, warmer environments. A shorter midpiece was favoured in moderate warming environments, while a wider midpiece was favoured at the more extreme, longer-term projections of warming.
Evatt and his supervisors think that since the midpiece contain the mitochondria that provide energy, it is probable that changes in temperature and viscosity will change the energy requirements of sperm during the location and fertilisation of eggs.
For the first time, the team show how projected changes in water temperature and viscosity may impact the fertility of marine populations and expose sperm to novel evolutionary pressures that may drive them to adapt in response.
In 2018, we published a number of papers that addressed a core area of research for the Centre. These papers considered the relationship between size and reproductive output and what that meant for our understanding of patterns of growth.
In order to do this, researchers compiled a database that accessed published work from the past 100 years that included data on fish size and reproductive output. When they examined the data from 342 species of fish they found that there was a hyper-allometric relationship between size and reproductive output in 95% of the species they looked at.
This information has massive repercussions for the way in which we manage our fisheries but also, if it is a more general rule, it may change the way we understand growth.
So, is it a more general rule: do larger individuals produce disproportionately more gametes / offspring than smaller individuals in taxa other than fish?
Michaela Parascandalo joined the team towards the end of 2018 to focus on gathering data to address this question. Initially she searched for data on invertebrates but has since expanded her search to include a total of 10 phyla. Michaela uses Google Scholar as the search engine and inputs a range of search terms that relate to body size and reproductive output. For each query entered, she looks at every paper displayed on the first 6 pages of results. She is looking for graphs of length/mass and reproductive output.
Michaela will then open the graph in Data Thief, a program that allows you to extract datapoints from a picture of a graph. In some cases, she has to do additional searches to get a conversion of length to mass for that species and latitudes and longitudes for the study.
All this information is entered into the database; the master copy has only one example of each species while the ‘duplicates’ file stores data from overlapping species’ that might be from different times or locations.
So far there are 75,000 data points in the master file, 30,000 data points in the duplicates file, 10 phyla, 978 species, a data span of 92 years and Michaela has repeatedly been identified as a ‘bot’.
There is more to do, but once Michaela has finished compiling the data, the team will be in a good position to assess the generality of hyperallometry in species other than fish. They will also be able to use the ‘duplicate’ database look at how relationships between size and reproductive output vary through space and time.