Research fellow position: ecologist / evolutionary biologist

  • Level A, research-only academic
  • $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

  1. A doctoral qualification in empirical ecology / evolutionary biology using microalgae as a model species.
  2. Demonstrated analytical and manuscript preparation skills; including an excellent track record of refereed research publications in high impact journals.
  3. Demonstrated experience in empirical research using cutting-edge quantitative approaches.
  4. Strong leadership, organisational and project management skills.
  5. Ability to work collaboratively with others

Enquiries to Professor Dustin Marshall on +61 3 9902 4449

For more information, or to apply, refer to the Monash University website

Research fellow position: marine larval biologist

  • Level A, research-only academic
  • $66,706 to $90,532 pa + 9.5% employer superannuation
  • Full-time, starting late 2018
  • Two-year, fixed-term
  • 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

  1. A doctoral qualification in larval biology
  2. Demonstrated analytical and manuscript preparation skills; including an excellent track record of refereed research publications in high impact journals
  3. Demonstrated experience in empirical research using cutting-edge quantitative approaches
  4. Ability to solve complex problems by using discretion, innovation and the exercise of diagnostic skills and/or expertise
  5. Well-developed planning and organisational skills, with the ability to prioritise multiple tasks and set and meet deadlines
  6. Excellent written communication and verbal communication skills with proven ability to produce clear, succinct reports and documents
  7. A demonstrated awareness of the principles of confidentiality, privacy and information handling
  8. A demonstrated capacity to work in a collegiate manner with other staff in the workplace
  9. 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

For more information, or to apply, refer to the Monash University website

Are there any advantages to being smaller in higher temperatures?

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.

This research was published in the journal Evolution.

Effects of two years of artificial size-selection (290 generations) on the cell volume of a phytoplankton species (Dunaliella tertiolecta).
Fitness increased with temperature (positive slope) in small-selected algae and decreased with temperature (negative slope) in large-selected algae. This finding confirms that reducing the size of a species automatically provides a fitness benefit at warmer temperatures.

Testing the drivers of the temperature-size covariance using artificial selection

Authors: Martino E Malerba, and Dustin J Marshall

Published in: Evolution

Abstract

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. Evolution PDF DOI

Physical and physiological impacts of ocean warming alter phenotypic selection on sperm morphology

Authors: Evatt Chirgwin, Dustin J Marshall, and Keyne Monro

Published in: Functional Ecology

Abstract

Global warming may threaten fertility, which is a key component of individual fitness and vital for population persistence. For males, fertility relies on the ability of sperm to collide and fuse with eggs; consequently, sperm morphology is predicted to be a prime target of selection owing to its effects on male function.

In aquatic environments, warming will expose gametes of external fertilizers to the physiological effects of higher temperature and the physical effects of lower viscosity. However, the consequences of either effect for fertility, and for selection acting on sperm traits to maintain fertility, are poorly understood.

Here, we test how independent changes in water temperature and viscosity alter male fertility and selection on sperm morphology in an externally fertilizing marine tubeworm. To create five fertilization environments, we manipulate temperature to reflect current-day conditions (16.5 °C), projected near-term warming (21 °C) and projected long-term warming (25 °C), then adjust two more environments at 21 °C and 25 °C to the viscosity of environments at 16.5 °C and 21 °C, respectively. We then use a split-ejaculate design to measure the fertility of focal males, and selection on their sperm, in each environment.

Projected changes in temperature and viscosity act independently to reduce male fertility, but act jointly to alter selection on sperm morphology. Specifically, environments resulting from projected warming alter selection on the sperm midpiece in ways that suggest shifts in the energetic challenges of functioning under stressful conditions. Selection also targets sperm head dimensions and tail length, irrespective of environment.

We provide the first evidence that projected changes in ocean temperature and viscosity will not only impact the fertility of marine external fertilizers, but expose their gametes to novel selection pressures that may drive them to adapt in response if gamete phenotypes are sufficiently heritable.

Chirgwin E, Marshall DJ, Monro K (2019) Physical and physiological impacts of ocean warming alter phenotypic selection on sperm morphology. Functional Ecology PDF DOI

Everybody loves good neighbours, but what makes a good neighbour?

Animals and plants compete for resources and traditionally we have held the view that competition drives interactions between species relying on the same resources. But Hayley Cameron and Dustin Marshall have shown it is not all about competition. Previously, we described Hayley’s PhD work where she demonstrated that large individuals did not outcompete their smaller neighbours, but instead facilitated their access to resources. But is that still true when the neighbours are a different species?

Hayley and Dustin have now looked at this in more detail. Collaborating with Tim Coulson from the University of Oxford, they varied the size and number of one species to see if it would affect the survival, growth and reproduction of a second species. They wanted to know how populations are affected when the size and numbers of neighbours vary.

The team used two common, filter-feeding, marine invertebrates shown to compete for food, space and oxygen. Watersipora is an ‘encrusting’ species that grows across surfaces, often growing over other organisms. Bugula has a ‘tree-like’ growth form and can efficiently harvest food and oxygen.

Both species are colonial invertebrates, made up of individual zooids. Watersipora and Bugula colonies were trimmed, creating a range of sizes and then a single Watersipora colony was placed on a small PVC plate and surrounded by different numbers of Bugula colonies. In total, they had 240 small plates hanging in Port Phillip Bay for 8 weeks.

Hayley, Tim and Dustin were interested in the consequences for populations when species of different sizes interact. So, they used a particular type of mathematical model called an Integral Projection Model. They entered data on survival, growth and reproduction of Watersipora for each size and number of Bugula neighbours. The model calculated the population growth rate for Watersipora with different neighbour combinations.

They found population growth of Watersipora was greatest when there were many, small Bugula neighbours. Large Bugula in the neighbourhood meant slow population growth of Watersipora; the species’ competed for resources and the more, large Bugulathere were, the greater the competition.

We know Bugula disrupts water flow and affects the delivery of food and oxygen to Watersipora. It seems, many small Buguladisrupt water flow and more food and oxygen reach the Watersipora. But while large Bugula also slow water flow, they consume more resources leaving a net negative effect on Watersipora.

So, both size and density played a part in determining whether a neighbour facilitated or competed with a target species. This means different population size structures will yield different outcomes in terms of species interactions. Hayley, Tim and Dustin emphasise that size should be included in studies of competition as any conclusions about how two species interact will depend on the size and density of the proposed competitor.

What is particularly exciting about these results is that the team may have uncovered an alternative pathway through which species using the same resources can co-exist. If body size mediates a switch between facilitation and competition then co-existence is more possible than previously simple experiments would imply.

This research was published in the journal Ecology Letters.

Size and density mediate transitions between competition and facilitation

Authors: Hayley Cameron, Tim Coulson, and Dustin J Marshall

Published in: Ecology Letters

Abstract

Species simultaneously compete with and facilitate one another. Size can mediate transitions along this competition–facilitation continuum, but the consequences for demography are unclear.

We orthogonally manipulated the size of a focal species, and the size and density of a heterospecific neighbour, in the field using a model marine system. We then parameterised a size‐structured population model with our experimental data.

We found that heterospecific size and density interactively altered the population dynamics of the focal species. Size determined whether heterospecifics facilitated (when small) or competed with (when large) the focal species, while density strengthened these interactions.

Such size‐mediated interactions also altered the pace of the focal’s life history. We provide the first demonstration that size and density mediate competition and facilitation from a population dynamical perspective. We suspect such effects are ubiquitous, but currently underappreciated.

We reiterate classic cautions against inferences about competitive hierarchies made in the absence of size‐specific data.

Cameron H, Coulson T, Marshall DJ (2019) Size and density mediate transitions between competition and facilitation. Ecology Letters PDF DOI

Powering ocean giants: the energetics of shark and ray megafauna

Authors: Christopher L Lawson, Lewis G Halsey, Graeme C Hays, Christine L Dudgeon, Nicholas L Payne, Michael B Bennett, Craig R White, and Anthony J Richardson

Published in: Trends in Ecology & Evolution

Highlights

Energetics studies have illuminated how animals partition energy among essential life processes and survive in extreme environments or with unusual lifestyles. There are few bioenergetics measurements for elasmobranch megafauna; the heaviest elasmobranch for which metabolic rate has been measured is only 47.7 kg, despite many weighing >1000 kg.

Bioenergetics models of elasmobranch megafauna would answer fundamental ecological questions surrounding this important and vulnerable group, and enable an understanding of how they may respond to changing environmental conditions, such as ocean warming and deoxygenation.

Larger chambers and swim-tunnels have allowed measurements of the metabolism of incrementally larger sharks and rays, but laboratory systems are unlikely to be suitable for the largest species.

Novel uses of biologging and collaboration with commercial aquaria may enable energetics of the largest sharks and rays to be measured.

Innovative use of technology and models derived from disparate disciplines, from physics to artificial intelligence, can improve our understanding of energy use in this group.

Shark and ray megafauna have crucial roles as top predators in many marine ecosystems, but are currently among the most threatened vertebrates and, based on historical extinctions, may be highly susceptible to future environmental perturbations. However, our understanding of their energetics lags behind that of other taxa. Such knowledge is required to answer important ecological questions and predict their responses to ocean warming, which may be limited by expanding ocean deoxygenation and declining prey availability. To develop bioenergetics models for shark and ray megafauna, incremental improvements in respirometry systems are useful but unlikely to accommodate the largest species. Advances in biologging tools and modelling could help answer the most pressing ecological questions about these iconic species.

Lawson CL, Halsey LG, Hays GC, Dudgeon CL, Payne NL, Bennett MB, White CR, Richardson AJ (2019) Powering ocean giants: the energetics of shark and ray megafauna. Trends in Ecology & Evolution PDF DOI