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.

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

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.

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

Mirth recognised with Crozier medal

The Centre for Geometric Biology’s Christen Mirth has been recognised for her research on how nutrition shapes development, having been awarded the Ross Crozier medal by the Genetics Society of Australasia.

When Christen first began working on this problem in 2003, using the fruit fly Drosophila melangoster as a model, researchers knew that nutrition had a role in the secretion of insulin-like peptides. These peptides, in turn, influenced the rates of body growth.  What they didn’t know, was what made insects stop growing.

During her postdoc, Christen and her colleagues discovered there was another hormone involved in regulating when growth should stop: ecdysone, the steroid that controls moulting in insects. It turned out that nutritional changes can control the timing of a critical pulse of ecdysone, which commits an insect to metamorphosis. In other words, ecdysone was the key they had been looking for, determining the developmental rate and the final size of the insect.

What’s more, the team found certain organs, such as the wings and the ovaries, require this ecdysone pulse for cells to acquire organ-specific identities and to grow. Organs also change the way they respond to nutritional cues with time by changing the combination of hormones required for growth, providing a further buffer against nutritional environments determining organ size. Such differences in the way organs respond to nutrition (and the associated hormone releases) are important as they allow for variation in animal shape and ensure that correct organ function is maintained in different nutritional conditions.

Christen has gone on to investigate other hormones and, in collaboration with colleague Associate Professor Alexander Shingleton of the University of Illinois, has found another developmental hormone that regulates body size but not developmental timing. This ‘juvenile hormone’ reduces insulin signalling and increases the concentration of ecdysone without altering the timing of ecdysone pulses.

Now as leader of the Mirth Lab, Christen emphasises how the group’s work provides a theory for the way nutrition might influence the growth of other animals. Nutrition may act as a stimulus, modifying insulin signalling and the synthesis of key developmental hormones like sex steroids in mammals.

The Ross Crozier medal was established by the Genetics Society of Australasia to recognise outstanding contributions to the field of genetics research by mid-career Australasian scientists. It has been awarded annually since 2011. The medal commemorates celebrated Australian evolutionary geneticist Ross Crozier (1943–2009).

The benefits of big neighbours

Larger offspring typically have higher survival, growth and reproduction than smaller offspring. So why then, do we see such a range in offspring size? PhD student Hayley Cameron tackles this conundrum and the results of her latest experimental study contradict accepted theoretical models by showing that bigger is not always better.

Classic life-history models assume a trade-off in the investment mothers make in the next generation; large offspring perform better but smaller offspring are ‘cheaper’ to make and so mothers make them in large numbers. These models predict that a single offspring size will maximise reproductive success in a particular environment. But, we don’t see single offspring sizes, we see a range of sizes.

Game-theory takes the models further and explains the variation in offspring size by generating a ‘competition-colonisation’ trade-off.  In these scenarios, larger offspring will win contests over smaller offspring, but smaller offspring are better able to colonise unoccupied areas because they are more abundant. This means, no single offspring size will maximise reproductive success for any given population and so variation in offspring size is maintained.

Hayley and her supervisor Dustin Marshall test the idea that larger offspring will out-compete smaller offspring in a well-studied model organism, the invertebrate Bugula neritina. This idea has received surprisingly little testing.

To do this Hayley collected larvae and measured each one before settling them on to acetate squares. She glued these acetate squares, with their newly settled offspring, onto PVC plates in pairs of different sizes. These plates were deployed at a field site and every week Hayley measured survival, growth and number of developing larvae for 336 individuals of known offspring size.

To their surprise Hayley and Dustin found, instead of being out-competed as predicted, small offspring received benefits from having larger offspring as neighbours. Large offspring did compete with large neighbours though, and these bigger offspring did best on their own.

The graph and table both show how reproduction (measured by the number of ovicells) in big and small Bugula changed depending on the size of its neighbour. Hayley found that when small Bugula were paired with big neighbours then the overall reproductive output of the small Bugula increased. In contrast, big Bugula did best with no neighbours at all.

Why did this happen? In this study, larger offspring grew into larger colonies and Hayley and Dustin think these larger colonies disrupt the flow which affects the supply of resources (food and oxygen) available to their neighbours.  A slower flow is likely to benefit smaller colonies which tend to be less efficient at capturing resources in high flows. Conversely, larger, more efficient, colonies may deplete the resources available for their large neighbours.

So, while life history theory has traditionally viewed offspring interactions through the lens of competition, Hayley’s PhD work suggests facilitation might also be important in maintaining variation in offspring size.

More of Hayley’s PhD work on variation in offspring size: Should mothers provision their offspring equally? A manipulative field test

This research is published in the journal Evolution.

Celebrating 500 generations of experimental evolution

It’s party time in the Centre for Geometric Biology: We’ve reached 500 generations of experimental evolution in our large and small algal cells!

Three and a half years ago Martino Malerba set up the first culture of the single-celled marine phytoplankton Dunaliella tertiolecta and begun to artificially select for large and small cells.  The selection process of separating the largest and the smallest cells has continued twice a week ever since.  Martino is now the proud ‘father’ of algal cells where the big cells are more than 10 times the size of the smaller cells.

The evolved algae lines have enabled Martino and his colleagues to look at the consequences of being a particular size without having to compare different species, which vary in many other ways than just size. This research directly supports the fundamental question the Centre for Geometric Biology seeks to understand “why do organisms grow to the size they do?”

The team has looked at the effects of size at both the level of the cell and the population. They have found that altering the size of a cell profoundly alters many fundamental traits of algal physiology and ecology. This affects how cells of different sizes will cope with fluctuating resources and, in turn, how those populations of cells use energy and grow.

Exciting research underway looks at community-level effects. What happens when grazers are only offered either large or small algal cells: does changing algal size have repercussions up the food chain?

Understanding how individuals grow and regulate their energy is critical in helping scientists predict how large-scale impacts, such as climate change, affect organisms.

Phytoplankton have critical roles in the ocean; they form the base of most food webs and fix large amounts of carbon.  So, while the research stemming from these artificially evolved algal cells is theoretically interesting, it also has a very direct and immediate application to how we understand and manage climate change.