Research fellow position: marine larval biologist

  • Level A, research-only academic
  • $66,706 to $90,532pa + 9.5% 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

How does size, fragmentation and food affect metabolic rates in a bryozoan?

We know that metabolic rate (a measure of energy use) tends to vary among individuals of different sizes and also with food availability. Lukas Schuster has been working with his PhD supervisors Craig White and Dustin Marshall to find out how metabolic rate changes with size in the colonial marine invertebrate Bugula neritina. They are also interested in how (or if) manipulating an organism’s size affects metabolic rate in individuals that were either starved or fed.

Lukas and his supervisors chose to work with Bugula because it is a colonial organism. These types of animals can be used for testing metabolic theories because the size of the colonies can be changed by removing fragments.

Lukas measured metabolic rate by measuring oxygen consumption of intact and size-manipulated colonies of Bugula that had been fed or starved as well as colonies that he had grown up in the field so he knew how old they were.

When Lukas measured metabolic rates for individuals of known age he found that rates increased proportionally with size across the different ages, that is, metabolic rate scaled isometrically with size when looking at individuals that ranged in age / developmental stage.  In contrast, when he looked at metabolic rates for a specific age or developmental stage he found that rates didn’t scale proportionally with size but, instead, had allometric scaling as has been found in previous studies.

Lukas and his supervisors point out that it is important to be aware of these differences. Measuring metabolic rates of field-collected specimens of unknown age may result in isometric scaling of metabolic rate with size. Conversely, measurements of specimens at the same developmental stage is likely to result in allometric scaling where larger individuals have proportionally lower metabolic rates compared to smaller individuals.

To the team’s surprise, they also found that when they measured metabolic rate in size-manipulated Bugula that has been collected from the field, metabolic rate reverted to allometric scaling. Manipulating size in Bugula may lead to a leaking of nutrients through the pores between the zooids that make up the colony and this may be driving the change in the relationship of metabolic rates with size.

Bugula responded to food deprivation by reducing its metabolic rate, and conversely responded to feeding by increasing its metabolic rate, which was consistent with what other researchers have found in other species. But, in comparison to other species, the rate at which Bugula increased its metabolic rate following feeding, was rather low. This may also relate to the fact that Bugula is a colonial species but as there are very few studies investigating the way metabolic rate responds to feeding in colonial organisms, it is hard to know for sure.

Clearly, the relationship between size and metabolic rates in Bugula is complicated and may relate, in part, to the fact that Bugula is a colonial organism. But to fully understand the effects of size manipulation on metabolic rates and biological processes within Bugula colonies, further studies will be needed.

This research is published in the journal Invertebrate Biology.

The origin and maintenance of metabolic allometry in animals

Authors:Craig R White, Dustin J Marshall, Lesley A Alton, Pieter A Arnold, Julian E Beaman, Candice L Bywater, Catriona Condon, Taryn S Crispin, Aidan Janetzki, Elia Pirtle, Hugh S Winwood-Smith, Michael J Angilletta Jr, Stephen F Chenoweth, Craig E Franklin, Lewis G Halsey, Michael R Kearney, Steven J Portugal, and Daniel Ortiz-Barrientos

Published in: Nature Ecology & Evolution

Abstract

Organisms vary widely in size, from microbes weighing 0.1 pg to trees weighing thousands of megagrams — a 1021-fold range similar to the difference in mass between an elephant and the Earth.

Mass has a pervasive influence on biological processes, but the effect is usually non-proportional; for example, a tenfold increase in mass is typically accompanied by just a four- to sevenfold increase in metabolic rate.

Understanding the cause of allometric scaling has been a long-standing problem in biology. Here, we examine the evolution of metabolic allometry in animals by linking microevolutionary processes to macroevolutionary patterns.

We show that the genetic correlation between mass and metabolic rate is strong and positive in insects, birds and mammals.

We then use these data to simulate the macroevolution of mass and metabolic rate, and show that the interspecific relationship between these traits in animals is consistent with evolution under persistent multivariate selection on mass and metabolic rate over long periods of time.

White CR, Marshall DJ, Alton LA, Arnold PA, Beaman JE, Bywater CL, Condon C, Crispin TS, Janetzki A, Pirtle E, Winwood-Smith HS, Angilletta MJ, Chenoweth SF, Franklin CE, Halsey LG, Kearney MR, Portugal SJ, Ortiz-Barrientos D (2019) The origin and maintenance of metabolic allometry in animals. Nature Ecology & Evolution PDF DOI 

Influence of food, body size, and fragmentation on metabolic rate in a sessile marine invertebrate

Authors: Lukas Schuster, Craig R White, and Dustin J Marshall

Published in: Invertebrate Biology

Abstract

Metabolic rates vary among individuals according to food availability and phenotype, most notably body size. Disentangling size from other factors (e.g., age, reproductive status) can be difficult in some groups, but modular organisms may provide an opportunity for manipulating size experimentally. While modular organisms are increasingly used to understand metabolic scaling, the potential of feeding to alter metabolic scaling has not been explored in this group.

Here, we perform a series of experiments to examine the drivers of metabolic rate in a modular marine invertebrate, the bryozoan Bugula neritina. We manipulated size and examined metabolic rate in either fed or starved individuals to test for interactions between size manipulation and food availability.

Field collected colonies of unknown age showed isometric metabolic scaling, but those colonies in which size was manipulated showed allometric scaling.

To further disentangle age effects from size effects, we measured metabolic rate of individuals of known age and again found allometric scaling. Metabolic rate strongly depended on access to food: starvation decreased metabolic rate by 20% and feeding increased metabolic rate by 43%.

In comparison to other marine invertebrates, however, the increase in metabolic rate, as well as the duration of the increase (known as specific dynamic action, SDA), were both low. Importantly, neither starvation nor feeding altered the metabolic scaling of our colonies.

Overall, we found that field‐collected individuals showed isometric metabolic scaling, whereas metabolic rate of size‐manipulated colonies scaled allometrically with body size. Thus, metabolic scaling is affected by size manipulation but not feeding in this colonial marine invertebrate.

Schuster L, White CR, Marshall DJ (2019) Influence of food, body size, and fragmentation on metabolic rate in a sessile marine invertebrate. Invertebrate Biology PDF DOI 

Why do cooler mothers produce larger offspring?

In a recently published letter, Amanda Pettersen, Craig White, Rob Bryson-Richardson and Dustin Marshall propose a simple model to explain a pervasive conundrum – why do cooler mothers produce larger offspring?

Life history theory maintains that mothers balance the costs and benefits of making a few larger and better performing offspring against making many smaller and poorer performing offspring.

A major challenge to the theory is the fact that temperature seems to alter the optimisation of this trade off. Observations indicate that across a wide range of taxa and systems, mothers in warmer conditions produce smaller offspring. What is more, experimental studies have also shown that increasing temperatures decrease offspring size.

Amanda and her PhD supervisors are proposing that linking life history theory and metabolic theory, which relates to energy use, can provide a widely applicable explanation to the offspring size / temperature relationship.

Their model is centred on the cost of development. Mothers must provision their offspring until they are able to feed for themselves, that is, attain nutritional independence. The time spent in this developmental phase coupled with the energy expended will comprise the ‘cost’ of development. The minimum offspring size that allows individuals to reach nutritional independence must, then, increase with increasing cost of development.

As temperatures increase, developmental rate is expected to increase so that less time is spent in the developmental phase and metabolic rates (rates of energy use) are also expected to increase. The research team are suggesting that we consider how sensitive these two components are to temperature. If developmental rate is more sensitive to changes in temperature than metabolic rate, then the cost associated with provisioning offspring to achieve nutritional independence will decrease with increasing temperatures.

Or to put it another way, if developmental rate increases more than metabolic rate as temperatures rise, so that the developmental time is shorter in relation to metabolic rate, then the developmental cost is lower and offspring are smaller at higher temperatures. If, however, metabolic rate is more sensitive to changing temperatures than developmental rate then the converse is true; the developmental cost will increase with increasing temperatures and offspring are predicted to be larger at higher temperatures.

In order to develop and test these ideas the team needed to generate measures of temperature dependence of metabolic rate and developmental rate simultaneously, something that hadn’t been done before in a systematic fashion.

They started by methodically searching published literature to determine the relationship between the temperature that mothers experience and the size of their offspring. They then experimentally manipulated temperature to examine how developmental rate and metabolic rates changed in two very different species – the bryozoan Bugula neritinaand the zebrafish Danio rerio. They used the data from these experiments to develop the mathematical functions for their model to determine how the costs of development change with temperature. Finally they searched the literature again to get data on the temperature dependence of developmental and metabolic rates for a wide range of species because they wanted to test whether their model could apply more generally.

Amanda and her colleagues found that the offspring size / temperature relationship is widespread. Also in the two species they collected experimental data for, they found that development time is more sensitive to temperature than metabolic rates. This means that the overall costs of development decrease with temperature. What is more, they found that this pattern applies more broadly – for 72 species across five phyla the costs of development are higher at cooler temperatures.

Combining life history theory and metabolic theory has allowed the research team to provide a general explanation for offspring size / temperature relationships. In colder temperatures mothers show an adaptive response whereby they offset the increased costs of development by making larger offspring that possess greater energy reserves.

This research is published in the journal Ecology Letters.

This figure shows the relationship between two biological rates; development time (D) and metabolic rate (MR). In the left-hand graph we can see that as temperature increases from T4 to T1, development time is expected to decrease and metabolic rate is expected to increase. The right-hand panels demonstrate what is predicted to happen when b) the developmental rate is more sensitive to temperature than metabolic rate and so c) the total costs of development (and therefore offspring size) should decrease with increasing temperature. In d) the converse is true – metabolic rate is more sensitive to temperature than developmental rate and so e) total costs of development (and offspring size) will increase with increasing temperature.

New projects 2019

A number of large new projects will be getting underway in 2019 as a result of ARC funding schemes. Dustin Marshall and Matt Hall are now Future Fellows and Giulia Ghedini has received a Discovery Early Career Researcher Award. Dustin and Giulia will be using marine invertebrates to look into impacts of global warming whilst Matt is tackling the importance of sex in the evolution of infectious disease.

Within a given species, often the greatest heterogeneity that a pathogen will encounter will be due to differences between males and females. Yet, up until recently, insight into this crucial topic was driven by research into one sex, typically males.

 

Matt’s recent work has shown that, in the water-flea Daphnia magna, not only is pathogen fitness lower in males, but so is a pathogen’s evolutionary potential. What is more, the relative proportion of males in a population can fundamentally alter the overall transmission potential of a pathogen.

A. The graph on the left demonstrates that pathogen fitness is sex specific; in this case pathogen fitness is greater in females. B. The graph on the right indicates how changes in the relative proportion of males can increase the burden of disease for every individual.

This project was stimulated by Matt’s recognition that there is an absence of theory that explicitly considers how males and females can impact on the evolution and epidemiology of infectious disease.  Matt is seeking to address this imbalance and integrate sex-specific effects into a general framework for disease evolution and epidemiology.

Matt will be using the water-flea Daphnia magnaand its associated pathogens to provide an experimental system in which he can manipulate infections in males and females, characterise the degree of differentiation, and generate predictive models.

 

Dustin will be investigating how temperature affects the life-history stages of feeding and non-feeding larvae. Marine life histories show strong biogeographic patterns: warmer waters favour species with feeding larvae and cooler waters favour species with non-feeding larvae. Warming could be particularly problematic for Australian species because in 2012, Dustin discovered that Australian coastal species predominantly have non-feeding larvae. This means that future temperatures increases could affect native Australian invertebrates disproportionately relative to other regions of the world. (Put in schematic from application here)

Schematic of the data pipeline to estimate developmental energetics across temperature regimes. For each species, parents will be exposed to a range of temperatures, after which the size and number of offspring that are produced will be measured. These offspring will then have every phase of their energy usage and acquisition, from fertilisation through to metamorphosis estimated across an orthogonal temperature range. Dustin will then integrate these estimates to create a thermal energy performance curve for each species and use these data to parameterise models of connectivity, viability and life history evolution.

At the end of an intensive experimental period, Dustin will have quantitative estimates of how temperature alters the success of a range of species from the gamete to the juvenile. At this stage Dustin will work with collaborators to generate predictive models to determine

  1. how does temperature alter the relative advantages for each of the two developmental modes?
  2. how does temperature affect dispersal and connectivity among populations for each developmental mode? and finally
  3. how does temperature affect the distribution of marine organisms with feeding or non-feeding larvae?

 

Giulia will be investigating how global warming will affect entire ecological communities.

We already know that warming can affect individuals by reducing their body size and speeding up energy use, as well as reducing water viscosity.  But what we don’t know is how these changes at the individual level might play out at the population and community level and affect the energy intake or expenditure of whole communities.

Giulia will be looking at how changes in body size at the individual level interact with population and community level ecosystem functions.

Giulia is particularly interested in this knowledge gap and will be investigating the implications of warming sea temperatures for important ecosystem functions such as productivity, food web stability or resistance to invasion.

Giulia has planned a series of experiments, using communities of easily manipulated, sessile, marine invertebrates, to explore 4 main questions.

  1. How do changes in community size-structure and composition under warming alter the energy intake (phytoplankton) and expenditure (oxygen) of marine invertebrate communities?
  2. Since the availability of energy can mediate biological invasions, does warming alter the energy usage of communities so that they are more susceptible to invasive species?
  3. Are the responses of invertebrate communities to warming mediated by changes in their food (phytoplankton)?
  4. Given that warming reduces water viscosity, how does this mechanical effect alter food consumption and metabolic expenditure in marine communities of different size-structure?

Debating growth and reproduction

In a recent post we described a paper written by Dustin Marshall and Craig White and published in Trends in Ecology and Evolution (TREE). The published article was called “Have we outgrown the existing models of growth?” In it, Dustin and Craig suggest that the growth dynamics that biologists have long sought to understand probably emerge simply from hyperallometric scaling of reproduction.

Daniel Pauly is a fisheries scientist from the University of British Columbia and is a proponent of the Gill-Oxygen Limitation Theory (GOLT) of growth. This theory applies to water-breathing animals and is structured around the proposition that growth is necessarily constrained by the size of the gills and the oxygen they are able to extract from the water.

Professor Pauly argues in a letter to TREE that there is a good reason why growth is not considered to be influenced by reproduction in the context of GOLT. While he agrees that reproductive output tends to scale hyperallometrically in fish, he does not agree that fish slow their growth because they allocate increasingly more to reproduction. Instead, he thinks that as growth slows (due to oxygen limitation caused by physical constraints on gill size) increased allocation of resources is directed to reproduction.

In their rebuttal, Dustin and Craig summarise their difference in opinion as one of causality; while Professor Pauly argues that body size in fish is limited by gill area, they believe that organs evolve to provide capacity to meet an organisms requirements. Or, in other words, the trait of body size is the product of selection whereby the size of an organisms is the best it can be to maximise fitness in a particular environment. Most importantly, taken to its logical extension, Dustin and Craig argue that Pauly’s own arguments imply fish reproduction should decrease with size.

In a separate letter, Michael Kearney from the University of Melbourne suggests that a radical revision of growth models is premature. In this case, Associate Professor Kearney suggests that a mechanistic modelling approach (such as Dynamic Energy Budget theory) based on a thermodynamically explicit theory of metabolism is better suited to understanding growth than the phenomenological modelling approach proposed by Dustin and Craig.

While Assoc. Prof. Kearney argues that the Dynamic Energy Budget model can incorporate hyperallometric scaling by adjusting the ‘rules’ governing how much energy is allocated to reproduction, Craig and Dustin say that to do this requires a phenomenological approach and is an unjustified post hoc model fitting solution. According to Craig and Dustin, this means that Assoc. Prof. Kearney’s model is not strictly mechanistic, with some parameters estimated by fitting mechanistic functions and some parameters requiring empirical data (a phenomenological approach).

But there is some agreement. Dustin, Craig and Michael Kearney are all interested in seeing studies of growth and metabolism that are conducted in the context of a full accounting of energy and mass balances (food in, changes in length and weight, respiration, faeces and eggs out) to continue improving our understanding of why organisms are the size they are.

You can read the original article and the follow-up letters.

Aquatic life history trajectories are shaped by selection, not oxygen limitation

Authors: Dustin J Marshall and Craig R White

Published in: Trends in Ecology & Evolution

Pauly1 argues that, as espoused in the gill-oxygen limitation theory (GOLT), growth slows as size increases because oxygen supply via the gills is unable to keep up with the oxygen demands of an increasingly large body. Thus, according to GOLT, growth determines the timing of reproduction, and fish reproduce when they become oxygen limited and growth starts to decline. GOLT has been critiqued on physiological grounds2,3 and we agree with those critiques. Large fish are no more oxygen limited than small fish, primarily because their respiratory surface area matches their metabolic demand for oxygen over a large size range…

Marshall DJ, White CR (2019) Aquatic life history trajectories are shaped by selection, not oxygen limitation, Trends in Ecology & Evolution. PDF DOI