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
Certain pathogens (disease-producing organisms) are stuck in a Catch-22; to survive they need to continue to find, and infect, new hosts. But infection makes their hosts sick and less likely to move to where there are new hosts to infect.
PhD student Louise Nørgaard and her supervisors Ben Phillips and Matt Hall have found evidence of a pathogen that resolves this issue by exploiting the differences in size and behaviour of male and female hosts to optimize its own chance of successful infection.
The team uses the freshwater crustacean Daphnia magna and its common pathogen Pasteuria ramosa as a model system to test the idea that a pathogen can exploit differences between the sexes of a host to its advantage. The pathogen P. ramosa is ingested by Daphnia after which it sterilises and kills the host, releasing transmission spores that are ready to infect a new host. Female Daphnia are bigger, live longer and are more susceptible to infection than males.
Louise set up two separate experiments, allowing her to monitor the probability that Daphnia would disperse from a crowded area to a less crowded area and to measure the rate and distance travelled by infected and uninfected male and female individuals.
In the first experiment Louise was able to capitalise on previous work that has shown that Daphnia will disperse when conditions are crowded. Exposure to water taken from high densities of Daphniais enough to encourage dispersal. Louise used ‘crowded-conditioned’ water and found infected male Daphnia were more likely to disperse than uninfected males. Infected females, on the other hand, were a lot less likely to disperse than uninfected females.
A second experiment found that infected females had four times the number of transmission spores than infected males and moved less far and more slowly than males or uninfected females. Infected males though, moved at the same rate and travelled the same distance as uninfected males.
So how do these differences between the sexes help the pathogen? Females are bigger and can host large numbers of transmission spores. Staying put when densities are high means they are releasing this large number of spores into a crowd – potentially maximising the chance of further infections. Smaller males have fewer spores to release and the chance of secondary infections may be maximised when they move to new areas where few individuals are already infected.
Importantly the differences in dispersal behaviour between infected males and females seem to relate directly to the way the pathogen interacts with each sex. Uninfected males and females had similar rates and distance of dispersal while uninfected females were more likely to move away from crowded habitats than males. These patterns disappear when both sexes are infected.
Do these different infection strategies in different sexes provide a form of bet-hedging for the pathogen? Louise and her supervisors think they do and, if widespread, will have important implications for disease dynamics.
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).
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.
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.
Offspring sizes vary within populations but the reasons are unclear. Game‐theoretic models predict that selection will maintain offspring‐size variation when large offspring are superior competitors (i.e., competition is asymmetric), but small offspring are superior colonizers. Empirical tests are equivocal, however, and typically rely on interspecific comparisons, whereas explicit intraspecific tests are rare.
In a field study, we test whether offspring size affects competitive asymmetries using the sessile marine invertebrate, Bugula neritina. Surprisingly, we show that offspring size determines whether interactions are competitive or facilitative — large neighbours strongly facilitated small offspring, but also strongly competed with large offspring. These findings contradict the assumptions of classic theory — that is, large offspring were not superior competitors. Instead, smaller offspring actually benefit from interactions with large offspring— suggesting that asymmetric facilitation, rather than asymmetric competition, operates in our system.
We argue that facilitation of small offspring may be more widespread than currently appreciated, and may maintain variation in offspring size via negative frequency‐dependent selection.
Offspring size theory has classically viewed offspring interactions through the lens of competition alone, yet our results and those of others suggest that theory should accommodate positive interactions in explorations of offspring‐size variation.
Cameron H, Marshall DJ (2019) Can competitive asymmetries maintain offspring size variation? A manipulative field test. Evolution PDFDOI
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.
About half of the coastline of Europe, the United States and Australasia is modified by artificial structures. In newly published research, we identified a new effect of marine urbanisation that has so far gone unrecognised.
When we build marinas, ports, jetties and coastal defences, we introduce hard structures that weren’t there before and which reduce the amount of sunlight hitting the water. This means energy producers such as seaweed and algae, which use light energy to transform carbon dioxide into sugars, are replaced by energy consumers such as filter-feeding invertebrates. These latter species are often not native to the area, and can profoundly alter marine habitats by displacing local species, reducing biodiversity, and decreasing the overall productivity of ecosystems.
Incorporating simple designs in our marine infrastructure to allow more light penetration, improve water flow, and maintain water quality, will go a long way towards curbing these negative consequences.
We are used to thinking about the effects of urbanisation in our cities – but it is time to pay more attention to urban sprawl in the sea. We need to better understand the effects on the food web in a local context.
Most animals that establish themselves on these shaded hard structures are “sessile” invertebrates, which can’t move around. They come in a variety of forms, from encrusting species such as barnacles, to tree-shaped or vase-like forms such as bryozoans or sponges. But what they all have in common is that they can filter out algae from the water.
In Australian waters, we commonly see animals from a range of different groups including sea squirts, sponges, bryozoans, mussels and worms. They can grow in dense communities and often reproduce and grow quickly in new environments.
How much energy do they use?
In our new research, published in the journal Frontiers in Ecology and the Environment, we analysed the total energy usage of invertebrate communities on artificial structures in two Australian bays: Moreton Bay, Queensland, and Port Phillip Bay, Victoria. We did so by combining data from field surveys, laboratory studies, and satellite data.
We also compiled data from other studies and assessed how much algae is required to support the energy demands of the filter-feeding species in commercial ports worldwide.
In Port Phillip Bay, 0.003% of the total area is taken up by artificial structures. While this doesn’t sound like much, it is equivalent to almost 50 soccer fields of human-built structures.
We found that the invertebrate community living on a single square metre of artificial structure consumes the algal biomass produced by 16 square metres of ocean. Hence, the total invertebrate community living on these structures in the bay consumes the algal biomass produced by 800 football pitches of ocean!
Similarly, Moreton Bay has 0.005% of its total area occupied by artificial structures, but each square metre of artificial structure requires around 5 square metres of algal production – a total of 115 football pitches. Our models account for various biological and physical variables such as temperature, light, and species composition, all of which contribute to generate differences among regions.
Overall, the invertebrates growing on artificial structures in these two Australian bays weigh as much as 3,200 three-tonne African elephants. This biomass would not exist were it not for marine urbanisation.
How does Australia compare to the rest of the world?
We found stark differences among ports in different parts of the world. For example, one square metre of artificial structure in cold, highly productive regions (such as St Petersburg, Russia) can require as little as 0.9 square metres of sea surface area to provide enough algal food to sustain the invertebrate populations. Cold regions can require less area because they are often richer in nutrients and better mixed than warmer waters.
In contrast, a square metre of structure in the nutrient-poor tropical waters of Hawaii can deplete all the algae produced in the surrounding 120 square metres.
Does it matter?
Should we be worried about all of this? To some extent, it depends on context.
These dense filter-feeding communities are removing algae that normally enters food webs and supports coastal fisheries. As human populations in coastal areas continue to increase, so will demand on these fisheries, which are already under pressure from climate change. These effects will be greatest in warmer, nutrient-poor waters.
But there is a flip side. Ports and urban coastlines are often polluted with increased nutrient inputs, such as sewage effluents or agricultural fertilisers. The dense populations of filter-feeders on the structures near these areas may help prevent this nutrient runoff from triggering problematic algal blooms, which can cause fish kills and impact human health. But we still need to know what types of algae these filter-feeding communities are predominantly consuming.
Our analysis provides an important first step in understanding how these communities might affect coastal production and food webs.
In places like Southeast Asia, marine managers should consider how artificial structures might affect essential coastal fisheries. Meanwhile, in places like Port Phillip Bay, we need to know whether and how these communities might affect the chances of harmful algal blooms.
One hectare of ocean in which fishing is not allowed (a marine protected area) produces at least five times the amount of fish as an equivalent unprotected hectare, according to new research published today.
This outsized effect means marine protected areas, or MPAs, are more valuable than we previously thought for conservation and increasing fishing catches in nearby areas.
Previous research has found the number of offspring from a fish increases exponentially as they grow larger, a disparity that had not been taken into account in earlier modelling of fish populations. By revising this basic assumption, the true value of MPAs is clearer.
Marine Protected Areas
Marine protected areas are ocean areas where human activity is restricted and at their best are “no take” zones, where removing animals and plants is banned. Fish populations within these areas can grow with limited human interference and potentially “spill-over” to replenish fished populations outside.
Obviously MPAs are designed to protect ecological communities, but scientists have long hoped they can play another role: contributing to the replenishment and maintenance of species that are targeted by fisheries.
Yet fishers remain sceptical that any spillover will offset the loss of fishing grounds, and the role of MPAs in fisheries remains contentious. A key issue is the number of offspring that fish inside MPAs produce. If their fecundity is similar to that of fish outside the MPA, then obviously there will be no benefit and only costs to fishers.
Big fish have far more babies
Traditional models assume that fish reproductive output is proportional to mass, that is, doubling the mass of a fish doubles its reproductive output. Thus, the size of fish within a population is assumed to be less important than the total biomass when calculating population growth.
But a paper recently published in Science demonstrated this assumption is incorrect for 95% of fish species: larger fish actually have disproportionately higher reproductive outputs. That means doubling a fish’s mass more than doubles its reproductive output.
When we feed this newly revised assumption into models of fish reproduction, predictions about the value of MPAs change dramatically.
Fish are, on average, 25% longer inside protected areas than outside. This doesn’t sound like much, but it translates into a big difference in reproductive output – an MPA fish produces almost 3 times more offspring on average. This, coupled with higher fish populations because of the no-take rule means MPAs produce between 5 and 200 times (depending on the species) more offspring per unit area than unprotected areas.
Put another way, one hectare of MPA is worth at least 5 hectares of unprotected area in terms of the number of offspring produced.
We have to remember though, just because MPAs produce disproportionately more offspring it doesn’t necessarily mean they enhance fisheries yields.
For protected areas to increase catch sizes, offspring need to move to fished areas. To calculate fisheries yields, we need to model – among other things – larval dispersal between protected and unprotected areas. This information is only available for a few species.
We explored the consequences of disproportionate reproduction for fisheries yields with and without MPAs for one iconic fish, the coral trout on the Great Barrier Reef. This is one of the few species for which we had data for most of the key parameters, including decent estimates of larval dispersal and how connected different populations are.
We found MPAs do in fact enhance yields to fisheries when disproportionate reproduction is included in relatively realistic models of fish populations. For the coral trout, we saw a roughly 12% increase in tonnes of caught fish.
There are two lessons here. First, a fivefold increase in the production of eggs inside MPAs results in only modest increases in yield. This is because limited dispersal and higher death rates in the protected areas dampen the benefits.
However the exciting second lesson is these results suggest MPAs are not in conflict with the interests of fishers, as is often argued.
While MPAs restrict access to an entire population of fish, fishers still benefit from from their disproportionate affect on fish numbers. MPAs are a rare win-win strategy.
It’s unclear whether our results will hold for all species. What’s more, these effects rely on strict no-take rules being well-enforced, otherwise the essential differences in the sizes of fish will never be established.
We think that the value of MPAs as a fisheries management tool has been systematically underestimated. Including disproportionate reproduction in our assessments of MPAs should correct this view and partly resolve the debate about their value. Well-designed networks of MPAs could increase much-needed yields from wild-caught fish.