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
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.
Authors: Martino E Malerba, Craig R White, and Dustin J Marshall
Published in: Frontiers in Ecology and the Environment
Artificial structures are proliferating along coastlines worldwide, creating new habitat for heterotrophic filter feeders. The energy demand of this heterotrophic biomass is likely to be substantial, but is largely unquantified.
Combining in situ surveys, laboratory assays, and information obtained from geographic information systems, we estimated the energy demands of sessile invertebrates found on marine artificial structures worldwide.
At least 950,000 metric tons of heterotrophic biomass are associated with commercial ports around the world, emitting over 600 metric tons of carbon dioxide into the atmosphere and consuming 5 million megajoules of energy per day.
We propose the concept of a trophic “footprint” of marine urbanization, in which every square meter of artificial structure can negate the primary production of up to 130 square meters of surrounding coastal waters; collectively, these structures not only act as energy sinks and carbon sources, but also potentially reduce the productivity of coastal food webs.
Malerba ME, White CR, Marshall DJ (2019) The outsized trophic footprint of marine urbanization. Frontiers in Ecology and the EnvironmentPDFDOI
Authors: Dustin J Marshall, Steven Gaines, Robert Warner, Diego R Barneche, and Michael Bode
Published in: Frontiers in Ecology and the Environment
Marine protected areas (MPAs) are important tools for managing marine ecosystems. MPAs are expected to replenish nearby exploited populations through the natural dispersal of young, but the models that make these predictions rely on assumptions that have recently been demonstrated to be incorrect for most species of fish.
A meta‐analysis showed that fish reproductive output scales “hyperallometrically” with fish mass, such that larger fish produce more offspring per unit body mass than smaller fish. Because fish are often larger inside MPAs, they should exhibit disproportionately higher reproductive output as compared to fish outside of MPAs.
We explore the consequences of hyperallometric reproduction for a range of species for population replenishment and the productivity of exploited species.
We show that the reproductive contribution of fish inside MPAs has been systematically underestimated and that fisheries yields can be enhanced by the establishment of reservoirs of larger, highly fecund fish.
Marshall DJ, Gaines S, Warner R, Barneche DR, Bode M (2019) Underestimating the benefits of marine protected areas for the replenishment of fished populations. Frontiers in Ecology and the EnvironmentPDFDOI
We have all heard the saying “live fast, die young” but it doesn’t only apply to film stars; smaller life forms also abide by this rule. Microscopic phytoplankton cells can double in numbers every few days, while the much larger elephant lives almost 100 years and reproduces slowly. This relationship between body mass and the ‘pace of life’ is well known. But the underlying mechanisms are far from resolved.
A number of years ago, scientists proposed a theory to link the ‘pace of life’ of individuals to the ecology of populations, communities, and ecosystems. The Metabolic Theory of Ecology predicts how energy use (metabolism) is affected by body size and how growth rate, development time and death rates change with energy use. These changes in individual rates will, in turn, affect rates of change in populations.
Metabolic Theory is very appealing because it explains many patterns we see in nature and is based on a fundamental rate which applies to all levels of organisation. But it is difficult to test. Detecting differences in energy use accurately requires having consistent differences in body sizes among organisms, while controlling for other variables (e.g. age, nutrition, health). This usually means comparing across species, which differ in more ways than just size.
Few tests of these predictions have been able to control for these variables but recently Martino Malerba and Dustin Marshall were able to do this and they found that the body size-metabolism relationship did not predict population change as expected.
Instead of the classical view, where body size determines the rate at which organisms use energy, which then determines demographics, Martino and Dustin suggest that body size can affect metabolism and populations at the same time.
IIf these laboratory cultures are representative of natural populations, we would predict that current trends of reduced body size (from global warming) could lead to lower rates of population increase, biomass productivity and maximum biomass. This is the opposite of what current theory would predict. This is particularly important when we consider the role of phytoplankton in fixing carbon and supporting food chains.
Body size often strongly covaries with demography across species. Metabolism has long been invoked as the driver of these patterns, but tests of causal links between size, metabolism and demography within a species are exceedingly rare.
We used 400 generations of artificial selection to evolve a 2,427% size difference in the microalga Dunaliella tertiolecta. We repeatedly measured size, energy fluxes and demography across the evolved lineages. Then, we used standard metabolic theory to generate predictions of how size and demography should covary based on the scaling of energy fluxes that we measured.
The size dependency of energy remained relatively consistent in time, but metabolic theory failed to predict demographic rates, which varied unpredictably in strength and even sign across generations.
Classic theory holds that size affects demography via metabolism – our results suggest that both metabolism and size act separately to drive demography and that among‐species patterns may not predict within‐species processes.
Malerba ME, Marshall DJ (2019) Size-abundance rules? Evolution changes scaling relationships between size, metabolism and demography. Ecology Letters PDFDOI
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.