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

Hot spots on the X chromosome? Testing a classic theory of sexual antagonism

Biologists have long been interested in sexually antagonistic selection, in which the genetic variants that provide an advantage for one sex are disadvantageous for the other. Sexual antagonism is important because it helps maintain genetic variation and represents one of several ways in which populations might remain maladapted with respect to their environments.

In 1984, a theoretical prediction was proposed by William Rice that said the X chromosome should be a ‘hot spot’ for sexually antagonistic genetic polymorphism. His mathematical models indicated that sexually antagonistic alleles were more likely to remain polymorphic when they were linked to the X chromosome than when they were on other types of chromosomes (i.e., autosomes). Rice’s prediction that polymorphism is easier to maintain on the X chromosome critically depends on the dominance relations between sexually antagonistic alleles. Other researchers have shown that the autosomes are more conducive to maintaining genetic variation under conditions that differ from Rice’s assumptions.

There have been numerous empirical studies that have demonstrated apparent support for Rice’s theory. But Filip Ruzicka and Tim Connallon argue that these studies share a common but incorrect assumption: that signals from sexually antagonistic genetic variation are equally detectable whether the variation is on sex chromosomes (ie X-linked) or on autosomes.

Instead, Filip and Tim found that the existing methods for testing this classic theory are all biased towards finding signals of sexually antagonistic variation on the X chromosome. They developed mathematical models to test how much X chromosomes and autosomes contributed to signals of sexual antagonism and found a considerable bias in existing studies.

When they revisited the experimental studies using their models, they found that most of them were actually consistent with scenarios in which the X chromosome is not a hot spot for sexually antagonistic polymorphism.

Drosophila melanogaster, the common fruit fly, is often used as a model system to study sexual antagonism. Image credit: Francisco Romero Ferrero via Wikimedia Commons

So how can we be sure whether William Rice’s theory is correct or not? Filip and Tim concede that experimentally testing this classic theory is difficult. They suggest the most feasible approach is to compare fitness components between fathers and sons (who do not inherit their fathers X chromosome) with fitness components between fathers and daughters (who do). Modern genomic approaches that directly estimate the fitness effects of individual genetic variation (genome-wide association studies or “GWAS” of fitness) are also promising avenues for testing the theory.

Filip and Tim hope that their predictions provide better guidelines for future tests. Their models can be used as baseline expectations against which experimental data can be compared.

While they acknowledge that there are limitations in their models, they maintain further attention to this issue will greatly improve our ability to predict the potential contributions of X-linked and autosomal genes to population genetic diversity and species divergence.

This research is published in the journal Proceedings of the Royal Society B: Biological Sciences

Winners and losers: why developmental strategy is important in determining marine invertebrate distributions under future climate

Global change will alter the distribution of organisms around the planet. Dustin Marshall and Mariana Álvarez-Noriega found rising ocean temperatures will impact early life stages of marine invertebrates and change the patterns in the distribution of species that we see today. In particular, species in which mothers invest heavily in offspring will be the biggest losers. These species occur predominantly at the poles.

In terrestrial environments seeds often disperse in the wind, with shape and size affecting how far they travel. It turns out much the same happens in the ocean but currents rather than wind carry marine larvae to their new homes. Larvae able to feed spend much longer in the water column meaning these species have far greater dispersal capabilities.

As with plants, dispersal is crucial to the survival of marine populations. Arriving larvae can seed new areas, re-seed vulnerable populations, and provide genetic variation for subsequent generations in far-flung regions. There is a downside: if temperature affects dispersal, it will also shape how species are affected by global warming.

Not all marine species use the same reproductive or life-history strategies which can mean differences in dispersal distances from centimetres to hundreds of kilometres for different species. Interestingly, there is a well-recognised relationship between latitude (or temperature) and reproductive strategy.

Species at higher latitudes (nearer the poles) tend to invest more heavily in their offspring and produce non-feeding larvae or bypass the larval stage altogether. This means these species don’t disperse very far. In contrast, tropical species tend to put little effort into provisioning their offspring and produce larvae that can feed. As a result, these larvae can spend a lot more time in the plankton and can be dispersed vast distances.

Dustin and Mariana wanted to know how these dispersal relationships might change as global temperatures change. To address this question, they revisited the database of marine invertebrates classified into feeding / development types from a previous study. They established relationships between temperature and development mode so they could then explore how predicted temperatures for 2100 would change patterns in distributions.

So, how will global warming affect these relationships? We know species’ in warmer waters are more likely to produce large numbers of feeding larvae able to remain in the water column for weeks at a time. As waters warm, these species are well placed to extend their range.

In contrast, species based in cooler waters tend to invest heavily in individual offspring, meaning that they develop quicker and settle closer to their parents. This reproductive strategy means that such species are more vulnerable to rapid global change as moving to new areas will, of necessity, be step-wise and slow.

Species at the poles will therefore be the biggest losers because not only will their lower dispersal lifestyles mean they will be slow to access cooler waters but also the options are limited; there is nowhere to go.

This figure shows the predicted change in prevalence of three different development modes in the southern hemisphere under a predicted scenario for global warming. The blue line shows an even increase of feeding larvae across all latitudes, while non-feeding planktonic larvae (purple line) will only increase at higher latitudes and there will be a loss of species that invest most heavily in their young and don’t have a planktonic larval stage near the poles (orange line).

This research was published in the journal Philosophical Transactions of the Royal Society B: Biological Sciences.

Projecting marine developmental diversity and connectivity in future oceans

Authors: Dustin J Marshall and Mariana Álvarez-Noriega

Published in: Philosophical Transactions of the Royal Society B: Biological Sciences

Abstract

Global change will alter the distribution of organisms around the planet. While many studies have explored how different species, groups and traits might be re-arranged, few have explored how dispersal is likely to change under future conditions.

Dispersal drives ecological and evolutionary dynamics of populations, determining resilience, persistence and spread. In marine systems, dispersal shows clear biogeographical patterns and is extremely dependent on temperature, so simple projections can be made regarding how dispersal potentials are likely to change owing to global warming under future thermal regimes.

We use two proxies for dispersal — developmental mode and developmental duration. Species with a larval phase are more dispersive than those that lack a larval phase, and species that spend longer developing in the plankton are more dispersive than those that spend less time in the plankton.

Here, we explore how the distribution of different development modes is likely to change based on current distributions. Next, we estimate how the temperature-dependence of development itself depends on the temperature in which the species lives, and use this estimate to project how developmental durations are likely to change in the future.

We find that species with feeding larvae are likely to become more prevalent, extending their distribution poleward at the expense of species with aplanktonic development. We predict that developmental durations are likely to decrease, particularly in high latitudes where durations may decline by more than 90%. Overall, we anticipate significant changes to dispersal in marine environments, with species in the polar seas experiencing the greatest change.

This article is part of the theme issue ‘Integrative research perspectives on marine conservation’.

Marshall DJ, Álvarez-Noriega M (2020) Projecting marine developmental diversity and connectivity in future oceans. Philosophical Transactions of the Royal Society B: Biological Sciences PDF DOI

Surviving starvation: feeding is not imperative to complete larval development for the copepod Tisbe sp.

Marine invertebrates display a range of complex life-history strategies. In general, larvae fall into one of two groups. They either meet the nutritional requirements of development by feeding during the larval phase or, they depend on nutrients supplied by the mother. Very rarely they can do both. These so-called ‘facultative feeders’ get a benefit from feeding but can, if necessary, complete larval development without food.

Alex Gangur and supervisor Dustin Marshall have found that a small crustacean can be added to this relatively short list of species that incorporate facultative feeding into the larval stage. Alex uses the copepod Tisbe sp. as his model species in a series of long-term experiments but he was surprised when he noticed that some of the larvae seemed to survive without food.

Alex and Dustin designed a series of experiments to determine if Tisbe was indeed a facultative feeder and, if so, what was the cost of completing development without food? They were also interested in how temperature might affect the outcomes as temperature is well known to have a strong relationship with larval development.

They set up a series of experiments where newly hatched larvae were assigned to vials with or without food and to one of two temperatures and individuals were monitored through metamorphosis and until they reproduced or died.

They found that a proportion of the starved copepods not only survived but went on to reproduce. But there was a cost. Development time was much longer in starved copepods compared to those that were fed and a higher temperature reduced development time in both feeding and starved copepods. The size of juveniles immediately after metamorphosis was smaller in starved copepods.

Starved copepods had reduced survival, longer development times and were smaller immediately after metamorpohosis.

Surprisingly, there was little carryover of the larval experience in the time to maturity or reproductive effort. Instead, the amount of food received as a juvenile was more important. But, more work is needed on the impacts of larval starvation on adult performance and, in particular, the effects of larval starvation on lifetime reproductive rate.

It is always difficult to extrapolate from lab experiments to real-world situations but the ability to complete larval development and metamorphosis in the absence of food likely provides an important buffer to populations experiencing fluctuating food availability.

This research is published in the journal Marine Ecology Progress Series.

Dietary preferences in filter-feeding animals might explain their crowded co-existence

An enduring concept in ecology is that space is the resource most in demand for communities living on hard substrates such as rocky shores and pier pilings. We have seen before how these communities can be extremely dense and diverse with little or no unoccupied space. But is space the whole story? These communities also need food and oxygen. How do such dense assemblages of animals manage to extract enough food to allow them to co-exist?

Belinda Comerford, Mariana Álvarez-Noriega, and Dustin Marshall have found different species of filter-feeders tend to consume one species of phytoplankton much more than others when offered a selection. They noticed studies looking at the role food plays in structuring filter-feeding communities tend to consider phytoplankton as a uniform resource. This makes no allowance for differences in size, shape or chemical make-up of the different algal species.

Belinda, Mariana and Dustin suspected that different species of filter-feeders will consume different components of the phytoplankton, reducing competition for food and allowing for the dense and diverse communities that we see in nature. So, they set about testing how different species of filter feeder consumed a mix of three different phytoplankton species that varied in size and shape and chemical make-up. 

They used 11 different species of invertebrate filter-feeding animals and offered them a mix of the three phytoplankton species. They measured the concentrations of each phytoplankton species in the animal chambers one minute and one hour after adding equal volumes of each species to the chambers. They also had control chambers that contained no animals which enabled them to estimate how much of the algae settled out to the bottom during the experimental period.

While most of the animals ingested all three phytoplankton species they did so at different rates. The encrusting bryozoan Watersipora subtorquata consumed the largest algal species at a much greater rate than it did the other two species while the sponge Sycon spp. favoured the smallest algal species. Some species such as the sea squirt Ciona intestinalis appear to be generalists, consuming all three algal species at the same rate. 

It seems that Belinda, Mariana and Dustin might be right. Thinking of phytoplankton as a homogenous resource underestimates the potential for reducing competition between filter-feeding species. If, instead of competing for a ‘common pool’ of phytoplankton, filter feeders target specific subsections then the diverse and densely packed communities that we see are more readily explained.

This research is published in the journal Oecologia.

The different invertebrates ingested the different phytoplankton species at different rates.

Is the X chromosome a hot spot for sexually antagonistic polymorphisms? Biases in current empirical tests of classical theory

Authors: Filip Ruzicka and Tim Connallon

Published in: Proceedings of the Royal Society B: Biological Sciences

Abstract

Females and males carry nearly identical genomes, which can constrain the evolution of sexual dimorphism and generate conditions that are favourable for maintaining sexually antagonistic (SA) polymorphisms, in which alleles beneficial for one sex are deleterious for the other.

An influential theoretical prediction, by Rice (Rice 1984 Evolution), is that the X chromosome should be a ‘hot spot’ (i.e. enriched) for SA polymorphisms. While important caveats to Rice’s theoretical prediction have since been highlighted (e.g. by Fry 2010 Evolution), several empirical studies appear to support it.

Here, we show that current tests of Rice′s theory—most of which are based on quantitative genetic measures of fitness (co)variance—are frequently biased towards detecting X-linked effects. We show that X-linked genes tend to contribute disproportionately to quantitative genetic patterns of SA fitness variation whether or not the X is enriched for SA polymorphisms.

Population genomic approaches for detecting SA loci, including genome-wide association study of fitness and analyses of intersexual FST, are similarly biased towards detecting X-linked effects. In the light of our models, we critically re-evaluate empirical evidence for Rice′s theory and discuss prospects for empirically testing it.

Ruzicka F, Connallon T (2020) Is the X chromosome a hot spot for sexually antagonistic polymorphisms? Biases in current empirical tests of classical theory. Proceedings of the Royal Society B: Biological Sciences PDF DOI

Differential resource use in filter-feeding marine invertebrates

Authors: Belinda Comerford, Mariana Álvarez-Noriega, and Dustin J Marshall

Published in: Oecologia

Abstract

Coexistence theory predicts that, in general, increases in the number of limiting resources shared among competitors should facilitate coexistence.

Heterotrophic sessile marine invertebrate communities are extremely diverse but traditionally, space was viewed as the sole limiting resource. Recently planktonic food was recognized as an additional limiting resource, but the degree to which planktonic food acts as a single resource or is utilized differentially remains unclear. In other words, whether planktonic food represents a single resource niche or multiple resource niches has not been established.

We estimated the rate at which 11 species of marine invertebrates consumed three phytoplankton species, each different in shape and size.

Rates of consumption varied by a 240-fold difference among the species considered and, while there was overlap in the consumer diets, we found evidence for differential resource usage (i.e. consumption rates of phytoplankton differed among consumers). No consumer ingested all phytoplankton species at equivalent rates, instead most species tended to consume one of the species much more than others.

Our results suggest that utilization of the phytoplankton niche by filter feeders is more subdivided than previously thought, and resource specialization may facilitate coexistence in this system. Our results provide a putative mechanism for why diversity affects community function and invasion in a classic system for studying competition.

Comerford B, Álvarez-Noriega M, Marshall D (2020) Differential resource use in filter-feeding marine invertebrates. Oecologia. PDF DOI

Facultative feeding in a marine copepod: effects of larval food and temperature on performance

Authors: Alexander N Gangur, and Dustin J Marshall

Published in: Marine Ecology Progress Series

Abstract

Most marine invertebrate larvae either feed or rely on reserves provisioned by parents to fuel development, but facultative feeders can do both.

Food availability and temperature are key environmental drivers of larval performance, but the effects of larval experience on performance later in life are poorly understood in facultative feeders. In particular, the functional relevance of facultative feeding is unclear. One feature to be tested is whether starved larvae can survive to adulthood and reproduce.

We evaluated effects of larval temperature and food abundance on performance in a marine harpacticoid copepod, Tisbe sp. In doing so, we report the first example of facultative feeding across the entire larval stage for a copepod.

In a series of experiments, larvae were reared with ad libitum food or with no food, and at 2 different temperatures (20 vs 24 °C). We found that higher temperatures shortened development time, and larvae reared at higher temperature tended to be smaller. Larval food consistently improved early performance (survival, development rate and size) in larvae, while starvation consistently decreased survival, increased development time and decreased size at metamorphosis. Nonetheless, a small proportion (3–9.5%, or 30–42.7% with antibiotics) of larvae survived to metamorphosis, could recover from a foodless larval environment, reach maturity and successfully reproduce.

We recommend that future studies of facultative feeding consider the impact of larval environments on adult performance and ability to reproduce.

Gangur A, Marshall D (2020) Facultative feeding in a marine copepod: effects of larval food and temperature on performance. Marine Ecology Progress Series PDF DOI

Conspecific chemical cues drive density-dependent metabolic suppression independently of resource intake

Authors: Melanie K Lovass, Dustin J Marshall, and Giulia Ghedini

Published in: Journal of Experimental Biology

Abstract

Within species, individuals of the same size can vary substantially in their metabolic rate. One source of variation in metabolism is conspecific density – individuals in denser populations may have lower metabolism than those in sparser populations. However, the mechanisms through which conspecifics drive metabolic suppression remain unclear. Although food competition is a potential driver, other density-mediated factors could act independently or in combination to drive metabolic suppression, but these drivers have rarely been investigated.

We used sessile marine invertebrates to test how food availability interacts with oxygen availability, water flow and chemical cues to affect metabolism.

We show that conspecific chemical cues induce metabolic suppression independently of food and this metabolic reduction is associated with the downregulation of physiological processes rather than feeding activity.

Conspecific cues should be considered when predicting metabolic variation and competitive outcomes as they are an important, but underexplored, source of variation in metabolic traits.

Lovass MK, Marshall DJ, Ghedini G (2020) Conspecific chemical cues drive density-dependent metabolic suppression independently of resource intake. The Journal of Experimental Biology PDF DOI

Food and chemical cues can both drive changes in metabolic rates

Metabolic rate, or energy use, changes with the size of the organism. This general pattern has been observed across different species, as well as among individuals of the same species. But while the broad pattern holds, individuals of the same species and the same size can also vary in the amount of energy they use.

Some studies have shown that individuals have lower metabolic rates as population numbers go up, but no one really knows why. Metabolic rates increase following food intake, so one plausible explanation is that competition for food in crowded conditions reduces food intake and, in turn, metabolic rates.

Melanie Lovass and her supervisors Dustin Marshall and Giulia Ghedini have run a series of experiments to investigate this possibility.

The team used the model species Bugula neritina to test their ideas. They ran a series of experiments where they were able to measure metabolic rates in individual Bugula colonies and they manipulated food, oxygen concentration, water flow and chemical cues to try and tease apart what was causing a reduction in metabolic rates in dense populations.

In the model system, each of these measures are influenced by how sparse or dense the population is. As expected, food availability affected metabolic rates but the team was surprised to find that chemical cues from individuals of the same species are also able to drive changes in metabolic rates.

Metabolic rates were lower in colonies that were starved, but metabolic rates were not affected by changes in water flow and oxygen concentrations.

This graph shows metabolic rates for <em>Bugula<em> colonies that have been exposed to chemical cues from other colonies (red) are lower than metabolic rates for the control colonies (blue).

More interestingly, Melanie and her supervisors found that metabolic rates were suppressed in Bugula colonies that were kept in ‘pre-conditioned’ water. This water had been exposed to other Bugula colonies overnight and so incorporated any chemical cues released from these other colonies. Melanie thought that chemical cues from fellow colonies might signal a reduction in feeding rates. To check if this was the case, she counted the number of feeding structures active in colonies exposed to ‘pre-conditioned’ versus normal seawater.

They found no differences in feeding rates indicating that the chemical cues from Bugula colonies were suppressing physiological processes rather than reducing feeding rates. So, while the chemical cues from other Bugula colonies reduce energy use, this reduction is in processes other than feeding activity.

While searching for food is energetically costly; keeping up feeding activity may be worth the costs and become even more important when access to food is very competitive.

Bugula are colonial organisms made up of ‘modules’. Each module has its own feeding structure – a crown of tentacles – known as a lophophore. There was no difference in the number of active lophophores in the colonies exposed to chemical cues compared to the control colonies. Photo credit: Amy Hall.

This research is published in the Journal of Experimental Biology.