Global environmental drivers of marine fish egg size

Authors: Diego R Barneche, Scott C Burgess, and Dustin J Marshall

Published in: Global Ecology and Biogeography

Abstract

Aim: To test long‐standing theory on the role of environmental conditions (both mean and predictability) in shaping global patterns in the egg sizes of marine fishes.

Location: Global (50° S to 50° N).

Time period: 1880 to 2015.

Major taxa studied: Marine fish.

Methods: We compiled the largest geo‐located dataset of marine fish egg size (diameter) to date (n = 1,078 observations; 192 studies; 288 species; 242 localities). We decomposed sea surface temperature (SST) and chlorophyll‐a time series into mean and predictability (seasonality and colour of environmental noise – i.e. how predictable the environment is between consecutive time steps), and used these as predictors of egg size in a Bayesian phylogenetic hierarchical model. We test four specific hypotheses based on the classic discussion by Rass (1941), as well as contemporary life‐history theory, and the conceptual model of Winemiller and Rose (1992).

Results: Both environmental mean and predictability correlated with egg size. Our parsimonious model indicated that egg size decreases by c. 2.0‐fold moving from 1 to 30 °C. Environments that were more seasonal with respect to temperature were associated with larger eggs. Increasing mean chlorophyll‐a, from 0.1 to 1 mg/m3, was associated with a c. 1.3‐fold decrease in egg size. Lower chlorophyll‐a seasonality and reddened noise were also associated with larger egg sizes – aseasonal but more temporally autocorrelated resource regimes favoured larger eggs.

Main conclusions: Our findings support results from Rass (1941) and some predictions from Winemiller and Rose (1992). The effects of environmental means and predictability on marine fish egg size are largely consistent with those observed in marine invertebrates with feeding larvae, suggesting that there are important commonalities in how ectotherm egg size responds to environmental change. Our results further suggest that anthropogenically mediated changes in the environment will have profound effects on the distribution of marine life histories.

Barneche DR, Burgess SC, Marshall DJ (2018) Global environmental drivers of marine fish egg size, Global Ecology and Biogeography PDF DOI 

Research fellow position: ecologist specialising in life history theory

Update: this position has now been filled.

  • Level A, research-only academic
  • $66,706 to $90,532 pa + 9.5% superannuation
  • Full-time, starting late 2018
  • Two-year, fixed-term
  • Monash University Clayton campus

The Centre for Geometric Biology is currently seeking to recruit an experienced ecologist who specialises in life history theory. This position will be with Professor Dustin Marshall and based within the School of Biological Sciences at Clayton Campus.

As the successful candidate, you will be expected to use existing datasets to investigate evolutionary patterns both within and across species, 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. The appointee will have a doctoral qualification in life history theory or evolutionary ecology
  2. Demonstrated analytical and manuscript preparation skills; including an excellent track record of refereed research publications in high impact journals
  3. Demonstrated experience in asking questions about life history theory 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 affect the maintenance of a constant body temperature?

Staying warm is a subject close to many of our hearts during winter and we probably wouldn’t be surprised to hear that animals from colder climes have higher rates of energy expenditure.  But is this true for all species, or is it more complicated than that?

Researchers from Uruguay, Daniel Naya and Hugo Naya, have joined forces with Craig White from the Centre for Geometric Biology to investigate how body mass in mammals affects the relationship between energy expenditure and climate. They found that, yes, energy expenditure was greater for species that live in colder regions but only in mammals smaller than 100 g. The effect became less marked as the animals got bigger. 

The Basal Metabolic Rate (BMR) is a measure that represents the minimum amount of energy needed to maintain a relatively constant body temperature through active heat production.  It has been repeatedly demonstrated that there is a negative correlation between temperature and residual BMR in mammals and birds.

So where does body mass come into all this?  Over half of all mammals weigh less than 100 g although the range in body mass for mammals scales over 8 orders of magnitude. Also, smaller animals are generally easier to work with in the laboratory and so it is likely that much of our data on BMR in mammals comes from smaller species.

In order to untangle the effects of size on the ‘Temperature – BMR’ relationship, Daniel, Hugo and Craig looked at existing data on 458 mammal species. They compiled data on body mass, BMR and temperature from each collection site.  Their data set, as expected, included many more small species than big ones. What is more, their prediction that smaller species would be more dependent on adjustments in BMR to cope with lower temperatures, was confirmed.

There are other ways to maintain constant body temperatures apart from exerting more energy or increasing the Basal Metabolic Rate. Some examples include physiological adjustments, such as the separation of core and outer temperatures through peripheral vasoconstriction.  Behavioural adjustments, such as building / using shelters or changing activity levels can also help maintain body temperatures. Body shapes (surface to volume ratio), body colour, and the properties of body fat and skin can all affect heat retention, absorbance and loss.

Smaller species may have less scope for accessing these alternative solutions. This is because their smaller size may place restrictions on their adoption; including both physical restrictions (fur thickness is limited by body size) and biological restrictions (colour change or activity changes can increase predation risk). Such factors may mean that smaller mammals are more dependent on basal heat generation as a means of maintaining a constant temperature than are larger mammals.

The research team are keen to see if the same pattern of strong Temperature – BMR relationships at smaller body mass but not at bigger body mass, hold true with birds as well.

This research was published in The American Naturalist.

Student session: Applying for a postdoc

Not all PhD students want to pursue a career in academia; some definitely do, while others feel that they would like to further their academic training through doing a postdoctoral fellowship before moving into industry or other fields.

But how do you go about getting a postdoc? 

Professors Dustin Marshall and Craig White will be speaking about their experiences as academics looking for postdocs, and we invite students and interested early career researchers to join us armed with questions about how to go about getting a postdoc, what to expect from a postdoc and ‘conversations you should have’ when starting a postdoc.

When: 2 pm, Thursday 23 August 2018

Where: Sanson Room (22 G01/02) Rainforest Walk, Monash University Clayton

Resources mediate selection on module longevity in the field

Authors: Karin Svanfeldt, Keyne Monro, and Dustin J Marshall

Published in: Evolutionary Biology

Abstract

The life histories of modular organisms are complicated, where selection and optimization can occur at both organismal and modular levels.

At a modular level, growth, reproduction and death can occur in one module, independently of others. Across modular groups, there are no formal investigations of selection on module longevity.

We used two field experiments to test whether selection acts on module longevity in a sessile marine invertebrate and whether selection varies across successional gradients and resource regimes.

We found that selection does act on module longevity and that the strength of selection varies with environmental conditions. In environments where interspecific competition is high, selection favours colonies with longer zooid (module) longevity for colonies that initially received high levels of maternal investment. In environments where food availability is high and flow rate is low, selection also favours colonies with longer zooid longevity.

These patterns of selection provide partial support for module longevity theory developed for plants. Nevertheless, that selection on module longevity is so context‐dependent suggests that variation in module longevity is likely to be maintained in this system.

Svanfeldt K, Monro K, Marshall DJ (2018) Resources mediate selection on module longevity in the field, Journal of Evolutionary Biology PDF DOI 

Sizing-up the impacts of fluctuating resources on body size

Recent work in the Centre for Geometric Biology has found that smaller algal cells had slower growth, lower storage of phosphorous and poorer recovery from phosphorous depletion but, interestingly, there was no effect of size when nitrogen was limiting.

Resource levels, such as food or nutrients, are rarely constant in nature but tend to fluctuate through time and across space. Such fluctuations in resources might have different impacts on organisms of different sizes but current ecological theories differ in their predictions of how the evolution of body size will be influenced by pulse inputs of food or nutrients.

While this is theoretically interesting, there is also a more pressing need for improving our understanding of such geometric biology. Phytoplankton cells are becoming smaller as a result of increased temperature and ocean acidification and we need to be able to better predict the consequences of this size shift under varying levels of resources.

Martino Malerba, Maria Palacios and Dustin Marshall have been able to test predictions from three different models of resource-use by using algae that have been genetically modified. They used 280 generations of artificial selection to create larger and smaller phytoplankton cells differing by as much as 1000% in mean body size.  

Cells were exposed to various resource levels by manipulating nitrogen (N) and phosphorous (P) in the growth media, to quantify how size can influence the ability of a species to cope with unpredictable nutrient conditions. 

Martino and his colleagues considered three different ecological theories that differ in their predictions on how size should mediate responses to fluctuating resources.  

  1. The ‘Fasting Endurance Hypothesis’ would predict that larger cells are more buffered against periods of nutrient limitation.
  2. The classic ‘r-K Selection Theory’ predicts that smaller cells with faster generation times will be better placed to take advantage of a nutrient pulse and so recover quickly from periods of nutrient limitation.  
  3. The ‘Metabolic Theory of Ecology’ would predict that tolerance to nutrient deprivation would decrease with increasing mass specific metabolic rate of an organism.

For the phytoplankton species used in this study (Dunaliella tertiolecta), the mass specific metabolic rate increases with size which means that larger cells should grow faster but be less tolerant to nutrient depletion than smaller cells. 

So which theory turned out to be correct?  The team found that periods of P depletion had a greater negative effect on smaller cells as predicted by the ‘Fasting Endurance Hypothesis’ but there was no effect of size on response to N depletion which was not predicted by any of the theories. 

Overall Martino, Maria and Dustin were able to determine that size interacts with stored resources in different ways. Increasing size can promote the ability to use stored P to supplement growth in D. tertiolecta, whereas the ability to store and utilise N does not change across sizes.

Transmission Electron Microscopy (TEM) photos with false coloring showing different densities of internal nutrient reserves between evolved algal cells from small-selected and large-selected lineages.
Mean population growth rate (with 95% confidence intervals) for each size-selected lineage of Dunaliella tertiolecta after experiencing nutrient-replete (red), nitrogen-deplete (green) or phosphorous-deplete conditions (blue) pre-trial conditions.

This research was published in Proceedings of the Royal Society B.

Do larger individuals cope with resource fluctuations better? An artificial selection approach

Authors: Martino E Malerba, Maria M Palacios, and Dustin J Marshall

Published in: Proceedings of the Royal Society B

Abstract

Size determines the rate at which organisms acquire and use resources but it is unclear what size should be favoured under unpredictable resource regimes.

Some theories claim smaller organisms can grow faster following a resource pulse, whereas others argue larger species can accumulate more resources and maintain growth for longer periods between resource pulses. Testing these theories has relied on interspecific comparisons, which tend to confound body size with other life-history traits.

As a more direct approach, we used 280 generations of artificial selection to evolve a 10-fold difference in mean body size between small- and large-selected phytoplankton lineages of Dunaliella tertiolecta, while controlling for biotic and abiotic variables. We then quantified how body size affected the ability of this species to grow at nutrient-replete conditions and following periods of nitrogen or phosphorous deprivation.

Overall, smaller cells showed slower growth, lower storage capacity and poorer recovery from phosphorous depletion, as predicted by the ‘fasting endurance hypothesis’. However, recovery from nitrogen limitation was independent of size—a finding unanticipated by current theories.

Phytoplankton species are responsible for much of the global carbon fixation and projected trends of cell size decline could reduce primary productivity by lowering the ability of a cell to store resources.

Malerba ME, Palacios MM, Marshall DJ (2018) Do larger individuals cope with resource fluctuations better? An artificial selection approach, Proceedings of the Royal Society B, PDF DOI