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

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.

Estimating energy use in communities

As ecologists, we often want to get an idea of how a community functions. For example, how much food does a community of animals consume every day? Or how much oxygen do plants produce every day? We can get an idea of these functions by measuring the energy use (or metabolism) of a community. But such functional measures can be difficult to collect, especially for an entire community. 

Since a community is made up of many species, estimating the total metabolism from the metabolism of each species separately is a way around this problem. But the methods we use, and the data required, vary and may not be validated against actual data because these are rarely available. 

Giulia Ghedini, Martino Malerba and Dustin Marshall have set out to test six different ways of estimating the energy use of a community. The authors measured actual metabolic rates in communities of phytoplankton (tiny marine microalgae) seeded with six different species of various sizes and left to change through time.

Previous work from the CGB meant the team had all the information they needed to estimate community energy use for the six methods. The different methods are summarised below.

There are a number of different ways we can estimate community metabolism that vary in complexity.

It turns out that when we are interested in whole community metabolism, size doesn’t matter. Why? Because larger species tend to be less abundant within communities, so even if each of them consumes more energy, the total energy use remains the same because there are fewer of them. While we knew this was the case for well-established communities, this new work shows that it is also true for communities as they change through time.

What is more, the team found that, in communities, the usual relationship between size and metabolism changed. Metabolism usually increases with size, but to a lesser extent for larger organisms (this is called an allometric relationship). This study showed that when measured across all species in the community, this relationship changed and the average energy use of species increased in direct proportion to their average size (this is an isometric relationship). This is why we can predict the total metabolism from the energy use per unit biomass. 

But towards the end of the experiment, when communities were dominated by larger cells, this ‘perfectly proportional’ relationship broke down. Giulia and her colleagues think that, when large species are very abundant, they suffer more from competition and reduce their metabolism more than smaller species. 

So, the good news is that we may be able to estimate community energy from easy to collect biomass data, but first, we need to see if this result applies to different communities. We also need more studies on how competition affects energy use, as the results from this study suggest they are important drivers of energy flow.

All of the methods are based on underlying theory so testing which methods are best at estimating actual metabolism can help us understand how energy use works in communities. This figure summarises what theory predicts we should have found compared to what we actually found.

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

How to estimate community energy flux? A comparison of approaches reveals that size-abundance trade-offs alter the scaling of community energy flux

Authors: Giulia Ghedini, Martino E Malerba, and Dustin J Marshall

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

Abstract

Size and metabolism are highly correlated, so that community energy flux might be predicted from size distributions alone. However, the accuracy of predictions based on interspecific energy–size relationships relative to approaches not based on size distributions is unknown.

We compare six approaches to predict energy flux in phytoplankton communities across succession: assuming a constant energy use among species (per cell or unit biomass), using energy–size interspecific scaling relationships and species-specific rates (both with or without accounting for density effects).

Except for the per cell approach, all others explained some variation in energy flux but their accuracy varied considerably. Surprisingly, the best approach overall was based on mean biomass-specific rates, followed by the most complex (species-specific rates with density).

We show that biomass-specific rates alone predict community energy flux because the allometric scaling of energy use with size measured for species in isolation does not reflect the isometric scaling of these species in communities. We also find energy equivalence throughout succession, even when communities are not at carrying capacity.

Finally, we discuss that species assembly can alter energy–size relationships, and that metabolic suppression in response to density might drive the allometry of community energy flux as biomass accumulates.

Ghedini G, Malerba ME, Marshall DJ (2020) How to estimate community energy flux? A comparison of approaches reveals that size-abundance trade-offs alter the scaling of community energy flux. Proceedings of the Royal Society B: Biological Sciences PDF DOI

Cell size, genome size and the selfish gene

Classic theories such as the ‘selfish gene hypothesis’ are not always easy to test. If true, then the theory proposes that cells accumulate extra DNA over time even when they provide no benefit to the organism – selfish genes. This means individuals with more DNA relative to their cell size should have reduced fitness. What is more, because the accumulation of DNA creates a burden on the organism, a good evolutionary strategy should be to reduce the amount of redundant DNA over time.

In one of the first tests of the theory using a single species, Martino Malerba, Giulia Ghedini and Dustin Marshall have found direct evidence that reduced amounts of total DNA (genome size) is associated with fitness benefits in a species of marine microalgae.

Scientists face a problem when they compare cell size and genome size between different species. It is hard to separate whether the slower rates of metabolism, development and growth are a result of genome size or the fact that larger organisms tend to have a slower pace of life anyway.

Martino and his colleagues were able to overcome this issue by using their evolved lines of the marine phytoplankton Dunaliella tertiolecta where cells have been artificially selected to be large or small. They were able to use a staining technique to measure the amount of DNA content within cells of different sizes but, importantly, of the same species and the same evolutionary age.

They weren’t surprised to find larger cells had more DNA. But they also found that, in cells of the same size, the cells with smaller genomes grew faster and accumulated more biomass; that is, had greater fitness.

So, this first finding was consistent with the prediction that reducing the amount of DNA in a cell can have positive effects for the fitness of a species. But having demonstrated that minimizing DNA can improve fitness, the team then wanted to test the prediction that species should decrease their DNA content as they evolve. Again, they were able to test this by using the Dunaliella and monitoring the evolving lines for a year or approximately100 generations.

Their results confirmed that cells decreased their DNA content by up to 11% across 100 generations of evolution. However, they also found that cells with already low amounts of DNA showed no change over time, which suggests the existence of an absolute lower limit in the DNA content of this species.

Overall, they have direct evidence for fitness benefits associated with reduced relative genome size consistent with the selfish gene hypothesis and as well as a minimum genome size below which an organism can’t maintain functionality.

This figure looks at how genome size or cell DNA content changes with cell size. The continuous black line shows the relationship between the two for the large, small and control algae after 350 generations of evolution. The broken line shows the relationship after 450 generations of evolution – 1 year later. Notice how the cell DNA content has reduced after 1 year of evolution.

This research was published in the journal Current Biology.

What role does oxygen supply play in determining a species’ thermal tolerance?

It is generally recognised that animals perform best at certain temperatures, so-called optimal temperatures. To understand how measures of performance, such as growth or running ability, will change with changing temperatures we need to understand the physiological processes limiting performance.

One compelling theory, known as the oxygen and capacity-limited thermal tolerance (OCLTT), suggests that a reduction in oxygen availability limits performance.

As temperature shifts away from optimal, the demand for oxygen in tissues is thought to outpace the rate of supply. This means a shift to anaerobic metabolism, a process far less efficient than aerobic metabolism, causing a reduction in performance.

Emily Lombardi, Candice Bywater and Craig White wanted to test the theory and used the speckled cockroach as their experimental animal because they are easy to breed and keep in the lab. Unlike other studies testing the OCLTT hypothesis, their interest was in the less extreme ends of the temperature range, which the OCLTT hypothesis specifically addresses.

Emily, Candice and Craig predicted that performance would be reduced (↓) in hypoxic environments and sub-optimal temperatures but increased (↑) in hyperoxic environments at optimal temperatures and stay the same (−) as atmospheric oxygen conditions at optimal temperatures when in a hyperoxic environment but at sub-optimal temperatures.

First, Emily and her colleagues calculated the temperature that optimised growth by allowing juvenile cockroaches to develop for 35 days at 8 temperatures ranging between 10 and 36 °C. They also determined the temperatures (both above and below the optimal temperature) where growth was reduced by 32%. These 3 temperatures (optimal, lower and upper developmental temperatures) were then used in oxygen manipulation experiments.

Next, they ran an experiment including the three temperatures plus three oxygen concentrations; atmospheric (21%), hypoxic (10%) and hyperoxic (40%). Cockroaches were assigned to one of the 9 treatments and growth rate was measured repeatedly over 5 weeks. Running performance – time on a treadmill – was measured at the end of the 5 weeks at the three temperatures. Tracheal morphology was also quantified because, in some species, changes to the oxygen delivery system can alleviate the demand for increased oxygen supply.

If the OCLTT theory held, then the team expected to see a difference in performance at the different oxygen concentrations at each temperature. But, instead, they found increasing oxygen concentrations did not mitigate the effects of sub-optimal temperatures. There was also no evidence that tracheal morphology changed as a result of the developmental temperature or oxygen environment. The team concluded that oxygen supply was not the main determinant of temperature-related performance limitations for the speckled cockroach.

It seems, the OCLTT may not provide the unifying theory its proponents hoped, but instead, a species’ thermal tolerance is likely shaped by a range of factors.

In fact, they found that growth did not differ between oxygen concentrations at the lower temperature, but growth was lower in hypoxic environments at optimal temperatures when compared to atmospheric oxygen concentrations and lastly growth was lower in hyperoxic environments at high temperatures compared to atmospheric oxygen concentrations. All points that share a letter are not significantly different from one another.

This research was published in the Journal of Experimental Biology.

Competition plays a part in maintaining variation in metabolic rates

We know that the rate at which organisms use energy (metabolic rate) varies substantially between individuals of the same species, even after accounting for size and temperature. What we are less sure about, is, why we see this variation.

When Amanda Pettersen and her colleagues thought about this question they considered it plausible that the competitive environments that individuals find themselves in might be important in determining whether a faster or slower metabolic rate is selected for. They wanted to find out whether variation in competition affects selection on metabolic rates and whether that could account for the variation in metabolic rate that persists more generally.

To test this idea Amanda used the model species Bugula neritina because it allowed the team to collect larvae in the lab, measure size and metabolic rate of larvae before assigning the larvae to one of three ‘competition’ treatments. Once the larval measurements were made, larvae were randomly assigned to either a no-competition environment, a competitive environment where they were put with other Bugula neritina (intraspecific competition) or, a competitive environment with an established mixed-species community (interspecific competition).

Individuals were then returned to the field and monitored weekly for survival, growth, age when first become reproductive and fecundity (total reproductive output).

Following measurements of size and metabolic rate, larvae were settled on small plates that were either kept bare (no competition), had other Bugula on the plate (intraspecific competition) or had an established community already there (interspecific competition). Plates were submerged in the field and monitored weekly.

The team already knew that higher metabolic rates are linked to faster growth, earlier onset of reproduction, and a shorter lifespan, while low metabolic rates are associated with a slow pace-of-life (slow growth, late onset of reproduction and long lifespan).

This experiment showed that individuals with higher metabolic rates were more likely to survive, more likely to reproduce and had greater numbers of ovicells at the start of reproduction, in the more intense, interspecific competition treatment. The team speculates that the individuals with higher metabolic rates grow more quickly enabling them to reach resources such as food and oxygen that are less available to smaller, slower growing organisms. There is a downside: these higher metabolic rate individuals also had a shorter lifespan.

The expectation amongst evolutionary biologists is that where there is strong selection pressure for a particular trait then the variation within that trait is reduced. So even accounting for the shorter lifespan, the overall increase in reproduction should mean that a higher metabolic rate is strongly selected for in competitive environments.

But the team also found that individuals with lower metabolic rates had a higher probability of living for longer in the absence of competition and would have continued to reproduce long after the individuals with a higher metabolic rate had died.

Bugula live on hard substrates and these areas commonly vary in their composition. Complex three-dimensional, highly diverse communities are interspersed with sparse populations of a few species and patches of bare areas. Individuals with lower metabolic rates that happen to arrive at a bare patch will have a long reproductive period and high overall fecundity. In contrast, individuals with a high metabolic rate that settle in amongst other species will be reproductive quickly and so successfully produce large numbers of offspring.

Because Bugula larvae are likely to find themselves in a variety of different competitive environments – even when they settle relatively close to each other – there is a strong evolutionary argument to explain the persistence of variation in metabolic rate, for Bugula at least.

This research is published in the journal Evolution Letters.

You are what you eat, but does it matter when you eat it?

We all know the expression “you are what you eat” but do juvenile diets determine your adult size or can adult diets change things? Gonçalo Poças, Alexander Crosbie and Christen Mirth have used a model organism – the fruit fly Drosophila melanogaster – to explore this question.

They found that while larval nutritional conditions play the dominant role in determining both adult body weight and appendage size in D. melanogaster, the adult diet can adjust body weight as the flies age.

The team manipulated both the quality and energy quantity of diets that larval and adult flies received in two experiments.

In the first experiment calorie content was held stable but the ratio of protein to carbohydrate was either 1:2 (high quality) or 1:10 (low quality). In the second experiment flies were fed a high quality diet but with two different calorie contents. The low calorie content diet was 25% of the high calorie diet.

Importantly, larvae were reared on one of the two diets in each experiment and then after “eclosing” (emerging as an adult) they either remained on the same diet or were switched to the alternative diet. This allowed the researchers to tease apart at what life-stage diet was most influential in determining measures of adult size.

Adult weights, wing area and femur size were measured at 3, 10 and 17 days old. The team predicted that adult weight would be more influenced by adult diet but that wing area and femur size would be determined by larval diet.

Schematic of the experimental design. Larvae were fed alternative diets in two experiments that looked at the quality of the diet or the energy content of the diet. Once flies emerged as adults they were kept on the same diet or switched to the other diet and measures of size were taken at 3, 10 and 17 days.

For the most part, the team found that the larval diet contributed more to differences in adult weight, wing area, and femur size in both males and females, but the quality of the larval diet had greater effects on adult size than the calorie content.

In addition to the effects of larval diet on adult size traits, Gonçalo and his colleagues found that animals subjected to poor larval nutrition were able to increase their body weight if maintained on good quality diets during the adult stages.

One surprising result the team uncovered was that the adult femur size changed with age, and depended on both the larval and adult diet. This suggests that the size of adult appendages might not be as fixed as previously thought.

So, to return to our initial question. Yes, the juvenile diet is very important in determining measures of adult size, but adult diets can make up for nutritional deficiencies in earlier life to some extent – in fruit flies at least.

This research is currently in press in the Journal of Insect Physiology.