Researcher spotlight

Life history of live-bearing fish in Rio de Janeiro

Hi everyone, it’s me

Welcome to Cardio & Conservation - Researcher Spotlight! In these posts I ask fellow students, academics, and industry professionals to make their science accessible. I want you to hear of the amazing work scientists are doing, in easy to understand terms - with the hope that this will help to increase interest and understanding of often complex studies.

Our first Spotlight comes from Oliwia Konecka - a lab-mate at RHUL who is embarking on her Master’s Research project on live-bearing fish.

Over to Oliwia!

Hello! My name is Oliwia and I am starting my masters journey this year (WOO)!

So, a little introduction to me. Earlier this year I graduated from Royal Holloway with a first-class Zoology degree. After 3 years of incredibly hard work and dedication, I was incredibly proud of myself and of so many of my friends.

My final year was definitely something.

I did my final year project on how light pollution affects the distribution of small mammals, finding that, unsurprisingly, small mammal populations tend to decrease with light pollution, however this effect was much more prominent with streetlights (rather than motorway lights), and with females rather than males.

I could write so much more but that research is not what this post is about.

I have a major interest in evolutionary ecology and how an animal's environment can impact them over time. With our current changing world, where never before seen temperatures are being reached, and habitats are being altered like never before, the importance of this research field is growing.

I am now starting my Masters in Biological Sciences by Research, looking at how differences in salinity and competition impact the life-history characteristics of two species of live-bearing fish, the Southern Molly (Poecilia vivipara) and the January Toothcarp (Phalloptychus januarius), in coastal Rio de Janeiro.

Rio de Janeiro, Brazil. Oliwia’s research will focus on two species of fish found in coastal Brazil. [Unsplash]

Okay, now what does that actually mean? Well, these fish live in waters that vary extremely in how salty they can be, and in some cases, these fish inhabit these waters together and in some alone. So, my goal is to see how some of the characteristics and traits they develop or show throughout their life differ in these different habitats. This will give us an idea as to how certain factors may affect certain organisms.

This will involve going to different lagoons in Brazil where these fish are present and testing the salinity (and other factors) of the water and whether one species is present alone or with the other. Then, net sampling a few males and females, to take back to the lab, where I measure, weigh, and dissect them. Dissection and desiccation of their reproductive organs and any offspring (because “live-bearing” fish produce live young rather than laying eggs) will tell me how much of their resources they will delegate to reproduction compared to their own survival.

For example, will fish in more saline environments prefer quantity or quality of offspring? Will they have a higher fat content? Higher relative mass? Similar questions can be asked regarding competition. Hopefully, I will find some answers, and maybe I can relay them back to you at some point.

This research is important because the results may allow us to predict how animals may respond to a change in their environment. In this case, how live-bearing fish respond to salinity and competition differences.

Freshwater habitats are quite vulnerable, as human and natural activity is resulting in salinisation of many freshwater bodies. Understanding how competition affects animals is also vital, due the global threat of invasive species (e.g. freshwater guppies in much of South and Central America) and how habitats are changing. Animals may be pushed to locations not previously seen before, dealing with new competitors and predators, and therefore being forced to adapt, or suffer the consequences.

Wish me luck on my journey!

I hope you enjoyed hearing about Oliwia’s project - Oliwia will be heading over to Brazil next month (November 2025) to undertake her field research. I’m hoping to catch up with Oliwia, and all of the other student researchers who we spotlight on the blog, to see the outcomes of their research. And, I will shortly be posting an author Q&A, watch out for that below.

In the meantime, if you have any questions for me or Oliwia, any suggestions, feedback or interesting topics for the blog, please get in touch.

Until next time!

Researcher Q&A

Following every Researcher spotlight post, I will ask our researchers a few questions about their post, their research, or interesting points which warrant further information. New spotlight posts will have a call for reader questions at the bottom - keep an eye out for these if you have something in particular you want to ask our Researchers!

For your final year project for your undergraduate degree, you mention the effect of light being more prominent in females than males. I'm interested to know what is likely the meaning of this?

My primary hypothesis for why females were more impacted by artificial light at night relates to how females provide all the parental care in these two small mammal species. Therefore, female fitness may be a big driver in the success and survival of young, as the female’s survival throughout the weaning stage of young is vital. A better adapted female is more likely to survive through the weaning stage, successfully wean her offspring, and then genetically those young are likely to have some of those advantageous adaptations. 

Could you provide an explanation of the term 'evolutionary ecology' for readers?

Evolutionary ecology is the blend of ecology and evolutionary biology (wow who could’ve guessed) - often looking at how evolution and its principles have led to the world we see today. A big part of this science is trying to identify the rules and patterns nature follows that result in certain characteristics or behaviours, allowing evolutionary ecologists to theorise how our biodiversity came about and what things may look like or do in the future. 

Why do you need to dissect the fish to obtain life-history characteristics? Is this necessary? And what are the ethical processes you have to consider?

Dissections are not always needed to obtain life-history characteristics. A life- history characteristic is any factor of an organism's stages of growth, reproduction, dispersal, and survival – more simply any characteristic of its life cycle. How fast does it grow? How large was it when it was born? How late is sexual maturation? How many offspring will it have? These factors are often impacted by its environment. 

I personally need to dissect the fish because part of the life history characteristics I am looking at is the fish’s reproductive allocation. I am looking partially at the relationship between a female and its offspring, and how much resources it employs for its own survival and how much the survival of its young in utero. This includes how many embryos it has, how many are fertilised, what different stages of development occur in it’s uterus, what portion of its resources has it deferred to the offspring, how large are the offspring at different stages? - All factors I need to dissect the fish for.

Many ethical considerations need to be made. An ethics form needs to be submitted and approved by an ethics board to ensure what I am doing doesn’t breach any major ethical concerns. Some ethical considerations are species specific – is this species endangered in any way? How will removing individuals of this species affect the local ecosystem? How many are suitable to take? Whilst some are more individual specific – will this individual undergo pain or discomfort? Is this research worth taking the life of this individual? 

In my circumstance the fish are very common, inhabiting a variety of locations across South America, I will also only be taking a handful of individuals, both males and females, as sample from each area, ensuring that both their populations and the ecosystem remain balanced. The individuals will be sacrificed upon removal from the water using anaesthetic – a certified humane practice that only takes a few seconds. Additionally, looking at how individuals may adapt to different habitats and environmental factors may provide science a better understanding of how organisms may adapt to climate change and the constant change in the environment humans are causing.  

What is the concern over invasive species, in Brazil and across the globe in general?

Invasive species are one of the largest threats to global biodiversity that does not get talked about enough. Some species are so well adapted to so many environments, or can adapt to so many, that they become a threat to the new land they inhabit. 

They may monopolise and take all the resources away from native organisms (mosquitofish). They may breed faster and have populations grow at a rate that native organisms cannot compete with (muntjac deer). They may predate native organisms (domestic cats). They may carry diseases that organisms in the habitat have not previously been exposed to (grey-squirrels). These are all world-wide problems, and mostly human-caused. Humans have released so many organisms to parts of the world they do not belong, having the interactions organisms have built between each other over millenia ruined by an unwelcome intruder. 

One of the best examples to me that explains the concern with invasive species is New Zealand bird populations. New Zealand, indigenously known as Aotearoa, has one native mammal; bats. Native New Zealand birds evolved over millenia with no native predators, meaning many don't have predator responses or defence mechanisms. Therefore, when colonisation brought about insane numbers of new species to New Zealand, many of the native wild birds did not stand a chance against their new predators, resulting in massive population declines. Predator Free 2050 projects aim to reduce the damage, however a large amount of the damage will never be reversed. This is why science into invasive species, and often what makes a species invasive, is incredibly important in our current climate.

Thank you so much Oliwia for taking the time to answer these questions!