Do sharks hang out with their siblings?
Show notes
In this episode of World of Sharks, we’re chatting all things spurdog, relatedness and genetics with molecular biologist and PhD student Fenella Wood! Fenella’s research delves into the fascinating world of spurdog population genetics to find out if the individuals that form large aggregations are related, which could have conservation implications if they are caught as by-catch. We learn all about shark DNA, life at sea, and what it was like to work at the legendary Bimini Biological Field Station Foundation, AKA Shark Lab.
We start by getting to know Fenella and her most memorable experience in the ocean [5.06]. As with all our guests, this is a tough question, but Fenella has managed to narrow it down to her first experience with a spectacular natural phenomenon: bioluminescence. While in the Bahamas, she was conducting fieldwork in the middle of the night when she realised that the boat’s wake was lit up by electric blue bioluminescent plankton. This became a regular occurrence during her time at Bimini but never lost its magic.
Before interning at the Shark Lab, Fenella spent her childhood obsessed with animals and the water [07.15]. At the age of ten, she visited an aquarium and discovered the world of marine biology after speaking to a staff member, and she became determined to become one herself. She also remembers watching a documentary on the shark fin trade at a young age and felt strongly that she needed to ‘fight their corner’. Years later, she studied Zoology at university and kept pursuing opportunities in marine science, which led to a placement at the Bimini Shark Lab [12.00]. This was a really formative experience for Fenella, as it was the first time she got to work with sharks in the field and exposed her to the field of genetics. She learnt about a previous research project on natal philopatry in lemon sharks and found it completely fascinating – from that moment, she focussed on molecular work.
Fenella’s work now looks at the tiniest but arguably the most important part of a shark. But what can we learn from a shark’s DNA [16.21]? Fenella describes DNA as the ‘molecular blueprint’ of an animal that dictates pretty much everything about it, from looks to behaviour. There is an incredible amount that we can learn from even just a small segment of DNA, and it can tell us things not just about individuals but also whole species and even groups of species. For example, on an individual level, we can identify what species a part of a shark, like a fin, belongs to (which is particularly useful when looking at international trade), learn about an individual’s health, or what an individual is eating. On a population level, we can also determine whether populations are mixing – in a very basic sense, this refers to whether sharks are mating within the same population or travelling and mating with individuals from different populations (we’ll explain why this is important later on). And we can tell who is related to who. And at a species level, we can identify species that look almost identical to one another [18.34]. In some cases, the only way to tell two species apart is by looking at their DNA, and many species have been distinguished in recent years thanks to advances in molecular techniques. And we can also understand the evolutionary history of a species or population just by looking at their genetics. It’s like unlocking a secret code that can tell us so much about an animal’s life – and that of its predecessors.
Fenella’s work focuses on population genetics, which revolves around questions about how populations in different places are connected to each other genetically [22.35]. For example, are individuals in different places related? Are these different populations mating or mixing? How are genes being passed and shared between populations? These questions are important to understand for conservation and management.
Fenella is investigating these kinds of questions relating to the spurdog, and before we get into her research, we talk about the species itself [26.30]. A spurdog, or spiny dogfish, is a relatively small species of shark that exists in temperate waters around the globe. Despite their small size, they have one of the longest gestation periods (the period of time between conception and birth) of any vertebrate – females can be pregnant for up to two years, which is similar to that of an elephant! They are ovoviviparous, meaning that the young grow inside the uterus and are nourished by an egg rather than via placenta, but the egg hatches inside the mother and the young are effectively born live. Another fun fact: they are venomous [29.00]! They have venomous spines located just behind the dorsal fin, which gives them their name.
Spurdogs are also a commercially traded species and were once heavily fished in UK waters to be sold in the popular British dish, fish and chips [29.15]. This is not so common now, as in 2010, there was a ban on retaining spurdogs in the North Atlantic. There is now a small quota, and they are not as targeted as they once were – but they are still caught in huge numbers as by-catch. The reason for this is because they aggregate in very large numbers, making them hard to avoid when using non-selective fishing methods [30.58]. According to IUCN, the spurdog is classed as Endangered in the North Atlantic, having experienced significant declines since the 1970s due to increased fishing pressure, to which their aggregating nature makes them particularly vulnerable [32.05].
For her PhD research and her SOSF project, Fenella is interested in finding out why these aggregations form and their genetic composition [32.51]. The spurdog is a highly mobile species, and previously the most common theory was that they aggregated due to the presence of food or for mating. But research led by James Thorburn at the University of Aberdeen found that, within a single aggregation that had been caught, siblings were present. Fenella wanted to delve into this further but lacked the specimens to do so – until a rare event in 2015 [36.05]. At that time, in the Isles of Scilly, in the South of England, there was an abnormally large by-catch event, where an estimated ten tonnes of spurdog was caught in a single net. This was so heavy that the fishermen were physically unable to move the net or pull it back on board and had to be towed back to shore! Unfortunately, the spurdog were unable to be released, and so were landed – but it did provide a rare opportunity to research such a large aggregation. Fenella was able to take tissue samples, which she is now using to investigate their genetics.
Although Fenella isn’t quite finished with her analysis (that’s a lot of samples to get through!), she can reveal that she is finding related sharks from that one aggregation, and more than she was expecting [41.51]. She has found half-siblings, first cousins, and half-first cousins. And she has found family clusters – meaning that one individual may be related to several others. This could mean that the sharks forming these aggregations have been together their whole lives, starting out together at nursery sites and progressing through life with sharks of a similar age. This doesn’t necessarily mean that they are actively choosing to hang out with their relatives (like you or I would) – more research is needed to understand why this is occurring. But it’s still looking like an exciting finding!
But why does this matter? As Fenella explains, we want to understand the extent of relatedness in aggregations because family groups travelling together may share adaptive traits [46.39]. These are the genes or sequences of DNA that may allow individuals to adapt to certain changes in their environment. For instance, some individuals may have genes that enable them to cope better in warmer conditions, and so those individuals may survive as the climate grows warmer, whereas those without that gene will not. So, if those family groups are all caught as by-catch, the chance of that adaptive trait being passed on reduces and threatens the species’ survival as a whole. Furthermore, understanding how populations are connected genetically allows us to understand how genetic material is passed between them, which can have implications for conservation [48.00]. If populations are mixing, then they will be healthier (genetically speaking) than isolated populations (you can read more about this here). So, different and management strategies will be needed to reflect this.
About our guest
FENELLA WOOD
Fenella is a marine biologist and is in the final year of her PhD studies at the University of Aberdeen. Her research interests are focused on molecular biology, using DNA to understand more about the animals she has studied. Her work has ranged from diet studies, environmental DNA, and now in population genetics. Fenella works in Scotland with two local species, spurdog and flapper skate, and loves being out on the water when she can.
Fenella is an SOSF project leader and her current project focuses on understanding the composition of spurdog aggregations, primarily by trying to identify if family groups are present.
Find Fenella on twitter – @Fenella_Wood
