The Lives of Sharks

SHOW NOTES

Dean has studied sharks and their relatives for over 30 years, and has had a number of memorable encounters [5.53]. One which stands out is a particularly adventurous research expedition into the deep waters of the Exuma Sound, in the Bahamas, to tag giant sixgill sharks (also known as bluntnose sixgill sharks). “Somehow I convinced some folks to let me strap spear guns with tagging tips on them to a submarine, to a submersible.” Grins Dean. “…and then tie like 150 kilos of fish to this expensive sub and go down to the bottom of the Xuma Sound at about 600 meters deep…and attract giant six-gill sharks, and tag them with spear guns, with satellite tags, which was fantastic, you know? You were watching these six huge sharks come swimming up and pushing the sub around. They were pushing us around like we were in a little tin can.”

Dean’s love of sharks began in childhood, during fishing expeditions with his father [11.00]. “I caught a little Atlantic sharp-nosed shark that was about 25, 30 centimeters long – a teeny tiny newborn – when I was seven years old. And I thought that was the coolest fish I’d ever seen.” After high school, Dean went to the University of Miami to study with the famous Samuel “Doc” Gruber, a working relationship that would continue for 30 years. His PhD focussed on the telemetry of sandbar sharks, for which he tagged almost 6,000 juveniles. Then, following a post-doc at the University of Hawaii, Dean returned to his home state of Florida and is now based at the Florida State University Coastal and Marine Laboratory, where he is associate director of research.

Much of Dean’s research is focused on understanding the life histories and population dynamics of threatened elasmobranchs, to better inform their management and conservation. This involves answering key “black holes” in our knowledge, such as: how long certain species live for? At what age do they reach sexual maturity? How many pups do they have, and how often do they have them? What’s the survival rate of those pups? And how long does that female keep producing pups? “These are all critical things to properly managing a population. But for many species, these are things we don’t know.” Explains Dean.

There is a general theme, however [13.24]. Sharks are, overall, long-lived, slow to grow, and slow to reproduce, in comparison to other fishes. They reach sexual maturity fairly late in life, reproduce less often, and invest more in fewer, but larger, offspring. These traits make them extremely vulnerable to human pressures, particularly overfishing, as it takes them a long time to recover from overexploitation. But, if humans are taken out of the equation, the “slow and steady” approach to life has some major advantages. “In the absence of fisheries, in the absence of us, the advantage of being slow growing like that is…the mortality, the natural rate of death on the juveniles is quite low compared to, you know, that of bony fish. You know, [in tuna] 99.9 % of all of the larvae are going to perish. But in the sharks, you generally expect, and that’s in their first year, you expect that at least a quarter of them are going to survive, if not more. So being big when you’re born creates a large advantage. Also by being long-lived and having sort of this long reproductive phase, it allows them to really target habitats that are stable and suitable for their offspring, and for mating, and for pupping, and for everything else. And of course, being large, while it has the disadvantage of requiring you to eat a lot to maintain that body mass, it has the advantage of having lower predation rates. There are not very many predators that can eat a large adult shark.”

But, there are over 500 species of shark – over 1,000 if you include their relatives, the rays – which means a lot of variation [16.40]. “There’s a bit of a continuum in the life histories of sharks.” Says Dean. On the ‘fast’ end of the spectrum are egg-layers, like catsharks, who can reproduce more frequently and have more offspring, because all development happens outside the mother. Most sharks have two reproductive tracts. And so [a female catshark] may fertilize and ovulate a pair of eggs in each reproductive tract every three to seven days for several months. And so she’s going to produce a huge number of offspring, you know, 40 to 60.” On the other end of the spectrum are those who really live life in the slow lane. The Greenland shark is perhaps the most obvious example, but there are also the gulper sharks. A deep sea species, the gulper shark takes decades to reach sexual maturity – 25-30 years – and have a gestation period (the length of time between conception and birth) of up to 3 years! And, after all that, they only produce one pup per reproductive cycle.

So, what are the main stages of a shark’s life [21.41]? “If you start from the beginning, when a shark is either hatched or born…if it’s born, we generally call that neonate, so newborn, just like neonatal humans. Or hatchling for one that’s hatched out of an egg.” Says Dean. Then, for the first year of its life, a shark is referred to by scientists as ‘Young of the Year’ (YOY). “The first year of life is often when your mortality is the highest. So it’s a critical life stage.” After this, the shark is known as a ‘juvenile’ – a stage that can last many years. “You are a juvenile from the time you’re born until you reach maturity.” Explains Dean. You can also have small juveniles, and large juveniles. “Large juveniles are those adolescents, so to speak. So the ones that are getting close to maturity and getting larger, and then [it’s] the adult life stage from then on.” There is also a later phase in life, shark old age, if you like. But scientists don’t know much about this time in a shark’s life, as larger, older adults are often hard to come by. These are some of the greatest “black holes” in our knowledge of sharks – how long do they live for? And do females continue reproducing into old age, or is there a sort of senescence, where she is no longer reproductively viable?

But before we ponder questions on the ageing of sharks, let’s go right back to the beginning of their life, because their embryonic development is really interesting [25.20]. Most groups of animals have one or two main ‘modes of reproduction’ – for example, birds lay eggs, whereas mammals generally give birth to live young (with the exception of one or two weirdos, like the platypus). But sharks have a staggering 12 different categories – and these are just the ones we know about! “Again, you can think of it all on a continuum, from yolk feeding, which is called lecithotrophy. Yolk feeding means that all of their nourishment of the embryo is just coming from the initial ovum, the initial egg. They’re not getting any extra nourishment from mom. And then at the other end of the spectrum is matrotrophy, which means mother feeding. And so there are several modes of extreme mother feeding where the mom is giving the offspring most of the nourishment that they need.” In between these two extremes is an astonishing diversity of variations. In some sharks, like sixgill sharks, dogfishes and angel sharks, the young still depend solely on the yolk sac for nourishment, but the mother does not lay the eggs. Rather, she retains the eggs in her uterus. Once the yolk sac is all used up, the young will hatch inside her, and are effectively born live – a mode known as ‘ovoviviparity’.

In other species, the female will keep producing unfertilised eggs throughout her pregnancy, which the developing embryos snack on until they are ready to be born [34.06]. This is called ‘oophagy’, and it is only known to occur in the lamniform sharks, the group that includes mako sharks, thresher sharks and basking sharks. This strategy results in large, well-fed babies: “If you look inside a white shark or mako shark at a late stage in their pregnancies, you’ll see embryos with these really large, very distended bellies.” Dean says. “A bit like Americans on Thanksgiving!”. In some cases, this also introduces sibling rivalry quite early on, as embryos compete for food while still in their mother’s uterus.

Sandtiger sharks, however, take the sibling rivalry to the extremes [36.34]. “In a lot of sharks, the teeth don’t really pop through the gum until pretty late in their development.” Says Dean. “But in sand tiger sharks, the teeth develop very, very early. And the reason this is needed is because the largest embryo then basically kills all of its brothers and sisters in the uterus, and then will feed on them. And so at the end, after this embryo has eaten all of its brothers and sisters and eaten all the unfertilized eggs, you get one pup that, when it’s born, is over a meter long, and it’s already learned to kill, so it doesn’t even have to learn to hunt.”

But eating your brothers and sisters in uteri aside, how do sharks learn to be a shark once they are born [38.48]? We know that they receive no parental care – once the eggs are laid or the sharks are born, there is no extra input from Mum and Dad. But are there any ways that sharks can learn the ropes before heading out into the big wide ocean? The truth is, we simply don’t know. Most likely, they act on pure instinct most of the time. But, they may be given certain ‘head starts’. While there are no schools for sharks, some species are known to have nurseries. Lemon and nurse sharks, for example, will give birth in shallow, sheltered areas that offer protection from predators, and have plenty of food. In this environment, Dean explains, the young sharks are able to learn how to hunt with minimal risk. There is also some limited evidence that sharks may learn from older, more experienced individuals. But, it’s another area we know relatively little about.

There is however, almost certainly, a teenage phase [43.23]. “There’s this whole phase for sure that lasts several years where in the male, the claspers are starting to get longer. The testes are getting bigger and heavier and getting ready to produce sperm… in the female, her ovaries are starting to develop. She’ll go through what’s called vitiligenesis, where the egg becomes larger and larger and yolkier and yolkier. During that period, that little gland that’s in the oviduct is where fertilization takes place, where an egg shell or a membrane or any casing gets put on the embryo. The gland starts developing, getting bigger and bigger and bigger during this teenage stage and their uterus starts to develop.” And like in humans, a teenage shark’s hormones are raging. The same hormones of oestrogen and testosterone are coursing through the shark’s body during this time, triggering sexual maturation. “Whether they, you know, whether the sharks, you know, act crazy when they’re young like teenage humans do, you know, I don’t know.” Laughs Dean.

And then, once adulthood is reached, the primary objective is to reproduce [49.21]. Although again, for many species, the specifics of mating and reproduction remain a mystery. We know that reproduction occurs via internal fertilisation. The male will grab onto the female – often on her pectoral or dorsal fin – and insert his claspers into the female’s cloaca. Female sharks therefore often have mating scars, and in some species have evolved a thicker skin. Needless to say, romance in the shark world is pretty non-existent!

We still have so much to learn about the lives of sharks. “There’s so, so many questions out there.” Says Dean. “In all the species, we often don’t even know how long they live. It’s hard to estimate. They don’t tell us, you know, how old their grandparents were when they died. In many, many species, the biggest animals in the population have never been aged. that’s why we’re pretty sure that most of our longevity estimates are underestimates.

But even just for fun…I want to know what the life history is and the function. I want to know if the pocket shark, if that gland, really emits, you know, this bioluminescent cloud. I want to know why,  if you look at all of the lantern sharks, they tend to have these relatively small pointed teeth and vertically migrate. And then you have the viper dogfish, they call it, but it’s a lantern shark. And this thing has crazy, protrusible dagger-shaped teeth like a goblin shark, but it’s a lantern shark…how’d that happen? That’s just crazy, you know? There’s so many, so many questions.”

You can find Dean’s latest book, co-authored with Daniel Abel, The Lives of Sharks: A Natural History of Shark Life, here. 

ABOUT OUR GUEST

Dr Dean Grubbs is a fish ecologist with interests in the biology of exploited and poorly studied estuarine and marine taxa. Much of his research addresses specific gaps in biological knowledge necessary for the management and conservation of coastal and deep-water sharks and rays. Dean specialises in the use of fishery-independent surveys to study population dynamics and the drivers of distribution patterns of fishes and to facilitate studies of life histories, reproductive biology, trophic ecology and systematics. Dean has also tagged and released more than 10,000 sharks representing over 40 species during the past 25 years. He employs a variety of tagging and telemetry techniques to examine movement, migration and patterns of habitat use and to delineate essential and vulnerable habitats for exploited, threatened or poorly studied species.

Associate Director of Research – Florida State University

Scientific Advisor – Save Our Seas Foundation

Shark Biology and Conservation: Essentials for Educators, Students, and Enthusiasts, a book by Dean, co-authored with Daniel Abel, can be found here.

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Twitter: @FSUMarineLab