Nurse Sharks: The Shark That Loves to Cuddle

SHOW NOTES

SOSF project leader Sergio Madrigal Mora has spent a lot of time around the much-beloved nurse shark, a shark that is sometimes affectionately nicknamed the ‘couch potato of the sea’. But before a love for these laid-back elasmobranchs came a deep interest in the natural world [05.04]. Growing up in Costa Rica, Sergio was surrounded by nature from an early age and encouraged by parents who nurtured and shared his curiosity. A particularly special memory comes from a family dive trip, during which Sergio first experienced the magic of the underwater world. “I remember taking the little boat off to the reefs and jumping in the water and just seeing all sorts of colour, all sorts of fish, all sorts of corals…it felt like a different world, and I was fascinated by that,” he recalls.

Those early experiences exploring reefs with family planted the seed for a career studying marine ecosystems [10.53]. At university in Costa Rica, Sergio first worked in reef ecology before gradually shifting toward researching fishes and eventually sharks. But what led him there wasn’t just a desire to work outdoors – while studying, Sergio also discovered a rather unusual passion. “The one connecting thing that got me from the reef ecology lab to the shark lab was not sharks, or knowing about sharks…it was coding,” he says. “Definitely what I see throughout these years of working in biology is that I want, and what I like to do, is take data and make some biological sense out of it.” This has led Sergio to study for a PhD at Flinders University in Australia, working with Dr Charlie Huveneers to answer fundamental biological questions about white sharks using an extensive dataset covering over ten years’ worth of tracking and telemetry.

But in this episode, Sergio is going to be talking about the subjects of his Master’s research, who receive far less of the spotlight than their more famous great white cousins [15.30]. Nurse sharks are a small family of sharks (the Ginglymostomatidae) that consist of just four species. Perhaps the most well-known is Ginglymostoma cirratum, a species popular with divers, to whom the name ‘nurse shark’ is usually ascribed. For many years, scientists believed all nurse sharks found throughout the tropical Americas were G. cirratum until, in 2016, closer examination of museum specimens revealed subtle morphological differences between individuals from the Atlantic and Pacific sides of the continent. This led to the description of a new species, Ginglymostoma unami, or the Pacific nurse shark. “When the Central American isthmus came out of the ocean – this was thousands of thousands of years ago – these two populations that were on the Pacific or in the Atlantic and Caribbean got completely split apart,” explains Sergio, “and they have not had contact with each other for all this time.”

Despite this, all nurse sharks – including the tawny nurse shark and shorttail nurse shark, the remaining two members of the family – share a similar lifestyle and body shape [20.45]. They belong to the carpet sharks, and spend much of their time on or near the seafloor, resting or slowly moving along reefs, sandy bottom and mangrove habitats in search of food.  As such, their mouths are located below their snout, and instead of large, slicing teeth like those of many oceanic sharks, nurse sharks have small teeth more suited for crushing prey such as crabs and shellfish. Similarly, their tail and body shape is adapted for slow, steady swimming near the seabed rather than high-speed pursuits in open water. Their sensory barbels — whisker-like structures near the mouth — help them detect prey hidden in sand or sediment, and combined with highly developed smell and electroreception suggest that nurse sharks are well adapted to hunting slow-moving animals buried in the seabed.

However, nurse sharks are perhaps most famous though for their laid-back attitude to life, taking long ‘naps’ together on the seabed and seemingly living life mostly in the slow lane [25.01]. They have even evolved adaptations to allow them to stay sedentary for extended periods of time, such as the ability to buccal pump – a form of respiration where water is actively drawn into the mouth and pumped over the gills using their throat muscles, which differs to other sharks like great whites that must swim constantly in order to breathe. Divers frequently encounter them lying motionless on reefs or stacked together in large groups, and as such, they have a reputation for laziness. But are they really the ‘couch potatoes’ everyone makes them out to be?

“The more we learn about them, the more we realise that’s probably not very true,” Sergio says [27.40]. Tracking and telemetry data have demonstrated that nurse sharks can travel far greater distances than scientists once believed; in Costa Rica, Sergio’s team tagged a shark that travelled nearly 400 kilometres along the coastline. Although this was just one individual, it suggests these sharks are at least capable of large-scale movements, possibly crossing international borders and connecting ecosystems across large areas of the eastern Pacific. “We’re likely just catching them when they’re resting,” Sergio concludes. “It’s not all the individuals that do it, but some of them definitely can move long, long distances.”

Defying stereotypes aside, another of their most intriguing behaviours is the nurse sharks’ propensity to ‘cuddle’ – or, in more scientific terms, form large aggregations on the seabed [35.10]. This was the focus of Sergio’s Master’s research and SOSF-funded project, Taking the temperature on nurse shark movement patterns. Pacific nurse sharks are known to form large aggregations with dozens of individuals resting together in shallow water. Some groups have even consisted of up to 50 sharks in a single aggregation. At first, scientists assumed these gatherings were linked to reproduction — a behaviour already documented in Atlantic nurse sharks. “The nurse sharks in the Atlantic – this is also the case in Brazil…It is females that go to the shallows. They get surrounded by males that want to court them. And then it is a whole courtship dynamic and a reproduction dynamic,” Sergio explains. But Pacific nurse sharks seemed to be behaving differently. These groups contained both males and females of mixed ages, including individuals too young to reproduce.

This raised an intriguing question: if the sharks weren’t gathering to mate, why were they gathering at all [40.12]? Sergio hypothesised that the answer may lie in the unique oceanography of his study site, Santa Elena Bay in northern Costa Rica. This region experiences seasonal upwelling events, where strong winds push warm surface water offshore and allow cold, nutrient-rich water from the deep to rise to the surface. During these events, temperatures can plummet: “During upwelling season it can drop down as low as 16 degrees… which is something tropical species would never be used to.”  Because nurse sharks are ectothermic — meaning their body temperature is influenced by the surrounding environment — they are sensitive to these fluctuations in temperature. Sergio’s research found that shark aggregations were far more likely to occur during colder periods associated with upwelling, leading to the theory that the sharks gather together in shallow, slightly warmer waters to conserve heat.

“What we think is that the reason they’re ‘cuddling’ is to get warmer and to share warmth together,” he says. This behaviour, known as behavioural thermoregulation, allows ectothermic animals to regulate body temperature by changing their behaviour.

Pacific nurse sharks are currently listed as Endangered by the IUCN, largely because so little is known about their populations, and partly because of fishing pressure that occurs across parts of their range [46.03]. Sergio’s research suggests that Santa Elena Bay may serve as an important refuge for the species. The area is protected through a collaborative marine management scheme developed with local fishing communities, which has helped maintain relatively healthy shark populations. “We get to study them in very near pristine conditions,” Sergio says. “And whatever we learn from them is going to be very valuable to any other researcher working on these species [in other regions].”

ABOUT OUR GUEST

SERGIO MADRIGAL MORA

Sergio was born and raised in San José, Costa Rica, and studied a Bachelor’s degree in Biology at the University of Costa Rica. He graduated in 2020 and was subsequently awarded a Fulbright Foreign Student scholarship to pursue a Master’s degree at California State University Long Beach. Sergio’s thesis research focused on Pacific nurse shark aggregations and movements in Costa Rica. He is now doing a PhD in Adelaide, South Australia, still studying aggregations and movements but now on white sharks.

Instagram: @sergiomm_77, @tiburones_ucr

Papers mentioned in this episode:

Madrigal-Mora, S., Chávez, E.J., Arauz, R., Lowe, C.G. and Espinoza, M., 2024. Long-distance dispersal of the endangered Pacific nurse shark (Ginglymostoma unami, Orectolobiformes) in Costa Rica revealed through acoustic telemetry. Marine and Freshwater Research, 75(2), p.MF23162.