Featured Image Caption: The deep sea can be an unrelenting place to live, but some animals have adapted to survive in these harsh environments (Image Source: “In The Blue” by sharkbait is licensed under CC BY-NC-SA 2.0.).
Reference: Botarro, M. (2022). Sixth sense in the deep-sea: the electrosensory system in ghost shark Chimaera monstrosa. Scientific Reports, 12, 9848. https://doi.org/10.1038/s41598-022-14076-2
Darkness. Silence. And, in a moment, chaos. The resulting bubbles from a struggle between predator and prey rise from depths of hundreds to thousands of meters from the light-devoid abyss to the water’s surface. This is the deep sea, where some of the rarest and most ancient life forms have dwelled. Here, sea creatures have evolved over hundreds of millions of years, adapting to the unrelenting conditions – total darkness, intense pressure, and extreme cold – of the ocean’s bottom. These stressors also make the deep-sea nearly impassable for humans and research equipment. As such, many of these aquatic animals remain an enigma to scientists, with many mysteries that remain unsolved.
When chaos emerges from these deceptively calm depths, the cause is often one of violence: the prey struggles to free itself from certain death by a predator clinging to what may be the only food source it finds that day. Without light or sound, predators may be challenging to detect and prey difficult to locate. Many, like the eerily named ghost shark, have adapted to the deep sea in unique and electrifying ways.
The ghost shark, also affectionately known as the rabbit fish, lives 300 to 1000 meters below the raging waves. As if being called a ghost shark isn’t spooky enough, its species name is sure to give quite a fright: Chimaera monstrosa. The mysterious shark lives in the Eastern Atlantic and Mediterranean Basin, inhabiting the ocean floor for 400 million years.
An Electrifying, Ghostly Tale
Researchers discovered that, without light or sound, ghost sharks use electroreception to navigate the deep sea. This phenomenon is made possible by tiny pores in the shark’s skin that allow it to detect the electromagnetic field in the saltwater and sense when that field changes due to the movement of other organisms. While this sixth sense has been studied in surface water fish, its mysterious past in the deep sea is left mostly untold. Recent discoveries have started to piece together the age-old story.
Surveying the sea bottom for food, this deep-sea shark feasts on bottom-dwelling crabs and mollusks. Ghost sharks (and their closest relatives) use a toothplate to grind the hard exteriors of their prey. Finding prey and positioning for attack can be challenging but is made easier by their electroreception. To dive deeper into how the ghost shark uses this unusual sense, researcher Massimiliano Botarro at the Genoa Marine Center in Italy took a closer look at the sensory pores and how they may use them. Botarro meticulously observed the pores of ghost sharks and categorized the receptors by size and body region.
Ghost sharks had an average of about 700 pores that ranged in size from 0.2 to 12 mm (about the thickness of a piece of paper to the size of dice). These pores are distributed all over the head of the ghost shark, with about 65% concentrated near the eyes, mouth, and snout. Each of these pores takes in electromagnetic information that can help the ghost shark assess the size of a detected object and either position itself for attack or scope out the best escape plan from a predator.
Forward-facing pores help the ghost shark navigate the ocean floor and detect its next meal. The highly concentrated pores around the ghost shark’s mouth likely help them position themselves relative to their prey to strike and capture their meal seamlessly. Fewer, larger pores on their sides might help the ghost shark from being caught off-guard by a stalking predator while their attention is aimed at finding food in a food-limited environment.
More Mysteries of the Ghost Shark
Predators aren’t the only thing causing ghost sharks toil and trouble. Typically, encountering predators threatens survival, while prey promotes survival. In the twisted tale of the ghost shark, prey may also threaten survival. Ghost sharks are often accidentally caught by deep-sea fishers searching for crabs, their most common prey. As the fishing cages and nets drag along the ocean floor, ghost sharks are whisked away with the prey they were so (un)lucky to find. These accidental captures have landed them on the Near-Threatened list provided by the IUCN (International Union for Conservation of Nature). Their additional listing as Data Deficient echoes that there is more of their story to be told.
Although the rising bubbles have held the secrets of this deep-sea creature for hundreds of millions of years, scientists are diving deep to uncover more mysteries of the ghost shark.