Sharks and rays have remarkable sensory systems. In fact, they actually boast seven different senses- two more than humans beings! One of these extras comes in their ability to sense electromagnetic fields. Electrosensory organs, called "Ampullae of Lorenzini", concentrated around the snout, conduct electrical signals through from the water directly into the brain. The ability is so sensitive that sharks are even able to perceive minute electrical currents generated by the contraction of muscles, so they can literally sense a heartbeat in the water! Some species use this superpower to navigate their way around the globe, whilst others use it to track down hidden prey when they are foraging. Scientists now know that some sharks, like the megamouth (Megachasma pelagios), are even able to use this electroreception to find microscopic food in the water! But how on Earth do they do this? Do animals as small as plankton really give off an electric field?
Meet the Megamouth
Megamouths are incredibly rare sharks, with less than 100 sightings or captures recorded in scientific history! A member of the Lamniform sharks, Megamouths are closely related to great whites (Carcharodon carcharias) and makos (Isurus species), and like their cousins, these sharks can grow to a huge size - up to 5.5 metres total length. Yet, despite their large size, these sharks are not voracious predators. In fact, like the basking shark (Cetorhinus maximus) and whale shark (Rhincodon typus), megamouths are "planktivorous" "filter feeders"; swimming through the water column and extracting small food stuffs, like zooplankton and jellyfish, out of the water to eat (Compagno, 1984; Nakaya et al, 2008; Kempster & Colin, 2011).
Unlike these other filter-feeding sharks, megamouths have a flabby body, with little musculature. They don't need it because they swim incredibly slowly through the water with their jaws wide open, occasionally closing their mouth, using their elastic skin and facial muscles to expel water through the gills, to swallow their food. This is known as "engulfment feeding" and is more commonly seen in whales (Nakaya et al, 2008; Kempster & Colin, 2011).
Slob Life
Based on their morphology, some scientists have even wondered whether megamouth sharks bother to swim at all when they are foraging! Megamouths have a stark, white strip around their lips and scientists have wondered whether this coloration might attract prey to swim directly into their mouths. They even hypothesised that it might be "bioluminescent" - glowing in the dark. to actually lure plankton directly into their mouths. This has, however, been disproven. To learn more you can check out Killer Lipstick (Nakaya et al, 2008; Kempster & Colin, 2011; Duchatelet et al 2020).
A mouth which is brightly coloured would certainly be advantageous to the megamouth, as unlike other planktivorous sharks, they live in quite deep, dark waters, up to 1000 metres down. Whale and basking sharks feed in relatively shallow waters, where sunlight penetrating from above and "upwellings" coming from below make the water extremely "productive"; with high concentrations of plankton and other life. Deep water on the other hand, with very limited light, is relatively "unproductive" in comparison (Nakaya et al, 2008; Duchatelet et al 2020).
Up and Down
Scientists studying their feeding habits in California, USA discovered that megamouths do not spend all of their time in these deep waters! They learned that megamouths undertake a "diel vertical migration", where they move up and down through the water column to spend time at varying depths on a daily schedule. During the night, the megamouths hung out between 12 - 25 metres, whereas they descended to 120 - 166 metres during the day. They suggested that megamouths probably move through different depths in this way in order to follow their microscopic prey (krill called Euphausia pacifica) which migrate up and down with the changing light levels. The sharks could be sensing the shifting light levels and using this as a "visual cue" to move to a different depth (Nelson et al, 1997).
It's Electrifying!
However, other researchers have suggested that a different sensory organ might also be at play... Scientists studying a megamouth shark specimen that was beached in Western Australia, discovered the Ampullae of Lorenzini are concentrated around the front and top of the shark's snout. Whilst these sensory organs were present in some of the lowest concentrations seen in any shark species, the scientists noted that their location would make them perfectly positioned to sense electrical signals coming from directly in front of the shark, in the direction they are swimming (Kempster & Colin, 2011).
Believe it or not, even though they are incredibly small, zooplankton can produce an electrical field of 1 mV per cm, which is certainly detectable by a shark's Ampullae of Lorenzini. When zooplankton are present in a large group this signal is amplified. Whatsmore, there are many other, larger species which also feed on zooplankton and these animals give off much larger electrical signals. The researchers hypothesised that the megamouth shark is able to either detect the bioelectrical fields given off by their prey or at least to sense the fields given off by other animals which are foraging for the same prey, and to use this as a way to locate their food. This would help them to track patchy prey items in deeper water and could drive their daily vertical migration; following high concentrations of food into shallower waters at night! Very cool! (Kempster & Colin, 2011).
There is so much we do not yet know about megamouths. Since their accidental discovery by the military in the 1970s, we have encountered so few of these animals, that to this day, they remain a little-studied mystery of the deep. It is exciting to know there are still things out there yet to be discovered and mysteries yet to be solved. Who knows what other amazing things we will learn about megamouths in the future... we can only wonder... and open our own mouths wide in awe!
References
Compagno LJV (1984). Sharks of the World: An Annotated and Illustrated Catalogue of Shark Species Known to Date. Volume 4, Part 1 - Hexanchiformes to Lamniformes. FAO Species Catalogue: Rome. Access online.
Duchatelet L, Moris VC, Tomita T, Mahillon J, Sato K, Behets C & Mallefet J (2020). The megamouth shark, Megachasma pelagios, is not a luminous species. PLoS One, 15(11): e0242196. Access online.
Kempster RM & Collin SP (2011). Electrosensory pore distribution and feeding in the megamouth shark Megachasma pelagios (Lamniformes: Megachasmidae). Aquatic Biology, 11, 225–228. Access online.
Nakaya K, Matsumoto R & Suda K (2008). Feeding strategy of the megamouth shark Megachasma pelagios (Lamniformes: Megachasmidae). Journal of Fish Biology, 73:1. Access online.
Nelson DR, McKibben JN, Strong WR, Lowe CG, Sisneros JA, Schroeder DM & Lavenberg RJ (1997). An acoustic tracking of a megamouth shark, Megachasma pelagios: a crepus- cular vertical migratory. Environmental Biology of Fishes, 49, 389–399. Access online.
By Sophie A. Maycock for SharkSpeak
