Chances are if you’re an animal, you need orientation in some way to navigate. It’s an integral part, actually, many using environmental cues like landmarks, sounds, odors, the position of the sun, or the direction of the Earth’s magnetic field as point of references to maintain the correct orientation to get where they are going. The Earth’s magnetic field, also called the geomagnetic field (GMF), extends several tens of thousands of kilometers into space, forms the Earth’s magnetosphere. Beneath our feet and above our heads, electric currents generate magnetic fields that are primarily dipolar (i.e. two poles, the geomagnetic North and South poles) on Earth’s surface.
Elasmobranch fishes (sharks, skates, and rays) are one species that exhibit a wide range of migratory behaviors – even preferences – to specific locations. They have been hypothesized to use the GMF to maintain a sense of direction as they navigate throughout their environment. However, it is difficult to test this ability in an environment as expansive as the ocean. But some sort of magnetic sensitivity has been demonstrated in the round stingray (Urobatis helleri; 1978), sandbar shark (Carcharhinus plumbeus; 2005 and 2017), scalloped hammerhead shark (Sphyrna lewini; 2005), short-tailed stingray (Bathytoshia brevicaudata; 2003), common stingray (Dasyatis pastinaca; 1974, 1976, and 1978), thornback ray (Raja clavata; 1976 and 1978), and yellow stingrays (Urobatis jamaicensis; 2017). But the sole example of spontaneous orientation by an elasmobranch to a magnetic field so far has been leopard sharks (Triakis semifasciata; 1974) that aligned to the north–south axis of the GMF in a captive setting.
A yellow stingray (Urobatis jamaicensis) swims beneath a pier in the waters off the coast of Belize. … [+]
A new study set out to determine is an elasmobranch could detect and use changes in GMF polarity to solve a spatial orientation task. Specifically, if an elasmobranch can use the polarity of a magnetic field as a cue to solve a T-maze task for a food reward, successfully complete a reversal learning procedure and learn subsequent spatial tasks in significantly fewer trials. “If an animal can use detect changes in GMF polarity then they could possibly use it as a cue to orient themselves, just like how we use the position of the sun to determine direction, or a smell to find a food source. A T-maze is a spatial navigation task and you can train an animal to use any cue (visual, chemical, etc.) as a way to indicate which direction to turn to find a reward,” explained Dr. Kyle Newton, postdoctoral research associate at Washington University in St. Louis, USA. “It is like training a dog to sit or fetch or whatever you want. Serial reversal learning is when you train an animal to associate two stimuli, such as GMF north indicates the location of food. Once they learn it, you switch the reward contingency and use GMF south to indicate the reward location. It is like sending mixed messages to someone – [like] all of a sudden ‘yes’ means ‘no.’ If the animal can relearn the task with the new reinforcement contingency, it gives you a measure of their cognitive flexibility. The thought is that an animal that relearns the task faster than before has more flexibility and might be better suited to adapt to rapid environmental changes when a given stimulus might now mean different things.”
Due to elasmobranchs possibly using the GMF as a navigational cue, the researchers believed they could detect and use the polarity of the GMF to orient themselves. They focused their work on yellow stingrays due to them having showcased their willingness to learn operant conditioning tasks previously. In the Urolophidae family, they are a variety of colors and patterns but are usually a yellow hue that allows for them to be practically invisible when camouflaged in the sand. Preferring sandy bottoms and shallow waters, their range includes North Carolina to Venezuela and the surrounding western Atlantic region. A known magnetically sensitive elasmobranch, they are small enough to use in spatial orientation experiments. In fact, this species only grows up to 26 inches (66 centimeters) in length with a maximum disc width of approximately 14 in (35 cm).
“Basically, I would put stingrays behind a barrier in a maze, lift the barrier, then as they approached the T-maze intersection the GMF polarity was rotated laterally so that N-S where switched to the left or right. As the stingray chose the right or left side, they would be rewarded with food if they chose correctly,” said Newton. “Some were trained to use GMF-S, others GMF-N. Of course the presentation was switched randomly to prevent them simply learning to turn left or right. Once they learned the task, I switched the reinforcement contingency until they learned it again. It is nothing more than operant conditioning, somewhat like Pavlov’s dogs. The difference is that I was not measuring a purely physiological response (e.g. dog salivation) and this was purely learning to associate two things (GMF polarity and food location) that are not associated in nature.” It was found that the yellow stingray can be trained to use magnetic polarity as a positive stimulus to solve a T-maze task, then it can relearn the task when the reward contingency is reversed.
A yellow stingray (Urobatis jamaicensis) lays in a seagrass meadow off the coast of Belize. This … [+]
“Although we do not understand how elasmobranchs detect magnetic cues, our results demonstrate that the yellow stingray can detect the polarity of a magnetic field and use it to spatially orient and solve navigational tasks. This supports the idea that Urolophid stingrays, and perhaps other elasmobranchs, might use the GMF as a cue to maintain a compass heading as they migrate towards a desired location,” said the study.
Migration can come with a cost, since elasmobranchs like the yellow stingrays receive injuries from a myriad of interactions as they navigate from point A to point B. Injuries can come from reproduction, fishing, and even scientific sampling. However, few studies have looked at healing rates even though anecdotal reports suggest elasmobranchs have a high capacity to recover from injury. It is unknown if environmental stressors impair wound healing, although their survival is generally unaffected by exposure to high CO2. A study to test the effects of high CO2 exposure on wound healing rates in the yellow stingray found that small dermal injuries (such as an 8 mm biopsy) closed by 22 days post wounding with a decrease in haematocrit (Hct or PCV), a measurement of the proportion of blood that is made up of cells.
It seems that high CO2 exposure (ΔpH = 1.4) does not influence healing rate or haematocrit, showcasing that small, minimally invasive procedures have a short‐term impact in this elasmobranch and that wound healing is not impaired by exposure to a chronic, environmental stressor. Therefore, it seems that wound healing rates may not be strongly impacted by ocean acidification (ΔpH = 0.4). But is this applicable to elasmobranchs as a whole? Only further studies will show!