It guides our daily and seasonal rhythms... It makes us feel safe and comforted... It allows us to navigate, to find our food, to choose a mate, to avoid threats. So vital is it to life that in the Bible God proclaims, 'Let there be light!'. It might sound like a really simple and obvious concept, but light is an incredibly important biological cue for basically every single species on the planet that is exposed to it. So how do sharks react to light? Do they need light? And how might light pollution have a negative impact on their lives?
I See You
It is pretty obvious that sharks can see. They have well-developed eyes and many species are very much visual predators; relying on their sight to hone in on their prey. But did you know that it is not only through their eyes that sharks they are able to detect light? As well as via the retina in their eyes (aka "ocular receptors"), they can actually sense light through the "pineal gland" in their brain and even through their skin (Carroll & Harvey-Carroll, 2023).
These "extra-ocular receptors" are not for vision, but are involved in detecting light in order to bring about "rhythmic activity" (Carroll & Harvey-Carroll, 2023).
Let There Be Light!
Light is an important signal in the environment that acts as a "cue" for many animals to alter their behaviour. Whether they are "nocturnal" (night owls), "diurnal" (awake during the day) or "crepuscular" (most active during dawn and dusk) the daily patterns of light provide a landmark for the "circadian rhythm". It is this internal 24 hour body clock that stimulates us to get up in the morning, eat at certain times of the day, and get tired and go to sleep at night (Carroll & Harvey-Carroll, 2023).
For example, studies of horn sharks (Heterodontus francisci) have shown that light is important for stimulating activity and locomotion. These sharks are nocturnal, but when kept only in darkness, their daily patterns of activity became muddled and they are active almost 24/7. However, when kept under constant illumination their locomotion stops almost completely and they barely move at all during a 24 hour period. This shows us that the light is a cue for these sharks - telling them when to be active and when to rest (Nelson & Johnson, 1970; Finstad and Nelson, 1975; Carroll & Harvey-Carroll, 2023).
Green Light
Yet it is not only on a diel cycle that light alters animal behaviour. The shifting light (brighter or darker) and the length of the day also has an seasonal pattern. This can trigger animals to start their breeding season or set off on their migrations annually (Grubbs et al, 2007; Nosal et al, 2014; Carroll & Harvey-Carroll, 2023).
Light has been implicated as a cue for seasonal rhythms in several species of sharks and rays. For instance, the "photoperiod" (the length of the day) has been linked to the start of the autumn and spring migrations of juvenile sandbar sharks (Carcharhinus plumbeus) (Grubbs et al, 2007). Similarly, the coming of the summer and winter solstices are the trigger for leopard sharks (Traikis semifasciatum) to aggregate in large groups (Nosal et al, 2014).
Guiding Light
Light even affects animals, including sharks, on a molecular level; with significant physiological and chemical changes happening in the body in response to illumination (Carroll & Harvey-Carroll, 2023).
For example, velvet belly lantern sharks (Etmopterus spinax) can glow in the dark. To learn more check out Twinkle, Twinkle, Little Shark. Scientists think that, rather than their eyes, it is the detection of light by extra-ocular receptors that is responsible for activating and controlling this "bioluminescence" (Delroisse et al, 2018).
For several species of sharks light has been implicated in affecting the production of reproductive hormones. In Australian sharpnose sharks (Rhizoprionodon taylori), for instance, the hormone 17β-estradiol is at its highest concentrations when the days are longer. This indicates that is it the photoperiod which controls when they become fertile and when the breeding season kicks off (Waltrick et al, 2014; Carroll & Harvey-Carroll, 2023).
Go Towards the Light
At the other end of the spectrum, being exposed to unnatural patterns or an excessive amount of light can also cause very serious physiological harm. The presence of Artificial Light at Night (shortened to ALAN) is known to disrupt both daily and seasonal biology, which can affect sharks on an individual and a population level (Carroll & Harvey-Carroll, 2023).
For instance, in the Maldives, Hawaii and Palau "light pooling" is used for night dives at ecotourism sights. Multiple bright lights are shone on the ocean surface to attract both microscopic animals, and megafauna like (whale sharks (Rhincodon typus) and manta rays (Mobula birostris). Whale shark prey availability is affected by unnatural light and therefore, there is great concern that exposing these animals and their habitat to light pooling may alter their natural diet and/or lead to physiological stress (Carroll & Harvey-Carroll, 2023).
It is only with increasing scientific study that we might begin to understand the true breadth of how light affects sharks. Yet we do not need to know all the mysterious ins-and-outs in order to be able to know that, morally, we should not be altering their natural habitats in this way. I for one would not be at my best if someone was shining a light in my eyes all night! So we certainly should not be subjecting marine creatures to the same.
References
Carroll D & Harvey-Carroll J (2023). The influence of light on elasmobranch behavior and physiology: a review. Frontiers in Marine Science. Access online.
Delroisse J, Duchatelet L, Flammang P & Mallefet J (2018). De novo transcriptome analyses provide insights into opsin-based photoreception in the lanternshark Etmopterus spinax. PloS One, 13:12, e0209767. Access online.
Finstad WO & Nelson DR (1975). Circadian activity rhythm in the horn shark, heterodontus francisci: effect of light intensity. Bulletin of the South California Academy of Science 74:1, 20–26. Access online.
Grubbs RD, Musick JA, Conrath CL & Romine JG. (2007). Long-term movements, migration, and temporal delineation of a summer nursery for juvenile sandbar sharks in the Chesapeake Bay region. In American Fisheries Society Symposium, Volume 50. Access online.
Nelson DR & Johnson RH. (1970). Diel activity rhythms in the nocturnal, bottom-dwelling sharks, Heterodontus francisci and Cephaloscyllium ventriosum. Copeia, 4:732. Access online.
Nosal AP, Caillat A, Kisfaludy EK, Royer MA& Wegner NC (2014). Aggregation behavior and seasonal philopatry in male and female leopard sharks Triakis semifasciata along the open coast of southern California, USA. Marine Ecology Progress Series 499. Access online.
Waltrick D, Jones SM, Simpfendorfer CA & Awruch CA (2014). Endocrine control of embryonic diapause in the Australian sharpnose shark Rhizoprionodon taylori. PloS One, 9:7, e101234. Access online.
By Sophie A. Maycock for SharkSpeak
