Are sharks drawn to the light?

posted: 04/11/12
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Many shark species — although not all — are by far our superiors when it comes to seeing in the dim depths of the ocean. Long after things go dark for even the keenest 20/20 humans, these sharks can still pick up a little light and see the contrasts around them. In part, they can thank their retinas, which contain huge numbers of photoreceptive rod cells, for this skill.

Additionally, lots of sharks have a reflective layer in their eyes, known as the tapetum lucidum. This layer helps bounce back any light that didn't register the first time. Think of seeing a cat's eyes glow in the dark, and then imagine encountering that eerie sight deep in the ocean.

Many shark species — in addition to other types of fish — can sometimes be lured in by patterns of light. Sharks see contrasts extremely well, and contrasts often mean potential food is in the area. For similar reasons, many shark safety lists mention that sharks may be drawn to things like high-contrast swimsuits and dramatic tan lines, as well as shiny objects and bright colors like orange and yellow.

Sharks might appreciate light for another reason, too. Vertebrate brains contain a small organ known as the pineal gland. For people, this gland is buried deep in the middle of our heads, but for sharks, it's front and center, covered by only a thin, translucent seal. Researchers can even see a shark's pineal gland by shining a flashlight up through its mouth.

The pineal gland is part of the endocrine system and works to regulate hormones, many of which are triggered by light and dark. Scientists don't know what specific purpose the pineal gland's placement might serve for sharks, but several possibilities exist, among them that it works like a calendar to help regulate their life cycles.

Another reason light can draw sharks is because it's been known to excite prey species like zooplankton. If a brightly lit cruise ship passes by, it can draw a whole portion of the food chain along after it. Of course, if your intention is to stir up a scene to get a good look at an area's marine life, floodlights might be just what you want to have along for the ride.

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