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Bioluminescence in animals is one of nature’s most fascinating phenomena and may greatly delight the casual observer. However, the showy displays of light produced by the creatures themselves, or by luminous symbiotic microorganisms, serve a range of important purposes. Predatory anglerfish may use bioluminescence to lure victims, while other species glow in order to attract mates or distract enemies.

Bioluminescence naturally occurs in an array of aquatic vertebrates and invertebrates along with land invertebrate species, but human intervention has brought about some other less likely glowing candidates as well. Read on to discover 10 incredible glow-in-the-dark animals – and a few surprises to boot.

10. Hawaiian Bobtail Squid

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Cephalopod molluscs are famed for their ability to spray clouds of ink and make quick escapes when threatened. However, the Hawaiian bobtail squid – which can be found off the coast of the Hawaiian Islands and Midway Island in the north Pacific Ocean – relies on bioluminescence for self-defense. In order for the squid to evade the attacks of predators, bioluminescent displays eclipse its silhouette, making it invisible rather than a ready meal. The squid also uses the same mechanism to feed on prey such as brine shrimp. As it lacks the chemicals to produce its own light, the Hawaiian bobtail squid collects bioluminescent Vibrio fischeri bacteria in a special light organ. This organ contains “lens” tissue to focus the light through the squid’s mantle and is surrounded by a modified ink sac that acts like a camera diaphragm. Interestingly, the squid jettisons its luminous bacteria during the day to save energy.

9. Deep Sea Anglerfish

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Deep sea anglerfish are surely some of the scariest-looking fish one could encounter. Yet anglerfish – both deep sea and shallow water – have also got to be some of the world’s most finely-adapted predators. There are over 200 species of anglerfish, and they vary in size, with some growing as long as 1.2 meters – although most are much smaller. As they stalk the waters of the oceans, female anglerfish flaunt one part of their body in particular: a small, fleshy lure at the end of a stem-like appendage that attracts prey.

Many deep sea anglerfish inhabit the sunless depths of the bathypelagic “midnight zone” as far as a mile below the waves. In order to attract prey, therefore, female deep sea anglerfish possess a lure specially adapted to allow bioluminescent bacteria to enter it and produce a glowing bright light. Unsuspecting prey are then drawn towards the curiously shining “fishing pole,” only to be attacked in an instant.

8. Quantula Striata (Glowing Snail)

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Most the Earth’s bioluminescent mollusc species – and indeed bioluminescent organisms generally – inhabit the oceans. However, one peculiar terrestrial snail has also developed this trait. Its bioluminescence is powered by an organ inside it, rather than through a symbiotic relationship with bacteria. Quantula striata – sometimes known as Dyakia striata – lives in forest and garden habitats in Southeast Asia, feeding on plants, detritus and even carrion, and it is the only land snail that glows this way. Still, the exact purpose of the unusual snail’s light show remains unknown. Scientists studying certain marine snails believe Quantula striata glows as a form of self-defense, perhaps aiming to attract the predator of its predator. Another possible theory is that it uses its strange luminous abilities to communicate.

7. Bioluminescent Jellyfish

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Jellyfish may induce both awe and fear. They glide through the water like graceful alien creatures, but all species also sting – sometimes with deadly results. Many species of jellyfish around the world also use bioluminescence as a form of defense against their natural enemies by apparently advertising their toxicity. It has been theorized that bioluminescent jellyfish parts may act as “sacrificial tags” that will make transparent predators stand out should they swallow the glowing bits – a trick thought to discourage attacks. Jellyfish and other bioluminescent organisms produce what is known as “cold light,” meaning that over 80 percent of the light is produced without thermal radiation. Bioluminescence enzymes also serve as antioxidants and may curb the proliferation of free radicals in jellyfish.

6. Fireflies

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Fireflies are perhaps the most widely known and commonly appreciated bioluminescent creatures on our list. Roughly 2,000 species of firefly light up the air over every continent except Antarctica, as they engage in spectacular mating and prey-attracting displays that resemble a natural light show. Interestingly, fireflies are not flies at all, but are types of beetle.

Many aquatic practitioners of bioluminescence use symbiotic bacteria to glow; the firefly, on the other hand, glows independently. Bioluminescent chemicals, including the enzyme luciferase, react with fireflies’ luciferin substrates to produce a spectacular glow-in-the-dark display. The reaction also features oxygen and magnesium, helping to create what have been called “nature’s fireworks.” Fireflies not only glow, but use a variety of flashes to communicate with one another during displays. In addition, the glow of firefly larvae is believed to warn predators of the grub’s toxic nature.

5. Scaleless Black Dragonfish

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Bioluminescent or not, the scaleless black dragonfish, also known as Melanostomias bartonbeani, is not a deep sea creature you’d like to bump into. The demonic-looking barbeled dragonfish is an aggressive predator with sharp, needle-like teeth, and once bothered, it lights up the length of its body – including its fins. Using photophores, the deep-sea dragonfish produces its own light. However, what makes this bioluminescent creature even more unique – and frightening – is the fact that it can glow red, as well as the more common bluish color associated with bioluminescence.

According to the World Wildlife Fund for Nature, “The light of the black dragonfish can be such long wavelengths that it is almost infrared and barely visible to the human eye. The ability to produce this type of light gives the black dragonfish an enormous advantage over its prey as it can find its way to unsuspecting prey through the deep dark depths of the ocean.”

4. Arachnocampa Gnats

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Arachnocampa gnats are native to Australia and New Zealand, and their maggots produce the ethereally-lit cavern displays that have created a multimillion-dollar tourist industry in Australia. While many species employ bioluminescence as a defense against predators, the gnat larvae uses a light-producing internal chemical reaction to attract prey into its web-like structure. To a human observer, the larvae resemble an entire galaxy of stars in the darkness, although more sinisterly they light the way for the final flights of their prey.

3. Googly-Eyed Glass Squid

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Around 60 species of glass squid make up the Cranchiidae, or glass squid, family. The largest glass squid is the colossal squid, the mantle of which can grow to almost 10 feet long. The family gets its name from the fact that most glass squid are transparent, which serves as a handy form of camouflage in the water. Many glass squid are also bioluminescent, including the colossal squid and its close relative the googly-eyed glass squid (pictured above), known more formally as Teuthowenia pellucida.

The googly-eyed glass squid is a rare deep-sea species found in the Southern Hemisphere. The squid’s cells form bioluminescent photophores that emit light and make it stand out in the gloom of the bathyal zone, deep beneath the surface of the ocean. As with several of the animals on this list, the light produced by these organs is generated by the chemical processes of symbiotic bacteria, which require energy to work. Interestingly, the googly-eyed glass squid’s stalked eyes have light organs, just like those of the colossal squid.

2. Cookiecutter Shark

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Don’t let the cute-sounding name fool you – the cookiecutter shark gets its name from the fact that it likes to bite circular plugs of flesh out of bigger animals. Its signature bite marks have even been found on submarines, subsea cabling and human bodies. The cookiecutter shark, which is known to inhabit warm waters around the world, uses its bioluminescent ability as a form of self-defense and as a means with which to feed.

The shark has purportedly been known to glow green for up to three hours once it has been removed from the water. It employs a series of ventrally-situated photophores to disrupt and disguise its outline in a method known as counter-illumination. It has been suggested that the shark’s tapered neck further enhances its deception by replicating the outline of a much smaller fish in the eyes of the cookiecutter’s unsuspecting victims. Unlike in other cases of counter-illumination, however, the cookiecutter’s belly has such a large amount of light cells that the luminescence still appears uniform close up.

1. Ultraviolet Scorpions

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While the glow-in-the-dark animals featured in this article operate through a natural process of bioluminescence, scorpions differ by glowing only under fluorescent ultraviolet light. Scorpions possess a number of small “eyes” along their bodies, prompting some researchers to suggest that the entire fluorescent exoskeleton itself functions as a light sensor. This has led to the widespread use of UV lights to quickly spotlight the predatory arthropods during scorpion research surveys. For the scorpions themselves, the ultraviolet fluorescing exoskeleton might be used to pick up the shadows of starlight and moonlight.

Like fireflies, scorpions inhabit every continent except Antarctica. They feed on a wide variety of small prey that they catch in their claws, following up with their notorious stinging tail. Interestingly, less than 2.5 percent of the more than 1,000 scorpion species in the world are deadly to humans.

Bonus: Manmade Bioluminescence: Monkeys, Sheep and Cats

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In 2011, scientists at the Mayo Clinic in Rochester Minnesota produced a litter of fluorescent cats by isolating the protein responsible for jellyfish bioluminescence and splicing it with feline DNA. The cats also possess genes that block the virus responsible for feline AIDS. It’s hoped that the study of these genetically modified cats will lead to progress in the study of HIV/AIDS as well as feline immunodeficiency virus (FIV).

More recently, in October 2012, researchers at the Institute of Animal Reproduction in Uruguay modified nine sheep, splicing their DNA with the glowing proteins derived from Aequorea victoria jellyfish. The sheep now glow greenish yellow under ultraviolet light, which makes them easier for night-working shepherds to locate. More importantly, the research has been described as a breakthrough as far as treating genetic conditions from diabetes to haemophilia is concerned.

In 2009, Japanese researchers at the Central Institute for Experimental Animals created glow-in-the-dark marmosets, triggering an ethical debate over messing with animal genetics. Scientists, however, claimed that the research might help provide advances in tackling diseases such as Parkinson’s and multiple sclerosis. They also proved that the monkeys could pass the gene responsible for their new, glow-in-the-dark look on to other generations.

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