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Milky seas are unusual phenomena which have been noticed by mariners for centuries, but which remain unexplained by scientists. These events are when the surface of the ocean, often from horizon to horizon, glows with a continuous uniform milky light. Although the origins of this light are not well investigated, the most plausible explanation is that it are caused by blooms of bioluminescent bacteria. Dinoflagellates, which cause red tides, flashing waves, and sparkling wakes behind boats, need to be physically stimulated to produce their brief bright flashes. This type of display does not match the kind of display seen in milky seas. Bacteria, on the other hand, will glow with the continuous light bacteria, under the right conditions.
The deep-dwelling squid Histioteuthis heteropsis is covered with photophores, perhaps used to mask its silhouette from predators and prey.
Anglerfish such as this Chaenophryne longiceps are classic examples of bioluminescent organisms, although it is rare to see them alive like this velvety-black specimen. Unfortunately, you will usually see images of dead-looking, preserved fish with milky white eyes, even on magazine covers and in the movie "Finding Nemo." These fish look like they could be large and scary, but the one shown here is only about 4cm long. Although most marine organisms do NOT use bacteria to make their light, the anglerfish is an exception. It cultures a glowing community inside its retractable lure (called an esca). Although prey attraction is a logical role for bioluminescence, and lures can be found in some other fish, demonstrated examples are quite rare.
The Pacific Black Dragonfish (Idiacanthus antrostomus) is one of the most amazingly bioluminescent animals in the sea. It is covered with photophores along lower and upper surfaces, and has photophores under its eyes and at the end of a long barbel. When disturbed it lights up all over, even down the lengths of its fins.
You may not have heard of it, but the hydromedusa Aequorea victoria is probably the most influential bioluminescent marine organism. Calcium-activated photoprotein and green-fluorescent protein (GFP) were first discovered and cloned from this cnidarian. This picture does not show bioluminescence or fluorescence. (See the link above for a fluorescence image, and read Claudia Mills web page for an in-depth discussion.)
The green-fluorescent protein (GFP) of the hydromedusaAequorea victoria is used widely for laboratory, clinical, and molecular applications. In the medusa itself, the protein is used to redirect energy from the photoprotein, which would normally emit blue light, to a longer wavelength green light. Although you may sometimes see photo captions that erroneously claim the entire jellyfish is glowing, this is not true. In living specimens such as this, GFP is actually located in discrete spots around the bell margin. Claudia Mills has created an excellent page dispelling many false notions about GFP in Aequorea.
The rainbow colors on ctenophores are not bioluminescence. They are merely diffraction acting on the ambient light. This shallow-dwelling species, Beroe forskalii, produces a bright luminescent display when disturbed. (Approx size 10 cm).
This small squid in the genus Abraliopsis has several different types of light organs. In addition to the bean-shaped ones at the tips of two central arms, it has small photophores covering the underside of its body.
Aequorea victoria is not the only hydrozoan that uses green-fluorescent protein, and GFP is not the only fluorescent molecule in the sea. Here is a different species of hydroid with GFP, and this photo also shows the red fluorescence that is produced by chlorophyll -- one of the light-absorbing pigments used in algae and phytoplankton.
There seems to be a perception that most organisms in the sea use bacteria to produce their bioluminescence, but this is generally not true (see the chemistry section for a list). However luminous bacteria are quite abundant in seawater. Most are blue or blue-green, but Ned Ruby found a strain of Vibrio that produces a longer wavelength glow. These culture plates show this yellowish strain (left) next to the normal wild type. The images have not been colorized, but may not be spectrally perfect due to the settings of the camera. Luminous bacteria make great (and inexpensive) classroom projects or demonstrations. We purchased some from Carolina Biological, and they glowed quite nicely.
Some lanternfish (myctophids) have very bright light organs near their tail (the white spots above and below the body at the left). These "sternchasers" produce a blinding flash at the instant that the animal darts away, leaving a confused predator in its wake.
This little octopod in the genus Japetella uses its bioluminescence in a unique way. Because light is produced only at certain times, by a ring around the mouth of females, it is thought to be private signal used in mating. The blue iridescence at the top right of the photo is not bioluminescence.
Nudibranchs are not generally thought of as bioluminescent organisms, but this pelagic form Phylliroe has the ability to produce light. (The head is on the left). (Length approx. 7 cm) See the SeaSlug Forum for more information.
The polychaete Tomopteris is one of the only marine creatures that makes yellow luminescence. When disturbed, a flurry of glowing sparks will erupt from the parapodia (the paddle-like structures). Eggs, visible in this photo, are also released into the water.
The siphonophore Praya dubia is said to be one of the longest animals on earth and can stretch for more than 40 meters. This picture shows just one of the two nectophores (swimming bells) and a little fragment of the long chain which it pulls through the water. Even this isolated piece is over 10 cm long. Scroll down a bit to see a picture of the whole animal taken from an ROV. It shows two of the nectophores and a long portion of the stinging curtain trailing behind. Like other Cnidaria, these creatures can deliver a powerful sting, and they also produce a beautiful blue bioluminescent glow.
There are very few reports of this bathypelagic ctenophore Bathyctena because it is typically found deeper than 2000 meters. This particular specimen was trawled with a special net designed by James Childress. The yellow spots along the body are sources of bioluminescence which are released into the water when the animal is disturbed. Also note the pigmented gut which helps block the glowing of organisms which it has eaten. (Length approx. 5 cm)
This physonect siphonophore Bargmannia was collected in excellent condition with the Johnson-Sea-Link submersible. This genus is interesting because it produces green and blue light, and it can also eject bioluminescent particles as it swims . (Nectosome [top part] approx. 9 cm long)
Much of the bioluminescence in the sea comes from single-celled algae such as this tropical dinoflagellate,Pyrocystis fusiformis. The red glow is chlorophyll fluorescence (visualized with a special technique called two-photon excitation microscopy) which has been superimposed over a view of the whole cell. (Length 1 mm)
This is Caecosagitta macrocephala, the only species of chaetognath (arrow worm) which isbioluminescent. The species is quite common at around 700 meters depth in the Pacific and Atlantic Oceans, but its luminescence was only discovered in the 1990s. (Read more about it here.) (Approx length 2.5 cm).
Even many single-celled organisms are bioluminescent. The radiolarianTuscaridium cygneum forms colonies such as this one in the deep-sea and glows when disturbed. (Diameter approx. 1.2 cm
This hatchetfish has many photophores beneath its body, presumably used in counterillumination. (Length approx. 3 cm)
Euphausia pacifica is a small vertically migrating species of krill. It is not clear whether the luminescence, concentrated in photophores along the bottom of the body, is used for counterillumination. (Length approx. 2 cm)
This ctenophore Beroe forskalii illustrates one of the reasons that many transparent organisms have pigmented guts. It has just ingested a lobate ctenophore Leucothea, and if the lights were off, the glow of the luminescing prey would be visible to passing creatures. The Beroe is swimming from right to left in this picture, and its mouth is at the left. (Approx. length 12 cm)
The ctenophore Deiopea is found near the surface around the world. Notice the thin filamentous branches of the tentacles which trail along the body from the mouth (top). (Approx length 5 cm).
This dramatic ctenophore lives near the bottom at depths around 1000 meters. It has not yet been named by scientists. Although it looks conspicuously colored when illuminated, there is practically no red light in its habitat, so red is as good as black. When disturbed, this species releases bioluminescent material into the water, causing a confusing swarm of sparkling lights. Notice also the darkly pigmented gut which masks the luminescence of its prey. (Length >15 cm)
This is the pigmented tissue which lines the gut of a deep-sea ctenophore. In environments where bioluminescence is the only source of light, it is important not to swim around with the remnants of your last meal glowing in your stomach.
The deep-sea scyphomedusa Atolla vanhoeffeni is abundant throughout the world. When disturbed it can produce an incredible perpetuated luminescence display. (Diameter approx. 3 cm)
The ctenophore Bathocyroë is one of the most abundant mesopelagic species, but because of its fragility, it was only described in 1978, when it was collected from a submersible. This genus, like the siphonophoreBargmannia, can produce blue and green luminescence (Approx. length 8 cm).
This ctenophore Ocyropsis is heavily parasitized by amphipods, undeterred by its ability to luminescence and by its escape response (clapping the lobes together). The rainbow color near the aboral end (bottom) is not bioluminescence, just diffraction of light. (Length approx. 5 cm)
The deep-sea medusa Paraphyllina has a crimson pigment which may help mask bioluminescence of organisms which it has eaten. (Diameter approx. 7 cm)
When people think of worms, they usually think "Yuck," but this annelid (segmented worm) must be one of the most beautiful deep-sea creatures. (The head is on the lower left.) As with other examples on this site, the blue color is not bioluminescence. It is iridescence caused when structures on the back of the worm are illuminated. It is not clear whether this effect has any function, since this particular worm lives 3000 meters below the surface, where the only light is bioluminescence. Closely related worms have scales which do produce bioluminescence when the animal is disturbed
Heteropods like this Carinaria are some of the very few planktonic organisms which arenot bioluminescent. However they still must deal with luminescence: Notice that the gut (the pink stripe below the eye) is pigmented, and is oriented vertically, probably to minimize its silhouette. (See the pteropod below for another non-luminous mollusc, and the squid for a mollusc which is luminescent.)
Pteropods, like this Clione, are one of the few kinds of planktonic invertebrates which do not have bioluminescent members.
Railroad worm . One of the few organisms that can emit light of two different colors.
