Bioluminescent secretions
In some animals, luminescent mucus material is secreted on the body surface and sometimes discharged into the water. Two examples are shown in the diagram on the right. Pholas, a boring shellfish found on the sea shore (right) and Chaetopterus, a parchment worm found in tubes on the lower shore (left) secrete such luminous mucus.
A section through the body of Chaetopterus is shown in the diagram on the left. The light cells (L.C.) are shown full of luminescent material and light cells that have discharged their contents are also shown (D.L.C.). The large mucus cells are also shown (M.C.).
Decapod shrimp, such as Oplophorus, Heterocarpus, Systellaspis and Acanthephyra, eject a bright luminous cloud from the base of the antennae and legs when stimulated. Members of Cypridina crustaceans sometimes known as seed shrimp also have bioluminescent secretions where the luciferin and luciferase are discharged separately and mix in the seawater to create the luminescence.
Light Organs
Some deep sea fish have light emitting cells called photocytes. These can be single or loosely arranged. They can also be part of a structure called a photophore. This can be quite complex in some species where these organs can control wavelength, angle and intensity of emitted light.
In the sea, such photophores are restricted to fish, cephalopods (octopus, squid), euphausiids (krill), and decapod crustaceans (shrimp, lobster, crabs), and are particularly common in members of the mesopelagic (mid-pelagic twilight zone) fauna, whose normal daytime depth distribution is between 200 m and 1000 m. Many of these animals direct their luminescence downwards as counterillumination as a form of camouflage.
Photophores can make up 12% of the body surface and up to 15% of the volume in some species of fish. These photocytes are found grouped together inside a chamber, known as the ‘photogenic chamber’, at the mouth of which there is a lens or ‘filter’. The function of the lens is reported to concentrate the light emitted from the organ and to guide the light towards the ventrally located mouth of the photophore. The diagram here shows a lens (l1, l2, l3) overlaying photocytes (ph) from the marine shrimp Sergestes prehensilis.
The entire photogenic chamber is surrounded by a layer of cells, rich in guanine crystals, immersed in an amorphous matrix, known as a ‘reflector’. The function of the reflector is to reflect the light emitted and direct it towards the lens. The reflector wall is covered by a layer of iridocytes containing melanin granules known as a ‘pigmented layer’ . Ventrally every photophore is bounded by a thick layer of cells, known as a ‘gelatinous layer’, which has dioptric properties. Some photophores have shutters to block the light when necessary or to aid in signaling.
These photophores can be of simple organization like those among several familes of decapod shrimp which can be on the cuticle or in modified hepatopancreas tubules.
One of the best-known bioluminescent squid, Taningia danae, carries the largest photophores in the animal kingdom – lemon-shaped organs up to 5cm in length – in the tips of its second (dorso-lateral) arms.
Symbiotic bacteria
Members of at least two families of squids and seven families of marine fishes are known to harbor a monospecific culture of luminous bacteria in specialized light organs. The bacteria benefit by getting nutrients and protection from their host. In return the host animal gets luminescence which they can then use for various purposes.
Only four species (Vibrio fischeri, Vibrio logei, Photobacterium leiognathi and Photobacterium phosphoreum) of the 10 described marine luminous bacteria have been isolated from light organs. P. leiognathi and P. phosphoreum occur primarily as symbionts of marine fishes. V. fischeri and V. logei has been identified as the specific light organ symbiont of at least five species of marine animal hosts, including several bobtail squids in the family Sepiolidae and fishes in the family Monocentridae. In most cases, the light organs of marine animals have been reported to contain only a single species of luminous bacteria.
Bioluminescence in freshwater
The limpet-like snail Latia neritoidesis only found in clear and shallow streams of North Island in New Zealand, and is the only bioluminescent animal which lives in fresh water.
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