The color change is beautiful, but your pet may already be scared half to death

"Clown lamp," "milk lamp," "crystal lamp," "Baolian lamp," "green lotus lamp"... At first listen, these names with "lamp" might make you think they come from a lighting wholesale market catalog. In fact, they are all trade names for ornamental fish.

Taxonomically, tetras (lamp fish) do not belong to a single unified group. They come from different taxonomic units, generally including several families within the order Characiformes, as well as some species in the Cyprinidae, Poeciliidae [jiāng], and Gastromyzontidae families. Most of them are small in size and brilliantly colored. Today's protagonistNeon tetraParacheirodon innesi is also a member of the tetra gang. It has an alias that commands the respect of drivers everywhere--"Traffic light"。

Neon tetra. Image: H. Krisp / wikimedia

A home on the far side of the Earth

Although the neon tetra is a regular in the domestic aquarium market, its true homeland isalmost the farthest place on Earth from us--in the tributaries of the Solim?es River in South America.

Map of the Amazon River and its major tributaries. Image: Park E, Latrubesse E M / Remote Sensing of Environment (2014)

The neon tetra lives inthe Solim?es Riverwhich flows through eastern Peru, southeastern Colombia, and Brazil, meeting the Negro River at Manaus in Brazil. At the confluence, the black water of the Negro River and the yellow water of the Solim?es River, laden with large amounts of sediment, are sharply distinct. The black Negro River is home to the neon tetra's close relatives in the same genus: the green neon tetra P. simulans and the cardinal tetra P. axelrodi.

The Negro River and the Solim?es River meet at Manaus, Brazil. The black water is the Negro River, and the more turbid water is the Solim?es River. Image: NASA (2008)

In the Solim?es River, some small tributaries oftenhave tannins released by decomposing humus dissolved in the water, giving them a brown color like strong tea; at the same time, because of the humus, the water here is also weakly acidic. Such rivers are usually calledblackwater rivers(Blackwater), and the neon tetra likes to live in exactly this kind of environment.

A blackwater river in Florida, its water stained brown by tannins. Image: Tim Ross / wikimedia

Neon tetras often swim in small groups in the water layer near the surface. They pursue and feed on small insects such as chironomid midges near the surface, or search in the water for mosquito larvae [jié jué] or water fleas to satisfy their hunger.

Performing a graceful "pairs skate"

The Solim?es River is located in the tropics, where temperatures are suitable year-round, and neon tetras can reproduce throughout the year. Although neon tetras areexternal fertilizers, their reproduction is not a careless release of gametes like that of some other externally fertilizing fish; instead, they show a considerable pursuit of beauty and delight. If I were to describe it, it would feel like twofigure skaters。

dancing gracefully on the ice. This pairs skate begins in the morning. The male appears first, the female a little later. After the two circle the "arena" several times, they begin to display their "scoring moves." The male presses the tip of his snout against the female's gill cover and abdomen, thenbends his body and presses closely side by side with the female. Immediately after, the female and male each fan one side's pectoral fin and caudal fin, with the male on the inside and the female on the outside,circling. One circle, two circles, and they have completed this most critical movement. Justas the movement is completed, the female and male each release their gametes, and a wonderful duet performance comes to an end.

This is a pairing of the cardinal tetra P. axelrodi. Image: Varna Malsyangal / youtube

After one perfect duet performance, the female and male leave behind 36-190 "spoils." These fertilized eggs are 0.9 mm in diameter. At 24-25°C, they hatch after about 24-36 hours, and then after 5 days of post-embryonic development, the newborn fry can swim freely in the water.

With a maximum body length of only 20-30 millimetersthe neon tetra also develops very quickly. If all goes well, these new lives that have just gained the ability to swim will mature in another 6-7 months, and as new blood they will join the great reproductive army of neon tetras.

Neon tetras in a home aquarium. Image: Tan Meng Yoe / wikimedia

How the "traffic light" changes color

The neon tetra has a slender body, and the teeth in its mouth and the small adipose fin on the rear upper side of its body announce its identity as a characin. If there is anything else special, it is the elongated bright band on its flank that gives off a blue-green metallic sheen, and the dazzling red band below the bright band. This is the origin of the name "traffic light."

The two color bands on the flank of the neon tetra. Image: J. de lange

But the colors of this "traffic light" are not so simple. If you have experience keeping neon tetras, you may have noticed that these bands seem to differ in color under different conditions--when the lighting changes suddenly or when they are startled, the color of the bright band on the flank changes. This is because the blue or green shown by the bright band is actually a structural color.

The biological colors we see actually fall into two major categories. One is pigment color, also calledchemical color. They are produced by pigments within organisms. Fish skin generally has melanophores, xanthophores, and erythrophores, which displaythe color of the pigment itself。

The other category isstructural color, also called physical color. Theyare formed by light being reflected and refracted in a medium and ultimately interfering. CD discs, oil films on water surfaces, and the colors of some birds' wings are all structural colors.

The different colors a bird shows from different angles are actually a kind of structural color. Image: Seyed Morteza Kamaly / github

The color of the neon tetra is structural color. In the skin at the bright band on their flank, there are closely arranged multiple layers ofiridophorescontaining guanine crystals. The guanine crystals within these iridophores, together with the cytoplasm, form layer upon layer ofa stacked structure arranged sparse--dense--sparse--dense. When light hits the skin of the neon tetra, after being continuously reflected and refracted by the stacked structure, the corresponding structural color is produced.

Not only that, when neon tetrassense stress or changes in light, they can change the permeability of their cells, causingthe iridophores to absorb water and swell. In the swollen cells, the guanine crystals are more loosely spaced, and in this way, another different structural color can be produced. This is how the neon tetra changes color.

The neon tetra produces structural color through iridophores and changes the structural color by altering the permeability of the iridophores. Image: Cong H, et al./Optics express (2011); Chinese localization: Species Calendar

Don't let them be lonely

The neon tetra's color-changing skill is a response to stress or changes in light intensity. So, for a small fish like the neon tetra that israther timid, observing a color change in the bright band on its flank is not a good thing. To properly serve your neon tetra master and let the old gentleman feel less frightened, you will have to think of a way.

Besides adding necessary shelter such as aquatic plants to the aquarium, can you also give them more companions? Well, the answer is yes.

Neon tetras need companions. Image: Corpse89 / wikimedia

Several British scientists once studied the animal welfare of neon tetras kept under home aquarium conditions. They placed neon tetras in aquariums in groups of 1, 2, 5, and 10, and studied the restlessness, aggression, affinity for conspecifics, and feeding performance of neon tetras under different group sizes. Overall,when the group was largest (10 fish), the neon tetras appeared calmer, showed the least aggression, showed a greater willingness to make contact with conspecifics, and took less time to begin feeding. This result generally also holds for some schooling small fish. So, to make your little fish happy, why not find them more companions?

A group of companions is the happiest thing of all. Image: Myfishroom / youtube

The beautiful neon tetra, ever since it first entered the ornamental fish market, has won the affection of hobbyists around the world. In the 1950s, breakthroughs in neon tetra breeding technology made themone of the most popular ornamental fish species。

In the United States in the 1990s, nearly every month about 1.8 million neon tetras worth 175,000 US dollars were imported from around the world. Most of these fish came from aquaculture farms in Southeast Asia, and a small portion from wild populations in their native countries of Colombia, Peru, and Brazil. Even today, people's affection for neon tetras continues to grow.

A stamp issued by Hungary in 1962, featuring a neon tetra. Image: Darjac / wikimedia

Over the past few decades, about hundreds of millions of neon tetras have left breeding farms and crossed the oceans to reach places around the world. These neon tetras have probably never seen the blackwater tributaries of the Solim?es River; theirmemory of their homeland is written only in their genes。

People do their best to create for these little creatures drifting in a foreign land an environment similar to their homeland--suitable temperature, pH, water hardness... Perhaps as long as the water flowing over their gill filaments still tastes like what is written in their memory, they will not be homesick.

This article is the 245th article of the 4th year of Species Calendar, from Species Calendar author @Sundayhao.