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The extravagant fish developed an explosion of color as the seas rose and fell

Hardware is jewelry that swims, floats, and jumps over coral reefs. The vibrant, heavy courtship displays of the fish are aimed at peers and rivals, and a new study suggests that the slow waxing and shrinkage of ice sheets and glaciers may be partly responsible for a variety of performances.

A new genetic analysis of more than three dozen fairy species details the approximately 12 million years of evolution that produced their wide assortment of shapes, colors, and behaviors. And the timing of these transformations implies that more than 60 species of hardware may be due to their great diversity to cyclical changes in sea level over the last million years, scientists report Feb. 23 in Systematic Biology.

Within the dizzying montage of colorful reef fish, the ironwork (Cirrhilabrus) can’t help but stand out. They are the most species-rich genus in the second-richest species of fish in the ocean, says Yi-Kai Tea, an ichthyologist at the University of Sydney.

“That’s pretty much biodiversity,” says Tea, who points out that new species of ferrets are identified each year. Despite this taxonomic footprint, Tea says, scientists knew “almost nothing” about the evolutionary history of fairy drawings or why there were so many species.

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To fill this knowledge gap, Tea and her colleagues turned to fish genetics, extracting DNA from 39 different ferret species. Many previous genetic studies on ocean animals in the region have focused on a handful of genes in individual species, but Tea and her team used a method that isolated nearly 1,000 genes from many species at once. Comparing DNA between species, the researchers reconstructed an evolutionary tree, showing how dozens of fairy species are interrelated. The team also estimated how long ago these species separated from each other.

Researchers have found that fairy tales diverged from others about 12 million years ago in the Miocene era. But many of the ferret species appeared only a million to three million years ago, in the late Pleistocene and Pliocene. It seems that fairy jugs first evolved in the coral triangle, a region of exceptionally high biodiversity in coral reefs in the western Pacific Ocean. From there, the fish exploded in many shrill forms, extending as far as the east coast of Africa to French Polynesia, a geographical range of about 17,000 miles.

Tea and his colleagues think that the timing of this rapid evolution is directly linked to the geological history of the region.

The Indo-Australian archipelago lies at the confluence of the basins of the Indian and Pacific Oceans – today, a splash of islands stretched between Asia and Australia. Tea describes this border as a “soft barrier,” with the spaces between the islands occasionally allowing marine species to pass into the neighboring oceanic kingdom.

But during the Pliocene and Pleistocene eras, when many fairy designs diversified, the ice ages drastically changed this seascape. When water became trapped in expanses of ice sheets and glaciers, sea level dropped, turning shallow reefs into land bridges. These changes may allow human ancestors access to Indonesia and Australia (SN: 1/9/20), but they have also cut off the movement of marine life. This isolation encouraged the evolution of new species on each side of the barrier.

fairy fish with lavender fringeA fairy male with lavender stripes (Cirrhilabrus lineatus) flashes its fins in an elaborate display of courtship.And Yi-Kai

When the glaciers melted again, the waters rose and the fish mixed again. Seas that rise and fall could act as a “species bomb,” Tea explains, creating new species of ferrets and turning them into the world every time the barrier dissolves. The team estimates that the fairies infiltrated the Indian Ocean at least five different times this way.

The findings are framed in an emerging image of the region as an engine of biodiversity in tropical seas. Genetic differences between Indian and Pacific Ocean populations have been documented in everything from starfish to giant clams, says Paul Barber, an evolutionary biologist at the University of California, Los Angeles, who did not participate in this research. Such divergences are signs of temporary isolation on both sides of the Indo-Australian archipelago.

Glacial cycles “sometimes lose explanations about fish diversification, especially in the tropics,” says Lauren Sallan, a paleobiologist at the University of Pennsylvania who is also not involved in this research. "That aspect is not presented enough."

Elaborate mating rituals from fairy drawings may also be the reason there are so many different species, Tea says.

Since scavengers live in large schools of mixed species, males are subjected to additional pressure to not only attract a mate, but also to make sure it is the right species. A performance with a recognizable arrangement of colors and, in some cases, fluorescence (SN: 29/05/14) would prove very useful. Doing that extra mile to find the right species may make crossing less likely, but it also encourages reproductive isolation. Over time, small genetic changes accumulate in each isolated group, so that the two wrassers become fundamentally different species, different in their DNA, appearance, and behavior.

“These fluorescence and color patterns are very species-specific,” says Tea, who explains that there are “heaps” left to explore about hardware, especially when it comes to how they choose peers and the evolution of their colors.

"You can dedicate your entire career to this if you want and you'll probably never learn everything there is to learn from these amazing fish."

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