29 November, 2013, benwupton
Image copyright: Kimiaki Ito
Take a dip in the warm waters off the subtropical island of Amami-Oshima, due south of mainland Japan, and you may notice an odd, circular sandcastle in the pristine seabed. It superficially resembles an upturned beer bottle cap, the regular peaks and troughs of its perimeter perhaps suggesting the stamp of some maritime machinery. At its centre you notice odd patterns, their irregularity contrasting with the outer rim. Perhaps it is at this point that unease creeps in, for surely great intelligence lies behind this design?
First reported in 1995 these structures remained a mystery until 2011 when veteran underwater photographer Yoji Okata at last caught a pufferfish in the act of making them. At the request of Hiroshi Kawase, curator of the Chiba Natural History Museum, Yoji returned to the site for further clues, finding eggs deposited in the fine sand in the centre. It was at this point that Dr. Kawase became convinced that it must be a nest. Along with Kimiaki Ito of the local Amami Marine Station the two set out to investigate further.
“It was unbelievable that a small puffer could construct such a large structure on the sandy bottom,” says Dr Kawase. The three men spent the rest of the summer diving and observing the nests first-hand, returning again the following year. Their findings, published this summer in Nature‘s online journal Scientific Reports, reveal a previously unknown level of sophistication in fish courtship.
Built at huge personal cost by the males of the species, the nests are intended to both attract a mate and provide a nurturing environment for the developing eggs. The fish begin by drawing a circle in the sand; they then use their fins to form shallow valleys and peaks around its edge. To do this they swim repeatedly into the circle at evenly-spaced intervals, each time deepening the valleys and raising the peaks. As they build these battlements the pufferfish are also washing fine sand particles into the centre of the nest.
Once the male is satisfied with the superstructure he will add the finishing touches. He places pieces of coral and shell fragments on the peaks, then, using his anal fins he generates currents across the centre – creating those primordial patterns in the fine sand. The whole process takes around a week.
It is now that the females arrive. They will view a number of nests before making their choice and may return to the same nest multiples times. Whilst she views the nest the male waits outside, occasionally rushing at her. After the fish have mated the male will remain in the nest to tend the eggs for 6 days. He performs no maintenance on the nest and so by the time the eggs hatch it has been worn away by the currents.
The creation of the fortifications around the nest (which reduce current speed by around 25%) and the use of shell fragments and fine sand patterns as decoration have never been reported before in fish. But what qualities do the females look for? This is the question Dr. Kawase and his colleagues hope to answer with their future work. “I want to verify which factor is most important for female mate choice,” he says.
The little fish are only 12cm long and are thought to be a new member of the genus Torquigener. They have so far only been found in a small area off Amami-Oshima, in a national park known for its endemic wildlife. With luck, their elaborate courtship rituals will continue for many years to come.
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