Geometry in Nature: Pufferfish Crop Circles

Geometry is evident in nature, such as in snowflakes, honeycomb, and plants.  A rather interesting example of geometry in nature has recently been discovered: geometric circular structures composed by the male pufferfish, Torquigener sp.  Male pufferfish have been found to construct large geometric circular structures on the seafloor, which plays a role in female mate choice.  The male pufferfish creates irregular patterns in the circular nest that are composed of fine sand particles, which is an important part of female mate choice.  These geometric circular nests are never reused because the valleys within the nest may not contain enough fine sand particles for multiple reproductive cycles.

                                  

Male pufferfish, Torquigener sp., digging a valley.

In 1995, a geometric circular structure was found on the seafloor near southern Amami-Oshima Island off the coast of Japan.  The circle was observed for a period of time, but it was unknown what actually made or caused the structure.  In 2011, the origin of the structure was discovered: a small male pufferfish, Torquigener sp., that was 120 mm long, was observed constructing a geometric circular structure.  After the discovery of this strange behavior of the male pufferfish, the structure’s function and the reasons for constructing a new structure for each reproductive cycle were investigated.

This study, done by Hiroshi Kawase, Yoji Okata and Kimiaki Ito, focused on ten male reproductive processes that were observed in two observation areas, one in Seisui and one in Katetsu, near the area that the first circular structure was found off the coast of Japan. The male pufferfish that were observed constructed a circular structure on the sandy seafloor that took between seven and nine days to complete.  The numbers of peaks and valleys within the structures were recorded along with the changes in the behavioral patterns of the pufferfish during the construction process.

a. Early Stage; b. Middle Stage; c. Final Stage; d. After Spawning.  Taken on June 23, 27, 29, and July 6, 2012, respectively.

            The early stage of construction consisted of the male creating a basic circular shape.  During the middle stage of construction the male pufferfish refined the circular structure.  In the final stage of construction, the structure was completed, and an irregular pattern of fine sand was organized in the central area of the structure.  Fragments of coral and shell were used to decorate the peaks of the valleys.  Females appeared only during the final stage of construction, and upon nearing the structure, the male would stir up the sand in the central area of the structure, retreat when the female entered the nest, then rush toward her several times.  After spawning took place, eggs were released in the central area of the nest. 

            The circular nests constructed by pufferfish exhibited three unique characteristics that have never been reported in fish:  radially aligned peaks and valleys created outside the nest site, shell and coral decorations on the peaks, and irregular patterns of fine sand particles in the nest site.  These three characteristics were maintained before mating but collapsed after spawning.  Therefore, it can be assumed that nest characteristics play an important role in the mate choice of the female pufferfish.  Definite factors affecting the females mate choice are unknown, such as size of structure, peak height, amount of fine sand particles, etc. 

a. Sand collected on the day before spawning; b. Sand collected on the day before hatching; White bar = 5mm

Although nest sites are usually retained, by fish and other animals, for the next reproductive cycle, new nest sites must be used by the pufferfish to create new circular nest structures for each reproductive cycle.  The valleys in the structure may not contain enough fine sand for multiple spawning cycles because one cycle consumes a large amount of fine sand particles.  A new site is needed to construct a new structure where there are sufficient fine sand particles available. 

The following video shows a male pufferfish constructing a nest: 

Resource

Role of Huge Geometric Circular Structures in the Reproduction of a Marine Pufferfish