Hiroshi Kawase

The reproductive behavior and nest-building activity of the sand-dwelling goby Hazeus ammophilus were investigated to examine its nesting characteristics and to determine how and why this species builds radial structures around its nests. Field observations revealed that males spawned with multiple females in open muddy-sand bottoms, using bivalve shells or fallen leaves as spawning substrates. Males cared for eggs after spawning and repeatedly mated with multiple females, suggesting a male-territory-visiting polygamous mating system. A distinctive feature of this species was the presence of radial ditches extending from the nest. These ditches developed through repeated male behaviors of digging from the nest toward the surrounding area and sweeping accumulated sand out of the nest, resulting in a crater-like structure around the nest. These behaviors may contribute to cleaning and stabilizing the spawning substrate, and the resulting structures themselves may also be involved in female mate choice. Taken together, these findings indicate that H. ammophilus has evolved a flexible reproductive strategy, and nest-building behavior possibly adapted to unstable open sandy environments, highlighting the behavioral diversity and ecological plasticity within gobiid fishes.

We investigated reproductive behavior and sexual patterns in two odacid fish—Rainbow cale Heteroscarus acroptilus and Herring cale Olisthops cyanomelas—inhabiting temperate reefs in Australia on the basis of underwater observations and histological studies. The males of both species established a territory and continuously courted females they encountered in the territory. The males and females went up in the water column to release gametes in pairs. We found ovarian cavities in the gonadal tissue of H. acroptilus males, suggesting that protogynous sex change occurred in this species. Dichromatism is reported in O. cyanomelas, with males having a dark blue body color while females have a brown body color; however, we found small mature males also included among brown individuals. Furthermore, we diagnosed O. cyanomelas with gonochorism, which is rare in closely related labrids. We compared the similarities and differences in reproductive behavior and sexual patterns between the two species and with labrids.

A small pufferfish, Torquigener albomaculosus, is known to construct an elaborate geometric circular structure, which has been referred to as a “mystery circle,” with a diameter of ~2 m in the sand of the seabed. We reconstructed a 3D model of this structure for the first time using a “structure from motion” (SfM) algorithm. The mystery circle constructed by the pufferfish may have potential applications for biomimetics similar to the structures constructed by termites and prairie dogs. To support the significance of its structural characteristics, it was observed that the water passing through the valley upstream always gathers in the center of the structure, regardless of the direction of water flow. Furthermore, it has the function of extracting fine-grained sand particles from the valleys and directing these to the center. Computational fluid analysis can be performed immediately using the quantified 3D data, and the structural features of the mystery circle is expected to be applied in a wide range of fields, such as architecture and engineering, via biomimetics.

A small (~10 cm) male pufferfish (Torquigener albomaculosus) builds a large (~2 m) sandy nest structure, resembling a mysterious crop circle, to attract females. The circle consists of radially arranged deep ditches in the outer ring region, and maze-like shallow ditches in the central region. The configuration is geometrical. Here, we examined the process of the outer ring construction, and extracted the ‘rules’ followed by the pufferfish. During construction, the pufferfish repeatedly excavates ditches from the outside in. Generally, excavation starts at lower positions, and occurs in straight lines. The entry position, the length, and the direction of each ditch were recorded. A simulation program based on these data successfully reproduced the circle pattern, suggesting that the complex circle structure can be created by the repetition of simple actions by the pufferfish.

Male pufferfish (Torquigener albomaculosus) construct “mystery circles”, geometric circular structures with radially aligned peaks and valleys, on sandy seabeds for reproduction. We discovered an earliest-stage mystery circle composed of dozens of irregular depressions. The structure changed to a primitive circular form with radially aligned valleys and a circular depression in the central area on day 2. The number of valleys increased, and the radius of the structure was much smaller than that of the final structure. These features were observed from days 2 to 4, considered the early stage of construction. The mystery circle had radially aligned peaks and valleys with a central flat area, while the radius and the differences in elevation between the peaks and valleys were increasing. These features, which were observed from days 5 to 8, were considered the middle stage of construction. On day 9, the mystery circle was completed. An irregular pattern was formed in the central area, and the radially aligned peaks were decorated with shell and coral fragments. These are the first recorded observations of construction of a mystery circle from the earliest stage.

We report that male pufferfishes (Torquigener sp., Tetraodontidae) constructed large geometric circular structures on the seabed that played an important role in female mate choice. Males dug valleys at various angles in a radial direction, constructing nests surrounded by radially aligned peaks and valleys. Furthermore, they created irregular patterns in the nest comprising fine sand particles. The circular structure not only influences female mate choice but also functions to gather fine sand particles in nests, which are important in female mate choice. Strangely enough, the males never reuse the nest, always constructing a new circular structure at the huge cost of construction. This is because the valleys may not contain sufficient fine sand particles for multiple reproductive cycles.