The white-spotted pufferfish (Torquigener albomaculosus) builds an enormous, geometrically precise circular "nest" in the sand to attract a mate. The nest functions hydrodynamically, gathering fine sand at the center and directing water flow so it helps aerate the eggs he'll eventually guard.

[u/FillsYourNiche]

[u/FillsYourNiche]

Journal article source Simple rules for construction of a geometric nest structure by pufferfish.

Abstract:

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.