Spatial Configuration and Multi-trophic Colonization in Modular Artificial Reefs: A Design-Research Field Study for Reef Restoration in the Gulf of Thailand

The ocean floor is not a flat, empty desert; it is a landscape of hidden rooms, narrow tunnels, and open plazas, each offering a different kind of safety and opportunity for the creatures that live there. For decades, scientists have known that the physical shape of a coral reef is the single most important factor in how many fish and other animals it can support. A reef with deep cracks and complex nooks provides shelter for small creatures hiding from predators, while open spaces allow larger fish to swim freely and hunt. When natural reefs are damaged by storms, pollution, or warming waters, communities often try to fix the problem by dropping artificial structures into the sea. These man-made reefs, usually made of concrete or repurposed materials, are intended to mimic the complexity of nature and jump-start the recovery of marine life. However, the designs for these structures have often been based on intuition or simple geometry, such as stacking cubes or dropping domes, without a clear understanding of how specific shapes attract specific types of animals. The question remains: does the exact arrangement of holes, walls, and open spaces matter more than the sheer size of the structure?

In the Gulf of Thailand, a team of researchers set out to answer this question by treating the underwater world as a design challenge. They built four different types of small, modular artificial reefs, each with a unique internal layout, and placed them on the seabed off the coast of Koh Khai. Instead of guessing which shape would work best, they applied a method called design research, which involves translating the biological needs of sea creatures into precise architectural rules. They focused on three key features: how enclosed a space is, the size of the openings, and the direction those openings face. The goal was to see if they could engineer a structure that specifically invited different kinds of life, from tiny crabs hiding in tight crevices to schools of fish swimming in the open water.

The researchers constructed four distinct prototypes using low-carbon concrete and 3D printing technology. The first design, called Randomized Lithic-Mimicry, looked like a pile of scattered rocks with irregular gaps of varying sizes, intended to create a maze of hiding spots. The second, Tiered Interconnected Refugia, was built like a stack of concrete layers with stepped platforms and wide, horizontal tunnels, offering shelter for creatures of different sizes. The third, Vertical Cavities, featured tall, chimney-like tubes running from top to bottom, designed to channel water flow and oxygen through the structure. The fourth, Horizontal Stratification Cavity, consisted of interlocking triangular blocks that created wide, flat passages running sideways through the unit. Each of these four structures was lowered to a depth of about 6 to 8 meters and left to be colonized by marine life. Over a period of three months, the researchers returned to the site every 30 days to take underwater photographs and videos, carefully counting the fish and invertebrates in different zones of each structure.

The results revealed a clear and surprising pattern: no single design was perfect for every type of animal. The study found that the degree of enclosure—the amount of open space versus the amount of hidden shelter—acted as a switch that favored one group of animals over another. The structures with the most open, exposed surfaces, particularly the one with the horizontal passages, attracted the highest numbers of fish. These fish, mostly migratory species and temporary visitors, preferred the open water where they could see their surroundings and swim freely. In contrast, the structures with the most enclosed, labyrinthine spaces, such as the pile of rocks and the stacked layers, were quickly filled with invertebrates like crabs, shrimp, and snails. These creatures, which often hide to avoid being eaten, thrived in the tight, dark crevices where they felt safe.

Interestingly, the researchers observed that the fish and the hiding creatures did not just prefer different spots; they actively avoided each other's ideal environments. The more enclosed a space was, the fewer fish were found there, but the more invertebrates settled in. Conversely, the most open areas were teeming with fish but had very few hiding creatures. This suggests that a single artificial reef cannot simultaneously maximize both fish attraction and the protection of small, hidden life. The study also showed that time played a crucial role. At the beginning of the experiment, most of the fish were just passing through, but as the structures aged, more of them began to stay and live there, especially in the sheltered zones. The tiny creatures, however, moved in much faster, often claiming the tightest spaces within the first month.

The findings challenge the common practice of building large, uniform artificial reefs. Instead of dropping one giant structure that tries to do everything, the researchers suggest that restoration projects should deploy a mix of different designs. By placing a structure designed to attract fish next to one designed to shelter invertebrates, communities could create a more diverse and resilient underwater neighborhood. The study also highlighted that the space between the structures matters just as much as the structures themselves. The open water surrounding the reefs became a gathering point for fish, showing that the entire arrangement of the reef, not just the individual blocks, shapes the ecosystem.

While the study was limited by the fact that only one of each design was tested, the patterns were strong enough to suggest a new way of thinking about reef restoration. The research indicates that the success of an artificial reef depends less on its total weight or size and more on the specific geometry of its holes and tunnels. By carefully designing the internal layout to match the needs of different species, scientists and engineers can create underwater environments that support a wider range of life. This approach moves beyond simply placing concrete in the ocean and toward a more thoughtful, architectural method of rebuilding the sea, where every crack and opening is placed with a specific purpose in mind. The work serves as a proof of concept, demonstrating that with the right design, we can engineer habitats that invite life back to the ocean floor.