This chapter establishes the technical and scientific foundation for the MARIS HABITATS project by situating it within the broader landscape of underwater monitoring and artificial reef design. While the field of marine biology has long utilized static structures for habitat restoration, the integration of real-time IoT (Internet of Things) capabilities remains a significant hurdle due to high costs and technical complexity.

By analyzing essential water quality parameters—such as pH, temperature, and turbidity—and the embedded systems required to track them, we identify the specific technical gaps that our modular approach aims to fill. This review serves not only as a state-of-the-art summary but as a justification for a cost-effective, sensor-integrated platform that moves beyond traditional, “passive” artificial reefs.

Artificial marine habitats can be designed in several ways to help restore marine ecosystems and support endangered fish species. One approach is the use of 3D-printed reef corals, which can be made from materials such as ceramic, limestone, or eco-concrete. These materials are durable and suitable for marine environments. Examples of projects using this approach include the Reef Design Lab in Australia and SECORE coral restoration projects, both of which focus on rebuilding reef structures that allow coral and marine organisms to grow again [1], [2].

Another commonly used solution is reef balls. These are concrete dome structures with holes that mimic natural reef caves. Because of their simple design they are easy to mass produce and very stable when placed on the seabed. The holes and cavities provide immediate shelter for fish and other marine animals, allowing the structures to quickly function as protective habitats [3], [4].

A different method is the creation of Bio-Rock or electric reefs. These reefs consist of metal structures through which a small electrical current is passed. This current causes minerals from seawater to deposit onto the structure, gradually forming a limestone-like coating. This process helps corals grow much faster than under normal conditions, and fish rapidly colonize these artificial reefs. This technique is known as Bio-Rock technology [5].

Artificial habitats can also be designed as modular “fish cities.” These structures include holes of different sizes so that multiple fish species can use them for shelter. Vertical elements are often incorporated to mimic natural reef cliffs, and the modules can be interconnected to create more complex ecosystems that support a greater diversity of marine life [6].

Another concept is the development of living seawalls. These are harbor walls or seawalls designed with textured panels and cavities so that marine organisms can attach to them and live on them. Instead of smooth concrete surfaces that support little life, these modified structures create habitats for algae, small invertebrates, and fish [7].

Several endangered fish species can benefit from these types of artificial habitats, including the Humphead wrasse, Nassau grouper, Atlantic Goliath Grouper, Smalltooth Sawfish, and the European eel. Although many of these species grow quite large, the habitats are especially important for juvenile fish. Young fish can use the structures as shelter and breeding areas, increasing their chances of survival. When more juvenile fish survive, adult populations can recover and thrive. Larger predators may also benefit by hunting around these habitats.

In habitat design, the shape of the structure is often more important than the material used. It is important to include many holes and cavities in different sizes so that different fish species can find suitable shelter. Vertical structures are also important because they mimic natural reef cliffs. In addition, rough surface textures help corals and algae attach and grow on the structures. Finally, ensuring good water flow around the habitat is essential, as it brings nutrients and oxygen that support marine life.

MEITEC

MEITEC is a company that designs and builds artificial reef structures for marine environments, mainly to support and improve fishery resources [8]. Their products are usually made from strong and durable concrete, which helps the structures stay stable on the seabed even when they are exposed to waves and ocean currents. These reef structures create spaces where marine species can hide, live, and reproduce, which can lead to an increase in fish populations over time.

In some cases, MEITEC also includes simple measuring devices, such as flow meters and water temperature sensors. However, these devices seem to be used separately rather than as part of a connected system. There is no clear indication that the system can collect or process data continuously, and advanced features such as real-time monitoring are not described [9].

Figure 1 presents the flow meter and water temperature meter.

ECOncrete

ECOncrete develops special types of concrete that are designed to be more environmentally friendly and better suited for marine life [10]. Instead of focusing only on strength, their technology also considers how the material interacts with the surrounding ecosystem. For example, they adjust the chemical composition and surface texture of the concrete so that it becomes easier for marine organisms to attach and grow. These solutions are commonly used in coastal structures such as seawalls and breakwaters, where they help support marine life while still performing their structural role.

Even though ECOncrete provides clear ecological benefits, their systems mainly act as fixed structures. They help marine life grow, but they do not include any built-in sensors or systems that can track environmental conditions. As a result, there is no real-time monitoring or data collection integrated into their designs [11].

Figure 2 presents armor block units of ECOncrete.

IntelliReefs

The IntelliReefs project, developed by the Reef Life Foundation, focuses on creating modular reef systems that do not rely on traditional concrete materials [12]. Instead, they use alternative materials that are more environmentally friendly and suitable for marine ecosystems. One of the main advantages of this system is its modular design, which allows different pieces to be combined in flexible ways. This makes it possible to create more complex structures that can better match different underwater environments.

Although IntelliReefs offers flexibility and uses sustainable materials, the available information does not show that the system includes any type of sensor or monitoring technology. Similar to the other solutions, it focuses mainly on providing habitat, without offering tools for real-time environmental data collection or analysis [13].

Figure 3 presents one of the projects of IntelliReefs

For our project involving a marine habitat at a maximum depth of 50 m off the Portuguese coast, the materials must withstand a pressure of approximately 5 bar while fostering biological growth and protecting sensitive sensors. To ensure the highest level of efficiency and environmental compatibility, we have analyzed various materials used in international restoration efforts.

The selection of materials and the structural design of artificial habitats are fundamental to ensuring both environmental compatibility and long-term viability. For this project, concrete has been identified as the primary material due to its exceptional durability and its proven track record in underwater construction. Its capacity to provide structural integrity against significant environmental stressors—such as salinity, strong currents, and wave action—makes it the industry standard for creating resilient marine foundations. While the chemical properties of concrete, particularly its initial pH levels, have historically been a point of debate, recent research has shifted the focus toward a more nuanced understanding of its behavior in open marine environments [14].

Studies indicate that the high alkalinity of newly submerged concrete (typically between 12–14) is rapidly diluted by seawater, resulting in no significant long-term detriment to coral growth or benthic colonization [15]. This suggests that ecological success depends less on extended curing periods or pH-neutral mixtures and more on the physical attributes of the habitat. Consequently, the following sections will detail how our project prioritizes structural complexity, substrate durability, and hydrodynamic stability [16]. By optimizing the weight-to-complexity ratio and ensuring low water absorption, we can guarantee that these structures remain stationary during extreme weather events while providing the necessary niches for biodiversity to thrive [17].

Based on the research and articles reviewed, the following table evaluates different material options—ranging from traditional foundations to innovative biocompatible substrates—from which we will select the most suitable components for our specific implementation:

A. Bacterial (Self-healing) High-Strength Concrete (HSC) This material incorporates bacterial spores, specifically Bacillus sphaericus (strain ATCC 14577), which remain dormant until a crack occurs. Water ingress activates the bacteria, which then precipitate calcium carbonate to seal the crack [18].

• Pros: Achieves 96 % recovery in water tightness within 56 days of seawater immersion [19]. It maintains structural integrity above 100 MPa, which is more than sufficient for the pressure at 50 m. It significantly reduces rebar corrosion by sealing entry points for chloride ions [20].
• Cons: Higher complexity in mixing and requires specific nutrients like calcium lactate and urea [21].
• Price: Estimated at 180 €/m³ – 260 €/m³.

B. Basalt Fiber-Reinforced Polymer (BFRP) Basalt fibers, derived from natural volcanic rock, are used to reinforce concrete or as standalone composite laminates [22].

• Pros: Naturally non-corrosive and chemically stable in aggressive saline environments [23]. Vacuum infusion manufacturing can produce laminates with flexural strength up to 400 MPa [24]. It provides a more resilient, damage-tolerant failure mode compared to the brittle collapse of traditional reinforced concrete [25].
• Cons: Slightly lower peak flexural strength compared to glass fibers, although superior in long-term durability and environmental footprint [26].
• Price: Estimated at 160 €/m³ – 220 €/m³ .

C. Geopolymer Gel Concrete A cement-free binder using materials like fly ash and metakaolin modified with nano-silica (SiO₂) [27].

• Pros: Significantly lower CO₂ footprint than Portland cement [28]. It shows superior resistance to chloride and sulfate attack in “wet-thermal” marine environments [29].
• Cons: Higher production costs currently limit wide adoption [30].
• Price: Estimated at 150 €/m³ – 200 €/m³.

D. ECOncrete® / Sulfoaluminate Cement (SAC) A proprietary concrete mix designed to reduce surface alkalinity to a neutral pH [31].

• Pros: Surface pH of 9–10 (closer to seawater's 8) promotes the settlement of “ecosystem engineers” like oysters, serpulid worms, and coralline algae. These organisms provide bioprotection, adding a calcified layer that strengthens the structure and limits oxygen/chloride penetration [32].
• Cons: Requires specialized design to ensure the lower pH doesn't compromise the protection of internal steel if used.
• Price: Estimated at 140 €/m³ – 180 €/m³.

E. Recycled Glass (Partial Aggregate Replacement) Crushed waste glass used to replace up to 30% of fine aggregates in the concrete mix [33].

• Pros: Improves chemical resistance and reduces water absorption [34]. It offers an eco-friendly way to utilize waste while maintaining sufficient compressive strength for marine applications [35].
• Cons: Replacing more than 30% of aggregate leads to a significant reduction in compressive strength [36].
• Price: Estimated at 90 €/m³ – 140 €/m³.

F. Biorock (Mineral Accretion) Uses low-voltage DC electricity to precipitate minerals (limestone) directly from seawater onto an iron frame.

• Pros: Accelerates biological growth by 400 % and allows the structure to self-repair after impacts.
• Cons: Requires constant power from your buoy; if the power is interrupted, the iron frame corrodes rapidly.
• Price: Base infrastructure 120 €/m³ – 160 €/m³ (excluding electrical components).

Table 1 …

Here we have another issue, since we have to present a prototype where it has to be functional and pass some test previously mentioned, we need to select a material with similar characteristics to the actual model, in order to be as close as possible. But also that is easy to get and handle since we are gonna be the ones using it.

TO BE CHECKED

• Option 1 : Basalt fabric-reinforced is attached to abandoned concrete or industrial waste. We need to check the pH level is neutral this process.
  • pros It is similar to the actual product and cuts down on costs.
  • cons It is impossible to modify the model design and there is no marketing advantage because it is not different from existing business.
• Option 2 : Polymer-clay is just shaped into the model we want and baked in the oven.
  • pros It is possible to be mini version of the actual model in any shape and cuts down on costs.
  • cons There are size limitations depending on the oven and it is different from the model of actual project so not sure if it will approve.

Designing a successful marine habitat involves a delicate technical paradox. On one hand, the project’s primary objective is to encourage biological colonization and the growth of marine life; on the other, the integrated sensors require direct, unobstructed contact with seawater to maintain accuracy. This necessity creates a significant challenge, as the very “bio-friendly” environment we aim to foster can lead to marine biofouling on sensitive equipment, which critically compromises data reliability and sensor sensitivity [37].

To resolve this conflict, our strategy focuses on three integrated design pillars. First, we must carefully select housing materials that support structural life while shielding internal components. Second, we are evaluating specialized anti-fouling coatings that can prevent accumulation on sensor windows without leaching harmful chemicals into the surrounding habitat. Finally, the spatial distribution of the sensors must be strategically planned to allow for clear measurements while minimizing their exposure to rapid biological growth. This balanced approach ensures that our ecological goals do not come at the expense of long-term monitoring precision.

Titanium alloy (TC4) or 316 L stainless steel are recommended for pressure resistance and durability [38]. For a 200 m depth, Titanium is preferred for long-term corrosion resistance [39].

• PDMS (Polydimethylsiloxane): A non-toxic, “fouling-release” coating that reduces the adhesion of algae and barnacles [40].
• CPT (Camptothecin)-based Paint: A natural compound that has shown virtually no macrofouling after nine months of immersion [41].
• SLIPS (Slippery Liquid-Infused Porous Surfaces): These provide exceptional resistance to organism attachment even in stagnant water [42].

Fish structure

Fish populations are generally associated with habitats that exhibit high structural complexity and spatial heterogeneity. Research suggests that complex environments provide essential ecological resources necessary for survival, including food availability, shelter from predators, and suitable areas for reproduction [43]. Structural features such as crevices, cavities, and irregular surfaces may create microhabitats that support a greater diversity of marine organisms and increase the overall ecological value of reef systems [44].

Artificial and natural reefs typically contain irregularities and indentations that form small shelters or “nooks”, which can serve as refuge areas for fish and other marine organisms [45]. These structural features are believed to reduce predation risk and provide protected spaces where fish can rest or reproduce.

Studies also indicate that fish communities tend to perform better in connected habitat mosaics rather than isolated structures [46]. Networks of habitats may facilitate movement, feeding opportunities, and ecological interactions between species, which can contribute to more stable and diverse marine ecosystems [47].

Location

The location of artificial reefs plays a key role in determining their effectiveness. The chosen site should encourage marine life to settle while avoiding interference with human activities such as shipping routes or commercial fishing areas. Water depth is another important consideration. Reefs placed too deep may not receive enough sunlight to support the growth of marine plants like algae, whereas reefs that are too shallow can be more vulnerable to damage from storms or human activity [48].
Most of the Artificial reef projects are placed at the depth of 10-30m but it all depends on which species and what type of marine life you are aiming the reefs for (see table 2).

The selected solutions were evaluated based on criteria such as cost, monitoring capability, structural complexity, sustainability, durability, and ecological performance.

Provide here the conclusions of this chapter and make the bridge to the next chapter.

Based on this study of the state of the art, the team decided to adopt the following <specify here the architecture, technique(s), material(s), component(s)> because <specify here the technical/scientific reasons>.

The comparison indicates that current artificial reef solutions primarily focus on structural and ecological aspects. Solutions such as ECOncrete and IntelliReefs demonstrate strong performance in sustainability and ecological enhancement, while MEITEC emphasizes structural stability and durability.

However, none of the analyzed solutions incorporate real-time monitoring or data transmission capabilities. This represents a significant limitation, as the effectiveness of marine restoration projects cannot be easily measured or optimized without continuous environmental data.

In addition, while modularity and structural complexity vary among solutions, scalability and long-term adaptability remain key challenges in existing systems.

Based on this analysis, it was observed that existing solutions provide valuable approaches to habitat design and ecological enhancement, but lack integration with data-driven technologies. These observations served as both a limitation and a source of inspiration for the proposed project.

The design direction was therefore developed to combine the strengths of current solutions—such as sustainability, structural performance, and ecological support—while addressing their limitations through the integration of real-time monitoring and data collection capabilities.