Why Shells?

How it works

Oyster larvae (spat) are free-swimming for approximately two to three weeks before permanently attaching to a hard surface.

Oyster larvae, known as spat are free-swimming for the first two to three weeks of life. During this time, they must find a safe and suitable surface to attach to permanently, a decision that will determine their survival and the future of the reef.

Larvae are highly selective about where they settle and the shells themselves play a key role in this process. Larvae are strongly attracted to settle on existing oyster shells, even if the oysters are no longer alive. This preference for settling near other oysters helps reefs grow in dense clusters, which supports reproduction and increases the resilience of the population. Whilst living oysters release specific peptides into the water that encourage larvae to settle nearby, the shells themselves provide a combination of chemical, biological and physical cues that make settlement far more attractive than other hard surfaces.

Chemical cues, come in the form of proteins and other organic molecules contained within the shell that slowly leach into the water. Larvae detect these compounds which signals that the surface is oyster habitat.

Biological cues are provided by microbial biofilms that rapidly form on shells, consisting of bacteria and microalgae. These microbes release additional compounds that reinforce the chemical signals from the shell, further encouraging larvae to settle.

Physical cues are offered by the shell’s ridges, grooves, and small pits, which provide secure attachment points and protection from predators and strong currents. Surfaces without these features, such as smooth rocks or plastic, are considerably less attractive to settling larvae.

Together, these cues guide oyster larvae to settle in optimal locations, promoting the formation of dense oyster reefs. These reefs are essential for successful reproduction, maintaining population stability and supporting the overall health and resilience of coastal ecosystems.

Step 1: Free-Swimming Larvae

• Larvae float in the water for 2–3 weeks.

• Searching for a safe surface to attach and grow.

Step 2: Detecting Chemical Cues

• Proteins and organic molecules slowly leach from oyster shells.

• Larvae sense these compounds, signaling “suitable oyster habitat.”

Step 3: Responding to Biological Cues

• Microbial biofilms (bacteria + microalgae) form on shells.

• Biofilms release compounds that reinforce chemical signals and attract larvae.

Step 4: Finding Physical Cues

• Shell ridges, grooves, and pits provide secure spots for attachment.

• Sheltered surfaces protect larvae from predators and strong currents.

Step 5: Settling and Forming Reefs

• Larvae attach to optimal locations.

• Dense clusters form, promoting reproduction, population resilience, and healthy coastal ecosystems.

x10 higher restoration success

Restoration projects using oyster shells can see up to 10 times higher oyster recruitment and survival.

Multiple large-scale restoration projects in the U.S. and globally have shown that oyster shell consistently outperforms alternative materials for spat settlement, recruitment and long-term reef health.

Global Shell Deficit

Oyster shells are widely recognised as the optimal substrate for reef restoration due to their biological compatibility, structural complexity, and chemical composition. However, in many parts of the world, these shells have become a scarce and highly sought-after resource, with demand consistently outpacing local supply.

This global imbalance has led to what is often termed a “shell deficit” - a limiting factor for restoration projects, where the lack of suitable hard substrate constrains the scale, success and ecological impact of reef restoration efforts.

In response, many large-scale initiatives have turned to oyster shell recycling programmes as a circular solution to recover shells that would otherwise enter the waste stream. In regions where local shell supply remains insufficient, some projects have even resorted to importing fossilised or dredged oyster shells to meet restoration needs.

Oyster shell recycling has emerged as a critical strategy in addressing these material constraints. Programmes typically source discarded shells from restaurants, seafood markets and aquaculture operations, which are then collected, cleaned (or cured) and stored for future use in restoration. Beyond solving the substrate shortage, these initiatives also serve broader environmental objectives, diverting organic waste from landfills and increasing public participation in marine conservation through outreach and education.

As the scale of global restoration accelerates, oyster shells are no longer treated as waste but as a precious natural resource, integral to coastal regeneration, circular resource use and ecosystem resilience.

Recycle your Oyster Shells here!

If you would like to donate shells or learn more about our Oyster Shell Recycling Programme
Please get in touch!