Oysters provide a wide range of well-documented ecosystem services, such as wave attenuation, a nursery function for many juvenile marine animals including commercially important (fish) species, carbon sequestration, water filtration, nutrient cycling etc. (Grabowski et al., 2007; Forrest et al., 2009; Mariculture, 2010; Scyphers et al., 2011). Although the relative importance of these services vary between locations, the ecological impact and ecosystem services provided by oysters make oyster reefs an important estuarine habitat (Jackson et al., 2001; Grabowski et al., 2007).

The three-dimensional, hard surface of established reefs can influence near-bed tidal flow and dissipate wave action (De Vries et al., 2007; Borsje et al., 2011). A flume study revealed that oysters can reduce wave heights by up to 40%, depending on water depth, reef height and width and wave height (Borsje et al., 2011). Field experiments reveal that oyster breakwaters can decrease shore erosion by up to 40% (Scyphers et al., 2011). Living oyster reefs show a wide range of overall morphologies and spatial orientations. The shell production on oyster reefs is a key determinant of the long-term continuity of the habitat and the oyster resource. The longevity of an oyster reef will depend on biogenic carbonate production in the form of shells, continued recruitment of new generations, adult mortality and reef expansion in both vertical and lateral directions (Walles and Mann, unpublished). Oysters can be used to protect other habitats such as intertidal flats, saltmarshes and submerged aquatic vegetation (De Vries et al., 2007; Grabowski et al., 2007; Borsje et al., 2011).

The Pacific Oyster is endemic to Japan, but has been introduced into a large number of other countries outside its natural range, also in Europe (Troost, 2010). The Pacific Oyster distribution is shown in the Figure 1. Most of these introductions have been for the sake of aquaculture. Worldwide, Pacific Oysters are one of the most widely cultured shellfish species. Pacific Oysters have very high growth rates (they can grow to over 75 mm in their first 18 months) and high rates of reproduction. Also in the Netherlands the Pacific Oyster was introduced for aquacultural purposes. The species was introduced by oyster farmers in 1964 in the Eastern Scheldt estuary (Southwest Netherlands). After its introduction, C. gigas spread rapidly through the estuary after natural spatfall events in the 1970s. At present C. gigas covers more than 9 km2 (8%) of the intertidal habitat, typically forming dense reefs of different sizes (Smaal et al. 2009). In recent years the expansion seems to slow down. By forming persistent reefs the oysters induced structural changes to the ecosystem.

As the Pacific Oyster C. gigas is a non-native species, most studies focused on the ecological consequences of this invasive species. C. gigas has a large filtration capacity and could compete with native bivalves for food, resulting in a shift in the benthic population (Smaal et al. 2005; Diederich 2006). Kochmann et al. (2008) shows that a change from native mussel beds to invasive oyster reefs does not pose a threat to species diversity, but results in a shift in abundance of the dominant species. A shift in the benthic population may have consequences for the food availability for bird populations (Smaal et al. 2005). These consequences are not necessarily negative. Zee et al. (2012) shows the positive effect of the presence of oyster reefs on feeding grounds for birds. Reefs not only protect the feeding grounds of protected bird species from erosion, they also have a positive effect on the food source birds feed upon. Donadi et al. (2013) indicate the importance of understanding the interaction between reef-forming ecosystem engineers (mussels) and surrounding benthic communities (cockles), for conservation and restoration of soft-bottom intertidal communities.

Both positive and negative effects of C. gigas need to be taken into account when considering the use of this species in a certain area for ecological engineering purposes. In the case of the Eastern Scheldt, the species is being cultured and is present throughout the system. The reefs that form in the intertidal have shown to mitigate the loss of tidal flats locally due to the ongoing erosion in this system after infrastructural works in the 1980s. The local use of this species for coastal protection purposes, i.e. mitigation of the ongoing erosion in the Eastern Scheldt, seems therefore justified, as long as the carrying capacity of the system is not affected.

Experiences with biogenic reefs show that the Pacific Oyster is a suitable ecosystem engineer for nature-based coastal defense strategies, if it were only because it occurs in great numbers in estuaries around the world and is a non-fastidious species creating extensive reefs in soft sediment intertidal habitats. One individual Pacific Oyster and an oyster reef are shown in the figures 2 and 3. Reefs are formed by densely packed individual oysters growing upward and outward on top of each other (Troost, 2009; Forrest, Keeley et al., 2009; Powers, Peterson et al., 2009). New oysters settle on top of old ones, creating reefs with a complex, three-dimensional, hard surface that is persistent over time and that is strong enough to withstand severe wave attack (de Vries, Bouma et al., 2007; Borsje, van Wesenbeeck et al., 2011; Scyphers, Powers et al., 2011).

In the Eastern Scheldt a large-scale pilot is in progress which studies and evaluates the use of Pacific Oyster reefs on an artificial substrate of dead oyster shells for the protection of tidal flats against erosion. Figure 4 show the construction of an artificial reef substrate.

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