Deeply hidden inside introduced biogenic structures – Pacific oyster reefs reduce detrimental barnacle overgrowth on native blue mussels

Abstract

In sedimentary coastal ecosystems shells of epibenthic organisms such as blue mussels (Mytilus edulis) provide the only major attachment surface for barnacle epibionts, which may cause detrimental effects on their mussel basibionts by e.g. reducing growth rate. In the European Wadden Sea, beds of native blue mussels have been invaded by Pacific oysters Crassostrea gigas, which transformed these beds into mixed reefs of oysters with mussels. In this study, we determined the spatial distribution of M. edulis and their barnacle epibionts (Semibalanus balanoides) within the reef matrix. Mean mussel density near the bottom was about twice as high compared to the mussel density near the top of an oyster reef, whereas barnacles on mussels showed a reversed pattern. Barnacle dry weight per mussel was on average 14 times higher near the top than at the bottom. This pattern was confirmed by experimentally placing clean M. edulis at the top and on the bottom of oyster reefs at two sites in the Wadden Sea (island of Texel, The Netherlands; island of Sylt, Germany). After an experimental period of five weeks (April and May 2015, the main settlement period of S. balanoides), the number of barnacles per mussel was at both sites significantly higher on mussels near the top compared to near the bottom. We conclude that the oyster reef matrix offers a refuge for M. edulis: inside reefs they are not only better protected against predators but also against detrimental barnacle overgrowth. This study shows that alien species can cause beneficial effects for native organisms and should not be generally considered as a risk for the recipient marine ecosystems.

Invading oysters and native mussels: from hostile takeover to compatible bedfellows

“…Furthermore, below the canopy of oysters, detrimental overgrowth by barnacles on mussels was low relative to exposed positions (Buschbaum et al. ). This may help mussels to overcome curtailed food in the association with oysters above them.…”

Selective byssus attachment behavior of mytilid mussels from hard- and soft-bottom coastal systems

“…These behavioral traits may not only determine the persistence of mussel beds in different habitats, but also their resistance to invaders (Buschbaum et al, 2016) or their own invasion success as shown in the Mediterranean mytilid mussel M. galloprovincialis (Branch and Steffani, 2004).…”

“…by increasing hydrodynamic forces for an overgrown individual (e.g. see Laudien and Wahl, 1999;Buschbaum and Saier, 2001;Buschbaum et al, 2016). Therefore, mussels seeking byssus attachment sites may prefer clean conspecifics.…”

Microscopic species make the diversity: a checklist of marine flora and fauna around the Island of Sylt in the North Sea

“…Currently, the number of non-indigenous species still seems low (39 out of 2758 equivalent to 1.4%) but some of the introduced species already had fundamental effects on the communities [41]. Examples are American razor clams Ensis leei which became a dominant bivalve in terms of abundance and biomass in the subtidal zone of the Wadden Sea [42], Pacific oysters Magallana (Crassostrea) gigas which turned the former mussel (Mytilus edulis) beds into combined oyster/mussel beds [43,44], or the Japanese seaweed Sargassum muticum which forms extensive beds in the shallow subtidal zone and nowadays functionally replaces former native seagrass (Zostera marina) beds that almost went extinct in the 1930s due to a parasitic slime mould [45] and failed to recover to their original extension, up to now. Most of the non-indigenous species recorded so far are either planktonic or large benthic species.…”