Man-made surfaces that spend extended periods of time submerged in marine waters (like ship hulls) frequently become fouled with attached organisms looking for a place to settle. In contrast, the surfaces of many marine organisms stay relatively clean in the same waters. One such example are intertidal mussels (Mytilus edulis and M. galloprovincialis), and biologists think their anti-fouling ability comes in part from the structure of a special external layer on their shells.
The outer layer of the mussels’ shells, the periostracum, is a strong but pliable material mainly composed of protein, which protects them from predators that might bore into their shells. The periostracum also appears to protect the mussel from smaller settling organisms. The shell’s surface topography consists of a repeating pattern of ripples ~1-2 μm wide and ~1.5 μm tall. In experiments, the intact intertidal mussel shells had the lowest level of fouling compared to other species of bivalves with either smooth or more randomly structured shell surfaces, synthetic molds of mussel shells, and smooth and sanded synthetic surfaces. Furthermore, the mussel shell has a high level of fouling release (ease of removal) for organisms that do end up attaching to the shell. Researchers studying various shell surfaces and their microtopographies found that the “waviness” (overall texture) of the surface correlates with both fouling resistance and fouling release.
The exact mechanism by which the blue mussel’s surface structure deters attachment is being studied, but the leading hypothesis is that the space between surface ridges is small enough that most fouling organisms cannot attach properly. However, because synthetic replicas aren’t as resistant to fouling as natural, intact mussel shells, it is likely that multiple strategies including surface chemistry and self-replenishment act together to reduce fouling.Edit Summary
“The homogeneous microtextured surface of M. galloprovincialis (1.94+0.03 μm), the smooth surface of the bivalve Amusium balloti (0 μm), and moulds of these surfaces (biomimics) were compared with controls of smooth (0 μm) and sanded moulds, (55.4+2.7 μm) and PVC strips (0 μm) in a 12-week field trial. The shell and mould of M. galloprovincialis were fouled by significantly fewer species and had significantly less total fouling cover than the shell and mould of A. balloti over a 12-week period…There was also no difference between the effect of the M. galloprovincialis mould and the sanded mould. The strong fouling deterrent effects of both these surfaces diminished rapidly after 6 to 8 weeks while that of M. galloprovincialis shell remained intact for the duration of the experiment suggesting factors in addition to surface microtopography contribute to fouling deterrence.” (Scardino and de Nys 2004:249)
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“The defence system of Mytilus appears to be highly complex: not only comprising complementary mechanisms (microtopography, surface chemistry and cumulative filtration), but also with the production of several bioactive components that potentially target the settlement or growth of more than one fouling organism.” (Bers et al. 2006:258)