2008 NORTHEASTERN NATURALIST 15(4):589–594 Adult Zebra Mussels (Dreissena polymorpha) Avoid Attachment to Mesh Materials Ashley E. Porter1,2 and J. Ellen Marsden3,* Abstract - Dreissena polymorpha (Zebra Mussel) is capable of attachment to a wide range of natural and man-made materials, but individuals tend to attach to hard, solid substrates. The effects of mesh substrates on Zebra Mussel attachment has not been studied. This study examined the attachment of adult Zebra Mussels to mesh substrates. Zebra Mussels >5 mm shell length were placed on trays constructed of mesh of different hole size and material, and a hard substrate (PVC) control. Their attachment choice was recorded after a 14-d period. The results indicated that Zebra Mussels do not tend to attach directly to mesh, and instead will move towards the closest hard substrate, usually another mussel. There was no significant difference between numbers of mussels attached to different mesh material types and mesh pore sizes. This study furthers our understanding of Zebra Mussel attachment and their preferences when attaching to substrata and has implications for fisheries biologists, aquarists, and others who use nets, mesh, or screens in fresh water. Introduction Dreissena polymorpha (Pallas) (Zebra Mussel) was first detected in North America in 1989, and since then, the rapid expansion of its abundance and distribution has had severe negative effects on native species, ecosystems, and human activities (Claudi and Mackie 1994). Zebra Mussels are a biofouling organism; they secrete proteinaceous adhesive byssal threads that create a strong attachment to substrates (Eckroat et al. 1993). They adhere to a wide range of materials, including rocks, sediments, water intake pipes, boat anchors and hulls, navigation buoys, and other man-made underwater structures (Ackerman et al. 1993, Claudi and Mackie 1994). The problems caused by biofouling have stimulated research on factors that affect Zebra Mussel attachment, such as substrate material, texture, orientation, and illumination (Kobak 2004, Marsden and Lansky 2000), with a focus on development of antifouling materials (e.g., de Lafontaine et al. 2002, Mussalli and Tsou 1990). Settlement can be deterred by use of toxic coatings such as copper, tin, or zinc, and removal of mussels can be facilitated with ablative paints and silicone coatings that reduce the attachment strength. However, use of such coatings is not applicable in all situations (e.g., paint cannot be applied to some research equipment), and toxic coatings are banned in some inland waters. Fouling of sampling and fishing nets or aquaculture pens is a particular concern, as 1Department of Biology, University of Vermont, Burlington, VT 05405. 2Current address - Oceanographic Center, Nova Southeastern University, 8000 North Ocean Drive, Dania, fl33004. 3Rubenstein School of Environment and Natural Resources, 81 Carrigan Drive, Aiken Center, University of Vermont, Burlington, VT 05405. *Corresponding author - ellen.marsden@uvm.edu. 590 Northeastern Naturalist Vol. 15, No. 4 this equipment is expensive, cannot readily be coated with antifouling material, and is challenging to scrub clean. Underwater screens used in a variety of applications, particularly water intake pipes, are also vulnerable to Zebra Mussel colonization. Fouling by settling juvenile mussels occurs over a period of weeks; however, adult mussels will voluntarily detach and relocate to new substrates in less than a day (J.E. Marsden, pers. observ.; Sandy Brown, Bellows Free Academy, Fairfax, VT, and Declan McCabe, Saint Michael’s College, Colchester, VT, unpublished data). While holding adult Zebra Mussels in the laboratory, we noted that mussels in shallow trays with a fiberglass mesh floor did not attach to the mesh, but clustered in a mass in which they were attached to each other. Here, we examine whether mussels preferably attach to solid or mesh substrates and investigate which characteristics of mesh affect attachment; specifically, we tested three types of mesh material and mesh with two sizes of holes. We hypothesized that mussels would attach more readily to solid than to porous (mesh) substrates, irrespective of the mesh material; we did not have a priori hypotheses about the effect of hole size. Methods Mussels were collected from a near-shore area in Lake Champlain, VT, approximately one week prior to initiation of the experiments, and held in aerated lake water. The mussels were manually removed from their attachment to the tank or each other just prior to use in the experiment; when possible the byssal mass was left attached to the mussel to avoid harming the mussel. Experiments were conducted in 1.3-m diameter, 1900-L circular static tanks filled with dechlorinated, aerated water; water temperature ranged from 18 to 20 °C. Tanks were exposed to ambient fluorescent laboratory lighting for eight hours of each day, and indirect sunlight for the remaining daylight hours. Circular, flat, wooden floating “trays,” 30 cm in diameter, were constructed using mesh as the base (Fig. 1a). The sides of the trays were 1 cm tall to confine mussels within the trays. The trays were uncovered, and floated at the surface of the tank at a depth sufficient to just immerse the mussels. Twenty mussels, picked randomly with respect to size, but all between 6 and 26 mm in length, were placed on each tray so they were approximately equidistant from each other and from the sides of the tray. The average size of all mussels in the experiment was 18.5 ± 3.4 mm; the average size of mussels in each tray ranged between 18 and 19 mm. At the end of two weeks, individual mussel response was recorded as attachment to the substrate, the wooden sides of the tray, another mussel, or no attachment. Each substrate was replicated twice in each of two tanks. The entire experiment was conducted three times over six weeks, but only one tank was used in the third period, resulting in a total of 10 replicates. Each mussel was used only once. A replicate included aluminum window screen (1 mm holes; Fig. 1b), fiberglass window screen (1 mm holes), Ace knotless nylon mesh with small holes (1 mm; Fig. 1c), Ace knotless nylon mesh with large holes (2 mm; Fig. 1d), gray polyvinyl chloride (PVC) alone, and PVC 2008 A.E. Porter and J.E. Marsden 591 covered with a layer of fiberglass screen. Four comparisons were of interest: attachment to (1) PVC versus mesh (nylon, aluminum, and fiberglass) material; (2) mesh with small versus large holes; (3) mesh of different materials (aluminum and fiberglass); and (4) solid substrate (PVC) versus PVC with a layer of fiberglass mesh over it. The final comparison examined whether it is the presence of mesh, or the absence of a hard underlying substrate, that is most important in the attachment of Zebra Mussels. PVC was used for the hard substrate control because Zebra Mussels will readily attach to this material (Kilgour and Mackie 1993, Marsden and Lansky 2000). Statistical analysis Initially, possible effects due to tank or experimental period were tested using one-way analysis of variance (ANOVA). For each comparison listed above, the response variable was the proportion of Zebra Mussels that attached to the substrate, each other, the side of the tray, or did not attach. All comparisons of interest were examined with ANOVA. Results Of 1184 Zebra Mussels, 70% formed an attachment to either a substrate or another mussel; only 3% of all mussels attached to mesh. Results were Figure 1. Experimental apparatus at the beginning of an experiment, and the three mesh types used in the experiments, showing mussels clustered on each substrate at the termination of an experiment. a = experimental tray, with floats around the edge; b = 1 mm fiberglass screening; c = 1 mm Ace mesh; and d = 2 mm Ace mesh. 592 Northeastern Naturalist Vol. 15, No. 4 consistent among tanks and experimental periods. Sixteen mussels were not accounted for at the end of the experiments, and likely crawled off the trays. There was no difference in attachment of Zebra Mussels between mesh with small and large holes (Fig. 2), between aluminum and fiberglass mesh (Fig. 2), or between the PVC and the PVC with an overlying layer of fiberglass mesh (Fig. 3). There was a significant difference in the proportion of mussels attached to all mesh substrates (nylon, aluminum, and fiberglass) versus solid substrate (PVC) (df = 5, F = 102.7, P < 0.0001; Fig. 2). Among all substrate types, there was no difference in proportion of mussels attached to the wooden sides of the trays (average per treatment 1–4 mussels, SD 1.1–3.3) and that did not form any attachment (average 4.2–7.7, SD 2.3–3.8; Figs. 2 and 3). However, the number of mussels attached to the substrate versus to other mussels varied inversely, and was related to substrate; significantly more mussels attached to each other on mesh substrates (50.7%) than on solid substrates (7.9%; P < 0.001). We did not measure attachment strength quantitatively; however, attachment to the solid PVC was detectably stronger than attachment to mesh substrates. Discussion This study demonstrates that adult, detached Zebra Mussels >5 mm shell length avoid re-attachment to mesh substrates. This avoidance was not affected by mesh material or the pore size of mesh. Byssal threads are produced from a gland in the foot, and are attached to substrate by an adhesive plaque at the terminal end of the thread (Eckroat et al.1993). The mussels may be Figure 2. Attachment of Zebra Mussels to four types of mesh vs. hard substrate (PVC). Bars are the mean of 10 replicates ± 1 standard deviation. 2008 A.E. Porter and J.E. Marsden 593 unable to achieve a secure attachment of their byssal threads to the porous materials. Mussels would tend to encounter voids as they began to establish their initial attachment on mesh, and this may be sufficient to stimulate them to find a better location. In the presence of solid material, the majority of Zebra Mussels attached to the substrate; on mesh substrates, mussels tended to attach to each other rather than the sides of the containers. This result may be simply a consequence of encounter rate; mussels in the center of the trays would likely find another mussel before they reached the edge of the tray. It could also indicate a tendency toward conspecific attraction (Cawein 1993); Wainman et al. (1996) noted that dreissenid larvae showed a preference for initial attachment to dreissenid shells in preference to other materials. Our study only examined the attachment tendencies of adult Zebra Mussels that had been detached from their substrate. A future study should examine preferences of settling juveniles, which are more numerous and can cause much greater fouling. Our observations suggest that juveniles also avoid mesh substrates. In an unrelated experiment, we left Ace mesh bags containing stainless steel mesh containers suspended underwater in Lake Champlain; after approximately three months, from June to September, the lines holding the bags were heavily colonized with newly settled Zebra Mussels, but there were few mussels on the mesh bags and none on the mesh portion of the steel containers. From a practical standpoint, these results suggest that mesh materials used for a variety of scientific, fishery, and industrial applications are Figure 3. Attachment of Zebra Mussels to hard substrates vs. hard substrate with mesh. Bars are the mean of 10 replicates ± 1 standard deviation. 594 Northeastern Naturalist Vol. 15, No. 4 unlikely to become badly fouled by Zebra Mussels. Moreover, the composition of the fabric is not important. The meshes we used were at the small end of the range of those used in most fishery work; gill nets and trap nets, for example, generally have meshes with holes from 1.3 to 15.2 cm. Acknowledgments We thank Kathy Chiang, who first noticed the phenomenon of mesh avoidance, and Declan McCabe, who assisted with data analysis. This study was conducted at the Rubenstein Ecosystem Science Laboratory at the University of Vermont. Literature Cited Ackerman, J.D., C.R. Ethier, and J.K. Spelt 1993. The biomechanics of byssal adhesion in Zebra Mussels (Dreissena polymorpha): Tests with a rotating disk. Pp. 265–282, In T. Nalepa and D. Schloesser (Eds.). Zebra Mussels: Biology, Impacts, and Control. Lewis Publishers, Ann Arbor, MI. Cawein, J. 1993. Zebra Mussel aggregation and migration: A study of mobility and micro-habitat site selection by the invading mollusc Dreissena polymorpha. M.Sc. Thesis. Northwestern University, Evanston, IL. Claudi, R., and G.L. Mackie. 1994. Practical Manual for Zebra Mussel Monitoring and Control. Lewis Press, Boca Raton, fl. de Lafontaine, Y., G. Costan, and F. Delisle. 2002. Testing a new anti-Zebra Mussel coating with a multi-plate sampler: Confounding factors and other fuzzy features. Biofouling 18:1–12. Eckroat, L.E., E.C. Masteller, J.C. Shaffer, and L.M. Steele. 1993. The byssus of the Zebra Mussel (Dreissena polymorpha): Morphology, byssal thread formation, and detachment. Pp. 239–263, In T. Nalepa and D. Schloesser (Eds.). Zebra Mussels: Biology, Impacts, and Control. Lewis Publishers, Ann Arbor, MI. Kilgour, B.W., and G.L. Mackie. 1993. Colonization of different construction materials by the Zebra Mussel Dreissena polymorpha. Pp. 167–175, In T. Nalepa and D. Schloesser (Eds.). Zebra Mussels: Biology, Impacts, and Control. Lewis Publishers, Ann Arbor, MI. Kobak, J. 2004, Recruitment and small-scale distribution of Dreissena polymorpha (Bivalvia) on artificial materials. Archiv für Hydrobiologie 160:25–44. Marsden, J.E., and D.M. Lansky. 2000. Substrate preferences of Zebra Mussels, Dreissena polymorpha, relative to material, texture, orientation, and sunlight. Canadian Journal of Zoology 78:787–793. Mussalli, Y.G., and J.L. Tsou. 1990. Successful coating applications based on nontoxic formulations. Pp. 1–20, In Workshop on Nonpolluting Control of Biosurface Fouling. Buffalo, NY. Wainman, B.C., S.S. Hincks, N.K. Kaushik, and G.L. Mackie. 1996. Biofilm and substrate preference in the dreissenid larvae of Lake Erie. Canadian Journal of Fisheries and Aquatic Sciences 53:134–140.