Data from: Invasive bryozoan alters interaction between a native grazer and its algal food

The epiphytic bryozoan Membranipora membranacea encrusts the surface of kelp blades, causing recurrent large-scale defoliation events in kelp beds off the Atlantic coast of Nova Scotia, Canada. The gastropod Lacuna vincta grazes kelp, creating perforations that weaken blade tissues and increase the fragmentation rate. We assess the interaction between M. membranacea and L. vincta by measuring the grazing rate of snails on bryozoan-encrusted and non-encrusted kelp (Saccharina latissima) in no-choice and choice experiments in the laboratory conducted in November and December 2010. There was no effect of diet on grazing rate in no-choice experiments. In choice experiments, however, L. vincta grazed significantly more non-encrusted than encrusted kelp (7.1 versus 1.1 mg snail−1 d−1), and grazing rate of non-encrusted kelp was almost twice that in the no-choice experiment (3.8 mg snail−1 d−1), indicating that snails may avoid colonies of M. membranacea on partially encrusted kelp blades. We found no effect of diet on growth, reproduction and survival of snails maintained for four weeks on encrusted or non-encrusted kelp. By concentrating grazing damage on non-encrusted areas of blades, L. vincta may act synergistically with M. membranacea to increase the likelihood of blade breakage and canopy loss. This indirect effect of the invasive bryozoan could augment its direct effect on the standing biomass of native kelp beds and detrital export to adjacent communities.附生苔藓虫（epiphytic bryozoan），膜孔苔虫（Membranipora membranacea）可附着于海带叶片表面，致使加拿大新斯科舍省大西洋沿岸的海带床反复出现大规模落叶事件。腹足类软体动物带纹凹顶螺（Lacuna vincta）会啃食海带，形成穿孔以削弱叶片组织并提高叶片破碎率。本研究于2010年11月至12月在实验室开展非选择与选择实验，通过测定该螺在被膜孔苔虫附着与未附着的长海带（Saccharina latissima）上的啃食速率，评估膜孔苔虫与带纹凹顶螺之间的相互作用。非选择实验结果显示，饵料类型对螺类的啃食速率无显著影响。但在选择实验中，带纹凹顶螺对未附着海带的啃食量显著高于附着海带（分别为7.1与1.1毫克·螺⁻¹·天⁻¹），且未附着海带的啃食速率几乎是非选择实验中该数值的两倍（3.8毫克·螺⁻¹·天⁻¹），这表明带纹凹顶螺会规避部分被膜孔苔虫附着的海带叶片上的苔虫群落。我们还发现，在分别以附着与未附着海带饲养螺类四周后，饵料类型对螺类的生长、繁殖与存活率均无显著影响。带纹凹顶螺通过将啃食损伤集中在叶片的未附着区域，可与膜孔苔虫产生协同作用，提升海带叶片断裂与林冠消失的概率。这种入侵性苔虫的间接效应，可能会增强其对本土海带床现存生物量的直接影响，并增加向邻近群落输出的碎屑通量。