Canopy-forming seaweeds of the genus Cystoseira (Fucales, Phaeophyceae) form diverse and productive habitats along temperate rocky coasts of the Mediterranean Sea. During the last decade, Cystoseira forests have retracted their range considerably due to many interacting environmental, biological and anthropogenic pressures. We investigated how reducing in patch-size of C. montagnei affects their associated molluscan communities at the shallow northwest rocky shores of Palermo (Sicily, Italy). Molluscs were sampled from the fronds of individual thalli, clumps of 3 and 5 thalli of C. montagnei over an annual vegetative cycle (May–September) in two sites within the Marine Protected Area “Capo Gallo-Isola delle Femmine”. We measured five substrate attributes of the alga (thallus volume, canopy volume, interstitial volume, algal surface, and biomass) and explored their relationships with the diversity of the associated molluscan assemblage. A total of 3756 individuals of molluscs were collected, belonging to 30 families and 57 species, being Rissoidae the most speciose family and Barleeidae the dominant in terms of abundance. The molluscan assemblage on C. montagnei displayed significant spatial and seasonal variations, with the maximum number of individuals and species in summer, whereas evenness and diversity displayed maximum values in spring. The abundance, species richness and diversity of the molluscan assemblage decreased with decreasing in patch-size of C. montagnei, regardless of the vegetative phase of the alga or the sites considered in the study, while evenness showed an opposite trend. The substrate attributes of C. montagnei changed over the vegetative cycle of the alga and showed a similar pattern across sites with values that tended to decrease in autumn during the quiescence phase of the alga. Variation in patch-size of C. montagnei affected also the substrate attributes of the individual alga, with individual thalli becoming smaller with increasing patch-size. The algal surface of a single thallus of C. montagnei ranged on average from 956 cm² in spring to 289 cm² in autumn. Furthermore, algal surface together with thallus volume and dry weight were the substrate attributes that explained better the variation of the number of molluscan individuals and species. Overall, our results suggest that the reduction of both patch-size and algal substrate of C. montagnei forests altered the structure and composition of its associated molluscan assemblage. We argue that a reduction or loss of Cystoseira forests could probably trigger bottom-up effects in rocky shores habitats, with consequences for the whole ecosystem structure, functioning and services provided to humans.

Cystoseira属（Fucales，Phaeophyceae）的形成冠层的海藻在地中海温带多岩石的海岸上形成了多种多样的生境。在过去的十年中，由于许多相互影响的环境，生物和人为压力，Cystoseira森林已经大大缩小了范围。我们调查了蒙太尼梭菌的斑块大小减少如何影响其在巴勒莫西北浅岩海岸（意大利西西里岛）的软体动物群落。软体动物从个体菌体的叶状体取样，第3块和5菌体C. montagnei在海洋保护区“ Capo Gallo-Isola delle Femmine”内的两个地点的年度植物生长周期（5月至9月）中。我们测量了藻类的五个底物属性（Thallus体积，冠层体积，间隙体积，藻类表面和生物量），并探索了它们与相关软体动物组合多样性的关系。总共收集了3756个软体动物，属于30个科和57个物种，其中Rissoidae是最特殊的科，而Barleeidae在数量上则占优势。在软体动物组合C. montagnei表现出明显的空间和季节变化，夏季最多的个体和物种数量，而春季的均匀度和多样性最多。的丰度，物种丰富度和软体动物组合的多样性随在贴片尺寸减小C. montagnei，无论藻类或在研究中所考虑的网站的营养阶段，而均匀显示出相反的趋势。的衬底属性C. montagnei改变了藻类的生长周期和显示跨站点类似的模式与倾向于在藻类的静止阶段中秋降低值。蒙塔尼亚梭菌的斑块大小变化单独藻类的底物属性也受到影响，随着贴片尺寸的增加，个体的沙门变小。的单个菌体的藻类表面C. montagnei范围上从956厘米平均²在春天289厘米²在秋天。此外，藻类表面以及th的体积和干重是底物属性，可以更好地解释软体动物个体和物种数量的变化。总体而言，我们的研究结果表明，蒙塔芒山毛榉森林斑块大小和藻类基质的减少都改变了其相关软体动物组合的结构和组成。我们认为Cystoseira减少或丢失 森林可能会在多石的海岸生境中引发自下而上的影响，从而影响整个生态系统的结构，功能和向人类提供的服务。