Variation of amphipod assemblage along the Sargassum stenophyllum (Phaeophyta, Fucales) thallus

ABSTRACTThe spatial distribution of fauna associated to marine macroalgae has mostly been investigated considering a horizontal plane. However, the macroalgal substrates can present a three-dimensional structure. In this sense, investigating how the associated fauna varies throughout a vertical plane can contribute to understanding the distribution of these organisms. The brown macroalga Sargassumpresents a vertical stratification along its thallus and harbors an amphipod fauna with a variety of feeding habits. In this work, we tested if the amphipod assemblage varies along different portions of the Sargassum thallus. We collected whole Sargassum stenophyllum thalli, as well as isolated basal and distal portions, from a rocky shore located on the north coast of São Paulo, in southeastern Brazil. The composition of amphipod families varied according to the Sargassum portion and the families Corophiidae, Caprellidae and Hyalidae accounted for most of the differences. Moreover, the basal portion of Sargassum had a higher diversity of amphipod families than the distal one, which may be related to differences regarding habitat complexity (herein, measured as algal biomass) between these portions. Detritivores (such as Corophiidae) were more associated to the basal portion and herbivores to the distal portion (Hyalidae) or along the whole Sargassum thallus (Ampithoidae). The variation of amphipod assemblage along Sargassum thallus seems to result from the interaction between the fauna natural history and the differences in conditions and physical structure along the algal thallus. In this sense, the vertical stratification of Sargassum can add another source of variation to the spatial distribution of associated fauna.摘要：以往针对与海洋大型藻类相关的伴生动物空间分布的研究，大多仅考虑水平平面维度。然而，大型藻类基质可呈现三维结构。有鉴于此，探究伴生动物群落在垂直平面上的分布变化规律，有助于深入理解此类生物的分布模式。褐藻马尾藻（Sargassum）的藻体存在垂直分层结构，且其附着的端足类动物类群具有多样的摄食习性。本研究旨在验证：马尾藻藻体的不同区段，其伴生端足类群落组成是否存在差异。我们从巴西东南部圣保罗州北海岸的岩岸生境中，采集了完整的宽叶马尾藻（Sargassum stenophyllum）藻体，以及分离得到的基部和远端区段样本。研究结果显示，端足类各科的组成随马尾藻藻体区段的不同而发生变化，其中蜾蠃蜚科（Corophiidae）、麦秆虫科（Caprellidae）和玻璃钩虾科（Hyalidae）是造成群落差异的主要类群。此外，马尾藻基部区段的端足类科多样性显著高于远端区段，这一现象或与两类区段的生境复杂度差异相关——本研究中以藻类生物量作为生境复杂度的衡量指标。食碎屑类动物（如蜾蠃蜚科）更倾向于附着于基部区段，而植食性动物则多分布于远端区段（如玻璃钩虾科），或广泛分布于整个马尾藻藻体（如合眼钩虾科Ampithoidae）。端足类群落在马尾藻藻体上的垂直分布差异，似乎是由动物自身的自然历史特性与藻体不同区段的环境条件、物理结构差异共同作用导致的。综上，马尾藻的垂直分层结构可为伴生动物群落的空间分布增添另一项变异来源。