Table S2a-b. ML+BI parameters from Sponge symbiosis is facilitated by adaptive evolution of larval sensory and attachment structures in barnacles

Symbiotic relations and range of host usage are prominent in coral reefs and crucial to the stability of such systems. In order to explain how symbiotic relations are established and evolve, we used sponge-associated barnacles to ask (1) Does larval settlement on sponge hosts require novel adaptations facilitating symbiosis? (2) How do larvae settle and start life on their hosts? (3) How has this remarkable symbiotic lifestyle involving many barnacle species evolved? We found that the larvae (cyprids) of sponge-associated barnacles show a remarkably high level of interspecific variation compared with other barnacles. We document that variation in larval attachment devices are specifically related to properties of the surface on which they attach and metamorphose. Mapping of the larval and sponge surface features onto a molecular-based phylogeny showed that sponge symbiosis evolved separately at least three times within barnacles, with the same adaptive features being found in all larvae irrespective of phylogenetic relatedness. Furthermore, the metamorphosis of two species proceeded very differently, with one species remaining superficially on the host and developing a set of white calcareous structures, the other embedding itself into the live host tissue almost immediately after settlement. We argue that such a high degree of evolutionary flexibility of barnacle larvae played an important role in the successful evolution of complex symbiotic relationships in both coral reefs and other marine systems.

共生关系与宿主利用范围在珊瑚礁（coral reefs）生态系统中极为显著，且对该类系统的稳定性至关重要。为阐明共生关系的建立与演化机制，本研究以海绵附生藤壶（sponge-associated barnacles）为研究对象，针对以下三个科学问题展开探讨：(1) 寄居于海绵宿主的藤壶幼体附着是否需要具备促进共生的新型适应性特征？(2) 幼体如何在宿主表面附着并开启其宿主相关生活史？(3) 这种涵盖众多藤壶物种的特殊共生生活方式是如何演化而来的？研究结果显示，相较于其他藤壶类群，海绵附生藤壶的幼体（cyprids，又称金星幼体）存在极高的种间变异水平。我们证实，幼体附着器官的变异与它们附着和变态的表面特性存在特异性关联。将幼体与海绵表面特征映射至基于分子构建的系统发育树（phylogeny）后发现，海绵共生习性在藤壶类群中至少独立演化了三次，且所有幼体均携带相同的适应性特征，与其系统发育亲缘关系无关。此外，两个藤壶物种的变态过程差异显著：其中一类始终附着于宿主表面，并发育出一组白色钙质结构；另一类则在附着后几乎立即嵌入活宿主组织内部。我们认为，藤壶幼体所具备的这种高度演化灵活性，在珊瑚礁及其他海洋生态系统中复杂共生关系的成功演化过程中发挥了关键作用。