File S1. SeqAilgment 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生态系统中极为突出，且对这类系统的稳定性至关重要。为阐释共生关系的建立与演化机制，我们以附生海绵的藤壶为研究对象，提出以下三个研究问题：(1) 海绵宿主表面的幼虫附着是否需要助力共生的全新适应性特征？(2) 幼虫如何在宿主表面附着并开启其生活史？(3) 涉及众多藤壶物种的这一特殊共生生活方式是如何演化而来的？我们发现，附生海绵的藤壶其幼虫（cyprids）展现出远高于其他藤壶的显著种间变异水平。我们证实，幼虫附着器官的变异与它们附着并发生变态的表面特性存在特异性关联。将幼虫与海绵表面特征映射到基于分子构建的系统发育树后显示，藤壶内的海绵共生习性至少独立演化过三次，且所有幼虫均呈现出相同的适应性特征，与其系统发育亲缘关系无关。此外，两个物种的变态过程差异显著：其中一种附着于宿主体表并形成一组白色钙质结构，另一种则在附着后几乎立即嵌入活体宿主组织内部。我们认为，藤壶幼虫如此高度的进化灵活性，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重要作用。