File S3. Concatenated 18S+28S sequences used for the maximum-likelihood analysis, reduced to 2440 positions (1–1458 for 18S; 1459–2440 for 28S) by removing alignment-ambiguous sites with Gblocks under “relaxed” parameters. from Flatworm cocoons in the abyss: same plan under pressure

While knowledge of early ontogeny in abyssal animals is highly limited in general, it was completely lacking for abyssal, free-living platyhelminths. We discovered flatworm egg capsules (or ‘cocoons') on rocks collected at depths of 6176–6200 m on the abyssal slope of the Kuril-Kamchatka Trench, northwestern Pacific. The egg capsules were black and spherical, around 3 mm in diameter, and contained three to seven individuals (<i>n</i> = 4) at the same developmental stage, either the spherical (putative early embryo) or vermiform (putative late embryo) stages. A molecular phylogenetic analysis based on 18S and 28S rRNA sequences revealed that the flatworms belong in suborder Maricola in Tricladida and suggested that they may have colonized from shallow to deep waters. This study provides the deepest record for free-living flatworms and the first information on their early life stages in the abyssal zone, which were very similar to those in shallow-water forms. This similarity in development between the relatively benign shallow-water and the extreme abyssal environments suggests that triclads adapting to the latter faced primarily physiological and/or ecological adaptive challenges rather than developmental ones.

总体而言，学界对深渊动物早期个体发育的认知本就极为有限，而对于自由生活的深渊扁形动物而言，相关研究更是完全空白。我们在西北太平洋千岛-堪察加海沟深渊斜坡6176~6200米水深采集的岩石上，发现了扁形动物的卵囊（或称“卵茧”）。这些卵囊呈黑色球形，直径约3毫米，内部包含3至7个处于同一发育阶段的个体（n=4），其发育阶段要么为球形（推定的早期胚胎），要么为蠕形（推定的晚期胚胎）。基于18S和28S核糖体RNA（rRNA）序列的分子系统发育分析显示，该扁形动物隶属于三肠目（Tricladida）海栖亚目（Maricola），研究结果还表明它们可能是从浅海向深海演化定植的。本研究不仅记录了迄今分布最深的自由生活扁形动物类群，还首次揭示了深渊带内该类群的早期生活史信息，其早期发育过程与浅海类群极为相似。相对温和的浅水环境与极端的深渊环境之间存在如此发育特征的相似性，这表明适应深渊环境的三肠目扁形动物所面临的适应性挑战主要来自生理和/或生态层面，而非发育层面。