Eye‐body allometry across biphasic ontogeny in anuran amphibians

Animals with biphasic lifecycles often inhabit different visual environments across ontogeny. Many frogs and toads (Amphibia: Anura) have free-living aquatic larvae (tadpoles) that metamorphose into adults that inhabit a range of aquatic and terrestrial environments. Ecological differences influence eye size across species, however, these relationships have not yet been explored across life stages in an ontogenetic allometric context. We examined eye-body size scaling in a species with aquatic larvae and terrestrial adults, the common frog Rana temporaria, using a well-sampled developmental series. We found a shift in ontogenetic allometric trajectory near metamorphosis indicating prioritized growth in tadpole eyes. To explore the effects of different tadpole and adult ecologies on eye-body scaling, we expanded our taxonomic sampling to include developmental series of eleven additional anuran species. Intraspecific eye-body scaling was variable among species, with 8/12 species exhibiting a significant change in allometric slope between tadpoles and adults. Across species, slopes of tadpoles were found to be more variable than those of their adult counterparts. Traits categorizing both tadpole (tadpole microhabitat, tadpole eye position, tadpole mouth position) and adult (adult habitat, adult activity pattern) ecology across species had significant effects on allometric slopes among tadpoles, but only tadpole eye position had a significant effect among adults. Our study suggests that relative eye growth in the preliminary stages of biphasic anuran ontogenies is somewhat decoupled and may be shaped by both immediate ecological need (i.e. tadpole visual requirements) and what will be advantageous during later adult stages.

具有双相生命周期（biphasic lifecycle）的动物，在个体发育（ontogeny）过程中往往会栖息于不同的视觉环境中。许多蛙类与蟾蜍（两栖纲：无尾目 Amphibia: Anura）拥有营自由生活的水生幼体——蝌蚪（tadpole），它们会经变态发育（metamorphosis）成为可栖息于多种水生与陆生环境的成体。不同物种间的生态差异会影响眼球大小，但目前尚未有研究在个体发育异速生长（ontogenetic allometric）的框架下，针对不同生命阶段探索这类关联。我们以兼具水生幼体与陆生成体的普通林蛙（Rana temporaria）为研究对象，依托一套采样完备的发育序列，探究了其眼体大小的缩放关系。研究发现，在变态发育阶段附近，个体发育异速生长轨迹发生了偏移，这表明蝌蚪的眼球存在优先生长现象。为探究蝌蚪与成体的不同生态特征对眼体缩放关系的影响，我们扩大了类群采样范围，新增了11个无尾目物种的发育序列。不同物种的种内眼体缩放关系存在差异，其中8/12的物种在蝌蚪与成体阶段的异速生长斜率发生了显著变化。跨物种分析显示，蝌蚪阶段的异速生长斜率相较于成体阶段更为多变。针对用于分类各物种蝌蚪生态特征（蝌蚪微生境、蝌蚪眼部位置、蝌蚪口部位置）与成体生态特征（成体栖息生境、成体活动模式）的相关指标分析表明：蝌蚪阶段的异速生长斜率受上述多项特征的显著影响；而成体阶段的异速生长斜率仅受蝌蚪眼部位置的显著影响。本研究表明，无尾目动物双相个体发育早期阶段的相对眼球生长存在一定程度的解耦现象，其既可能受当下的生态需求（即蝌蚪的视觉需求）调控，也可能受未来成体阶段的适应性优势所塑造。