Data from: Size structuring and allometric scaling relationships in coral reef fishes

Temperate marine fish communities are often size structured, with predators consuming increasingly larger prey and feeding at higher trophic levels as they grow. Gape limitation and ontogenetic diet shifts are key mechanisms by which size structuring arises in these communities. Little is known, however, about size structuring in coral reef fishes.
Here, we aimed to advance understanding of size structuring in coral reef food webs by examining the evidence for these mechanisms in two groups of reef predators. Given the diversity of feeding modes amongst coral reef fishes, we also compared gape size—body size allometric relationships across functional groups to determine if they are reliable indicators of size structuring.
We used gut content analysis and quantile regressions of predator size—prey size relationships to test for evidence of gape limitation and ontogenetic niche shifts in reef piscivores (n=13 species) and benthic invertivores (n=3 species). We then estimated gape size—body size allometric scaling coefficients for 21 different species from four functional groups, including herbivores/detritivores, which are not expected to be gape-limited.
We found evidence of both mechanisms for size structuring in coral reef piscivores, with maximum prey size scaling positively with predator body size, and ontogenetic diet shifts including prey type and expansion of prey size. There was, however, little evidence of size structuring in benthic invertivores. Across species and functional groups, absolute and relative gape sizes were largest in piscivores as expected, but gape size—body size scaling relationships were not indicative of size structuring. Instead, relative gape sizes and mouth morphologies may be better indicators.
Our results provide evidence that coral reef piscivores are size-structured, and that gape limitation and ontogenetic niche shifts are the mechanisms from which this structure arises. Although gape allometry was not indicative of size structuring, it may have implications for ecosystem function: positively allometric gape size—body size scaling relationships in herbivores/detritivores suggests that loss of large-bodied individuals of these species will have a disproportionately negative impact on reef grazing pressure.

温带海洋鱼类群落通常具有体型结构特征：捕食者随生长会捕食体型更大的猎物，并在更高的营养级（trophic levels）进行摄食。口限约束（gape limitation）与个体发育食性转变（ontogenetic diet shifts）是这类群落形成体型结构的核心机制。然而，学界对珊瑚礁鱼类的体型结构特征仍知之甚少。
本研究旨在通过在两类礁栖捕食者类群中检验上述机制的相关证据，加深对珊瑚礁食物网体型结构的认知。鉴于珊瑚礁鱼类摄食模式的多样性，本研究还比较了不同功能群（functional groups）的口部尺寸-体型异速生长（allometric）关系，以判断其是否可作为体型结构的可靠指示指标。
我们采用胃含物分析（gut content analysis）与捕食者体型-猎物体型关系的分位数回归（quantile regressions）方法，检验了礁栖肉食性鱼类（piscivores，n=13种）与底栖食虫动物（benthic invertivores，n=3种）存在口限约束与个体发育生态位转变的相关证据。随后，我们估算了4个功能群共21个物种的口部尺寸-体型异速生长缩放系数，其中植食性/腐食性类群（herbivores/detritivores）理论上不受口限约束。
研究结果显示，珊瑚礁肉食性鱼类存在上述两种体型结构形成机制的相关证据：最大猎物体型随捕食者体型呈正相关缩放，且存在食性类型转变与猎物体型范围扩大的个体发育食性转变现象。但底栖食虫类群几乎未表现出体型结构特征。跨物种与功能群来看，肉食性鱼类的绝对与相对口部尺寸均如预期般最大，但口部尺寸-体型的缩放关系并不能指示体型结构。相较而言，相对口部尺寸与口部形态或可作为更合适的指示指标。
本研究结果证实，珊瑚礁肉食性鱼类具有体型结构特征，且口限约束与个体发育生态位转变正是该结构形成的核心机制。尽管口部尺寸异速生长无法指示体型结构，但其对生态系统功能或具有重要意义：植食性/腐食性类群的口部尺寸-体型呈正异速生长缩放关系，意味着这类物种的大型个体消亡，将对珊瑚礁牧食压力产生不成比例的负面影响。