Gut morphology frequently reflects the food organisms digest. Gizzards are organs of the gut found in archosaurs and fishes that mechanically reduce food to aid digestion. Gizzards are thought to compensate for edentulism and/or provide an advantage when consuming small, tough food items (e.g., phytoplankton and algae). It is unknown how widespread gizzards are in fishes and how similar these structures are among different lineages. Here, we investigate the distribution of gizzards across bony fishes to (1) survey different fishes for gizzard presence, (2) compare the histological structure of gizzards in three species, (3) estimate how often gizzards have evolved in fishes, and (4) explore whether anatomical and ecological traits like edentulism and microphagy predict gizzard presence. According to our analyses, gizzards are rare across bony fishes, evolving only six times in a broad taxonomic sampling of 51 species, and gizzard presence is not clearly correlated with factors like gut length or dentition. We find that gizzard morphology varies among the lineages where one is present, both macroscopically (presence of a crop) and microscopically (varying tissue types). We conclude that gizzards likely aid in the mechanical reduction of food in fishes that have lost an oral dentition in their evolutionary past; however, the relative scarcity of gizzards suggests they are just one of many possible solutions for processing tough, nutrient-poor food items. Gizzards have long been present in the evolutionary history of fishes, can be found in a wide variety of marine and freshwater clades, and likely have been overlooked in many taxa.

中文翻译：

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真正的毅力？鱼类砂囊的比较解剖学和进化


肠道形态经常反映食物生物的消化情况。砂囊是古龙和鱼类的肠道器官，可以机械地减少食物以帮助消化。砂囊被认为可以补偿缺齿和/或在食用小而坚硬的食物（例如浮游植物和藻类）时提供优势。目前尚不清楚鱼类砂囊的分布范围有多大，以及这些结构在不同谱系中的相似程度如何。在这里，我们研究了骨鱼砂囊的分布，以（1）调查不同鱼类砂囊的存在，（2）比较三个物种砂囊的组织学结构，（3）估计鱼类砂囊进化的频率，（4） ）探索解剖学和生态特征（如缺牙和小食）是否可以预测砂囊的存在。根据我们的分析，砂囊在硬骨鱼中很少见，在 51 个物种的广泛分类采样中仅进化了六次，而且砂囊的存在与肠道长度或牙列等因素没有明显相关性。我们发现砂囊形态在存在的谱系之间存在差异，无论是宏观上（作物的存在）还是微观上（不同的组织类型）。我们得出的结论是，砂囊可能有助于机械地减少在进化过程中失去口腔齿列的鱼类的食物。然而，砂囊的相对稀缺表明，它们只是加工坚硬、营养不良食品的众多可能解决方案之一。砂囊长期以来一直存在于鱼类的进化史上，可以在各种海洋和淡水分支中找到，并且可能在许多类群中被忽视。