Department of Integrative Biology and Museum of Vertebrate Zoology, University of California, Berkeley, California, USA.
Department of Biology, University of North Carolina at Chapel Hill, North Carolina, USA.
J Fish Biol. 2020 Jul;97(1):163-171. doi: 10.1111/jfb.14344. Epub 2020 May 3.
Dietary specialization on hard prey items, such as mollusks and crustaceans, is commonly observed in a diverse array of fish species. Many fish consume these types of prey by crushing the shell to consume the soft tissue within, but a few fishes extricate the soft tissue without breaking the shell using a method known as oral shelling. Oral shelling involves pulling a mollusc from its shell and it may be a way to subvert an otherwise insurmountable shell defence. However, the biomechanical requirements and potential adaptations for oral shelling are unknown. Here, we test the hypothesis that a novel nasal protrusion is an adaptation for oral shelling in the durophagous pupfish (Cyprinodon brontotheroides). We first demonstrate oral shelling in this species and then predict that a larger nasal protrusion would allow pupfish to consume larger snails. Durophagous pupfish are found within an endemic radiation of pupfish on San Salvador Island, Bahamas. We took advantage of closely related sympatric species and outgroups to test: (a) whether durophagous pupfish shell and consume more snails than other species, (b) if F1 and F2 durophagous hybrids consume similar amounts of snails as purebred durophagous pupfish, and (c) if nasal protrusion size in parental and hybrid populations increases the maximum size of consumed snails. We found that durophagous pupfish and their hybrids consumed the most snails, but did not find a strong association between nasal protrusion size and maximum snail size consumed within the parental or F2 hybrid population, suggesting that the size of their novel nasal protrusion does not provide a major benefit in oral shelling. Instead, we suggest that the nasal protrusion may increase feeding efficiency, act as a sensory organ, or is a sexually selected trait, and that a strong feeding preference may be most important for oral shelling.
鱼类广泛存在以硬壳猎物（如软体动物和甲壳类动物）为食的特化现象。许多鱼类通过压碎外壳来摄取内部的软组织来消耗这些类型的猎物，但有几种鱼类在不破坏外壳的情况下利用一种称为口腔脱壳的方法提取软组织。口腔脱壳涉及从壳中取出软体动物，这可能是一种颠覆原本无法克服的外壳防御的方法。然而，口腔脱壳的生物力学要求和潜在适应机制尚不清楚。在这里，我们检验了一个假设，即一个新的鼻腔突起是食硬鱼（Cyprinodon brontotheroides）口腔脱壳的适应特征。我们首先证明了该物种的口腔脱壳行为，然后预测更大的鼻腔突起可以使食硬鱼消耗更大的蜗牛。食硬鱼分布在巴哈马圣萨尔瓦多岛上的一个特有辐射种群中。我们利用密切相关的同域物种和外群来检验：（a）食硬鱼是否比其他物种吃更多的蜗牛；（b）F1 和 F2 食硬杂种是否与纯种食硬鱼消耗相似数量的蜗牛；（c）亲代和杂种种群中鼻腔突起大小是否会增加消耗的蜗牛最大尺寸。我们发现食硬鱼及其杂种消耗的蜗牛最多，但在亲代或 F2 杂种群体中，鼻腔突起大小与消耗的蜗牛最大尺寸之间没有很强的关联，这表明它们新的鼻腔突起大小在口腔脱壳中并没有提供主要优势。相反，我们认为鼻腔突起可能提高了进食效率，充当了感觉器官，或者是一种性选择特征，而强烈的摄食偏好可能是口腔脱壳最重要的因素。
