The volume of the olfactory bulbs (OBs) relative to the brain has been used previously as a proxy for olfactory capabilities in many vertebrate taxa, including fishes. Although this gross approach has predictive power, a more accurate assessment of the number of afferent olfactory inputs and the convergence of this information at the level of the telencephalon is critical to our understanding of the role of olfaction in the behaviour of fishes. In this study, we used transmission electron microscopy to assess the number of first-order axons within the olfactory nerve (ON) and the number of second-order axons in the olfactory peduncle (OP) in established model species within cartilaginous (brownbanded bamboo shark, Chiloscyllium punctatum [CP]) and bony (common goldfish, Carassius auratus [CA]) fishes. The total number of axons varied from a mean of 18.12 ± 7.50 million in the ON to a mean of 0.38 ± 0.21 million in the OP of CP, versus 0.48 ± 0.16 million in the ON and 0.09 ± 0.02 million in the OP of CA. This resulted in a convergence ratio of approximately 50:1 and 5:1, respectively, for these two species. Based on astroglial ensheathing, axon type (unmyelinated [UM] and myelinated [M]) and axon size, we found no differentiated tracts in the OP of CP, whereas a lateral and a medial tract (both of which could be subdivided into two bundles or areas) were identified for CA, as previously described. Linear regression analyses revealed significant differences not only in axon density between species and locations (nerves and peduncles), but also in axon type and axon diameter (p #x3c; 0.05). However, UM axon diameter was larger in the OPs than in the nerve in both species (p = 0.005), with no significant differences in UM axon diameter in the ON (p = 0.06) between species. This study provides an in-depth analysis of the neuroanatomical organisation of the ascending olfactory pathway in two fish taxa and a quantitative anatomical comparison of the summation of olfactory information. Our results support the assertion that relative OB volume is a good indicator of the level of olfactory input and thereby a proxy for olfactory capabilities.
Brain Behav Evol

中文翻译：

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软骨鱼和硬骨鱼中枢神经系统内嗅觉输入的收敛：嗅觉敏感性的解剖学指标

嗅球 (OB) 相对于大脑的体积以前曾被用作许多脊椎动物类群（包括鱼类）嗅觉能力的代表。尽管这种粗略的方法具有预测能力，但更准确地评估传入嗅觉输入的数量和端脑水平的这些信息的收敛性对于我们理解嗅觉在鱼类行为中的作用至关重要。在这项研究中，我们使用透射电子显微镜评估了软骨（褐带竹鲨, Chiloscyllium punctatum [CP]) 和骨（普通金鱼，Carassius auratus[CA]) 鱼。轴突总数从 ON 的平均值 18.12 ± 750 万到 CP OP 的平均值 0.38 ± 21 万不等，而 ON 为 0.48 ± 16 万，CA OP 为 0.09 ± 02 万。这导致这两个物种的收敛比分别约为 50:1 和 5:1。根据星形胶质细胞鞘、轴突类型（无髓鞘 [UM] 和有髓鞘 [M]）和轴突大小，我们在 CP 的 OP 中没有发现分化的束，而外侧束和内侧束（两者都可以细分为两个束）或区域）被确定为 CA，如前所述。线性回归分析显示，不仅物种和位置（神经和花梗）之间的轴突密度存在显着差异，而且轴突类型和轴突直径也存在显着差异（p#x3c; 0.05）。然而，OPs 中的 UM 轴突直径大于两个物种的神经 ( p = 0.005)，物种间ON ( p = 0.06)中的 UM 轴突直径没有显着差异。这项研究提供了对两种鱼类分类群中上行嗅觉通路的神经解剖学组织的深入分析，并对嗅觉信息的总和进行了定量解剖比较。我们的结果支持这样的断言，即相对 OB 体积是嗅觉输入水平的良好指标，因此是嗅觉能力的代理。
大脑行为进化