Department of Coastal and Marine Systems Science, Coastal Carolina University, Conway, South Carolina, USA.
Department of Marine Science, Coastal Carolina University, Conway, South Carolina, USA.
J Fish Biol. 2019 Nov;95(5):1331-1341. doi: 10.1111/jfb.14139. Epub 2019 Oct 7.
Discovery of an unusual rectal gland in the Atlantic sixgill shark Hexanchus vitulus led us to examine the rectal glands of 31 species of sharks to study diversity in rectal-gland morphology. Twenty-four of 31 species of sharks had digitiform glands (mean width-length ratio ± SD = 0.17 ± 0.04) previously assumed to be characteristic of all elasmobranchs regardless of habitat depth or phylogenetic age. Rectal glands from the family Somniosidae were kidney bean-shaped (mean width: length ± SD = 0.46 ± 0.05); whereas those from families Echinorhinidae and Hexanchidae were lobulate (mean width: length ± SD = 0.55 ± 0.06). Rectal gland width: length were different among species with digitiform morphology and lobulate morphology (ANOVA; R = 0.9; df = 15, 386; 401, F = 219.24; P < 0.001). Histological and morphological characteristics of the digitiform morphology from deep-sea sharks were similar to those from shallow-water sharks. Histology of lobulate rectal glands from hexanchids were characterised by tubule bundles separated by smooth muscle around a central lumen. Additionally, we examined plasma chemistry of four species of sharks with digitiform rectal glands and two species with lobulate rectal-gland morphology to see if there were differences between morphologies. Plasma chemistry analysis showed that urea and trimethylamine N-oxide (TMAO) followed the piezolyte hypothesis, with TMAO being highest and urea being lowest in deep-sea sharks. Among electrolytes, Na was highest in species with lobulate rectal glands. Hexanchids and echinorhinids both have lobulate rectal glands similar to those of holocephalans, despite the more than 400 million years separating these two groups. The morphological similarities between the lobulate rectal-gland anatomy of primitive sharks and the secretory morphology of holocephalans may represent an intermediate state between Holocephali and derived shark species.
在大西洋六鳃鲨（Hexanchus vitulus）中发现了一种不寻常的直肠腺，促使我们检查了 31 种鲨鱼的直肠腺，以研究直肠腺形态的多样性。31 种鲨鱼中有 24 种具有先前被认为是所有软骨鱼类特征的指状腺（平均宽-长比 ± SD = 0.17 ± 0.04），无论栖息深度或系统发育年龄如何。Somniosidae 科的直肠腺呈咖啡豆形（平均宽度：长度 ± SD = 0.46 ± 0.05）；而 Echinorhinidae 和 Hexanchidae 科的直肠腺呈小叶状（平均宽度：长度 ± SD = 0.55 ± 0.06）。具有指状形态和小叶状形态的物种之间的直肠腺宽度：长度不同（ANOVA；R = 0.9；df = 15, 386；401，F = 219.24；P < 0.001）。深海鲨鱼的指状形态的组织学和形态学特征与浅海鲨鱼相似。来自六鳃鲨的小叶状直肠腺的组织学特征是管束被中央管腔周围的平滑肌分离。此外，我们检查了具有指状直肠腺的四种鲨鱼和具有小叶状直肠腺形态的两种鲨鱼的血浆化学，以观察形态之间是否存在差异。血浆化学分析表明，尿素和三甲胺氧化物（TMAO）符合压离子假说，深海鲨鱼中的 TMAO 最高，尿素最低。在电解质中，Na 在具有小叶状直肠腺的物种中最高。尽管这两组之间相隔超过 4 亿年，但六鳃鲨和棘鲨都具有类似于全头类的小叶状直肠腺。原始鲨鱼的小叶状直肠腺解剖结构与全头类的分泌形态之间的形态相似性可能代表了 Holocephali 和衍生鲨鱼之间的中间状态。
