魚嘴

此條目介紹的是魚類的生理結構。關於其他用法，請見「魚嘴鎮」和「魚嘴站」。

魚嘴（fish mouth）也稱魚顎或魚頜（fish jaw），是構成魚類的口腔的骨骼、軟骨、肌肉和韌帶等軟組織的組合結構，是魚類消化道和呼吸道的開端。絕大多數硬骨魚有兩套頜骨——主要的口頜（oral jaws）負責開合嘴部併吞咬食物，而位於咽部後方的咽頜（pharyngeal jaws）則負責咀嚼和吞咽食物^[2][3]。軟骨魚類（比如鯊魚和鰩魚）只有由軟骨組成的口頜。通常情況下，魚類的頜骨由關節連接、垂直相對的上頜和下頜組成，並且可以擁有規則排列的牙齒。軟骨魚的口頜則會長出數套多次從內向外移動更換的牙齒。

慈鯛的頭骨中口頜（紫色）和咽頜（藍色）的側視圖^[1]

展現一條幼年扁頭恐怖麗魚下咽頜和口頜中的鰓弓（咽弓）和角鰓硬組織（弓骨）的背視圖，白色星號所指的帶有牙齒的咽頜（圖中的比例尺為500微米^[1]）

魚類的頜骨（特別是硬骨魚）演化出了類似連杆機構的複雜結構，來適應水域生態系統中的各種競爭需求。比較明顯的是能夠迅速協調的將頜骨向前突出張開的平面四杆機構，使得魚嘴可以迅速擴大口腔體積並產生負壓將獵物吸入口中。魚類的前上頜骨也配有這種結構^[4]，使得整個魚嘴擁有三套四杆機構可以前後、上下、左右的口腔直徑都產生擴大^[4][5][6]。

演化

魚類和其它脊椎動物綱的演化紡錘圖^[7]，最早發展出頜部的是已經滅絕的盾皮魚綱和棘魚綱

魚嘴的頜骨結構可能來源於支撐無頜魚類鰓部的咽弓（pharyngeal arches）。最早的魚頜出現在約4億3000萬年前的志留紀的（已滅絕的）盾皮魚^[8]和棘魚之中^[9]。擁有可積極開合的顎部最初的選擇優勢可能並不與進食有關，而是增加呼吸效率——顎部開合產生的「頰泵」（buccal pump）效應可以讓更多的新鮮水在單位時間內流過魚鰓以便增加納氧量。用開合頜骨進行吞咬很可能只是一個連帶產生的副功能，但隨後變成了許多從早期魚類演化出的脊椎動物的主要生存技能，被一些演化生物學家譽為「脊椎動物歷史上最深刻並最激進的演化步伐」^[10][11]和「至關重要的創新」^[12]。相比之下，無頜魚類的生存難度更高，因此大部分都在三疊紀滅絕沒能存活至今，對少數存活至今的圓口綱魚類（盲鰻和七鰓鰻）的研究也沒能幫助解釋早期頜骨的演化對脊椎動物頭骨深層重塑的影響^[13][14]。

通常的看法是脊椎動物的頜骨與魚類的鰓弓（branchial arch或gill arches）同源^[15]，都來自於胚胎階段發展出的咽弓。無頜魚的鰓裂開口在嘴後方，由軟骨組織支撐，而第一組鰓弓則環繞嘴口。這第一組鰓弓在有頜魚中發生對摺彎曲變成了上下頜；第二鰓弓的上部在則變成了負責將頜骨和顱骨連接的舌頜骨（hyomandibula）^[16]，在真骨魚中還負責懸掛鰓蓋（operculum）^[17]。現在被普遍接受的看法是有頜魚類的祖先是身上有骨質的甲板覆蓋、無頜的甲冑魚^[18][19]。

另見

• 有頜下門

參考

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15. For example: (1) both sets of bones are made from neural crest cells (rather than mesodermal tissue like most other bones); (2) both structures form the upper and lower bars that bend forward and are hinged in the middle; and (3) the musculature of the jaw seem homologous to the gill arches of jawless fishes. (Gilbert 2000)
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額外閱讀

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• Gilbert. Figure 1.14. Jaw structure in the fish, reptile, and mammal. (illustration). 2000.

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外部連結

外部影片連結
Video of a slingjaw wrasse catching prey by protruding its jaw
Video of a red bay snook catching prey by suction feeding

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• Myers, PZ. Evolution of the jaw. Pharyngula. 13 March 2007 [2022-06-27]. （原始內容存檔於2021-05-09）.
• Barford, Eliot. Ancient fish face shows roots of modern jaw. News. Nature. 25 September 2013 [2022-06-27]. （原始內容存檔於2013-10-31）.
• Zhu, Min; Yu, Xiaobo; Erik Ahlberg, Per; Choo, Brian; Lu, Jing; Qiao, Tuo; Qu, Qingming; Zhao, Wenjin; Jia, Liantao; Blom, Henning; Zhu, You’an. A Silurian placoderm with osteichthyan-like marginal jaw bones. Nature. 2013, 502 (7470): 188–193. Bibcode:2013Natur.502..188Z. PMID 24067611. S2CID 4462506. doi:10.1038/nature12617.