“眼高于顶”的鱼类——异目鱼 （原创 克尔-纽曼黑洞）

异目鱼属Ipnops属于异目鱼科Ipnopidae，共3种。模式种为Ipnops murrayiGünther, 1878。该属幼体于海水表层浮游生活，亚成体即沉入深海，成体分布于水深1392~4970米。

Ipnops murrayi/ NOAA Office of Ocean Exploration and Research, Gulf of Mexico 2018, Public domain

Ipnops murrayi/ NOAA Office of Ocean Exploration and Research, Exploring Deep-Sea Habitats off Puerto Rico and the U.S. Virgin Islands, 2018, Public domain

异目鱼最显著的特征即为其头顶的在硬骨鱼中形状独一无二的器官。该器官曾被认为是发光器或同时具有眼和发光功能，后经解剖确认为高度特化的眼。成年异目鱼的眼位于头顶，表面覆盖一层薄的透明膜，该膜由左右额骨（frontal bone）和顶骨（parietal bone）融合而成。眼无晶状体，虹膜退化，视网膜极扁平。眼本身固定，因此眼肌高度退化。这些特征意味着异目鱼的眼睛为探测极微量光线而牺牲了所有成像装置。

异目鱼幼体在海水表层浮游生活，此时其眼睛较为正常，位于头部两侧。在成长过程中，异目鱼栖息地逐渐变深，双眼90°转向并移至头顶，额骨和顶骨逐渐融合，最终形成成年异目鱼结构极为特殊的眼。

Ipnops meadi/ Astrid Leitner, NOAA Office of Exploration, Public domain

异目鱼骨骼钙化程度较低，嗅觉器官严重退化，但侧线系统特别是头部的感觉器官发达。鳃盖膜和鳃盖结构使得从鳃腔排出的水必须通过鳃盖角斜向上涌出，这样可避免身体侧线受到不必要的刺激，也不会过度搅动沉积物。异目鱼主要栖息于深渊底部，通过头顶的感光器观测多毛类或小型甲壳类活动时产生的微弱光芒——这些生物可能自身不发光，但其活动时会惊扰附近的发光微生物导致发光。

Ipnops meadi/ Thiel et al, 2022

异目鱼雌雄同体，通常集群出现。其眼部对光线的独特反射使其在深海观测中极为显眼，因此成为克拉里昂-克利帕顿断裂带（Clarion-Clipperton Zone，CCZ）最常见的一属鱼类。

参考文献及部分图片来源：

Anderson W W, Berry F H, Böhlke J E, et al, 1966. Fishes of the Western North Atlantic: Orders Iniomi and Lyomeri: Part 5[M]. New Haven:Yale University Press: 147.

Marshall N B, Staiger J C, 1975. Biological results of the University of Miami deep-sea expeditions. 110: Aspects of the structure, relationships, and biology of the deep-sea fishIpnops murrayi(Family Bathypteroidae)[J]. Bulletin of Marine Science, 25(1): 101-111.

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Okiyama M, 1981. A larval Ipnops and its possible metamorphosing process[J]. Japanese Journal of Ichthyology, 28(3): 247-253.

Okiyama M, 1986. Bathypelagic capture of a metamorphosing juvenile ofIpnops agassizi(Ipnopidae, Myctophiformes)[J]. Japanese Journal of Ichthyology, 32(4): 443-446.

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Prirodina V P, Neyelov A V, 2020. The osteological features ofIpnops agassiziiGarman, 1899 (Aulopiformes: Ipnopidae) from Bathyal and Ultra-Abyssal Depths of the Australia–New Zealand Region, with remarks on the biogeographical significance of these findings[J]. Russian Journal of Marine Biology, 46(1): 22-28.

Theisen B, 1966. On the cranial morphology ofIpnops murrayiGunther, 1878: With special reference to the relations between the eyes and the skull[J]. Galathea, 28: 7-18.

Thiel R, Christodoulou M, Pogonoski J J, et al, 2022. An application of morphological analysis and DNA barcoding to identifyIpnopsfrom the Clarion-Clipperton Zone (CCZ) as I. meadi Nielsen, 1966 with notes on other species of the genus (Aulopiformes: Ipnopidae)[J]. Marine Biodiversity, 52(6): 68.

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