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鱼虱科

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鱼虱科
雄性及雌性的鲑疮痂鱼虱
科学分类 编辑
界： 动物界 Animalia
门： 节肢动物门 Arthropoda
亚门： 甲壳亚门 Crustacea
纲： 桡足纲 Copepoda
目： 管口水蚤目 Siphonostomatoida
科： 鱼虱科 Caligidae
Burmeister, 1834[1]
属[2]

见正文

异名
  • Eirgidae
  • Euryphoridae
  • Pargulidae

海虱是桡脚类下的鱼虱科生物。其下共有36个属，当中最著名的疮痂鱼虱属及鱼虱属就分别约有42和300个物种。[2]它们是水生的 体外寄生虫，吃寄主的黏液、表皮组织及血液。其下的疮痂鱼虱属及鱼虱属会寄生在海鱼身上，尤其对饲养的鲑鱼造成很大的问题。

多样性

就海虱的认识主要是来自对饲养鲑鱼的感染。不过，就其生物学及与寄主间的关系所知甚少。

很多海虱都有特有的寄主，例如鲑疮痂鱼虱就特别喜欢寄生在鲑鱼当中，尤其是饲养的大西洋鲑。鲑疮痂鱼虱也可以寄生在其他鲑鱼，但程度各有不同。太平洋的鲑鱼，如银大麻哈鱼、大麻哈鱼及驼背大马哈鱼等对于鲑疮痂鱼虱会产生强烈的组织反应，在感染的首周就会将之排出。[3]太平洋的鲑疮痂鱼虱也可以发展到寄生到三刺鱼[4]，不过大西洋的则没有出现这种情况。

海虱在浮游阶段如何扩散及寻找寄主仍然是迷。温度、光线及水流都是主因，其生存倚赖25‰以上的盐度。[5][6][7][8]有指鲑疮痂鱼虱会向著光源向上迁徙，正好会遇到向下游的鲑鱼鱼秧。[9]在潮间带浮游阶段的海虱可以被送到几十公里以外的地方。[7][10][11]

海虱会从溯河的鱼类身体上掉下及死去，例如回到淡水河流中繁殖的鲑鱼。不过这些鲑鱼回到海中重新受感染的原因则不明。有指海虱可能是在河口生活的鱼类身体上存活下来，或是转往其他的寄主。至于海虱如何在野外鱼类之间传播也不详。成年的疮痂鱼虱属在实验室环境下可以在不同鱼种间传播，但并不频繁；而鱼虱属则随时可以传播。

形态

鲑疮痂鱼虱的大小较鱼虱属大一倍。它们的身体有4个部份，包括头胸部、第四足体节、生殖部份及腹部。[12]头胸部是一块阔板，当中包含了头三足的体节，可以像吸盘般将它们吸在寄主鱼类的身体上。所有的海虱都有像吸管的口器。第二触角及口附肢都进化到可以帮助将它们找著寄主。雄虱也会用第二触角来找住雌性进行交配。[13]成年雌性比雄性大，生殖部份也非常大。鲑疮痂鱼虱会生两串共500-1000颗卵，成熟后卵会变深色。一只雌虱一生可以产6-11对卵串，估计寿命只有7个月。[7][9][14]

生命周期

海虱有浮游及寄生的阶段。所有阶段之间都会脱壳。[12][13][15][16]其中鲑疮痂鱼虱只要17-72日就可以由卵成长到成年。

海虱的卵会孵化出第一期无节幼体，继而脱壳成长至第二期无节幼体。两期的无节幼体阶段都不会进食，只靠卵黄供应能量，而且懂得游泳。到了成年阶段就会寻找寄主。水流、盐度、光线等都可以帮助它们寻找寄主。[7]海虱多会寄生在鱼类身体受水流影响较少的地方，特别是鳍及其他受保护的地方。[6][17]当幼体附在寄主身上后，就会觅食一段时间脱壳成第一期的附著幼体。海虱会继续发育成另外的3期附著幼体。所有四个附著幼体阶段都有一个前部丝体的结构，用来附著在寄主身上。不同的海虱有不同的繁殖时间及方法。亚成体及成体可以在鱼类的身体上移动，有时甚至可以在寄主之间转移。雌性成体的身体较大，也较为扁平。[18]

食性

无节幼体及幼体期都是不会进食的。当附在寄主身上时，它们就会发育到附著幼体阶段。附著幼体都有胃肠道，且会吃寄主的黏液及组织。亚成体及成体都是主动觅食者，有时甚至会吸食寄主的血液。鲑疮痂鱼虱会分泌大量胰蛋白酶到寄主的黏液，帮助觅食及消化。[3][19]其他的物质包括前列腺素E2，可以帮助觅食及避开寄主的免疫反应。[3][20]海虱是否疾病媒介则不明，但它们肯定带有细菌及病毒。[21]

疾病

病理学

在大西洋鲑上的雌性鲑疮痂鱼虱。

海虱会在附著的位置对寄主造成物质及酶促的破坏。它们在吃食的时候会造成像割开的伤口，严重程度会受到几个因素所影响，如寄主物种、年龄及健康。海虱感染会对寄主鱼造成慢性压力，包括破坏其表皮出现失血、电解质的改变及释放皮质醇。这样会降低寄主鱼的免疫反应，令它们易于染病。[22][23]

不同海虱的物种、发育阶段及数量也会造成不同程度的破坏。大西洋鲑的组织似乎对海虱没有任何反应。相反，银大麻哈鱼及驼背大马哈鱼对于鲑疮痂鱼虱则有强烈的反应，包括表皮增生及炎症。在感染后的第一周就会排斥海虱。[3]鲑疮痂鱼虱可以令饲养的大西洋鲑及野生的红大马哈鱼出现深深的损伤，尤其是在头部，严重的程度可以见到头颅骨。

饲养与野生鱼类的关系

有指受到海虱感染的饲养鲑鱼会将病传播到周边野生的幼鱼，令它们的数量大减。[24]海虱，尤其是鲑疮痂鱼虱及多种鱼虱属都会令饲养及野生的鲑鱼致命。[25][26]海虱会迁徙及附在野生鲑鱼的皮肤上达几日之久。[27][28][29]大量受感染的鲑鱼可以令海虱的浓度攀升，游经此地的野生幼鲑鱼就会受到感染及死亡。[30][31]在加拿大的太平洋海岸，差不多有80%的驼背大马哈鱼死于海虱的感染。[24]

褐鳟的数量因感染海虱已于近年出现大幅下降。[32]

多项研究都指饲养鲑鱼会积聚海虱，并会破坏周边的野生鲑鱼群落。[31]其他研究却指饲养鲑鱼所感染的海虱，并不会对野生群落造成影响。[33]

下属分类

鱼虱科包括以下属：

参考文献

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外部链接

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