扁水虱科 Serolidae 属于节肢动物门软甲纲等足目。虽然很像三叶虫,但它们和三叶虫实际上没多少关系。扁水虱科共 22 属 106种,但除 5 种(Heteroserolis carinata、H. tropica、H. mgrayi、Atlantoserolis agassizi、A. vemae)外,皆分布于南半球,许多种聚集于南极和亚南极区域。在那里,一些物种的密度极高,如生活于南极阿德米拉蒂湾(Admiralty Bay)6 米深的海底的 Paraserolis polita (Pfeffer, 1887),密度高达每平方米 274 个个体。扁水虱的生活深度范围很大,从潮间带至 5500 米深都有发现。
扁水虱仅有很弱的游泳能力,主要依靠步足尖在底质(主要是软底质)上行走移动。它们不会挖洞,仅会令沉积物浅浅地覆盖部分体表。扁水虱是肉食性动物,以各种小型无脊椎动物为食,南极的幼体还有同类相食的习性。
一些扁水虱物种对于温度和盐度变化非常敏感。例如,南极的 P. polita 的行动和觅食能力在温度升高 5℃、盐度降低 4 的环境中会受到极为严重的损害;而分布于巴拿马运河太平洋入口附近潮间带和浅海区域的 H. tropica (Glynn, 1976) 只会在有上升流的三天出现于 Naos 岛的潮间带,上升流会令当地海水温度下降 4~5℃。
Brucerolis sp.
扁水虱幼体从卵黄获取营养,没有浮游幼体期。卵被存放在雌性的育儿袋中孵化,但是这并不绝对安全。例如,线虫 Chromadorita ceratoserolis (Lorenzen, 1986) 就可以感染南极的 Ceratoserolis trilobitoides (Eights, 1833) 育儿袋中的卵。南极的扁水虱的卵在育儿袋中孵化的时间非常长,如 C. trilobitoides的卵需要 23 个月才能孵化,而 P. polita 的卵需要约 20 个月。卵袋中的卵并非同时孵化。从 C. trilobitoides 的第一个卵孵化到最后一批幼虫出生,间隔大约 9 周,孵出的幼体会在育儿袋中停留几周才离开母体,释放幼体的过程需要 18~33 天。C. trilobitoides 一生只产一次卵,但 P. polita 可以多次产卵。
这种在无脊椎动物中最长的孵化时间之一的现象主要是低温和食物因素导致的。雌性积累卵黄和幼体成长都需要大量营养,而南极温暖的夏季会令浮游植物的数量显著增多,进而导致扁水虱的食物如多毛纲幼体的数量大幅增加。雌性扁水虱利用这个机会快速补充营养。卵子 75% 的质量都在夏季积累,并于秋季被产于卵袋中。因此,如果扁水虱的卵不能在 8 个月内完成孵化,那么它们就必须再等一年才能让海中的食物足够幼体生长发育。
H. carinata (Lockington, 1877) 是扁水虱科中第一个被发现于北半球的物种,也是将近一个世纪中唯一已知的北半球扁水虱。它分布于太平洋南部加利福尼亚州海岸和下加利福尼亚州的浅水。直到 1966 年,人们才发现了第二种北半球扁水虱——分布于北大西洋乔治亚州沿海大陆架水深 33~44 米处的 H. mgrayi (Menzies & Frankenberg, 1966)。这两个分别分布于太平洋和大西洋的物种拥有共同的祖先,直到被中美洲陆桥分隔并最终演化为两个物种。而所有南极物种来自同一个南美洲祖先。该物种从南美扩散至南极,并在德雷克海峡分离南美洲与南极洲后逐渐演化出现存的南极扁水虱物种。位于大陆架上的南极扁水虱至少三次独立向深海(2000 米以深)扩散并演化,造就了深海的 Acutiserolis 属和主要生活于大陆架的 Ceratoserolis 属的深海物种 C. meridionalis (Vanhöffen, 1914),而其他深海扁水虱如 Caecoserolis 属的其他物种、Atlantoserolis属 和 Glabroserolis 属的物种可能起源于南美洲大陆架物种的一次向深海的扩散。
这三个属的深海扁水虱眼睛完全退化,而浅海的 Serolis 属拥有大而凸起的眼睛,这使其眼睛尽管位于背部,但依旧能够积极地改变它们的颜色。它们能够根据照明条件和底质类型,甚至根据挖掘程度来改变它们的颜色模式。
Acutiserolis spinosa (Kussakin, 1967) / Angelika Brandt
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