把木头当饭吃，它们是深海木材分解者（上）（克尔-纽曼黑洞）

食木蛤科Xylophagaidae的成员是一类专性栖息于深海沉木的双壳纲生物，与其“近亲”船蛆科由同一祖先演化而来。这两科是双壳纲“唯二”的专性木食类群，也是海洋中最主要的木材降解生物类群。但与船蛆不同，食木蛤的内脏完全在壳瓣之内，仅水管伸出，贝壳没有壳内柱。与栖息于温带与热带浅海浮木和潮间带木材内的船蛆不同，食木蛤生活于深水沉木（最深可达7250米）和高纬度地区浅海沉木中，极少数会在风暴冲刷上岸的沉木中被发现。因此，船蛆能耐受盐度与温度的剧烈波动，通过分泌钙质蛀道内壁和用铠板封闭孔口抵御在潮间带暴露于空气中时的干燥环境；而深海食木蛤不需要耐受干燥环境，因此没有用于封闭蛀道口的铠板，仅Xyloredos属的两个物种具有在蛀道末端分泌钙质蛀道内壁的能力。

Xyloredo teramachii，2023年2月拖网采集于东海1956海区4小区370~420米深 / 作者拍摄自有标本

食木蛤能将沉木能量和养分输送到深海环境，是连接陆地与深海生态系统的桥梁。其通过特化的带齿脊的壳前缘将木材锉磨成碎屑，并将碎屑储存在胃盲囊中，利用共生的γ-变形菌分解纤维素。作为深海木材降解的主要生物类群，食木蛤能在数月内形成高密度种群。例如，Xylophaga alexisi用7个月就能在木材表面形成170孔/厘米2的钻孔密度，Xylophaga depalmai在云杉中的密度6个月就能达（1484.5±259.8）个/分米³。一些物种在半年内即可将木材蛀至可用手压碎的程度。例如，Xylophaga washingtona仅用2.4个月就能让木材被用手压碎，而Xylonora atlantica用8~10个月即可使木材崩解，Xylophaga depalmai在6个月内可降解51.88%的木材。

Xylonora atlantica。 PAS. 后闭壳肌痕；PR. 缩足肌痕；SR. 水管收缩肌痕 / Romano et al，2020

纤维素分解会形成硫化环境，船蛆可通过钙质蛀道内壁抵御有毒的硫环境，而食木蛤则演化出多种方式，如水管的角质层、角质保护锥，或用黏液混合粪便填满水管与蛀道之间的空隙，隔绝水管与环境中的硫。Xylophaga dorsalis支系成员更进一步，会在木材表面形成粪便堆积的烟囱状结构。“粪烟囱”富含细菌，能诱导新幼虫定殖并减弱水流，加之食木蛤的高密度集群，导致局部含氧量下降。因此，被Xylophaga dorsalis支系成员蛀食的木材生物多样性普遍较低，仅有耐受低氧环境的生物如拥有血红蛋白的Xylophaga dorsalis存活。Xylophaga dorsalis支系成员还能通过滤食辅助获取能量，这些特征使其成为非常强大的竞争者。例如，胡安·德富卡海脊（奋进段）[Juan de Fuca Ridge (Endeavour)]的样本最初以广布性及高丰度高度特化于新基质快速定殖的Xylophaga microchira为主，但在24个月后则以Xylophaga dorsalis支系的Xylophaga oregona为主。而另一项研究的同支系的Xylophaga indica则占该样本个体总数的95.7%。【待续】

Xylophaga dorsalis。A. 带软体的生物体背视图；B. 带软体部的生物体侧视图；C. 正视图；D. 贝壳内面 / Romano et al，2014

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