把木頭當飯吃，它們是深海木材分解者（上）（克爾-紐曼黑洞）

食木蛤科Xylophagaidae的成員是一類專性棲息于深海沉木的雙殼綱生物，與其“近親”船蛆科由同一祖先演化而來。這兩科是雙殼綱“唯二”的專性木食類群，也是海洋中最主要的木材降解生物類群。但與船蛆不同，食木蛤的內臟完全在殼瓣之內，僅水管伸出，貝殼沒有殼內柱。與棲息于溫帶與熱帶淺海浮木和潮間帶木材內的船蛆不同，食木蛤生活于深水沉木（最深可達7250米）和高緯度地區淺海沉木中，極少數會在風暴沖刷上岸的沉木中被發現。因此，船蛆能耐受鹽度與溫度的劇烈波動，通過分泌鈣質蛀道內壁和用鎧板封閉孔口抵御在潮間帶暴露于空氣中時的干燥環境；而深海食木蛤不需要耐受干燥環境，因此沒有用于封閉蛀道口的鎧板，僅Xyloredos屬的兩個物種具有在蛀道末端分泌鈣質蛀道內壁的能力。

Xyloredo teramachii，2023年2月拖網采集于東海1956海區4小區370~420米深 / 作者拍攝自有標本

食木蛤能將沉木能量和養分輸送到深海環境，是連接陸地與深海生態系統的橋梁。其通過特化的帶齒脊的殼前緣將木材銼磨成碎屑，并將碎屑儲存在胃盲囊中，利用共生的γ-變形菌分解纖維素。作為深海木材降解的主要生物類群，食木蛤能在數月內形成高密度種群。例如，Xylophaga alexisi用7個月就能在木材表面形成170孔/厘米2的鉆孔密度，Xylophaga depalmai在云杉中的密度6個月就能達（1484.5±259.8）個/分米3。一些物種在半年內即可將木材蛀至可用手壓碎的程度。例如，Xylophaga washingtona僅用2.4個月就能讓木材被用手壓碎，而Xylonora atlantica用8~10個月即可使木材崩解，Xylophaga depalmai在6個月內可降解51.88%的木材。

Xylonora atlantica。 PAS. 后閉殼肌痕；PR. 縮足肌痕；SR. 水管收縮肌痕 / Romano et al，2020

纖維素分解會形成硫化環境，船蛆可通過鈣質蛀道內壁抵御有毒的硫環境，而食木蛤則演化出多種方式，如水管的角質層、角質保護錐，或用黏液混合糞便填滿水管與蛀道之間的空隙，隔絕水管與環境中的硫。Xylophaga dorsalis支系成員更進一步，會在木材表面形成糞便堆積的煙囪狀結構。“糞煙囪”富含細菌，能誘導新幼蟲定殖并減弱水流，加之食木蛤的高密度集群，導致局部含氧量下降。因此，被Xylophaga dorsalis支系成員蛀食的木材生物多樣性普遍較低，僅有耐受低氧環境的生物如擁有血紅蛋白的Xylophaga dorsalis存活。Xylophaga dorsalis支系成員還能通過濾食輔助獲取能量，這些特征使其成為非常強大的競爭者。例如，胡安·德富卡海脊（奮進段）[Juan de Fuca Ridge (Endeavour)]的樣本最初以廣布性及高豐度高度特化于新基質快速定殖的Xylophaga microchira為主，但在24個月后則以Xylophaga dorsalis支系的Xylophaga oregona為主。而另一項研究的同支系的Xylophaga indica則占該樣本個體總數的95.7%。【待續】

Xylophaga dorsalis。A. 帶軟體的生物體背視圖；B. 帶軟體部的生物體側視圖；C. 正視圖；D. 貝殼內面 / Romano et al，2014

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