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(nèi)臟完全在殼瓣之內(nèi)，僅水管伸出，貝殼沒有殼內(nèi)柱。與棲息于溫帶與熱帶淺海浮木和潮間帶木材內(nèi)的船蛆不同，食木蛤生活于深水沉木（最深可達(dá)7250米）和高緯度地區(qū)淺海沉木中，極少數(shù)會(huì)在風(fēng)暴沖刷上岸的沉木中被發(fā)現(xiàn)。因此，船蛆能耐受鹽度與溫度的劇烈波動(dòng)，通過分泌鈣質(zhì)蛀道內(nèi)壁和用鎧板封閉孔口抵御在潮間帶暴露于空氣中時(shí)的干燥環(huán)境；而深海食木蛤不需要耐受干燥環(huán)境，因此沒有用于封閉蛀道口的鎧板，僅Xyloredos屬的兩個(gè)物種具有在蛀道末端分泌鈣質(zhì)蛀道內(nèi)壁的能力。

Xyloredo teramachii，2023年2月拖網(wǎng)采集于東海1956海區(qū)4小區(qū)370~420米深 / 作者拍攝自有標(biāo)本

食木蛤能將沉木能量和養(yǎng)分輸送到深海環(huán)境，是連接陸地與深海生態(tài)系統(tǒng)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(chǔ)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(shù)月內(nèi)形成高密度種群。例如，Xylophaga alexisi用7個(gè)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(gè)月就能達(dá)（1484.5±259.8）個(gè)/分米3。一些物種在半年內(nèi)即可將木材蛀至可用手壓碎的程度。例如，Xylophaga washingtona僅用2.4個(gè)月就能讓木材被用手壓碎，而Xylonora atlantica用8~10個(gè)月即可使木材崩解，Xylophaga depalmai在6個(gè)月內(nèi)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(huì)形成硫化環(huán)境，船蛆可通過鈣質(zhì)蛀道內(nèi)壁抵御有毒的硫環(huán)境，而食木蛤則演化出多種方式，如水管的角質(zhì)層、角質(zhì)保護(hù)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(huán)境中的硫。Xylophaga dorsalis支系成員更進(jìn)一步，會(huì)在木材表面形成糞便堆積的煙囪狀結(jié)構(gòu)。“糞煙囪”富含細(xì)菌，能誘導(dǎo)新幼蟲定殖并減弱水流，加之食木蛤的高密度集群，導(dǎo)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(huán)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(qiáng)大的競爭者。例如，胡安·德富卡海脊（奮進(jìn)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(zhì)快速定殖的Xylophaga microchira為主，但在24個(gè)月后則以Xylophaga dorsalis支系的Xylophaga oregona為主。而另一項(xiàng)研究的同支系的Xylophaga indica則占該樣本個(gè)體總數(shù)的95.7%。【待續(xù)】

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

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