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鲸落(whale fall)是指须鲸(Balaenoptera)和抹香鲸(Physeter macrocephalus)等大型鲸类死亡后,尸体沉入海底,形成的一个长期以鲸尸为营养源的生态系统。鲸落一词最早出现于1991年,Allison等首次使用该词分析鲸化石生成原理[1]。从字面上理解,鲸落的“落”指的是鲸尸从海面下沉到海底的过程,但如果视为一个生态学概念,笔者认为,将“落”理解为以鲸尸为营养源支撑起的一个独特深海生物群落更为贴切。此外,鲸落的“鲸”也并非泛指所有鲸目动物,外文文献涉及该词时主要指14种须鲸和抹香鲸。由于大型须鲸和小型齿鲸在个体大小和骨骼脂肪含量方面存在很大的差异,导致两者在尸体分解过程、持续时间、所支撑起的生物群落和在深海生态系统所扮演的角色迥然不同[2]。虽然个别学者在开展小型齿鲸海底植入实验时也使用“whale fall”一词[3],但多数学者还是使用“large food fall” [4-5]或“cetacean fall” [6]等加于区分。目前,“鲸落”的中文名词概念还不够明确,使用较混乱,相对于须鲸和抹香鲸等大型鲸类死亡后形成的具有化能自养阶段的鲸落,笔者建议使用“豚落”来表示小型齿鲸尸体在深海形成的不具化能自养阶段的独特生物群落。
并非所有大型鲸类生命的最终归宿都形成鲸落。只有当鲸营养不足或长期生病死亡时,身体浮力因脂肪层消耗降低,鲸尸才会下沉并伴随水压的增大引起肺坍塌,从而进一步降低浮力并快速地掉落到海底。鲸尸的下沉还是上浮主要取决于脂肪含量,除了抹香鲸和露脊鲸(Eubalaena japonica),其他大型鲸类的身体密度都略大水,一般死亡后都会下沉[1,7]。特别是许多大型须鲸都具有季节性迁徙习性,由于迁徙距离长且途中一般不进食,很容易导致营养不良和脂肪层的过度消耗,死亡后形成鲸落。据估计迁徙期死亡灰鲸(Eschrichtius robustus)形成鲸落的比例高达90%[8]。另外,热带海域是许多鲸类冬季的育幼场所,生崽和长时间育幼行为也会导致营养物质过度消耗,一旦个体死亡也容易形成鲸落。因此大洋边缘大型鲸类的迁徙路线和热带鲸类的育幼场所是全球鲸落密度最高的海域[8]。
在19世纪中期,人们就偶尔在鲸尸残骸中找到一些从未见过的奇特深海生物[8]。随着深海底拖作业的日益频繁,越来越多的新物种甚至新属在鲸骨中被发现,人们逐渐意识到大型鲸类死亡后可能会在海底形成独特的生物群落[9]。然而拖网所能获得的毕竟只有一些残骨碎肉,作业过程也会对生物体形态产生破坏,难于反映鲸落的全貌。由于深潜技术的发展,1987年11月10日,由美国Alvin号深潜器在加利福尼亚州的San Catalina Basin水深1 240 m处发现第一个完整的鲸落[10]。这是1头长约20 m的蓝鲸或长须鲸形成的鲸落,处于化能自养阶段,体表和附近沉积物表层覆盖一层厚厚的硫化菌菌膜,除了观察到一些和冷泉和热液相同的无脊椎动物外,研究人员还发现了6种从来没有在该海湾发现过的动物。该鲸落和其独特生物群落的发现立刻引起学术界极大的关注[10]。由于自然鲸落的发现具有很大的偶然性,自1992年开始,夏威夷大学在加利福尼亚海域持续开展人工植入鲸落实验,大大促进鲸落研究的发展[8]。截至2022年,全世界已经发现了45个自然鲸落,较深入研究的有7个,另外还开展了38个人工鲸落植入实验[11-13]。
Review and prospect of whale fall ecology
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摘要: 鲸落是指须鲸(Balaenoptera)和抹香鲸(Physeter macrocephalus)等大型鲸类死亡后,尸体沉入海底,形成的一个长期以鲸尸为营养源的生态系统。鲸落研究始于1987年,迄今已经积累了一些生态认识,对这些认识的梳理将为我国即将兴起的鲸落研究提供借鉴。鲸落的演替可分为移动清道夫、机会主义者、化能自养和礁岩阶段,促进、耐受和抑制作用是推动不同阶段演替的主要生态因子,而水深、水温和地理位置对各演化阶段的物种组成和群落结构具有重要影响。目前尚未在移动清道夫阶段发现鲸落专性物种,但已经在机会主义者和化能自养阶段发现了包括食骨虫在内的129种专性物种,这些物种通过洋流将生活史早期产生的浮游幼虫在鲸落间进行扩散。基于全球大型鲸类种群数量和死亡率,有学者粗略估算出相邻且演化阶段相同鲸落的平均距离为5~16 km,这些鲸落随机散布于各大洋海底,可为生物在位置固定、区域距离较远的冷热泉间扩散提供垫脚石,共同维持深海生物多样性水平。有研究认为,捕鲸业对大型鲸类的大肆捕杀导致全球鲸落减少了65%~90%,这已引起深海物种生物多样性水平的严重下降甚至大规模灭绝。鲸落研究极具挑战性,无论探测自然鲸落还是人工植入鲸落都要求配备高性能潜水器和具备熟练深潜作业能力,是一个国家对深海探索能力和综合海洋科技水平的体现。根据国际鲸落研究进展和我国深海探测技术现状,提出我国鲸落研究应充分利用区域优势、聚焦共性科学问题和注重新技术应用的建议。Abstract: A whale fall occurs when great whales such as baleen whales (Balaenoptera) and sperm whale (Physeter macrocephalus) die and their carcasses fall to the ocean floor. Compared with other organisms dropped from the euphotic zone to the sea floor, great-whale carcasses highlight with giant body sizes and high bone-lipid content which allow them to support a sequence of heterotrophic and chemosynthetic microbial assemblages in the energy-poor deepsea for decades. The succession of whale fall can be divided into mobile-scavenger, enrichment opportunist, sulphophilic and reef stages. Successional mechanisms of these stages are driven by facilitation, tolerance and inhibition, while species composition and community structure are affected mainly by water depth, temperature and geographical location. No whale-fall specialist has been found in the mobile-scavenger stage to date, but 129 species that are specialized to live on whale remains have been identified in the opportunistic and chemoautotrophic stages. Based on the population size and mortality of great whales around the world, the estimation of average distance among adjacent whale falls with the same successional stage is 5-16 km. These whale falls share the similar chemoautotrophic communities and species with those of vents and seeps, and thus are considered ecological and evolutionary stepping-stones in deep-sea floor. Unfortunately, the serious decline of great whales populations caused by commercial whaling have led to a 65%-90% reduction of whale falls in the world ocean, which may have reduced species diversity and caused species extinctions in deep-sea ecosystems. The detection of natural whale falls and experimentally implanted whale remains are considered to be the two main methods for whale falls research which requires the high-performance submersible and skilled deep diving operation ability. Finally, based on cetacean resources and the development of deep-sea exploration technology in China, we highlight future research interests of whale fall.
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Key words:
- baleen whale /
- sperm whale /
- Osedax worm /
- cold seep /
- hydrothermal vent /
- hadal trench
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