海洋生态与环境科学重点实验室知识库

上世纪六十年代以来，全球多地发生了水母数量上升、种群聚集，或不规律暴发的现象，对沿海核电设施、渔业资源、渔业生产捕捞、人类生命安全和旅游业、生态系统结构与功能等造成了严重的危害，引起了公众和政府的广泛关注。

水母具有非常复杂的生活史，以浮游和底栖两种形态世代交替存活。在东亚海域，浮游阶段的水母体仅在每年夏季存活2~4个月，而底栖阶段的水螅体则可在海底常年固着生存。水螅体通过出芽、足囊等无性繁殖方式可以扩充自身种群，也可通过横裂产生碟状体，进而发育成水母。“故事发生在海底”，水螅体在海底环境中的数量变化和繁殖方式对水母种群数量具有重要、决定性的控制作用，是解开水母暴发机理的关键一环，也是对水母暴发进行预测、预报和综合防控的基础。

海底是水母水螅体真正的家园，水螅体在野外面临着非常复杂的环境。水螅体的生存、发育和繁殖，取决于其自身生理特性，和来自其他底栖生物的竞争和捕食威胁，同时又与底栖生态系统共同受到气候变化和人类活动的直接和间接影响。水母水螅体个体微小，仅有1~2毫米，且透明、柔软、脆弱，观测和取样都很困难，因此有关水螅体的研究相对比较匮乏，且大多集中于实验室内。此类研究多为针对水螅体个体或种群的人工控制变量实验，以温度、盐度、溶解氧、饵料等一至两个因素为变量，探究水螅体在实验因素不同水平下的性状特征。实验条件较为单一，与野外实际情况差距较大，因此从实验室获得的实验结果很难解释自然状态下的水母暴发机理。

本研究聚焦水母水螅体以及底栖生态系统，从种群、群落和生态系统三个层次，全面研究了全球变暖和底栖生态系统变动对于水母水螅体无性繁殖、生存策略、种群变动的影响，这些变化与底栖生物群落之间的相互关系，以及底栖生态系统对水螅体种群的控制作用。本研究将室内实验与野外原位现场实验相结合，分别开展了超过8个月的室内培养模拟、野外挂板和野外原位人工加热的实验，并通过显微观察、高像素拍照等手段相机记录了水螅体及其他底栖无脊椎生物的种群动态。研究得到了以下主要结果：

一、水螅体在冬季越冬期间，底层温度、春季升温速度和投饵频率等对水母水螅体的无性繁殖的影响不是单向的、单一的，而是二元的、多面的。暖冬对水螅体的无性繁殖总量有积极的促进作用，使水螅体释放更多的碟状体，产生更多的芽体，这分别对应了水螅体种群向水母体种群发展以及扩展自身水螅体种群的两个方向。而冷冬带给水螅体更强的越冬低温刺激，导致更密集的横裂和更集中的碟状体出现。升温较慢、低温持续时间较长的春天，使水螅有足够的时间在适宜的温度范围内进行横裂，促进二次横裂的发生，进而释放出更多的碟状体。快速变暖的春天可以导致水螅体的横裂更加集中，迅速完成水螅体种群到水母体种群的转换。总体而言，冬季温度高、升温速度慢时，水螅体会发展出较高的横裂频率以及释放更多的碟状体。

饵料数量在水螅体整个越冬和无性繁殖过程中也起到重要作用。当水螅体在温度适宜的条件下快速密集地横裂时，需要在短时间内获得大量的饵料作为能量基础，而食物的缺乏就可能成为限制因素。当水螅体在较长时间内多次横裂时，更多的饵料充当了“加油站”，为水螅体的无性繁殖提供了持续的能量。同时，充足的饵料还具有放大其他环境因子作用效果的作用，促进更多的芽体和碟状体的产生。

二、通过对海月水母水螅体在野外的实际生存状况、水螅体自身的繁殖和发育策略，以及与其他底栖无脊椎生物的相互作用的研究发现：对于水螅体自身来说，温度仍然是控制其无性繁殖数量的最重要的因子。温度的升高可以促进水螅体大量出芽，扩充种群规模。尽管横裂等短期损耗过程会降低水螅体的种群密度，但是在温度和饵料条件适宜的情况下，水螅体种群会很快恢复。

水螅体与固着类生物的空间竞争和捕食与被捕食以及食物竞争等是他们相互作用的主要方式。具有硬质外壳或硬质表面的底栖无脊椎生物物种，如藤壶、贻贝、蛇螺、缝栖蛤，可以为水螅体的生长提供大面积的附着基质，也可以为水螅体提供更复杂的空间结构来寻求庇护。以海绵、玻璃海鞘和菊海鞘为代表的表面黏滑或柔软的底栖无脊椎生物与水螅体产生严峻的空间竞争，其可以完全覆盖于水螅体种群之上，导致水螅体大量死亡。底栖端足类构筑的巢穴或多分支生物所形成的生物结构也可以占据大量的基质空间，与水螅体发生空间竞争，但它们与水螅体的空间相互作用取决于它们的自身特征。钩虾产生的泥管和泥穴最初非常柔软和松散，占据了水螅体种群的部分空间。然而，随着时间的推移当它们硬化的时候，水螅体可以逐渐重新附着在它们之上。绿藻和褐藻等大型藻类最初可以与水螅体在同一空间内一起生长，但由于基质下表面缺乏光线而逐渐死亡。此外，像水螅和草苔虫等丛状生物也在附着基上生长，与水螅体产生空间竞争，且其表面不利于水螅体附着，因为它们的结构太纤细，而且随着水流摆动，不利于水螅体的定殖。

本研究还发现，水螅体对于饵料缺乏的耐受性极强，在仅有少量微微型饵料供应的情况下，水螅体可以以一个较低的活跃状态和较小的体型耐受超过6个月，直到夏季高温来临时缓慢死亡。

三、在正常条件下的健康生态系统中，水母水螅体作为弱小的生物，在底栖无脊椎生物群落中艰难求生。而在底栖生态系统在气候变化和人类活动的共同胁迫下发生动荡的时候，如海洋温度升高时，底栖无脊椎生物群落内的种间关系开始发生改变，这为水母水螅体带来了机会和挑战。水母水螅体因其自身特性而抓住机会大量繁殖，占据竞争优势和适应先机，逐渐发展为主导性物种，并在夏季带来水母暴发。

具体来说，仅1.5℃的升温就会在演替早期改变底栖无脊椎生物群落结构，同时，温度升高会导致生物多样性和丰富度的下降，突出了某些生物对群落结构差异的主导性地位，并强化了主导性物种的优势地位。然而，不同种生物对于海洋升温的响应具有差异性，随着温度的升高，水螅体的定殖显著增加，而海鞘的覆盖受到抑制，这可能与两种物种的热适应差异有关。

水螅体因其多样化的繁殖方式，可以覆盖在具有硬质或粗糙外壳的生物的表面，入侵其他种群，占据空间优势。我们推测全球变暖和近海人工基质的引入将导致沿海有害生物在机会主义生物博弈中占据主导地位。以水母为代表的灾害性生物具有以下共同特征:（1）复杂的生命周期，（2）多样化的无性繁殖方式，（3）对环境变化的快速反应，（4）较强的耐受性和较宽的温度适应范围。此外，海岸带工程和人工基质的引入也给水母水螅体提供了种群扩张的条件。

本文的研究将以往针对于水母水螅体单一生物的人工室内控制实验推广到原位生态系统，将全球变化的作用效果落实到局地海域，将温度对于水螅体的直接作用拓展到经由生态系统介导的间接作用。研究受底栖生物群落控制的水母水螅体的群落动态以及其对气候变化的响应，从系统的角度来理解和解释了灾害性生物的暴发原理。对灾害性生物的动态解释、预测预报，以及对生态系统动力学和全水体耦合的生物海洋学研究，提供了更坚实的实验证据和理论基础。

Since the 1960s, the phenomenon of abundance increases, population gathering, and irregular outbreaks of jellyfish have occurred globally. The harmful jellyfish blooms cause serious damage to coastal nuclear facilities, fishery production and commercial fishing, human life safety, tourism, and ecosystem structure and function, attracting widespread concern from the public and governments.

Jellyfish have a very complex life cycle, with a metagenic lifecycle consisting of floating form and benthic form. The floating jellyfish only survive for 2~4 months during summer, while the benthic polyp can colonize on the sea bottom all year round. Polyp can expand its population through asexual reproductions such as budding and podocyst, and can also produce ephyra through strobilation, which then develop into adult jellyfish. Early studies by scientists indicate that "the story happens on the sea bottom, " as fluctuations in polyp populations and its reproductive strategies in the benthic environment play a crucial and decisive role in controlling the population of jellyfish, which is the key to unraveling the mechanism of harmful jellyfish bloom, and is the basis for forecasting and prevention of harmful jellyfish outbreak.

The ocean bottom serves as the habitat of jellyfish polyp. Polyp faces a highly complex environment in the wild. The survival, growth, and production of polyp not only depend on its physiological characteristics, but also be influenced by competition and predation threats from other benthic organisms, and the directly and indirectly effects mediated by benthic ecosystems caused by climate change and human activities. Jellyfish polyps are minuscule, measuring only 1~2 mm, transparent, soft, and fragile, which is difficult to observe and sample. Therefore, studies on polyp are relatively scarce, and mostly confined to laboratory settings. Such studies predominantly involve controlled experiments on individual polyps or populations, manipulating variables like temperature, salinity, dissolved oxygen, and food to explore the characteristics of polyps under varied experimental conditions. Given the simplistic experimental setups compared to the complexities of natural environments, interpreting jellyfish outbreak mechanisms based on laboratory findings proves challenging.

This study focused on the jellyfish polyp and the benthic ecosystem to comprehensively investigate the effects of global warming and benthic ecosystem changes on the asexual reproduction, survival strategy and population dynamic of jellyfish polyp at the population, community, and ecosystem levels, as well as the control effect of benthic ecosystem on polyp population. In this study, laboratory experiments were combined with field in-situ experiments. Experiments of indoor culture simulation, field hanging plate and field in-situ artificial heating were carried out for more than 8 months, respectively. The population dynamics of polyp and other benthic invertebrates were recorded by means of microscopic observation, high pixel photography. The following main results were obtained:

1. The effects of benthic winter temperature, spring warming rate, and feeding frequency on the asexual reproduction of jellyfish polyps during the overwintering period are not one-way or singular, but rather dual and multifaceted. A warm winter positively promotes the asexual reproduction of jellyfish polyps, leading to more ephyrae through strobilation and more new polyps through budding, corresponding to the development of jellyfish polyp populations towards medusa populations and the expansion of their own polyp populations. A cold winter provides stronger overwintering stimuli with low temperature to jellyfish polyps, resulting in more intensive strobilation and concentrated ephyrae production. A slow warming in spring and prolonged period of low temperatures can extend the duration of the optimum temperature for jellyfish polyps to strobilate and release ephyrae, result in more secondary strobilation and ephyrae. A rapid warming in spring causes jellyfish polyps to strobilate more rapidly, completing the transition from jellyfish polyp populations to medusa populations. Overall, in warmer winters with slow warming rates, jellyfish polyps exhibit higher strobilation frequencies and release more ephyrae.

The availability of food also plays a crucial role throughout the entire overwintering and asexual reproduction process of jellyfish polyps. When jellyfish polyps strobilate rapidly and intensively under suitable temperature conditions, a large amount of food is needed as an energy source within a short period, and food scarcity may become a limiting factor. When jellyfish polyps strobilate multiple times over a longer period, more food acts as a “refueling station,” providing continuous energy for the asexual reproduction of jellyfish polyps. Adequate food also amplifies the effects of other environmental factors, promoting the production of more buds and ephyrae.

2. Through systematic surveys of the actual status of jellyfish polyps in the field, investigations into their reproductive and developmental strategies, and interactions with other benthic invertebrates found that temperature remains the most important factor controlling the quantity of asexual reproduction of jellyfish polyps. Higher temperatures promote extensive budding of jellyfish polyps, expanding the population size. Although short-term loss processes such as strobilation may reduce the population density of jellyfish polyps, under suitable temperature and food conditions, polyp populations quickly recover through budding.

Interactions with other benthic invertebrates are also important factors determining the density of jellyfish polyps. Interactions between jellyfish polyps and benthic invertebrates, such as ascidians, gastropods, and mussels, are primarily characterized by spatial competition for attachment sites, predation, and resource competition. Benthic invertebrates with hard surfaces or shells provide additional attachment substrates for polyp growth and complex spatial structures for shelter. Smooth or slimy-bodied benthic invertebrates like sponges, ascidians compete intensely with jellyfish polyps for space, who can completely covering the polyps, and leading to significant mortality. Burrows constructed by benthic crustaceans or complex structures formed by branching organisms can also occupy substantial substrate space, engaging in spatial competition with jellyfish polyps. The interaction outcomes between these organisms depend on their respective characteristics. For instance, the soft and loose mud tubes and burrows produced by crustaceans may initially impede polyp settlement but eventually provide attachment sites as they harden over time.

Jellyfish polyps exhibit remarkable resilience to food shortages, capable of surviving in a low-active state with minimal microplankton supply for over six months until gradual mortality ensues with the onset of summer heat.

3. In a stable and healthy ecosystem, jellyfish polyps struggle to survive amidst benthic invertebrate communities. However, during ecosystem disturbances induced by climate change and human activities, such as rising ocean temperatures, interspecies relationships within benthic invertebrate communities begin to shift, presenting both opportunities and challenges for jellyfish polyps. Exploiting their inherent characteristics, jellyfish polyps seize the opportunity to proliferate extensively, gain competitive advantages, adapt opportunistically, and evolve into dominant species, culminating in jellyfish blooms during the summer.

Even a modest temperature increase of 1.5°C can alter the structure of benthic invertebrate communities in the early stages of succession, leading to decreased biodiversity and richness, emphasizing the dominance status of certain species and reinforcing the advantageous position of dominant species. Responses to ocean warming vary among different organisms, with jellyfish polyp settlement significantly increasing as temperatures rise, while ascidians coverage is suppressed, possibly due to thermal adaptation differences between the two species. Due to their diverse reproductive strategies, jellyfish polyps can cover the surfaces of organisms with hard or rough shells, invade other populations, and gain spatial advantages.

It is speculated that global warming and the introduction of nearshore artificial substrates will enable harmful coastal organisms to dominate in opportunistic biological interactions. Disastrous organisms like jellyfish share common features, including complex life cycles, diverse asexual reproduction methods, rapid responses to environmental changes, strong tolerance, and broad temperature adaptability. Additionally, coastal engineering and artificial substrates provide conditions for the population expansion of jellyfish polyps.

Our research extends previous laboratory-controlled experiments focusing on jellyfish polyps to in-situ ecosystem settings, applying the effects of global change into localized marine environments, and expanding the direct impact of temperature on jellyfish polyps to indirect effects mediated by ecosystems. Our research investigates the community dynamics of jellyfish polyps controlled by benthic communities and their response to climate change, providing a systematic understanding and explanation of the harmful blooms of disastrous organisms from a systems perspective. This study offers a more solid empirical evidence and theoretical foundation for the dynamic interpretation, prediction, and forecasting of disastrous organisms, as well as for the study of ecosystem dynamics and coupled processes in multiple water layers in marine biology.