1 Introduction

whale fall refers to the isolated ecosystem formed when large cetaceans die and sink to the bottom of the sea. The deep-sea environment is characterized by a scarcity of energy and food, and has long been referred to as the "Marine desert". However, when a huge whale carcass falls into the deep sea, it is like an oasis emerging in the desert, suddenly providing rich nutrient supply and habitat for deep-sea creatures (Yin et al., 2023). The large amount of organic matter carried by whale falls can be decomposed and utilized for decades to hundreds of years, and is hailed as the "gift" of deep-sea life (Silva et al., 2021; Li et al., 2022).

Research shows that a 30-ton whale carcass contains approximately 1.2×10 ³ kilograms of organic carbon, equivalent to the amount of carbon received by a 100-square-meter deep-sea bed over 1,000 years. Such concentrated organic matter input has broken the normal oligotrophic state in the deep sea, significantly increased local biomass and biological activity, and made whales an important hotspot for deep-sea biodiversity (Martin et al., 2021). For instance, at least 43 species, totaling approximately 12,490 biological individuals, were recorded on a whale fall on the seabed of the North Pacific Ocean, far exceeding the biological abundance of the surrounding background environment (Li et al., 2022).

Whale falls not only provide a food source, but also the sulfides released from the decomposition of lipid in their bones can support chemoautotrophic biomes, similar to chemoautotrophic ecosystems such as cold seep and black chimney (Silva et al., 2021; Pearson et al., 2023). Meanwhile, whale falls may act as "stepping stones" to promote the diffusion of deep-sea sulfide-dependent organisms among dispersed geochemical habitats (Pereira et al., 2020; Silva et al., 2021). Due to the contingency and spatial limitations of whale fall formation, there have been relatively few related studies in the past (Li et al., 2022). However, in recent years, with the development of deep-sea technology and the rise of interdisciplinary research, whale fall ecology has received increasing attention (Yin et al., 2023).

This study aims to review the main progress in the research of whale fall ecosystems in recent years. It will explore the ecological significance, community succession patterns and driving mechanisms of whale falls, and look forward to future research directions and application prospects. Studying the dynamics of whale fall communities not only holds significant scientific importance, deepening the understanding of life adaptation and evolution in extreme environments, but also helps assess the role of whale falls in the deep-sea carbon cycle and material transport, providing new ideas for deep-sea ecological protection.

2 An overview of the Ecological Significance and Research Progress of Whale Falls

2.1 Definition and discovery history of whale falls

The term "whale fall" summarizes the carcass of a whale, its sinking process, and the deep-sea ecosystem formed thereby (Li et al., 2022). In 1987, humans witnessed a whale fall for the first time in the deep sea of the northeastern Pacific Ocean: The USS Alvin manned submersible discovered a blue whale skeleton about 21 meters long off the coast of California. The skeleton was covered with a dense "carpet of life", including a large number of decomporers such as bacterial biofilms and worms (Smith and Baco, 2003; Smith et al., 2015; Georgieva et al., 2023. This is the first recorded natural whale fall ecosystem in science, confirming the existence of a special biological community in the deep ocean that uses whale remains as an energy source. Since then, scientists have successively discovered new whale falls in deep seas such as the Pacific Ocean and the Atlantic Ocean, and found traces of whale fall fossils in the Neogene strata about 30 million years ago in the geological record (Yin et al., 2023).

Whale falls are thus regarded as an important "stepping stone" and "relay station" for the evolution of deep-sea organisms (Xie et al., 2023). Whaling records since the 19th century also offer clues that there might have been more whale falls in history than in modern times, providing an important food source for deep-sea creatures. However, large-scale industrial whaling in the 20th century led to a sharp decline in the population of large whales. It is estimated that the frequency of natural whale falls in contemporary times has decreased by at least 50 to 95% compared with that before whaling. Some deep-sea exclusive species that rely on whale falls may thus face the threat of extinction (Smith et al., 2019). In recent years, with the development of technologies such as deep-sea remotely operated vehicles (ROVs), scientists in China and other countries have also begun to actively search for and study whale falls. In 2020, a Chinese scientific research team discovered the remains of a sperm whale about 3 meters long at a depth of approximately 1,500 meters in the South China Sea and conducted in-situ observations. This was the first time that natural whale fall was recorded in China (Figure 1) (Yin et al., 2023).

Figure 1 The first discovery of the whale fall of Stenella longirostris on the Zhongnan seamount in the South China Sea in March 2020 (Adopted from Yin et al., 2023) Image caption: Some megafauna observed at the dolphin fall during April–June 2020: (B) Munidopsis sp., (C) caridean shrimp, (D) sea urchin, (E) lithodid crab, (F) snailfish, (G) grenadier, (H) cusk-eel, and (I) Halosauridae sp. (Adopted from Yin et al., 2023)

2.2 The significance of whale falls for deep-sea ecosystems

Whale falls bring a large amount of organic matter input to the barren deep-sea environment and are hailed as the "oasis" of deep-sea life (Li et al., 2022; Yin et al., 2023). Under normal circumstances, the deep sea mainly relies on the slow sedimentation of "Marine snow" on the surface to supply nutrients. The amount of organic carbon deposited each year is less than a few grams per square meter. In contrast, the thousands of kilograms of organic carbon carried by a large whale instantly sank into the deep sea after its death, equivalent to the amount of organic carbon that would normally sink over hundreds or even thousands of years. The nutrients rich in whale carcasses quickly gathered various scavengers and microorganisms, forming a vigorous biological hotspot on the originally quiet and desolate seabed (Chen and Wang, 2020). Whale falls provide a rich and continuous source of nutrition. They not only allow large scavengers to feed on soft tissues, but also nourish specialized microorganisms through the high lipid content in bones, and then support subsequent chemical autotrophic communities through microbial products (such as hydrogen sulfide) (Silva et al., 2021; Li et al., 2022).

Many new species discovered in whale fall environments have unique adaptations, such as the bone-eating worm Osedax belonging to polychaetes, which feeds exclusively on bones, and mollusks that live in symbiosis with sulfides, etc. (Shimabukuro and Sumida, 2019; Souza et al., 2021). The existence of these species expands our understanding of the extreme adaptability of life. On the other hand, whale falls also connect scattered deep-sea chemogenic habitats by providing "stepping stones". Many invertebrate groups (such as large mussels, tubular worms, etc.) found on whale falls also appeared in hydrothermal vents and cold seeps, supporting the hypothesis that whale falls act as relay stations to promote species diffusion (Pereira et al., 2020; Silva et al., 2021).

3 The Stage Division of Ecological Succession in Whale Fall

3.1 Mobile scavenger stage

This is the initial stage after the formation of a whale fall, lasting from several months to several years (Bolstad et al., 2023; Xie et al., 2023). When the huge whale carcasses sink to the bottom of the sea, they first attract large deep-sea scavengers such as sleeping sharks, rays, deep-sea fish and large scavengers (Zhou et al., 2020). For instance, in a simulated "Niu Luo" experiment in the South China Sea, eight Pacific sleeper sharks were observed to come in turn to tear up corpses (Tian et al., 2024).

During this process, a large amount of organic debris and nutrients seep into the adjacent seabed, forming a eutrophic sedimentary environment. This provides the basic conditions for the subsequent stages of settled organisms. The main ecological function of the meat accumulation stage lies in rapidly transferring most of the organic matter of the whale body to the deep-sea food web: scavengers convert whale meat into their own energy and growth, enabling efficient energy transfer in the deep sea (Aguzzi et al., 2018; Dasgupta et al., 2024). It is worth noting that the duration of this stage varies among different whale species. Whales with high fat content and large body size (such as blue whales) have more soft tissue, and their meat accumulation stage may be relatively prolonged and gather more large scavengers. The soft tissues of smaller or leaner whales (such as small toothed whales) are consumed more quickly, and the meat accumulation stage is shorter (Li et al., 2022; Xie et al., 2023).

3.2 Enrichment opportunist stage

When the soft tissues of the whale are exhausted and the large scavengers disperse, the whale fall enters an opportunistic stage. This stage lasts for several months to several years and is characterized by the proliferation and growth of a large number of small and medium-sized benthic invertebrates in the sedimentary environment rich in organic matter (Bolstad et al., 2023). The organic debris and fat that seep out during the decomposition of whale carcasses significantly enrich the surrounding seabed sediments, forming a nutrient "halo". Opportunistic species (often organisms that tolerate or even prefer organic enrichment) will settle in large numbers here (Xie et al., 2023).

Typical opportunistic organisms include some small polychaetes (such as worms of the Hirudinidae family and sarcandelids, etc.) and crustaceans (such as isopods, amphipods), as well as small bipedal mollusks among mollusks (Bolstad et al., 2023; Xie et al., 2023). They multiply rapidly by taking advantage of the excessive organic debris in the sediment, with individual density and biomass rising sharply in a short period of time, forming high-density communities. It is reported that the density of polychaetes worms in the sediments within a 1-meter radius around whale falls can increase by tens of times compared to the background value (Onishi et al., 2020; Li et al., 2022; Ibrahim et al., 2024).

3.3 Sulfophilic stage

The energy transformation stage is the most remarkable and longest-lasting stage in the ecological succession of whale fall, which can last for decades to hundreds of years (Xie et al., 2023). The core of this stage is that the abundant lipids in whale bones decompose and release reducing chemicals such as hydrogen sulfide under the action of anaerobic bacteria, thereby establishing a chemoautotrophic ecosystem similar to cold seeps and hydrothermal fluids (Silva et al., 2021; Pearson et al., 2023). Specifically, the fat rich in whale bone undergoes anaerobic decomposition (sulfate reduction) in the bone marrow buried in sediment, generating a large amount of sulfides that exudate from the bone surface and the surrounding sedimentary environment (Fujiwara et al., 2007; Li et al., 2022).

At this stage, numerous peculiar creatures that were unique to whale falls or shared with cold seeps/hot fluids emerged. Among them, the most representative one is the "bone-eating worm" - the tube worm of the Osedax genus. Osedax has no gastrointestinal tract. Instead, it buries into bones through a root-like structure and acquires the products of bone fat decomposition from symbiotic bacteria to nourish itself. Since their first description in 2004, more than 30 species of Osedax have been discovered worldwide. They are landmark species in the fossil energy stage of whale fall (Shimabukuro and Sumida, 2019; Eilertsen et al., 2020). Whale falls have formed miniaturized energy ecosystems similar to cold seeps and hydrothermal vents in the deep sea, which are called "whale fall cold seeps" by some scholars (Danise et al., 2016; Xie et al., 2023).

3.4 Poor nutrition stage (Reef Stage/Decay Stage)

When the energy substances in whale bones are almost exhausted, the whale fall ecosystem enters the final oligotrophic stage, also known as the decaying reef stage (Xie et al., 2023). At this point, the main body of the whale bone has been eroded and perforated by a large number of organisms, leaving very little organic matter and reducing the supply of sulfides to levels close to the background. Whale bone remains more often act as hard substrate "reefs", providing a surface for deep-sea benthic organisms to attach to and inhabit. At this stage, chemosymbiotic organisms gradually disappeared, replaced by some filter-feeding or attached later settlers. For example, suspended feeding groups such as sponges, barnacles, corals, and polyps may attach to bare skeletons, while migratory barnacle hermit crabs, small polychaetes, etc. move around the skeletons (Danise et al., 2014; Bolstad et al., 2023).

However, even after the whale bones have almost completely decomposed, there may still be mineralized bone fragments slightly above the seabed left, serving as "micro-reefs" to continue providing habitats for deep-sea creatures until they are eventually buried by sediments or completely decomposed. The oligotrophic stage marks the end of a whale's life cycle, with energy output approaching its conclusion, but its impact on deep-sea organisms may persist for many years. Some researchers speculate that during the history of frequent whaling, a large number of whale bones sank to the seabed and might have continuously provided a hard substrate habitat for the deep sea, which to some extent connected the deep-sea sedimentary environment with the reef environment (Smith et al., 2019).

4 The Driving Mechanisms of Community Succession

4.1 Nutrient decomposition and material circulation

Whale fall ecological succession is initially driven by huge nutrient pulses. The high amount of organic matter carried by whale carcasses is a huge energy input for the deep sea, and its rapid decomposition and circulation affect every stage of community dynamics. During the meat accumulation stage, large scavengers convert the organic matter in the whale body into their own biomass and metabolic products by consuming whale meat. Some nutrients are deposited in the form of feces and debris and enriched in the local environment (Zhou et al., 2020). Subsequently, in the opportunistic stage, small benthic invertebrates consume a large amount of organic debris and decays in the sediment, accelerating the mineralization and burial of organic matter (Li et al., 2022).

At the chemoenergetic stage, the material cycle mode changes from direct decomposition to chemical synthesis: anaerobic bacteria decompose whalbonite lipids to produce H_2S, etc. Reducing compounds are utilized by chemoenergetic autotrophic bacteria to fix inorganic carbon into organic matter, which is then consumed by higher-level consumers, achieving a unique chemoenergetic food chain (Silva et al., 2021). Therefore, whale fall has established a combined cycle system of "heterotrophic and autotrophic". This cycle greatly enhances the local productivity of the deep sea, causing a "short circuit" in the deep-sea carbon cycle - a large amount of carbon rapidly settles and is permanently stored in deep-sea biomass and sedimentary reservoirs (Tulloch et al., 2018; Li et al., 2022; Pearson et al., 2023).

4.2 The fundamental role of microbial communities in ecological succession

Microorganisms are the fundamental driving force of the material cycle in whale landing and play a key role at each stage. During the meat accumulation and opportunistic stages, aerobic heterotrophic bacteria multiply in large numbers on the surface of whale meat and in eutrophic deposits, accelerating the decomposition and spoilage of organic matter. These bacteria not only directly degrade complex organic matter, but also provide delicious bacterial membranes and residues for subsequent consumers (such as small worms, crustaceans), thereby promoting the forward development of community succession (Li et al., 2022). As the sedimentary environment tends to be anaerobic, a series of anaerobic microorganisms and chemoautotrophic bacteria make their appearance in the chemoenergy stage.

In addition, microorganisms further participate in community building by symbiosis with animals. For example, the Gram-negative heterotrophic bacteria symbiotic in the bone-eating worm Osedax can assist it in dissolving whale bones to obtain nutrients (Shimabukuro and Sumida, 2019); Chemoautotrophic bacteria symbiotic to the gills of mussels and clams provide the main source of nutrition for the hosts (Silva et al., 2021). These symbiotic relationships enable the invertebrates on the whale landing to fully utilize the microbial products to thrive. The succession of microbial communities is closely linked to the changes in the whale landing environment. For instance, in whale fall sediments, heterotrophic bacteria such as Bacteroidetes and Firmicutes dominated in the early stage of eutrophication, while sulfur-oxidizing bacterial communities such as Campylobacteria emerged with the increase of H_2S (Li et al., 2022).

4.3 Interspecies interactions (Competition, Symbiosis, predation)

The dynamic succession of whale fall communities is accompanied by rich interspecific interactions, including competition, symbiosis and predation, etc. Competition: In nutrient-rich whale landing environments, different species may compete for the same resources. Predation: The predation relationship runs through all stages of a whale's descent. From the accumulation stage when large scavengers tear whale meat, to the opportunistic stage when crustaceans prey on small worms, and then to the energy stage when some predatory polychetes (such as worms of the Hesionidae family) prey on filter-feeding crustaceans (Shimabukuro and Sumida, 2019), these predatory behaviors help regulate the population size. Maintain the balance of the community structure. The temporary visits of top predators also play a role - deep-sea fish, sleeping sharks, etc. occasionally return to whale lands to prey on small animals, further transmitting energy upwards (Tian et al., 2024).

Symbiosis: Whale falls provide a stage for various symbiotic relationships. A typical example is the chemogenic stage of nutritional symbiosis. Symbiosis such as mussel - sulfur-oxidizing bacteria and Osedax- heterotrophic bacteria makes it possible for the host to obtain nutrients in extreme environments (Shimabukuro and Sumida, 2019; Silva et al., 2021). In addition, there are spatial symbiosis and attachment relationships. For instance, some barnacles attach to whale bones to filter food, and attached barnacles can provide hiding places for small crustaceans. These complex interaction relationships enable whale fall communities to have a certain self-sustaining function. Even if external nutrition decreases, internal energy can still partially circulate.

4.4 Regulatory effects of environmental factors on communities

The physical and chemical factors of the Marine environment also play an important regulatory role in the succession of whale fall communities. Water depth and temperature can affect the decomposition rate and community composition. In shallower waters (such as 500~1000 meters), where the temperature is higher and scavengers are more active, the soft tissue decomposition of whales is faster, the meat accumulation stage is shorter, and the duration of the subsequent energy depletion stage is also shorter (Zhou et al., 2020). Ocean currents and oxygen supply: The intensity of ocean currents affects the diffusion of nutrients around whale falls. Under strong current conditions, the organic matter of whale falls may be dispersed and diluted more quickly, and the opportunistic stage is not very obvious. In weak current basins, nutrient enrichment is concentrated around the bones, which is conducive to the emergence of a large number of settlers (Li et al., 2022).

Sediment type: The seabed substrate affects the settlement patterns of whale fall organisms. In soft and muddy sedimentary environments, whale bones are prone to partial burial, sulfides accumulate within the sediments, and opportunistic organisms mostly burp for food. In hard or rocky environments, whale bones are exposed in water, attracting more attached filter-feeding organisms to enter the late stage (Bolstad et al., 2023). Geographical and biogeographic factors should not be ignored either. The differences in species pools among different ocean basins will lead to differences in community composition. For example, the species richness of Osedax is high in whale falls in the North Pacific, while the diversity of bone-eating worms is relatively low in whale falls in the Southern Ocean, but many invertebrate groups specific to cold water have emerged (Stauffer et al., 2022; Bolstad et al., 2023). This reflects the restrictive effect of regional species supply on community structure.

5 The Connection Between Whale Falls and Deep-Sea Ecosystems

5.1 Whale falls as deep-sea "ecological hotspots"

Whale falls are regarded as important "ecological hotspots" or "nutrient islands" in the deep sea and play a significant role in enhancing local biodiversity. Whale falls gather a variety of scavenging and opportunistic species from the surrounding wide area, allowing them to appear simultaneously in a small area. Whale falls have also given rise to a large number of new species. Scientists have discovered over 100 species that set new scientific records on global whale falls, many of which are considered "desperate" creatures highly specialized in the whale fall environment (Souza et al., 2021). In addition, whale falls have significantly increased the functional diversity of deep-sea ecosystems. Originally, deep-sea benthic communities mainly decomposed sedimentary organic matter, while whale falls simultaneously contain multiple nutritional functional groups such as filter eaters, scavengers, and chemoautotrophs, coexisting in the same system (Li et al., 2022).

5.2 Spillover effects on surrounding habitats and neighboring communities

The impact of whale falls is not confined to the small area where their skeletons are located; there is also a certain "spillover effect" on the surrounding deep-sea habitats and neighboring communities. The nutrient sources provided by whale falls overflow into the surrounding sedimentary environment, benefiting the adjacent seabed biological communities. During the feeding process of large animals attracted by whale falls (such as sleeping sharks and crustaceans), some food residues or metabolic products scatter around and are utilized by other benthic organisms, thereby transmitting energy to a wider area. Some animals on the whale landing will spread outward at different stages. For example, in the later stage of the meat accumulation phase, many scavengers disperse after the whale meat is eaten up and may continue to wander in the adjacent sea area in search of other food, indirectly transporting the energy of the whale fall away (Zhou et al., 2020). In the long term, a large amount of whale bone remains are deposited on the seabed, which also has a certain impact on the characteristics of the deep-sea bottom.

5.3 Similarities and differences between whale falls and cold seep and hydrothermal vent ecosystems

Whale fall ecosystems share interesting similarities with deep-sea cold seep and hydrothermal vents and other chemical energy ecosystems, but also show unique differences. In terms of similarity: These three types of ecosystems all rely on chemical energy as the energy source for some biological communities. In terms of ecological functions, all three can be regarded as "nutrient islands" in the deep sea, attracting the aggregation of surrounding organisms and having the effect of enhancing local diversity (Li et al., 2022). These systems all contribute to the diffusion of deep-sea populations. Scientists have proposed the "stepping stone" hypothesis, suggesting that whale falls may connect cold seep and hot fluid, providing a relay station for chemoenergy-dependent habitats (Silva et al., 2021).

In terms of differences: The sources of energy are essentially different. The energy of hydrothermal vents and cold seeps comes from geological activities (mantle hydrothermal, oil and gas leakage), which is relatively stable and can last for hundreds or even thousands of years, while the energy of whale falls comes from organic reserves and is a one-time supply, usually exhausted within several decades (Xie et al., 2023). Whale fall begins with a large input of organic matter and undergoes a distinct heterotrophic decomposition stage in the early stage, which is not present in cold seeps and hydrothermal fluids. There are also differences in species composition. Although there are some shared groups, each also has highly specialized species. From the perspective of spatial distribution, hydrothermal and cold springs are mostly fixed in specific geological zones, while whale falls are randomly scattered in the ocean. Whale falls may occur in all ocean basins, but their distribution is scattered and difficult to predict (Yin et al., 2023).

6 Regional and Species-Specific Case Studies

6.1 Research on whale fall communities in the north pacific

The North Pacific (especially along the coast of California) is one of the earliest and most in-depth regions for whale fall research. Here, a considerable number of natural whale falls have been discovered, and the succession process of their communities has been detailedly recorded through multiple deep-sea expeditions (Yin et al., 2023). Research shows that the whale fall communities in the North Pacific are extremely rich and diverse, giving birth to numerous novel species. For instance, a total of 18 different species of Osedax bone-eating worms were recorded on a series of whale falls in Monterey Bay, California, making it the area with the most known Osedax species at present (Eilertsen et al., 2020). In addition, artificial simulation experiments have also been carried out in the study of whale falls in the North Pacific. Smith and other scholars conducted whale bone simulation experiments in the Santa Catalina Basin and found that the succession path was basically the same as that of natural whale falls, only the time was compressed. Through these experiments, they estimated the time scales and community succession rates of different stages of whale falls, establishing a quantitative basis for whale fall ecology (Xie et al., 2023).

6.2 Comparative analysis of the north atlantic and the southern ocean

The North Atlantic and the Southern Ocean (Antarctic Sea) are two other regions with whale fall records. Studies have shown that their community characteristics differ from those of the North Pacific (Bolstad et al., 2023). North Atlantic: In 2016, one of the world's deepest natural whale falls was discovered at a depth of approximately 4,200 meters in the Southwest Atlantic. The research team described the community composition of the whale fall and discovered 41 species, most of which were new scientific species (Shimabukuro et al., 2022). Southern Ocean: Research on whale falls in the Antarctic waters is quite limited, but some discoveries have already been made. Palmer Deep accidentally discovered a complete minke whale skeleton (963 meters deep) during a diving voyage in the western Antarctic Peninsula in 2017. This was the first time that a natural whale fall had been observed on-site in Antarctica (Figure 2) (Sumida et al., 2016; Bolstad et al., 2023). The community composition of this whale fall is different from that in temperate or tropical regions: During the mobile scavenging stage, there are almost no large predators such as sharks, and mainly deep-sea fish, shrimp and isopods endemic to Antarctica clean the soft tissues (Zhou et al., 2020). Overall comparison shows that the North Atlantic whale fall community lies between the Pacific Ocean and the Southern Ocean: it has chemogenic groups similar to those in the Pacific Ocean, but is also limited by its own species pool. The Southern Ocean, on the other hand, exhibits characteristics of low diversity and a bias towards background communities, with relatively few chemoenergy-specialized species.

Figure 2 Remains of an Antarctic minke whale (Balaenoptera bonaerensis) and associated biota, observed at 963 m in Palmer Deep, Western Antarctic Peninsula (Adopted from Bolstad et al., 2023) Image caption:2, Entire skeleton, largely intact, with baleen plates downslope (lower left) and caudal vertebrae upslope (upper right); 3, Anterior view; 4, Enlargement of disarticulated caudal vertebrae; a, Anemone (Actinaria indet.); b, Brachioteuthid squid Slosarczykovia circumantarctica; d, Drift algae (Cystosphaera jacquinotii and Himantothallus grandifolius); m, ‘Sea pig’ Protelpidia murrayi; n, Barracudina Notolepis coatesi; r, Rhodaliid siphonophore (novel taxon); s, Salpa thompsoni; z, Deep-sea eelpout (Zoarcidae indet.) (Adopted from Bolstad et al., 2023)

6.3 Differences in whale settlement communities of specific whale species

The differences in body size, physiology and fat content among different whale species can also lead to certain variations in their whale fall ecology. Body type and fat reserves are key factors. Large filter-feeding whales (such as blue whales and fin whales) are large in size and have fat-rich bones, and their whale falls tend to be larger in scale and last longer (Li et al., 2022). Behavioral differences: Whether some whale species sink after death depends on their density and oil content. For instance, the right whale (one of the large baleen whales) sometimes floats instead of sinking to the bottom after death because it is rich in fat. Even if these whales die, they do not form deep-sea whale falls, making it impossible to establish deep-sea communities that depend on them (Smith et al., 2019). Bone structure also has an impact. The skull of filter-feeding whales is huge and rich in oil, which can form large "skull colonies", and their chemogenic communities may occur separately from other parts of the body (Yin et al., 2023). There are also decomposition products: Some toothed whales (such as sperm whales) contain large amounts of waxy oil (whale wax), and its decomposition may release methane, which might form additional small methane nutrient communities similar to cold seep, but the current evidence is limited.

7 The Application Prospects and Challenges of Whale Fall Research

7.1 Deep-sea ecological protection and biodiversity awareness

Whale fall research provides new perspectives and opportunities for deep-sea ecological protection and the understanding of biodiversity. Whale fall communities are highly specialized, containing a large number of novel species and unique adaptation mechanisms, expanding human understanding of life diversity (Souza et al., 2021). At the conservation level, whale fall reminds people to pay attention to the occasional habitats in the deep sea and their vulnerability. Restoring whale populations not only restores surface ecological functions but also may enhance deep-sea carbon sinks and biodiversity, which is part of the "natural solution" (Pearson et al., 2023). Therefore, strengthening the protection of cetaceans has a dual significance: it not only safeguards the macroscopic flagship species but also maintains the health of the hidden deep-sea ecosystem. Whale fall research also prompts us to re-examine the scope of deep-sea protection. At present, the establishment of most deep-sea protected areas focuses on hydrothermal, coral, fishery resources, etc. The intermittent small habitat of whale falls is often overlooked, but it is crucial for some organisms (Smith et al., 2019).

7.2 Assessment of contribution to the carbon cycle and material deposition

Whale landing plays a unique role in the Marine carbon cycle and material deposition process, making certain contributions to global carbon sinks. As large creatures, whales store a large amount of carbon in the form of a "carbon pool" during their lifetime. When they die and sink, it is equivalent to rapidly transporting the carbon from the surface to the deep sea and storing it. On the other hand, the massive deposition of organic matter from whale falls has also promoted local carbon burial. The organic carbon enriched in whale bones and surrounding sediments is partially converted into inorganic carbon and enters the lithofacies cycle under the action of microorganisms, while the rest remains in the sedimentary layer in a form that is difficult to decompose. In addition, the biological communities attracted by whale falls also store carbon within the organisms. The bones and shells of a large number of organisms that are deposited after death also serve as carbon sinks. It should be pointed out that there is still controversy over the efficiency of whale fall carbon sinks. The role of whale fall in the carbon cycle requires a comprehensive assessment: It is significant in terms of local deep-sea carbon sequestration and sediment carbon burial, but has a limited overall impact on the global carbon cycle (Pearson et al., 2023).

7.3 Development and limitations of research methods

The methodology of whale fall research has made significant progress in recent years, but it still faces many challenges and limitations. The advancement of deep-sea exploration technology. Modern remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs) are equipped with high-definition cameras, enabling scientists to actively search and closely observe whale landing sites (Yin et al., 2023). The application of novel analytical methods, such as molecular biology and environmental DNA (eDNA) technology, is helping to discover whale fall species and potential whale fall locations. Through metagenomics, researchers can detect the succession of whale fall microbial communities in sediments and discover bacterial changes that cannot be captured by traditional cultures (Li et al., 2022).

Despite this, the study of whale falls remains fraught with difficulties. The biggest challenge lies in the randomness and rarity of whale falls - natural whale falls are hard to come by. At present, there are less than 50 cases of natural whale landings reported in the literature (Zhou et al., 2020). There is also the issue of time scale. A complete whale fall succession can last for decades or even hundreds of years. However, scientific research projects have limited cycles and mostly can only be observed intermittently for several years (Xie et al., 2023). Deep-sea field experiments and sampling themselves are challenging. Deep-sea conditions are harsh, each dive is costly, and sampling is often destructive and has a limited coverage. If traditional box-type sampling is adopted, it is very likely to miss the small-scale heterogeneous distribution of whale falls. The integration of multiple disciplines still needs to be strengthened. Whale fall ecology involves biological, chemical, geological and other processes. Currently, most research focuses on the description of biological communities, while paying insufficient attention to the details of the chemical environment and the geological changes of sediments. The uncertainties brought about by human activities also affect research.

8 Concluding Remarks

Whale falls are unique and significant "nutrient islands" in deep-sea ecosystems, and their formation injects tremendous organic energy into the originally barren seabed. The ecological succession process of whale fall shows distinct stages: from the initial stage of large-scale scavengers competing for a feast, to the flourishing of opportunists in the middle stage, then to the peak of chemical energy symbiotic communities in the later stage, and finally to the stage of sporadic attached organisms after nutrient depletion. The species alternate and change in each stage, reflecting the complex interspecific interaction and energy flow mechanism.

Whale falls not only give birth to a rich and diverse biological community, including a large number of newly discovered endemic species, but also connect the scattered deep-sea energy ecosystems through a "stepping stone" effect, which is of profound significance. Studying the dynamics of whale fall communities helps deepen our understanding of the life laws in deep-sea extreme environments: whale falls offer an observable deep-sea "ecological experiment", allowing scientists to gain insights into how deep-sea life responds and evolves under concentrated resource pulses. Meanwhile, whale fall research also has potential value in application. It reminds us that large cetaceans play a hidden yet important role in the Marine carbon cycle and the maintenance of deep-sea biodiversity. Protecting cetaceans and their habitats not only saves macroscopic species but also indirectly safeguards numerous strange lives that inhabit the dark deep sea and survive on whale falls.

With the advancement of deep-sea technology and research methods, more breakthroughs are expected in whale fall research. For instance, the discovery of more whale falls in various regions will fill the biogeographic gap, long-term continuous observations will reveal the complete succession process, and molecular biological methods will uncover the secrets of the interaction between microorganisms and macroorganisms. This new knowledge will further enrich the theory of deep-sea ecology and provide scientific support for the protection of deep-sea ecosystems. In a sense, the "whale fall" contains the mystery of "all living things" - just as the popular science saying goes: "When a whale falls, all living things come to life." This brief yet splendid feast of life in the deep ocean is worthy of continuous exploration and cherishing by humanity.

Acknowledgments

The author appreciates two anonymous peer reviewers for their comments on the manuscript of this study.

Conflict of Interest Disclosure

The author affirms that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

References

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