Anglerfish are among the most distinctive and highly adapted organisms of the deep-sea environment, flourishing in conditions marked by complete darkness, immense pressure, and limited food sources. Commonly residing below 1,000 meters, they possess remarkable traits such as bioluminescent appendages, reduced musculature, and specialized cellular compounds like piezolytes that enable survival under intense pressure. Despite their evolutionary specialization, recent reports of anglerfish appearing near the ocean surface have puzzled scientists and prompted deeper investigation.

A striking instance involves the live documentation of a black sea devil (Melanocetus johnsonii) near Tenerife—an observation never before recorded for this species in such shallow waters. Similar sightings of humpback anglerfish indicate that these events may reflect a broader behavioral trend rather than isolated incidents. These movements might result from physiological distress, attempts to escape deep-sea predators, or responses to environmental disturbances like rising temperatures or changing prey distributions.

Because anglerfish are not strong swimmers and are finely tuned to life in the abyss, their presence near the surface may reflect critical challenges to their survival. This article examines various explanations for these phenomena, considering both internal physiological drivers and external ecological pressures. Understanding these rare surface appearances is crucial for broadening our knowledge of deep-sea species behavior and for identifying the wider implications of climate change on marine life.

Dear Editor,

Anglerfish, typically associated with deep-sea habitats, have recently been observed venturing into shallower waters, capturing the attention of researchers. A notable occurrence involved a female black sea devil (Melanocetus johnsonii) being filmed near the surface close to Tenerife in the Canary Islands, a rather rare event. Furthermore, scientists documented a humpback anglerfish swimming near the ocean’s surface during daylight. These observations raise questions regarding the motivations for this atypical behavior.

Numerous theories have been suggested to explain these unusual surface behaviors. One possible reason is that health issues or physical discomfort may compel anglerfish to ascend to the surface. Anglerfish are known for their limited swimming capabilities, often relying on passive drifting in their deep-sea environment. A study recorded a female ceratoid anglerfish from the genus Oneirodes drifting at a depth of 1,474 meters near Monterey, California. The fish quickly retreated when approached but spent 74% of the observation time drifting passively at various angles. When it did swim, it moved erratically at a speed of 0.24 body lengths per second, coordinating the movement of its pectoral fins. This lethargic behavior is well-suited to the energy-limited conditions of the deep sea[1]

An alternative hypothesis posits that surface excursions may function as a strategy for avoiding predators, enabling anglerfish to utilize the upper ocean layers as a refuge from threats present in deeper waters. Additionally, certain researchers suggest that these movements could be associated with spawning behaviors, as the surface environment offers ideal conditions for reproduction. Variations in oceanographic elements, such as temperature changes or shifts in prey availability, might also drive anglerfish to explore new habitats, including the surface waters [2,3]

The recorded occurrences of deep-sea anglerfish, especially the black sea devil (Melanocetus johnsonii) and humpback anglerfish, emerging into surface waters warrant a reassessment of our comprehension of their behavioral ecology. These unusual surface sightings, which were not previously noted for some species, prompt important inquiries regarding the environmental and physiological elements that drive these movements. The significance of these discoveries goes beyond the observations themselves, providing valuable insights into possible stressors and adaptations among deep-sea organisms.

A likely explanation for this behavior is physiological distress. Deep-sea fish are subjected to intense pressure in their natural environments, and any decline in health—whether from illness, aging, or parasitic infestation—may compel them to rise to the surface, where survival is increasingly difficult. For instance, the black sea devil spotted near Tenerife was in poor condition, indicating that its rise may have been involuntary and linked to an underlying health issue. These occurrences are consistent with earlier research showing that deep-sea species display changes in movement and erratic behavior when under physiological stress[1].

In addition to health-related considerations, the avoidance of predators presents another hypothesis that deserves attention. Although the deep sea is typically viewed as an environment rich in predators, certain dangers may compel anglerfish to momentarily retreat to surface waters. This idea is consistent with predator-prey interactions noted in various marine species, where vertical migrations act as a protective strategy. Nevertheless, due to the anglerfish’s limited swimming speed, the efficacy of this approach is still questionable.

Anglerfishes experience hydrostatic pressure that increases by 1 atmosphere for every 10 meters of water depth. This elevated pressure disrupts various cellular functions, affecting membrane rigidity, altering protein folding and enzyme activity, and hindering ligand binding to receptors. However, deep-sea organisms mitigate many of these effects through the presence of small organic compounds known as piezolytes. These molecules bind closely to water, preventing it from infiltrating the proteins’ interiors and maintaining their structural integrity. Generally, the deeper an organism resides, the greater the concentration of piezometers found within its cells[4]

 Additional studies investigating the reproductive cycles and environmental factors influencing ceratioid anglerfish would be beneficial in evaluating this potential.

Changes in the environment, especially alterations in oceanographic conditions, constitute another element that may affect these movements. Increased ocean temperatures, changes in the distribution of prey, and differences in the stability of the water column have been associated with modified behaviors in numerous marine species [5]. Should anglerfish be reacting to alterations in deep-sea conditions, it may have considerable consequences for their ecological functions and conservation status. Subsequent research that includes long-term environmental monitoring and telemetry tracking will be crucial in ascertaining whether these surface sightings are mere anomalies or indicative of a larger trend. The sighting of the black sea devil near Tenerife holds particular importance as it marks the first recorded live observation of this species at the surface. Although the poor condition of the specimen limits the ability to draw firm conclusions regarding its behavior, its preservation at the Museum of Nature and Archaeology in Santa Cruz de Tenerife presents a significant opportunity for further morphological and genetic research. Comparative analyses with other deep-sea anglerfish species could offer additional insights into these uncommon surface occurrences.

In summary, although anglerfish primarily inhabit deep-sea environments, their rare appearances at the surface highlight the importance of ongoing research into the ecological and physiological factors influencing this behavior. Future studies should broaden their focus to include in situ observations, physiological evaluations, and environmental monitoring, which are vital for revealing the fundamental mechanisms at play. As human activities increasingly affect marine ecosystems, gaining insight into these behavioral irregularities is crucial for guiding conservation efforts and assessing deep-sea biodiversity. The recent sighting of deep-sea anglerfish at the ocean’s surface is not merely an unusual occurrence; it serves as a significant alert regarding substantial ecological changes taking place in our seas. This phenomenon, potentially caused by increasing ocean temperatures, changes in food supply, or disturbances in deep-sea habitats, necessitates immediate scientific scrutiny. These elusive species, which have adapted to thrive in the extreme darkness of the deep ocean, should not be found in shallower waters. Their appearance underscores the urgent need to evaluate the health of our marine ecosystems to prevent irreversible harm. Investigating the factors driving this unprecedented behavior is essential for the preservation of ocean biodiversity. The secrets of the deep are becoming more apparent—now is the critical moment to take action.

References

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2.     Currey MC, Walker C, Bassham S, Healey HM, Beck EA, Cresko WA: Genome-wide analysis facilitates estimation of the amount of male contribution in meiotic gynogenetic three-spined stickleback (Gasterosteus aculeatus). J Fish Biol. 2023, 102:844–55. 10.1111/jfb.15321

3.     Reid CH, Patrick PH, Rytwinski T, Taylor JJ, Willmore WG, Reesor B, Cooke SJ: An updated review of cold shock and cold stress in fish. J Fish Biol. 2022, 100:1102–37. 10.1111/jfb.15037

4.     Isakov N: Histocompatibility and reproduction: Lessons from the anglerfish. Life (Basel). 2022, 12:113. 10.3390/life12010113 5.         Ramirez-Llodra E, Tyler PA, Baker MC, Clarke A: Deep-sea ecology in a changing world. Trends in Ecology & Evolution. 2010, 25:599–607.