Comb jellies, also known as ctenophores, are captivating marine invertebrates that drift through ocean waters worldwide, distinct from true jellyfish despite their similar appearance. There are between 100 and 150 known species, each playing a role in marine ecosystems.
Their Global Habitats
Comb jellies are remarkably widespread, found in all major oceans, including the Arctic, Atlantic, Pacific, Indian, and Southern Oceans. They thrive in a broad spectrum of marine environments, from the frigid waters near the poles to warm tropical seas. Their presence extends from coastal areas, such as estuaries and bays, to the vast expanses of the open ocean.
Many species are commonly found in nearshore waters, particularly along the eastern coasts of the Americas. They can also inhabit brackish water and tolerate a wide range of salinity, temperature, and water quality, contributing to their ubiquity and success. Certain species, like Mnemiopsis leidyi, have become invasive in new regions, such as the Black, Caspian, and Baltic Seas, often transported through ship ballast water, demonstrating their resilience and ability to establish populations in diverse ecosystems.
Life in Varied Depths
Comb jellies exhibit a wide vertical distribution, populating the ocean from the sunlit surface to profound depths exceeding 7,000 meters. Some species reside in the epipelagic zone, the ocean’s uppermost layer, while many others are adapted to the darker, colder realms below.
These deeper habitats include the mesopelagic, or twilight zone, and the bathypelagic, also known as the midnight zone. Some comb jellies have even been discovered in abyssal plains and deep-sea trenches, like a unique benthic species found at 7,217 meters in the Ryukyu Trench. Different species have specialized to these varying depths, encountering distinct environmental conditions such as light, pressure, temperature, and food sources. For instance, Beroe abyssicola is found down to 2,800 meters in midwater habitats.
Adaptations to Their Homes
Comb jellies possess biological features that allow them to inhabit diverse marine environments successfully. Many species have transparent bodies, which serve as effective camouflage in the open water, helping them avoid predators and ambush prey. A defining characteristic is their ciliary combs, rows of fused cilia that beat rhythmically to propel the animal through the water. These combs also scatter light, creating a shimmering, rainbow effect that is often mistaken for bioluminescence.
True bioluminescence, the ability to produce light through chemical reactions, is common in many comb jelly species. This light serves multiple purposes, including attracting prey, startling predators, and possibly facilitating communication among individuals in the dark deep sea. In deep-sea species, red pigmentation in their stomachs can mask the light from ingested bioluminescent prey, preventing them from glowing from the inside and revealing their presence to larger predators. Most comb jellies capture prey using sticky cells called colloblasts, located on their tentacles. However, some species, like those in the genus Beroe, lack tentacles and instead use large, expandable mouths to directly engulf prey, often feeding on other comb jellies.