Beneath the ocean’s surface, ctenophores are gelatinous invertebrates known for their shimmering, comb-like structures. Often called “comb jellies,” they are frequently mistaken for jellyfish but belong to a distinct and ancient lineage of marine life. Found across all the world’s oceans, from coastal waters to the deepest abysses, ctenophores play a significant role in marine ecosystems.
What Are Ctenophores?
Ctenophores are marine invertebrates with gelatinous, transparent bodies. They exhibit biradial symmetry, unlike the radial symmetry of true jellyfish, allowing division into two planes. Their most distinguishing feature is eight rows of fused cilia, called ctenes or comb rows. These comb rows beat in a coordinated, wave-like fashion, propelling the ctenophore through water.
The rhythmic beating of these cilia refracts light, creating a shimmering, rainbow effect. This unique locomotion makes them the largest animals to move using cilia. Unlike true jellyfish (phylum Cnidaria), ctenophores do not possess stinging cells (nematocysts) for defense or prey capture. Instead, they use different mechanisms to secure food.
Life and Survival Strategies
Ctenophores are active predators, with most species feeding on small planktonic organisms like copepods, larval fish, and other zooplankton. Many species use specialized sticky cells called colloblasts, located on their tentacles, to capture prey. These colloblasts release an adhesive substance that traps prey, which is then reeled into the ctenophore’s mouth.
Other species, such as Beroe, lack tentacles and possess large mouths, allowing them to engulf prey directly, often feeding on other ctenophores. Ctenophores inhabit a wide range of marine environments, from surface waters to the deep sea. Most ctenophores are hermaphroditic, possessing both male and female reproductive organs. They release eggs and sperm into the water column, where external fertilization occurs, leading to a larval stage that can resemble a tiny, transparent bell.
Unique Biological Marvels
Ctenophores possess several unique biological features. One notable trait is bioluminescence, their ability to produce light through biochemical reactions. This internal light production serves various ecological functions, such as startling predators, attracting prey, or potentially facilitating communication within their species. The intensity and color of the light can vary among different ctenophore species.
Their nervous system is also distinct, characterized by a diffuse nerve net rather than a centralized brain. This decentralized network allows for coordinated movements and responses to stimuli. At the aboral (opposite to the mouth) end of the ctenophore’s body lies the apical organ, a specialized sensory structure. It controls movement and orientation in water and contains a statolith, a calcareous structure that helps them sense gravity and maintain balance.
Ecological Roles
Ctenophores hold a significant position within marine food webs, primarily as predators of zooplankton and larval fish. By consuming these organisms, they help regulate the population sizes of other marine animals, influencing the ecosystem’s balance. Their feeding habits can have a ripple effect throughout the marine food chain, impacting ocean health.
While natural components of their native ecosystems, certain ctenophore species can become invasive when introduced to new environments. A well-documented example is Mnemiopsis leidyi, introduced into the Black Sea in the early 1980s, likely via ship ballast water. This rapidly reproducing feeder outcompeted native planktonic fish, severely disrupting the local food chain and devastating the region’s fishing industry within a decade. This illustrates how changes in ctenophore populations can have far-reaching consequences for marine ecosystems.