Comb jellies are marine invertebrates belonging to the phylum Ctenophora. These organisms inhabit ocean waters worldwide, yet their delicate, gelatinous bodies often lead to confusion about their biological status. Their fragile, transparent appearance belies a complex and unique physiology. Modern science recognizes comb jellies as active, successful predators that play a substantial role in marine ecosystems across the globe.
What Defines a Comb Jelly
Comb jellies are classified within the phylum Ctenophora, a name derived from the Greek words ktene (comb) and phora (to carry). This phylum is entirely distinct from Cnidaria, the group that includes true jellyfish, sea anemones, and corals. A primary difference is that comb jellies do not possess the stinging cells, or cnidocytes, found in cnidarians.
Instead of stinging cells, ctenophores rely on specialized structures called colloblasts to secure their prey. These unique cells are found along their tentacles and release a sticky, adhesive substance that glues the food item to the tentacle. Their bodies exhibit biradial symmetry, meaning they have two planes of symmetry, unlike the multiple planes seen in true jellyfish. The body structure consists of a gelatinous middle layer, the mesoglea, sandwiched between two layers of cells, making them among the structurally simplest animals with organized tissues.
The namesake feature is the eight rows of ciliated plates, or ctenes, that run longitudinally along the outer surface of their bodies. These rows are composed of thousands of fused, giant cilia, which are the largest structures of their kind used for locomotion. While most species are spherical or oval, some, like the Venus’s girdle (Cestum veneris), can be elongated and ribbon-like, reaching lengths of up to one meter.
How Ctenophores Move and Function
Comb jellies are the largest animals that rely on cilia for movement. They achieve this through the synchronized beating of the ctenes in their eight comb rows, which propels the organism through the water, usually mouth-end first. This constant, rhythmic motion is visible on the surface and causes the mesmerizing iridescent, rainbow-like refraction of light for which they are famous.
Their nervous system is decentralized, lacking a true brain. It operates through a subepidermal nerve net that is denser around the comb rows and mouth. At the aboral end, opposite the mouth, is a specialized sensory organ called the statocyst, which functions as a balance sensor. The statocyst contains a statolith, a tiny grain of calcium carbonate, which helps the organism sense gravity and maintain orientation in the water column.
The digestive system is unusual; while they possess a mouth and a gut, some species, such as Mnemiopsis leidyi, have a “transient anus” that only forms momentarily for the expulsion of waste. Furthermore, many ctenophore species exhibit an extraordinary ability for whole-body regeneration, rapidly healing wounds and replacing missing organs, including their statocyst. This regenerative capacity, which is absent in some species like those in the genus Beroe, makes them a focus of study for understanding the evolution of tissue repair in animals.
Comb Jellies’ Role in the Marine Ecosystem
Comb jellies are found in all marine environments, ranging from the open ocean surface to abyssal waters, and from polar regions to tropical seas. They are voracious carnivores that consume a wide variety of prey, including copepods, fish eggs, fish larvae, and smaller gelatinous organisms. Their ability to consume up to ten times their own body weight in plankton per day establishes them as significant predators.
This predatory nature gives them a substantial influence on plankton communities, impacting local fisheries by competing with larval fish for food and directly preying on eggs and young. The ctenophore Mnemiopsis leidyi, often called the sea walnut, has become a notorious invasive species in various European seas, including the Black Sea. Its introduction led to a collapse of local fisheries by dramatically reducing the populations of zooplankton and fish larvae.
Despite their delicate appearance, comb jellies are part of the diet for several marine animals. Specific species of fish, sea turtles, and even certain seabirds, such as the Magellanic penguin, are known to consume them. Other comb jellies, most notably the Beroe species, are specialized predators that feed almost exclusively on other ctenophores, often helping to control the population of invasive species like Mnemiopsis.
An Ancient Branch on the Tree of Life
The lineage of comb jellies stretches back deep into geological time, with fossils suggesting their presence as far back as the early Cambrian period, approximately 525 million years ago. This makes them one of the most ancient groups of animals still existing today. Their deep ancestry has led to a major ongoing debate among scientists about their exact placement on the animal family tree.
For many years, sponges (phylum Porifera) were considered the first animal group to branch off from the Animalia kingdom, known as the Porifera-sister hypothesis. However, recent genomic analyses support the Ctenophora-sister hypothesis, suggesting that comb jellies were the first to diverge from the common ancestor of all other animals. This controversial placement implies that complexity, such as a nervous system, may have evolved in the ctenophore lineage and then been lost in sponges, or that nervous systems evolved independently in different animal groups.
The unique genetic makeup of the ctenophore nervous system relies on different neurotransmitters and signaling pathways than those found in other animals. This lends credence to the idea of an independent evolutionary path. Regardless of which group proves to be the true basal metazoan, the comb jelly’s existence highlights a successful, independent evolutionary experiment in animal biology.