Comb jellies, also known as ctenophores, are fascinating marine invertebrates that captivate with their delicate, often shimmering appearance. These animals drift through the global ocean, representing a distinct and ancient lineage within the animal kingdom. Their unique adaptations allow them to thrive in diverse marine environments, from surface waters to the deep sea.
What Are Comb Jellies?
Comb jellies belong to the phylum Ctenophora, a group of exclusively marine invertebrates. The name “Ctenophora” originates from Greek words meaning “comb-bearing,” referring to their distinctive rows of ciliary combs. With approximately 100 to 200 described species, these gelatinous creatures inhabit oceans worldwide.
Comb jellies have radial symmetry and a body composed largely of a jelly-like substance. This gelatinous material, known as mesoglea, provides structural support and buoyancy, making them appear delicate and transparent. Their bodies typically consist of two main cell layers: an outer epidermis and an inner gastrodermis, with the mesoglea sandwiched between them.
Comb jellies range in size from a few millimeters to as large as 1.5 meters (5 feet) in some species. Despite their soft bodies, fossils believed to be ctenophores have been found dating back to the early Cambrian period, approximately 525 million years ago, indicating their long evolutionary history.
Distinctive Appearance and Features
A defining characteristic of comb jellies is their “comb rows,” which are eight longitudinal bands of large, fused cilia arranged along their bodies. These combs beat in coordinated waves, propelling the comb jelly through the water. The movement of these cilia scatters light, creating a mesmerizing, iridescent rainbow effect that is often mistaken for bioluminescence.
While the rainbow colors are a result of light refraction, many comb jelly species are indeed bioluminescent, producing their own light through chemical reactions. This light, typically blue or green, serves various purposes, including defense. This internally generated glow adds to their ethereal appearance in the dark ocean depths.
Comb jellies possess specialized feeding cells called colloblasts, which are unique to their phylum. These cells are found on their tentacles or body surface and work by secreting a sticky substance that adheres to prey. Unlike the stinging cells of true jellyfish, colloblasts do not inject venom but instead trap small organisms like plankton, fish larvae, and small crustaceans. Once prey is ensnared, the tentacles retract, guiding the food towards the mouth for digestion.
Where They Live and How They Thrive
Comb jellies are found in all marine environments across the globe, from the sunlit surface waters to the cold, dark deep sea. Their widespread distribution highlights their remarkable adaptability to diverse oceanic conditions. While some species prefer warmer, temperate, tropical, or subtropical waters near the surface, others thrive in deeper or nutrient-sparse regions.
As predators, comb jellies play a significant role in marine food webs. They are carnivorous and opportunistic, primarily feeding on zooplankton, small crustaceans, and even other comb jellies. Their feeding strategies vary among species; some use long, retractable tentacles armed with colloblasts to capture prey, while others engulf their food directly with their mouths.
Their locomotion, powered by the rhythmic beating of their comb rows, allows them to move through the water column in search of food. This method of movement makes them the largest animals known to swim primarily using cilia.
Not Your Average Jellyfish
Despite their common name and gelatinous appearance, comb jellies are not true jellyfish. They belong to entirely separate phyla: comb jellies are in Phylum Ctenophora, while true jellyfish are in Phylum Cnidaria. This distinction reflects fundamental differences in their biology and evolutionary paths, even though they share a superficial resemblance.
A primary difference lies in their method of movement. Comb jellies use their distinctive comb rows, composed of fused cilia, to propel themselves through water. True jellyfish, in contrast, move by rhythmically contracting and relaxing their bell-shaped bodies, pulsating through the water.
Another significant distinction is their feeding mechanism. Comb jellies employ colloblasts, which are sticky adhesive cells, to capture prey. Conversely, true jellyfish are characterized by specialized stinging cells called nematocysts, which they use to immobilize their prey with venom.