Ctenophora

Ctenophora, commonly known as comb jellies, are a phylum of marine invertebrates that are distinct from jellyfish and other cnidarians. They occur in oceans worldwide, from coastal to open waters, often drifting in the plankton. Many species are transparent or translucent and exhibit striking iridescence.

Anatomy and biology: Ctenophores have gelatinous, mostly transparent bodies with two tissue layers and a jelly-like mesoglea. They possess eight rows of ciliary plates called ctenes, which beat in coordinated waves to propel the animal and create the shimmering light visible on their bodies. Unlike cnidarians, they capture prey with colloblasts—specialized adhesive cells on tentacles or tentacle-like organs—rather than nematocysts. Some species lack tentacles entirely (the group Nuda).

Locomotion and feeding: The comb rows provide locomotion, allowing precise, slow movement or rapid drifting. Feeding typically involves catching small planktonic organisms with colloblasts; prey is transported to the mouth and into a simple digestive system. While many ctenophores bear tentacles, others have reduced or no tentacles, relying on their body surface for prey capture in some cases.

Reproduction and development: Most ctenophores are hermaphroditic, though reproductive strategies vary by species. Fertilization is usually external, with embryos developing into free-swimming larval forms called cydippids in many species, while others show direct development.

Ecology and classification: Ctenophora are divided into two main classes: Tentaculata (with tentacles) and Nuda (without tentacles). They play a role as planktonic predators in marine ecosystems and can influence local populations of other zooplankton. The exact evolutionary position of ctenophores among animals is debated, with some molecular studies suggesting they are among the earliest-diverging animal lineages, a conclusion that remains contested. Notable ecological events include invasive species such as Mnemiopsis leidyi in some seas, which can impact fisheries and native communities.