Comb jellies may be the oldest surviving branch of the animal family tree
Comb jellies row through the sea on beating bands of tiny hairs, and at up to 1.5 metres long they are the biggest animals to swim that way. Some biologists argue that their lineage split off before sponges did, which would make them the earliest-branching animals alive today. The argument is still unresolved.
Their scientific name, Ctenophora, joins the Greek words for comb and carry. Most of the 186 known species have eight rows running the length of the body, each lined with stacked bands of cilia called ctenes that beat in sequence, each comb brushing the one beneath. Sizes range from about a millimetre to that 1.5-metre maximum.
Nearly all are predators, taking anything from microscopic larvae to small adult crustaceans. Many trap prey with colloblasts, sticky cells found in no other animal. Body plans vary widely: egg-shaped cydippids fish with two retractable tentacles, flat platyctenids mostly lack combs, and big-mouthed beroids eat other comb jellies. The lone exceptions to the hunting habit are the young of two species that live as parasites on salps.
Their bodies are mostly jelly sandwiched between two thin cell layers, a design they share with jellyfish, though they have muscle types that in other animals come from a third layer, so textbooks disagree on how to classify them. They also have more than 80 cell types, more than sponges or cnidarians. Rotate one through half a turn and it looks the same, yet it has no mirror symmetry, because only two of its canals end in anal pores.
Fossils thought to be comb jellies turn up in exceptionally preserved beds from the early Cambrian, about 525 million years ago. Where they sit relative to sponges has flip-flopped: one reanalysis blamed misleading, unusual genes, but a 2017 study by Whelan and colleagues backed the comb-jellies-first view. Many open-ocean species are so fragile that they fall apart when caught and are known mostly from photographs.
Source: Ctenophora
