Comb jellies are a phylum of animal that can be found living in marine waters around the world. Their most distinctive feature is their ‘combs’. They are also the largest animal that swim by means of cilia, with adults ranging from a mere few millimetres to a whopping 1.5 metres in size.
Almost all comb jellies are predators. They take prey ranging from as small as microscopic rotifers and larvae, to small adult crustaceans. However, there are 2 exceptions which live as parasites. When conditions are favourable, comb jellies can eat 10 times their own weight on a single day!
There are about 100 to 150 species that are currently classed as comb jellies. Another 25 have been identified but not yet described and named. The textbook look of these species are that they have egg-shaped bodies and retractable tentacles. These tentacles have mini tentacles called tentilla on them, which are covered with sticky cells that capture prey. These cells are called colloblasts. Comb jellies come in different shapes and forms, including flattened deep-sea platyctenids, to coastal beroids that do not have tentacles and actually prey on other comb jellies by using their huge mouths that have stiffened cilia that act as teeth.
Most comb jelly species are actually hermaphrodites. Fertilisation is usually external, and the fertilised eggs are kept inside of their parents’ bodies and kept inside the body until they hatch. Young comb jellies look almost like planktons and they gradually shape as they grow. The only exceptions are the beroids found on the coast as well as deep-sea platyctenids. In addition, some juveniles in certain comb jelly species can actually reproduce before they reach adult shape and size. As a result, the combination of being able to reproduce early as well as hermaphroditism enables even the smallest comb jelly population to grow at an explosive rate.
Even though comb jellies have soft, gelatinous-like bodies, fossils have been found that have been thought to be of comb jellies. These fossils show the comb jellies without any tentacles, but much more comb-rows than modern-day comb jellies. These fossils have been discovered with a carbon-dating of at least 525 million years ago in the early Cambirna. As a result, the placement of the comb jellies in the evolutionary family tree of animals has always been debated. The current view is based on molecular phylogenetics, is that all modern comb jellies most likely appeared after the Cretaceous-Tertiary extinction which took place 65.5 million year ago.