Although comb jellies look like jellyfish, they actually make up a group called Ctenophores (“tee no fours”) which means comb bearing in Greek. The most distinctive feature of comb jellies is, not surprisingly, the “combs” that run down the side of its body. There are usually eight combs and the teeth of these combs are called cilia. These are used for swimming. Comb jellies are the largest organisms to use this method of movement and can be anything from a few millimeters to up to 1.5 meters long!
All comb jellies are predatory, taking plankton from the water column by swimming mouth first. They use an organ called a colloblast that squirts a glue like substance onto the prey. When food is plentiful, a comb jellie can eat up to ten times its own body weight in one day!
Their rabid hunger has caused some problems in areas where they have been unintentionally introduced. When the species Mnemiosis leidyi from the West Atlantic was introduced to the Black Sea and Sea of Azoz through bilge water from container ships, there was a collapse of local fish populations. It is thought that this was caused by comb jellies eating larval fish and removing the food source for adult fish by eating the crustacean larvae.
Most species have blue/green bioluminescence that makes them glow at night. However, the rainbow effect that is seen during the day travelling up and down the side of a comb jelly is not caused by bioluminescence but when the cilia move slightly out of sync with the next. This causes a scattering of light making them look like out-of-this world spaceship.
For a long time it was thought that nothing ate comb jellies because of their low nutrient value. However, it has since been discovered that some fish species, turtles and jellyfish predate them upon. Jellyfish can be so prevalent that sometimes jellyfish predation can temporarily wipe out an entire population of comb jellies.
Some comb jellies are sequential hermaphrodites, meaning they change gender as they get older. They release eggs and sperm into the water column, where fertilization occurs. Comb jellies start life as small plankton (except for two species that begin life as parasites), some then settle on the seafloor, while other species continue to float on the currents into adulthood.
Comb jellies are often seen around Arran in the summer months, along with jellyfish. They can form large congregations at the surface, where there is the most plankton.
Latin name: Ctenophores
Distribution: Comb jellies are often seen around Arran in the summer months,
Size: A few millimeters to up to 1.5 meters long!
Photos by: Fiona Cumming, taken in Sannox Bay, Isle of Arran