A comb jelly lures in prey with a dazzling light show of vibrant colours pulsing rhythmically down its entire body.

Often easily confused with jellyfish due to similarities in their transparent bodies - made up almost entirely of water, comb jellies are in fact very different to their jellyfish lookalikes.

Ctenophores, or ‘comb jellies’ belong to their own phyla, Ctenophora (meaning ‘comb bearing’ in Greek). The name refers to the cilia (similar to small hairs) which have a longitudinal arrangement, often resembling the appearance of a hair comb. Comb jellies are in fact the largest ciliated animals in the world. The cilia are used when feeding, where beating of the cilia creates a current, moving food items (plankton) towards the animal’s mouth. Here, plankton are greeted by a collection of specialised sticky cells, which trap prey, and allow for it to be ingested.

Comb jellies are voracious predators, often consuming several times their body weight each day. Unlike jellyfish, comb jellies have a continuous digestive system – in one way and out the other – which allows them to feed more efficiently. As pelagic (open water), nocturnal predators, comb jellies can often be observed feeding when participating in black water night dives (open ocean diving at night), where a dazzling light show of many colours is often first spotted, running rhythmically down the animal’s body. The vibrant colours are created through the refraction of light as it passes through cells, and the colourful lightshow is used by comb jellies to entice their prey.

These organisms have an important role across all marine ecosystems they are found to exist in. Comb jellies are carnivores, feeding on zooplankton such as small crustaceans and fish larvae, helping to control populations and maintain biodiversity across plankton communities.

Comb jellies are extremely hardy animals, with many species often tolerating temperature ranges from below 2 degrees Celsius to above 32 degrees Celsius. They are also able to survive in a range of salinities, and low oxygen environments. As a result of their hardiness, they have a wide distribution around the world, however for similar reasons, in addition to them often being able to survive for weeks without feeding, this has led to their invasion of certain marine ecosystems.

Perhaps the most famous of comb jellies amongst the scientific and fishing communities is Mnemiopsis leidyi, known for its invasion of the Black Sea in the early 1980’s, and the Caspian Sea in the late 1990’s. As a self-fertilising hermaphrodite, M. leidyi is capable of rapid population growth when living in favourable conditions. After the species was introduced into the Black Sea from the east coast of America through ships’ ballast water, the population of M. leidyi quickly exploded in numbers, and began feeding on zooplankton, fish eggs and fish larvae. Shortly after its introduction and population growth, there was a drastic reduction in anchovy catches as well as reduced catches in other pelagic fish including horse mackerel and sprat. The collapse in fisheries was linked to M. leidyi acting as a direct competitor to the larval stages of these fish by feeding on similar zooplankton prey, as well as directly consuming the eggs and larvae of these fish, therefore reducing their adult population sizes. In the Caspian Sea, similar effects on fish populations caused significant concerns regarding the population reduction of Caspian Seals – an extremely important member of the Caspian marine ecosystem.

The above examples of comb jelly invasions highlight how large populations of these animals are able to have a significant effect on marine ecosystems given the right environmental conditions, and also demonstrates how this can affect an entire ecosystem. This is largely due to their hardiness, reproductive biology, and high feeding rates on zooplankton including fish larvae and fish eggs.

Marine environments are often becoming more favourable for comb jellies as a result of anthropogenic (human) activity resulting in ocean warming, and elevated nutrient levels from agricultural, industrial, and domestic waste – both of which often allow for rapid population growth of certain comb jelly species. This has raised significant concern as to how the populations of comb jellies will respond to future changes to marine environments, and what the effects may be on the populations of fish and other marine organisms, as well as the biodiversity of entire marine ecosystems.

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