Comb jellies

Sea gooseberries (Cydippida) are an order of comb jellies, the “primal form”. They have a rounded, mostly egg-shaped body which in the case of the common species Pleurobrachia pileus becomes a few centimetres in diameter. Over the body run the eight rows of ciliated combs with which they swim. Sea gooseberries have long tentacles with often even finer tentacles branching off them, which they can spread out like a drift net. The tentacles are covered with adhesive cells (colloblasts) consisting of blisters with a kind of glue in them, just like on a post-it paper. When a prey swims against the blister, it snaps and sticks. The gooseberry can then retract the tentacle and swallow it with its oesophagus to eat the prey.

The life cycle of Pleurobrachia pileus

Gooseberries produce eggs from which a small larva with two tiny tentacles develops. This larva grows into an adult with similar shape. In the Netherlands you can find sea gooseberries all year round in the sea, but especially in spring.

Sea gooseberries are eaten by fish like the lumpsucker, but especially by other comb jellies. The beroid species Beroe gracilis is specialised in eating sea gooseberries.

Lobed comb jellies (lobata) are an order of comb jellies that have developed lobes on either side of the mouth opening. In addition, many species have strongly reduced tentacles. Often the tentacles are stuck in grooves inside the body, where they work as a filter and to transport prey towards the oesophagus. Some species of lobed comb jellies can grow to a size of tens of centimetres. They are often very fragile and almost always damaged when caught in a plankton net. Many species emit light, which is often greenish/yellowish in colour.

The life cycle of the lobed comb jelly Mnemiopsis leidyi

When lobed comb jellies hatch, they first develop two tentacles, just like the sea gooseberry. Only later the tentacles disappear and the lobes are formed.

Two species of lobedcomb jellies occur in Dutch waters. The species Bolinopsis infundibulum is an indigenous species that mainly occurs in cooler waters, in the North Sea a little further from the coast. Close to the coast and in brackish inland waters, the American comb jelly Mnemiopsis leidyi has been a common species since at least 2006.

Mnemiopsis originally occurs along the east coast of North and South America. The species is notorious for outbreaks in the Black Sea and Caspian Sea where the comb jelly probably hitchhiked with ballast water from ships. In the over-fertilised and over-fished Black Sea of the 1980s Mnemiopsis had a huge amount of food available and little competition from fish that also eat zooplankton. Because they are able to reproduce extremely quickly, the numbers of comb jellies exploded and ate away the food for the remaining fish. Later on, the problems subsided somewhat because overfertilisation and overfishing decreased, but also because a natural enemy of the American rib jellyfish, the beroid Beroe ovata, appeared in the area.

Mnemiopsis in the Netherlands

For my PhD I carried out extensive research into Mnemiopsis leidyi and its possible impact on aquatic ecosystems in the Netherlands. In waters like the Wadden Sea, Oosterschelde and Grevelingenmeer, the species is sometimes extremely common and exerts a highpredation pressure on zooplankton. Fortunately, so far this has been limited to the summer and autumn, and not in spring when they could cause the greatest damage by competing with- and predating on fish larvae and native jellyfish. As Mnemiopsis likes warm water, this could change if climate change causes coastal waters to warm up more quickly in spring.

Our research showed that adult Mnemiopsis in the Netherlands eat the same food as other jellyfish and fish like herring and sprat. Larvae and smaller comb jellies eat much smaller plankton, making them less of a competitor to native zooplankton-eating fish.

At present, Mnemiopsis is a particularly problem for fishermen working with fine-mesh gear, such as traps, which can get clogged by massive numbers of comb jellies.

Mnemiopsis caught in a canal behind Amsterdam Central Station!

What is special is that Mnemiopsis can survive at very low salt levels. You can sometimes even find them around Amsterdam in the polders and canals! Experiments we carried out on “Amsterdam” rib jellyfish show that they are better adapted to brackish water than the Mnemiopsis in the North Sea. Do they come from somewhere else, or have they adapted very quickly?

Melon jellies (Beroidae) are the only family within the comb jelly class Nuda. They are animals that are fully adapted to eating other comb jellies. They no longer have tentacles and are in fact nothing more than a swimming stomach with a large mouth. On the lips of the mouth opening there are special hook-shaped teeth that grab into the tissue of the prey so it can’t escape, and when the prey is ingested, the teeth can close like a zipper!

Most melon jellies specialise in one type of prey comb jelly. For example, the slender lemon jelly Beroe gracilis mainly eats sea gooseberries and the larger species Beroe cucumis mainly eats Bolinopsis infundibulum. However, if they encounter another kind of rib jellyfish, they will attack them just as ferociously! See the picture and movie below. The native melon jellies of the Netherlands also seem to eat the invasive Mnemiopsis leidyi, but this doesn’t seem to have any effect on populations of either predator or prey.

A slender melon jelly Beroe gracilis eats an American comb jelly Mnemiopsis leidyi, both caught in the Wadden Sea.

The large melon jelly Beroe cucumis from the Wadden Sea, full of Pleurobrachia pileus. This picture was taken in an aquarium where Beroe could easily catch the gooseberries, I wonder whether this happens in nature as well?