Invertebrates
Comb Jelly
Pleurobrachia pileus · also called comb jelly, sea gooseberry, ctenophore
A comb jelly looks like a jellyfish and is often called one, but it belongs to a completely separate branch of the animal kingdom. It has no stingers. It swims instead on eight rows of beating comb plates, and as those plates move they split daylight into shimmering bands of colour that seem to run along its transparent body.
Overview
The comb jelly best known to European beachgoers is the sea gooseberry, a marble-to-walnut-sized transparent ball trailing two long branched tentacles. Comb jellies as a group — the phylum Ctenophora — drift in every ocean, from the surface to the deep sea and the tropics to the poles, and range from millimetres to the 1.5-metre ribbon of the Venus’s girdle.
Swimming on combs
Eight rows of comb plates, each built from thousands of fused cilia, run down the body and beat in coordinated waves. This makes the comb jelly the largest animal to move using cilia. As the plates beat in daylight they act like tiny prisms, producing the running-rainbow effect — a structural light trick, separate from the true blue-green bioluminescence many comb jellies also make in the dark.
Glue, not venom
Instead of stinging cells, comb jellies have adhesive cells called colloblasts on their tentacles that burst and glue prey — copepods, fish larvae, and other comb jellies — before the tentacle is wiped across the mouth. Tentacle-less species like Beroe simply engulf other comb jellies whole.
A troublesome traveller
The warty comb jelly Mnemiopsis leidyi, carried from the Americas in ballast water, exploded in the Black and Caspian Seas in the late twentieth century, devouring fish eggs and larvae and helping collapse local fisheries.
Status
Comb jellies have not been assessed as a group. They are abundant worldwide, and several species are increasing where warming water and disturbed food webs favour them.
Comb Jelly: common questions
Is a comb jelly a type of jellyfish?
No, though the two are easy to confuse. True jellyfish belong to the phylum Cnidaria, along with corals and sea anemones, and share the defining feature of that group, stinging cells. Comb jellies belong to a completely separate phylum, Ctenophora. They have no stinging cells, they swim with rows of beating comb plates rather than by pulsing a bell, they catch prey with glue rather than venom, and their overall body organisation is different. The two groups are both gelatinous, transparent, and planktonic, which is why they look alike and why comb jellies are often loosely called jellyfish, but they are only distantly related.
Why do comb jellies flash rainbow colours?
The rainbow effect and the glow are two different things. The shifting bands of colour that seem to race along a swimming comb jelly in daylight are caused by its comb plates. Each plate is a row of fine cilia that scatters and splits light like a diffraction grating as it moves, so an observer sees rippling spectral colours. This happens only when there is light to split and stops when the animal stops beating its combs. Separately, many comb jellies are genuinely bioluminescent and produce their own blue-green light from chemical reactions in their tissues when disturbed in the dark, which is a true glow rather than a reflection.
How does a comb jelly catch its food without stingers?
Most comb jellies fish with two tentacles that can be extended far beyond the body and are lined with side branches. These branches carry adhesive cells called colloblasts. When a small planktonic animal brushes against a tentacle, the colloblasts rupture and release a strong glue that holds the prey fast. The comb jelly then contracts the tentacle and wipes it across its mouth to transfer the catch. Some comb jellies, such as those in the genus Beroe, have no tentacles at all and instead swim with their large mouth open, engulfing other comb jellies whole and gripping them with rows of stiffened cilia that work like teeth.
Why are comb jellies important to scientists?
For two main reasons. First, their position on the animal family tree is disputed. Some genetic analyses place comb jellies as the earliest-branching group of all animals, even before sponges, which would have major implications for how features such as nerves and muscles evolved, and the question is still debated. Second, they are powerful and sometimes destructive players in marine ecosystems. Because some species reproduce very fast and eat fish eggs and larvae as well as the plankton that young fish depend on, an introduced comb jelly can transform a sea, as happened when Mnemiopsis leidyi invaded the Black and Caspian Seas.