What are they?
The Ctenophora, known as the comb jellies, are an ancient phylum of gelatinous zooplankton that look superficially like jellyfish. Comb jellies are likely some of the earliest animals to ever exist, along with sponges, and are named for their characteristic rows of ‘combs’ located across their bodies. These combs are composed of small cilia which pulse along the length of the row to propel the comb jelly through the water column, refracting light as they do so giving the comb jellies a shimmering effect. All ctenophores are generally quite small, and to date, around 150 species have been described. These species can be split into two classes, Class Tentaculata (has two paired tentacles covered in sticky-glue secreting cells, colloblasts) and Class Nuda (No tentacles, often have large mouths for swallowing prey whole). There are only several species of ctenophore found in Irish waters. This includes Beroe spp. of the class Nuda and Pleurobrachia pileus and Bolinopsis infundibulum of Class Tentaculata.
Comb jellies are a part of many ocean ecosystems and are an important invertebrate predator of planktonic species, and act as a food source for other planktonic and non-planktonic organisms, including other ctenophores. In fact, the Beroe, a group of Ctenophores in Class Nuda, use their streamlined, torpedo-like bodies to propel themselves towards other smaller Ctenophora, like the sea gooseberries, engulfing them whole with their large mouths. Indeed, most species of comb jellies are incredibly effective predators of zooplankton, able to control the populations of planktonic crustaceans, arthropods and cnidarians, with the Tentaculata using their sticky tentacles to capture and consume prey. No species of Ctenophora feeds on phytoplankton, with their presence ensuring that phytoplankton consumers do not become too numerous and diminish phytoplankton numbers, making the comb jellies an incredibly important part of planktonic food chains as a top predator. Ctenophores have the potential to be invasive, for example, in the Black Sea, the ctenophore Mnemiopsis leidyi after an accidental introduction decimated the zooplankton community with particular effect on fish eggs and larvae.
Most ctenophores have a simple life cycle, without a distinct larval stage. Many are hermaphrodites, and are often observed performing external fertilisation similar to other gelatinous plankton groups, with one genus capable of self-fertilisation. Ctenophores are holoplanktonic, spending all of their lifecycle in the water column, with one exception. The Platyctenids are bottom-dwelling ctenophores, often being mistaken for sea slugs. They are also unusual among ctenophores in that they reproduce via asexual cloning.
By Jack Smith