The phylum of ctenophores, whose translation from Greek means ‘comb-bearers‘, is comprised of just over 150 species, all of which are marine and predominantly from warm waters. It is a little known phylum, although extremely abundant in seas around the world, accounting for a very significant percentage of the total placton biomass.
With the exception of a few sessile species, ctenophores are mostly free swimmers that are at the mercy of tides and ocean currents. However, in adverse conditions such as large storms, they have the ability to descend and take refuge in deeper water using their 8 rows of radially arranged combs (ctenos) as a means of locomotion.
Many ctenophores are similar to jellyfish, and have historically been confused with cnidarians. Ctenophores differ in having specialized cells known as coloblasts that secrete a sticky substance with which they capture their prey. There are some species of ctenophores that can present, in addition to the characteristic coloblasts of ctenophores, cells of the cnidocyte type, that is cells characteristic of cnidarians. These cnidocytes are not characteristic of the animal itself, but species such as Haekelia rubra that feed on cnidarians, are able to reuse the stinging cells (cnidocytes) and incorporate them into the surface of their own body as a defensive mechanism against possible predators.
The body of ctenophores, spherical to elliptical, is translucent and gelatinous, with biradial symmetry. The 8 rows of combs present on its body give it bright iridescent shades during the day and thanks to its synchronous movement allows specimens to move through the water. During the night, most ctenophores are capable of emitting pulses of luminescence. They may have more or less long extensible tentacles, which they use to capture their prey, including small planktonic organisms such as crustaceans.
The vast majority of ctenophor species are simultaneous hermaphrodites, that is, the same individual is capable of generating both male and female gametes. These gametes are released into the water column, where fertilization will occur and planktonic larvae will form.
Traditionally, ctenophores have been divided into two main classes according to the presence or absence of tentacles. Those with tentacles make up the class Tentaculata and are by far the most common and well known group of ctenophores. The second class, called Nuda, is formed by those species that do not have tentacles.
