Phylum Ctenophora

One of the many deep-sea species found in Phylum Ctenophora. This species is bioluminescent.

It's been a while since I took a look at one of the animal phyla. I've spoken about this particular phylum in some detail before, so some of you may have already read parts of this post.

A larvae with a colored body

The name Ctenophora is derived from the Greek for "comb bearing," and this phylum includes approximately one hundred and fifty species. Though they superficially resemble jellyfish and other Cnidarians, the two groups are quite different. For one, Ctenophores lack the stinging cells that the Cnidarians have that cause the painful sensation when their tentacles brush against you. Instead of using those stinging cells to capture food, Ctenophores used structures called colloblasts, which secrete a sticky, adhesive substance. They are actually quite successful predators, to the point where some are troublesome invasives. These animals can also be distinguished from jellyfish and their relatives thanks to their complete digestive tract. While Cnidarians have only a blind-ended sack, comb jellies have a distinct mouth for ingestion of food and anal pores for elimination of waste.

Ctenophores are commonly referred to as comb jellyfish thanks to another of their characteristic features. Every Ctenophore has rows of cilia known as ctenes or combs organized into eight bands along the body called costae or comb rows. This is their primary method of movement and most species are only weak swimmers, unable to swim against a strong current. This makes them planktonic, despite the large size of some species. In combination with the combs, a sense organ opposite of their mouth is used to detect their orientation and controls how quickly the different costae beat.

A bioluminescent species

These creatures can be quite colorful, though many are transparent. In addition to body pigment, nearly every of species is bioluminescent. This is common in organisms that live in deep ocean water, though the biological process that produces the light in this group is unique. It's difficult to say what exactly the glow is for in this Phylum, but a number of purposes have been theorized. In addition to this bioluminescence, all comb jellies also show iridescence, the rainbow sheen that is so frequently seen when viewing them. The sheen is actually cause by light reflecting off the comb rows as they move, creating the colorful, flashing appearance.

Class Tentaculata

Class Nuda

There are two Classes in this phylum, and the division has been made based on whether or not the species posses tentacles at any point during their lifecycle. Species with tentacles are placed into Class Tentaculata, a group which includes the vast majority of comb jellies. All other species are placed into Class Nuda. Overall, species in Nuda have simpler body plans than those in Tentaculata. This includes the lack of oral lobes, which aid in prey capture as well as movement through the water. In addition, those tentacles seen in Tentaculata are used exclusively for prey capture. Nuda, in contrast, has developed a very stretchy mouth to help capture prey.

There is still much that isn't known about Ctenophores thanks to most species only being found in open ocean. Often, observations are chance encounters where individuals are caught in fishing nets. It isn't uncommon for scientific expeditions that are specifically looking to study these animals to discover new species during their searches.

Source is Biology of the Invertebrates. Images are from Wikimedia Commons and are under Creative Commons licenses or are copyright free: one, two, three, four, five.