CATHENOPHORES: CHARACTERISTICS, REPRODUCTION AND NUTRITION - BIOLOGY - 2025

The ctenóforos (phylum Ctenophora) are almost exclusively planktonic marine organisms. They owe their name to the fact that they have bands of cilia on their surface arranged in the shape of combs (ctenes).

Ctenophores are made up mostly of water, so their body has a gelatinous appearance, which is why they are located within the gelatinous plankton.

Ctenophora. Taken and edited from Orin Zebest, via Wikimedia Commons.

They are a very small group, as there are only about 150 living species described. They are quite variable in size, ranging from a few millimeters to more than two meters.

Only a few species are benthic, and they are all taxonomically located within the order Platyctenida. They do not have their own stinging cells, however some species can use, for their defense, the non-triggered nematocysts of the jellyfish that have served as food.

characteristics

They are diblastic organisms, that is, they develop from two embryonic leaves, ecto and endoderm. Additionally, they have a cellular mesoglea between both embryonic leaves.

All ctenophores have 8 bands of long cilia fused at the base that are called the swim paddle, ctene or comb. The ctenes are arranged southern.

They have a pair of tentacles that in almost all species can retract into a tentacular sheath. The tentacles have branches called tentacles.

These organisms have adhesive cells called coloblasts. These cells are exclusive to the ctenophores, they are located in the tentacles and they serve to capture food.

Their symmetry is biradial, they lack excretory, respiratory, circulatory organs, as well as a skeleton. The digestive system is complex and ends in a pair of tiny anal pores.

They do not present alternation of generations, nor sessile forms. They have a characteristic larva, called cidipoid, which is exclusive to ctenophores, although in some species it is absent and development is direct.

Differences with jellyfish

Despite their superficial similarities, considered evolutionary convergences (similar characters in species from different ancestors), jellyfish and ctenophores present numerous and important differences. Among them the following can be noted:

-Jellyfish have nematocysts and ctenophores have coloblasts. Nematocysts are organelles used to inject toxins. Coloblasts are non-stinging cells.

-Some jellyfish present alternation of generation with a sessile polyp phase, others are colonial. Ctenophores do not present sessile or colonial forms.

-The musculature of jellyfish is of ectodermal or endodermal origin. The musculature of the ctenophores, on the other hand, originates from the mesoglea.

-Swimming paddles are exclusive to ctenophores.

Taxonomy

The phylum Ctenophora was erected by Eschscholtz in 1829. It is made up of two classes with current species and one made up of extinct species.

The class of extinct ctenophores is called Scleroctenophora. This class is made up of four genera, which differ from the current ones by presenting a sclerotic cover and paired bands of ctenes.

Classes with current forms are called Nuda and Tentaculata. This classification depends on the absence (Nuda) or presence (Tentaculata) of tentacles. Some authors suggest that these groups are not monophyletic, so their validity is under discussion.

Currently nine orders and more than 160 species are recognized.

Ctenophora mesopelagico Bathocyroe fosteri. Taken and edited from: Photo courtesy of Marsh Youngbluth, via Wikimedia Commons.

Reproduction

Asexual

Some ctenophores of the order Platyctenida are able to reproduce asexually by a process of fragmentation. In this process, organisms shed small pieces of their bodies as they move. Each piece will then develop as a complete organism.

Sexual

Hermaphroditism is the norm in ctenophores, with only a few dioecious species existing. The gonads are made up of bands of cells that develop on the walls of an internal cavity called the meridional canal.

Gametes are generally released into the environment through the mouth. Fertilization can be crossed or self-fertilizing and is external, except in some benthic species that present internal fertilization. In these last species, the incubation of the egg is also internal.

The egg hatches into a larva called cidipoid, which is ciliated and plactonic. The larva becomes an adult after gradual changes. There is no metamorphosis.

Nutrition

Ctenophores are carnivorous, they feed mainly on zooplankton, although some species can feed on larger prey, such as jellyfish.

Tentaculated cathenophores trap their prey thanks to the colobrasts located in their tentacles. Those without tentacles capture them directly with their mouths.

Coloblasts consist of a hemispherical head made up of adhesive granules and two filaments, one straight and the other spiral, wound around the rectum like a spring. When the tentacle comes in contact with the prey, the coloblasts shoot up and adhere to the victim thanks to the adhesive granules.

The main prey for ctenophores are part of zooplankton, such as copepods. Other species prefer larger prey such as salps (tunicates) or jellyfish.

Ecological impacts

In recent years, gelatinous plankton populations, including ctenophores, have increased their densities in some areas, leading to serious ecological impacts.

The causes of these population increases are still unknown, but some authors suggest increases in ocean temperatures and eutrophication. They can also be due to introductions of species in areas other than their original ranges.

An example of the latter cause is the accidental introduction of the species Mnemiopsis leidyi into the Black Sea. This species, coming from the western Atlantic, was introduced in the 80's of the last century, in the Black Sea, by ballast waters of ships.

This species rapidly proliferated, affecting the entire food web of the Black Sea, causing the collapse of the anchovy fishery. In the Caspian Sea, it affected both density and zooplankton diversity.

In the Caspian Sea, it affected fisheries for fish that fed on zooplankton, in a similar way to what happened in the Black Sea. It also invaded the Mediterranean Sea.

Due to its strong negative impact on fisheries and the environment, it has been classified by the IUCN as one of the 100 most harmful invasive species in the world.

Ctenophora Mnemiopsis leidyi. Taken and edited from: Steven G. Johnson, from Wikimedia Commons.

References

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3. EE Ruppert, RD Barnes & RD Barnes (1994). Invertebrate zoology. Fort Worth: Saunders College Pub.
4. RC Brusca, W. Moore & SM Shuster (2017) Invertebrates. Third Edition. Oxford University Press.
5. CE Miles (2019). Ctenophore. Recovered from marinespecies.org/
6. Ctenophora (2019), On wikipedia. Recovered from en.wikipedia.org