Comb jellies, or ctenophores, are soft-bodied marine invertebrates belonging to the phylum Ctenophora. Recognized by their gelatinous bodies, these creatures swim using rows of cilia, known as ‘combs,’ which are the largest cilia used for locomotion in the animal kingdom. The jelly-like mass is situated between two cellular layers, each two cells thick. Ctenophores exhibit various body forms, including egg-shaped cydippids and large-mouthed beroids.
They are generally carnivorous, preying on organisms ranging from microscopic larvae to adult crustaceans. Many species possess colloblasts – specialized structures that release adhesive substances to capture prey.
Adult comb jellies vary in size from a few millimeters to 5 ft (1.5 m), with species like Magellania venosa being among the largest extant forms. In contrast, the smallest living brachiopod, Gwynia, measures just about 1 mm (0.039 inches).
Comb jellies are unsegmented and mostly radially symmetrical; thus, their primary body axis is oral to aboral (mouth to the opposite end).
Like cnidarians, they are composed of a jelly-like or gelatinous mesoglea sandwiched between two epithelial layers containing over 80 cell types. A fibrous basement membrane and adhesive inter-cell connections bind these layers. Because of this two-layer cellular arrangement, these animals are traditionally considered diploblastic.
Most species possess eight tissue strips called comb rows or swimming plates that run along the length of their bodies. These rows are equipped with comb-like bands of cilia called ctenes. These cilia confer the name ‘ctenophore’ to these animals.
The outer layer of the epidermis comprises two cell types: sensory cells secreting mucus and interstitial cells that transform into other cell types. In some parts of the body, like tentacles, the outer epidermal layer also contains specialized, mushroom-shaped cells called colloblasts that secrete adhesive substances to entrap prey.
The inner cellular layer lines a cavity that houses a mouth, pharynx, stomach, and a system of internal canals, which branches throughout the body and terminate into anal pores. The inner surface of this cavity is lined by an epidermal layer called the gastrodermis.
Although comb jellies have relatively few species, they exist in a remarkable range of body forms.
| Cydippids | Have spherical to cylindrical or egg-shaped bodies. A pair of long and slender tentacles extend from the opposite ends of their bodies and are usually fringed with tentilla or tiny branching tentacles. |
| Lobates | As their name suggests, they are shaped as a pair of lobes or muscular, cuplike extensions projecting beyond the mouth. |
| Beroids | They are sac-like and lack tentacles. At the oral end of the body lies macro cilia, a bunch of fused bundles of thousands of cilia that help these animals bite off huge chunks of prey that are difficult to swallow. |
| Other Forms | Members of the order Ganeshida have circular bodies with a pair of small oral lobes and tentacles.Those in the order Platyctenida have oval bodies flattened along the oral-aboral axis. Members of the order Cestida are ribbon-shaped, whereas those of Thalassocalycida resemble the medusoid stage of cnidarians. |
Comb jellies possess two large blood vessels, with blood flowing forward in one and backward in the other.
Although they lack gills or lungs, gaseous exchange takes place across the cellular layers of the body.
These invertebrates possess a muscular pharynx, where the ingested food is liquified by enzymes and passed on to the canal system by the beating of the cilia. The ciliary rosettes within the canal may assist in transporting nutrients to muscles in the mesoglea.
While the excretory system in comb jellies is not well understood, it is believed that the ciliary rosettes in the gastrodermis may play a role in expelling waste from the mesoglea. Although some waste may be excreted through anal pores, most of it is regurgitated through the mouth.
Although comb jellies lack a distinct brain or centralized nervous system, they possess a subepidermal nerve net that encircles the mouth and is most dense around the comb rows, pharynx, and tentacles. In addition to the subepidermal nerve net, these invertebrates also possess a less organized mesogleal nerve net comprising single neurites.
The neurons in the nerve net are fused to form a syncytium and lack synaptic interconnections. Some Cambrian fossils indicate that ancient comb jellies had long nerves. However, in modern forms, only members of the genus Euplokamis have long nerves.
At the opposite end of the mouth lies the aboral organ, which has its own underlying nerve net. This organ comprises a balance sensor called the statocyst, which contains a tiny grain of calcium carbonate, the statolith. Four bundles of cilia or balancers support this organ, which is covered by a transparent dome-like case made of long, immobile cilia.
Unlike most animals, the nervous system of comb jellies lacks neurotransmitters like serotonin, dopamine, nitric oxide, octopamine, and noradrenaline. Instead, they use L-glutamate, which is received by an unusually wide variety of ionotropic glutamate receptors.
Initially, comb jellies were grouped with cnidarians under a single phylum, Coelenterata. Such classification stemmed from the similarities between the two groups. Like cnidarians, comb jellies are composed of a gelatinous mesoglea sandwiched between an inner and an outer cellular layer. However, unlike in cnidarians, these two layers in comb jellies are two cells deep. Moreover, the neurons of comb jellies have evolved independently from those of other animals, emphasizing the need to classify the two groups into separate phyla.
Although the exact position of comb jellies in the animal evolutionary tree is still unclear, it is believed that cnidarians and bilaterians are more closely related to each other than either is to comb jellies.
Approximately 180 living comb jelly species are currently recognized, though the exact number remains uncertain. This uncertainty is due to the fact that many formally described species are identical to species with different scientific names.
Traditionally, these invertebrates are classified into two broad classes: Tentaculata (with tentacles) and Nuda (without tentacles). These two classes are further subdivided as follows:
Since most modern comb jellies, except beroids, possess larvae resembling cydippids, it was assumed that the last common ancestor (LCA) of comb jellies was a cydippid. However, in 1985, Richard Harbison’s morphological analysis of the group revealed that cydippids are not monophyletic (do not include all descendants of a single common ancestor).
A molecular phylogeny analysis (2001) using 26 species (including four recently discovered ones) proved that cydippids are non-monophyletic, and the LCA of comb jellies was indeed cydippid-like. The genetic differences between these comb jelly species were so negligible that the interrelationships between the classes Lobata, Cestida, and Thalassocalycida remained uncertain. This relatedness suggests that the LCA of modern ctenophores lived relatively recently and may have even survived the most recent Cretaceous–Paleogene extinction event (65.5 million years ago).
Comb jelly fossils are rare due to their extremely soft bodies and are only found in lagerstätten, which are sites with exceptional conditions for preserving soft tissue. Until the mid-1990s, only two fossil specimens of crown group comb jellies from the Devonian Period were known and suitable for analysis. Since then, three additional species have been discovered and studied in the Burgess Shale and other similarly aged Canadian rocks from the Mid-Cambrian Period, dating back 505 million years. All these fossils lacked tentacles but had between 24 and 80 comb rows, as opposed to just eight in present-day forms.
The youngest fossil outside the crown group is Daihuoides from the late Devonian period, which is a member of the basal group that apparently went extinct over 140 million years ago.
The fossil Eoandromeda from the Ediacaran Period has an eightfold symmetry, with eight spiral arms, just like comb jellies, and may also be a member of the phylum Ctenophora. Additionally, a sessile frond-like fossil, Stromatoveris, unearthed from China’s Chengjiang lagerstätten, dates back to about 515 million years ago and had cilia on its branches for filter feeding. Scientists believe that Stromatoveris could be ancestors of present-day comb jellies, which later became swimmers and modified their cilia for propulsion underwater. Other fossils, such as Dinomischus, Daihua, Xianguangia, and Siphusauctum, provide evidence for the evolution of comb jellies from sessile ancestors.
Around 520 million-year-old (Cambrian) fossils discovered from Chengjiang in China represent Scleroctenophora, an extinct class of comb jellies. Their members were characterized by a complex internal skeleton with long spines and eight soft-bodied flaps for both feeding and swimming.
These invertebrates are exclusively marine, with no individual ever spotted in any freshwater environment. They are found in various oceanic zones, ranging from coastlines to surface waters and depths over 7,000 m.
Although comb jellies sometimes occupy brackish-water estuaries and coastal lagoons connected to oceans, some, like members of Mnemiopsis, are found in saline lakes without connection to the ocean. In some coastal areas, these animals are more commonly spotted during the summer.
Humans have introduced some comb jellies to certain regions. For example, in the early 1980s, Mnemiopsis leidyi was accidentally introduced to the Black Sea.
Though most comb jellies are carnivorous (except a single, partly parasitic genus) and mostly feed on mollusks, fish larvae, cnidarians, and crustaceans, dietary choices vary among the different groups.
Comb jellies propel underwater by synchronized beating of the cilia in the comb rows. Although the propulsion or power stroke is away from the oral end of the body, the direction is sometimes reversed when they wish to escape from a predator lurking nearby. In such situations of threat, these invertebrates can accelerate to six times their normal swimming speed.
Although exact information on the lifespans of comb jellies is still being determined, they are assumed to live anywhere from less than a month to three years.
Most comb jellies are hermaphroditic, either producing eggs and sperm simultaneously or at different times within the same individual. However, a few exceptions, like Ocyropsis crystallina, Ocyropsis maculata, and Bathocyroe foster, are dioecious, meaning they have distinct male and female individuals.
For reproduction, sperm and eggs are released into the surrounding water via pores in the epidermis, and fertilization is usually external. However, platyctenids fertilize internally and retain their eggs in brood chambers until they are ready to hatch. Most hermaphroditic species, including members of the genus Mnemiopsis, are believed to undergo self-fertilization.
The fertilized eggs usually develop directly into juveniles, bypassing the larval form; however, in some comb jellies, there may be an intermediate cydippid stage. The juveniles are planktonic and usually resemble miniature adult cydippids, with an egg-shaped body and a pair of retractable tentacles. Juveniles of the genus Beroe (under the class Nuda), however, lack tentacles, just like their adults. Moreover, some juveniles, like those of bottom-dwelling platyctenids, behave like true larvae and develop into the adult form only after dropping to the sea floor.
Members of the classes Lobata and Cydippida undergo dissogeny, in which the animals have two sexually mature stages, first as larvae and second as juveniles and adults. As larvae, they can only produce gametes periodically and usually stop doing so after their first reproductive period.
Comb jellies are usually preyed upon by fish, such as the chum salmon (Oncorhynchus keta), turtles, jellyfish, and other comb jellies, like beroids. Additionally, larvae of sea anemones and flatworms parasitize these invertebrates.
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