Platyctenida, also known as benthic comb jellies, is an order of

comb jellies Ctenophora (; : ctenophore ) is a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), and they are ...

in the class

Tentaculata Tentaculata is a class of comb jellies, one of two classes in the phylum Ctenophora. The common feature of this class is a pair of long, feathery, contractile tentacles, which can be retracted into specialised ciliated sheaths. In some species, ...

. Platyctenids display a generally benthic lifestyle in contrast to most ctenophores being largely pelagic. Platyctenids display widely differing morphological characteristics from their pelagic counterparts, being highly flattened on their oral-aboral axis and having lost many key traits associated with the phylum.

Taxonomy

Platyctenida is the only

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

group of organisms in the phylum Ctenophora. Platyctenida are considered to be a phylogenetically young group along with the orders Lobata and Beroida and are believed to have stemmed from an ancestral version of the order Cydippida, after some kind of bottleneck effect in the phylum. This has been supported by strong morphological and developmental data, specifically the sharing of what has been termed a "Cydippida-like" larva form in all 4 orders. Platyctenida is thought to be a

polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...

group.

Description

Ranging in size 15 cm and below, they have dorso-ventrally flattened, oval and secondarily bilaterally symmetrical bodies. Platyctenids look very much like

nudibranch Nudibranchs () are a group of soft-bodied marine gastropod molluscs, belonging to the order Nudibranchia, that shed their shells after their larval stage. They are noted for their often extraordinary colours and striking forms, and they have b ...

s or flatworms and are often confused for them. All but one species of platyctenids lack the iconic ctene rows (the

cilia The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...

ted comb-rows) that distinguishes the ctenophores but they still possess the pair of tentilla-bearing tentacles and adhesive collocytes that also characterize the phylum in pores along the dorsal surface. They cling to and creep on surfaces by everting the

pharynx The pharynx (: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea (the tubes going down to the stomach and the lungs respectively). It is found in vertebrates and invertebrates ...

and using it as a muscular "foot". They are usually cryptically colored, live on the seabed and rocks, and as epibionts on other benthic organisms like

algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...

, soft coral, and other invertebrates; primarily certain species of

cnidarians Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...

and

echinoderms An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as larv ...

(primarily the genus ''Coeloplana sp''.). They are often revealed by their long tentacles with many side branches, seen streaming off the back of the animal into the current. They tend to be ectosymbiotic with the organisms they live on. Whereas most

ctenophores Ctenophora (; : ctenophore ) is a phylum of marine invertebrates, commonly known as comb jellies, that marine habitats, inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs ...

are hermaphroditic, certain platyctenids have been found to be asexual, and furthermore, where other ctenophores have been found to reproduce using external fertilization, certain species of platyctenids have been found to use brood pouches.Harbison, G. R. 2001. Ctenophora. Encyclopaedia of Life Sciences

References

Rudman, W. B. 1999. Benthic ctenophores. Sea Slug Forum, Australian Museum, Sydney

Eeckhaut, I., Flammang, P., Lo Bue, C., & Jangoux, M. 1997. Functional morphology of the tentacles and tentilla of Coeloplana bannworthi (ctenophore, platyctenida), an ectosymbiont of Diadema setosum (echinodermata, echinoida). Zoomorphology, 117:165-174.

Harbison, G. R. 2001. Ctenophora. Encyclopaedia of Life Sciences

Podar, M., Haddock, S., Sogin, M., & Harbison, R. 2001. A Molecular Phylogenetic Framework for the Phylum Ctenophora using 18S rRNA Genes. Molecular Phylogenetics and Evolution. Vol: 21 (2), 218-230
{{Authority control Tentaculata Ctenophore orders