DESCRIPTION OF THE PHYLUM CTENOPHORA (ESCHSCHOLTZ 1829)
| EUKARYA>UNIKONTA>OPISTHOKONTA>ANIMALIA>DIPLOBLASTEA>CTENOPHORA |
PHYLUM CTENOPHORA LINKS
| Ctenophora (ten-O-pho-ra) is a combination of two Greek roots meaning comb bearers [cteno (κτένα) comb; and phoro (φέρω) bearers]. The name is a reference to the comb-like ciliary rows. |
| INTRODUCTION TO THE CTENOPHORA The ctenophores are diploblastic animals with a modified radial or biradial symmetry. Their tentacles (see Figure 1) have adhesive structures called colloblasts (text with tooltip) A colloblast is a Ctenophore feeding cell that is analogous to the nematocyst of Cnidaria. that do not seem to be homologous to the nematocysts (=cnidae) of the Cnidaria. Mnemiopsis is a common member of this phylum that has exploded in numbers and become an exotic pest in the waters of the Black and Caspian Seas (Figure 2). Some, like Beroe (Figure 3), have no tentacles but entrap their prey (usually other ctenophores) with their large gaping mouths. Although most ctenophores are planktonic, many species are benthic (text with tooltip) A benthic (adj.) organism is one that lives in or on the bottom of marine or freshwater environments. . One such genus, Coeloplana (Figure 4) is decidedly bilaterally symmetrical. In addition to their characteristic symmetry and tentacles with colloblasts, Simion et al. (2015) list the following characters for the ctenophores: 1) They have comb-like ctenes (see Figure 5), which are found in 8 longitudinal rows and give the group its name. 2) The aboral surface (the part of the animal away from the mouth) has a sensory organ that detects the direction of the pull of gravity. 3) In embryogenesis, they have an unusual type of unilateral cleavage. 4) Many ctenophores have a characteristic cydippid larval (text with tooltip) A cydippid larva is the free-swimming, planktonic larval form of some Ctenophores. stage (Figure 6). Reviews of basal metazoans (Collins et al. 2005; Halanych 2004) also provide evidence that the ctenophores are the sister group to the Cnidaria+Bilateria (see Figure 7). That is, the ctenophores are more basal or primitive relative to the cnidarians, a view that is not supported by morphological work like that of Nielsen (2001) who interprets the body plan of the ctenophores as a reduced triploblastic organism that has affinities with the deuterostomes. Genomic work on basal animals appears to support a view in which ctenophores are sisters to all other metazoans, including sponges (e.g. Dunn et al. 2008; Hejnol et al. 2009; Ryan et al. 2013)*. The basal or sister position of Ctenophora seemed to be supported by the interpretation of a ctenophore-like fossil from the ediacaran fauna by Tang et al. (2011). Indeed, Ou et al. (2015) redefined a set of Cambrian fossils as ctenophores with skeletons. In their interpretation, all modern ctenophores either lost their skeletons or came from a line that never had skeletonized forms. Their cladistic analysis (using morphological data) does appear to support two extant monophyletic groups, here considered to be classes. Indeed, Hinde (2001) presents a classification system for the Ctenophora in which the animals with tentacles occupy a single class and the few without tentacles (the Beroids or Nuda) occupy the other class. However, molecular work by Podar et al. (2001) and Simion et al. (2015) suggests that the taxonomic structure of the ctenophores is much more complex than the dichotomy of two classes. *See the arguments in the Animal Kingdom Description, especially those of Pisani et al. (2015), for rejecting the Ctenophore-sister hypothesis. |
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| FIGURE 1. Pleurobrachia swimming with trailing tentacles. Image from http://faculty.washington.edu/cemills/Ctenophores.html | FIGURE 2. Mnemiopsis, whose native range is along the tropical Atlantic coast of South America has become an invasive in the Black and Caspian Seas. Image from http://www.zin.ru/projects/invasions/gaas/mnelei_i.htm | FIGURE 3. Beroe, a ctenophore without tentacles, feeds on other ctenophores by swimming with its large gaping “mouth” open. Image from http://faculty.washington.edu/cemills/Ctenophores.html |
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| FIGURE 4. Coeloplana, a benthic ctenophore with an appearance more like a sea slug than a jelly fish. This animal was observed off Darwin in Australia. Image from http://www.seaslugforum.net/factsheet.cfm?base=ctenopho | FIGURE 5. A high magnification of the ciliary rows making up part of a single ctene. Image from http://www.microscopy-uk.org.uk/mag/indexmag.html? http://www.microscopy-uk.org.uk/mag/artmay98/comb.html | FIGURE 6. A labeled image of a cydippid larva. Image from http://www.reefkeeping.com/issues/2004-03/rs/index.php |
FIGURE 7. A cladogram showing a simple separation between the major morphological groups of ctenophores. The topology of the figure is informed by Collins et al. (2005) and Halanych (2004).
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| By Jack R. Holt and Carlos A. Iudica. Last revised: 02/02/2016 |
