Ctenophora

(Comb jellies)

Phylum Ctenophora

Number of families 20

Thumbnail description
Primarily pelagic animals, and the largest organisms that use cilary propulsion for their main locomotory mode

Evolution and systematics

Determination of phylogenetic relationships within the phylum Ctenophora and with other phyla is a difficult task. Although the Ctenophora have been closely aligned with Cnidaria, numerous alternative schemes exist. Ctenophores have been linked with several phyla (Cnidaria, Platyhelminthes, Porifera, and Echinodermata), but molecular evidence supports them as a unique phylum linked most closely to the Cnidarians (based on ribosomal RNA analysis). Ctenophores were once classified as coelenterates based on the presence of nematocysts in one species (Haeckelia rubra), which were later found to be kleptocnidae. There are currently two classes (Cydippida and Nuda) in this phylum, seven orders, including Lobata, Cydippida, Cestida, Thalassocalycida, Ganeshida, Platyctenida, and Beroida, 20 families, and 100–150 described species. There have been several new species described since 1995 and there are many more awaiting descriptions. There are two recognized fossil species found dating from the Devonian age (417–354 million years ago) that are recognizable as cydippids.

Physical characteristics

Ctenophores, or "comb bearers," are named for the characteristic eight rows of macrociliary plates that they all possess at some point during their life. The body consists of two tissue layers, the endodermis and the ectodermis, which enclose a poorly differentiated gelatinous layer of acellular mesoglea. Ctenophores have a statocyst that consists of a pit of modified ciliated epithelium containing a calcareous statolith. This statocyst is mechanically coupled to the ctene rows by four pairs of ciliary tracts and four sets of balancer cilia. The subectodermal nerve net is associated with this statocyst. Ctenophores lack the epithelial-muscle cells characteristic of cnidarians, but do have both smooth and striated muscle. The gastro-vascular system does have two openings, however secretions of material out of the anal pores is rare, with defecation of undigested material occurring primarily through the mouth. There is a complex network of circulatory canals that often form the basis for ctenophore taxonomy. The phylum Ctenophora is unique in the possession of specialized adhesive structures called colloblasts. These organelles are utilized to capture prey in an analogous fashion to the nematocysts in the Cnidaria, but the colloblast and nematocyst morphologies are very different. The atentaculate ctenophores lack colloblasts.

Distribution

Ctenophores are exclusively marine and can be found in all of the world's oceans, from the poles to the equator and from the surface to as deep as 23,950 ft (7,300 m) in the ocean. It is likely that they occur even deeper, but researchers have not been able to spend much time below this depth.

Habitat

The majority of ctenophores are pelagic species, free swimming in the water column. They can be found in patches or as individuals. One order (Platyctenida) is benthic in nature. Species in this order lose their ctene rows as they settle onto the bottom, but retain their tentacles for feeding. At least one genus is parasitic (Lampea), feeding on salps by locating them with tentacles and then settling down and feeding on the salp with the oral region pressed against them.

Behavior

Ctenophores are predators either actively foraging or serving as an ambush predator. Ctenophores are capable of a wide suite of complex behavioral patterns that belie their apparent structural simplicity. Water flow patterns around a number of ctenophore species have been observed in the field and in the laboratory, with the findings that ctenophores manipulate water flow in a manner that can enhance locomotion (jet propulsion and "flying"), feeding ability (capture and handling of prey), and the ability to escape from predators.

Feeding ecology and diet

Ctenophores are predators feeding on a wide variety of prey items, including but not limited to jellies, copepods, ctenophores, mollusks, fish larvae, and salps. Some ctenophores exhibit prey selectivity, feeding along thermoclines or along the ocean bottom. Metabolic rates vary among ctenophores, with respiratory and excretory rates a direct linear function of animal weight (ash-free dry weight) and these rates are very temperature sensitive. The differences in metabolic rates between genera are exemplified by the basic metabolic requirement of 1.5 mg carbon (C) per day for Beroe, 260 mg C per day for Bolinopsis, and 2,600 mg C per day for Mnemiopsis.

Reproductive biology

Ctenophores do not possess an alternation in morphologies (as seen in most scyphozoans and many hydrozoans) nor has the formation of a colonial structure ever been documented. Reproduction is primarily external, with the majority of ctenophores being simultaneous hermaphrodites. Pelagic ctenophores seem to be able to self-fertilize with close to 100% normal development achieved in the laboratory, although the sequential release of sperm and eggs and the mass spawning behavior that is often noted may reduce the percentage of self-fertilization in the field. Protandry has been documented for some platyctenids. Platyctene ctenophores are also unusual in that internal fertilization, brooding of embryos, and asexual reproduction by laceration is common. Hermaphroditism is postulated as the ancestral state for the phylum and one genus is known to be dioecious, Ocyropsis. Breeding season for most ctenophores is year-round, with spawning peaks in the spring and the summer. Ctenophores may be utilizing photoreceptor cells for spawning cues, with different species spawning in response to different light/dark regimes. Other factors affecting gametogenesis are nutrients and possibly endocrine secretions. Diisogamy or paedogenesis has been noted for Pleurobrachia, Bolinopsis, Dryodora, and Mnemiopsis.

Conservation status

No species of ctenophore is listed by the IUCN.

Significance to humans

Most ctenophores have very little effect on humans, but the introduction of Mnemiopsis leidyi into the Black Sea during the 1970s (presumably from ballast water discharge) has been implicated in the collapse of fisheries in that area. Since then, Beroe ovata has also been introduced into the area and appears to be controlling Mnemiopsis leidyi populations.

Species accounts

List of Species

No common name

Beroe forskalii

order

Beroida

family

Beroidae

taxonomy

Beroe forskalii Milne Edwards, 1841.

other common names

None known.

physical characteristics

Flat and triangular in shape with the aboral end at the apex of the triangle and the oral region being the base. There are eight rows of cilia running the length of the body and numerous canals under and between the ctene rows.

distribution

Worldwide distribution. A related species (Beroe ovata) was recently introduced into the Black Sea and Sea of Azov.

habitat

Open ocean and near shore, from the surface waters to depths of 1,640 ft (500 m).

behavior

An active predator, foraging often includes a spiral swimming pattern. Once food is located, members of this family use specially modified macrocilia to manipulate and cut prey items. These macrocilia are very distinct in morphology and, for Beroe forskalii, these macrocilia are arranged in stripes inside the oral region.

feeding ecology and diet

Primary food is other ctenophores.

reproductive biology

Hermaphroditic.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Sea walnut

Mnemiopsis leidyi

order

Cydippida

family

Bolinopsidae

taxonomy

Mnemiopsis leidyi A. Agassiz, 1865.

other common names

None known.

physical characteristics

Lobate ctenophore reach 3.9 in (10 cm) in length and with four deep lateral furrows from the edges of the oral lobes to the statocyst. The oral lobes are smooth and elongate, as are the auricles.

distribution

West coast of the Atlantic Ocean and, more recently, the Sea of Azov and the Black Sea.

habitat

Shallow waters, near shore, and in bays and estuaries.

behavior

There are two distinct feeding behaviors: either swimming (oral end first) or being stationary (with the oral end down). The majority of time is spent actively swimming vertically, with the latter behavior being seen primarily in the lab and likely being an energy-conserving behavior.

feeding ecology and diet

Feeds on barnacle nauplii, small zooplankters, copepods, and both fish eggs and larvae.

reproductive biology

Nothing is known.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Venus's girdle

Cestum veneris

order

Cydippida

family

Cestidae

taxonomy

Cestum veneris Lesueur, 1813 (= Cestus veneris Chun, 1879 and 1880).

other common names

None known.

physical characteristics

Ribbon-shaped, reaching lengths of 4.9 ft (1.5 m) but only 3.1 in (8 cm) in width. The ctene rows are all on one side of this ribbon, with the mouth on the other.

distribution

Atlantic, Pacific, Antarctic, and Mediterranean waters.

habitat

Surface waters.

behavior

Has an escape behavior that consists of a snakelike undulation of the long body enabling the ctenophore to move several body lengths in seconds.

feeding ecology and diet

Swims horizontally in the water column, moving 3.2–6.5 ft (1–2 m) before moving vertically (up or down 1.9–3.9 in [5–10 cm]) and reversing direction. This behavior results in the cestid retracing its original path offset by 1.9–3.9 in (5-10 cm) above or below. Prey capture is on the tentacles lying over the body and the ctenes generate small vortices that may enhance prey movement and capture as the cestid moves back and forth through the water. Prey includes copepods and small mollusks.

reproductive biology

Eggs and sperm develop in the meridional canals in a similar fashion as most other ctenophores.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Vallicula multiformis

order

Cydippida

family

Coeloplanidae

taxonomy

Vallicula multiformis Rankin, 1956.

other common names

None known.

physical characteristics

Nothing is known.

distribution

First described from Jamaica; has been found in warm shallow waters around the Pacific.

habitat

Living on mangroves or tunicates in Jamaica; often found on manmade objects such as piers. Also found on sea grasses (and often exported to aquaria accidentally).

behavior

Glides over the substrate, stopping and forming small peaks through which the tentacles rise into the waters to forage for food. Most benthic ctenophores are highly substrate selective, but Vallicula is more of a generalist.

feeding ecology and diet

Copepods and larval decapods have been observed being caught and ingested. Diatoms have also been found within the stomach pouches. Unwanted food is wrapped up in mucus and ejected.

reproductive biology

Viviparous, shedding embryos that are almost competent to settle.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Bloody belly

Lampocteis cruentiventer

order

Cydippida

family

Lampocteidae

taxonomy

Lampocteis cruentiventer Harbison, Matsumoto, and Robison, 2001. Also known as Lampoctena sanguineventer.

other common names

None known.

physical characteristics

In the order Lobata, but differs from other families by the presence of a deep notch between adjacent subtentacular ctene rows and by the blind aboral ending for all meridional canals. The gut is always a deep red color while the body coloration varies from clear to deep purple (and all shades in between).

distribution

Currently known only from the east coast of the Pacific, however it is believed to have a much wider distribution in the Pacific and in other oceans.

habitat

Inhabits the deep sea from 984–3,320 ft (300–1,012 m).

behavior

Nothing is known.

feeding ecology and diet

Nothing is known.

reproductive biology

Nothing is known.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Sea gooseberry

Pleurobrachia bachei

order

Cydippida

family

Pleurobrachiidae

taxonomy

Pleurobrachia bachei A. Agassiz, 1860.

other common names

None known.

physical characteristics

Globular in shape with two tentacles (each of which have secondary tentacles). The tentacle sheaths are found at a distance from the infundibulum rather than next to it. Of particular note is that species in this family do not appear to have bioluminescent capabilities (unlike other ctenophores).

distribution

Worldwide distribution.

habitat

Surface waters both near shore and open ocean.

behavior

Has the oral end at the opposite end to where the tentacles originate. Due to this, it exhibits a stereotypical feeding behavior consisting of contracting the tentacle with prey and rapidly spinning around, wrapping the tentacle around the body, and eventually swiping the tentacle across the mouth region and ingesting the prey.

feeding ecology and diet

Predators on small crustaceans, it exhibits some prey selectivity. Patchy in distribution and have the potential to strip a water column of small zooplankton.

reproductive biology

Hermaphroditic, with direct development.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Thalassocalyce inconstans

order

Cydippida

family

Thalassocalycidae

taxonomy

Thalassocalyce inconstans Madin and Harbison, 1978.

other common names

None known.

physical characteristics

Ctenophore with tentacles that have lateral filaments, a medusa-like body, short ctene rows, and a mouth at the apex of a central conical peduncle. The order Thalassocalycida has one family, one genus, and one species currently described. It is morphologically distinct from the Cydippida and the Lobata, appearing to be an intermediate step between these two orders.

distribution

Originally described from the slope waters off New England; found in other oceans as well. It is more of an open-ocean species, its relative fragility making it too delicate to survive near shore.

habitat

Found from the surface waters down to 9,070 ft (2,765 m).

behavior

Likely an ambush predator, waiting for and capturing prey within its bell. When startled, it rapidly squeezes the bell, forcing water out and propelling the animal backwards. This escape behavior is limited to that single clap and distances traveled are 1–2 body lengths.

feeding ecology and diet

Copepods.

reproductive biology

Presumably hermaphroditic.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Resources

Books

Harbison, G. R. "On the Classification and Evolution of the Ctenophora." In The Origins and Relationships of Lower Invertebrates, edited by S. C. Morris, J. D. George, R. Gibson, and H. M. Platt. Oxford: Oxford University Press, 1985.

Matsumoto, G. I. Phylum Ctenophora (Orders Lobata, Cestida, Beroida, Cydippida, and Thalassocalycida): Functional Morphology, Locomotion, and Natural History. Los Angeles: University of California, Los Angeles, PhD Dissertation, 1990.

Periodicals

Harbison, G. R., G. I. Matsumoto, and B. H. Robison. "Lampocteis cruentiventer gen. nov., sp. nov.: A New Mesopelagic Lobate Ctenophore, Representing the Type of a New Family (Class Tentaculata, Order Lobata, Family Lampoctenidae, fam. nov.)." Bulletin of Marine Science, 68 (2)(2001): 299–311.

Madin, L. P., and G. R. Harbison. "Thalassocalyce inconstans, New Genus and Species, an Enigmatic Ctenophore Representing a New Family and Order." Bulletin of Marine Science 28 (4) (1978): 680–687.

Rankin, J. J. "The Structure and Biology of Vallicula multiformis, gen. et sp. nov., a Platyctenid Ctenophore." Journal of the Linnean Society, 43 (1956): 55–71.

Stanley, G. D., and W. Sturmer. "A New Fossil Ctenophore Discovered by X-rays." Nature Vol. 327, No. 6125 (1987): 61–63.

Organizations

Monterey Bay Aquarium Research Institute. 7700 Sandholdt Road, Moss Landing, CA 95039 Phone: (831) 775-1700. Fax: (831) 775-1620. E-mail: [email protected] Web site: <http://www.mbari.org>

Other

Mills, C. E. "Phylum Ctenophora: List of All Valid Species Names." March 1998 [June 10, 2003]. <http://faculty.washington.edu/cemills/Ctenolist.html>.

George I. Matsumoto, PhD