Entoprocta

Diversity

Phylum Entoprocta (also known as Kamptozoa ) includes nearly 200 currently known species of sessile, solitary (family Loxosomatidae ) or colonial (families Loxokalypodidae , Pedicellinidae and Barentsiidae ), primarily marine organisms, although two freshwater species, Loxosomatoides sirindhorne and Urnatella gracilis have been identified. Marine species are found throughout the world’s oceans, and Urnatella gracilis is found on all continents except Antarctica. These organisms superficially resemble cnidarian hydroids and bryozoans (Phylum Ectoprocta ); the main body consists of a cup-like calyx that bears a ring of ciliated tentacles, which is attached on its dorsal surface to the substrate (or stolon in colonial species) by a long, thin stalk. Whether solitary or colonial, all entoprocts are sessile suspension feeders. Entoprocts exhibit a range of reproductive modes and behaviors, including asexual clonal reproduction, as well as hermaphroditic and dioecious sexual reproduction.

Geographic Range

The majority of entoprocts live in coastal, marine environments throughout the world. Urnatella gracilis is found in rivers on every continent except Antarctica, and Loxosomatoides sirindhorne has been identified from rivers in Thailand.

• Biogeographic Regions
• nearctic nearctic
  • native native
• palearctic palearctic
  • native native
• oriental oriental
  • native native
• ethiopian ethiopian
  • native native
• neotropical neotropical
  • native native
• australian australian
  • native native

• Other Geographic Terms
• holarctic holarctic
• cosmopolitan cosmopolitan

Habitat

Colonial entoprocts are found living on rocks, shells, algae, and underwater structures, preferring more solid substrates to softer sediments. Solitary species are found living on other animals such as sponges, sessile annelids, and ectoprocts, taking advantage of the feeding currents produced by these animals. They are found from intertidal areas to depths of up to 500 m. While marine species may be found in brackish environments, the two freshwater species are found only in fully freshwater environments.

• Habitat Regions
• temperate temperate
• tropical tropical
• saltwater or marine saltwater or marine
• freshwater freshwater

• Aquatic Biomes
• benthic benthic
• reef reef
• rivers and streams
• coastal coastal
• brackish water brackish water

• Other Habitat Features
• intertidal or littoral intertidal or littoral

Systematic and Taxonomic History

Following their discovery, these organisms were alternatively classified as special, polyp-like rotifers, or they were classified along with Ectoprocta , in phylum Bryozoa . The name Entoprocta was first applied to these organisms by the German zoologist Hinrich Nitsche in 1870 to separate them from Ectoprocta , based upon the position of the anus (inside the lophophore ring, rather than outside), embryonic cleavage pattern (spiral vs. radial), and differences in body cavity type (acoelomate vs. coelomate). These differences resulted in Hatschek elevating Entoprocta to phylum level in 1888. The name Kamptozoa would later be applied to this group by Carl Cori in 1929, and is still used interchangeably with Entoprocta .

The monophyly of Entoprocta , as well as that of its two orders, Solitaria and Coloniales , is well supported by morphological characters and a recent molecular phylogenetic analysis. However, the evolutionary relationships of entoprocts to other phyla remain a matter of some debate. Recent molecular phylogenetic studies have suggested that entoprocts and ectoprocts should in fact be considered classes within a single taxon called Polyzoa , in agreement with their original classification. These, and additional molecular studies have suggested that Polyzoa should also include the obscure phylum Cycliophora , which appears to share a sister relationship with Entoprocta . These studies also suggest that Polyzoa be placed in a larger superphylum, Lophotrochozoa , based on the common presence of the trochophore larval state in member phyla (which include annelid worms, mollusks, flatworms, and nemertean worms, among others). There is no consensus, however, on which of the lophotrochozoan phyla represent the closest relative of modern polyzoans.

Physical Description

These organisms superficially resemble cnidarian hydroids and bryozoans (phylum Ectoprocta ), with the main body consisting of a cup-like calyx that bears a ring of 6 to 36 ciliated tentacles, attached on its dorsal surface to the substrate (or stolon in colonial species) by a long, thin stalk. The calyx and stalk are covered by a thin, collagenous cuticle, which does not extend over the tentacles, and is underlaid by a cellular epidermis. Muscle bands beneath the epidermis allow the organism to compress its calyx and stalk, extend its tentacles, and bend its stalk. These organisms are functionally acoelomate, lacking a fluid-filled body cavity. It is uncertain whether this condition results from a secondary filling of an embryonic blastocoel with mesenchyme, or whether these organisms lack a blastocoel throughout development. The viscera are located entirely in the calyx, with the mouth and anus found on the ventral surface (vestibule), surrounded by the tentacular crown. The gut is U-shaped and lined with a layer of ciliated cells, widening into a stomach near the internal base of the calyx, followed by an intestinal segment that leads to the anus. Entoprocts are quite small, with individual zooids ranging from 0.1 to 7 mm in length.

• Other Physical Features
• heterothermic heterothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• sexes alike

Development

Both colonial and solitary species are capable of asexual clonal reproduction by budding; species may reproduce sexually and be dioecious, simultaneous, or protandrous hermaphrodites. Eggs either hatch into planktonic larvae, or in some species, complete early development in a brood chamber (located in the atrium), attached by secretions of cement glands to the chamber's wall. In some brooding species, nutrition is provided to developing embryos via special placental cells; in others, the egg yolk nourishes the growing larvae. Planktonic larvae may remain free swimming for up to 7 months before settling, while those raised in a brood chamber will settle relatively quickly (within a few days of hatching).

Entoproct embryonic development follows the holoblastic, spiral cleavage pattern typical of protostome organisms, with the mesoderm forming from the 4d mesentoblast. Development continues to a coeloblastula stage, after which, the embryos of most species proceed into a free swimming, feeding planktonic larval stage strongly resembling the trochophore larva of protostome species. Some species produce lecithotrophic or benthic crawling larvae. Trochophore larvae have equatorial ciliary bands (used for feeding on suspended particles), apical and ventral sensory tufts of cilia, pigment-cup ocelli that serve as light-sensing organs, a complete digestive system, and a pair of protonephridia for waste excretion. After their larval period, most entoprocts settle, attach to the substrate, and undergo metamorphosis. Unequal growth of the body directs the vestibular surface away from the substrate and the mouth, anus, and gut may rotate up to 180°, to face the vestibular surface; however, no rotation or unequal growth is necessary for some species. In these, an asexual bud forms from the attached larvae to form the zooid, which is already oriented in the correct position.

Some species in family Loxosomatidae produce free swimming larvae that may produce adult buds precociously; the adults are held in a body pocket of the larvae until their release through the body wall, usually a few days. After release, the larvae die. In some species, adult buds have been noticed developing from larvae while the larvae were still developing in their parents' ovaries. In at least one species, males and females may be produced via budding.

• Development - Life Cycle
• metamorphosis metamorphosis

Reproduction

Gonads, when present, are located just beneath the vestibular surface and empty into the water via the gonopore. Male zooids release sperm into the water, where it can be drawn into the reproductive tract of female zooids. Fertilization occurs in the ovaries or oviducts.

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

Entoprocts exhibit a wide range of reproductive modes and behaviors. Both colonial and solitary species are capable of asexual clonal reproduction by budding. In colonial species, new zooids are formed from the stalk or the stolons joining individuals, while solitary species produce buds from the base of the calyx. Sexual reproduction also occurs and individuals may be dioecious, simultaneous, or protandrous hermaphrodites. Mating seasons depend on location; entoprocts from warmer waters may reproduce year-round, while many of those in more temperate waters show a breeding peak in late summer and early fall (experimental evidence supports increased water temperature as a trigger for sexual maturity).

• Key Reproductive Features
• seasonal breeding seasonal breeding
• year-round breeding year-round breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• simultaneous hermaphrodite
• sequential hermaphrodite
  • protandrous protandrous
• sexual sexual
• asexual asexual
• fertilization fertilization
  • internal internal
• oviparous oviparous

Beyond production of gametes and, in some cases, development of larvae in a brooding pouch (sometimes fed by placental cells), entoprocts do not display parental investment.

• Parental Investment
• no parental involvement
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • female

Lifespan/Longevity

Although no definitive studies regarding the lifespan of entoprocts are currently available, feeding planktonic larvae of some species live for more than 7 months (at which point they settle and attach to the substrate).

Behavior

Entoprocts may be colonial or solitary. An entoproct may shed (typically under adverse environmental conditions) and regenerate its calyx, sometimes changing sex when it does; most entoprocts with this ability are colonial (only one species of solitary entoproct, Loxosomella antarctia is noted to regenerate the entire calyx in this way). It is possible to fold the tentacles and move the calyx using subepidermal musculature of the body and stalk. Once larvae have settled, they are most typically sessile, using a "foot" to attach to a substrate. A few species are reportedly capable of movement even after adulthood, for instance members of genus Loxasomella inside the tubes of marine annelids.

• Key Behaviors
• diurnal diurnal
• nocturnal nocturnal
• crepuscular crepuscular
• sessile sessile
• motile motile
• sedentary sedentary
• solitary solitary
• colonial colonial

Communication and Perception

Entoprocts have greatly reduced nervous systems, consisting of a single subenteric ganglion between the vestibular surface and stomach. This enervates the tentacles, calyx, and stalk via several pairs of nerves arising from the central ganglionic mass. Tactile receptors are scattered over the body and concentrated on the tentacles. Some species in family Loxosomatidae have ciliated papillae, which form lateral sense organs as well.

• Communication Channels
• tactile tactile

• Perception Channels
• tactile tactile

Food Habits

All entoprocts are sessile suspension feeders. The cilia on their tentacles generate water currents that carry small food particles (particularly phytoplankton, as well as diatoms and algae) to a ciliated food groove at the base of the tentacular crown. The tentacles also have glandular cells that secrete a sticky substance to capture larger food particles, which are also moved to the food groove. The cilia in the food groove carry food to the mouth, where a combination of ciliary action and muscle contractions move the food through the digestive system. One species of entoproct, Loxosomella brochobola , has nematocyst-type organs; their function is unknown but may be related to feeding.

• Primary Diet
• planktivore planktivore

Predation

As very small invertebrates, entoprocts are likely prey to many larger marine and freshwater animals. Known predators include small crustaceans and mollusks , flatworms , and nudibranchs .

Ecosystem Roles

Solitary entoprocts are most often commensal on invertebrates such as sponges , annelids , sipunculans , ascidians , and ectoprocts . There is a high level of host specificity. Entoproct colonies may also be found on mollusk shells. There are no parasitic forms currently recognized and there is no data currently available regarding parasites of these animals.

A phoretic relationship has been identified between Urnatella gracilis and larval Cordalus cornutus (commonly called hellgrammites, the larval form of Eastern dobsonflies); as a result of this relationship, entoprocts gain a means of dispersal, protection from predators, and nutrition.

Economic Importance for Humans: Positive

Beyond the potential for scientific research, there are no known positive effects of entoproct species on humans.

• Positive Impacts
• research and education

Economic Importance for Humans: Negative

Entoprocts may be a cause of water and vessel fouling.

Conservation Status

There is currently no concern regarding entoproct species becoming threatened or endangered.

Other Comments

While rare, entoproct fossils date back to approximately 520 million years ago, during the Cambrian period.

Additional Links

Encyclopedia of Life

Contributors

Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Texas State University.

References

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