Acoela

(invertebrate zoology) An order of marine flatworms in the class Turbellaria characterized by the lack of a digestive tract and coelomic cavity.

(Acoels)

Phylum: Platyhelminthes

Class: Acoela

Number of families: 20

Thumbnail description
Tiny marine wormlike metazoans; the most primitive living animals with bilateral symmetry

Evolution and systematics

The name Acoela comes from two Greek words that mean "without a body cavity"; it refers to a distinguishing feature of this order (or phylum) of tiny wormlike multicellular marine invertebrates. Species in this group have no true body cavity or coelom. A true coelom is a fluid-filled body cavity formed from mesodermal tissue. It lies between the outer body wall of epidermal tissue and the gut or digestive tract.

The Acoela have long been included as an order within the phylum Platyhelminthes, the flatworms, and the class Turbellaria, the marine flatworms. On the other hand, very recent morphological, developmental, and molecular studies, including comparisons of 18S ribosomal DNA from 61 species representing 25 animal phyla, support the hypothesis that the Acoela are a separate phylum representing the most primitive living animals showing bilateral symmetry, or regularity of body form on the right and left sides (Ruiz-Trillo et al.). Bilateral symmetry is a basic characteristic of all triploblastic animals, those with three tissue layers in their embryonic stages: the outer ectoderm, middle mesoderm or mesenchyme, and inner endoderm. In contrast to this pattern, diploblastic animals have only two layers of tissue as embryos, lacking the mesodermal layer.

The Acoela may be direct descendants of the earliest line of animals to diverge from diploblastic organisms with the beginnings of triploblastic features: a middle tissue layer and bilateral symmetry. The DNA studies suggest that the acoels or their direct ancestors diverged from the diploblasts much earlier than did the main line of triploblastic animals (including all living triploblasts other than Acoela), in pre-Cambrian times, well before the so-called Cambrian Explosion era of 540–500 million years ago, when most if not all modern animal phyla appeared.

The work of Ruiz-Trillo and his colleagues has been challenged by Tyler and his working group, who maintain that the order Acoela should be considered the earliest or most primitive group within phylum Platyhelminthes. Lundin considers the Acoela to have likely evolved from ancestors within or related to the Nemertodermatida.

Nevertheless, other features of the Acoela that support their being a phylum unto themselves include their simple nervous system, as compared with platyhelminth species, and their manner of embryonic development. According to a study by Raikova et al., which compared the brains of acoels with those of other platyhelminths, the acoel brain and nervous system are simpler and much different in structure from those of other platyhelminths, suggesting major differences between the two groups and supporting the classification of Acoela as a separate phylum.

As of 2003, Acoela is included as an order together with Nemertodermatida in the species complex Acoelomorpha within the phylum Platyhelminthes on the basis of similarities between Acoela and Nemertodermatida. According to the European Register of Marine Species, there are 20 families within the order Acoela.

Physical characteristics

Acoels are tiny; the members of most species are no longer than 0.078 in (2 mm), although Convolutriloba retrogemma can reach lengths of 0.23–0.28 in (6–7 mm) and Convoluta roscoffensis can grow up to 0.59 in (15 mm) long. The bodies of acoels may be either oval or cylindrical in shape, and flattened dorsoventrally.

Acoels have either a simple pharynx or none at all; the pharynx or mouth is situated on the ventral (lower) surface. They have no digestive tract (gut), no protonephridia (primitive kidneys for excretion and osmotic balance), and no distinct gonads. The brain is quite simple, unlike the more complex bilobed brain found in most platyhelminth species. The acoel nervous system is a loose net of nerve fibers strung throughout the body. Most species also have simple eyes known as ocelli. Individuals of nearly all species carry a statocyst, a tiny, spherical organ for balance and orientation.

Distribution

Acoela are found worldwide in mostly shallow waters in all oceans from tropical to polar regions.

Habitat

Acoela live in marine or brackish water. Some species drift or swim in the open sea, or live unobtrusively among sand grains on sea bottoms in shallow coastal waters. Some even live in cold brine channels within Antarctic ice floes.

Behavior

Acoela are free-living, either planktonic (swimming or drifting) or interstitial (living between sand grains on the sea bottom). A few are commensals on other invertabrates. They move by means of cilia, or tiny hairlike projections, covering the entire outer side of their epidermis. The species that live among sand grains reportedly behave much as do ciliated protozoans, particularly Paramecium and Opalina, which share space with the Acoela.

Feeding ecology and diet

Acoels have no true gut, but have instead a digestive syncytium, a simple, not formed inner cavity. The digestive syncytium is not formed from mesodermal tissue as is a true coelom. Various acoel species feed on algae, microorganisms and detritus, ingesting food through a simple pharynx or even simpler mouth located on the ventral surface. Some carry endosymbiotic algae within their epidermis and absorb nourishment manufactured by the algae.

The mouth or pharynx of an acoel leads to a packed or loose mass of endodermal cells that serves as the digestive organ. Food particles ingested by the animal are absorbed and digested by individual cells in the endodermal mass. This mode of digestion is known as phagocytosis.

Reproductive biology

Acoels reproduce sexually, although some species also reproduce asexually by fission (asexual reproduction). Acoela have no distinct gonads; gametes are formed directly from the mesenchyme, or middle tissue layer. Acoel spermatozoa bear two flagella on each sperm cell, another primitive feature.

Acoels differ from other bilaterally symmetrical animals in the way in which their embryonic cells divide during development. A fertilized acoel egg divides once; then the two resulting cells subdivide further into many smaller cells. This pattern contrasts with the eggs of all other bilateral animals, each of which divides first into two and then into four cells that go on to divide into many smaller cells. According to Henry et al., the acoel pattern of embryonic development supports the hypothesis that the acoels branched off from the ancestral line of bilateral animals very early, and may thus represent an earlier evolutionary experiment in body structure.

Some adult acoela reproduce asexually by fission in one of three possible ways: architomy, in which smaller pieces separate from the maternal animal prior to organ differentiation; paratomy, or transverse fission, in which organ differentiation takes place within the fragments before separation; and budding, or local tissue reorganization in which a small bud or outgrowth detaches itself from the parent organism and lives independently.

Conservation status

Acoels are quite obscure yet so widespread and numerous that no species are considered threatened.

Significance to humans

Acoels have no known direct significance to humans as of 2003. Their main value to humanity is scientific, in that studies of them may shed light on the earliest evolution of bilateral triploblastic animals.

Species accounts

Resources

Balzer, I. "Symbiotic Association Between the Acoel Convolutriloba longifissura and the Alga Tetraselmis sp." In Endocytobiology VII, edited by E. Wagner, J. Normann, H. Greppin, J. H. P. Hackstein, R. G. Hermann, K. V. Kowallik, H. E. A. Schenk, and J. Seckbach. Geneva: Geneva University Press, 1999.

Caira, Janine N., and D. Timothy J. Littlewood. "Worms, Platyhelminthes." In Encyclopedia of Biodiversity, vol. 5, edited by S. Levin. San Diego: Academic Press, 2001.

Littlewood, D. T. J., and R. A. Bray, eds. Interrelationships of the Platyhelminthes. London: Taylor and Francis, 2001.

Akesson, B., R. Gschwentner, J. Hendelberg, P. Ladurner, J. Müller, and R. Rieger. "Fission in Convolutriloba longifissura: Asexual Reproduction in Acoelous Turbellarians Revisited." Acta Zoologica 82, no. 3 (2001): 231–239.

Balzer, I. "Symbiotic Association Between the Plathelminth Convolutrilova longifissura and the Alga Tetraselmis sp." Nova Hedwigia 112 (1996): 461–475.

Bartolomäus, Thomas, and Ivonne Balzer. "Convolutriloba longifissura nov. spec. (Acoela)— The First Case of Longitudinal Fission in Plathelminthes." Microfauna Marina 11 (1997): 7–118.

Gschwentner, Robert, Sanja Baric, and Reinhard Rieger. "New Model for the Formation and Function of Sagittocysts: Symsagittifera corsicae n. sp. (Acoela)." Invertebrate Biology 121, no. 2 (1999): 95–103.

Gschwentner, Robert, Peter Ladurner, Willi Salvenmoser, Reinhard Rieger, and Seth Tyler. "Fine Structure and Evolutionary Significance of Sagittocysts of Convolutriloba longifissura (Acoela, Platyhelminthes)." Invertebrate Biology 118, no. 4 (1999): 332–345.

Henry, J. Q., M. Q. Martindale, and B. C. Boyer. "The Unique Developmental Program of the Acoel Flatworm, Neochildia fusca." Developmental Biology 220 (2000): 285–295.

Janssen, Hans Heinrich, and Rolf Gradinger. "Turbellaria (Archoophora: Acoela) from Antarctic Sea Ice Endofauna— Examination of their Micromorphology." Polar Biology 21, no. 6 (1999): 410–416.

Raikova, Olga, M. Reuter, Elena Kotikova, and Margaretha K. S. Gustafsson. "A Commissural Brain! The Pattern of 5-HT Immunoreactivity in Acoela (Plathelminthes)." Zoomorphology 118, no. 2 (1998): 69–77.

Ruiz-Trillo, I., M. Riutort, T. J. Littlewood, E. A. Herniou, and J. Baguna. "Acoel Flatworms: Earliest Extant Bilaterian Metazoans, Not Members of Platyhelminthes." Science 283 (1999): 1919–1923.

"Acoel Flatworms Misrepresented?" University of Maine, Department of Biological Sciences. 1999 (July 15, 2003). "Order Acoela." Alien Travel Guide (July 15, 2003). . "Project: Morphology and Phylogeny of the Acoelomorpha (Platyhelminthes)." Göteborgs Universitets Marina Forskningscentrum. .

[Article by: Kevin F. Fitzgerald, BS]

An order of marine Turbellaria, generally regarded as primitive, and lacking protonephridia, oviducts, yolk glands, permanent digestive cavity, and strictly delimited gonads. The nervous system is epidermal in the most primitive representatives, but it is generally submuscular and consists of three to six pairs of longitudinal strands without anterior concentrations recognizable as a brain. The anterior or median ventral mouth opens into a simple pharynx which lacks elaborate musculature and leads directly to the syncytial vacuolated mass of endoderm forming the spongy intestinal tissue and lacking any true lumen.

Eyes are often absent; when present they are of the usual paired turbellarian pigment-cup type, with one to a few pigment and retinal cells. The epidermis, typically, is uniformly ciliated, and locomotion is accomplished by means of rapid, smooth ciliary gliding or swimming.

The acoels (see illustration) are mostly small (one to several millimeters in length). They are virtually worldwide in distribution, living beneath stones, among algae, on the bottom mud, or interstitially, from the littoral zone to deeper waters. A few are pelagic (in tropical and subtropical zones), and others have become symbiotic, living entocommensally in various invertebrates but mainly in echinoderms. See also Coelenterata; Platyhelminthes; Turbellaria.

Acoela: Childia spinosa.

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