Phylum Echiura is comprised of over 230 species of unsegmented, sausage-shaped, coelomate worms, traditionally placed in three orders: Echiuroinea, by far the largest order, with two families (Bonelliidae and Echiuridae) and many species; Xenopnuesta, with one family (Urechidae) and 4 species; and Heteromyota, which is made up of one monotypic family (Ikedaidae), containing two species. Echiurans range widely in body length, color, and skin texture. They are characterized by a highly mobile, extendable proboscis (although it cannot retract into the body cavity), whose use in feeding gives them the common name "spoon worms". These worms inhabit marine or brackish water environments and can usually be found inside the burrows they create in the substrate. Many species deposit feed on detritus, but filter feeding varieties are also known. Echiurans of genus Bonellia are toxic and exhibit extreme sexual dimorphism as well as an unusual sex determination mechanism. ("Echiura", 2013; Brusca and Brusca, 2003; Goto, et al., 2013; Lehrke, 2011; Nishikawa, 2002; Piper, 2007)
Echiurans are benthic, marine invertebrates, typically found burrowed into sand or living on surface substrates, from intertidal areas to depths of 10,000 m (most commonly in shallow waters). In the Atlantic Ocean, the greatest echiuran species diversity occurs in the northern temperate region. (Biseswar, 2009; Brusca and Brusca, 2003; Shapiro, 2012)
The name Echiura was first applied to these worms in 1940 by William Wallace Newby. Inter-relationships of echiuran groups remain a contentious area of study. The traditional classification divides this phylum into three orders based on the presence or absence of a closed respiratory system, the presence or absence of a thin-walled, enlarged cloaca, and arrangements of the body wall musculature; these orders include Xenopneusta (containing family Urechidae), Heteromyota (containing the monotypic family Ikedaidae), and Echiuroinea (containing families Bonelliidae and Echiuridae). This classification appears to be based on incorrect information, however, and a later revision eliminated order Heteromyota, placing genus Ikeda in Echiuroinea, while also maintaining order Xenopneusta. Recent analyses have recognized two major clades within Echiura; a so-called Bonellia-group, containing Bonellidae and Ikedaidae, and a Urechis-group, containing Urechidae and Echiuridae. Support for this classification appears strong, and comes from both molecular data and the presence of multiple putative morphological and life history synapomorphies for both clades. (Ax, 1999; Bock, 1942; Goto, et al., 2013; Lehrke, 2011; Nishikawa, 2002; Purschke, 2007; Stephen and Edmonds, 1972)
Although they are distinctive and widely accepted as a monophyletic group, many studies (both morphological and molecular phylogenetic analyses) have indicated that echiurans actually represent derived segmented worms (Phylum Annelida), rather than their own phylum, a hypothesis that has since been almost universally accepted by biologists. (Bleidorn, et al., 2003a; Bleidorn, et al., 2003b; Bourlat, et al., 2008; Dunn, et al., 2008; Goto, et al., 2013; Hessling and Westheide, 2002; McHugh, 1997; Struck, et al., 2007; Zrzavy, et al., 2009)
Echiurans are unsegmented worms with two body sections; the trunk and an anterior, preoral, extendable proboscis, which is often much longer than the trunk. They range widely in body length, from under 10 mm to over 2 m, and their skin surfaces may be smooth or warty, sometimes with chitinous bristles. They range widely in color as well, from dull beige or gray to bright green, as seen in members of genus Bonellia due to the production of the green-colored toxin known as bonellin. (Brusca and Brusca, 2003; Shapiro, 2012; Waggoner, 1995)
Echiuran body walls are formed by layers of muscle (circular, longitudinal and oblique), supported by a hydrostatic skeleton formed by the coelom. The body wall is covered with a fibrous dermis, epidermis (made of gland cells and cubular cells) and a thin cuticle. A peritoneum lines the body wall internally. The epidermis along the proboscis groove (also known as the “gutter”) is ciliated. This gutter is located at the anterior end of the trunk and leads to the mouth. The coelomic cavity is large and unsegmented, although there are partial mesenteric separations between the gut and body wall. This construction allows for peristaltic movements of the body wall during burrowing or other locomotion. They also have a set of small posterior hooks, which help to anchor the worms in their burrows. (Brusca and Brusca, 2003; Shapiro, 2012; Waggoner, 1995)
Most echiurans are not sexually dimorphic; members of genus Bonellia, however, are extremely dimorphic, with females reaching lengths of up to 2 meters, while males are only a few millimeters long, have greatly simplified body systems, and live inside the bodies of females. (Brusca and Brusca, 2003; Piper, 2007; Shapiro, 2012)
Ripe gametes are released through the nephridia. Epidemic spawning is typical and fertilization occurs externally. Cell cleavage is holoblastic and spiral. Free swimming trocophore larvae develop and, over the course of up to three months, elongate into young worms. Sex determination in bonelliid species is mainly influenced by environmental factors (metagamic), although genetic factors determine the sex of larvae not exposed to appropriate cues. If a larva settles near the proboscis of a female, it will develop rapidly into a male, while if it settles away from a female, it will develop into a female. If two larvae clump together, one will likely be female and the other male. Larvae settling on a female's trunk rather than proboscis may appear to remain undifferentiated, apparently representing an intermediate sex. (Brusca and Brusca, 2003; Jaccarini, et al., 1983; Shapiro, 2012)
Gametes are produced in specialized regions of the peritoneum, often located at the base of the ventral blood vessel. Ripe gametes are released into the coelom and accumulate in the nephridia. Echiurans typically breed via epidemic spawning, with fertilization taking place externally. In many bonelliid species, males live parasitically within the female gonoducts, fertilizing eggs as they are released. There is currently no data available regarding triggers for echiuran spawning. (Brusca and Brusca, 2003; Margulis and Chapman, 2009)
Spawning season for echiurans varies depending on location; some populations breed during the winter, others during the summer. These worms are dioecious, and are not known to reproduce asexually. (Brusca and Brusca, 2003; Fish and Fish, 1996; Margulis and Chapman, 2009)
Although data regarding lifespan of echiurans is not widely available, it has been reported that members of family Urechidae may live up to 25 years. (Anker, et al., 2005)
Euchiurans are most often found with their trunks buried, anterior end upward; members of genus Urechis create U-shaped burrows. Burrowing and other locomotion is accomplished through peristaltic movements of the body wall. These worms are typically solitary and sessile; male bonelliids are considered to be parasitic on females. (Brusca and Brusca, 2003)
Echiurans have simple nervous systems, comprised of an anterior nerve ring extending around the gut (dorsally to the proboscis and ventrally to a nerve cord extending along the body). They lack most sensory systems, having only weak tactile sensitivity and possibly some chemoreceptive capabilities. Modes of communication between echiurans are unknown, but given their limited sensory capabilities, chemical cues appear to be the most likely means by which these animals communicate. (Brusca and Brusca, 2003)
Most echiurans deposit feed by extending the proboscis, which can expand and contract, over the substrate near their burrows. Gland cells in the epithelium of the proboscis secrete adhesive mucus, and food particles (mainly detritus) are moved through the mucus to the mouth by cilia that line the proboscis groove (also called the gutter). Members of genus Urechis are considered filter feeders. These species produce a funnel-shaped mucus net from glands at the base of the proboscis, drawing water through the mucus net and into the burrow, causing particles of detritus to be caught in the net. The mucus net is periodically drawn in by the proboscis and ingested. (Brusca and Brusca, 2003; Goto, et al., 2013; Piper, 2007)
Echiurans may withdraw into their burrows to hide from potential predators; however, animals including bony fishes, sharks, rays, and sea otters are known to prey on them; humans may also use these worms as bait. Bonelliid species produce a toxin, bonellin, which lends them a bright green color and is likely an antipredatory adaptation. (Anker, et al., 2005; Brusca and Brusca, 2003; Cowles, 2010)
Echiurans, especially members of genus Urechis (commonly known as "innkeeper worms") often have commensal species sharing their burrows; these commonly include shrimp, small crabs, polychaetes, bivalves, and copepods. Less commonly commensals include nematodes, nemerteans, flatworms, entoprocts, gastropods, and fishes. There is evidence that some echiuran species, such as Bonellia viridis, are commensal with rock-burrowing mudshrimp. Common parasites of echiurans include gregarinid sporozoans, ciliates, trematodes, nematodes, polychaetes, and cestodes. Male bonelliids are considered to be parasitic on females. (Anker, et al., 2005)
There are no known adverse effects of echiurans on humans.
As a broadly cosmopolitan phylum, echiurans in general are not currently in danger of becoming endangered, nor is any individual species within the phylum. (IUCN, 2013)
Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Animal Diversity Web Staff.
the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.
Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.
living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
living in the southern part of the New World. In other words, Central and South America.
body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.
living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.
on or near the ocean floor in the deep ocean. Abyssal regions are characterized by complete lack of light, extremely high water pressure, low nutrient availability, and continuous cold (3 degrees C).
Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
areas with salty water, usually in coastal marshes and estuaries.
uses smells or other chemicals to communicate
the nearshore aquatic habitats near a coast, or shoreline.
having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.
active at dawn and dusk
an animal that mainly eats decomposed plants and/or animals
fertilization takes place outside the female's body
union of egg and spermatozoan
a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.
Referring to a burrowing life-style or behavior, specialized for digging or burrowing.
having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.
a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.
Found in northern North America and northern Europe or Asia.
fertilization takes place within the female's body
the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.
the area in which the animal is naturally found, the region in which it is endemic.
active during the night
islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.
found in the oriental region of the world. In other words, India and southeast Asia.
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death
an animal which has a substance capable of killing, injuring, or impairing other animals through its chemical action (for example, the skin of poison dart frogs).
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
mainly lives in oceans, seas, or other bodies of salt water.
breeding is confined to a particular season
remains in the same area
non-motile; permanently attached at the base.
Attached to substratum and moving little or not at all. Synapomorphy of the Anthozoa
reproduction that includes combining the genetic contribution of two individuals, a male and a female
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
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