Echiura

Diversity

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.

Geographic Range

Echiurans are found in marine and brackish environments throughout the world.

• Biogeographic Regions
• nearctic nearctic
  • native native
• palearctic palearctic
  • native native
• oriental oriental
• ethiopian ethiopian
  • native native
• neotropical neotropical
  • native native
• australian australian
  • native native
• oceanic islands oceanic islands
  • native native
• indian ocean
  • native native
• atlantic ocean atlantic ocean
  • native native
• pacific ocean pacific ocean
  • native native
• mediterranean sea
  • native native

• Other Geographic Terms
• holarctic holarctic
• cosmopolitan cosmopolitan

Habitat

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.

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

• Aquatic Biomes
• benthic benthic
• coastal coastal
• abyssal abyssal
• brackish water brackish water

• Other Habitat Features
• intertidal or littoral intertidal or littoral

Systematic and Taxonomic History

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.

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.

Physical Description

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.

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.

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.

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

• Sexual Dimorphism
• sexes alike
• female larger

Development

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.

• Development - Life Cycle
• metamorphosis metamorphosis

Reproduction

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.

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

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.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • external external
  • internal internal
• oviparous oviparous

As broadcast and epidemic spawners, echiurans are not known to exhibit any parental investment beyond the production of gametes.

• Parental Investment
• no parental involvement

Lifespan/Longevity

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.

Behavior

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.

• Key Behaviors
• fossorial fossorial
• diurnal diurnal
• nocturnal nocturnal
• crepuscular crepuscular
• parasite parasite
• sessile sessile
• sedentary sedentary
• solitary solitary

Communication and Perception

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.

• Perception Channels
• tactile tactile
• chemical chemical

Food Habits

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.

• Primary Diet
• detritivore detritivore

• Foraging Behavior
• filter-feeding filter-feeding

Predation

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.

Ecosystem Roles

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.

Economic Importance for Humans: Positive

Beyond scientific research, there are no known positive effects of echiurans on humans

• Positive Impacts
• research and education

Economic Importance for Humans: Negative

There are no known adverse effects of echiurans on humans.

Conservation Status

As a broadly cosmopolitan phylum, echiurans in general are not currently in danger of becoming endangered, nor is any individual species within the phylum.

Additional Links

Encyclopedia of Life

Contributors

Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Animal Diversity Web Staff.

References

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