Acorn worm

Enteropneusta
Scientific classification
Kingdom: Animalia Phylum: Hemichordata Class: Enteropneusta
Families
Harrimaniidae Protoglossidae Ptychoderidae Spengelidae

Acorn worm is the common name for any of the worm-shaped marine invertebrates comprising the hemichordate class Enteropneusta, characterized by three body parts (protosome or proboscis, mesosome or collar, and metasome or trunk), a covering of cilia, and a solitary, benthic lifestyle. As a member of the phylum Hemichordata, they are closely related to the chordates. While most are small, Balanoglossus gigas of Brazil reaches 1.5 meters (4.5 feet) in length (Mertz 2004).

Overview and description

Acorn worms comprise one of three classes within the phylum Hemichordata, a group of bilaterally symmetrical marine invertebrates. Hemichordata are deuterostomes; that is, they have true coeloms (body cavities), with the coelom forming through enterocoely (the mesoderm forms as evaginations of the developed gut that pinch off, forming the coelom) and the first opening becomes the anus rather than the mouth as in protostomes.

The acorn worms comprise the class Enteropneusta. This class is characterized by a three-part body plan, consisting of an anterior proboscis or protosome, followed by a short, fleshy collar or mesosome, and ending with a long, wormlike trunk, or metasome (Mertz 2004; Cameron et al. 2000). One theory is that this three-part body originates from an early common ancestor of all the deuterostomes, and maybe even from a common bilateral ancestor of both the deuterostomes and protostomes. The acorn shape of the front end, with the proboscis and collar, is the source of their common name.

The acorn worm's body is cylindrical, with cilia present over all body areas (Mertz 2004). Acorn worms move by cilia movements and body contractions. The skin, in addition to being covered with cilia, also is covered with glands that secrete mucus, and the cilia also helps in distributing this proteinaceous mucus (Mertz 2004). Some acorn worms produce a bromide compound that gives them a medicinal smell and might protect them from bacteria and predators.

TThe creature's mouth is located at the collar behind the proboscis.

Acorn worms, or enteropneusts, are considered more highly specialized and advanced than other similarly shaped worm-like creatures. They have a circulatory system with a heart that also functions as a kidney. Acorn worms have gill-like structures that they use for breathing, similar to the gills of primitive fish. They breathe by drawing in oxygenated water through their mouth, and the water then flows out the animal's gills, which are on its trunk. Thus, the acorn worm breathes about the same way as fish. While acorn worms share with other hemichordates the lack of a dorsal postanal tail and lack of segmentation of the muscular and nervous systems, adult enteropneusts share such chordate characteristics as pharyngeal gill pores, a partially neurulated dorsal cord, and a stomochord, which is similar to a chordate notochord (Cameron et al. 2000). Hence, acorn worms are sometimes said to be a link between classical invertebrates and vertebrates.

Some species of acorn worms do exhibit a postanal tail, which sometimes shows weak signs of segmentation. An interesting trait is that its three-section body plan is no longer present in the vertebrates, except for the anatomy of the frontal neural tube, later developed into a brain, which is divided into three main parts. This means some of the original anatomy of the early chordate ancestors is still present even if it is not always visible.

There are about 70 species of acorn worm in the world. The largest species is Balanoglossus gigas, found in Brazil. It reaches 1.5 meters in length (4.9 feet) and inhabits a burrow that is longer than 3 meters (9.8 feet). Most acorn worms are much, much smaller, with some Saccoglossus species only reaching a length of 5 centimeters (2 inches). The main species for research is Saccoglossus kowalevskii', whose members range from ten to 15 centimeters (4.0 to 5.9 inches) in length (Grzimek et al. 2004). One genus, Balanoglossus, is also known as the tongue worm.

Habitat, behavior, and feeding

All species of acorn worms are marine infaunal benthos, typically found in intertidal or shallow marine areas, but occasionally in deeper water, including deep abyssal plains (Mertz 2004; Twitchett 1996). In shallower areas they commonly inhabit U-shaped burrows, whereas in the abyssal plans they have been photographed freely moving on the surface of the substrate (Twitchett 1996). The U-shaped burrows have the two ends of the burrow open on the ocean floor and the rest of the U underground (Mertz 2004). Acorn worms have been found in oceans throughout the world, from the shoreline down to a depth of 10,000 feet (3,050 meters).

While acorn worms generally live in burrows on the sea-bed, they also can be found in the sand inside shells, or in thick seaweed, or under rocks, or between roots (Mertz 204). Burrowing species like Balanoglossus clavigerus use their proboscis to burrow into sand or mud and may line their U-shaped burrows with epidermal secretions that add strength to the burrow walls (Mertz 2004). They may lie in their environment with the proboscis sticking out of one opening in the burrow, but may also spend much of their time underground. They are rarely seen because of this lifestyle. Acorn worms are generally slow burrowers.

When threatened, members of the Saccoglossus genus may expand their proboscis, anchoring the animal in the burrow or vegetation, while drawing in the rest of its body (Grzimek et al. 2004).

Acorn worms tend to be solitary animals and they are either sediment feeders or suspension feeders. To obtain food, many acorn worms swallow sand or mud that contains organic matter and microorganisms in the manner of earthworms (this is known as sediment feeding or deposit feeding). At low tide, they stick out their rear ends at the surface and excrete coils of processed sediments (casts). Another method that some acorn worms use to obtain food is to collect suspended particles of organic matter and microbes from the water. This is known as suspension feeding.

Saccoglossus kowalevskii is known to eat bacteria, diatoms, and microalgae that live in the sediment, as well as dissolved and particulate organic matter from the water (Grzimek et al. 2004). Individuals of this species eat as much as 300 times their body weight in sediment each day (Grzimek et al. 2004).

Reproduction

Acorn worms have separate genders and reproduce sexually, releasing eggs and sperm into the water for external fertilization. In some, eggs develop into free-swimming larvae that look very similar to echinoderm larvae. (This suggests vertebrates and echinoderms are closely linked phylogenically.) The larvae eventually settle down and change into tiny acorn worms on the surface and take on the burrowing lifestyle. Others lack a larval stage, but develops directly into small juveniles. Indirect developers, which have tornaria larvae, such as species of Balanoglossus and Ptychodera, are in the majority (Mertz 2004). Saccoglossus kowalevskii is the example of a direct developer (Mertz 204). Acorn worms also have been known to under asexual reproduction by fragmentation of the adult's body, although is form of reproduction is uncommon (Mertz 2004).

The reproductive activity of Saccoglossus kowalevskii, a direct-developing species, appears to be influenced by seawater temperature, with a temperature shift from 27°C to 22°C (80.6°F to 71.6°F) stimulating spawning (Grzimek et al. 2004). The eggs average about 0.4 millimeters (0.02 inches) in diameter. They are released by the females into the water and the males release sperm, with fertilization in the sea water. The eggs hatch after seven days into worm-like young that immediately began a sessile lifestyle (Grzimek et al. 2004). They lack a planktonic larval state (Grzimek et al. 2004).

Classification and origin

Generally four families of acorn worms are recognized: Harrimaniidae, Protoglossidae, Ptychoderidae, and Spengelidae (ITIS 2008; Myers et al. 2008). Within Harrimaniidae, four extant genera are recognized (including Saccoglossus), while 1 genus is recognized in Protoglossidae, three genera (including Balanoglossus) in Ptychoderidae, and four genera in Spengelidae (ITIS 2008).

The fossil record of acorn worms is extremely poor, although fossils have been identified from the Lower Triassic some 250 million years ago (Twitchett 1996).

References

ISBN links support NWE through referral fees

• Grzimek, S. F. Craig, D. A. Thoney, N. Schlager, and M. Hutchins. 2004. Grzimek's Animal Life Encyclopedia, 2nd edition. Detroit, MI: Thomson/Gale. ISBN 0787657786.

• Integrated Taxonomic Information System (ITIS). 2005a. Enteropneusta ITIS Taxonomic Serial No.: 158617. Retrieved May 24, 2008.

• Mertz, L. A. 2004. Hemichordata. In B. Grzimek, S. F. Craig, D. A. Thoney, N. Schlager, and M. Hutchins. Grzimek's Animal Life Encyclopedia, 2nd edition. Detroit, MI: Thomson/Gale. ISBN 0787657786.

• Myers, P., R. Espinosa, C. S. Parr, T. Jones, G. S. Hammond, and T. A. Dewey. 2008. Class Enteropneusta (acorn worms). The Animal Diversity Web (online). Retrieved May 28, 2008.

• Twitchett, R. J. 1996. The resting trace of an acorn-worm (Class: Enteropneusta) from the Lower Triassic. Journal of Paleontology 70(1): 128-131.

Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

• Acorn_worm  ^history

The history of this article since it was imported to New World Encyclopedia:

• History of "Acorn worm"

Note: Some restrictions may apply to use of individual images which are separately licensed.