Acorn worm
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Harrimaniidae |
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 are infaunal benthos that either may be deposit feeders or suspension feeders.
Acorn worms are rarely seen by humans because of their lifestyle. They live in U-shaped burrows on the sea-bed, from the shoreline down to a depth of 10,000 ft. (3,050 m). The worms lie there with the proboscis sticking out of one opening in the burrow. Acorn worms are generally slow burrowers.
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 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.
Reproduction
Acorn worms have separate genders that release 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 don't have a larval stage, but develops directly into small juveniles.
ReferencesISBN links support NWE through referral fees
- 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.
- 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.
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