Acorn worm is the common name for any of the worm-shaped marine invertebrates comprising the hemichordate class Enteropneusta, characterized by three body parts (an acorn-shaped proboscis, a short fleshy collar behind it, and a long trunk), a covering of cilia, and a solitary, benthic (bottom-dwelling) lifestyle. While some are even as small as five centimeters (two inches) in length, Balanoglossus gigas of Brazil reaches 1.5 meters (4.5 feet) in length (Mertz 2004). Acorn worms are widespread in the ocean floors with their range extending from shallow intertidal regions out to the oceanic trenches to depths of 10,000 feet (3,050 meters).
Although acorn worms as a group tend to be poorly known, they are considered important because both their distinctive physiology and their phylogenetic location intermediate between the invertebrates and vertebrates make them a valuable source of scientific information about the origin of chordates and the bilateral body plan, (Mertz 2004). These fascinating creatures also add to the wonder of nature for humans. Ecologically, they are important in marine food chains.
Acorn worms comprise one of the three classes within the phylum Hemichordata, a group of bilaterally symmetrical marine invertebrates. Hemichordata are deuterostomes; that is, they have true coeloms (body cavities), which form from the embryonic mesoderm as evaginations of the developed gut that pinch off; also the first opening of the coelom becomes the anus rather than the mouth as in protostomes.
The acorn worms comprise the class Enteropneusta whose three-part body plan consists 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.
The 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.
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 three meters (9.8 feet). Most acorn worms are much, much smaller, with some Saccoglossus species only reaching a length of five centimeters (two 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.
All species of acorn worms are part of the marine infaunal benthos (animals dwelling below the surface of the sea bottom), 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 as part of the (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).
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 develop directly into small juveniles. Indirect developers, which have tornaria larvae (which resemble the larvae of starfish), 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 undergo asexual reproduction by fragmentation of the adult's body, although this 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).
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 one genus is recognized in Protoglossidae, three genera (including Balanoglossus) in Ptychoderidae, and four genera in Spengelidae (ITIS 2008).
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