Hemichorodata

(Hemichordates)

Phylum Hemichordata

Number of families 8

Thumbnail description
Small wormlike marine animals that live individually or in colonies, depending on the species

Evolution and systematics

The extant members of the phylum Hemichordata (formerly called Stomochordata) number about 92 and are typically divided into three classes:

• Enteropneusta or acorn worms: four families;
• Pterobranchia or pterobranchs: three families; and
• Planctosphaeroidea: one family.

Hemichordates are closely related to the chordates, sharing many chordate or chordate-like features. In the enteropneusts, these features include lateral openings, which are pharyngeal gill pores that connect the pharynx or airway to the exterior; and a stomochord that is somewhat analogous to the chordate notochord, although it has been described as "no more than an anterior extension of the buccal cavity." Despite these similarities, genetic studies, morphological comparisons of the larvae and anatomy of the nervous system, and biochemical evidence have recently indicated that hemichordates may be more closely related to echinoderms (starfishes and sea urchins) than to chordates.

In addition, recent genetic analyses of the extant hemichordates suggest that the pterobranchs are not plesiomorphic (similar in form) as previously thought, but have actually evolved from a forerunner to the enteropneusts. Under this arrangement, the lineage of the Ptychoderidae family within the enteropneusts split off first in the course of evolution, followed by a later separation of the Harrimaniidae family of the enteropneusts and the pterobranchs.

In addition to these three extant classes of Hemichordata, there is an extinct class, Graptolithina, known from fossils found in rocks dating from the Ordovician and Silurian periods (505–410 million years ago).

Physical characteristics

Hemichordates have two major body plans. One of the most notable characteristics of the enteropneustan hemichordates is their three-part body plan, which includes a protosome, or anterior proboscis (sometimes called a pre-oral lobe); followed by a mesosome or collar; and finally a meta-some, or trunk. Cilia, which are present over all body areas, play roles in locomotion and in distributing the proteinaceous mucus secreted by the acorn worms. The largest species is the enteropneust Balanoglossus gigas of Brazil, an acorn worm that reaches 4.9 ft (1.5 m) in length and lives in long burrows stretching over more than 9.8 ft (3 m).

Pterobranchs have a three-part body plan like the enteropneusts, but with a shorter, shield-shaped proboscis and a more complex collar. In some species, the collar has tentacled arms. Pterobranchs form colonies, often with individuals attached by so-called stolons or stems. The individual animals are called zooids and are quite small, typically less than 0.04 in (1 mm) long. Groups form and live within a coenecium, which is a network of proteinaceous tubes built with secretions from each animal's proboscis.

The class Planctosphaeroidea has only one species, Planctosphaera pelagica, and it is known only from its larvae. Although several times larger at 0.3–1 in (8–25 mm) long, the almost-spherical, transparent P. pelagica larva is otherwise quite similar to enteropneust tornaria, having a gelatinous body covered with cilia. Unlike tornaria, however, the epidermis of P. pelagica has two deep invaginations (pouchlike formations) as well as numerous glands that secrete mucus.

Distribution

Enteropneusts, pterobranchs, and planctosphaeroids occur in oceans throughout the world. In general, the acorn worms live in shallower areas and the pterobranchs in deeper waters. The single species known from the class Planctosphaeroidea is found in both the Atlantic and Pacific Oceans at depths between 246 ft (75 m) and about 3,280 ft (1,000 m).

Habitat

Habitat varies by class. Adult acorn worms are typically found in either intertidal or shallow marine areas, although they are occasionally found in deeper water. They generally inhabit burrows in the sea bottom but also live sometimes in the sand inside shells, under rocks, in thick seaweed, or between root tangles. Adult pterobranchs are colonial forms that live in secreted tubular coenecia, and the planctosphaerids are planktonic.

Behavior

The acorn worms are solitary animals that are generally found sheltered in burrows, under rocks, or in thick vegetation. The burrowing species, like Balanoglossus clavigerus, use their proboscis primarily to fashion U-shaped burrows. They line the burrow walls with epidermal secretions that provide added strength. Each end of the burrow lies at the surface of the sea bottom and the remainder of the "U" is underground. One end is a cone-shaped depression in the sand bottom, and the other can be identified by a several-inch-tall pile of worm castings a short distance away. Besides this main burrow, Balanoglossus also employs a few side tunnels. Frequently, acorn worms will stretch their proboscis and collar out of the tunnel, but they spend the bulk of their time underground. When threatened, acorn worms respond by expanding the proboscis, effectively anchoring the animal in its burrow or tangle of vegetation while withdrawing the rest of the body. Studies of phototaxis (movement toward or away from a light source) reveal that illumination stimulates some species, like Saccoglossus ruber, to burrow deeper.

Because of the burrowing nature of most hemichordates, little is known about the reproductive and other behaviors of many species.

Feeding ecology and diet

Hemichordates may be either suspension- or sediment-feeders. The latter, like Balanoglossus clavigerus, take in sediment and obtain nutrients from the organic matter contained

in it. The suspension-feeding adult hemichordates, as well as the filter-feeding tornaria larvae, gather their meals by generating currents with the cilia located on their bodies and drawing in organic matter. There is some uncertainty about the role their mucus plays in prey capture. Some scientists believe food sticks to the mucus-covered proboscis, and the cilia then beat in a pattern that draws the mucus and the food together to the mouth at the bottom of the proboscis. Researchers studying such species as Rhabdopleura normani, on the other hand, have found that normal feeding does not involve mucus; instead, the organism relies on the cilia to change direction in movements called local reversals and thereby directs food particles to its mouth.

Indirect-developing species have free-swimming tornaria larvae that live on plankton for weeks to months. Some species, like Saccoglossus horsti, have free-swimming larvae that obtain all their nutrition from their yolk, and within a few days take up the sessile (permanently attached) lifestyle. Studies of P. pelagica larvae indicate that their mucus may facilitate feeding, although the details are unclear and several alternative hypotheses have been suggested for the mucus.

Reproductive biology

Enteropneusts normally reproduce sexually via external fertilization, and develop either directly or via tornaria larvae. The indirect developers, including Balanoglossus and Ptychodera species, are in the majority. These species develop from egg to planktonic tornaria larva to adult form. The tornaria larvae eventually become sessile, with the burrow-dwellers developing tails behind the anus that they use to anchor themselves in their mucus-lined tunnels. Direct developers, on the other hand, hatch into adult animals, bypassing the planktonic phase. An example is Saccoglossus kowalevskii. Enteropneusts are also known to reproduce asexually by fragmentation of the adult's body, but this mode of reproduction is uncommon. Typically, the females lay up to 3,000 eggs at a time, and the males release sperm that appear to find the eggs by following chemical cues. Reproduction in many species is cyclical. Saccoglossus horsti, for example, breeds in late spring to midsummer. Water temperature and tides appear to affect reproductive timing in hemichordates.

The pterobranchs reproduce via short-lived larvae in a fashion similar to the enteropneusts, but more often resort to reproduction by asexual budding. Many, perhaps all, of the hemichordates are able to regenerate portions of their trunks.

Conservation status

No hemichordates are listed as threatened by the IUCN.

Significance to humans

The hemichordates are perhaps most important to humans for the information they can provide about the origin of chordates, deuterostomes, and bilateral animals.

Species accounts

List of Species

No common name

Cephalodiscus gracilis

order

Cephalodiscida

family

Cephalodiscidae

taxonomy

Cephalodiscus gracilis M'Intosh, 1882, Straits of Magellan.

other common names

None known.

physical characteristics

Cephalodiscus gracilis is a three-part animal with a cephalic shield, complex collar, and trunk. The collar has two rows of up to five arms each. The arms have ciliated tentacles that reach 0.2–0.3 in (5–7 mm) in length.

distribution

Originally found at the southern tip of South America, they have since been found in Atlantic and Indo-Pacific waters.

habitat

These marine hemichordates live in hollow coenecia, but are able to travel freely inside each coenicium and on its tubes. They are typically found in waters less than 65.6 ft (20 m) deep.

behavior

Little is known about the behavior of this species.

feeding ecology and diet

These organisms are filter feeders, using their cilia to create currents to direct the food. Their tentacles also flick food into the food canal. Their diet consists of organic food particles.

reproductive biology

Embryos are brooded in the coenecium tubes. Cephalodiscus has a free-swimming larval stage followed by a enteropneust-like adult form. Adults eventually take on the characteristic U-shape.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Saccoglossus kowalevskii

order

No order designation

family

Harrimaniidae

taxonomy

Saccoglossus kowalevskii Agassiz 1873.

other common names

None known.

physical characteristics

Adults are wormlike animals with a pointed proboscis, followed by a short collar and a long trunk. They range 4.0–5.9 in (10–15 cm) long. The proboscis is generally whitish yellow or light pink, the collar is orange, and the trunk can be pink to orange-brown.

distribution

North Atlantic, off the coast of Europe and the United States from Massachusetts to South Carolina.

habitat

Intertidal zones, typically sandy or silty areas.

behavior

When threatened, an individual of this and other species of the genus Saccoglossus will swell its proboscis to serve as a holdfast in its burrow while drawing in the rest of its body. A mostly subterranean animal, Saccoglossus is most often seen with its proboscis poking out from its sea-bottom burrow. Its reproductive activity appears to be affected by seawater temperature, with spawning stimulated when temperatures shift from 80.6 to 71.6°F (27 to 22°C).

feeding ecology and diet

Saccoglossus kowalevskii gathers food from the uppermost layer of sediment in its immediate surroundings. Food items may include bacteria, microalgae and diatoms that live in the sediment, and dissolved or particulate organic material in the water. Each day, individuals of this species may take in as much as 300 times their body weight in sediment. Research indicates that this acorn worm is induced to feed by the presence of microalgae and diatom populations as well as other food sources that are high in chlorophyll.

reproductive biology

Fertilization is external. Females release mucus-bound eggs into the water, males respond by releasing sperm, and fertilization occurs in the sea water. The eggs, which average about 0.02 in (0.4 mm) in diameter, hatch in seven days into worm-like young that begin a sessile lifestyle immediately. Saccoglossus kowalevskii is a direct-developing species that hatches from eggs into the adult form without the planktonic larval stage common to many other hemichordates.

conservation status

Not listed by IUCN.

significance to humans

None known.

Hawaiian acorn worm

Ptychodera flava

order

No order designation

family

Ptychoderidae

taxonomy

Ptychodera flava Eschscholtz, 1825.

other common names

None known.

physical characteristics

The adult is a yellowish brown animal with a small coneshaped proboscis, short collar, and long trunk.

distribution

Pacific Ocean, particularly near Hawaii and Japan.

habitat

Coastal marine waters.

behavior

Spawning times are related to sea water temperatures. In Hawaii, these acorn worms spawn in late November to early December, usually around 6 p.m. or dusk.

feeding ecology and diet

The diet of Ptychodera flava consists of organic food particles.

reproductive biology

Fertilization, which is external, occurs when females release mucus-bound eggs into the water, and the males respond by releasing sperm. Fertilization occurs in the sea water. The eggs, which measure about 0.00433–0.00472 in (110–20 µm) in diameter, hatch into tornaria larvae in about two days. Tornaria larvae have a keyhole shape with a sphere above a broadened bell-shaped bottom. Two eyespots are present on the dorsal (upper) surface.

conservation status

Not listed by IUCN.

significance to humans

None known.

No common name

Rhabdopleurida normani

order

Rhalodopleurida

family

Rhabdopleuridae

taxonomy

Rhabdopleura normani Allman, 1869, Norway.

other common names

None known.

physical characteristics

Adults have a cephalic shield, complex collar, and trunk. They can reach 0.1 in (3 mm) long, but typically are about 0.04 (1 mm) in length. There are two tentacled arms rising from the collar. The tentacles may reach 0.06 in (1.5 mm) in length.

distribution

Atlantic and Arctic Oceans, Mediterranean Sea.

habitat

These marine hemichordates are sessile colonial organisms found in shallow water up to 33 ft (10 m) deep, clinging to the bottoms or other protected nooks and crannies of rocks, corals, and other hard underwater surfaces. They live in and are confined to a coenecium because of their attachment to a continuous organic stem or stolon.

behavior

Adult individuals are known as zooids. They live in separate, translucent tubes within a coenecium built out of secretions from their cephalic shields. The zooids are attached to a stolon and are unable to leave the coenecium. The larvae, on the other hand, use their cilia for swimming.

feeding ecology and diet

Individual adult R. normani worms feed by poking their proboscis, or cephalic shields, out of the coenecium and extending their two ciliated "arms" into the water. Although it has been suggested that other hemichordates use the mucus they secrete to transport food items, recent research indicates that individuals of this suspension-feeding species beat their cilia to draw food particles toward the mouth and do not rely on mucus to trap their food.

reproductive biology

R. normani engages in sexual and asexual reproduction. Asexual budding occurs at the stolon, which runs along the bottoms of the tubes and forms connections among the zooids. Sexual reproduction occurs throughout the year. Females secrete tubes that are coiled at the base. The coils serve as brood chambers for one to seven eggs, which are deposited at different times, and therefore are at different developmental stages. The eggs are yolky, a creamy yellow in color, and about 0.0078 in (200 µm) in diameter. Each larva hatches and grows to a size of 0.016–0.018 in (400–450 µm). At that point, it swims past the other eggs and exits the tube. The larva continues swimming in search of its own place to settle on the bottom.

conservation status

Not listed by IUCN.

significance to humans

None known.

Spaghetti worm

Saxipendium coronatum

order

No order designation

family

Saxipendiidae

taxonomy

Saxipendium coronatum Woodwick & Sensenbaugh, 1985, near "Rose Garden" geothermal vent, Galápagos Rift, at a depth of 8130 ft (2478 m).

other common names

Italian: Verme tentacolato.

physical characteristics

A long thin yellowish-white hemichordate, this species can reach 6.6–9.8 ft (2–3 m) in length. The proboscis is tapered to a soft point toward its front, and the collar is short.

distribution

Near the Galápagos Islands.

habitat

Found near hydrothermal vents in the deep sea, loosely attached to rocks.

behavior

Little is known about the behavior of this species, but individuals are typically seen in "tortuous coils, wrapped upon themselves and welded in mucus."

feeding ecology and diet

Little is known about the diet of S. coronatum.

reproductive biology

Fertilization is external; otherwise, little is known about this species' mode of reproduction.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Resources

Books

Barrington, E. J. W. The Biology of Hemichordata and Protochordata. San Francisco: W. H. Freeman and Co., 1965.

Periodicals

Cameron, C. B., J. R. Garey, and B. J. Swalla. "Evolution of the Chordate Body Plan: New Insights from Phylogenetic Analyses of Deuterostome Phyla." Proceedings of the National Academy of Sciences 97, no. 9 (April 25, 2000): 4469–4474.

Halanych, K. M. "Suspension Feeding by the Lophophore-Like Apparatus of the Pterobranch Hemichordata Rhabdopleura normani." Biology Bulletin 185 (December 1993): 417–427.

Hart, M. W., R. L. Miller, and L. P. Madin. "Form and Feeding of a Living Planctosphaera pelagica (Phylum Hemichordata)." Marine Biology 120 (1994): 521–533.

Lester, S. M. "Ultrastructure of Adult Gonads and Development and Structure of the Larva of Rhabdopleura normani (Hemichordata: Pterobranchia)." Acta Zoologica (Stockholm) 69, no. 2 (1988): 95–109.

Organizations

British & Irish Graptolite Group. c/o Dr. A. W. A. Rushton, The Natural History Museum, Cromwell Rd., South Kensington, London, SW7 5BD United Kingdom. Web site: <http://www.graptolites.co.uk/>

The Graptolite Working Group of the International Palaeontological Association. c/o Dr. Charles E. Mitchell, Department of Geology, State University of New York at Buffalo, Buffalo, NY 14260-3050 United States. E-mail: [email protected] Web site: <http://www.geology.buffalo.edu/gwg/index.htm>

Other

"Chris Cameron's Homepage." (15 July 2003). <http://cluster3.biosci.utexas.edu/faculty/cameronc/CBC.htm>.

"Hemichordate Phylogeny." University of Washington Faculty Web Server. (15 July 2003). <http://faculty.washington.edu/bjswalla/Hemichordata/hemichordata.html>.

Leslie Ann Mertz, PhD