What are Sipunculans?

Sipunculans are commonly known as peanut worms. The live in oceans burrow under rocks and in tight crevices between rocks. Peanut worms are bilaterally symmetrical and unsegmented unlike Annelids. The body is usually divided into two regions: an anterior region called an introvert which can be retracted into the body and a psterior region. At the tip of the introvert is a mouth that is surrounded by tentacles. The introvert is withdrawn by retractor muscles attached to the midregion of the trunk. Sipunculans are coelomates, and use hydrostatic pressure to keep the body firm. Sipunculans are about 6-7 cm long when stretched out. The body wall is strong and muscular; when threatened, sipunculids can retract their body into a shape resembling a peanut kernel.

Two examples of sipunculids are: Sipunculus nudus and Phascolosoma perlucens.

Cool Fact! Sipunculid worm jelly is a delicacy in the town of Xiamen in Fujian province of China

Feeding and Digestion

Food is captured on branching cilliated tentacles that surround the mouth. The food particles are then transported to the mouth. Sipunculans have an unusual complete U-shaped gut. The mouth is surrounded by a crown of tentacles that collects food particles from surrounding surfaces. Sipunculans are surface deposit feeders that gather food from the mud and sand that they live in. Because the digestive tract is U-shaped, the gut forms a twisted loop and so the anus is on the side of the body!




F=anus
B=retractor muscles
E=compensation sac
C=inverted proboscis

Respiration and circulation

Sipunculans do not have a resiratory or circulatory system. However, branching cilliated tentacles act as a surface for respiratory exchange of oxygen and CO2. The fluid in their coelom transports nutrients and oxygen throughout the body. Floating within the coelomic fluid are cells containing a respiratory pigment called hemerythrin which help transport oxygen.

Excretion

Excretory cells are found floating free in the coelomic fluid. Also, a pair of annelid-like nephridia open ventrally. The regulate volume and serve as an outlet for gametes.

Reproduction

Floating free in the coelom are developing gametes. Most sipunculids reproduce sexually. They are also broadcast spawners, releasing their gametes to be spawned in the sea. As in annelids, the fertilized egg undergoes spiral cleavage. A free-swimming trochophore larvae usually develops. In some sipunculans, the trochophore larvae is followed by a large rounded larval stage. It may feed on plankton or stored yolk. The larvae eventually settles and develops into the adult. Some sipunculids reproduce asexually by unequal body division or budding. Sipunculans are quite good at regeneration. Some sipunculans can regenerate dammaged tentacles, or even regenerate trunk or digestive regions.

Movement

Sipunculans use their fluid filled bodies as a hydrostatic skeleton. The animals are basically a muscular bag of incompressible fluid and by controlling where fluid in their coelom flows, they can control body movement.

Sipunculids have much more control than this however. They have muscles called longitudinal muscles (muscles that run the length of the body from anterior to posterior)within the body and the body wall just under the skin which they can contract to bring the end of the body up to have the body curl. The longitudinal muscles assist in retraction, body curling, and moving the body from side to side.

Response

Many sipinculans live in permanent burrows or rock crevices for protection. When harassed by predators, the worm can withdraw the anterior part of the body for protection in the trunk and is called the introvert.

Comparison to other phyla

Sipunculans were once grouped with the annelids, even though they lack segmentation and bristles on the body. Later, Sipunculans were grouped with the molluscs because of similar developmental and larval characteristics. Sipunculans are now in their own phylum, but the close relationship between molluscs and annelids is still recognized.

Sipunculans have been recognized for their role in bioerosion of coral reefs and as an indicator of environmental health. They are also an important food source for fish and invertebrates.

References

Lester B. Peason College. "Phascolosoma agassizii". Racerocks. 7 December 2002. taxalab/bio2002/phascolosomaa.htm>.

Miller, Kenneth R. and Joseph Levine. Biology. 4th ed. Upper Sadle River, NJ: Prentice Hall, 1998.

Pearse, Viki., and others. Living Invertebrates. Boston, Massachusetts:Blackwell Scientific Publications, 1987.

Rice, Mary E. "Possible Boring Structures of Sipunculids". Integrative and Comparative Biology. 1969. icb.oxfordjournals.org/cgi/content/abstract/9/3/803>

Ruppert, Edward, E., Richard S. Fox and Robert D. Barnes. Invertebrate Zoology: A functional evolutionary approach.
7th ed. Belmont, CA: Thompson Brooks/Cole, 2004.