- Sipuncula means little tube.
- Sipunculans are commonly called "peanut worms" based on their appearance. When their introvert is retracted they look like shelled peanuts.
- There are 320 known species, all of which are marine.
- They are found in all oceans, in all temperatures, and can be found in depths ranging from intertidal to approximately 7000 meters.
- Some species make their home by burrowing into sand/mud. Other species live in rock crevices, empty shells, or bore into rock.
- Sipunculans, in the past, were considered part of the Annelid phyla, despite lacking segmentation and bristles among other annelid characteristics.
- Later, they were thought to be more closely related to the phyla Mollusca based on their developmental and larval characteristics.
- In 1959, they were placed in their own phylum
- There are four families within the phyla
- Kingdom Animalia
Eumetazoa (Metazoans)
Billateria
Protostomia
Lophotrochozoa
Phylum Sipuncula
Family Aspidociphonidae
Family Golfingiidae
Family Sipunculidae
Family Phascolosomatidae
- Sipunculans exhibit bilateral symmetry and are coelomic.
- They are gray/brown in color and some species are reddish purple or green.
- They are vermiform and resemble annelids in structure.
- They have a rubbery cuticle covering their body.
- Most species are less than 10 cm long, but a few species can be up to 70 cm long.
- Their mouth is surrounded by a ring of mucus-covered tentacles which are used for feeding.
- Longitudinal and circular muscles allow them to burrow using peristaltic movements.
- The surface of the posterior portion of the body is covered with bumps, warts, tubercles, or spines.
- Tactile receptors are concentrated mainly on the tentacles, but are also found all along the body length.
- Chemosensory organs (nuchal organs) are located dorsally.
- For defense, they can pull their anterior portion, also called the introvert, inside the posterior portion.
- They have no circulatory or respiratory system.
- coelomic fluids transport nutrients and oxygen to all parts of the body
- A kidney like organ, the nephridium, filters the waste of the coelomic fluid.
- Gas exchange takes place across the body wall.
- They have a complete, U-shaped digestive system with a mouth, twisted intestines, and anus.
- Their nervous system is composed of a bundle of nerves inside the tip of the introvert, and a nerve chord running ventrally down the body.
- Some species reproduce asexually by fragmenting itself into a larger anterior portion and smaller posterior portion and re-growing the missing parts.
- Other species are gonochoristic and reproduce with external fertilization.
- Due to their soft body, they have an extremely sparse fossil record.
- Lecathylus gregarious is a peanut worm fossil from the early Silurian period
- A group of fossils known as hyoliths have been found in rocks through the Paleozoic period. These organisms had conical shells with an operculum linking them to molluscans. However, a few fossils of Hyoliths have been found to show traces of a looped and coiled intestine comparable to living sipunculans. Furthermore sipunculans secrete a calcified anal plate, so it is not hard to think that past relatives may have secreted more extensive shells.
- They are capable of regenerating portions of their body including portions of the tentacles, and digestive system.
- They are able to absorb dissolved organic material directly across their body wall. It is speculated that they meet 10% of their nutrient requirement this way.
- Larger species are used as fishing bait in some places
