- 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
Ph. Sipuncula, Fam. Golfingiidae
Purschke, G., et. Al.1997. Ultrastucture of the nuchal organ and cerebral organ in Onchnesoma squamatum (Sipuncula, Phasolionidae). Zoomorphology 117:23-31.
The position of Sipunculans in the phylogenetic tree is still being discussed. Two general hypotheses link these organisms to either the annelids or the molluscans. An in depth look at the nuchal and cerebral organ of a Onchnesoma squamatum is done to determine whether these structures may be homologous to the well known nuchal and cerebral structures of the polychaetes (annelids).
This is the first in depth study of the nuchal and cerebral organs. By determining whether these structures are homologous to those of the polychaetes, a firmer link to either hypothesis can be made. If they are similar, this could be more evidence to link the sipunculans to the annelids.
1828 species were collected, anaesthetized and examined under both scanning and transmission electron microscopes. Nuchal organs are found to always be unpaired, and these organisms lack an olfactory chamber.
When compared to the polychaete nuchal and cerebral organ, major differences were found. Nuchal organs are paired in polychaetes and unpaired in sipunculans. Furthermore, the sensory nerves to the nuchal organ in polychaetes have microvillia in the olfactory chamber. It is concluded that not only does the study not support the homology of the nuchal organs in the Sipuncula and Annelida, but it also suggests that the organ has a different function in the two taxa.
Ph. Sipuncula, Fam. Golfingiidae
Blanco, G. A., et. Al. 2005. Hydrogen Peroxide Induces Apoptotic-Like Cell Death in Coelomocytes of Themiste petricola (Sipuncula). Biological Bulletin. 209:168-183.
Do the species of Sipuncula and other coelomocytes use apoptosis to respond to infected, injured, or mutated cells? Will exposure to hydrogen peroxide cause apoptotic-like processes in sipunculans?
Apoptosis is important in immunity, and its regulation in coelomocytes could be significant to the sipunculan immune system. Hydrogen peroxide is known to produce apoptosis in the cells of animals from protozoa to mammals. It induces apoptosis by a massive depolarization of mitochondria.
Adult specimens with an average weight of 100mg were collected and their coelomocyte fluid drained. Suspensions of 1x16 cells/ml were exposed to 100mM H2O2 and assayed for different apoptotic indicators at several points. It was observed that several characteristics of apoptosis were taking place such as chromatin condensation, nuclear segmentation, cell volume decrease, membrane blebbing, and formation of apoptotic bodies.
Hydrogen peroxide did induce apoptotic like processes. This indicates that apoptosis may be used for the regulation of cell numbers during times of stress and for immune responses.
Ph. Sipuncula, Fam. Phascolosomatidae
Tzetlin, A. B. and G. Purschke. 2006. Fine structure of the pharyngeal apparatus of the pelosophera lava in Phascolosoma agassizii (Sipuncula) and its phylogenetic significance. Zoomorphology. 125:109-117.
The buccal organ in sipunculans is considered homologous to either the pharyngeal organ in polychaetes or the radular apparatus of the mollusks. The larva of phascolosoma agrassizii studied to determine which homologous structure the buccal organ is more closely related.
The determination of whether this organ is more like the homologous structures found in the molluscs or the annelids can help support either as being more closely related to the sipunculans in hopes of placing them correctly on the tree of life.
Adults were collected and kept until gametes were released. The larva which resulted were fixed, dehydrated and prepared into slides of ultra thin sections. These slides were observed using transmission electron microscopy. The results show great structural similarities between sipunculan’s buccal organ and the polychaetes pharynx. No similarities were seen between the buccal organ and the radular apparatus of the molluscs.
This lends to the evidence that sipunculans are more closely related to annelids than to molluscs.
