Chaetozone are a family of segmented worms belonging to the class Polychaeta. They reside in soft substrate in coastal waters around the globe, which has prompted the adaptation of a unique body plan. The body is surrounded by branchiae filaments that extend out of each segment and act as gills, and tentacular filaments extend from the head that are used in feeding. Musculature and nervous systems are well-established and have been shown to regenerate anterior regions following decapitation, which can occur from predation. There is also evidence that the tentacular filaments and branchiae have regeneration capabilities, and this was explored by staining of actin filaments and nuclei to map the musculature system. Chaetozone are opportunistic animals, and can withstand extreme environmental conditions where other organisms would deteriorate. This enables their use for measuring levels of pollutants in the ocean and ecosystem health. 

Distinctions between species of the Cirratulid genus have been confounded, and as a result have a number of common names (Bush, 2006). Some of the literature has assigned names to the juvenile form of a previously defined species. There are still major revisions to the taxonomy of Cirratulidae, however this is difficult due to the lack of research. (Bush, 2006).

Classification 
Phylum Annelida
Class Polychaeta
Subclass Sedentaria
Order Terebellida
Suborder Cirratliformia
Family Cirratulidae
Genus Chaetozone
Species Unknown

Chaetozone play a significant role in the endobenthic community, ensuring a connectedness of detritus and grazing food chains (Bush, 2006). They are deposit feeders that recycle organic matter found in sediment into a form usable to higher trophic levels. The animals are found embedded in soft, grainy sediment with branchial gills and tentacular filaments protruding out into the water column as the only visible body areas (Bush, 2006). 

Chaetozone can also indicate ecosystem health by being highly prevalent in areas of either moderate or high pollution levels (Jorgensen et al., 2005). The composition of pollution, such as domestic waste, has elevated levels of organic matter depending on the type of industrialisation involved (Jorgensen et al., 2005). Due to the accumulation of pollution sources at coastal regions, eutrophication occurs in sediment and the water column, which develops into tonic compounds and places significant stress on the organisms in these areas (Colombo et al., 1992). As a result, there is a decline in the numbers of native but less resilient species, and the increase of opportunistic animals such as Chaetozone (Jorgensen et al., 2005).

Cirratulids, as a general model for Chaetozone, live beneath the substrate in J-shaped tubes bound by mucous excretions (Pardo et al., 2004). 
They feed with grooved, ciliated filaments that extend across the sediment surface, catching food particles which move down through the groove in each filament and into the mouth (Pardo et al., 2004). This process restricts particle size to only fine sand grains, however enables the animal to remain in its tube while feeding. 

Sexual Reproduction 
Cirratulids are believed to reproduce asexually from architomic fragmentation, and then become sexually mature (Petersen, 1999). There is a relationship between size of individual and presence of natatory chaetae as well as state of maturity, where natatory chaetae (appendages for swimming) are an indication of sexual maturity (Petersen, 1999). 

Gonads appear ventrally beneath the gut and above the ventral nerve cord at the start of gametogenesis (Petersen, 1999). Gametocytes multiply from septa and the related blood vessels, however a direct relationship does not exist between them. Oocytes and spermatocytes disconnect from the gonads in groups during the early stages of development and mature to full size while in coelomic fluid (Petersen, 1999(. At the beginning of vitellogenesis, the oocytes detach from one another and spermatocytes separate from the testes, forming morulae (Petersen, 1999). The sperm display spherical heads, which suggests that they externally fertilise (Rouse, 2001). The entire process of gametogenesis occurs within a year, from start to termination at spawning, and as a result there are well-coordinated spawning seasons. 

Particular species produce green bio-luminescence from photophores found in the epidermis of abdominal segments. This suggests that spawning occurs during pelagic swarming, and luminescence plays a role in maintaining cohesion of the swarms (Petersen, 1999). 

Asexual Reproduction and Regeneration 
Regeneration has been shown in a number species, and is likely to be a common trait within the family and becomes essential during years of poor sexual success (Bush, 2006). In the event of a decapitation, the wound would close and a blastemal form within a week. The blastema continues to lengthen in an anterior direction in the following days, and at day 10 the mouth will become distinguishable (Petersen, 1999). After 14 days, the tentacles begin to extrude at the same as the head extends forward which induces early segmentation. Elongation of the anterior regions continues into the fourth week following the injury (Petersen, 1999). After approximately one month, the regenerated head is the same shape and size as the original, however there are significantly fewer branchiae and tentacular filaments which have decreased in length (Petersen, 1999).  

The initial response after decapitation is for the circular muscle to contract to close the wound. Longitudinal muscles will begin growth towards the blastemal after six days, and shortly after the blastema lengthens and combines with the muscle fibres to form into strands (Petersen, 1999). Additionally, there is growth of the intestinal muscle into the blastema area, which reaches the anterior body surface after ten days and forms a ring that becomes a primitive mouth (Petersen, 1999). Two weeks following decapitation, this ring surrounding the mouth opening creates a pouch directly behind the mouth. Despite the original shape of the muscle in the body wall, there are fewer fibres and a thinner layer (Petersen, 1999). 

Within a week of decapitation, a nervous system structure appears in the blastema area. Three nerve loops are visible; the middle is connected to the inner neurite bundle between the two strands of ventral nerve cord (Petersen, 1999). Both lateral loops are attached to the outer neurite bundles in the ventral nerve cord. Twelve days after decapitation, the lateral loops can no longer be detected and circumesophageal connectives appear (Petersen, 1999). A regeneration region of the ventral nerve cord is noticeable between the circumesophageal connectives and ventral nerve cord. Following two weeks, the brain commissures redevelop, and after twenty days the nervous system has completely regenerated (Petersen, 1999). 

The tentacles of Cirratulids are also capable of regeneration in the event of decapitation (Weidhase et al., 2015). The first pair of tentacles appears between six and eight days following the original injury, and a second develops after nine days. There is no overall pattern or time frame for the growth of further tentacles and branchiae, and the numbers of these never return to the original number. As the nervous system and musculature begins to extend through the body of the animal, it also develops in time with tentacles, which is important to maintain the feeding and sensory capabilities of the animal (Weidhase et al., 2015). 

The current understanding of Chaetozone defence shows a lack of any active mechanisms, and uses avoidance as its only method. When stimulated, both tentacular filaments and branchiae will immediately contract towards the body and into the animals’ burrow (Pardo et al., 2004). 

The only other technique to avoid predation is to burrow deeper into the sediment layer (Pardo et al., 2004). A study of the defence mechanisms in Chaetozone used increased light intensity to simulate a threat and found that the animal responded by drawing sediment close to its body and ceased movement. Once light decreased to the baseline level, the worm resumed moving (Pardo et al., 2004). There is also evidence that the mucus used to bind the sediment into a tube contains a flavour that is distasteful to fish (Pardo et al., 2004).

Chaetozone exist in a highly specialised environment, which requires a number of unique adaptations to ensure survival. Tentacular filaments and branchiae are positioned on the sediment surface in the water column to allow the majority of the animal to live in the anoxic conditions beneath sediment (Pardo et al., 2004). This allows a constant oxygen supply to be maintained, even at low tides when the tentacles may not be covered with water. However, it does increase the risk of predation on the animal (Pardo et al., 2004). 

Chaetae are also an adaptation to the particular environment to the genus, and can be used for classification since they are species specific. Chaetozone display spine-shaped chaetae splaying out from the body wall at each segment. This enables the animal to manoeuvre through soft sediments and adhere to the walls of its burrow. Due to the importance of these appendages for survival of the organism, chaetae development is tightly controlled and the genome sequences are highly conserved (Hausen, 2005). 

The most ancestral form of an annelid is comparative to the body plan of a polychaete, which was described as displaying distinct body segments, chaetae, septation and reduced parapodial folds (Purschke et al., 2014). It also spent the majority of its lifestyle burrowed in sediment, although the head lacked appendages (Purschke et al., 2014). This suggests that adaptations over time have only slightly altered the ancestral form, and enabled Chaetozone to be as specialised as possible to its habitat. 

A nervous system resembling a rope-ladder exists in Cirratulids. The ventral nerve cord is comprised of two threads, with one ganglion per segment, exhibiting three main nerves, and can be broken into multiple neurites (Petersen, 1999). At the initial chaetigerous segment, both strands of the longitudinal ventral nerve cord are joined, and then diverge into the circumesophageal connective either side of a particular nerve (Petersen, 1999). It is assumed that this nerve is part of a bundle that connects the dorsal root and ventral body. The brain is located at the furthermost anterior region of the circumesophageal connectives (Ruppert, 2004). 

Cirratulids consist of a longitudinal muscle layer, which creates a muscle plate that surrounds the dorsal body, and extends from prostomium to pygidium (Petersen, 1999). Muscle strands are prevalent running parallel along the ventral surface of the animal, and are covered by a circular muscle network. There is also a scaffold of circular and longitudinal muscles in the intestine, and surrounding the mouth opening. Prominent longitudinal muscle fibres are found in both branchiae and tentacular fibres (Petersen, 1999). 

Research on tentacle structure is only detailed to the level of family, and as a result the information provided is common across Cirratulids. Tentacular filaments function as grooved feeding appendages containing a single blood vessel, and can either be present in single or multiple pairs within a larger group (Bush, 2006).

Chaetozone, like all Polychaetes, have filtration nephridia to excrete waste from the body. A metanephridial system works alongside the haemal system with filtration sites enclosed by podocytes, epithelial cells, and septum-associated metanephridia (Ruppert, 2004).

The body of Chaetozone are generally brown, dark grey or blueish-black in colour with a lighter underside compared to the top colour (Blake, 2006). Tentacular filaments are red or orange, banded with black, and the brightest colours appear closest to the anterior (Blake, 2006). Branchiae can be different colours on a single individual, commonly ranging from red to dark brown. Chaetae are transparent. 

When submerged in sediment, only the tentacular filaments can be seen, making the animal appear a deep red colour (Blake, 2006). However, colour is particular to the individual, life-stage as well as a species, and consequently there is likely to be a variation along this continuum (Blake, 2006).  

Chaetozone are distributed globally, having been identified at various depths between the Poles and Tropics (Bush, 2006). Despite the majority being located in shallow coastal waters in soft sediment, there is evidence suggesting that individuals exist below 500 metres in the Faroe-Shetland Channel in the cold waters off Scotland (Pardo et al., 2004). However, another paper stating that they are limited to less than 82 metres (Bush, 2006). 
Since Chaetozone are deposit feeders, their abundance is highest in regions of organic enrichment such as up-welling zones (Bush, 2006).

Chaetozone distribution is confined due to the limited range of sediment composition that are suitable for survival (Bush, 2006). The animals are particularly sensitive to granule size, silt and clay content. Highest animal presence across a number of species occurs in areas with very fine sand, and moderate levels of silt and clay (Bush, 2006). This is consistent with observations in a lab environment where the animal was buried beneath sediment with a fine particle size.