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Examples of pseudocompound chaetae: ( A ) Lumbrinereis inflata , ( B ) Nothria elegans , ( C ) Diopatra ornata . All scales 10 m m.
Source publication
In this article we review the phylogenetic distribution of major chaetal types within the Polychaeta, discuss what has been demonstrated about chaetal function in modern worms, and examine what is known about the evolution of chaete through the fossil record. We conclude with specific cautions about how chaetae are treated in phylogenetic analyses...
Context in source publication
Context 1
... if this unusual tube has attributes that make it difficult for the worm to be extracted from it and to understand how acoetids anchor their bodies as they extend from and retract back into their tubes. Eisig (1887, in Pettibone 1989) describes fishing for Polyodontes with bait and collecting only the anterior portions of the worms; the posterior sections had presumably pulled back into their tubes! The second exception is that some members of the Glyceridae construct semipermanent galleries ( Glycera alba : Ockelmann and Vahl 1970) yet do not have hooked chaetae (Table 2), and it would be informative to learn how they maintain position within their galleries. Third, the Hesionidae are for the most part free-living, actively mobile worms. Some hesionid species (for example, Ophiodromus ), however, are known as facul- tative commensals (or perhaps parasites) on the bodies of various hosts, including echinoderms (Hickok and Davenport 1957). Hesionids are not typically described as having hooked chaeta, although Ophiodromus does have small hooked ends at the tips of their compound chaetae; how they maintain their positions on their hosts is unknown. This general examination of the potential match between specific lifestyles and the chaetae that are characteristic of specific families is likely to become much more informative as we examine finer-scale covariation between the morphology of species and their natural histories. There are hints that at least in some cases there is a tight coupling; for instance we know of two terebellids that do not build tubes: Telothelepus capen- sis , in which hooks are reduced in number and found only on the abdomen, and Enplobranchus sanguineus, which lack hooks entirely (Day 1967). Finer-scale examinations will likely provide intriguing and instruc- tive examples in which morphology and function are uncoupled. The function of rows of hooked chaetae (including uncini) as anchors for tube-dwelling worms has been implicated in a number of different taxa by a variety of techniques. Morphologically, the orientation of hooks on the bodies of tube-dwelling worms is predictable by knowing the shape of the tube, irrespective of whether the worm lives head-up (for example, sabellids, oweniids, or terebellids) or head-down (for example, maldanids or pectinarids) in the tube. The hooks of tube-dwellers are oriented such that the tip of the hook is facing the direction in which the worm could be extracted from the tube (Woodin and Merz 1987). In this orientation the hooks are positioned to engage the tube wall and prevent removal of the worm from the tube by forces such as those produced by water movements or predators (Lauder 1980; Denny 1988; Nemeth 1997). The mechanical effectiveness of this mechanism has been demonstrated in various species of sabellids, oweniids, and maldanids, in which removal of the worm from a tube takes on average three times more force in the direction in which the tips engage the tube wall than in the reverse direction (Woodin and Merz 1987; Merz and Woodin 2000). That worms are actively employing this function is demonstrated by the observations that (i) anaesthetized worms present very little resistance to extraction from their tubes (Woodin and Merz 1987; Merz and Woodin 2000); (ii) when worms are forced out of their tubes in the direction that engages their hooks, those hooks and hook rows are damaged on the widest portion of the body, where the hooks would be expected to most rigorously engage the tube wall (Merz and Woodin 2000) (Fig. 3C); and (iii) worms removed from their tubes by carefully cutting open the tubes show scars and worn chaetae in these same regions (Merz and Woodin 2000). If or how the hooks of other taxa perform an anchoring function has not been demonstrated; of particular interest is how solitary hooks might function. The terms “jointed” or “compound” chaetae have not been used consistently in the literature. In general, they are applied when the chaetal shaft is interrupted by a morphological elaboration or thinning (Fig. 4). The joint consists of a socket in which the distal blade of the chaeta is anchored by both a ligament and a hinge (Fig. 4). The joint is external to the body and is not directly connected to either muscles or nerves, and the distal blade is free to move, governed by the shape of the socket and the position and length of the ligament. This type of jointed chaeta (identified as ones with single ligaments by Rouse and Fauchald 1997) is known only from families within the Phyllodocida (Table 1). The development of this classic jointed chaeta was beautifully described by Gustus and Cloney (1973) and O’Clair and Cloney (1974) for a nereidid polychaete. Members of the Eunicida, Flabelligeridae, and Acrocirridae are sometimes described as having compound or pseudocompound chaetae. In the case of the Eunicida, the distal blade is not free to move, either because it is limited by double ligaments (Rouse and Fauchald 1997; Fig. 5) or because there is no socket and the folds of the chaetal shaft that have been assumed to be a joint do not in fact bend (personal observation; Diopatra : Fig. 5C). We agree with Rouse and Fauchald (1997) that the details of morphology of compound chaetae have not been thoroughly explored and that examination of them will be important for both systematics and functional morphology. They would also be good subjects for developmental study. Bartolomaeus (1995, 1998) and Bartolomaeus and Meyer (1997) have demonstrated that hooks (including uncini) from a variety of families develop in basically the same way and that different hook morphologies are the result of variations in timing of the actin-filament system of the chaetoblast and surrounding follicle cells. We suspect that compound and pseudocompound chaetae may similarly be the result of slight modifications of the developmental pro- gram of chaetal formation. Knowing the developmental basis for these morphologies would help confirm or reject the proposed relationship among the Eunicida, Phyllodocida, and Terrebellida (Fig. 1), the three clades that have families with compound or pseudocompound chaetae. There has been only one experimental study of the function of compound chaetae ( Ophiodromus pugettensis: Hesionidae: Phyllodocida: Merz and Edwards 1998); however, several elegant descriptions do discuss chaetal extension, parapodial position, and gaits in the Phyllodocida (for example, Gray 1939; Marsden 1966; Mettam 1967, 1971). Many of the members of the Phyllodocida are active worms (Table 1) that display a variety of gaits from simple walking to swimming. Such movements involve the coordination of the parapodia, chaetae, and body wall musculature and produce changes in parapodial flexure and chaetal protrusion depending upon the position of the parapodium (Gray 1939; Marsden 1966; Mettam 1967, 1971). It is interesting that jointed chaetae are primarily associated with the ventral neuropodium in biramous taxa or the ventral side of the parapodium in uniramous taxa (Table 1), where they would be the attachment points to the substrate and bear the weight of the animal. The exception to this ventral orientation of jointed chaetae is found in forms that are holopelagic (for example, Lopadrorhynchidae, Pontodoridae, and Iospilidae) and in the Nereididae, which have swimming epitokes (the Syllidae may also develop jointed chaetae in the notopodium during epitoky, although different authors vary in their description of the swimming chaetae; see footnotes to Table 1). There is very little known about how chaetae specifically con- tribute to the mechanics of swimming in polychaetes. The arrangement of chaetae in the parapodium might suggest that they need to be distributed laterally (rather than ventrally) and that in the absence of interaction with a solid substratum, there is less selection for dorsal-ventral regionalization of chaetal types within the parapodium. The ventral position of jointed chaetae in crawling forms may allow the tips of the chaetae to indepen- dently orient to the rugosities of the substratum, thus decreasing slippage and increasing stepping efficiency. This hypothesis was tested in O. pugettensis (Hesionidae), where jointed chaetae were trimmed either just distal to or immediately proximal to the joint. As expected, loss of the joint significantly reduced locomotory performance (measured as the maximum speed within a gait or stride distance) (Merz and Edwards 1998). Thus for compound chaetae that flex at the joint, the joint may have a clear function. The function, if any, of compound or pseudocompound chaetae that do not appear to bend at the joint is unknown. The compound (or pseudocompound) chaetae of Diopatra magna (Onuphidae: Eunicida), for example, do not bend at the joint (R. A. Merz and S. A. Woodin, unpublished data) (Fig. 5C). Without flexure they would appear to be the equivalent of simple, though thickened, capillaries or hooks. Another way to view the evolution of chaetae is to examine the fossil record. Even though chaetae and jaws are the polychaete features most likely to be fos- silized (Colbath 1986, 1988; Briggs and Kear 1993), only in cases of exceptional preservation (lagerst ̈tten) is it possible to see detailed structure of individual chaetae and their exact position on the body. There are relatively few such lagerst ̈tten that provide whole- body polychaete fossils from which we can draw much information (see Briggs and Kear 1993 for a summary of whole-body polychaete fossil collections). The earliest undisputed examples of polychaete body fossils are from the Burgess Shale of the Cambrian (543–490 mya). Dickinsonia , Spriggina, and Marywadea from the Ediacaran fauna of the Vendian (600–543 mya) have been discussed as polychaetes (Glaessner 1976; Conway Morris 1979), but this view has been rejected, in part because of their lack of chaetae (Conway Morris 1979). The remarkable preservation of the ...
Citations
... In the majority of sedentarian annelids, hooked chaetae serve a primary function in anchoring to the tube, facilitating the worm's stability and movement within its habitat [3,4,42]. However, in Osedax, these structures clearly serve a different function. ...
Background
This study sheds light on the genetic blueprints of chaetogenesis (bristle formation), a complex biomineralization process essential not only for the diverse group of bristle worms (annelids) but also for other spiralians. We explore the complex genetic mechanisms behind chaetae formation in Osedax japonicus, the bone-devouring deep-sea worm known for its unique ecological niche and morphological adaptations.
Results
We characterized the chaetal structure and musculature using electron microscopy and immunohistochemistry, and combined RNAseq of larval stages with in-situ hybridization chain reaction (HCR) to reveal gene expression patterns integral to chaetogenesis. Our findings pinpoint a distinct surge in gene expression during the larval stage of active chaetogenesis, identifying specific genes and cells involved.
Conclusions
Our research underscores the value of studying on non-model, "aberrant" organisms like Osedax, whose unique, temporally restricted chaetogenesis provided insights into elevated gene expression across specific larval stages and led to the identification of genes critical for chaetae formation. The genes identified as directly involved in chaetogenesis lay the groundwork for future comparative studies across Annelida and Spiralia, potentially elucidating the homology of chaetae-like chitinous structures and their evolution.
... The early larvae of P. dumerilii exhibit a highly stereotypical set of compound bristles that are also referred to as spiniger-type bristles. Bristles of this type are thought to be relevant for locomotion through different substrates 5,12,13 . These bristles arise in specialised tissue pouches-chaetal sacs (schematised in Fig. 1a)-that line the lateral aspects of the larvae (Fig. 1b). ...
... Taken together, our quantification not only supports the idea that the deposition of blades, joints, and shafts is driven by changes in the biogenesis programme on the scale of hours/days, but that additional dynamics on the scale of minutes/hours are responsible for depositing repetitive, submicrometric features of the blade and shaft. The stereotypicity of bristle structures in Platynereis, as well as the documented species-specific variations of bristle patterns in other polychaetes 4,5 indicate that bristle biogenesis is driven by a genetically programmed cellular mechanism. In other contexts, such as the vertebrate hair cells or lateral line neuromasts, elaborate cellular geometries rely on the occurrence of specialised apical microvilli 22 . ...
Organisms generate shapes across size scales. Whereas patterning and morphogenesis of macroscopic tissues has been extensively studied, the principles underlying the formation of micrometric and submicrometric structures remain largely enigmatic. Individual cells of polychaete annelids, so-called chaetoblasts, are associated with the generation of chitinous bristles of highly stereotypic geometry. Here we show that bristle formation requires a chitin-producing enzyme specifically expressed in the chaetoblasts. Chaetoblasts exhibit dynamic cell surfaces with stereotypical patterns of actin-rich microvilli. These microvilli can be matched with internal and external structures of bristles reconstructed from serial block-face electron micrographs. Individual chitin teeth are deposited by microvilli in an extension-disassembly cycle resembling a biological 3D printer. Consistently, pharmacological interference with actin dynamics leads to defects in tooth formation. Our study reveals that both material and shape of bristles are encoded by the same cell, and that microvilli play a role in micro- to submicrometric sculpting of biomaterials.
... In such specimens, the head becomes effectively concealed under the posterior part of its body, while the longer notochaetae project outward protecting the posterior part of the body and pygidium. In extant benthic annelids, shorter capillary chaetae may be used for stabilizing the body inside the burrows or tubes in which they dwell [36]. However, the notochaetae of Ursactis seem less likely to be used for such a purpose because they have no structures that can be used for anchoring (e.g. ...
Early annelid evolution is mostly known from 13 described species from Cambrian Burgess Shale-type Lagerstätten. We introduce a new exceptionally well-preserved polychaete, Ursactis comosa gen. et sp. nov., from the Burgess Shale (Wuliuan Stage). This small species (3–15 mm) is the most abundant Cambrian polychaete known to date. Most specimens come from Tokumm Creek, a new Burgess Shale locality in northern Kootenay National Park, British Columbia, Canada. Ursactis has a pair of large palps, thin peristomial neurochaetae and biramous parapodia bearing similarly sized capillary neurochaetae and notochaetae, except for segments six to nine, which also have longer notochaetae. The number of segments in this polychaete range between 8 and 10 with larger individuals having 10 segments. This number of segments in Ursactis is remarkably small compared with other polychaetes, including modern forms. Specimens with 10 segments show significant size variations, and the length of each segment increases with the body length, indicating that body growth was primarily achieved by increasing the size of existing segments rather than adding new ones. This contrasts with most modern polychaetes, which typically have a larger number of segments through additions of segments throughout life. The inferred growth pattern in Ursactis suggests that annelids had evolved control over segment addition by the mid-Cambrian.
... Different kinds of compound (or "jointed" chaetae) occur in many polychaete taxa; however, a similar hinge and ligament structure as the one described herein is only known for clades within Phyllodocida (Rouse and Fauchald 1997;Merz and Woodin 2006;. This type of compound chaeta only has a single ligament, is flexible, and plays a significant role in the locomotory performance of the animal (Merz and Edwards 1998;Hesselberg and Vincent 2006). ...
... Eunicida, also has compound chaetae ). These, however, lack a clear socket and are therefore also referred to as pseudo-compound chaetae with double ligaments (Merz and Woodin 2006;Rouse and Fauchald 1997). The formation of joints and ligaments in Eunicida is yet to be studied. ...
Annelid chaetae are extracellular chitinous structures that are formed in an extracellular epidermal invagination, the chaetal follicle. The basalmost cell of this follicle, the chaetoblast, serves like a 3D-printer as it dynamically shapes the chaeta. During chaetogenesis apical microvilli of the chaetoblast form the template for the chaeta, any structural details result from modulating the microvilli pattern. This study describes this process in detail in the model organism Platynereis dumerilii and clarifies some aspects of chaetogenesis in its close relative Nereis vexillosa, the first annelid in which the ultrastructure of chaetogenesis had been described. Nereid species possess compound chaetae characteristic for numerous subgroups of errant annelids. The distal most section of these chaetae is movable; a hinge connects this part of the chaeta to the shaft. Modulation of the microvilli and differences in their structure, diameter and number of microvilli, and their withdrawal and reappearance determine the shape of these compound chaetae. Chaetal structure and pattern also change during life history. While larvae possess a single type of chaeta (in addition to internal aciculae), juveniles and adults possess two types of chaetae that are replaced by large paddle-shaped chaetae in swimming epitokous stages. Chaetogenesis is a continuous process that lasts during the entire lifespan. The detailed developmental sequence of chaetae and their site of formation are very similar within species and species groups. We expect that similarity results from a conserved gene regulatory network making this an optimal system to test the phylogenetic affinity of taxa and the homology of their chaetae.
... At ancient seeps, non-calcareous tubeworms are often very abundant within the deposits in which they occur and must therefore have made effective use of seep chemosynthetic production and must also have been key components of these ancient ecosystems, providing habitat structure as well as mediating carbon ow to higher trophic levels. Progress in understanding the evolutionary timings of when different annelid lineages moved into seeps is, however, largely hindered by the lack of morphological details observable on many seep tube fossils, as well as the more limited utility of tubes for identi cation, in which homoplasies can often exist (Merz and Woodin 2006;Kiel and Dando 2009). The situation is also not helped by a poor fossil record for non-calcareous tubicolous annelids in general. ...
Animals that construct tubular dwelling structures were regular colonisers of ancient hydrocarbon seep environments and remain so to the present day. At modern seeps, tubes are most commonly built by animals belonging to the phylum Annelida, or the segmented worms, and have an important ecological role in enhancing local heterogeneity and providing habitat for other seep fauna. The most prominent tubeworms within modern seeps belong to the families Siboglinidae, which build their tubes from organic secretions, and Serpulidae, which construct calcareous-walled tubes. Tubeworms have occupied ancient hydrocarbon seeps since at least the Devonian, and their abundance and diverse morphologies within seep deposits, especially of tubes with an originally organic composition, attest to their important membership of these communities throughout evolutionary history. Calcareous tubes are generally present in ancient seep deposits at lower abundances and therefore likely represent opportunistic colonizers of these settings. Many fossil seep tubeworms are ascribed to the annelids, with calcareous tubes being in general more straightforward to place taxonomically, while identification of tubes of an originally organic composition is more challenging. Serpulids colonised seep environments sometime during the Late Jurassic or Early Cretaceous, while the siboglinids may have occupied ancient seeps as early as the Cretaceous.KeywordsAnnelidaEvolutionChemosynthesisSiboglinidaeVestimentiferaChaetopteridaeHydrothermal ventFossilisationOrganic chitin-protein compositionHabitat-forming speciesAgglutinated tube
... To pump fresh oxygen-rich water through their tubes periodically, annelids perform peristaltic movements (Sendall et al. 1995). Capillary chaetae have been identified to serve as stabilizing elements during these movements to avoid the animal from changing its position within the tube (Merz and Woodin 2006). However, these hypotheses remain speculative and need to be tested experimentally with living specimens. ...
... These vesicles are formed in a Golgi-stack associated process; the contents are released into the gap between the chaeta and the follicle cell (e.g., Bartolomaeus 1995Bartolomaeus , 1998 In their simplest construction chaetae, thus, are distally tapering bristles that mechanically resemble glass fiber sticks. Such chaetae are termed capillary chaetae; they can be found in nearly all annelid groups except for Sipuncula (Capa et al. 2019;Merz and Woodin 2006;Weigert and Bleidorn 2016). Transverse sections show wide canals with thin lamellae in the center. ...
Phylotranscriptomic studies of the past decade have repeatedly placed Oweniidae together with Magelonidae, as the sister group to remaining annelids. This newly established placement clearly makes them a key-lineage for understanding annelid evolution and morphology. One of the most prominent morphological features of all annelids are their chaetae. The arrangement and formation process (chaetogenesis) of these chitinous bristles have been studied extensively in hooked chaetae that are arranged in rows. However, the information on other types of chaetae is still scarce. In this study, we investigated the scaled capillary notochaetae of Owenia fusiformis, looking both into the formation process that causes the scaly surface ornamentation and into their arrangement within tight bundles. Our results demonstrate the incredible plasticity of chaetogenesis that allows forming a vast array of three-dimensional structures. The capillary chaetae of Owenia fusiformis are unique in lacking an enamel coating and the scales covering the apical surface of each chaeta are formed by a single microvillus of the chaetoblast. Furthermore, the bundle of chaetae has a peripherally located formative site and a central degenerative site and it appears to result from a secondary curling of the chaetal sac.
... Capitella also has chitinous bristles called chaetae, a hallmark annelid character (Fig. 2C). Chaetae are used in burrowing, swimming or anchoring the body within its tube (Merz & Woodin, 2006). Chaetal morphology varies widely across species and is often used in species identification (Fauchald, 1977;Rouse & Pleijel, 2001). ...
Over the last few decades, the annelid Capitella teleta has been used increasingly as a study system for investigations of development and regeneration. Its favorable properties include an ability to continuously maintain a laboratory culture, availability of a sequenced genome, a stereotypic cleavage program of early development, substantial regeneration abilities, and established experimental and functional genomics techniques. With this review I tell of my adventure of establishing the Capitella teleta as an emerging model and share examples of a few of the contributions our work has made to the fields of evo-devo and developmental biology. I highlight examples of conservation in developmental programs as well as surprising deviations from existing paradigms that highlight the importance of leveraging biological diversity to shift thinking in the field. The story for each study system is unique, and every animal has its own advantages and disadvantages as an experimental system. Just like most progress in science, it takes strategy, hard work and determination to develop tools and resources for a less studied animal, but luck and serendipity also play a role. I include a few narratives to personalize the science, share details of the story that are not included in typical publications, and provide perspective for investigators who are interested in developing their own study organism.
... The microvilli of the chaetoblast leave a system of hollow canals in the chaeta, resulting in a unique honeycomb-like pattern, constituting a stable as well as flexible structure (Dnestrovskaya & Jirkov, 2011;Hausen, 2005). The dynamic nature of the microvilli during chaetogenesis allows the formation of various types of chaetae, such as capillary, hooked, or even jointed ones (Bartolomaeus, 1998;Merz & Woodin, 2006;Tilic et al., 2016). ...
The annelid clade Amphinomidae, commonly known as “fireworms,” is notorious for causing skin inflammations after physical contact. These injuries are caused by the chaetae of the animals, which are calcified and particularly brittle. The incorporation of calcium carbonate particles and their disintegration cause the chitinous lamellae within a chaeta to break apart and result in a prominent inner cavity. Although the chaetae of Amphinomidae are well studied, data concerning their sister group, Euphrosinidae, are scarce. In this study, we examined the chaetae of two species of Euphrosine for the first time regarding the variety of chaetal types, their ultrastructure, elemental composition, and arrangement. This allows us to get a better understanding of chaetal characteristics in the ground pattern of Amphinomida. We were able to determine bifurcate chaetae as the primary chaetal type in Amphinomida and provide evidence that their aciculae must have evolved secondarily. Calcification is a common feature among amphinomidan chaetae and regarded as autapomorphy of Amphinomida. Finally, a single dorsal formative site in each notopodium and neuropodium, found in Euphrosine foliosa, likely represents the ancestral state in Amphinomida. There is no evidence for venom production or secretory cells associated with the chaetae of Euphrosine, which is in accordance with what was previously observed in Amphinomidae. Our study also complements further studies into chaetal characteristics such as the number and arrangement of formative sites.
... Through the development of transmission electronic microscopy, two major studies describing the general morphology of these intriguing organs were carried out by Boletzky (1973) and Brocco et al. (1974) who pointed out the similarities of KO setae with those of other invertebrate animals. The bristle-like setae or chaetae, are composed of longitudinally oriented filaments present in groups of lophotrochozoans such as annelids (Gustus and Cloney, 1973;Merz and Woodin, 2006); the mantle of brachiopods (Gustus and Cloney, 1972;Lüter, 2000); sensory organs of polyplacophoran molluscs (Leise and Cloney, 1982), and the gizzard teeth of bryozoans (Gordon, 1975). The chaetae or setae described for all these groups, including the KO of octopods, are similar but most probably not homologous structures (Hausen, 2005), with a huge range in the form, size and degree of arrangements. ...
The entire skin surface of octopus embryos, hatchlings and juveniles bears scattered tufts of tiny chitinous setae within small pockets, from which they can be everted and retracted. Known as Kölliker’s organs (KO), they disappear before the subadult stage. The function of these structures during the early life of the octopus is unknown, despite having been first described nearly two centuries ago. To investigate these organs further, general trends in size of KO distribution and density were analyzed in hatchlings and juveniles of 17 octopod species from all oceans, representing holobenthic, holopelagic and meropelagic modes of life. The size of the KO is fairly constant across species, unrelated to mode of life or hatchling size. The density of KO is similar on ventral and dorsal body surfaces, but hatchlings of smaller size tend to have a higher density of KO on the aboral surface of the arms. Analysis of a series of post-hatching Octopus vulgaris shows KO size to be constant throughout ontogeny; it is therefore a consistent structure during the octopus early life from planktonic hatchling to benthic juvenile. New KO are generated on the skin of the arm tips during the planktonic period and initial benthic lives of juveniles. Their density, on both the mantle and arms, gradually decreases as the octopus grows. In older benthic juveniles, the KO degrades, losing its setae and the base of its follicle becomes exposed as a nearly circular stump of muscle. It is estimated that fully everted KO increase the body surface area by around two-thirds compared to when the KO are retracted. This modular mechanism of body surface extension and roughness probably influences flow-related forces such as drag and propulsion of the moving surface of the young octopus while it is of small size with a relatively large surface area. In addition, the distribution of these organs on the aboral surface of the arms of the octopus and their birefringent properties suggest a role in camouflage. Further research is needed to test these hypotheses of KO function in live animals.
... Besides simple capillary chaetae, a vast array of chaetal types occur in Annelida . They are mostly used by the animals for defense or locomotion, but also for foraging, drilling into hard substrates, or anchoring to the tube (Merz & Woodin, 2006;Pernet, 2000;Tilic et al., , 2017. Now, the use of chaetae as construction material for tubes can be added to this list. ...
Sthenelanella is an unusual genus of tube‐dwelling scale worms, with fine fibrous threads that appear on either side of the body. These fibers emerge dorsally in long silvery bundles that are then tightly interwoven to construct the felt‐like material of the tube. In the literature, these fibers are described as the products of so‐called “spinning glands.” In this article, we investigated the ultrastructure of these notopodial fibers and show that they are annelid chaetae. Three or four dynamic microvilli of a basal chaetoblast form each of these feltage chaetae, making them the thinnest known annelid bristles. Our results show that the spinning glands of Sthenelanella uniformis are additional, highly modified notopodial chaetal sacs. We also show that the follicle cells, by their secretion of the enamel layer, play an active role in shaping the final chaeta. These findings not only increase the known morphological diversity of chaetae but also demonstrate the apparent plasticity of the machinery that form these chitinous structures. Our results are compared with chaetae in other annelids, with a particular focus on similar fibrous chaetae in Aphroditiformia.
