Phyllochaetopterus cf. verrilli.
Phyllochaetopterus Grube, 1863

Taxonomic Categories:

Phylum: Annelida
Sub-phylum: Pleistoannelida
Class: Chaetopterida
Order: Chaetopterida
Family: Chaetopteridae
Genus: Phyllochaetopterus

Note: The specimens described in this page are likely to belong to the species Phyllochaetopterus verrilli Treadwell, 1943; however during consultation with Professor Greg W. Rouse (a leading expert in marine annelid worms) it was determined that confirmation of the species would require DNA sequencing. For this reason, the specimens are referred to as Phyllochaetopterus cf. verrilli; with the ‘cf’ designation meaning ‘confer’ or ‘compare with’ P. verrilli.

Phyllochaetopterus cf. verrilli is a member of the Chaetopteridae, a small family of marine tube-dwelling annelid worms (Nishi et al. 2009). Chaetopterid worms are identified by their fragile bodies with three distinct regions (A, B and C), pair of long grooved palps (see Figure 1), and paper-like tubes (Nishi et al. 2009). The construction of tubes from mucous combined with mud, silt or sand give the Chaetopteridae their common name; parchment tube worms (Barnes,1965; Wilson, 2000). Tubes are commonly buried in sediment or attached to rocks, and may be straight, L-shaped, J-shaped, or U-shaped, depending on the species (Barnes, 1965; Moore et al. 2017).

P. cf. verrilli is a very small chaetopterid worm, with all specimens observed measuring less than 3 cm long (see Figure 2). It constructs and lives within a thin sand-coated tube attached to rocks or other available substrates (see Figure 3). It has a pale cream body, with a dark colouration towards the posterior gut region due to chaetopterin; a pigment mixture of chlorophyll and phaeophorbides (Rouse & Pleijel, 2001; Britayev & Martin, 2016). This pigmentation is thought to indicate a possibly symbiotic flagellate micro-organism by some researchers, though this is uncertain (Wilson, 2000). P. cf. verrilli has no eye spots, and explores its surroundings using its long palps, which can be extended from its tube.

The Phyllochaetopterus Grube, 1863 genus is divided into four species groups based on features of A4 chaetae, along with the number of anterior and mid-body chaetigers (Nishi & Rouse, 2007). There are currently 24 species accepted as valid within the genus (Britayev & Martin, 2016). 

All members of the Chaetopteridae have a tripartite body plan with three morphologically distinct tagmata (see Figure 4); which are differentiated by distinct types of notopodia (Moore et al. 2017). A ciliated groove (shown in Figure 5) runs the length of the body dorsally (Rouse & Pleijel, 2001; Wilson, 2000). The mid-body region features highly modified foliaceous notopodial lobes (see Figure 6 and 7), which contain bordering membranelles that beat to facilitate water currents (Wilson, 2000; Barnes,1965).

In chaetopterid worms the body is usually pale or yellowish, with the chaetae often translucent (Britayev & Martin, 2016). A thick glandular field or ‘plastron’ is located ventrally (see Figure 8), and is pierced with pores producing mucous (Britayev & Martin, 2016). The head may have dark reddish pigmentation and palps may be striped dark brown, green or reddish bands; however none of these colourations were observed in P. cf. verrilli.

The head is composed of the prostomium and peristomium, along with anterior segments in some species (Rouse & Pleijel, 2001; Wilson, 2000). The prostomium is the first body segment, in front of (and not including) the mouth; it is usually a small dorsal lip extension (Wilson, 2000). The peristomium is the second anterior body segment directly behind the prostomium, and contains the mouth, antennae and sometimes the palps (Wilson, 2000). P. cf. verrilli shows no clear distinction between the prostomium and peristomium, which appear to be fused. As shown in Figure 8, P. cf. verrilli has a rounded prostomium with a petal-like large dorsal lip and cleft ventral lip.

The anterior region (A) of chaetopterids will have between 9 and 20 broad, flattened segments (Britayev & Martin, 2016); P. cf. verrilli exhibits 9 segments and corresponding chaetigers. In chaetopterids the mid-body region (B) may contain 2 or up to more than 30 segments, and will be highly specialized and elongated (Britayev & Martin, 2016). P. cf. verrilli displays 2-3 foliaceous lobes in the mid-body. The posterior region (C) has the largest number of segments, with the notochaetae often internal (Britayev & Martin, 2016). The posterior segments can rapidly contract, allowing the worm to withdraw quickly into its tube (Britayev & Martin, 2016). The terminal pygidium is a simple lobe (Britayev & Martin, 2016).  

Palps & Antennae

P. cf. verrilli displays the characteristic long pair of perisotomial palps just behind the mouth, extending from the anterior dorsal surface (see Figure 5). Each palp contains a longitudinal groove lined with cilia. The palps function primarily in ejecting debris and fecal pellets from the tube, along with minor feeding roles (Barnes, 1965). P .cf. verrilli specimens were also seen to use their palps to collect and adjust sand and shell debris while constructing and modifying the tube.

Members of the Phyllochaetopterus genus can be identified by their pair of small antennae located behind the palps (Barnes, 1965; Rouse & Pleijel, 2001). These structures contain internal chaetae, and are therefore considered to be elongated notopodia from segment A1 (Rouse & Pleijel, 2001; Britayev & Martin,2016).

Mucous Secretion

The Chaetopteridae are characterized by a unique feeding mechanism, where water is strained through a mucous bag secreted by mid-body notopodia (Barnes, 1965). This method allows the worm to obtain fine food particles trapped in the mucous (Barnes, 1965). The mucous bag is secreted in the mid-body region (see Figure 6), and produces a food ball from obtained particles (Wilson, 2000). The food ball is then carried to the mouth along the mid-dorsal ciliary groove; the entire process usually takes between 30 seconds and 2 minutes (Wilson, 2000). Phyllochaetopterus worms may form as many as eight mucous bags at one time (Barnes, 1965; Britayev & Martin, 2016).

Chaetopterids also use mucous string feeding when there are high concentrations of suspended particles; the mucous string is spun by the notopodial rings, and forms a twisting strand in the water current (Barnes, 1965). Mucous strings are then rolled up by the foliaceous notopodia and brought to the mouth (Wilson, 2000; Barnes,1965). When removed from their tubes, P. cf. verrilli specimens were found to produce large quantities of mucous (see Figure 8).

Chaetae

Chaetae are found on the majority of body segments in chaetopterids (Merz & Woodin, 2006). Capillary chaetae are simple, tapering pointed chaetae that are likely to have a major role in mechano-reception; with the free ends of capillary chaetae exposed to water movements in epifaunal worms, which may transmit information about their surroundings to the body of the worm (Merz & Woodin, 2006). Capillary chaetae may also assist with movement and stabilization of body segments, especially during irrigation of tubes (Merz & Woodin, 2006). Hooked chaetae are found in tube-dwelling worms that need to anchor; including multidentate hooked uncini, which may occur singly, in bunches or in rows and may be deeply embedded in the body (Merz & Woodin, 2006). Hooks are positioned to contact the tube wall and prevent removal from the tube by water movements or predators (Merz & Woodin, 2006).

All chaetopterids possess uniquely modified chaetae bristles on the fourth notopodia, used for cutting the tube wall or removing partitions to allow modifications, extensions and branching of the tube (Barnes, 1965; Merz & Woodin, 2006). Phyllochaetopterus may have a single pair of cutting spines (Rouse & Pleijel, 2001). In P. cf. verrilli the fourth notopodia can be clearly seen due to their dark colouration (see Figure 5).

The Chaetopteridae are widely distributed, with most species inhabiting shallow shelf depths (Nishi et al. 2009). Some species of Phyllochaetopterus have been found in abyssal hydrothermal vent sites (Britayev & Martin, 2016). P. cf. verrilli specimens were collected from pieces of rubble found in shallow depths from the Moreton Bay region. 

Many species are solitary; however some chaetopterids form dense aggregates (Britayev & Martin, 2016). Populations can reach high densities in shallow waters, with their parchment tubes forming persistent structures (Wilson, 2000). Chaetopterids may act as hosts for symbiotic organisms and facilitate the growth of aerobic organisms, with some bacteria found to cluster around the tubes (Britayev & Martin, 2016). Parchment tube worms have also been known to provide sediment trapping, sediment stabilization and bioturbation benefits within marine ecosystems (Wilson, 2000); although as an epifaunal worm P. cf. verrilli is likely to have a smaller role than infaunal chaetopterids.

Symbiotic associations are known for many chaetopterid species; with their tubes providing shelter, continuous water flow and accessible food (Britayev & Martin, 2016). In large species the tubes may be inhabited by decapods, scaleworms and carapid fish (Britayev & Martin, 2016), however as a very small chaetopterid, symbionts of P. cf. verrilli are likely to be predominantly scaleworms or other minute marine annelids.

In some Phyllochaetopterus species up to four worms may share a single (often branched) tube, constructing partitions to separate themselves (Barnes, 1965). P. cf. verrilli tubes were observed with one occupant only. 

Tube Construction

Unlike the majority of the Chaetopteridae, P. cf. verrilli is epifaunal; attaching its straight or irregular U-shaped tube to submerged substrata rather than living within sediments. The tube is made by a thin inner mucous, externally coated with sand and shell debris (see Figure 9). While some species of Phyllochaetopterus construct branched tubes with internal partitions (Barnes, 1965), all P. cf. verrilli worms were found inhabiting straight or slightly curved tubes without branching.

The tube is secreted by a thick glandular epidermis, which covers the rounded ventral surface of the anterior body region (Barnes, 1965). In many chaetopterids the ventral lip collects sand grains and molds the outer sand layer during tube construction, sometimes aided by the palps (Barnes, 1965). P. cf. verrilli specimens frequently used their palps for collecting and positioning sand and shell debris during tube construction or modification (see Video 1 and 2 below). 

In the laboratory setting I removed several P. cf. verrilli worms by consistently tapping on and breaking up small parts of the sandy tube at one end. The worm would then gradually come out of the opposite end, secreting a large amount of mucous. Worms that had been removed from their tube were seen to gather available sand grains using their palps, and would presumably construct another tube – however they were not observed for a long enough period of time to see an entirely new tube being formed.

As a sedentary tubicolous annelid, P. cf. verrilli is dependent upon a water current moving through the tube, bringing oxygen and food, while removing waste (Barnes, 1965). Like other species within the Phyllochaetopterus genus, P. cf. verrilli generates the water current by beating membranelles bordering the foliaceous notopodia of the mid-body region (Barnes,1965).

Predator Avoidance

Chaetopterids are likely to be an important food source for fish and crustaceans, however their trophic position has not been well studied and is somewhat obscure (Britayev & Martin, 2016). The tube that P. cf. verrilli inhabits is vital for avoiding predators; with the worm only leaving its tube when forcefully removed, or when the tube is partially or wholly destroyed.

Among the Chaetopteridae, some species within the Chateropterus and Mesochaetopterus genera exhibit bioluminescence, producing a bright blue luminescent mucous from certain notopodia and parapodial flaps (Britayev & Marin, 2016). The blue colouration is unusual for a shallow-water benthic invertebrate, and its purpose is debated (Britayev & Martin, 2016). Bioluminescence in chaetopterids may be used to attract small prey, to deter other animals from colonizing the tube, or act as defense mechanism against predators (Britayev & Martin, 2016). When disturbed, Chaetopterus specimens have been found to emit a bioluminescent mucous cloud from one end of their tube, and will move to the opposite end – this may confuse potential predators or act as a diversion (Britayev & Martin, 2016). Bioluminesce is not known to occur in the Phyllochaetopterus genus, and was not observed in P. cf. verrilli specimens.

Reproduction & Development

Chaetopterids have separate sexes and fertilization is external, with fertilization taking place when gametes are released into the water column (Britayev & Martin, 2016). In sexually mature chaetopterids the posterior region contains the gametes (Wilson, 2000). The parapodia of females contain yellow coiled ovaries and eggs, while males have white parapodia (Wilson, 2000; Rouse & Pleijel, 2001).

Larvae

Larvae are up to 1mm in diameter and spherical, with barrel-shaped or globular compact bodies (Bhaud & Fernandez-Alamo, 2000). Chaetopterid larvae are planktotrophs, with a pelagic life lasting over several months (Bhaud & Fernandez-Alamo, 2000; Wilson, 2000). Larvae become active filter feeders even while some yolk remains, and may feed on planktonic algae and small crustaceans; including prey up to half their size (Britayev & Martin, 2016).

In larval form the modified chaetae of chaetiger 4 are identifiable, and the body is already divided into three distinct regions (Wilson, 2000; Bhaud & Fernandez-Alamo, 2000). The anterior region exhibits the prostomium and peristomium, terminal mouth, short palps and usually one to three pairs of eyes (Bhaud & Fernandez-Alamo, 2000). The middle region shows one or two ciliated rings, and the posterior is tapered with a cylindrical pygidium (Bhaud & Fernandez-Alamo, 2000). Larvae can also be identified based on two morphological types of the neuropodial uncini found in the middle and posterior regions (Bhaud & Fernandez-Alamo, 2000). In Phyllochaetopterus larvae it can be difficult to identify the characteristic antennae, depending on the stage of development (Bhaud & Fernandez-Alamo, 2000).    

During development the posterior region becomes more elongated, possibly facilitating metamorphosis and settlement (Britayev & Martin, 2016). Larvae have two or three ciliary bands, sensory cilia along the body and fine cilliation at the oral opening (Britayev & Martin, 2016). Planktonic larval phases are shown in Figure 10.

Settlement

Settlement occurs when the larvae become large bodied; they use mucous parachutes to reach bottom sediments and attach by the holdfast – an adhesive area in the anal region (Britayev & Martin, 2016). Juveniles quickly build mucous-coated tunnels and begin to construct their tube (Britayev & Martin, 2016). In laboratory conditions some recently-settled Chaetopterus larvae have been found to revert from long-bodied forms back to oval planktonic forms if conditions are not adequate for settlement (Britayev & Martin, 2016). This process may be repeated several times, and could possibly allow a secondary return to the plankton; facilitating higher dispersal, greater possibilities of settlement success and colonisation, and also higher predation risks (Britayev & Martin, 2016). It is unknown if P. cf. verrilli possesses this reversion ability.

Asexual Reproduction & Regeneration

Asexual reproduction has been described in Phyllochaetopterus through architomy; where the organism splits into fragments and regenerates missing segments (Rouse & Pleijel, 2001). In some cases up to six individuals have been found in a single tube as a result of architomous splitting; this may allow populations to reach large densities (Rouse & Pleijel, 2001).

Asexual reproduction in chaetopterids may be related to the number of segments in the mid-body region, with the worm dividing once segments reach a certain number, and then regenerating the missing parts of each section (Rouse & Pleijel, 2001; Bhaud, 1998). Researchers have found that fragments containing mid-body segments may fully regenerate the remaining regions (Britayev & Martin, 2016). It was also noted that regeneration of the anterior region is faster than regeneration of the posterior region (Britayev & Martin, 2016). In some cases regeneration may be a result of accidental breaking or non-lethal predation rather than asexual reproduction (Britayev & Martin, 2016). 

Regeneration & Tube Building - Mini Experiment

Introduction:

Within the Annelids many genera are capable of extensive regeneration; including a complete head, complete tail, or regeneration of both head and tail from a small fragment (Bely, 2014). The earliest steps of regeneration include wound healing and blastema formation (Bely, 2014). Once a transverse division has occurred, most annelids can seal the body by rapid muscular contraction, which stems the loss of fluids and possibly offers a safeguard against infection (Bely, 2014). Different types of cells migrate towards the wound and persist for at least the first day after wounding has occurred; these cells phagocytize damaged tissue, form a tissue plug at the wound and contribute to regeneration (Bely, 2014). Severed edges of the outer epidermis fuse, gut epithelium fuses, and the mouth or anus forms secondarily (Bely, 2014). The nerve cord is important for initiating and sustaining early regeneration, and in chaetopterids posterior regeneration has been found to be possible (and even stimulated) by removing the brain (Bely, 2014).

This experiment aimed to examine (1) if both the anterior and posterior regions of P. cf verrilli could regenerate; (2) if both the anterior and posterior regions could construct a new tube.

Methods:

A single specimen of P. cf verrilli was measured in centimetres using a ruler, then divided approximately half way along the mid-body using a small pair of dissection scissors (see Figure 11). The two sections were labelled as “A/B” (anterior/mid-body) and “B/C” (mid-body/posterior). Each section was placed in a separate glass container filled with a small quantity of sand. Glass containers were then placed in an aquarium filled with salt water for a period of one week. After one week, the A/B and B/C sections were removed from their containers, and examined under a dissecting microscope to determine if regeneration and/or tube building had occurred. 

Results:

During initial observation, section A/B was found to have produced mucous and become covered in sand grains within 20 minutes; presumably this was the beginning of tube construction. Section B/C showed less movement and remained uncovered on top of the sand.  

One week after division, section A/B was encased in a fully completed tube, partially buried in the sand (see Figure 12). Section B/C did not have a tube, and was still located on top of the sand. Once removed from its tube, section A/B moved using its notopodia and palps (see Video 3 below), while section B/C appeared to move using its posterior foliaceous notopodia along with periodic contractions of the muscles and gut (see Figure 13). Advanced regeneration did not appear to have occurred within one week; with no clear growth segments observed; however both sections appeared to have evidence of wound healing and were still active.  

Discussion:

P. cf. verrilli can produce mucous from both the anterior and posterior regions; however after division had occurred only section A/B had constructed a new tube. This is presumably due to the critical role of the palps in collecting and positioning sand grains. Section A/B also contained sensory organs and structures which would have allowed the segment to obtain information about its environment, and chaetae-bearing notopodia allowing it to position itself accordingly. 

Section B/C showed less activity than section A/B; although it was mobile the B/C section was observed to remain in approximately the same position within the container as it was originally placed. Section B/C did not construct a tube or move into the sand, and remained on the surface. This is most likely due to the absence of palps and chaetae-bearing notopodia to facilitate more complex movements; without these structures section B/C was limited to simple segment contractions.    

Each section had evidence of wound healing and was able to move continuously. Neither section displayed signs of new growth, which would be identifiable by smaller size in relation to the intact segments (Britayev & Martin, 2016). The results of this experiment indicate that a period of one week is not sufficient for advanced growth to occur; full regeneration is likely to require several weeks to complete. Researchers have noted that Phyllochaetopterus worms will regenerate a missing anterior region faster than a missing posterior region (Britayev & Martin, 2016); however as posterior segments are unable to construct a protective tube, these segments will be more vulnerable to predation than anterior segments, and therefore may be less likely to achieve full regeneration.

Chaetopterids possess a terminal mouth and a simple, non-eversible pharynx (Britayev & Martin, 2016). The gut is straight, and may pouch within the mid-body region (Britayev & Martin, 2016). The circulatory system is closed, with the heart located at the start of the mid-body, and a ventral vessel extending along the entire body (Britayev & Martin, 2016). In most marine annelids, blood flows along the dorsal vessel and is pumped around the head, and then flows towards the pygidium (Rouse & Pleijel, 2001). 

Many marine annelid have elaborate branchiae, which increase the surface area for gaseous exchange (Rouse & Pleijel, 2001). In tube dwelling worms, branchiae are often regionalised where there is maximum water flow (Rouse & Pleijel, 2001). 

Tube dwelling annelids usually possess statocysts; which are small gravity receptors allowing the worm to orient itself (Rouse & Pleijel, 2001). Statocysts may be a single pair, or more than 20 segmentally arranged pairs, and are always located dorsally on the anterior region (Rouse & Pleijel, 2001). Small granules in the statocyst (either sand grains called 'statoconia' or hard secretions called 'statoliths') fall against receptor cells and provide stimuli to facilitate positioning and balance (Rouse & Pleijel, 2001).

The nervous system in the genus Chaetopterus includes two dorso-lateral cerebral ganglia (Britayev & Martin, 2016); this is presumed to be similar in P. cf. verrilli. The nerve cords converge at the first segment of the mid-body, forming a nerve cord with segmental, repetitive ganglia (Britayev & Martin, 2016). Segmental organs in the posterior region are used to expel the gametes (Britayev & Martin, 2016; Rouse & Pleijel, 2001).

The body can be categorized according to the parapodia, with the anterior body region containing notopodia only; the mid-body containing prominent achaetous notopodia, neuropodia and biramos parapodia; and the posterior segments containing less prominent parapodia (Wilson, 2000). The chaetae are lancelet-shaped, with uncini (chitinous hooks) in the median and posterior neuropodia (Wilson, 2000).

​Tissue Section

One P. cf. verrilli specimen was sectioned longitudinally, the H & E stained tissue is shown in Figure 14. Sections were examined using a compound microscope, at a magnification of 4X/0.10. The section shown is at a slightly oblique angle; a photo of a living P. cf verrilli specimen in a similar position has been placed alongside the section for reference. The section has been labelled with reference to Freeman & Bracegirdle (1971), with suggestions where interpretation of anatomy is uncertain. 

The Chaetopteridae clade is globally distributed and found from intertidal zones to abyssal depths (Moore et al. 2017). Most species live in infaunal tubes, however P. cf. verrilli is one of a small number that attach their tubes to hard substrates (Rouse & Pleijel, 2001). Morphological variations (such as chaetal hard structures) have been observed across different geographical regions in chaetopterids, leading to both classification difficulties and the identification of new species (Bhaud et al. 2006).

Once thought to be cosmopolitan, the Chaetopteridae are now known to have higher biodiversity than originally believed, with morphological variations showing geographical divergences (Bhaud et al. 2006). Phyllochaetopterus species have been found around the world, including parts of Africa, Britain, Hawaii, Italy, Japan and the Pacific (Rouse & Pleijel, 2001).

The fossil record of marine annelid worms mainly features chaetae, jaws, and tubes, feeding traces or burrows; with rare cases of exceptional preservation showing detailed whole-body fossils (Merz & Woodin, 2006). The earliest undisputed examples of marine annelids bearing chaetae are from the Cambrian, (543 – 490 mya), with recognizable species belonging to modern families found in the Carboniferous (354 – 290 mya) (Merz & Woodin, 2006). Tube dwelling lifestyles have persisted for many millions of years; however tube dwelling is likely to be convergent and to have arisen multiple times within the marine annelids (Merz & Woodin, 2006). There are no recorded fossils of chaetopterid worms (Rouse & Pleijel, 2001; Britayev & Martin, 2016). 

Phylogenetic studies have identified the Chaetopteridae as an early-diverging annelid lineage; however this is debated, with the placement of the clade unstable (Moore et al. 2017). Within the Chaetopteridae, the genera Phyllochaetopterus and Spiochaetopterus are paraphyletic, and form a sister clade to Chaetopterus and Mesochaetopterus (Moore et al. 2017).