In general, Syllidae species are small and slender (Beesley, Ross & Glasby 2000). On average, Syllidae are less than 10 mm long and 1 mm wide (Beesley, Ross & Glasby 2000). The specimen falls within this typical range (Fig. 1). Syllidae’s small size enables it to move into interstitial spaces and crevices (Martins et al. 2013). The members of the genus Syllis have cylindrical bodies (Lucas, Sikorski & San Martín 2018). Many Syllidae species are colourful due to the pigmentation of their gut, epidermis,and oocytes (Beesley, Ross & Glasby 2000). The specimen is pale orange, but its middle segments are dark yellow (Fig. 2A). Moreover, there is a dark brown marking in the middle of the specimen’s body (Fig. 2B). The dark yellow and brown are likely due to the contents of the specimen’s gut or oocytes. The specimen also has a black epitoke and blue spots on its dorsal cirri (Fig. 2A; Fig. 3). There are black lines across the specimen’s body (Fig. 2B). The epidermis of Syllidae may be smooth or papillated (Beesley, Ross & Glasby 2000). The specimen has a smooth epidermis.

 

Syllidae have a rounded prostomium which is usually wider than long (Beesley, Ross & Glasby 2000). The prostomium of the specimen has eyes, lateral antennae, palps, and nuchal organs (Fig. 3). 

The specimen has two pairs of lensed eyes which are arranged in a trapezium (Fig. 3). The anterior pair of eyes is larger than the posterior pair. The lenses of the eyes would have developed from the extension of supporting cells (Glasby 1993). Syllidae also often have a smaller anterior pair of ocelli, however, this is not apparent in the specimen.

A pair of lateral antennae, a median antenna and palps are a defining feature for Syllidae (Beesley, Ross & Glasby 2000). Syllinae have separate palps and Syllis have their palps fused at the base, which can be seen in the specimen (Fig. 3; Beesley, Ross & Glasby 2000; Lucas, Sikorski, San Martín 2018). The specimen’s palps are longer than its prostomium (Fig. 3). The specimen may utilise its palps as one of its methods of feeding. Palps can be used to sort organic material from sediment (Beesley, Ross & Glasby 2000).

The nuchal organs in the specimen are not prominent (Fig. 3). Nuchal organs are a defining feature for Syllidae and the indistinctness of the organs is a defining trait for Syllinae (Beesley, Ross & Glasby 2000).

The segments of the specimen have appendages. These appendages include dorsal cirri, ventral cirri and neuropodia with protruding chaetae (Fig. 4). 

The specimen’s dorsal cirri, also referred to as tentacular cirri, are long and appear on each segment (Fig. 5). They are one cell thick and have blue spots on them (Fig. 3; Fig. 4). One of the defining features of Syllinae is their long and articulated dorsal cirri (Beesley, Ross & Glasby 2000).

As is common for Syllidae, the ventral cirri are below the neuropodia, closer to the ventral side of the specimen (Beesley, Ross & Glasby 2000). The specimen’s ventral cirri are smaller than the dorsal cirri and do not have spots on them (Fig. 5).

The muscles of Typosyllis antoni are arranged in a similar way in the antennae, dorsal cirri, and ventral cirri (Aguado et al. 2014). These appendages all have inner longitudinal muscle fibres that run from the base to the end (Aguado et al. 2014) However, unlike the other appendages, the base of the dorsal cirri has cirral muscle bundles with a basal muscular socket (Aguado et al. 2014). Typosyllis antoni shares traits such as chaetae shape with Syllis parturiens (Aguado et al. 2014). Due to the shared traits of these species and Typosyllis being closely related to Syllis, it is possible that the specimen has a similar muscular arrangement in these appendages.

The class Polychaeta derives its name from chaetae (Merz & Woodin 2006). The chaetae are in the ventral neuropodia. The chaetae play a key role in the identification of Syllis species (San Martín & Wosfold 2015). In general, Syllis have falciger compound chaetae and thick simple chaetae (Passos Ribeiro et al. 2020). The specimen has prominent compound falciger chaetae which can be seen by the dentate blades (Fig 5). These chaetae, which have hooked tips, are usually found in polychaetes that need to anchor themselves (Merz & Woodin 2006). This includes polychaetes that live in tubes or are symbiotic or interstitial (Merz & Woodin 2006). Syllis are interstitial and some live-in association with other organisms. This may be the reason why the specimen requires these chaetae. Although the anterior and posterior have the same type of chaetae, there are some differences between them. The anterior chaetae have longer falcigers than the posterior chaetae (Fig. 5). Moreover, the end of the posterior chaetae is more hooked than that of the anterior (Fig. 5).

Other key physical features in the specimen’s segmentation include its musculature, coelomic cavities, integument, and its lack of notopodia.

The main muscular elements of polychaetes are the circular and longitudinal fibres in the body wall, as well as parapodial, chaetal, oblique, diagonal and dorsoventral fibres (Tzetlin & Filippova 2005). Polychaetes also have muscular aspects associated with their septa and mesenteries (Tzetlin & Filippova 2005). Studies conducted on the species Syllis gracilis found that its dorsal muscle band had at least 10 muscle bundles next to each other and two ventral bands (Parapar et al. 2019). Moreover, it had well developed parapodial muscular fibres, a few muscles associated with the parapodial wall and muscles attached to the aciculae and chaetae (Parapar et al. 2019). The muscular features of Syllis gracilis have also been described in other Syllidae species (Parapar et al. 2019). This suggests that these muscular elements are common in Syllidae and so the specimen is likely to have these features.

Syllidae have fluid-filled coelomic spaces between their body wall and gut wall (Parapar et al. 2019).  The coelomic peritoneum defines the dorsal and ventral mesenteries, however it does not enclose the foregut which is involved in the movement of the eversible pharynx (Parapar et al. 2019). The species Syllis gracilis has a wide coelomic cavity which is divided by septa (Parapar et al. 2019). The specimen is likely to share the same feature since it is in the genus Syllis.

The integument is the tough outer protective layer of an organism. The integument of polychaetes has one epithelium layer which has multiple cell types (Storch 1968). The epithelium layer is on top of a thin fibrous extracellular layer (Storch 1968). The integument of the antennae and cirri of Syllidae are distinctive to other polychaetes (Parapar et al. 2019). The antennae and cirri have a central core of musculature and nervous tissue which is surround by different types of integumental cells (Parapar et al. 2019). An example of one of the types of integumental cells is the gland cells which are a long and slender corkscrew shape (Parapar et al. 2019). The function of these integumental glands, which also occur on the dorsal and ventral surfaces of Syllidae, is not known (Beesley, Ross & Glasby 2000).

The specimen is lacking notopodia which is arguably an autapomorphy of Syllidae (Glasby 1993).

In many species of the genus Syllis, the pharynx starts in the third chaetiger which is posterior to the peristomium (Fig. 6B; San Martín & Lopez 2003). The complex Syllidae digestive track consists of four parts: a buccal cavity, pharyngeal tube, proventricle and ventricle (Fig. 6A; Tzetlin & Purschke 2005). 

The buccal cavity is short and leads to the longer pharyngeal tube (Beesley, Ross & Glasby 2000).

A defining feature for the subfamily Syllinae is the straight pharyngeal tube (Fig. 6A; Beesley, Ross & Glasby 2000). In Syllidae, the pharyngeal tube is the non-muscular part of the pharynx and often has a circle of papillae with sensory cells at its opening (Tzetlin & Purschke 2005). In Syllis, this is then followed by a mid-dorsal tooth which does not have a trepan (Lucas, Sikorski & San Martín 2018; Tzetlin & Purschke 2005). Syllidae teeth are often made of a cuticle or sclerotised cuticle (Beesley, Ross & Glasby 2000). The pharyngeal tube leads to the proventricle.

The proventricle is the muscular part of the pharynx and can clearly be seen through the specimen’s body (Beesely, Ross & Glasby 2000; Fig. 6). It has often been considered as a synapomorphy of Syllidae (Teresa, San Martín & Siddal 2012). However, Sphaerodoridae, Naurilinielidae and Pilargidae all have a similar structure (Aguado, Nygren & Siddall 2007). These structures may have formed due to convergent evolution or have originated in a distant ancestor (Aguado, Nygren & Siddal 2007). In any event, Syllidae’s proventricle should not be considered as a synapomorphy (Aguado, Nygren & Siddall 2007). However, Syllidae’s proventricle is one of its defining characteristics (Glasby 1993). The proventricle can vary greatly in length among species (Haswell 1921). It is cylindrical and has slight lateral compression (Haswell 1921). It is made up of striated muscles cells with sarcomeres and calcium concretions (Weidhase et al. 2016). At the proventricle’s anterior, there are chitinous plates which are thought to be homologous to the sick-shaped jaws of Nereididae due to their position (Glasby 1993). The wall of the proventricle has 6 layers: the thin splanchnic layer of the coelomic epithelium, the thin outer fibrous membrane, the main layer composing of radial muscle-columns and annular muscle bands, the inner fibrous membrane, the enteric epithelium and the cuticle (Haswell 1921). As shown in figure 6A, the proventricle has ring-shaped fine lines which are non-striated muscles fibres and dots which are the outer ends of the cores of the radial columns of cross-straited muscle (Haswell 1921; Tzetlin & Purschke 2005).

One role that the proventricle plays is that it can act as a suctorial pump for feeding (Teresa, San Martín & Siddal 2012). It can also play a role in regeneration and the reproductive process (Teresa, San Martín & Siddal 2012). In Syllinae, the proventricle and prostomium work together to facilitate reproduction (Franke 1999). The proventricle releases a hormone that inhibits sexual development and maintains the growth of segments and somatic cells (Franke 1999). This was determined by removing the proventricle in a non-reproductive Syllinae which induced schizogamy and then reimplanting the proventricle which stopped the schizogamy process (Franke 1999). The prostomium hormone stimulates sexual development by inhibiting the release of the hormone from the proventricle (Franke 1999). Moreover, it is thought that if the hormone is above a certain level at the time of sex determination, the gametes will be female (Heacox & Schroeder 1982). A study on Typosyllis prolifera also found that the hormone released from the proventricle is cyclic and high levels of it at the start of the schizogamy period can assist with regeneration after the stolon detaches (Heacox & Schroeder 1982). Another study that involved the removal of the proventricle from Typosyllis antoni found that it caused schizogamy to be accelerated, reduced the posterior segment regeneration and resulted in poor quality stolons (Weidhase et al. 2016). This supports the idea that the proventricle plays a key role in reproduction and subsequent regeneration. However, since the proventricle does not have any glandular tissue, it is unclear how it could have direct involvement in these processes (Weidhase et al. 2016).

The ventricle of Syllidae has thin walls and is anterior to a ciliated epithelium (Beesley, Ross & Glasby 2000). The ventricle of Syllidae sometimes has paired water filled caeca which are thought to control buoyancy (Tzetlin & Purschke 2005; Beesley, Ross & Glasby 2000). The ventricle leads into the intestine and is often viewed as being a part of it (Tzetlin & Purschke 2005).

The specimen's epitoke formed at the posterior of its body and has chaetae (Fig. 7.) The chaetae appear to be capillary chaetae which are for locomotion, stabilisation and sensing the environment (Merz & Woodin 2006). The epitoke is involved in the process of schizogamy which is a form of sexual reproduction (Teresa, San Martín & Siddal 2012). This means that the specimen is an adult as it can reproduce. The epitoke separates from the adult after it has undergone morphological changes (Weidhase et al. 2017). After the epitoke separates, the specimen will regenerate its posterior end (Weidhase et al. 2017). In Syllinae, the epitoke may develop a head appendage before or after it separates (Beesley, Ross & Glasby 2000). Once it has separated, it becomes a stolon that will actively swim to the open sea to spawn (Teresa, San Martín & Siddal 2012). After spawning, the stolon will die (Beesley, Ross & Glasby 2000).

The specimen appears to be similar to the species Syllis maganda, however there are several differences. They both have palps that are longer than the prostomium and 4 eyes in a trapezoidal arrangement (José Martínes & San Martín 2020). S. maganda is bright orange and blue with red lines across its body and red spots on it dorsal cirri (José Martínes & San Martín 2020). The specimen is also orange, however, is not as brightly coloured. The specimen has black lines and blue spots, rather than red lines and spots. S. magnada has thin ventral cirri that are shorter than the parapodial lobes (José Martínes & San Martín 2020). Comparatively, the specimen’s ventral cirri appear to be slightly longer than its parapodial lobes (Fig. 4). The specimen has a shorter pharynx and shorter dorsal cirri but longer proventricle than S. maganda (P Álvarez-Campos 2021, pers. comm., 22 May). Moreover, the specimen’s anterior and posterior chaetae have shorter blades and its posterior chaetae also have thinner shafts than those of S. maganda (Fig. 6; Fig. 8; P Álvarez-Campos 2021, pers. comm., 22 May). Given the differences to S. maganda, the specimen is likely to be a new species (P Álvarez-Campos 2021, pers. comm., 22 May).

Syllidae are common in Australian waters and usually dominant marine habitats in terms of number of species and individuals (Álvarez-Campos et al. 2015). The success of Syllidae is likely due to their wide range of body sizes, feeding mechanisms and reproductive strategies (Serran, San Martín & López 2006). The genus Syllis is the largest and most diverse genus in Syllidae (Álvarez-Campos et al. 2015). Syllis has more than 120 species globally and 30 species in Australia (Álvarez-Campos et al. 2015). The abundance of Syllis species depends on the type of environment. For instance, Syllis armillaris and Syllis variegata dominate subtidal areas, whereas, Syllis gracilis dominates intertidal and shallow subtidal areas as it is suited to less structured environments (Serrano, San Martín & López 2006).

Syllidae are found in areas with soft substrates where they generally have an interstitial life (Granados-Barba et al. 2003; San Martín & Worsfold 2015). They are also found in areas with hard substrates where they have an errant life and live among organisms such as algae, as well as biogenic structures (Granados-Barba et al. 2003; San Martín & Worsfold 2015). Syllidae are especially abundant in shallow marine habitats but are also found in deeper waters (San Martín & Worsfold 2015). They are not found in fresh water and do not play an important role in estuaries (San Martín & Worsfold 2015). Syllidae are also found in cryptic environments, such as sponges and dead corals, where they generally have the highest species richness (Granados-Barba et al. 2003). For example, Syllis mayeri is associated with the sponge Ircinia strobilina (Musco & Giangrande 2005). Additionally, Syllidae are found in biofouling communities on artificial substrates (Diaz-Castaneda 2000). The presence of seaweed and other invertebrates may assist with the colonisation of Syllidae because they prefer holes and crevices (Ba-Akdah 2018). However, studies have found that Syllidae are one of the dominant groups in the early stages of colonisation which suggests that they do not need the assistance of other organisms to colonise an area (Diaz-Castaneda 2000). Another study found that Syllis species were some of the first to colonise an artificial area (Ba-Akdah 2018). The ability of Syllis species to colonise artificial substrates explains why the specimen was present on the ARMS plate that was hung off One Mile Jetty in Dunwich, North Stradbroke Island from the 27^th of January to the 24^th of March.

Most species in Syllinae are carnivorous (Franke 1999). There is a lack of consensus on whether most Syllidae are generalist or specialist feeders (Martins et al. 2013; Franke 1999). The argument for Syllidae being generalist feeders is that they are active and live amongst a wide range of organisms (Martins et al. 2013). This means that Syllidae have many different preys available to them and they can move around to obtain their food. An example of a generalist Syllis species is Syllis gracilis (Serrano, San Martín & López 2006). It is likely that Syllidae feed on corals, hydrozoans, bryozoans, sponges, and ascidians (Franke 1999; Beesley, Ross & Glasby 2000).

Syllis feed through the use a few structures. First, the pharyngeal tooth is used to pierce prey (Beesley, Ross & Glasby 2000). It has been suggested that the pharyngeal tooth also has a basal poison gland (Beesley, Ross & Glasby 2000). Secondly, the proventricle acts as a pump to suck up the contents of the prey (Haswell 1921; Beesley, Ross & Glasby 2000) Thirdly, the chitinous plates in the proventricle may facilitate feeding by mechanically digesting food (Haswell 1921).

Given the abundance of Syllis species, they play an important role in the environment. Syllidae live in association with many organisms and have relationships that range from symbiotic to parasitic (López et al. 2001). Symbiotic relationships benefit both parties, whereas parasitic relationships negatively impact the host. Despite the negative implications for the host, parasitic relationships are still important as they can affect the food web and interactions in the environment. As established prior, Syllidae are good at colonising empty spaces, which means that they would facilitate the colonisation of other species (Diaz-Castaneda 2000). Syllidae are predated on by marine benthic organisms and provide food for pelagic organisms when their stolons migrate to the pelagic environment (Ertan Cinar 2003). Syllinae are also predators and so play a key role in population management (Ertan Cinar 2003).

Syllidae can reproduce sexually and asexually (Beesley, Ross & Glasby 2000). In general, species have separate sexes, however there are some Syllinae species that are hermaphrodites (Beesley, Ross & Glasby 2000).

Asexual reproduction is not very common in Syllidae because usually the posterior cannot replace the pharyngeal system (Franke 1999). Syllis gracilis is one of the few species that can completely regenerate and so is able to reproduce asexually (Franke 1999). Since Syllis gracilis is from the same genus as the specimen, it is possible that the specimen can asexually reproduce.

Sexual reproduction in Syllidae occurs by epitoky which involves the adult undergoing metamorphosis before spawning (Ribeiro, Bleidorn & Aguado 2018). There are two types of epitoky: epigamy and schizogamy, however Syllinae only undergo schizogamy (Fig 9; Ribeiro, Bleidorn & Aguado 2018; Teresa, San Martín & Siddal 2012). This is a very successful mode of reproduction in Syllinae (Franke 1999).

The process of schizogamy begins when the atokous form of Syllinae metamorphoses into an epitokous form (Fig 9(2)). The specimen is in this stage of its lifecycle as it has an epitoke (Fig. 7). The epitoke, which is in the posterior segments of the body, is detached as a stolon (Teresa, San Martín & Siddal 2012). The adult must then regenerate its posterior segments (Fig 9(7)). The stolons in Syllinae do not display sexual dimorphism, however, there is variation in stolon structure (Franke 1999). There are five types of stolons in Syllinae. First, acephalous where the stolon has no anterior appendages (Teresa, San Martín & Siddal 2012). Secondly, acerous where the stolon has two pairs of eyes (Teresa, San Martín & Siddal 2012). Thirdly, dicerous where the stolon has two pairs of eyes and a pair of unarticulated antennae (Teresa, San Martín & Siddal 2012). Fourthly, tetracerous where the stolon has two pairs of eyes and a pair of well-developed antennae (Teresa, San Martín & Siddal 2012). Fifthly, pentacerous where the stolon has two pairs of eyes, a pair of lateral articulated antennae and a central antenna (Teresa, San Martín & Siddal 2012). The specimen has an acephalous stolon as it does not have any eyes or appendages (Fig. 7). Moreover, Syllinae have sexual lability where individuals can produce stolons of different sexes in a subsequent schizogamy event (Franke 1999).

Once the stolon is detached, it actively swims to the pelagic environment (Fig 9(3); Teresa, San Martín & Siddal 2012). Broadcast spawning occurs when Syllinae stolons discharge eggs of about 70 μm and sperm (Fig 9(4); Franke 1999). After releasing eggs or sperm, the stolon dies (Fig 9(5)). The eggs then sink to the bottom and within 24-48 hours, the larvae hatch as trochophore (Franke 1999). Despite having small eggs, Syllinae larvae have a small pelagic phase which only lasts a few hours or days (Franke 1999). This is likely due to the fact that the epitoke already provides some dispersal in the pelagic environment and it is important that the larvae do not disperse too far as Syllinae only have a few suitable habitats (Franke 1999).The larvae then move into the metatrochophore stage where the larvae swim near the bottom and become completely benthic (Franke 1999). The larvae develop into juveniles and then adults (Fig 9(6)).

Another potential reproductive strategy that the specimen may employ is the one of Syllis rosea which involves the female stolon taking care of the offspring (Langeneck et al. 2020). The female stolon releases the eggs in a gelatinous cluster which eventually compacts into a cocoon and is attached to the middle-posterior chaetigers (Langeneck et al. 2020). The larvae remain associated with the female stolon until it dies 7 days later (Langeneck et al. 2020). Although this is not a common reproductive strategy, it occurs within the specimen's genus and so the specimen potentially uses this method.

The initiation of schizogamy has many potential causes. The external and internal factors that might cause the commencement of schizogamy are thought to target the proventricular and prostomial hormones (Franke 1999). The first potential trigger is the season. During summer, the prostomium goes through short recurring phases of high hormone release and as a response, the proventricular endocrine activity reduces (Franke 1999). This results in reproductive phases throughout summer. The specimen was collected from Dunwich on the 24^th of March. The temperature around the time the specimen was collected was above the temperature used in an experiment that found summer temperatures could trigger reproduction (Franke 1999). The specimen's reproduction could have been caused by it being summer. The second potential trigger is light. It has been found that in Typosyllis prolifera the photoperiod determines whether reproduction occurs and a temperature of around 13˚C is a mere prerequisite (Franke 1999). Experiments in Syllis amica also found that photoperiod was important in triggering reproduction and so this may be a trigger for the specimen too (Franke 1999). The third potential trigger is the phases of the moon, however there is no conclusive evidence in the genus Syllis to support this idea (Franke 1999). The fourth potential trigger is night-time as it has been found that swarming tends to occur in the evening for Syllis amica due to the tidal cycle and so this is also a potential trigger for the specimen (Franke 1999). This trigger is likely to be a strategy to reduce predation risk (Franke 1999). The fifth potential trigger is a pheromonal interaction between swarming males and females (Franke 1999). However, there has been no evidence of any Syllis species undergoing reproduction due to a pheromonal interaction and so this is unlikely to be a trigger for the specimen.

33 species of Syllidae live in close association with other marine invertebrates (López et al. 2001). The commensal Syllidae species are associated with sponges, cnidarians, sipunculids, echinoderms, bryozoans, and decapods (López et al. 2001). The parasitic Syllidae species are associated with sponges but can also be parasites of cnidarians, other polychaetes, nemerteans, and tunicates (López et al. 2001).

An example of Syllis species that live in association other invertebrates are Syllis cf. armillaris, Syllis ferrani, and Syllis pontxioi (López et al. 2001). They live inside gastropod shells which are occupied by hermit crabs and Pionosyllis magnicca (López et al. 2001). These Syllis species do not have any morphological adaptations to this mode of life and have been reported as free-living as well as living in association with colonial or reef-building invertebrates (López et al. 2001). More research is required to determine the nature of the association and whether these species have any behavioural adaptations (López et al. 2001).

Syllis species have also been found to host protozoans (Álvarez-Campos et al. 2014). Protozoans were found on the parapodial bases of Syllis magdalena and Syllis prolifera (Álvarez-Campos et al. 2014). They were also found on the dorsal surface, nuchal organs, mouth opening and anterior cirri of Syllis elongata (Álvarez-Campos et al. 2014)

Generally, Syllidae regenerate from the pygidium, prostomium or midbody segments (Ribero, Bleodorn & Aguado 2018). However, many Syllinae species demonstrate the ability to regenerate their anterior (Ribero, Bleodorn & Aguado 2018). This increased regenerative ability is linked to their use of schizogamy which has a regenerative aspect (Weidhase et al. 2017). The most common method of regeneration in Syllidae is an epimorphic process, however some species use morphallaxis (Ribero, Bleodorn & Aguado 2018). Epirmorphosis involves somatic cell activity, dedifferentiation, and re-differentiation (Ribero, Bleodorn & Aguado 2018). Dedifferentiation involves the formation of a blastema that contains undifferentiated cells and acts as a growth zone (Ribero, Bleodorn & Aguado 2018). Comparatively, morphallaxis does not involve blastema formation and so the body remodels itself so that the morphology matches the role of the new segment position (Ribero, Bleodorn & Aguado 2018). Regeneration time varies between species (Ribero, Bleodorn & Aguado 2018).

Syllis has a metanephridial system for excretion of waste products. As in Syllis gracilis, the specimen likely has metanephridia with metameric disposition (Parapar et al. 2019). Metanephridia occur in all body segments except for the most anterior and posterior ones (Goodrich 1945). The metanephridial duct opens externally via a ventral nephridiopore and internally into the coelom of the anterior segment by a nephrostome (Beesley, Ross & Glasby 2000). According to Goodrich (1945), in species with stolons, such as Syllis species, the metanephrida and gonoducts fuse to form a metanephromixia. However, Bartolomaeus (1999) does not think that that the metanephromixia is formed by the fusion of two types of tissues but rather from just one tissue. In any event, the metanephromixia is involved in excretion and the release of gametes (Parapar et al. 2019).

The general features of an Annelida central nervous system can be observed in Syllis gracilis and so are likely to be present in the specimen (Parapar et al. 2019). The Annelida central nervous system consists of an anterior prostomial brain, ventral nerve cord, segmentally arranged ganglia and circumpharyngeal nerves (Orrhage & Müller 2005). Capillary chaetae are also thought to play a role as mechano-receptors (Merz & Woodin 2006).

Polychaetes have a closed circulatory system which usually consists of capillary beds, gut lacuna, and medial, dorsal, and ventral longitudinal vessels that are linked by smaller vessels (Beesley, Ross & Glasby 2000). The blood flows towards the posterior in the ventral vessel which is below the gut and has branches that link to the body wall muscles and epidermis (Beesley, Ross & Glasby 2000). The blood then flows through the lateral vessel to the anteriorly flowing dorsal vessel, after which the blood goes around the gut and to the ventral vessel again (Beesley, Ross & Glasby 2000). Body wall movements and muscles that surround the dorsal blood vessel control blood flow (Beesley, Ross & Glasby 2000).

The circulatory system of Syllis gracilis is less complex than other polychaetes (Parapar et al. 2019). S. gracilis has two major dorsal and ventral blood vessels surrounded by coelomic peritoneum, as well as thin vessels that connect the two major longitudinal vessels (Parapar et al. 2019). Moreover, the circulatory system of S. gracilis appears to be less complex than Syllis garciai as it lacks a well-developed pair of longitudinal lateral vessels on either side of the gut (Parapar et al. 2019). This suggests that the circulatory systems in Syllis are likely to vary between species. Therefore, the exact nature of the circulatory system in the specimen cannot be determined without further examination. However, it is likely that the specimen has a simple circulatory system.

Due to the size of Syllidae, they generally do not possess respiratory organs (Purschke et al. 2017). Instead, it is likely that the specimen has respiratory pigments dissolved in its blood that transport oxygen to its tissues (Beesley, Ross & Glasby 2000). In general, polychaetes can draw 50-60% of oxygen from the water that touches their body (Beesley, Ross & Glasby 2000).

The digestive system of polychaetes includes a foregut, midgut, and hindgut (Tzetlin & Purschke 2005). Despite the metameric segmentation in Syllidae, its foregut is highly specialised (Haswell 1921). The foregut includes a pharyngeal sheath, pharyngeal tube, proventricle, ventricle, and lateral caeca (Haswell 1921). These features are described in the pharynx section. The midgut of polychaetes is adapted to the species’ needs and consists of a stomach and intestine (Tzetlin & Purschke 2005). It is likely that the intestine would not be longer than the specimen’s body and would not be coiled (Tzetlin & Purschke 2005). Septa and mesenteries attach the gut system to the body wall (Tzetlin & Purschke 2005). Further examination of the specimen would be required to find out about its midgut and hindgut.

The monophyly of Syllidae has been supported by the incorporation of new taxa and the secondary structure of 18S rDNA (Teresa, San Martín & Siddal 2012). Moreover, the subfamilies of Syllidae are all monophyletic, except for Eusyllinae (Aguado, Nygren & Siddall 2007).

The families Calamyzidae and Levidoridae are no longer valid and are now considered to be members of Syllidae (Beesley, Ross & Glasby 2000). Comparatively, Phyllodocida used to be considered a clade, however, is now classified as an order (Fauchald 1977).

Syllidae was suggested to be part of the superfamily Nereidoidea, however this was challenged by the fact that Nereidoidea may be paraphyletic (Beesley, Ross & Glasby 2000). It has been proposed that Syllidae is part of new superfamily (Aguado & Bleidorn 2010). This proposition stems from the idea that Syllidae evolved rapidly which led to the creation of two ancient evolutionary lines (Aguado & Bleidorn 2010). The first consists of Anoplosyllinae and is characterised by its chaetal and dorsum features (Aguado & Bleidorn 2010). The second includes the rest of the Syllidae and is characterised by an increased rate of evolution in the 18S molecule (Aguado & Bleidorn 2010). The superfamily, Sylloidea, and two families, Anoplosyllidae and Syllidae, have been proposed (Aguado & Bleidorn 2010).

Syllidae reproduction provides important phylogenetic information (Garwood 1991; Franke 1999). Epigamy is thought to be the primitive condition and schizogamy the derived condition (Teresa, San Martín & Siddall 2012). Schizogamy is hypothesised to have evolved independently in Syllinae and Autolytinae (Franke 1999).

A study was conducted to compare the muscular systems of Syllidae and Sphaerodoridae (Filippova et al. 2010). It was found that despite the families’ proventricles being similar, there were differences between the structures, including their fibre composition (Filippova et al. 2010). It was concluded that the families were not a sister group and that the proventricle is a homoplasy of the families (Filippova et al. 2010; Teresa, San Martín & Siddal 2012).

There is a lot of uncertainty surrounding the phylogenetic relationships of Syllis (Álvarez-Campos et al. 2015). The first reason for this is because Syllis has no clear synapomorphies and was traditionally thought of as a genus with a mixture of species that had no distinctive features (Álvarez-Campos et al. 2015). The second reason is that there is a lack of detail in the old descriptions of Syllis species, the old specimens have been lost and there is a lack of molecular data for many species (Álvarez-Campos et al. 2015).

In 1879, Langerhans divided Syllis into four subgenera: Typosyllis which have falcigerous chaetae, Haplosyllis which have thick simple chaetae, Ehlersia which have falcigerous and elongated compound chaetae, and Syllis which have pseudo-simple and falcigerous chaetae (Álvarez-Campos et al. 2015). Haplosyllis is now considered a distinct genus in Syllinae, however, the placement of the other groups of species is debatable (Álvarez-Campos et al. 2015).

Licher (1999) considered Typosyllis and Syllis to be separate genera (Álvarez-Campos et al. 2015). It was suggested that Typosyllis had 213 taxa, including the Ehlersia species (Álvarez-Campos et al. 2015). Licher (1999) thought that only species with pseudo-simple chaetae could be Syllis (Álvarez-Campos et al. 2015). However, Aguado et al. (2014) found that this analysis was inconsistent with the relationships between the species. This is because simple chaetae evolved several times by the fusion of shafts and blades, and other species also evolved simple chaetae by the loss of blades (Aguado et al. 2014). Moreover, San Martín (2003) thought that Syllis was the only valid genus (Álvarez-Campos et al. 2015).  Some of the Syllis and Typosyllis species in Australia were synonymised, sometimes due to error, which has created more confusion surrounding the classification of these species (Álvarez-Campos et al. 2015). One phylogenetic study suggested that Syllis and Typosyllis are paraphyletic, however, the results were inconclusive (Álvarez-Campos et al. 2015). If future research supported these results, it is likely that Typosyllis would become invalid (Álvarez-Campos et al. 2015).

To resolve the uncertainty surrounding Syllis and Typosyllis, more information about stolons, molecular data and sampling is required (Teresa, San Martín & Siddal 2012; Aguado et al. 2014). Until further clarification, the original classification of species as Syllis or Typosyllis may be used (Passos Ribeiro et al. 2010).

Kingdom: Animalia

Phylum: Annelida

Class: Polychaeta

Order: Phyllodocida

Family: Syllidae

Subfamily: Syllinae

Genus: Syllis

Thank you, Patricia Álvarez-Campos, Greg Rouse, and Guillermo San Martín for assisting me with the identification of the specimen and providing me with lots of useful information. Thank you, Pat Hutchings, for suggesting the use of POLiKEY for the identification of the family.

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