The Typosyllis genus generally abides by the normal polychaete body plan with a segmented body and a high number of segments most of which have parapodia. There are a few key morphological features such as the prostomium and the proventricle which define this family from other polychaetes (Rouse & Pleijel, 2006).

The prostomium of Typosyllis contains two pairs of eyes and three pairs of antennae composing of two pairs of lateral antennae and one pair of median antennae (see Figure 3). Along with the antennae there is also a pair of simple palps at the anterior end (Hutchings, 1984). Also utilised for sensory purposes is a soft tissue layer which contains one sometimes two layers of papillae (Rouse & Pleijel, 2006). At the posterior end, between the prostomium and the peristomium, are the nuchal organs which appear as two-ciliated grooves (Aguado & Martin, 2009; Rouse & Pleijel, 2006).

The first segment is the only segment that lacks setae (achaetous) and parapodia with dorsal and ventral cirri (Rouse & Pleijel, 2006). There are also two pairs of tentacular cirri present on the first segment (Hutchings, 1984; Rouse & Pleijel, 2006). Characteristic of the Syllinae subfamily are pores on the ventral cirri (Aguado & Martin, 2009). Starting at the second segment and continuing posteriorly through the rest of the body, uniramous parapodia is present with dorsal and ventral cirri situated on the notopodium (see Figure 1 and Figure 2) (Beesley et al., 2000; Rouse & Pleijel, 2006). These latter segments also have neurochaete present (Rouse & Pleijel, 2006).

The Syllidae family vary in length from a couple mm to a few cm. Overall they are very thin with their width not reaching more than a few mm and normally being 1mm or less. Typosyllis typically have more than 15 body segments (Beesley et al., 2000). 

One of the key features of the Syllidae family is the muscularised pharynx (Hutchings, 2000). This pharynx comes in the form of an axial proboscis with a variety of teeth like adaptations. Identification of individual genus and species within the Syllidae family often require dissection of this pharynx and examination of the tooth like structure (Rouse & Pleijel, 2006). Generally, most Syllidae including the Typosyllis genus have a singular tooth medio-dorsally located. The eversible pharynx is also lined with a chitinous layer (Rouse & Pleijel, 2006).

Typosyllis are well adapted to a variety of habitat types. They are normally free-living and hence have adapted to live on hard substrata, soft sediments, algae surfaces or even seagrass blades (Beesley et al., 2000; Hutchings, 1984; Rouse & Pleijel, 2006). Coral reefs and shelf depths are other locations they have habituated (Rouse & Pleijel, 2006). There have been some suggestions that species of Typosyllis are associated with specific hosts such as echinoderms, sponges and cnidarians (Rouse & Pleijel, 2006). Typosyllis like many other polychaetes, play a vital role in maintaining ecosystem services. In particular, estuarine ecosystems benefit greatly from polychaetes due to their ability to produce detritus (Hutchings, 1984). Initially, other larger organisms break down the leaves from mangroves and seagrass. Polychaetes then play a large part in breaking these leaves down further, producing detritus. This detritus is crucial in estuarine ecosystems as many other organisms feed on these smaller particles keeping the food chain of these ecosystems sustainable (Hutchings, 1984).

It is thought that most Syllidae species have adapted a carnivorous lifestyle due to their strongly muscularised proventricle. Additionally, they use their pharyngeal tooth to pierce their prey and then proceed to remove the contents using their proventricle (Hutchings, 1984; Rouse & Pleijel, 2006). However, there has been suggestions that some syllids may also feed on algae, sponges and cnidaria meaning they could also be detritiviorous (Hutchings, 1984). 

It is now known that Typosyllis larvae do not feed in the water colomun as there is no evidence of feeding larvae within the Syllidae family. Rouse & Pleijel (2001) suggested that this disproves evidence from Wilson (1991), who believed there were some species of Syllidae which had planktotrophic larvae. There is not enough research into this area either way to conclude if there are or are not planktotrophic larvae within the Syllidae family. 

The Syllidae family is renowned for their diversity in reproductive modes (Aguado et al., 2012; Rouse & Pleijel, 2006). All Syllids are classified as gonochoric though hermaphroditism has been seen in some species of Exogoninae and Syllinae (Rouse & Pleijel, 2006). Syllidae exhibit opportunistic sex change and there are multiple hypothesised triggers which could potentially start this process.

Before discussing the details of this ability to change sex, it first important to understand what little is known about the early syllidae life history. Syllidae larvae hatch from the egg capsule after parapodia have started to develop. During this young stage the larvae have three prostomial antennae, both pairs of eyes but have not yet developed palps or ventral cirri (Beesley et al., 2000). Within four to six weeks of fertilization metamorphosis is completed in most syllids, but the exact timing of fertilisation for the Typosyllis genus is largely unknown (Beesley et al., 2000). 

As like many polychaetes, syllids have the ability to leave their benthic habitat and swarm to the surface in order to reproduce (Rouse & Pleijel, 2006). To be capable of accomplishing such feats, morphological adaptations are clearly evident (Rouse & Pleijel, 2001). The ability to express these changes is known as epitoky. Reproduction of these species occurs in free water either by broadcast spawning or by a process whereby the male stolon swims around the female and entangles her in sperm mucus threads (Rouse & Pleijel, 2006).

Most syllids display some form of epitoky which can be divided into two forms: epigamy and schizogamy. These two variations are vastly different and hence imply that radical evolutionary change has occurred in both morphology and behaviour of polychaetes (Aguado et al., 2012).

Epigamy is one of two form of eptioky where the whole individual is transformed into the epitoke (Rouse & Pleijel, 2001). Some of the morphological adaptations evident when epigamy occurs include the enlargements of anterior appendages and eyes as well as notochaete in the mid to posterior segments of the body (Aguado et al., 2012; Rouse & Pleijel, 2006). Epigamy is common in many polychaetes and is evident in the Syllidae family (Rouse & Pleijel, 2006). 

The second form of epitoky is known as schizogamy which entails the posterior end of the worm forming what is known as a stolon before detaching from the body. These polychaetes like those that exhibit epigamy, also have modified mid-posterior body segments becoming reproductive units (stolon) (Aguado et al., 2012). These stolons develop their own eyes and other sensory appendages before detachment occurs. Once detached they swim to spawn in pelagic habitat while the adult (or original worm) remains in benthic habitat (Aguado et al., 2012). 

Schizogamy is characteristic for both the Syllinae and Autolytinae subfamilies within the Syllidae family (Musco et al., 2010). However, both of these subfamilies demonstrate different stolons. With the Autolytinae subfamily, the stolons have clear sexual dimorphism, emerge from the mid body regions and have three distinct regions (Aguado et al., 2012; Aguado & Martin, 2009). Alternatively, the Syllinae subfamilies’ stolons have no differentiation in sex, emerge from the posterior segments and are non-differentiated (Aguado et al., 2012; Aguado & Martin, 2009). In order to classify Syllinae into species, the appendages they develop on the anterior part of their body are often utilised (Aguado et al., 2012).

Within the Syllidae family, schizogamy is present in two forms. The first is scissiparity which is the normal form of schizogamy whereby the stolon is formed from existing segments (Rouse & Pleijel, 2001). The second form is known as gemmiparity and is only displayed in the Syllidae family. It involves the production of stolons from newly produced body segments that are grown specifically for the purpose of being a stolon (Rouse & Pleijel, 2001). With the Syllinae subfamily, and hence Typosyllis genus, it is the former mentioned scissiparity which is most commonly displayed (Rouse & Pleijel, 2001). 

One species of Typosyllis has been extensively studied in regard to the ability to change sex. Typosyllis prolifera are “stolonising free-spawners” with a planktonic larval recruitment stage before eventually settling in a sedentary habitat (Bartolomaeus & Purschke, 2005; Rouse & Pleijel, 2006).  Typosyllis prolifera individuals that are born as males stay male during their reproductive cycles. The individuals born as females may at one of their reproductive cycles undergo irreversible sex change (Bartolomaeus & Purschke, 2005; Rouse & Pleijel, 2006). Rouse & Pleijel (2001) researched the reasoning behind why the female sex is as easily altered as it is within Typosyllis prolifera. They suggested that the utilisation of external fertilisation may effect the level of efficiency of fertilisation unless there is a large quantity of sperm produced. Hence, in order to have enough sperm in the water column, many females need to change to males to produce the large amount required (Rouse & Pleijel, 2001). This was supported by another study which found that this change was delayed in high density populations probably assuming that there is enough sperm to keep the population viable (Bartolomaeus & Purschke, 2005; Rouse & Pleijel, 2006). 

Another suggestion was that a male stolon may be less costly than carrying a female stolon and therefore many chose to change to male for the smaller cost (Bartolomaeus & Purschke, 2005; Rouse & Pleijel, 2001). The effect of successive reproductive cycles has also been proposed as to the reasoning behind the sex change. The very first reproductive effort for a female may leave little reproductive potential for following spawnings and therefore after the first one, a change may be induced (Bartolomaeus & Purschke, 2005). Finally, Franke (1986) suggested early in the literature that the change is a temporal pattern whereby the individual is going to change based on what sexual adaptation is going to be best to pass on their genetics to the next generation.

There are many environmental and physiological factors which influence the start of the sex change. Studies have shown that light-temperature effects may influence reproduction by altering the mediation of the endocrine system (Beesley et al., 2000). As aforementioned the proventicle is a key morphological feature of the Syllidae family and may play a vital role in the release of an inhibitory hormone which controls stolonisation. This process within Typosyllis prolifera starts once the level of ‘stolonisation-inhibiting hormone’ decreases periodically during the warmer parts of the year (Beesley et al., 2000). This is why it has been suggested that the stolonisation is likely to be linked to the summer lunar cycles. Typosylis prolifera under laboratory controlled conditions were found to stolonise up to 18 times each circalunar interval (Franke, 1985). This equates to roughly every 31 days where the caudal part of the individual becomes the stolon and detaches. Franke (1985) suggested this cycle consists of two parts. The first is a post reproductive phase where the caudal segment detaches. The second is the stolonisation phase itself (Franke, 1985).

Kingdom: Animalia

Phylum: Annelida

Class: Polychaeta

Subclass: Errantia

Order: Phyllodocia

Suborder: Nereidiformia

Family: Syllidae

Subfamily: Syllinae

Genus: Typosyllis

It is estimated that there are approximately 20 000 species of polychaetes and over 70 families have been described (Hutchings, 1984). Within the Syllidae family alone there are over 70 genra and over 700 species (Aguado et al., 2012; Musco et al., 2010). Due to this large number of species, identifying syllids to the species level is often extremely difficult with many species still being undescribed (Hutchings, 1984). 

Rioja (1925) furthered the classification of syllids by dividing them into four initial subfamilies:

1. Syllinae
2. Autolytinae
3. Eusyllinae
4. Exogoninae

These subfamilies have widely been supported and accepted by subsequent researchers but many believe these distinctions were made based on practical reasoning rather than on phylogenetic studies (Beesley et al., 2000). 

The evolution of the Syllidae family itself is hypothesized to be similar to other polychaetes evolving rapidly. It is thought that during the evolution of this family two distinct lines appeared. One was the Anoplosyllinae and the second contained the rest of the syllids like the aforementioned four subfamilies (Aguado et al., 2012). It was since this discovery of the split evolutionary lines that an extra subfamily was added, the Anoplosyllinae which makes the fifth subfamily of Syllidae (Aguado & Martin, 2009). The latter evolutionary line consisting of the four subfamilies was characterized by the accelerated rate of evolution thought to have occurred (Aguado et al., 2012). At this stage there is no real consensus as to the phylogenetic relationships among these five subfamilies. Aguado & Martin (2009) suggested that Syllinae, Autolytinae and Exogoninae were monophyletic whilst Eusyllinae was thought to be polyphyletic. Musco et al. (2010) then alternatively suggested that the five subfamilies can be distinguished due to their reproductive characteristics and are all monophyletic (Aguado et al., 2012; Musco et al., 2010). 

Reproductive mechanisms play a vital role in differentiating the monophyletic subfamilies of Syllidae (Aguado et al., 2012). Amongst the varying theories of evolution within this family, the evolution of the different reproductive modes of Syllidae draw a large consensus. Many state that epigamy is the primitive reproductive mode. It hence has been suggested that schizogamy was the derived condition having appeared twice in Autolytinae and Syllinae as convergent processes (Aguado et al., 2012). 

Perhaps the key defining feature of the Syllidae family is the presence of a proventricle. It has now been proposed multiple times as being a synapomorphy of the Syllidae family (Aguado et al., 2012). Although widely accepted, further research into the anatomy and physiology of the proventricle within each family needs to be exhaustively studied to fully support the claim of synapomorphy. Making this comparison would also provide suggestions as to the possible homology of this structure as well as to its evolutionary origins and convergent appearance (Aguado & Martin, 2009). 

The complicated nature of the phylogenetic relationships within the Syllidae family continues with the relationships among the subfamily, Syllinae. The Syllinae has been defined as a group of syllids with articulate appendages (Aguado et al., 2012). In order to classify and organism to genus level within the Syllinae the type of stolon and the role it plays in reproduction is extremely phylogenetically important, with most genus having slightly morphologically different stolons (Aguado et al., 2012). The Typosyllis genus itself has sparked much controversy amongst phylogenetic researchers. This genus was brought about by grouping together all the species of Syllinae that do not fall under any other genus due to their lack of defining morphological features. However, initially this genus was created by Langerhans (1879) to differentiate species with compound chaete and those with simple chaete (Aguado et al., 2012).