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Summary | |
Polychaeta is a class of segmented worms belonging to the phylum Annelida; mostly comprised of marine representative but are known to also inhabit terrestrial freshwater systems (Pamungkas et al., 2019). At present there are approximately 85 extant families within Polychaeta, comprising of approximately 11,456 extant species (Pamungkas et al., 2019; Glasby et al.,2000).
Trypanosyllis zebra, a species first described by Grube (1860), is an errant organism belonging to the Syllidae family within class Polychaeta (Day & Hutchings, 1979). Members of this family are characterised by a heavily muscularised pharynx within a dorsoventrally flattened, ribbon-like body (Alvarez-Campos et al., 2017). This family of worms are known to have a global distribution; inhabiting inshore coastal environments amongst coral rubble and other such substrates (Alvarez-Campos et al., 2017; Glasby et al., 2000).
The T. zebra specimen examined within the present study was obtained from autonomous reef monitoring structures (ARMs) collected from the Manly boat harbour, QLD, Australia.
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Physical Description | |
Similar to other members of the Syllidae family, T. zebra exhibits a distinctive flattening along the dorsal-ventral axis (Figure 1); displaying a ribbon-like body plan (Matos Nogueira & Fukuda, 2008).
As a member of the phylum Annelida, T. zebra displays clear segmentation with numerous short and large segments; growing to an approximate maximum length of 100mm with up to 500 individual segments (Matos Nogueira & Fukuda, 2008; San Martin et al., 2008). A principal characteristic of T. zebra is the distinctly unique colouration which runs the length of the body.This colouration consists of one to two transverse bars ranging in shades of red to purple, these appear on the dorsal side of each segment of the body (Figure 2) (Matos Nogueira & Fukuda, 2008; San Martin et al., 2008).
T. zebra displays clear cephalisation at the anterior end of the body in the form of a pre-segmental prostomium and peristomium (Matos Nogueira & Fukuda, 2008; Aguado & San Martin, 2009). Between the prostomium and peristomium sits a pair of densely ciliated pits known as nuchal organs; these act as a chemoreception apparatus and are only present within polychaete worms (San Martin & Aguado, 2012). The prostomium of T. zebra contains two prostomial lobes that house two pairs of eyes (See Figure 3) in a trapezoidal arrangement (San Martin et al., 2008). Three antennae projections extend from the prostomium of T. zebra, present are a pair of lateral antennae and a singular median antennae (Figure 3). The occurrence of three antennae projections is a unique characteristic only found within members of the Syllidae family (Glasby et al., 2000). These prostomial antennae vary in size, with the median antennae generally the longest of the three (San Martin et al., 2008). At the anterior end of the animal, syllid worms exhibit a pharyngeal opening usually surrounded by a crown of papillae or teeth known as a Trepan (San Martin & Aguado, 2012). In the case of T. zebra , San Martin (2008) found that the number of papillae and teeth within the trepan is size-dependent and can vary between 10-20 pharyngeal teeth for small to large individuals.
T. zebra displays obvious cirri, both peristomal and dorsal (Figure 3), along the length of the body. These cirri are generally thick and vary in length but do not often exceed the width of the given individual (San Martin et al., 2008: Glasby et al., 2000). Similar to other marine polychaete species, T. zebra exhibits a pygidium with two distinct anal cirri extending from the body wall (San Martin et al., 2008)
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| Figure 1 |
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| Figure 2 |
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| Figure 3 |
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Ecology |
Habitat | |
T. zebra is a free-living errant organism that is known to inhabit shallow marine environments, both of soft and hard substrate (Glasby et al., 2000; Alvarez-Campos et al., 2017). T. zebra, as with most polychaete species, is unable to osmo-regulate and is consequently constrained to a marine existence (Glasby et al., 2000). Alvarez-Campos et al. (2017) notes T. zebra inhabiting biological substrates such as algal matting, dead coral, sponges, bryozoans, ascidians and hydroids..
The contours and crevices within these substrates provide quality shelter from predators and for reproductive purposes for invertebrate organisms such as syllid worms, consequently these environments often support a high abundance of organisms (Simboura, Nicolaidou, & Thessalou-Legaki, 2000). T. zebra is predominately found within temperate and tropical seas (Glasby et al., 2000)
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Predation | |
Similar to other marine invertebrate organisms, T. zebra experiences significant predation-based mortality during juvenile stages (Bell & Coull, 1978; Cowden, Young, & Chia, 1984). This predation pressure extends into adulthood; T. Zebra utilises highly porous substrates such as sponges and deceased coral as a refuge from predators (Simboura, Nicolaidou, & Thessalou-Legaki, 2000).
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Life History and Behaviour |
Life History | |
Within the family Syllidae, gonochoric species employee a highly specialised form of sexual reproduction known as epitoky (Aguado, San Martin & Siddall, 2012; Ribeiro, Bleidorn & Aguada, 2018). Epitoky involves adult individuals undergoing a metamorphosis prior to reproduction and can be achieved in one of two ways; epigamy or schizogamy (Ribeiro, Bleidorn & Aguada, 2018). The presence of these two different methods of epitoky within the Syllidae family suggests radical evolutionary changes in both morphology and behaviour at some point throughout the lineage of the syllid worms (Aguado, San Martin & Siddall, 2012).
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Sexual Reproduction | |
T. zebra is a species which undergoes schizogamous epitoky. Schizogamy is a process whereby sexually mature benthic individuals develop swimming appendages, known as notochaetae, throughout the mid and posterior regions of the body (Aguado, San Martin & Siddall, 2012). This metamorphosed posterior region of the body develops eyes and other sensory apparatus and is known as a stolon (Ribeiro, Bleidorn & Aguada, 2018; Aguado, San Martin & Siddall, 2012). T. zebra develops acerous stolons meaning that two pairs of eyes are developed, similarly to the adult body form (Aguado, San Martin, & Siddall, 2012). The stolon (Figure 4), which also carries gametes from the sexually mature individual, is then detached from the main body to drift in the water column.
Whilst adrift in the pelagic zone, spawning occurs via interaction with other stolons. Franke (1985) showed that many syllid worms undergo synchronous stolon formation and release, which eventually leads to stolonal swarming, based on the lunar cycle and other abiotic factors such as temperature. T. zebra undergoes schizogamous epitoky via scissiparity (Figure 5); the formation of a singular non-differentiated stolon (Aguado, San Martin, & Siddall, 2012; Aguado & San Martin, 2009). This schizogamous mode of reproduction enables T. zebra to only subject part of the body (the stolon) to pelagic predation risk, whilst ensuring the sexually mature adult remains active in the benthos for future reproductive activity (Aguado & San Martin, 2009).
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| Figure 4 |
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| Figure 5 |
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Larval Development | |
The specific fertilisation method of T. zebra is currently unknown; however, it is most likely similar to the majority of species within the subfamily Syllinae, occurring via broadcast spawning (Musco et al., 2010; Franke, 1999). Fertilisation in this way occurs during stolonal swarming and involves the mass release of gametes. Once fertilised, eggs descend to the benthos where they remain until they hatch as very developmentally early trochophore larvae approximately 24-48 hours later (Franke, 1999). These eggs are small in size and do not undergo an extended pelagic phase. This is in contrast to fundamental theories proposed by Thornson (1950), whereby marine invertebrates which generally produce eggs of a small size experience a pelagic phase. Franke (1999) attributed this to the presence of a pelagic stage during epitoky and also to the restricted habitat preferance of many syllid worms; particularly in reference to juvenile dispersal.
Specific growth and developmental rate have not been recorded for T. zebra, however studies have investigated the growth of other closely related syllid worms. Schiedges (1979), observed syllid worms of the genus Autolytus sp. reaching sexual maturity, implied by stolon formation, at approximately 80 to 160 days post hatching. The same study found syllid worms of the genus Autolytus sp. to have an approximate life span of between 200 and 250 days, however a singular specimen within the study persisted for 550 days post hatching (Schiedges, 1979).
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Anatomy and Physiology |
Internal Anatomy | |
T. zebra, similar to other syllid worms, has simple internal structures consistent with that of the general polychaete body plan. The internal musculature is characterised by two distinct muscle groups (Figure 6); the outer transversal muscle fibres and the inner longitudinal muscles (Aguado et al., 2015). These muscle groups are present throughout the segmented region of the body and are situated on both the dorsal and ventral sides of the body, this musculature does not extend to the pygidium (Aguado et al., 2015).
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| Figure 6 |
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Circulatory and Digestive System | |
T. zebra exhibits a closed circulatory system (Figure 7), whereby the blood flows anteriorly via the dorsal vessel and posteriorly towards the pygidium via the ventral vessel (Glasby et al., 2000). Blood flow is dependent on movements of the body wall and the surrounding muscle; enabling blood to reach all areas of the body (Glasby et al., 2000). Specialised extensions of the body wall, known as branchiae, contain a section of the vascular system and numerous capillaries; acting as sites for efficient respiratory gas exchange (Glasby et al., 2000).
Characteristic of the Syllidae family, T. zebra exhibits a highly muscularised eversible axial pharynx (Alvarez-Campos et al., 2015; Glasby et al., 2000). This pharynx (See Figure 6) generally extends through 11 segments of the anterior end of the animal (San Martin et al., 2008). Similarly to other syllids, T. zebra exhibits a specialisation of the digestive tube known as the pro-ventricle; an apparatus used as a suctorial pump during feeding and also as a secretory gland during reproduction (Aguado, San Martin, & Siddall, 2012). The pro-ventricle is generally long and thin, similar in length to the pharynx and is often visible via transparency in the epidermis (San Martin et al., 2008; Aguado et al., 2015).
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| Figure 7 |
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Feeding | |
Within the Syllidae family, there is a broad diversity of diets, likely due to their benthic existence (Glasby et al., 2000). Examinations of faecal samples by Giangrande, Licciano, & Pagliara (2000) found T. zebra to have a detritivorous diet; consisting primarily of vegetal detritus, sediment particles, diatoms and bacteria. It is also likely that T. zebra consumes colonial invertebrates such as corals, hydrozoans, bryozoans and sponges (Glasby et al., 2000). Feeding in T. zebra most likely occurs via suctorial feeding, whereby the pro-ventricle is involved in creating a suctioning pressure (Aguado, San Martin, & Siddall, 2012).
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Biogeographic Distribution | |
T. zebra has a wide global distribution, this species is known to occur throughout all Australian states as well as in both the Mediterranean Sea and Atlantic Ocean (San Martin, Hutchings, & Aguado, 2008; Matos Nogueira & Fukuda, 2008). It is for this reason that many studies have considered T. zebra to be a cosmopolitan species (Alvarez-Campos et al., 2017).
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Evolution and Systematics |
Phylogeny | |
According to the World Register of Marine Species (WORMS, 2019), Trypanosyllis zebra can be classified taxonomically in the following way:
Kingdom – Animalia
Phylum – Annelida
Class – Polychaeta
Subclass – Errantia
Order – Phyllodocida
Suborder – Nereidiformia
Family – Syllidae
Sub-family – Syllinae
Genus – Trypanosyllis
Species – Trypanosyllis zebra
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Distinction from other Syllid Worms | |
T. zebra, unlike other cosmopolitan species, does not have an extended pelagic phase; facilitating a broad distribution (Alvarez-Campos et al., 2017; San Martin, Hutchings, & Aguado, 2008). Due to this, Alvarez-Campos et al. (2017) proposed that T. zebra may actually be two or more morphologically indistinguishable cryptic species belonging to the genus Trypanosyllis sp. This highlights the potential benefits of using molecular tools for identifying cryptic species from geographically different samples within a potential species complex; enabling understanding of true biodiversity and distribution (Alvarez-Campos et al., 2017; Aguado & San Martin, 2009).
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Conservation and Threats | |
Polychaete worms are highly abundant in almost all marine habitats; currently 11,456 extant species have been identified and conservative estimates predict actual species abundance to be between 25,000 and 30,000 (Pamungkas et al., 2019). T. zebra is an abundant species and has a broad distribution throughout many of the world’s oceans (WoRMS Editorial Board, 2019; Glasby et al. 2000; San Martin, Hutchings & Aguado, 2008). As a result, there are currently no known conservation efforts targeting T. zebra and no immediate threats facing this species.
However, a study conducted by Simboura et al. (2000) recognised that polychaete worms are heavily dependent on sediment, depth, hydrodynamics and other such abiotic factors. Meaning that future changes in marine abiotic factors due to climate change may influence this species. Organisms belonging to the Syllidae family, such as T. zebra, have been identified as good faunal and ecological indicators of the physical environment; potentially providing important ecological information in the future (Martins et al., 2013; Simboura, Nicolaidou, & Thessalou-Legaki, 2000).
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References | |
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