Spirorbis is a speciose genus of polychaete tubeworms within the family Serpulidae. Members of this genus are globally distributed and live across a range of habitats from marine to freshwater environments (Gierlowski-Kordesch and Cassle, 2015; Zatoń et al., 2012). Although species are native to many regions, some have become biofoulers on ship hulls and even pests in aquaria (Henschel & Cook, 1990).

Species of Spirorbis can be defined by their coiled calcareous shell firmly attached to the substrate which bears a crown of feeding tentacles and a modified operculum, protecting the organism from predators and desiccation (Brinkmann and Wanniger, 2009; Schively, 1897) (Figure 1). The majority of species have a short pelagic larval phase, facilitating dispersal of the species, but once settled individuals remain sessile for the remainder of their lives. (Bell et al. 2001) Members of the Spirorbis genus are suspension feeders that use their extended tentacles to filter particles from the surrounding water. 

This webpage functions as a summary of the Spirorbis genus, using inferences from defined species around the world, although species may not be present in Australian waters.

Species within genus Spirorbis are sessile, tube-building serpulid polychaetes (Brinkmann and Wanniger, 2009). They comprise of a soft body, for which colour varies by species, encompassed by a coiled calcareous shell (Rzhavsky et al., 2014) (Figure 3). In some species, such as Spirorbis inornatus, as the shell coils whorls overlap each other, as shown in Figure 4. Other significant features defining Spirorbis are their tentacular crown and operculum, created from the fusion of their anterior ends (Brinkmann and Wanniger, 2009) (Figure 5). The operculum in Spirobis is in fact a modified tentacle (radiole) that functions as a plug to the entrance of the tube to protect the organism. This head region is then followed by a series of segments of similar appearance, along the remainder of its body (Rush, 2013) (Figure 5). 

The average size of adult Spirorbis worms varies between species, however most members of the Spirorbis genus reach sizes of around 1.5 - 5mm in coil diameter when fully developed (Rzhavsky et al., 2014) (Figure 4). For example. smaller species such as Spirorbis coralline rarely exceed 1.5mm whereas larger species such as Spirorbis rupestris can grow up to 4.5mm (de Silva and Knight-Jones, 1962; Gee, J. M. and Knight-Jones, W. 1962). 

Species example – Spirorbis spirorbis

The body size of an individual Spirorbis spirorbis is usually around 3-4mm and is orange-red in colour, it is encased in an evenly-coiled, white calcareous tube (Ballerstedt, 2001) (Figure 6). Coiling of this tube is sinistral (coiling to the left) with a small, peripheral flange (Ballerstedt, 2001) (Figure 6). 

Organisms can be classified to genus level according to the following features (Fauchald, 1977):

ORDER: Sabellida

• Tentacular crown
• Setae present 

FAMILY: Spirorbidae

• Completely coiled tube
• Assymetrical body
• Four thoratic segments present 

GENERA: Spirorbis

• External segmentation
• Body is not a flattened disc
• Generally distinct segmentation, but if indistinct body is longer than wide
• Anterior end (incorporating part of the prostomium) altered into a tentacular crown

The World Register of Marine Species (WoRMS) currently recognises 215 species within the genus Spirorbis and 14 species within the sub-genus Spirorbis (Fauchald, 2020). 

Most of the work on Spirorbis spp. relates to a few common species such as Spirorbis spirorbis (Linnaeus, 1758), Spirorbis borealis (Daudin, 1800) and Spirorbis inornatus (L'hardy and Quievreux, 1962) (Brinkmann and Wanninger, 2009; Rzhavsky et al., 2014; Schively, 1897). These species have contributed greatly to the foundation of knowledge on the genus, though features identified may differ between species. 

Unfortunately, species of Spirorbis are very challenging to recognise and even trained taxonomists struggle to identify Spirorbis specimens to a species level. This is highly apparent in Australian species, as although common, difficulties in classification have led to many species remaining undefined and thus limited information at species-level (Atlas of Living Australia, 2020). 

Members of Spirorbis are commonly found on rocky shores in shallow intertidal and sublittoral zones (Atlas of Living Australia, 2020; Ballerstedt, 2001). Species within the genus typically live attached to seaweed, but some favour attachment directly to bedrock, shells or other substrates (Gee, 1963; O’Connor and Lamont, 1978). 

Preference towards specific substrates can be seen in a number of species (Bell et al., 2001, Schively, 1897). For example, observations made by Gee in 1963 showed that S. borealis was generally found on the brown algae Fucus serratus, S. rupestris on encrusted bedrock and S. tridentatus under surfaces of boulders. Larvae of each species are likely inclined to search for slightly different substrates to each other to avoid inter-specific competition, which can be a big problem in marine communities (O’Connor and Lamont, 1978). 

The majority of species in genus Spirorbis release their larvae at a relatively advanced stage of development (Gee, 1963). Figures 7 and 8 show larval development in further detail. It is common for larvae to have a short pelagic phase after hatching, although settlement often occurs within a few hours (Brinkmann and Wanninger, 2009). Experiments have shown if larvae do not settle and metamorphose within 24 hours, they rarely remain viable (Gee, 1963). This planktonic phase also aids in the dispersal of species, thus maintaining genetic diversity in populations (Rothlisberg, 1974). Individuals can also locate available substrate, in what can sometimes be an extremely competitive environment (Gierlowski-Kordesch and Cassle, 2015) (Figure 9). Once an individual has settled on the substrate it will remain attached for the remainder of its life (Bell et al. 2001). During its lifetime these tubeworms will experience many breeding periods, often throughout the entire year (Rothlisberg, 1974).

Species of genus Spirorbis are suspension feeders that extend their ciliated tentacles to capture particles in the water column (Rzhavsky, 2014). Once captured, particles are transported by ‘conveyor belts’ of cilia in grooves of the radioles (Rush, 2013) (Figure 10). These ciliated grooves function to sort particles into different size categories. Small particles are transported to the mouth, moderately sized particles are stored as reserves and large particles rejected by the organism (Rush, 2013).

As Spirorbis species are sessile organisms they are not able to physically escape any evident threats (Gierlowski-Kordesch and Cassle, 2015). Therefore, tubeworms must have other forms of defence to protect themselves. Their calcareous tube, which an individual begins secreting upon settlement, acts as a physical barrier and provides protection from the surrounding environment (Fauchald 1977). Worms also have a modified operculum which acts as a plug, blocking the opening of the tube and reducing the chance of harm from predators (Potswald, 1968) (Figure 11). Their operculum also functions to seal moisture in the tube, which is extremely important in their harsh inter-tidal habitats where desiccation is a prevalent risk (Schively, 1897). 

As members of genus Spirorbis are sessile, movement only occurs within their tube (Rouse, 2001). Tubeworms move by either setal movements or by slow, peristaltic action of their body (Rouse, 2001). When extended, the tentacular crown of the animal is exposed, but when contracted, the operculum covers the opening of the tube to protect the animal (Bergan, 1953).

Reproductive strategies differ between species in the genus Spirorbis. The majority of species appear to be hermaphrodites, utilising methods of self-fertilisation or cross-fetilisation, and sometimes both (Lucey et al., 2015). These tubeworms also protectively brood their eggs and larvae (Gee and Williams, 1965). The method of brood projection utilised is species-specific, either taking place within the modified operculum or within the parental tube (Potswald, 1968). Reproductive periods have been shown to vary amongst species and by geographical location (Rothlisberg, 1974; Mook, 1983). 

Reproductive anatomy

Figure 12 shows the gonads of species Spirorbis mörchi. The first two abdominal segments of the body are female, but the remaining segments are male (Potswald, 1967) (Figure 12). However, the number of female segments can differ between species, for example, the first 3 or 3 ½ segments are female (Potswald, 1967). 

Global warming caused by enhanced emissions of greenhouse gases is becoming an increasing problem for marine organisms (Gao et al., 2019). Large volumes of carbon dioxide are absorbed by the ocean daily and have led to ocean acidification and rising sea temperatures (Gao et al., 2019). An experiment carried out by Ni et al. in 2018 showed that ocean acidification and temperature rise cause highly destructive corrosion to the calcareous shells of species Spirorbis spirorbis (Figure 17). As carbon emissions are predicted to continue rising, this will become a more prevalent threat to species in within genus Spirorbis (Gao et al.; 2019, Ni et al., 2018).