Idanthyrsus australiensis is a marine tube worm that was discovered by Haswell in 1883. I. australiensis has no one common name but various sources refer to them aseither bristle worms or sand-mason worms. Both are an accurate description of the worm as it has bristles (chaetae) and lives in a sandy tube.

 

Classification:
Kingdom - Animalia
Phylum - Annelida
Class - Polychaeta
Subclass - Sedentaria
Infraclass - Canalipalpata
Family - Sabellariidae
Genus - Idanthyrsus
Species - Idanthyrsus australiensis
(WoRMS 2008).

 

Annelidais the phylum of the segmented worms; polychaetea is the class of worms with parapodia and chaetae, subclass sedentaria as they are not free living, and family sabellariidae, the tube builders.

 

I. australiensis are found on all surrounding Australian coasts and are generally found on hard substrate. The specimens I collected for observation were found on the rocky shores of Hastings Point, New South Wales, Australia.

There were two species of tube worms found at Hastings Point: I. australiensis and Galeolaria gemineoa (Halt, Kupriyanova, Cooper & Rouse, 2009). I knew my species had a sandy tube and G. gemineoa had a calcareous tube. My species was also larger than the other. Beesley (2000) claims there is only one species of sabellariidae found in Australia (although this is under revision), which was I. australiensis meaning I wasn’t likely to mistake it for another species.

I observed tubes in clumps (there were also some solitary individuals near by), which were constructed from sediment that resembled its surroundings. Tubes in clumps were bent and twisted around each other. Solitary tubes were generally straight but followed the contouring of the substrate. The tubes were ridged to touch due to cement secreted from glands (Curtis 1973).

  

Description of morphology to aid in identification of I. australiensis will be described in the following paragraphs and figures.

Like other polychaetes (fig. 2), its body retains distinct segmentation and the presence of chaetae and parapodia.

Unlike other polychaetes the peristomium and prostomium are fused (fig. 3). The prostomium is fused dorsally but is free ventrally and the peristomium are only visible as lips (Beesley 2000).Sabellariidae also lack antenna (Beesley 2000).

Sabellariidae are recognised by their well-developed operculum (fig. 4), which has one to three rows of golden paleae (fig. 5 and fig. 6) (Beesley 2000 and Rouse & Fauchald 1997). The operculum is the anterior tentacle like structure that blocks the entrance of the tube and also acts as a respiratory surface. The oppercular palaea and nuchal hooks are deciduous meaning they can be shed periodically.

 

The body can be divided into four main regions: the anterior, para-thoratic, abdominal, and caudal.

The prostomium, peristomium and first two uniramous segments comprise the anterior. The next three to four segments makeup the para-thoratic region. The parapodia (fig. 7) in this region are biramous with dorsal cirriform branchiae, oar-shaped notochaetae (fig. 8, g) and capillary neurochateae (fig. 8, f).

The abdominal region contains multiple biramous segments that have dorsal ciriform branchiae (fig. 7), notopodal unici(fig 8, h), and neuropodial capillary chaetae (fig 8, f).

The caudal region bears achaetous parapodia and the anal tube and folds under the body (fig. 3).

I. australiensis has a bathymetric range of 0-60m depth and can be found in solitary form, small clusters and some families have been found to be reef builders. Other members of sabellariidae have been found on rocky shores, shells, stones, corals, bryozoans, sponges, ascidians, timber, sandy substrates, and man made infrastructure (Beesley 2000; Curtis1973; Feroni-Perez et al 2016). Idanthyrsus trochophore larvae have a highly innervated dorsal hump (DH) (Brinkmann & Wanninger 2008) that has been hypothesized to chemically detect appropriate settlement substrate (Feroni-Perez et al., 2016). 

 
Annelid larvae have been reported as showing preference in many studies (Beesley 2000; Curtis 1973; Feroni-Perez et al., 2016; Pawlic 1992; Wilson 1929). Sabellariid larvae have been known to examine substrate by crawling over it (Amieva & Reed 1978; Curtis 1973). If conditions are unfavourable,the larvae can postpone metamorphosis but will eventually die if the right substrate is not found (Curtis 1973). Favourable sites would have clean, stable substrate with currents capable of dislodging sand particles (Curtis 1973).

Multiple studies concur that presence of adult tubes potentially play a role in chemical cues for larval settlement ( Beesley 2000; Curtis 1973; Feroni-Perez et al., 2016; Pawlic 1992; Wilson 1929). Pawlic asserts that a large variety of marine invertebrates receive chemical cues from adult conspecifics. The cues inform larvae whether or not the substrate is suitable and settlement ensures sustainable population numbers for colonies.

 

Studies on Sabellariidae species found specific chemicals in the cement of the tubes that attracted conspecific larvae to settle (Pawlic 1992). Gas chromatographical analysis and experiments revealed three main free fatty acids that induced larval settlement:palmitoleic, linoleic (18:2); arachidonic (20:4); and eicosapentaenoic acid (Pawlic 1992).

 

Once a suitable site has been chosen, a mucoid layer is secreted around the animal, in which it can metamorphose in (Beesley 2000; Curtis 1973). The mucoid layer can also begin to collect sediment, which will be the beginnings of the tube (Curtis 1973).

Sabellariidae occur as individuals, in clumps, or as reef builders (Feroni-Perez et al. 2016). Their close proximity to other conspecifics allows them to be free spawners. This means all individuals become gravid at the same time (usually in the warmer months) and release their gametes in a spawning event (Feroni-Perez et al. 2016). Gametes have been observed to abruptly eject from tubes in bursts (Curtis 1973). The ova in females grow in a dark cellular mass on stemlike structures. Once the ova are mature, they detach from the stems and are collected in the coelom cavity which are expelled upon muscle contraction through the nephridiopores (fig. 9) (Curtis 1973). Spawning occurs at high tide because at low tide, the individuals are often exposed (Waterman 1934). Once fertilised, the zygotes undergo spiral cleavage (Ruppert et al. 2004).  The video below will explain the life cycle of I. australiensis in detail. The video also briefly describes a small experiment to determine if I. australiensis larvae are photolytic. A study done by Marsden (1985) on Spirobranchus giganteus (Pallas 1976) found a positive photolytic response. I also found a positive response to exposure to light, meaning the larvae moved towards the light source.  Idanthyrsus australiensis reproduction and Sabellariidae trochophore development  N.B: the significance of dorsal humps will be described in the Anatomy and Physiology section, below. When larvae have settled, a mucoid tube is excreted around them (Curtis1973). Once the mucoid tube is formed, the larvae undergo metamorphosis and become juveniles (Curtis 1973). Using their opercular structure, they selectively gather grains, place them carefully onto the mucoid tube then cement them on (Curtis 1973).

There is little information on the physiology of the sabellariidae family let alone I. australiensis. However, they are polychaete worms. Each segment in polychaetes contains a ventral nerve chord, ganglioin, nephridium (kidney), coelomic cavities, connective tissue and blood vessels,muscle (circular, longitudinal and parapodial), gut, and parapodia (fig 9) (Ruppert et al 2004). However, Beesley (2000) does note that the gut of sabellariids differentiates along its length.

 

There are, however, studies that have focused on the sensory organs structure and function, which I will briefly summaries. Sensory structures have been studied more than other anatomical features perhaps because they play such a key role in the organism’s survival and are a great morphological feature to observe in species evolution (Feroni-Perez et al.2016).

 

Palpal structures have been well examined in multiple sabellariidae species. Amieva & Reed (1987) describe the larval palps in particular, with great detail. They observed the use of the palps to search for suitable substrate to settle on and described sensory structures within the palps. They found two types of motile cilia that ran the length of the premodial palps and that each type of cilia would beat at different rates. Such behaviour of the cilia would suggest their movement was under neuronal control (Amieva & Reed 1987).

 

The larval tentacle resembles a hollow cylinder comprised of four tissue layers; the epidermis, connective tissue, muscle, and peritoneum (lining of the coelom)(Amieva & Reed 1987). The epidermal layer was imbedded with glands, sensory cells and nerves.

 

Though there is limited information on I. australiensis regarding physiology, the description above should highlight the complexity and importance of sensory structures.

 

Structures of particular interest in I. australiensis include the median organ (MO), median ridge (MR) (fig 5) and in larvae, the dorsal hump (DH) (see video) (Feroni-Perez et al. 2016).  The MR extends from the upper lip of the dorsal crest to the opercular lobes (Feroni-Perez et al. 2016). The MO is a continuation of this structure and protrudes outwards. Depending on the species, the MO and MR can bear differing numbers of cilia and/or eyespots (Feroni-Perez et al. 2016).  The DH only occurs in larval form but is also considered to be a very important sensory structure used to aid in settlement. All of these structures are thought to be sensory as they are highly innervated (Feroni-Perez et al. 2016).

The map below was pieced together using locations of specimen collection from Feroni-Perez et al. (2016), Kirtly (1927), and the Atlas of Living Australia species occurrence map. Note that this map does not note absence of I. australiensis, merely presence. As of the year 2000, I. australiensis was thought to be the only Sabellariidae species found in Australia. However, some of these occurrences are being revised as different species (Beesley 2000).

In sabellariids, sensory organs are a key feature used to identify species radiation. In particular, the morphology of the dorsal hump and median organ were important structures that told us that they are a derived trait unique to its taxon (autapomorphic).

 

Feroni-Perez et al­ (2016) studied these structures in particular and hypothesised there could be more than one species of Idanthyrsus in Australia. The morphological features they focused on included: median organ, median ridge, eyespots,size, and colour pattern. They compared individuals from various locations in Western Australia and multiple locations in New South Wales. Morphological differences were found in the number of eyespots along the median ridge and morphology of the median organ. Additionally, phylogenetic analysis revealed that the further away populations were from each other, the greater genetic differences were found.

 

Feroni-Perez et al­ (2016) did not conclude that this meant they were different species but urged more research be completed before asserting facts. 

I. australiensis is not on record in the IUCN Red-list database and there are no reported incidences of the species being under threat. One could assess their state of wellbeing by examining their habitat: the rocky shore. A study by Coutinho et al. (2016) highlighted two main factors that could affect a species distribution: biotic and abiotic factors. The main abiotic factors on the rocky shore would be energy levels (wave exposure) and time spent out of the water (for example in low tides). Biotic factors include competition and predation.

 

Over the years of visiting Hastings Point, NSW, where I collected my specimens, I have observed a decline in I. australiensis numbers. Though I cannot assert a cause for this decline, I can speculate that an imbalance of biotic and/or abiotic factors could cause this. I will also say that over the years, I have noted major changes in species dominance and composition but also know that the system is resilient enough to always recover to some extent. 

Amieva, M., & Reed, R. (1987). Functional morphology of the larval tentacles of Phragmatopoma californica (Polychaeta: Sabellariidae): Composite larval and adult organs of multifunctional significance. Marine Biology, 95(2), 243-258.

Atlas of Living Australia website at http://spatial.ala.org.au/?q=lsid:urn:lsid:biodiversity.org.au:afd.taxon:2448301a-66fb-40b7-b913-d5f0b6230a86 Accessed 19 May 2017.

Beesley, P. L. (2000). Fauna of Australia: Polychaetes & Allies, The Southern Synthesis (Vol. 4A). Canberra, ACT: Australian Government Publ. Service.

Brinkmann, N., & Wanninger, A. (2008). Larval neurogenesis in Sabellaria alveolata reveals plasticity in polychaete neural patterning. Evolution & Development, 10(5), 606-618.

Coutinho, R., Yaginuma, L., Siviero, F., Dos Santos, J., Lopez, M., Christofoletti, R., . . . Arruda Goncalves, L. (2016). Studies on benthic communities of rocky shores on the Brazilian coast and climate change monitoring: Status of knowledge and challenges. Brazilian Journal Of Oceanography, 64(2), 27-36.

Curtis, L.A. (1973). Aspects of the Life Cycle of Sabellaria vulgaris verrill (Polychaeta: Sabellariidae) in Delaware Bay. Xerox University Microfilms, 74, 1-246.

Faroni-Perez, L., Helm, C., Burghardt, I., Hutchings, P., & Capa, M. (2016). Anterior sensory organs in Sabellariidae (Annelida). Invertebrate Biology, 135 (4), 423-447. doi: 10.1111/ivb.12153.

Kirtley, D. W. (1974) The geological significance of the Polychaetous annelid family Sabellariidae. Diss. Florida State University.

Marsden, J. (1985). Light responses of the trochophore larvae of the serpulid polychaete, Spirobranchus giganteus . Bulletin of Marine Science. 1985, Bulletin of Marine Science. 1985.

Pawlik, J.R. (1992). Chemical ecology of the settlementof benthic marine invertebrates. Oceanography Marine Biology Annual Review, 30, 273-335.

Reece, J. B. (2011). Campbell biology (9th ed.). Frenchs Forest, N.S.W.: Pearson Australia.

Rouse, G. W., & Fauchald, K. (1997). Cladistics and polychaetes. Zoologica Scripta, 26(2), 139-204.

Ruppert, E. E., Barnes, R. D., & Fox, R. S. (2004). Invertebrate zoology: a functional evolutionary approach(7th ed.). Belmont, CA: Thomson-Brooks/Cole.

Waterman, A. J. (1934). Observations on Reproduction, Prematuration, and Fertilisation in Sabellaria vulgaris. Biological Bulletin, 67 (1), 97-114.

Wilson, D. (1929). The Larvæ of the British Sabellarians. Journal of the Marine Biological Association of the United Kingdom, 16(1), 221-268.

WoRMS (2008). Idanthyrsus australiensis (Haswell, 1883). In: Read, G.; Fauchald, K. (Ed.) (2017). World Polychaeta database. Accessed through: World Register of Marine Species at http://marinespecies.org/aphia.php?p=taxdetails&id=333663 on 19 May 2017.