As a Serpulidae, Hydroides Elegans is a tube building species and as such is not often visible. By some reports the tubes can reach lengths of up to 80mm (Imajima, 1976), but they are more commonly around 50mm. The tubes are sub-trapezoidal and almost circular with a width of between 1 and 2 mm. They are thin and white in colouration with many transversal growth rings along the sides. There are also a pair of longitudinal keels running along them (Sun et al., 2015; van Nieuwenhuijzen et al., 2016).

The actual creature itself is far smaller than the tube in which it lives. The organism is usually around 10 to 20 mm long with the branchiae and operculum making up nearly a quarter of the animal. H. Elegans has a variable number of radioles with 11-16 pairs present, all possessing pinnules along their lengths. This organ is often the most apparent feature when in the tube as only it and the operculum extends past the rim of the tube (Sun et al., 2015; van Nieuwenhuijzen et al., 2016; Day, 1967).

The operculum is one of the key defining characteristics of the H. Elegans. It a derived radiole and extends above the other radioles when the organism is feeding. When it is in its tube the operculum is able to block the lumen. It is roughly circular and only about 1 mm wide and tall. It is made up of 13-21 spines which are fused to each other about halfway along to form a wide shallow cup. These spines are have pointed tips and 2-4 lateral spinules and 1-3 internal spinules, both also pointed. The funnel is made up of 21-28 blunt radii and is not chitenous. The peduncle is circular in crossection and connects to the funnel (Sun et al., 2015; van Nieuwenhuijzen et al; Day, 1967).

H. Elegans has a low collar which is continuous with the thoracic membrane and forms an apron over the abdominal chaetigers. There are also collar chaetae with a well denticulated zone at the knee. The thorax has 6 unicigerous chaetigers, with each unici possessing 6-8 curved teeth. The abdomen is made up of 42-62 chaetigers, with up to 8 capillary chaetes of the anterior of the animal (Sun et al., 2015; van Nieuwenhuijzen et al; Day, 1967).

H. elegans utilizes sexual reproduction, with gametes being released and the fertilisation occurring externally in the water column. The trochophore larvae can begin feeding at only 9 hours of age and they are capable of developing a sensory organ after 12 hours. They become capable of metamorphosis after 5 days, but require exposure to biofilm bacteria in order for settlement to occur. Almost immediately after finding a suitable settlement substrate the larvae attach themselves via a sticky thread, after which they will secrete their primary tube. This is secreted from almost all segments of the body and pushed away from the larvae by the setea to form a tube around them. This entire process is very quick and sometimes will take even in less than 10 mins from start to finish. Construction of the secondary tube begins soon after the beginning of metamorphosis from the anterior end of the primary tube. Metamorphosis will be concluded after around 12 hours and the larvae will be a juvenile. The primary tube is not calcareous while the majority of the adult secondary tube is (Flemming et al., 2009).

As a member of Annelid H. elegans demonstrates clear segmentation along its body. There is a definitive separation between the thorax and abdomen, seen by faecal groove inversion and chaetal inversion among other things. As a polychaete, H. elegans possesses poorly developed biramous parapodia, no doubt a result of the sedentary lifestyle of the organism (Beesley, Ross and Glasby, 2000).

H. elegans are a part of the Serpulidae family, all of which create tubes of some sort. In the case of H. elegans, this tube is calcareous and the organism will likely never leave the tube. Throughout its life it will continue to make additions to this secondary tube by the way of a peristomial collar. The collar can fold over the rim of the tube and secrete the calcareous substance which is then grafted to the existing structure. The material for this secretion made in the ventral surface of the anterior segments (Day, 1957). In Serpulidae there are a pair of calcium-carbonate secreting glands in the collar of the organism. These glands secrete a mucopolysaccharide matrix which induces the precipitation of calcium from the water. A mixture of the calcium and the mucopolysaccharide is then used to extend the tube’s length in the way discussed above (Beesley, Ross and Glasby, 2000).

The main external organ of this organism is its tentacular crown, and Hydroides elegans uses this radiolar crown for a number of purposes. Respiration primarily occurs here through the peripheral blood system (see below) though there are some suggestions of auxiliary respiration across the body wall via generated water flow through the tube (Knight-Jones and Fordy, 1979). In addition the crown is used for feeding, the radioles have pinnules for filtering phytoplankton and organics. The crown also has an internal ‘skeleton’ as found in all Serpulidae. (Rouse and Pleijel, 2001).

Like most polychaetes, H. elegans has a through gut from the mouth to the anus. There is no proboscis, pharynx or buccal cavity, while the oesophagus and gut are ciliated to facilitate transportation. There is a faecal grove running along the dorsal side of the thorax (ventral on abdomen). This is also ciliated and assists in faecal movement as well as the release and control of gametes (Rouse and Pleijel, 2001; Beesley, Ross and Glasby, 2000). In addition there are a pair of excretory nephridia are located at the anterior of the animal with a single exit (Orrhage, 1980). These nephridia have been determined to be mixonephridia by Goodrich (1945).

As a sedentary worm, H. elegans does not have an extreme amount of musculature for locomotion. However it does exhibit one very specific muscular action. When threatened it will withdraw itself within its tube with incredible speed. This is cause by the sudden contraction of the longitudinal muscles in conjunction with the anchoring of the abdominal uncini and the backwards thrusting of the collar chaetae. All together this makes for a very fast retreat (KNIGHT-JONES and FORDY, 1979).

Orrhage (1980) has found that Serpulidae possess a nuchal organ just above the mouth. They have complex brains for sedentary worms and many have a pair of peristomial eyes (Goodrich, 1945). H. elegans has very well developed nerve fibres but does not possess any eyes at all in its adult form (Day 1957).

The Haemal system of all Serpulidae consists of a central blood system and a peripheral system. In the central system blood moves from the abdomen to the thorax before finding a ventral vessel back to the abdomen. The peripheral system has branchial elements in the tentacles of the crown as well as vessels in the collar and lips. The blood is propelled from segment to segment by the contraction of the body wall in the previous segment as there is no heart body present (Hanson 1950).

H. elegans is one of the most common and widely distributed virulent foulers in tropical and subtropical harbours around the world. It naturally inhabits lagoon areas but will also foul fish farms, harbours and ships hulls. It is most comfortable on hard, rough and dark substrate (natural or artificial) with a water depth of 0.4-7m (van Nieuwenhuijzen et al., 2016; Sun et al., 2015). Since the initial identification of H. elegans they have been noted to have a range spanning both sides of the Atlantic, and includes well established populations in the southern parts of the Caribbean and Florida (Masterton 2007). At the time of its discovery in 1883 in Port Jackson, Sydney Australia, it occurred rarely other than on specific seaweeds at depths of up to 20 metres. This is thought to indicate natural occurrence rather than introduction by shipfouling. With 3 out of the 5 of the complex nominal taxa which H. elegans belongs to occurring in tropical Australia and Indonesia it can be inferred that this is a naturally occurring population (Sun et al., 2015). This conclusion in conjunction with the biographical axiom for H. elegans strongly suggests that the point of origin for this species is somewhere on the east coast of Australia (Sun et al., 2015).

H. elegans is a very refined organism and fits very well into the niche it has occupied. It is a virulent competitor and very good at spreading. There is no fossil record of H. elegans and it was not officially recorded until Haswell in 1883.

There are three prevalent theories for the evolution of segmentation in annelids: Gonocoel theory; Cyclomerism theory; and the Corm theory. The Gonocoel theory states that the annelid ancestor did not display segmentation, but had the same organs repeated along the length of its body. Over time these organs enlarged and moulded themselves into the paired segmental coeloms displayed in modern annelids. Cyclomerism theory states that the common ancestor was segmented, but only had a few segments (thought to be three), the last of which would have eventually subdivided to give us the modern form of annelids. The third theory is the Corm theory. This theory states that the common ancestor reproduced clonally and that eventually its zooids became the body segments (Ruppert, Fox and Barnes, 2014). It could be theorised that the H. elegans supports the Cyclomerism theory with three segments spontaneously developing in the larval stage (Wisely, 1958), before further segmentation mainly occurring from the posterior segment with the exception of four thoracic segments produced from somewhere in the middle of the body (Seaver, Thamm and Hill, 2005).

H. elegans itself has been an important part of the study into the origin of segmentation in annelids. Seaver and Kaneshige (2005) conducted a study to determine whether the segmentation seen in annelids originated at the same place as the segmentation in arthropods. Using genetic data from H. elegans alongside other polychaetes and arthropods it was confirmed that the origin of segmentation was distinct between the two (Seaver and Kaneshige, 2005).

H. elegans is a problematic fouling organism due to its ability to quickly colonize newly submerged surfaces, grow as much as 1/5mm a day, and having a short larval period (Flemming et al., 2009). They also have very few natural predators, with the exception of some parasitism by copepods and unicellular (Ruppert, Fox and Barnes, 2014). As a result the biggest threat to H. elegans is physical removal and destruction by humans as a result of biofouling. It is quiet common to find any Hydroides species fouling the underside of a ship and therefore is easily transported between locations (Masterton 2007). As aggregations of H. elegans is common and the organism has a short larval period and reaches sexual maturity in as short as 9 days, they are extremely quick to spread and fill an area quite densely. As discussed above, H. elegans are very heavy competitors, an example of which can be found in a report written by NIMPIS (2016) show that competition by H. elegans for food and oxygen has been associated in up to 60% mortality for cultured oysters in Japan. There is currently no considerable threat to the general population of H. elegans.

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