The tube-dwelling genus of worms, Hydroides Gunnerus, 1768 is a common occurrence in places such as the Mediterranean and the Indo-Pacific (El Hedney et al. 2020). Like many tube worms, they have colourful tentacular crowns used for filtering oxygen and food particles from the water but it’s their uniquely shaped operculum that distinguishes them. The double-layered modified tentacle is used to close off their tube when avoiding predators and has diversified so that their shapes are species-specific (Bastida-Zavala 2002). They are found attached to hard substrates either natural or manmade and the larvae settle on areas with a high abundance of biofilms and conspecifics (Lema et al. 2019, Toonen & Pawlik 1996). This has allowed them to dominate vulnerable habitats impacted by humans. As they are associated with structures such as ships and ports, human-mediated transport has brought them to areas where they can outcompete struggling native species. Their fouling of manmade structures has significant economic impacts for transport and aquaculture industries (Piola & Johnston 2008). In more balanced communities they provide habitats with the calcareous tubes and a food source either directly or via their faeces, for many other organisms(Connell & Orias 1964, Haines & Maurer 1980). Like many marine organisms, the changing environmental conditions of climate change will have direct and indirect effects on Hydroides sp. such as a decrease in tube robustness (Meng et al. 2019). 
The species of this genus identified on the ARMS plates, based of operculum appearance, are Hydroides elegans (Haswell, 1883), Hydroides diramphus (Mörch, 1863) and Hydroides ezoensis (Okuda, 1934). 

Operculum

The distinguishing feature of the genus Hydroides is the two-tiered operculum connected by a long peduncle (Bastida-Zavala 2002)(Figs. 1&2 ). The first tier of the operculum is usually referred to as the funnel with distal sections named radii. The second tier is referred to as the verticil which is made up of chitinous spines (Kupriyanova et al. 2015) (Figs. 1&2). The colour and shape of opercula range across species and this variation is used to identify most species in the genus(Sun et al. 2015). The species found on the ARMS plates are good examples of how especially the verticil of the operculum can be species-specific and their identification was made mostly from this (Fig.1). A pseudoperculum is found opposite to the operculum. It is a smaller and less developed operculum that can be used as a back up if the first operculum is lost (Kupriyanova et al. 2020) (Fig. 2). 

Branchial crown

The lophophore of these organisms is referred to as the branchial crown and is usually constructed from two lobes of radioles(branchiae) off which branch pinnules (Fig.2b) (Bastida-Zavala 2002). The branchial crown can be a range of colours and patterns including, white, brown, purple, yellow and orange and this can vary between and within species and over individual lifetimes (Sun et al. 2015) (Fig 1a-c.). Unlike other Serpulidae genera, no branchial eyes or stylodes are present (Kupriyanova et al. 2015). 

Body

The thorax of Hydroides is made up of 7-9 chaetigers with the first being referred to as the collar which is usually longer and has an anterior membranous flap at the base of the branchial crown. On the thoracic segments, chaetae are positioned on dorsally and are sorted into several types based on morphology which can helpful for species identification. On the ventral side of the thorax uncini which are deeply set chaetae, are present expect on the collar, in a rasping or saw-shaped form (Bastida-Zavala 2002). The abdomen can have up to 200 segments depending on the size and age pf the organism. The uncini are this time positioned dorsally and the ventrally positioned chaetae often get more elongate towards the pygidium (Kupriyanova et al. 2020). All these features can be seen in figure 2. 


Tube

The tube of this genus are typically white and are calcium carbonate in a glycosaminoglycan matrix (Fig. 1). The tube is formed by calcium glands in the collar and water temperature that it is produced in may affect its thickness and shape (Li et al. 2016). Lone individuals may have a tube mostly attached to the substrate with a sub-circular shape but in aggregations, as they are often seen in, the tubes may rise upwards and become more circular (Thorp et al. 1996). 

Reproduction

Most species in the genus Hydroides are gonochoristic except for Hydroides elegans which is a protandric hermaphrodite (Miles & Wayne 2009). True gonads are found in most species excluding Hydroides dianthus in which the gametes are produced from a germinal epithelium. The gametes exit via the nephridiopores and ciliary beating moves them to the tube opening (Kupriyanova et al. 2001). All members of the genus use broadcast spawning usually over an extended period, for example between May and September in Hydroides ezoensis off Japan. The eggs are typically small and released in high numbers (Miura & Kajihara 1981). Fertilisation in this species has been reported to have fast and successful rates over a range of densities (Qian & Pechenik 1998). 

Larval Stage 

The zygote then undergoes spiral cleavage forming a blastula which matures and becomes a larva (Arenas-Mena 2007). This is usually a trochophore feeding larva that uses a prototroch, a band of cilia, to swim through the water column. The larva forms simple gut with a mouth and anus separated and the cilia on the hyposphere form a neurotroch which runs ventrally (Fig. 3). Especially large anal vesicles are found in Hydroides. Left and right ocellus are developed from clusters of red pigment cells, protonephridia and ventral longitudinal and metatroch circular muscles form. Species such as Hydroides ezoensis have shown to have a positive phototrophic response (Wisely 1958). After a few days developing in the water column, these features enable the larvae to analyse several environmental factors that determine a suitable substrate for settlement. Their behaviour will then change from free-swimming to exploration of the benthos with some pausing to test locations (Kupriyanova et al. 2001). Bacterial biofilms have been shown to be a strong influencing factor of larval settlement, namely the density (Lema et al. 2019). One possible reason for this is the ability of larvae to feed on bacteria when phytoplankton levels are low (Gosselin & Qian 1998). Possible mechanisms for the settlement and metamorphosis of the larvae include changes in phosphorylation of proteins related immune responses and digestion (Zhang et al. 2010). Other factors affecting settlement are likely light, hydrology, substratum structure, dissolved oxygen and salinity (Kupriyanova et al. 2001). In some species such as Hydroides dianthus, a second major settlement cue are inductive short-range soluble compounds released from the bodies of conspecific adults or juveniles. A biofilm is usually required for settlement but as in the case of H. dianthus individuals may not begin to metamorphose until receiving these conspecific cues (Toonen & Pawlik 1996).  This causes the aggregation of particular species that is common for this genus (Kupriyanova et al. 2001). 

Metamorphosis

To begin metamorphosis the larvae produce a sticky thread posteriorly from which a the primary tube is secreted and then shaped within fifteen minutes. Metamorphosis is complete after lengthening of the body, formation of the collar and the initial branchial lobes. The calcareous tube is secreted after about 2 hours and juvenile development is signalled once the anterior branchial radioles grow. In the juvenile stage, the branchial crown is differentiated (Carpizo-Ituarte & Hadfield 1998). Growth of these species such has been shown to faster in the warmer months and is affected by temperature, population density, salinity and food availability. Maturation is reached once an individual reaches the desired size so these factors may affect the time until maturation (Kupriyanova et al. 2001). The lifespan of Serpulids is positively related to the size of individuals with Hydroides elegans lasting one year in aquariums (Grave 1933). 

General

The body plan of Hydroides sp. is unusual in that the operculum shape makes the organisms asymmetrical and the side to which it develops is seemingly random. The digestive system of Hydroides is made of a foregut, midgut and hindgut like typical Annelid body plans and nephridia are present for excretion. The musculature consists of circular and longitudinal muscles as well as muscles around the digestive system. They control are involved in digestion and circulation of blood. As for the nervous system, the branchial crown has two large nerves in each lobe called the inner and outer branchial nerves. From these branchial nerves stem nerves that innervate the radioles and opercula. Those originating from the outer branchial nerve run dorsally, and those from the inner run ventrally. These branchial nerves originate from the supraeosophageal ganglion and the nervous system extends posteriorly with ventral nerve cord (Schochet 1973). 

Circulation and Respiration

The circulatory system of the Hydroides genus shares most of its characteristics with members of its family Serpulidae. It consists of a central blood system that is confined medially in the thorax and abdomen which is made of larger blood vessels. The dorsal blood vessels carry blood anteriorly and the ventral blood sends blood back posteriorly. This direction is created by the gut muscle coat using antiperistatical movement. The peripheral blood system includes vessels in the branchial crown, including the operculum and distal areas of the thorax and abdomen such as the parapodia. These smaller vessels are unique as the end blindly and alternatively full and empty. The blood will reverse back to the central blood system instead of travelling circularly. When the crown is in the tube this circulation will stop and only the central system carries on. The radioles of the branchial crown are the site of respiration for these organisms. It is also this peripheral system that varies between Serpulidae families, for example, the blood vessels in the operculum are spiralled in the Pomatoceros subfamily and branched in most others including Hydroides (Hanson 1950). 

The genus Hydroides is a globally distributed genus and Australia is home to both invasive and native species (Sun et al. 2015). The highest occurrences are found in areas largely populated by humans and artificial structures such as docks, ships and aquaculture structures have been largely colonised by this genus (El Hedney et al. 2020) (Fig. 4). The distribution can be attributed to transport that is human-mediated such as ship ballast (Hewitt & Campbell 2010). The three species found on the ARMS plates are likely non-native species. The origin of dispersal of Hydroides elegans, however, is considered to be Australia as that is where it is most prominent however it is considered cryptogenic. The best estimate at an origin for Hydroides diramphus is the Caribbean but due to transport on shipping vessels, it reached Australia by 1983. Hydroides ezoensis was first identified and is common in the Sea of Japan and was recorded as becoming abundant in Australia by the 1980s. It’s distribution however appears in higher than the location of the ARMS plates and this is the main argument against this identification. While it is a subtropical species no records of the species are higher than Newcastle where the water temperature is lower on average by several degrees (Sun et al. 2015). The difference in latitude indicates an environmental similarity of approximately 0.35 according to Hayes et al. (2004). Another well-dispersed species of Hydroides is the Hydroides dianthus which has wide temperature tolerance that has enabled it to expand into many latitudes. While much dispersal of this species has been recorded, Sun et al. (2017) explore the presence of cryptic species shows the adaption to local environments. This could be the case in other widely distributed species that could be shown using similar analysis. 

Regardless of their origins each of the species identified on the plates is recognised as alien to some area of the world. They likely share factors that make them more successful disperses even with the large human input as they have established populations with broad ecological conditions, this also indicated as Serpulids make up 15% of alien polychaete species (Çinar 2014). The combination of areas such as harbours with increased water pollution and infrastructure that see vessels travelling from other ports has allowed these species to invade and aggregate. In these areas, native species are often already struggling because changing conditions making the communities vulnerable to invasives (Piola & Johnston 2008). Hydroides species can quickly colonise artificial substrates and aggregate through conspecific cues leads their domination of many ports and harbours (Toonen & Pawlik 1996). These changes in community structure have knock-on effects such as changes to phytoplankton population structures and other alterations to trophic interactions. 

As well replacing native ecosystems with low diversity invasive communities the fouling done by this genus has caused major economic damage. The fouling on ships hulls creates extra weight that increases fuel usage as well as compromises the hulls structural integrity (Xu et al. 2006). Other areas hurt by aggregations of Hydroides include increased costs to aquaculture and losses in tourism because of destroyed habitats (Hayes 2004). 

This economic loss especially has driven research into methods of removal and population management strategies of Hydroides. One common strategy in the past for Hydroides and other biofouling species removal is the use of specialist hull coatings that are toxic because of their metallic compounds (Alberte et al. 2009). For example, copper was used in hull coatings and infrastructure as it has negative impacts of Hydroides survivorship across larval and embryology stages (Xie et al. 2005). The use of these antifouling methods however not only target Hydroides and other hard fouling species but expose entire ecosystems to toxic chemicals. Several studies have been done on the negative ecological impacts of another major type of chemicals used for antifouling, TBTs(Tributyltin). The TBT compounds cause mortality in oyster and gastropod species that are crucial to the rest of the ecosystem for habitat, sedimentation, algal grazing (Roach & Wilson 2009, Underwood & Barrett 1990). Their absence causes a reduction in other invertebrate species because of these services that they provide (Alzeiu 2000). For this reason, they were banned in many areas of the world. Current research is now into naturally released inhibitors that have isolated from marine invertebrates. One paper Xu et al. (2009) found a branched-chain fatty acid found in Streptomyces sp. inhibited Hydroides settlement by changing protein expression. Problems facing strategies like this include the cultivability and the yield of organisms producing these compounds (Xu et al. 2009). 

The genus Hydroides belongs to a family of Annelids called Serpulids that are a group of tube building worms that are distinguishable by the presence of the operculum in comparison with other tube dwellers in Sabellida (Ten Hove & Kupriyanova 2009). Another apomorphy of this family is multiciliated terminal cells in the larval protonephridia (Brinkmann & Wanniger 2009). Within Serpulidae the genus Hydroides is believed to part of the subfamily Serpula, closely related to the genera Serpula and Crucigera (Lehrke et al. 2007)(Fig. 5). While the operculum is a common feature of Serpulidae between subfamilies how it was derived is believed to be different. In Serpula it was derived from a radiole which can be inferred from the neuronal innervation (Müller 1864). The last common ancestor of this group has been dated the Cenozoic period around 66 million years ago (Vinn 2007). Since then great diversification has occurred in the operculum namely during the Miocene. This was not in the pattern of adding complexity, the most complex group including Hydroides elegans diverged during the Mid Paleocene. Hydroides ezoensis’s ancestors diverged during the Mid Eocene (Sun et al. 2018). The modern-day diversity in the Indo-Pacific region is posited to be due to the ocean activity after this diversification (Briggs 2006). The genus may also be much more specious than currently represented because of possible cryptic species (Sun et al. 2017). 

Phylum: Annelida
Class: Polychaeta*
Infraclass: Canalipalpata
Order: Sabellida
Family: Serpulidae 
Subfamily: Serpulinae 
Genus: Hydroides 
Species: Hydroides elegans, Hydroides ezoensis and Hydroides diramphus 

The genus name reverted back to original feminine Hydroides in 2000. Most commonly it had been previously been referred to as Eupomatus (Read et al. 2017). 
*Polychaetes: The group taxa polychaeta is paraphyletic and the Annelid classes are still unclear (Weigert & Bleidorn 2016). 

As well as threatening other species through their invasion of vulnerable habitats Hydroides face also face certain threats such as pollution and climate change. As they are often found in industrial areas such as ports and harbours they are exposed to run off that is made of harmful chemicals. One of these is the chemical phorate that is used in pesticides and has been found to decrease the success of embryological development in H. elegans (Vijayaragavan & Raja 2018). Another toxic pollutant carbendazim also decreased embryological success in that species (Vijayaragavan & Raja 2020). An additional threat to organisms that depend on biomineralization for structures like tubes and shells is ocean acidification. This event caused by increased carbon dioxide levels affects ocean chemistry and therefore the ability of species like Hydroides sp. to build their homes through calcification. Simulation of projected ocean acidification levels shows that the genus may be able to survive but would experience reduced robustness of their tubes. In this study, it was also predicted they will be able to recover fast if ocean acidification levels lower because of this continued survival (Meng et al. 2019). Furthermore, another study showed that the increased temperature levels associated with climate change may counteract the effect of ocean acidification by increasing tube robustness (Li et al. 2016). Indirectly, the genus Hydroides may be vulnerable to changing environmental conditions due to their trophic interactions, such as the settlement signals they receive. The paper Hung et al. (2005) suggested that an increase in exposure to ultraviolet radiation impaired signalling from biofilms which decreased larval settlement. Likewise, the species relying on Hydroides species will be impacted by their absence in communities as they provide habitats and are a source of food for many other organisms. 

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