Sessile marine invertebrate assemblages are changing due anthropogenic activities such as shipping-induced invasions and artificial substrate providing marine infrastructure. To understand how local environments will be affected we must identify colonisation and growth patterns of individual taxa, in this study we focus on the Didemnum genus. The aim of this study was to identify Didemnum spp. growth and colonisation patterns on artificial substrate over time, and it was predicted that 1) Didemnum spp. would quickly colonise plates, 2) increased colonisation with time and 3) increased growth with time. This was assessed by deploying ARMS units for nine weeks and a year, with the presence of colonies noted and % colony cover measured using ImageJ software. The results identified rapid colonisation of nine-week units (79.6%, S.E ± 4.6%) but no increase in colonisation with time. Likewise, no increase in colony cover was observed with time. It is likely the colonisation and growth trends are result of a mixture of community ecological effects and annual water temperature variation.

Marine sessile communities have long been impacted upon by anthropogenic activities (Halpern et al, 2007), in particular shipping and coastal infrastructure have had significant impacts (Henderson et al, 2020 & Sardain et al, 2019). Shipping has been well established as a source of invasions of non-indigenous sessile invertebrates, which subsequently displace indigenous species (Floerl and Inglis, 2005 & Sardain et al, 2019 & Seebens et al, 2013). Additionally, studies have identified artificial substrates provided by marine infrastructure facilitate changes in community assemblages, whilst also providing an increase in substrate available for colonisation (Sanabria-Fernandez et al, 2018 & Sedano et al, 2019). As these anthropogenic impacts continue to influence the assemblages of sessile benthic communities, there is a need to understand and thus predict future changes to these communities.

Before community changes can be predicted it is imperative to understand the strategies for colonisation and growth of taxa. Given the myriad of strategies utilised by taxa to grow (e.g. colonial vs solitary) and colonise (e.g. differences in chemo-, photo- and geotaxis) substrates (Jackson, 1977 & Rodríguez et al, 1993) It is believed assessing the growth and colonisation of individual taxa is the most effective method to predict future changes. One significant taxon is the globally distributed genus Didemnum (Casso et al, 2019 & Jaffar et al, 2016 & Smale and Childs, 2011).  Didemnum spp. are described as a colonial Ascidian consisting of genetically identical zooids (Lafargue and Wahl, 1987). Invasions from this genus have caused substantial ecological and economic damage in both tropical and temperate coastal waters (Bullard et al, 2007 & Kremer and Rocha, 2011 & Switzer et al, 2011). Two invasive species of particular concern in Australia are D. perlucidum (associated with tropical waters) (Smale and Childs, 2011) and D. vexillum (associated with temperate waters) (Casso et al, 2019), the former of which has already been detected in West Australia. How these species will impact Australian ecosystems is largely unknown.

Whilst Ascidian recruitment and development strategies are diverse (Lambert, 1982), numerous studies into Didemnum spp. seem to suggest strategies are conserved within this genus (Hurlbut, 1992 & Ritzmann et al, 2009 & Ryland et al, 1984 & Valentine et al, 2009 & Kremer et al, 2009). Common themes identified by these studies include continual spawning throughout the year with periods of increased spawning typically in summer (Kremer et al, 2009 & Ritzmann et al 2009) but sometimes in winter (Valentine et al, 2009). Lecithotrophic larvae are produced and colonisation occurs soon after spawning (Kremer et al, 2009 & Hurlbut, 1992 & Ritzmann et al 2009). Furthermore, colony growth and morphology seem to be influenced by the prevailing conditions rather than species-specific differences (Fletcher et al, 2013). In tropical areas cooler months result in periods of increased growth (Ryland et al, 1984), whilst in a temperate study colony growth increased in warmer months (Valentine et al, 2009 & Ritzmann, 2009). The conservation of colonisation and growth strategies in Didemnum spp. allows for the use of indigenous Didemnum spp. to predict the impact of invasive species once they colonise.

Moreton Bay is an estuarine system on the East coast of Australia and is regarded as an ecologically significant system that sustains both temperate and tropical invertebrate species (Davie and Hooper, 1998). Moreton bay has experienced significant coastal development in the last couple of decades, with numerous marinas, jetties and other artificial structures developed. Increases in artificial structure increase the susceptibility of invasions within a system, such as Moreton Bay (O’Shaughnessy et al, 2020 & Ruiz et al, 2009). As of 2010 Moreton was known to have five indigenous Didemnum spp. all of which inhabit primarily tropical environments (Kott, 2010). The aim of this study is to use these indigenous species to generalise colonisation and growth patterns of Didemnum spp. including harmful invasive species on the artificial substrates of Moreton Bay. It is predicted Didemnum spp. will 1) quickly colonise ARMS units, 2) increased colonisation as time progresses and will consequently result in 3) increased occupancy of space on the plates as time progresses.

Data Collection  

Growth and colonisation trends of Didemnum spp. on artificial substrate were assessed via the deployment of Automated Reef Monitoring Systems (ARMS) for nine weeks and a year. ARMS are a standardised method of surveying marine invertebrates developed by NOAA (National Oceanic and Atmospheric Administration, NA). ARMS units consist of nine 23cm x 23cm plates and one larger base plate of 35cm x 45cm, all plates were made of type 1 PVC (fig. 1). Six ARMS units were strung upside down from a local jetty, three for a year commencing on 26/03/2019 and three for nine weeks commencing on 22/01/2020. The units were collected and disassembled, with the top and bottom of each individual plate photographed from a set distance using a photography stand. 

Photographs were analysed using ImageJ software to assess the relative % coverage of the Didemnumcolonies. The area (in pixels) of the entire plate and the area occupied by the Didemnum colonies was identified using the polygon function in ImageJ. The % coverage of Didemnum was calculated by dividing the colony area via the plate area. The base plate was excluded because its larger size meant the % coverage wouldn’t be comparable with the smaller plates. The top of the plates were excluded as Didemnum did not colonise the top of the plates at all. Colonisation of the ARMS units was assessed by identifying the percentage of individual plates within the ARMS unit that had Didemnum spp. present.

Study Site

ARMS units were deployed from One Mile Jetty (27⁰29’34.9’’ S, 153⁰24’10.8” E) in Dunwich, North Stradbroke Island (fig.2). This pontoon is largely surrounded by soft substrate particularly sandflat and seagrass; however, a small pontoon and rock wall are present on the adjacent shore. One Mile jetty is a floating jetty; thus, the ARMS units always remain at the same depth. This jetty is frequented by small recreational vessels as well as a larger passenger ferries, the latter of which has been observed creating turbulent water flow. The average water temperature of nearby Point Lookout ranges from 28⁰c in summer to 20⁰c in winter.

Statistical Analysis

Colonisation of the ARMS units was assessed by identifying the percentage of plates within the ARMS unit that had Didemnum spp. present. The mean colonisation percentages for the two deployment times were then analysed for statistically significant differences using a One-way ANOVA, with an alpha value of 0.05. The mean coverage percentage of Didemnum spp. was also calculated for each ARMS unit, but plates without colonisation were excluded from this calculation. A One-way ANOVA was also used to identify any statistical significance for coverage percent among the two deployment periods.

Time did not result in increased colonisation of the ARMS units by Didemnum spp. (ANOVA, F_1,4= 1.18, P = 0.35). However, both ARMS units deployed for nine weeks and a year had high average colonisation rates of 79.6% (S.E ± 4.6%) and 70.4% (S.E ± 7.4%) respectively (fig. 3).

The average size of Didemnum colonies also did not increase with time (ANOVA, F_1,4= 1.64, P= 0.27). Despite this, Didemnum colonies on the ARMS units deployed for nine weeks (13% S.E ± 4.7%) were much more variant in regard to coverage than units deployed for a year (6.7% S.E ± 1.4%) (fig. 4). This variance included some particularly large values of average colony coverage of 22.1% and 10.7%.

The presence of Didemnum spp. on artificial substrates, a proxy for colonisation, did not change between units deployed for short and long time periods (fig.3). This does not support the prediction of increased colonisation on the year-long deployed units. Despite the lack of change, both sets of units had a high colonisation rate, with the average colonisation rate above 70%. This supports the hypothesis that Didemnum would quickly colonise the ARMS units. There was also a lack of statistically significant differences in colony coverage, a proxy for growth, among the ARMS units (fig. 4), not supporting the prediction of increased colony cover with time. Despite this, the colonies of the units deployed for a short amount of time had greater average colony cover and variation.

The high colonisation rate on the ARMS units deployed for nine weeks indicates Didemnum spp. are fast colonisers of new artificial substrate. This supports the status quo that Didemnum spp. continually produce larvae (trickle spawning) that are lecithotrophic in nature and consequently fast colonisers (Kremer et al 2009, Ritzmann et al 2009, Valentine et al 2009, Hurlbut 2019). Indeed, Fletcher et al (2018), observed D. vexillumwas able to quickly colonise artificial substrate during peak reproductive output. However, it must be noted the nine-week ARMS units were deployed in January and year-long ARMS units were deployed in March, when water temperatures averaged 26⁰c (Bureau of Meteorology, 2020). Interestingly, given the tropical nature of Moreton Bay’s Didemnum spp. (Kott, 2010) it could be reasonable to assume increased colonisation of the year-long ARMS units as they were deployed over winter. However, this did not occur as demonstrated by the lack of difference between the colonisation rates.

There are several possible explanations for the lack of colonisation difference between ARMS unit deployment times. One possible explanation is the that as more time passed, more inhibiting species colonised the units and prevented further larval settlement. It is well understood marine sessile invertebrates deploy chemical and sometimes physical defences to inhibit the colonisation of competitors (Kremer, 2009 & Krug, 2006). A Study from Kremer et al (2009) identified that the colonisation of D.perlucidum was reduced when a more established community was present. There was a notable increase in the community biodiversity on the year-long deployed units compared to the nine-week deployed units (fig.5). Thus, it is likely there was an increase in inhibiting species present on the plates, but further analysis of the plates’ community composition is required to confirm this hypothesis. Whilst community ecology would impact the colonisation rates it is also possible the reproductive output reduced in the cooler months.

Given tropical Didemnum spp. are usually associated with increased larval output during cooler months it was surprising to see a lack of increase in colonisation rates on the year-long ARMS units (Ryland et al 1984, Lins et al 2018). However, Moreton Bay is at best sub-tropical and is known for the overlap in temperate and tropical species (Davie and Hooper 1998). In fact, Moreton Bay is the southern limit for at least four of its five indigenous species (Kott 2010). Given prevailing conditions determine the timing and magnitude of reproductive output (Fletcher et al 2013, Auker and Oviatt 2008). It could be the case that Moreton Bay’s cooler winters do not facilitate increased reproductive output as they exceed lower temperature thresholds of the tropical species. Indeed a similar observation has been made with the tropical D.perlucidum in Perth, W.A. (Muñoz et al 2015). Perth’s average water temperature is similar to Brisbane, with average winter lows reaching 18⁰c. Muñoz et al (2015) identified significant increases in recruitment and growth of D.perlucidum in summer compared to winter. Therefore, it is possible reproductive output and thus colonisation of Didemnum spp. in Moreton Bay decreases in winter. Further deployment of ARMS units at different times of the year are required to support this rationale.

The growth of Didemnum colonies followed an unexpected trend of on no increased growth on the year-long deployed units compared to the nine-week deployed units (fig.4). Furthermore, if not for the large variation observed on the nine-week units it is possible the nine-week units had a significantly greater colony cover. The possible explanations of this trend relate to changes in the community ecology as time progresses as well as the prevailing temperature conditions.

As with colonisation it is highly probable changes in community ecology over time, particularly the reduction in free space but also the increase in inhibitors, prevent increased Didemnum colony growth. Previous research involving ARMS units deployed in Moreton Bay identified Didemnum spp. were outcompeted by Bryozoans as well as other Ascidians (Blanks, 2019). Competitive interactions between these taxa increase as the community develops. Additional research in New England, USA, identified a positive correlation of Didemnum spp. colony cover with a reduction in other Ascidians (due to increased predation) (Janiak et al 2013). However, the results from this study could have occurred due to increased free space. Indeed, a study from the same location identified Didemnum spp. abundance increased with greater free space (at time of colonisation) and was not affected by changes in species richness (Osman and Whitlatch 2007). The exact relationship between Didemnum spp. growth and the effect of inhibitors are poorly understood and requires further investigation. However, the effect of free space is well understood, as free space increases so does colony cover (Osman and Whitlatch 2007 & Tebbett et al 2019 & Dias et al 2008).

Temperature is well established as a factor controlling the growth of Didemnum spp. colonies (McCarthy et al 2007 & Fletcher et al 2013 & Ryland et al 1984 & Valentine et al 2009 & Ritzmann 2009). Generally, with tropical species (occupying tropical areas) experiencing growth peaks in cooler months and reduced growth during warmer months (Ryland et al 1984 & Lins et al 2018 & Kremer and Rocha 2011). However, the results from this investigation reports no extra increase in colony size despite year-long plates being deployed over winter. These results reflect the trends observed with Didemnum colonisation, and it is possible that tropical species persisting outside tropical areas perform better in summer than winter. There is some evidence for this as identified by D.perlucidum growth patterns in Perth, W.A (Muñoz et al 2015). Again, more research is needed to confirm or reject this hypothesis.

Didemnum spp. have shown to be rapid colonisers of the provided artificial substrate in this investigation. However; this rapid colonisation has not translated to increased colonisation on year-long deployed ARMS plates as well as no increased colony cover. Ultimately this seems to infer communities quickly reach a natural equilibrium that may resemble natural communities. To confirm this, analysis of naturally occurring communities must occur in order to identify community differences between new artificial substrate and well-developed natural substrate. Currently the majority of research suggests there is such a difference (Tyrrell and Byers 2007 & Sanabria-Fernandez et al 2018). The fast colonisation rate of Didemnum spp. also allow marine infrastructure to be early indicators of invasion. Regular surveys of marine infrastructure could identify invasive D. perlucidum or D. vexillum before they establish themselves within natural communities (Sanabria-Fernandez et al, 2018 & Sedano et al, 2019). Once established, eradication of invasive Didemnum spp. in Moreton Bay may be most effective during cooler months as growth and colonisation may be reduced. This requires additional research but some evidence from D.perlucidum growing in Perth (Muñoz et al 2015) suggests winter could be the ideal time to focus eradication efforts.

I would like to acknowledge and thank Bernie and Sandie Degnan for guidance they provided in particular their knowledge of marine invertebrates and assistance in analysing the plates. Thanks is also given to the staff of the University of Queensland who were involved in the deployment and collection of ARMS units as well as their transportation to the lab for analysis. Finally I would like to thank the tutors and students of BIOL3211 who assisted in the analyse of the ARMS units and their plates.

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