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Colonisation and Competitive Potential of Didemnum on Moreton Bay ARMs Plates


Jorgia Blanks 2019

Abstract
A particularly prolific and aggressive colonizer of the genus Didemnum was detected growing on marine settlement plates deployed at two sites within Moreton Bay. The colonial ascidian was the most abundant species that appeared on the plates and its overgrowth abilities posed a potential threat to individuals of other species. Extensive image analysis of the plates was conducted to quantify and categorize the abundance and growth patterns of the Didemnum. Subsequent data analysis revealed that the ascidian exhibits significant settlement preferences for the underside of surfaces. Many artificial and disturbed environments such as ship hulls and jetties are compatible with this growth pattern, therefore an affinity for this orientation can prove problematic for the prevention and management of species dispersal. In addition, the Didemnum was significantly more abundant at the Amity Point site, which may be an indicator of its mode of arrival or alternatively suggest preferences for higher energy environments with less sediments. The relationship between the phylum of competitors and the competitive ability of the Didemnum is strong, with the focal ascidian being a poor competitor against bryozoans and other chordates, known for their anti-fouling capabilities. In contrast, the ascidian was efficient at overgrowing molluscan and arthropod individuals which is consistent with other literature on the subject. Overall, this species is a highly effective colonizer, especially in disturbed environments, and has the potential to spread elsewhere becoming an invasive pest. It likely poses less of a threat, however, to already established local benthic communities due to a weaker competitive ability in comparison to its counterparts.

Introduction

Marine pests are of increasing global concern due to high rates of shipping traffic interconnecting the oceans more than ever before (Bax et al 2003 and Hayes & Sliwa 2003), and climate change resulting in increasingly homogenized landscapes promoting the success of a few highly adaptable colonizers and competitors to the detriment of all other species (Sutherhurst 1995). The latter point can, in certain cases, also result in previously benign native species becoming increasingly prolific and posing a threat to their own native ecosystems.

Members of the genus Didemnum, a colonial encrusting ascidian, have repeatedly been observed to display highly invasive tendencies which are of concern in both Australia (Dias et al 2016) and worldwide (Fofonoff et al 2018). This is in part due to its fast growth rate and efficient reproductive capacity (Muñoz etal 2015), however, the major threat from this genus stems from its high colonization capacity in disturbed environments (Kremer et al 2010), and its strong competitive abilities (Kremer et al 2010 and Muñoz &McDonald 2014). A number of exotic Didemnum species appear on the National Introduced Marine Pest Information System (NIMPIS) list of species which are of concern for introduction or have already been introduced into Australian waters (Australian Government NIMPIS). In 2010, Didemnum vexillum was identified inTwo-Fold Bay, New South Wales, and appropriate quarantine action was swiftly taken in accordance with the Australian Government Marine Pests Rapid Response Manual (NSW DPI 2010). The outbreak is currently under control. Didemnum perlucidum, which most closely resembles the focal Didemnum species for this study found in Moreton Bay, is of unknown origin but has systematically invaded parts of North America, the Caribbean, East and West Africa, and the Pacific (Fofonoff et al 2018) (Figure 1). In 2010 it was discovered along the coast of Western Australia (WA) (Dias et al 2016) and is currently known to be present from the south coast of WA all the way along the west coast and into the Northern Territory (Muñoz & McDonald 2014). The outbreak is currently being managed in state marine parks and reserves, however, attempts to contain and stop the spread and establishment of D. perlucidum have been unsuccessful (WA DPI 2017). This species is of particular concern due to potential ecological impacts as it has been found to grow on and subsequently smother seagrass (Simpson et al 2016), which is of particularly high ecological importance and economic value in many regions. In Brazil, the outbreaks of the species have threatened aquaculture facilities due to its capacity to quickly overgrow shellfish in farms (Baptista et al 2007).

Many endemic and native species of Didemnum exist in Australian waters which are generally present in small colonies and have little to no significant impact on the surrounding environment (Kott 2004). It is often challenging to confidently taxonomically distinguish invasive Didemnum species from endemic (Kott 2004). The identification process requires molecular identification such as DNA barcoding in combination with extensive observation of internal anatomy and larval forms (Muñoz & McDonald 2014), which are beyond the scope of this laboratory. In lieu of precise species identification, however, a potentially problematic member of the Didemnum genus was detected growing on Autonomous Reef Monitoring Structures (ARMS) deployed at Amity Point and Dunwich in Moreton Bay in 2018. ARMS were also deployed at Manly Boat Harbour, however, no Didemnum was present on these plates. Despite significant environmental and species composition differences between the two sites, Didemnum was incredibly prolific in both, present on all ARMS and on the majority of plates. It was frequently observed to exhibit concerning growth patterns including growing over much of the plate (Figure 2), including other species that had settled there. This research project has thus been undertaken in order to understand the environmental and growth pattern preferences, and colonization and competitive abilities of this Didemnum species. The Didemnum is expected to exhibit strong preferences for an ‘upside-down' orientation which would confer more safety and protection in the natural environment. Based on its invasive behaviors, Dunwich is predicted to be the site as which it is most abundant, due to high rates of boat traffic. Didemnum species are generally effective colonisers and competitors (Muñoz & McDonald 2014, Smale & Childs 2011). Should this principle hold true with this species, it is expected to see low rates of avoidance compared with overgrowth. These studies will examine the full extent of the Didemnum threat to the Moreton Bay ecosystem, and enable informed management of the potential pest. 
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Figure 1
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Figure 2

Materials and Methods

ARMS Plates

Autonomous Reef Monitoring Structures (ARMS) are three dimensional structures consisting of nine 25cm x 25cm settlement plates and a larger base plate which mimic the structure of reef habitat so as to attract and encourage colonization of a variety of marine species (NOAA 2017 and Marraffini et al 2017). For this experiment ARMS were deployed on November 20, 2018 for four months at each North Stradbroke Island site (Figure 3) to assess the abundance, distribution, and competitive and colonization abilities of Didemnum in the Moreton Bay region over the summer period. At Dunwich, three ARMS were mounted upside-down on the underside of One Mile Jetty (27°29’35”S 153°24’11”E), a floating pontoon which fluctuates with tidal motion. Due to the mounting orientation, the ‘top’of the plate which would traditionally be the bottom is denoted by the side oriented towards the surface of the water. This site is frequented by the Stradbroke Flyer ferry which departs from Cleveland, as well as many other irregular vessels. ARMS were retrieved on 13 March, 2019.

At Amity Point, three ARMS were mounted in a similar fashion on the underside of Amity Point Caravan Park Jetty (27°24’05”S 153°26’13”E), a fixed structure so the ARMS depth will fluctuate with tidal movements. This jetty is not accessible to boats, and is primarily utilized for recreational activities including fishing. ARMS were retrieved on 20 March, 2019.

Following collection, each plate was comprehensively photographed on each side in a University of Queensland St Lucia laboratory and logged in the Great Barrier Reef Invertebrates (GBRI) ARMS database prior to further analysis.

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Figure 3

Image and Plate Analysis

Plate photographs were analyzed using ImageJ/FIJI trainable segmentation software. Figure 4 demonstrates the process by which images were analyzed by first increasing contrast and decreasing brightness to more clearly demarcate the white Didemnum from the light grey plate background, implementing the trainable segmentation tool to recognize and select for the Didemnum colour and pattern, and finally analyzing particles greater than 15mm2 to give the percentage plate coverage while excluding ‘noise’ from the analysis.


Didemnum in each plate was additionally assessed for its competitive ability by noting the number of times and the phylum of organisms it was clearly growing around (avoiding) (Figures 5 & 6) and growing over (Figures 7, 8 & 9).

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Figure 4
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Figure 5
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Figure 6
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Figure 7
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Figure 8
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Figure 9

Data Analysis

Welch Two Sample T-tests were performed in RStudio to assess differences between sites and orientation. Chi-squared analysis was performed on categorical growth pattern variables. All figures generated in RStudio and Excel.

Results

Orientation Preferences

The Didemnum showed a very strong and highly significant preference for the bottom of the growth plate (side orientated towards ocean floor) across both deployment sites (p<<0.001,t=7.7257, df=57.05 at 95% CI), as depicted in Figures 10 and 13. Over both sites the mean area of plate covered on the underside was 21.79%, (sd=17.29, n=53), while the topside equivalent mean was 2.99% (sd=3.82, n=53).

This same result was repeated when each site was measured individually. At Amity Point the mean coverage on top was 5.48% (sd=4.01, n=26), and below 27.74% (sd=18.18,n=26), with a significant p<<0.001 (t=6.0957, df=27.43 at 95% CI). Similarly, at Dunwich p<<0.001 (t=5.5139, df=26.366 at 95% CI), with a very low average topside coverage of 0.60% (sd=1.22, n=27), and comparatively higher bottom coverage of 16.05% (sd=14.51, n=27).

From the standard deviations, variance was very high across all treatments. This can be accounted for when results are broken down plate by plate (Figures 12 & 13) and considering the small sample size.


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Figure 10

Site Preferences

Amity Point had the greater abundance of Didemnum as measured by average percent coverage of all plates (Figure 11 & 12) (p=0.006184, t=2.8009, df=94.262), with a mean of 16.61% coverage (sd=17.21, n=52), while the mean coverage at Dunwich was just 8.32% (sd=12.84, n=54). The difference was more significant(p<<0.001, t=5.9464, df=29.405) when comparing only the topsides of plates, with Amity still the more abundant site (Figure 12). The Amity Point mean remained consistently higher but the disparity between sites was less apparent when comparing the coverage of the underside of the plates, with a p-value of 0.013 (t = 2.5805, df = 47.789) (Figure 12).
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Figure 11
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Figure 12
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Figure 13

Competitive Ability

There were 125 clearly discernible cases of a Didemnum interaction with another species. Almost twice as many instances of avoidance behaviour were observed from the Didemnum than organism overgrowth (Figure 14). In particular, the ascidian tended to avoid members of the phylum Bryozoa ten times more often than it was able to overgrow them. A similar interaction was observed between the Didemnum and fellow urochordates, with avoidance behaviour observed four times more frequently than competitive overgrowth.

Competition was more successful against molluscs and arthropods, though instances of these interactions were less frequently observed. Didemnum succeeded in overgrowing molluscan species 21 times from 25 observed interactions, and arthropods 11 from 14 meetings.

Chi-squared analysis confirmed a significant relationship (p<<0.001, df=3, chi-square=51.4) between the Didemnum competitive growth pattern and phylum of competitor.
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Figure 14

Discussion

Orientation and Settlement Preferences

As expected, the Didemnum displayed a strong preference for settlement and growth on the underside of ARMS plates.Once invertebrate larvae reach competency, they will have developed the capacity for chemotaxis, phototaxis and geotaxis: the ability to respond with motile action to these stimuli (Rius et al 2010). Most of the organisms collected were found to have settled on the underside of plates, so this is either a convergent or homologous adaptive trait and supports a conclusion that a bottom orientation is advantageous. Didemnum larvae utilize geotaxis to select for the underside of plates, which likely translates to a better chance of metamorphosis and survival. The underside of surfaces is in general a safer settlement option. The organism is less visible to predators in the water column, and the underside offers shelter from exposure due to tidal changes or weather. In some cases, it can be more exposed to nutrient rich upwelling,which is beneficial to the many sessile marine invertebrates, including Didemnum, that are filter feeders(Riisgaard & Larsen 1995). The higher rates of plate coverage which were observed within the ARMS rather than on the outer plates follow a similar logic, by offering greater physical protection from exposure and predation.

This outcome has certain implications for the spread and dispersal of this Didemnum species should it become a marine pest. Jetties, pontoons and many other man-made structures and disturbed environments offer excellent ‘undersides’ for the ascidian to colonise. The most concerning of these is the biofouling of ship hulls, colonization of which will lead to the transport of Didemnum from one location to another.

Colonisation

At Amity Point, Didemnum on average covered close to one third of the undersides of plates. On individual plates,this number was as high as 60%, and across all sites 20% of the plate bottom was covered by Didemnum. A similar pattern has been demonstrated on many occasions (Kremer et al 2010, Muñoz et al 2010, Muñoz & McDonald 2014, Smale and Childs 2011 and Simpson et al 2016), and is simply due to the fact that Didemnum are very good colonisers (Kremer et al 2010, Muñoz et al 2010, Muñoz & McDonald 2014, Smale and Childs 2011 and Simpson et al 2016). Lecithotrophic larvae enable them to proliferate and settle very quickly (Muñoz & McDonald 2014). In this instance, embryos fertilised by individuals on one plate would be able to settle and metamorphose within a matter of hours on an adjacent plate or ARMS before they could be carried away by currents or tidal movement to a less ideal settlement location. The colonization and spread is also aided by asexual budding of zooids across the plate surface before many other species have even had the chance to settle (Muñoz & McDonald 2014 and Simpson et al 2016). Individuals are highly fecund (Kremer 2008), even once the plates had returned to the lab the ascidian was observed on multiple occasions to release high volumes of sperm,despite experiencing extreme disturbance. The major problems with invasive Didemnum species in have occurred as a result of this superior colonization ability (Kremer et al 2010, Muñoz et al 2010, Muñoz & McDonald 2014, and Simpson et al 2016). Instances of D. perlucidum growing on top of and smothering seagrass have been reported in Western Australia and the Northern Territory (Simpson et al 2016), and both D. perlucidum and D. vexillum have threatened shellfish farming programs across Central and South America (Rocha et al 2009). These are extreme cases,but in order to prevent a similar outcome in Moreton Bay, the problem should be monitored.

Site Preferences

There are two potential hypotheses to explain why Didemnum abundance was significantly higher at Amity Point than at Dunwich, the first of which being that site preferences are merely indicative of mode of arrival. Amity Point is exposed to the open ocean through which hundreds of large international ships pass regularly in deeper water. If the first colonizer for this particular ascidian in the area was carried in by ballast water, on the hull of a ship (Bax et al 2003 and Hayes & Sliwa 2003), or Pacific Ocean currents, it would pass through Amity long before reaching Dunwich. If these truly are just the beginnings of an invasive front, we would expect to see progressively more growth and prevalence of the Didemnum in Dunwich and throughout Moreton Bay (Bridgwood et al 2014). This also offers a potential explanation for the complete absence of Didemnum in Manly Boat Harbour.

Disparities in site prevalence are symptomatic of environmental suitability is an alternative hypothesis, that is not necessarily in contradiction with the first. It would, however, also be applicable if the Didemnum is not an invasive, but rather a native species that has had a particularly successful spawning season and growth period in Moreton Bay.There are obvious distinctions between the two sites. As above, Amity is exposed to input from wider oceanic currents, and is a higher energy environment. The ARMS stationed here were static and thus affected by tidal movement. In contrast, Dunwich is sheltered and has more input from the bay area which in turn is regularly exposed to flows from the Brisbane River during high rain periods. The Dunwich ARMS were attached to a floating pontoon and so tidal motion was far less apparent. Any one or none of these differences could constitute a superior habitat for Didemnum at Amity, and this is a crucial area for future research into preventing a Didemnum outbreak (Muñoz & McDonald 2014).

Competitive Ability

The competitive ability of the Didemnum was not as effective as initial observations had suggested. The growth pattern tended to avoid more species than it was able to overcome, particularly bryozoan and chordate phyla. Generally, studies place ascidians as superior competitors to bryozoan and most other phyla (Russ 1982 and Todd & Turner 1988). Current theory points to their species-specific symbiotic relationships with bacteria acting as an anti-foulant (Schmidt 2015). On a species level,however, in many instances the Didemnum was particularly weak or ineffective against most other ascidians and encrusting bryozoans. Potential conclusions that can be drawn from this point towards this ascidian being a particularly weak competitor, however, further investigation into the species level interactions is advised. The instances of Didemnum overgrowing molluscs and arthropod barnacles are consistent with other international studies (Baptista et al 2007 and Rocha et al 2009), although it was at times unclear whether the organism within the shell was still alive. There remains a possibility that the ascidian may also be ineffective against live individuals of mollusk and arthropod phyla.

Implications and Recommendations

Didemnum species have the potential to become highly invasive in the marine environment and present unique management challenges (Kremer et al 2010, Muñoz et al 2010, Muñoz & McDonald 2014, Smale and Childs 2011 and Simpson et al 2016). Effective colonisers, particularly in disturbed environments, the major concern with regards to Moreton Bay is a conceivable threat to seagrass beds and reef habitats (Muñoz & McDonald 2014 and Simpson et al 2016). The Didemnum species which was abundant on ARMS within Moreton Bay,however, was a far better colonizer than it was a competitor against other major sessile and encrusting phyla. The threat, therefore, remains mostly limited to new and highly disturbed environments, rather than the already established benthic communities which make up the majority of biodiversity of Moreton Bay. The ascidian should, however, continue to be monitored on future ARMS to be properly identified and to ensure it remains under natural biological control within the ecosystem.

Acknowledgements
I would like to thank Bernie and Sandie Degnan for their assistance and guidance with this project. I would also like to acknowledge the tutors and UQ staff who were involved in deploying and retrieving the ARMS plates from North Stradbroke Island.

References

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