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Student Project

Comparison of the abundance and distribution of settled solitary ascidians on ARMS plates deployed in Dunwich and Amity Point, Northstradbroke Island QLD

Pallavi Singh 2019


In a protected marine park such as Moreton Bay it is important to understand the biodiversity that reside there. Ascidians are common marine benthic organisms found here however they possess various characteristics which make certian species ideal invaders of different habitats. In this study I investigated the abundance and distribution of solitary ascidians in Dunwich and Amity Point, North Stradbroke Island, QLD. Using photos of ARMS plates deployed in these sites for four months, I counted the abundance and position on plate of the ascidians. Afterwards I completed a series of pairwise t tests and found that abundance was significant between the two sites as well as preference for settling on the edge (p<0.05). The outcomes of these types of settlement experiments are important in many aspects such as monitoring biodiversity, identifying invasive species and even influencing plans to build marine structures such as marinas.


Moreton Bay is a protected marine park spanning over 3400km2. It is a semi closed embayment located between the southeast coast of Queensland and three dune island barriers commonly known as, Moreton Island, North Stradbroke Island and South Stradbroke Island (Gibbes et al. 2014). Moreton Bay has four entrances that facilitate oceanic exchange through tidal flushing; it is also adjacent to the East Australian Current which allows the exchange of warm tropical water and associated biota (Gibbes et al. 2014). As a result the unique dynamics of Moreton Bay has allowed it to become a diverse and productive ecosystem which supports a variety of temperate and subtropical marine life (Gibbes et al. 2014).

Benthic marine invertebrates are animals which live on the ocean floor (Brusca et al. 2016). In their life cycles most have a motile larval stage which they use to find ideal substratum to settle and metamorphose into adults on (Brusca et al. 2016). Settlement is a vital part of invertebrate life cycles and occurs in response to various stimuli. Stimuli include environmental conditions such as water currents (velocity), wind patterns, topography and luminosity as well as ecological processes like competition, predation and disturbances (Rodriguez et al. 1993). Moreover factors such as the number of larvae and other chemical cues from conspecifics for example can also influence settlement (Rodriguez et al. 1993). In environments such as Moreton Bay where anthropogenic activities like driving ferries are prominent, the influences of the aforementioned factors on larvae settlement can change in varying degrees.

Ferries and recreational boating are a common site in Moreton Bay. Boating is a disturbance to natural environments and its ecological impacts are increasing with more recreational fishing, larger vessels and coastal developments providing better access to more waterways (Bishop 2003). Impacts of boating include: the addition of contaminants such as organic and inorganic chemicals as well as physically damaging, displacing and/or decimating species like corals and sea grasses (Bishop 2003). Wave action is also altered by the wash (loose water left behind a vessel as it moves) of boats and the waves created by travel speed (Bishop 2003). The chemical and physical disturbances created by boats influence the stimuli larvae use during their settlement process.

One marine invertebrate family common to Moreton Bay are ascidians, commonly known as sea squirts. Ascidians are sessile organisms that can be found at all depths and on almost any substratum (Brusca et al. 2016). They are suspension filter feeders and exist in both solitary and colonial forms (Brusca et al. 2016). Colonial forms grow as uniform masses smothering the surface they are settled on (Brusca et al. 2016). Solitary ascidians live attached to hard substrata (Rius et al. 2010) and can be easily identified by their two distinct incurrent and excurrent siphons (Brusca et al. 2016). Solitary ascidian larvae have short lifespans and therefore need to settle quickly (Rius et al. 2010). The settlement behaviour of this family differs between species, this makes it difficult to generalise the factors which effect their settlement (Brusca et al. 2016). However, from the existing research on particular species it has been found that settlement occurs as a result of larvae possessing sensory receptors possibly involved in substratum selection (Brusca et al. 2016), other factors such as light, gravity, salinity, presence of adults or competitors, biochemical properties and energy limitations have all been found to be linked with the settlement of solitary ascidians (Rius et al. 2010).

The relationship between solitary ascidian settlement and boat traffic has not been extensively researched. Of the few studies that exist many focus on the communities in marinas. For example Webb and Keough (2000) deployed settlement plates inside and outside of two marinas to survey the composition of fouling assemblages. They found that there was more solitary ascidians inside the marinas and attributed it to decreased competition. Another study by Rius et al. (2010) aimed to study the settlement rates of six solitary ascidian species while testing for the effects of phototaxis (light), geotaxis (gravity) and presence of conspecifics (adults). They found that most species preferred to settle when dark and had a position preference. No relationship was found between settlement and the presence of conspecifics.

Settlement influences habitat selection and in doing so determines the abundance and distribution of a species (Rius et al. 2010). Investigating the abundance and distribution of ascidians is important. Due to their tolerance of a range of temperatures, salinities and pollutants (Smale and Childs 2012) ascidian species have become successful in invading natural habitats by taking advantage of areas with low predator abundance and increased food availability (Brusca et al. 2016). In protected areas such as Moreton Bay it is highly likely that boat activity may be directly and indirectly facilitating the invasion of solitary ascidians. This is precisely why settlement studies need to be done. Using settlement plates to assess the abundance and distribution (on and among plates) of species on a small scale can provide information such as, species richness, relative abundances of indigenous and non-indigenous species and interactions between species which can be applied on a larger scale for example, the construction of marine structures like marinas (Pati et al. 2015). Along with this ascidians are also classic biofouling organisms. Settlement plates can be useful to antifouling research and assessing biofouling communities (Pati et al. 2015).

In this study I aimed to evaluate and compare the abundance and distribution of solitary ascidians on settlement plates deployed in Dunwich and Amity Point, North Stradbroke Island, QLD. I hypothesised that (1) Dunwich would have higher abundance and denser distribution because of the increased disturbance caused by boat traffic allowing ascidians to become opportunistic during settlement, (2) the ascidians would prefer to settle on the edges to facilitate filter feeding.

Materials and Methods

ARMS (Automated Reef Monitoring System)

These settlement plates are simple built structures that are usually fixed onto marine substrates. They are used to survey biodiversity, largely on coral reefs but now in other benthic habitats such as in Moreton Bay. The ARMS contain 10 levels labelled A-I for clarity in this report (Figure 1).


Dunwich and Amity Point are two major locations on North Stradbroke Island (Figure 2). Dunwich receives heavy boat traffic predominantly from water taxis transporting people to and from the mainland. Amity Point is a leisurely area with no traffic from ferries however it is frequented by swimmers and recreational fishers.

ARMS Analysis

Using a camera top and bottom photos were taken of plates A-I from each of the ARMS. I then used the photos for my analysis focusing on the bottom side of the plates. For each plate I identified the number of solitary ascidians and their position on the plate. Solitary ascidians were identified by their two distinct siphons as well as, for some, their gelatinous appearance (Figure 3). To measure position on the plate, using my computer and a ruler I drew an 11cm x 11cm square and fit it onto the middle of the pictures which were 20.5cm x 20.5cm (Figure 3). I then counted and recorded the number of ascidians in the square (middle) and outside of it (edge).

Statistical Analysis

For the statistical analysis I completed a series of pairwise t-tests using R studio. I used the t.test function to compare the significance of ascidian abundance amongst the plates across all three ARMS; in other words I completed tests comparing plate A&B, A&C, A&D etc. until all plates had been tested with each other. I also used t-tests to compare the significance of position on plate within (tested the average of middle against edge abundance for each site) and between the two sites (tested the overall average abundance for middle in Amity Point against Dunwich and repeated it for edge abundance).

Figure 1
Figure 2
Figure 3


Counting of the ascidians revealed Dunwich had 128 specimens and Amity Point had 51. Pairwise t-tests on the number of ascidians revealed that the difference between plate levels A&B, A&C, A&D and A&G was significant (CI=95% p<0.05) (figure 4). The same analysis was completed on the Amity Point plates and plate levels A&B, A&e, B&G and E&G had significant differences (CI=95% p<0.05) (figure 4). Pairwise t tests were also completed on the position data and it was found that the abundance of ascidians located on the edges were significantly greater than those in the middle, both within and between the sites (CI=95% p<0.05) (figure 5).
Figure 4
Figure 5


The aim of this study was to compare and evaluate the abundance and distribution of solitary ascidians across two sites in Moreton Bay. Following four month deployments in Dunwich and Amity Point, using photos of each of the plates the average number of ascidans and position per level (A-I) across the three ARMS was counted. After collating and analysing the results the hypothesised outcome was proven to be correct. Dunwich had a higher abundance and somewhat denser distribution then Amity Point and the ascidians also seemed to prefer to settle on the edges of the plates.

In general the number of ascidians counted on the plates from Dunwich had more than double compared to Amity Point (n=128 and n=51). The Dunwich plates also showed a trend in regards to average distribution among the plates- the most significant differences in distribution were among the bottom few plates A-D (p<0.05). The plates from Amity Point showed some significance but there was no identifiable trend. Analysis of positions showed that the ascidians preferred to settle on the edges of the plates with both locations having over 80% of individuals positioned outside of the 11 x 11cm square. Preference for settling on the peripherals of the plates is also supported by highly significant p values found both between and within the sites (p<0.05).

Since this study only focused on evaluating and comparing the abundance of ascidians there is no data relating to specific factors which may have affected settlement, therefore inferences explaining the results will be made based on background research. The intensity of light may have influences the abundance of individuals. For this study abundances were counted from the bottom side of the plates because on observation they had more individuals (Dunwich 99%, Amity Point=65% of individuals on the bottom). This may be related to the bottom of the plates having more shade, along with this the constant presence of boats may have lowered the intensity or periods of exposure to light, allowing more suitable settlement conditions for the ascidians. The opposite may be true for Amity Point as the lack of large points like ferries and calmer waters may have increased light exposure. It is also possible in both of these sites that chemical cues from conspecifics may have occurred; this is because there are peaks in the data (figure 4) where significantly more individuals were counted. The trend and lack of relationship between seen between the distribution of individuals among the plates in Dunwich and Amity Point respectively may have been attributed to mixing in the water column. Boats create waves which allows some mixing in the water column distributing larvae to lower levels. It can be predicted that in Dunwich more individuals were located on plates B,C and D because the constant wave action created by the boats allowed a deeper level of mixing with more larvae reaching lower plates. Amity Point had a patchier distribution which would correspond to its lower wave action compared to Dunwich and occasional mixing event where larvae would be taken to deeper waters. Positioning preference on the edge is most likely due to the biological requirement of ascidians to filter feed in order to survive. It is appropriate to assume that water flow would be unobstructed and the strongest along the edges of the plates, this is supported by the observation that 100% of the individuals on the peripherals where orientated outwards. It is suitable to assume that the factors which affect the settlement of solitary ascidians in Dunwich and Amity Point expand beyond the scope of what is inferred here.

There are several studies that investigate the factors affecting solitary ascidian recruitment however, there are very few that compare abundance and distribution amongst sites similar to Amity Point and Dunwich. Of the literature that does exist Webb & Keough (2000) looked at the settlement of fouling communities inside and outside of two marinas. The conditions inside marinas may be slightly similar to those at Dunwich in the sense that there is a lot of movement from boats. In relation to this study Webb and Keough (2000) found that solitary ascidians were more abundant inside the marinas however they concluded that it may be because of decreased competition. Decreased competition may offer an alternate explanation to the high abundance of ascidians in Dunwich. Sahu et al. (2015) completed a study on the biofouling communities around a jetty in India. They calculated ascidian abundance as percentage covered on wood panels. They found that solitary ascidians were more abundant in the summer months where the sea temperature was also higher. These results are more closely related to what was found in Moreton Bay since the plates were deployed during the summer, which seems to indicate a trend for the settlement of solitary ascidians.

This study was limited by the use of photos and a small number of replicates, as a result the accuracy of the data collected may have been compromised due to unclear pictures and the inability to perform complex statistical analyses. For future studies I would recommend deploying more ARMS and using the physical plates as well as pictures to gather data, by doing this ascidians can be accurately identified and counted. Other data related to factors affecting settlement should be measured for example, from the plates species richness- to test for the effects of competition-, environmental parameters such as temperature, water turbidity and wind speed as well as data on disturbances like ferries and recreational fishers. Obtaining these additional sets of data can provide a more precise analysis on the factors which effect abundance and distribution of solitary ascidians.

To summarise, it was found that solitary ascidians were more abundant in Dunwich compared to Amity Point. It was also seen that individuals preferred to settle on the edges of the plates. These findings support my hypothesis and may be explained by various factors including level of light exposure, chemical cues and wave action. For future studies it is recommended that the factors which may affect settlement of solitary ascidians be specified and investigated thoroughly. Studying the abundance and distribution of solitary ascidians is important especially to asses the biodiversity of areas and to recognise invasive species.


Thank you to Sandie, Bernie Degnan and all of the tutors for being supportive and extremely helpful during this project.


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