Single Transect Studies of a Sheltered/Muddy Site and a

Exposed/Sandy Site on Austrovenus stutchburyi and Macomona liliana Size and

DensityJ. J. Cole and the class of 2015 in the Marine Biology and Monitoring paper of the

Bachelor in Applied Science at the University of Waikato

1.0 INTRODUCTION1.1 Topic IntroductionTwo sites on at Tuapiro Point in Tauranga Harbour, New Zealand, have been selected for this study. One is exposed to wave action and the prevailing winds and the other is sheltered from the prevailing winds and has muddy sediment. Invertebrates have been counted using sample cores and the data analysed. From the bivalve data, Macomona liliana and Austrovenus stutchburyi data was further analysed to give size and density statistics.

Hewitt et. al., (1996) has been chosen as a comparison study. That study also analysed the size and density statistics of M. liliana and A. stutchburyi and was conducted in the Manukau Harbour next to the Auckland International Airport and also had two similar sites to this study. The differences to this study’s methods is thus: Hewitt et. al., (1996) chose mid tide sites, whereas this study was conducted at three tidal elevations and they took more samples over a larger area of the Manukau Harbour estuary. They also performed more advanced statistical analyses.

The Hewitt et. al. (1996) study bears more significance in that it is becoming increasingly apparent in ecology that the distribution of organisms often change over different scales of study (for example, Allen and Starr, 1982; Dayton and Tegner, 1984; Powell, 1989; Legendre, 1993; Ardisson and Bourget, 1992; Horne and Schneider, 1994). Although the three different tidal elevations in this study will give a wider scope to Hewitt et. al., (1996).

1.2 Literature ReviewHewitt et al., (1997) had samples 1 m apart nested within each 5 m, which were in turn, 1 km apart. This was repeated 3 times. This way, their data was not as homogeneous as it is in this study and it represented a larger scale and therefore gave a clearer picture of the ecology of the entire sand flat. The data collected in this study only represents a very narrow band of sand flat.

Legendre et al., (1997) found that large scale experiments are needed to explain the spatial structure of M. liliana and A. stutchburyi. It is only at these larger scales that makes it possible to observe physical factors that influence the spatial structure of these bivalves. Therefore, it is difficult to explain the distribution of the bivalves that were sampled in this study. More samples are needed to be taken as per the method used by Hewitt et al., (1997) to make this possible.

Hewitt et. al. (1996) conducted a study at two mid tide sites in the Manukau Harbour next to the Auckland International Airport. They called their two sites, muddy and sandy. This study was also conducted at two sites with one being muddier than the other. These two sites are called the sheltered (muddy) and exposed (sandy), although this study sampled at three tidal elevations. In Hewitt et. al. (1996), both A. stutchburyi and M. liliana were more abundant in the sandy (exposed) site, which is also true in this study, especially at mid tide. Another similarity between this and the Hewitt et. al. (1996) study is that there were very few predators present. Just some seabirds and low densities of crab as indicated by the crustaceans count in figure 6.

Hewitt et. al. (1996) suggests that M. liliana are not very mobile as adults but A. stutchburyi are. Although Legendre et al., (1997) found that both adult bivalves are less mobile than juveniles. They also found that clusters of adult bivalves occur more often because of a variation of effects including predation, competition and advection etc. High tide clusters of larger bivalves indicate the hydrological history over several years. The distribution of smaller bivalves tend to be more random due to wide ranging wave action being solely responsible for the deposition of larva. They then, over a period of some years, congregate in areas that advection has placed them and then fine tune their position by moving on their own to sites that serve them well in terms of food availability and lack of predators. Indeed, in this study, the presence of the seagrass Zostera marina, increases the size and density of all marine invertebrates since it serves as a predator hideout and is hot spot for feeding (Reed & Hovel, 2006).

1.3 Aims and ObjectivesThis study aims to provide a wider scope to Hewitt et. al., (1996) in studying the three different tidal heights.

This study also aims to examine the two communities of the sheltered and exposed sites at three different tidal elevations (low, mid and high) within each site along a transect line, to decide whether their taxonomies are statistically significantly different. Special attention will be paid to the density and size of the bivalves. Those being the clam A. stutchburyi – a suspension feeder and the wedge shell M. liliana – a deposit feeder.

2.0 METHODS2.1 Study site Location and Description

Figure 1. Map of the study sites at Tuapiro Point in Tauranga Harbour in the North Island of New Zealand. The wheltered site to the west is well protected, while the exposed site to the east, while still protected by Matakana Island, is more exposed. This map is the same one as used by Lelieveld, Pilditch, & Green, (2004), but the sheltered and exposed sites have been added in as well as the transect lines.

Figure 2. Low tide at the sheltered site showing mangrove Avicennia marina var. resinifera seedlings (Sveda, G., 2015b).

Figure 3. Photo of the exposed site at low tide taken from high tide mark. This shows the Z. marina seagrass meadow (Sveda, G., 2015a)

2.2 Description of MethodsA semi-systematic sampling scheme was used to quantitatively analyse community arrays across the shore (Fig. 1). In this sampling scheme, positions were distributed systematically across the transect line, at low, mid and high tidal elevations. At each position, 21 replicate core, (13 cm diameter by 15 cm deep), samples randomly selected were collected.

The cores were then sieved using a 1 mm mesh, and invertebrates were assigned to the taxonomic groups of gastropods, polychaetes, crustaceans and bivalves. Each group was then counted. Within the bivalves, A. stutchburyi and M. liliana were identified and measurements of the maximum shell length of two common species were taken to the nearest mm. The double headed arrows in fig. 4 and fig. 5 show exactly what shell length was measured and care was taken to ensure everyone was taking the same measurements.

All data was then collated and entered into a spreadsheet for statistical analysis.

2.3 PredictionsThe first prediction is that shell size may vary with tidal elevation because of a decrease in feeding opportunities. The second, that M. liliana will be more densely populated in the sheltered site.

Figure 5. A. stutchburyi (Bould, G., 2008a)

Figure 4. M. liliana (Bould, G., 2008b)

3.0 RESULTS3.1 Community Composition3.1.1 Total Mean Densities

High tide Mid tide Low tide High tide Mid tide Low tideEXPOSED SHELTERED

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l mea

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Figure 6. Total mean density of invertebrates (Polychaetes, Crustaceans, Gastropods and Bivalves) in the exposed (black bars) and sheltered (white bars) sites at high, mid and low tide. Standard deviation error bars are, on the whole, relatively short.

The highest density of invertebrates are at the mid tide exposed site. This would e because of sightings of the seagrass, Z. marina. Also, the densities are relatively high at both low tide sites.

Apart from that mid tide exposed site, as the tidal height increases, there is a proportionate decrease in invertebrate mean densities.

Figure 7. Close up photo of Zostera marina in the mid tidal zone of the exposed site

3.1.2 Proportional Density of all Invertebrates

High tide Mid tide Low tide High tide Mid tide Low tideEXPOSED SHELTERED

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Polychaetes Crustaceans Gastropods Bivalves

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Figure 8. The proportion of density that each invertebrate group exists in.

Bivalves are most dominate at mid and low tide on the sheltered site. Gastropods are most dominant at the mid tide in the exposed site and high tide at the sheltered site. Crustaceans are not dominant at all but occur at the more exposed tidal areas and less in the more sheltered sites. Polychaetes are in higher densities in higher tidal sites. The exception is the mid tide, exposed site, where they are in very low numbers and seem to be replaced by Gastropods because of the observed presence of a seagrass, Z. marina.

3.2 Bivalve Mean Size

High tide Mid tide Low tide High tide Mid tide Low tideEXPOSED SHELTERED

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Figure 9. Mean size of Austrovenus stutchburyi (black bars) and Macamona liliana (white bars) at each tidal zone in the exposed and sheltered sites. The standard deviation error bars are long in most cases, especially for M. liliana.

M. liliana are a larger species than A. stutchburyi. The size of each species does not change very much over each site. However, M. liliana are largest in the sheltered, high tide site and smallest at the exposed mid tide and sheltered low tide sites. A. stutchburyi are notably largest in the exposed low tide site but it does not have a notable site at which they are at their smallest.

3.3 Bivalve Density

High tide Mid tide Low tide High tide Mid tide Low tideEXPOSED SHELTERED

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Figure 10. Mean density of A. stutchburyi (black bars) and M. liliana (white bars) at each tidal zone within the exposed and sheltered sites. Standard deviation error bars are extremely long indicating that there is a very wide variability of densities in the core samples at each site.

A. stutchburyi (686 total individuals) is more densely populated than M. liliana (266). A. stutchburyi is most densely populated in the exposed mid tide site. It is also densely populated in both low tide sites.

There is a spike of both species at mid tide on the exposed transect because of the presence of the seagrass, Z. marina. Apart from that site, it is apparent that increases in tidal height represent proportionate decreases in both species mean densities. This is a similar pattern to figure 6.

M. liliana it is mostly populated at the sheltered low tide site. It is also densely populated in the exposed mid tide site.

Overall, this graph is a similar shape to figure 6. The main differences is the length of the error bars, which is because there is mare data in figure 6, which makes it more precise.

Table 1. Total counts of A. stutchburyi and M liliana at each site

Exposed

Sheltered

A. stutchburyi

431 255

M. liliana 112 154

There are significantly more A. stutchburyi in the exposed site sample cores than in the sheltered site’s cores. There does not look like that there is much difference between the M. liliana counts for each site.

3.4 Statistical Analysis of Bivalve Size and AbundanceTable 2. ANOVA p values of the data sets within each site. All p values indicate very significant differences in the values of all sites.

Data sets Sites p valuesDensity Exposed 0.00

Sheltered 0.00Sheltered Exposed 0.01

Size 0.00This indicates that more analysis needs to be performed to find out exactly where the significant differences lie within the sites.

Table 3. Density T-test p values between sheltered and exposed sites within each species.

A. stutchburyi 0.00 significant difference

M. liliana 0.11 no significant difference

There is a very significant difference in the densities of A. stutchburyi between the exposed and sheltered sites. This is not the case for M. liliana.

Table 4. Size T-test p values between sheltered and exposed sites within each species

A. stutchburyi 0.04 significant difference

M. liliana 0.67 no significant difference

There is a statistically significant size difference of A. stutchburyi between the exposed and sheltered sites and no significance for M. liliana. Both data sets are less significantly different than the density data sets.

Table 5. T-test p values for overall densities at all combinations of tidal heights at each site. Non-significant differences are heavily shaded.

Exposed ShelteredTidal heights A. stutchburyi M. liliana A. stutchburyi M. lilianaHigh/Mid 0.00 0.00 0.02 0.00High/Low 0.01 0.01 0.00 0.00Mid/Low 0.00 0.22 0.06 0.00

All tidal combinations (except for between mid and low tides for M. liliana on the exposed transect and for the same tidal combination for A. stutchburyi on the sheltered transect – heavily shaded) have significantly different densities between each tidal combination.

Table 6. T-test p values for overall sizes at all combinations of tidal heights at each site. Non-significant differences are heavily shaded.

Sites Exposed ShelteredTidal heights A. stutchburyi M. liliana A.

stutchburyiM. liliana

High/Mid 0.00 0.03 0.01 0.01High/Low 0.04 0.19 0.01 0.17Mid/Low 0.06 0.57 0.94 0.05

There are significant size differences for both species between the high and mid tides at both exposed and sheltered sites. M. liliana has no other significantly different sizes in any other tidal comparison at either site. The size of A. stutchburyi are significantly different between high and low tides at both sites. There are no significant size differences at either site, for either species when comparing mid and low tides.

3.5 Size FrequenciesIt seems that at all sites that the sizes of the bivalves are very similar at each tidal elevation.

3.5.1 A. stutchburyi Exposed site

A. stutchburyi are very infrequent on the exposed high tide site, very frequent at mid tide (because of the presence of the seagrass, Z. marina) and somewhere in the middle at low tide.

The trend lines show a normal distribution at high and mid tides. At low tide, the size is very scattered, but the bars still show a very slight normal distribution curve.

The true mean gradually decreases as the tidal elevation decreases.

The total frequency of A. stutchburyi in the exposed site is 431 individuals.

Figure 11. Size frequency of A. stutchburyi on the exposed site. High tide (left), mid tide (centre), and low tide (right). Trendline shows the moving average for every fifth data entry.

3.5.2 A. stutchburyi Sheltered site

There are higher frequencies of each size class as the tidal elevation decreases.

All graphs do show a normal distribution trend line. Although at low tide, there seems to be two cohorts. One smaller cohort with a mean centring 12 – 13 mm as per the exposed site. The other larger cohort mean centring on 13 – 14 mm.

The overall frequency of A. stutchburyi is lower in the sheltered site with 255 individuals.

3.5.3 M. liliana Exposed site

Figure 12. Size frequency of A. stutchburyi on the sheltered site. High tide (left), mid tide (centre), and low tide (right). Trendline shows the moving average for every fifth data entry.

Figure 13. Size frequency of M. liliana on the exposed site. High tide (left), mid tide (centre), and low tide (right). Trend line shows the moving average for every fifth data entry.

As with A. stutchburyi, there are higher frequencies of individuals in the mid tide elevation. The size classes represented at all tidal elevations are more scattered than A. stutchburyi.

There were also sightings of the seagrass, Zostera marina at mid tide

Cohorts seems to feature more in the M. liliana data. It is not clear where the true means lie at high or low tides, but at mid tide, there seems to be one true mean at 6 – 8 mm and another at 24 – 25 mm.

3.5.4 M. liliana Sheltered site

As the tidal elevation decreases, the frequencies of M. liliana increases. This is a similar pattern to A. stutchburyi at the sheltered site although there are less overall M. liliana individuals.

The trend lines do show a resemblance of normal distribution curves. The mid tide trend line seems to show at least two possible cohorts although it is not possible to find the true mean of the smaller cohorts. The largest one seems to have a true mean size of 26 – 37 mm. There also seems to be a small cohort of smaller individuals at low tide with a true mean centring on 9 – 10 mm. The larger cohort is more obvious with a true mean of 22 – 23 mm.

The total frequency of M. liliana in the sheltered site is 154 individuals.

Figure 14 .Size frequency of M. liliana on the sheltered site. High tide (left), mid tide (centre), and low tide (right). Trend line shows the moving average for every fifth data entry.

4.0 DISCUSSION4.1 PredictionsThe first prediction that shell sizes of A. stutchburyi and M. liliana would vary with tidal elevation is partly justified. Table 6 shows that there are significant size differences for both species between the high and mid tides at both exposed and sheltered sites. M. liliana has no other significantly different sizes in any other tidal comparison at either site. The size of A. stutchburyi are significantly different between high and low tides at both sites. There are no significant size differences at either site, for either species when comparing mid and low tides. A. stutchburyi, but not M. liliana has significant differences in shell sizes between exposed and sheltered sites (Table 4). M. liliana shell size has been explained by Hewitt et. al., (1996), who found that increase in sediment grain size was positively correlated to increases in shell sizes for M. liliana, but not A. stutchburyi.

The second prediction made that M. liliana would be more populated in the sheltered site is disproven since the density T-test p value in Table 3 examining significant difference between the sheltered and exposed sites is insignificant.

4.2 Species diversityThe Hewitt et. al. (1996) study only found bivalves and a two species of polychaetes, whereas this study found a much more species diverse site (fig. 8).

It is interesting to note that where the Z. marina meadow is located at mid tide on the exposed site that polychaete abundance has fallen dramatically and out of sequence. The normal sequence for polychaetes is for their numbers to fall as the tidal height decreases but in this case, the mid tide at the exposed site, it is at its lowest proportional abundance. They seem to have been replaced by gastropods, which are predicted to be feeding of Z. marina.

The opposite is true at high tide on the exposed site where polychaete numbers are much higher and gastropods are much lower where there is less vegetation and the only sustenance for herbivores is in the phytoplankton that is delivered by high tides. Predation from the highest proportion of crustacean (crabs) numbers, is also likely to be contributing to the lower proportion of bivalves and gastropods. On the sheltered side, there are also lower numbers of bivalves and gastropods that have probably been predated on by birds. Another factor could be that the sand at high tide is more compacted by human recreational and scientific activity. It is harder for these organisms to burrow into more compacted sand as described by Lelieveld, Pilditch & Green, (2004).

In the sheltered site, higher numbers of gastropods at high tide is assumed to be because of the presence of A. marina var. marina, which they are feeding from.

4.3 Bivalve size and densityTable 4 and figure 9 combine to show that the difference between the mean sizes of both species of bivalves, especially M. liliana is not all that significant. The p value of the T-test comparing both site’s sizes of A. stutchburyi is 0.04, which is only just significantly different. Factors that contribute to different sizes of M. liliana is grain size (Hewitt et. al., 1996). This would tend to suggest that the difference in grain size between the exposed and sheltered site does not tend to vary very much. A

personal observation of these two sites is that the sheltered site did seem to be muddier than the exposed side, which was sandier.

The density of these species at each site gives a clearer picture of the ecology of these species. The T-test p value in table 3 and the total counts of each species in table 1 combine to shows that there is a significantly higher density and count of A. stutchburyi in the exposed site than M. liliana.

4.4 Size frequenciesFigures 11 – 14 show the spread of sizes that occur for each species of bivalve in each site.

In the exposed site, A. stutchburyi sizes (figure 11) do not tend to vary much except at low tide, where each size class is present. This shows that recruitment is occurring here at low tide. They then move up to higher elevations to feed on Z. marina at mid tide, where there is a high abundance.

In figure 12, the lower the tidal elevation, the higher the abundance of A. stutchburyi. The true means to don’t seem to vary much across the tidal elevations. The spread of sizes are relatively tight meaning that the individuals are all around the same age and size.

Figure 13 shows that the sizes of M. liliana in the exposed site are much more spread out indicating a high variety of different life stages of this bivalve. The recruitment of this species is therefore spread out across all tidal elevations. This tends to agree with Table 4, which illustrates that the p values of T-tests on M. liliana sizes are not significant.

Figure 14 also shows a wide spread of M. liliana sizes in the sheltered site.

4.5 Size of this studyThis is a very small scale study, which presents many problems. We only surveyed a small section of Tuapiro Point and as such, it is unwise to assume that the patterns discovered here are typical of Tuapiro Point, or any other sand flat. It is therefore recommended that the methods used in this study to be replicated as per the methods used in Hewitt et. al. (1997). Furthermore, numerous studies also indicate the importance for large scale to enable the analysis of important natural processes, abiotic and biotic (for example, Allen and Starr, 1982; Dayton and Tegner, 1984; Powell, 1989; Legendre, 1993; Ardisson and Bourget, 1992; Horne and Schneider, 1994).

The spike in density and size for all invertebrates including M. liliana and A. stutchburyi relates to sightings of the seagrass, Z. marina (Fig. 7 and Fig. 3). Reed and Hovel (2006) found that a certain threshold of Z. marina densities positively correlated to increases in densities of all epi-benthic communities. They also found that Z. marina tends to occur in areas that are disturbed by humans. This coincides with sightings of horses and a sled at mid tide on the exposed site. The Hewitt et. al., (1996) study does not mention the presence of any vegetation.

Figure 10 has very long standard deviation error bars, which again indicate that more data is reqiuired by utilising the survey design by Hewitt et al., (1997). This time, to get a better representation of the mean densities. The reletively insignificant size differences in bar graph in figure 9 gives more weight to the reason why there needed to be a bigger study. With the small amount of time available, this was as much as could be done. Any future study should take into account the study design by Hewett et. al (1997) and if possible, to allow more time for more data collection along more transect lines.

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Sveda, G. (2015a). Exposed Site. In E. Site.jpgSveda, G. (2015b). Sheltered Site. In S. Site.jpg