Do the fish abundance and diversity vary with the size and the type of corals?
I.BACKGROUND
Nowadays, coral reefs are considered as the most diverse marine habitat [1]. Therefore, coral reefs can be called as the rainforest of the sea [1]. This marine environment has complex communities of various types of species which occupy different ecological niches. Moreover, as coral reefs can be encountered in any ocean, they have a great ecological importance for many species. Thus, the Red sea is also a favourable habitat to analyse the diversity of the different species present in coral reefs. The entire Red sea coastal reef is about 2’000 km long [2] and the major corals growing are branched species of corals, belonging to the gender Acropora and Porites [2]. In the red sea, 300 hard coral species have been recorded. This is four times more than the species found on Caribbean reefs, and can be comparable to the species richness found in the Maldives and the Seychelles. The red sea coral reef is characteristic, and has features that lack in other reefs around the indo-pacific oceanic region. This is due to the poor and slow water and larval exchange between the sea and the surrounding ocean [2]. This confers a protection against climate change and coral bleaching. More in details, this coral reef has developed high tolerance to high temperatures and salinity (and this is very important in the light of global warming). Moreover, the Red sea lacks rivers which can discharge sediments and nutrients that can damage the coral reef [2]. Therefore, the Red Sea can be considered as one of the most healthy coral reef barrier and is not concerned by mass coral bleaching event [2]. Another consequence of the isolation of the red sea from the ocean is the presence of a high endemism of marine life. Indeed, 10% of recorded coral reef fish species (Fig. 1) are endemic to the Red Sea [2].
​
​
​
​
​
​
​
​
​
​
​
Figure 1: Chaetodon semilarvatus, part of the Chaetodontidae Family, ©Fotolia [2]
II.RESEARCH QUESTION AND HYPOTHESES
The aim of our project is to quantify the fish abundance and diversity in relation to the size and the type of coral. So our research question is, do coral size and type affect coral reef fish diversity and abundance? Our hypotheses are the following ones:
H0= There is no effect of the type and size of coral on the fish abundance and diversity
H1= There is no effect of the size of coral on the fish abundance and diversity
H2= There is no effect of the type of coral on the fish abundance and diversity
We predict that fish will be more abundant in a certain type of coral which provides more chance to hide from predators. Moreover, the coral size will also determine the fish abundance present in a certain type of coral. We predict that there is a positive relationship between fish abundance and coral size. This is simply due to the fact that bigger corals have the capacity of passively attracting more individuals and thus, just by chance, of accumulating more species than smaller ones [3]. Moreover, more common species (in our case families) have a greater chance to also occur on smaller corals, and rarer species, or families, have a higher chance to just occur on bigger corals (a so-called nested pattern) [3]. Fish diversity will also increase as the coral size increases, as some family cohabits in the coral, while others just pass through to find food or use them as a temporary shelter. We predict that both abundance and diversity are highly dependent from the type of the coral, as some can be used as shelter, temporary home or as food resource.
III. MATERIALS AND METHODS
3.1. Study site and Study subjects
The study was conducted in the Red Sea at Abu Sauatir (Fig. 2) which is situated at 14 kilometres north of El Quseir, Egypt, over five days during June 2016 (http://www.rootsredsea.com). The coral reef of Abu Sauatir is a fringing reef, a kind of coral reef which grows directly from the shore [2].
Figure 2: Abu Sauatir coral reef map
3.1.1 Corals
The variety of coral shapes and sizes largely depend on the species and their emplacement on the reef. Different coral growth forms can be found on a coral reef and descriptive words are used to describe growth form. Some common terms used are: branching, upright, massive, encrusting, plating, and solitary [4,5,6]. During our study, we focused on the most common types of corals present in Abu Sauatir coral reef, without considering their species, but characterizing them according to their shape. Thus, we focused on 5 different types (shapes) of corals: Fire coral, Finger coral, Foliate coral, Massive coral and Plate coral. It is important to mention that we considered the “Fire Coral” as one coral type even if it is not a true coral. Indeed, “Fire corals” were abundant at the study site, thus we considered important to include them in our data collection. Moreover, we will only consider the branched form of this coral, which has a mustard brown colour and is the most common one. Then, we considered Finger corals and Branching corals as a same category and we will only use the term Finger during our study. Finally, we did the same with Plate and Tabulate corals. We considered both as a same category and only use the term Plate coral in our study.
Below is a brief description of the 5 coral types considered in our study. All of them are hard coral type, which means they have a calcium carbonate skeleton, with the exception of the “Fire corals”.
FIRE CORALS (Fig. 3): Fire corals belong to the genus Millepora and are not true corals, because they belong to the taxonomic group of the hydrozoans, while all the true stony corals are anthozoans [7]. However, they are calcareous species. They can grow and encrust rapidly on the reef crests, also when there is high water movement [7]. Since they belong to the hydrozoans, fire corals also have nematocysts, which can sting. However, the sting is not dangerous [7]. Fire corals are present worldwide in tropical seas, a part for the Hawaii reefs [7]. Fire corals belonging to different species can take various forms [7].
​
​
​
​
​
Figure 3: Fire coral
​
FINGER CORALS (Fig. 4): These corals grow in a tree like shape with relatively long primary and secondary branches and small polyps. Often, damsel fish and hermit crabs can be found living within the branches of the branching coral [8]. Growing in a ‘dendritic’, or tree like, shape. This is one of the fastest growing forms, as all of the polyps on a branch contribute to skeletal growth in a single direction. Generally, these corals grow fast when conditions are good, but are easily susceptible to mortality due to temperature increases, algae overgrowth, or predation [9]. Branching means forming branches while arborescent means tree-like [6].
Figure 4: Finger coral
FOLIATE CORALS (Fig. 5): Thin, plate like corals which grow in 3-D Shapes, often resembling plants (foliage). These corals tend to grow in areas with high levels of sunlight. In some shallow reef areas of Koh Tao, a specific genus of Foliose coral, Pavona, is taking over the entire reefs. This may be due to the ability of this coral to deal with high levels of sedimentation, giving it the ability to outcompete all other corals [9].
Figure 5: Foliate coral
​
MASSIVE CORALS (Fig. 6): Massive are dense, spherical or hemispherical corals [9]. This term means solid and similar in shape in all dimensions [6]. They are very slow growing, but tend to be very resilient. They are a major contributor to the long-term, solid structure of the reefs, and live to be tens of thousands of years old [9].
​
Figure 6: Massive coral
PLATE CORALS (Fig. 7): Thin, plate-like corals which grow in thin, horizontal sheets resembling shelves. These corals grow in areas with low wave action and clean water. These corals will not survive well in areas with high structural threats such as anchors, snorkelers, etc [9]. Large, table shaped corals which create very diverse habitat for both small fishes, and large fishes underneath. They are generally made up of corals of the branching, corymbose, or digitate shapes, which grow horizontally at one depth, instead of increasing the height [9].
​
Figure 7: Plate coral
3.1.2 Fish species
The fish families which we could more often observe in the Red Sea coral reef during our study are the Labridae, Pomacentridae and Serranidae families.
LABRIDAE (Wrasses)
Labridae is the second largest family of marine fishes, which is distributed in the Indian, Pacific and Atlantic oceans, usually in shallow water, such as coral reefs [10]. This family is very diversified, it includes species of different size (from less than 15 cm, to about 2.3 m), shape and colour (which is usually very bright)[11]. Most species in this family are sand burrowers, but some are carnivores, planktivores and some can also feed on corals. Some smaller species are also ectoparasites removers like cleaners fish. Many species live in harems with several females for one male. Species in the indo-Pacific oceans are pelagic spawners [11].
​
POMACENTRIDAE (Damselfishes or anemonefishes)
They are one of the most abundant families of tropical reef fishes. They are usually small (but can range from 5 to 36 cm), territorial and can be therefore aggressive. They are usually herbivorous but sometimes they build “gardens” of algae. They can also live in association with sea anemones. They are found in multiple habitats: sea-grass beds, corals or rocky places, or in the water column. They are usually benthic [10,11,12].
​
SERRANIDAE ( Groupers, sea basses, fairy basslets)
This family show attractive colour, and can feed on zooplankton. Groupers can grow up to 3 m length and 400 kg. They are benthic predators, and can also feed on crustaceans and fishes. Large groupers are less represented in this family than small, colourful basslets and anthias. These small groupers live in harems with smaller females and are commonly found under ledges and caverns [11,12]. We only considered Groupers in our data collection.
However, we also considered other families which are present in the Red Sea, but less abundant, such as the Chaetodontidae, Pomacanthidae, Cirrhitidae, Holocentridae, Blenniidae, Gobiidae, Acanthuridae, and Tetraodontidae.
​
3.2. Data collection technique
We did snorkeling at the edge of the coral reef to collect our data. To collect them we proceeded each time in 3 steps. First we chose randomly one coral type and tried to not select a second time the same coral. Moreover, for the same type of coral we selected different sizes. Therefore, we sometimes selected deeper corals and other times shallower ones to avoid bias due to a possible variation in species composition and/or abundance. However, as we were snorkeling, we limited the coral choice at maximum 3 meters depth. Secondly, the three of us looked at the selected coral for a couple of minutes to record the fish species and number of individuals present on it. Finally, we measured the coral size using a measuring tape by taking the width and the length. After one day of training of our data collection method, we decided to separate between us the data collection to be more efficient. If the coral was too large, two of us measured it and we did the observation all together. At the end of each day, all data were stored in an Excel table and with the length and the width of each coral we calculated the area of an ellipse (E) to have an estimation of the coral's size (E=π * R/2 * r/2, with R the length and r the width). Per day, we collected observations about more or less 15-20 corals. We tried to have the same number of observations per type of coral, which were around 20 in the end. In total, we collected 101 observations during the study. We had to remove some data from the analysis because we collected data from some cleaning stations, so it was too hard to take them in consideration and could cause a bias in the analyses, due to the very high abundance of cleaner fish and clients.
3.3. Statistical analyses
We wanted to calculate the abundance and the diversity of fishes present in the different corals. To do so, we calculated these variables separately using two different methods. First, we wanted to calculate the abundance of fishes. Thus, we have two response variables: the number of individuals and the number of fish families. These two response variables are continuous and are calculated using two different generalized linear model (GLM). The models have two explanatory variables: the size (continuous), and the type (categorical) of coral. These two models were calculated using R version 2.13.2.
Secondly, to calculate the fish diversity, we used the Shannon’s diversity index (H). This index will give us information about the family richness in the area of interest. To be more accurate about the real family richness, this index also takes into account the abundance of each family, when calculating the diversity of a certain area.
With S the number of families and pi denoting the proportion of individuals belonging to a family S.
From this index, we can calculate the Shannon’s equitability (EH): EH = H/Hmax = H/ln(S)
This equitability represents the evenness, in other words, the maximum richness of the data. This index assumes a value between 1 and 0.
IV.RESULTS
As we stored all our 101 data in an Excel table, we could already observe some interesting results. As we expected, certain type of coral had much more fishes than others. In (Fig. 8), we can observe, that Fire and Finger corals have the higher amount of individuals. At the opposite, Plate corals have less than 30 individuals in total.
​
​
​
​
​
​
​
​
​
​
​
​
​
​
Figure 8: Abundance of fishes per types of coral during field observations.
​
When we look at the fish abundance per family, there were also some interesting results. Some fish families were much more present than others in all our data. The Labridae and the Pomacentridae families have the highest amount of individuals in total, as we can see on (Fig. 9) below. The Serranidae family is the third most abundant family, although most of the listed individuals were categorized in only 3 different observations. At the opposite, the Acanthuridae, the Tetraodontidae and the Gobiidae families have the lowest number of individuals (< 5 individuals).
​
​
​
​
​
​
​
​
​
​
​
​
​
​
​
​
Figure 9: Abundance of fishes per families observed in total.
​
​
Now that we have an overview of the results from the Excel table, we calculated the fish abundance using Rstudio, version 2.13.2. We analysed two models, one with the number of individuals and another with the number of families of fishes.
The model corresponding to the number of individuals is defined as below:
model02=glm(N.Individual~Type*Size,quasipoisson,data=coral) DF= 101-10= 91
With this model, we can observe only one significative result. The corals’ type Plate (p=0.0282) has significative effect on the number of individuals. On the other hand, there is no effect of the size of coral and no effect of the interaction between the type and the size of coral on the number of individuals.
We did the same with the number of families. The model is the following:
model03=glm(N.Family~Type*Size,poisson,data=coral) DF= 101-10 = 91
Here, there were no significant effect of the type and the size of corals on the number of family.
Now, concerning the diversity of fishes, we calculated it using the Shannon’s diversity index (H) and the Shannon’s equitability (EH), as explained in the statistical analyses section.
H= 1.51 and EH = 0.63
V.DISCUSSION
Globally, we have some interesting results although we have only one significative result according to our statistical analyses. During the analysis of the fish abundance, we concluded that both parameters (the type and the size) do not have any effect on the abundance of fishes present on the different types of corals. Thus, this result is not supporting our prediction, which supposed that the type and the size of corals have an influence on the fish abundance. However, with the only significative result, we can conclude that the coral’s type Plate has a small effect on the number of fishes. This means that the number of individuals we found is representative of the normal amount we can find on this type of coral. At the opposite, we could not find any significative result for the coral’s type Fire and Finger, even if they seemed to be the type having the highest abundance of fishes (Figure 8). Thus, the fact that we found the greatest amount of fish on both types of corals (Fire and Finger) is not explained by our models. In other words, it is not because we found a high abundance of fishes on these two types that we would always find the greatest amount of fish on them. However, we can explain the high amount of fish we found on Fire corals by their properties of painful stings. Those corals can be used by small fishes as shelter against predators or just because the structure is favourable to host a great amount of individuals. Moreover, as these types of corals grow rapidly in building coral reefs and are widespread in each ocean, they have a lot of colonies to host fishes.
Now, if we look at the families of fishes we found (11 in total), we can observe that some families are more represented than others, although there are no significative results. These founding can be explained by the properties that confer coral to fishes (e.g shelter). Or maybe also serve as food resources for some species (Butterfly fish). The two more represented families (Labridae and Pomacentridae) from our study are the most diverse families and are represented with individuals that are territorial and live on corals. However, as we do not have any significant results, we cannot say that one family prefer one type of coral more than another, and also that the size of corals is not a factor explaining the abundance of certain families on corals. At last, the 8 other families considered in this study are mostly represented by pelagic or benthic species which just pass through corals, eating them or are hiding in cavities. This explains why most of these families have less than 15 observed individuals.
Concerning the diversity of fishes, by calculating the Shannon’s diversity index, we found that the area has a small diversity (species richness) but the equitability (evenness) is good. We can conclude that there is 63% of diversity of fishes in the area of our study.
Figure 10: The group in action (Lara Bernasconi, Alessia Wolf and Jennifer Gier)
VI.REFERENCES
1) Knowlton, N., Brainard, R. E., Fisher, R., Moews, M., Plaisance, L., & Caley, M. J. (2010). Coral reef biodiversity. Life in the World’s Oceans: Diversity Distribution and Abundance, 65-74.
2) William Alevizon, Ph.D. (2010-2013), Red Sea coral reefs: http://www.coral-reef-info.com/red-sea-coral-reefs.html, 16.08.16.
3) Gotelli, N.J. (2001). A Primer of Ecology. Published by: Sinauer Associates, Inc., Sunderland, Massachusetts, Third Edition, p.170.
4) Coral Reef Alliance (2014), hard corals: http://coral.org/coral-reefs-101/coral-reef-ecology/hard-corals/, 16.08.16.
5)Swierts and Vermeij (2016), Competitive interactions between corals and turf algae depend on coral colony form. PeerJ 4:e1984; DOI 10.7717/peerj.1984.
6) Australian Institute of Marine Science (2013), Colony formation: http://coral.aims.gov.au/info/structure-colony.jsp, 16.08.16.
7) Reefkeeping Magazine, Reef central, Coralmania with Eric Borneman (2008). The Fire Corals: http://reefkeeping.com/issues/2002-11/eb/, 16.08.16.
8) Big Blue Conservation, Hard coral growth forms: http://www.bigblueconservation.com/marine-life/corals/49-hard-coral-growth-forms.html, 16.08.16.
9) New Heaven diving School, Coral Growth Forms: http://www.newheavendiveschool.com/marine-conservation-thailand/learning-resources/coral-growth-forms/, 16.08.16.
10) Nelson, J. S., Grande, T. C., & Wilson, M. V. (1994). Fishes of the World. John Wiley & Sons.
→ Jonna, R. (2003). "Labridae", Animal Diversity Web: http://animaldiversity.org/accounts/Labridae/, 16.08.16.
11) Berg, L.S., 1958. System der rezenten und fossilen Fischartigen und Fische. VEB Verlag der Wissenschaften, Berlin. → Christian Elloran (2010), Fish base, Family Labridae-Wrasses: http://www.fishbase.org/summary/FamilySummary.php?ID=362, 16.08.16.
→ Christian Elloran (2010), Fish base, Family Serranidae: http://www.fishbase.org/summary/FamilySummary.php?ID=289, 16.08.16.
12) Keoke & Yuko Stender (2014), Fish Families: http://www.marinelifephotography.com/fishes/fishes.htm, 16.08.16.
​
Data stored on figshare. Data.txt: https://dx.doi.org/10.6084/m9.figshare.3599112.v4, R script: https://dx.doi.org/10.6084/m9.figshare.3600504.
