Diversity and abundance of giant clams in Anambas Islands, Indonesia

Giant clam is one of the most common types of bivalves found in coral ecosystem. Geographically, these clams have limited distribution in the Indo-Pacific tropics, from the Red Sea to the Pacific Islands of Tuamotu. Ecologically, the clam acts as bio-filtration and natural bio-deposition. The presence of clams becomes a marker that the waters are still good and the absence of harmful pollutants. The population of this clam in Indonesian waters continues to decline. Anambas archipelago are known for its rich diversity of marine life. Since the establishment of this area became NACA as an ATP in 2014, marine tourism activities grew rapidly. Therefore it is necessary to do this research which aims to know the diversity of species and abundance of clams in the waters of Anambas Islands. Field survey was conducted in August 2015 by using Belt Transect method. The research results found 6 species with D 1 value is 0.593. The overall abundance is 0.144 indv.m -2 . Tridacna crocea has the highest abundance and relative abundance of the highest species of the other species of 60.54%. Meanwhile, the smallest relative abundance is Tridacna gigas which is only 2.36%.


Introduction
Tridacnidae or giant clam is one of the most common types of bivalves found in coral ecosystem waters. Giant clam lives in association with coral reefs by immersing themselves in the substrate (drill) or sticking to the sandy bottom of the water. These animals live in symbiosis with the dinoflagellata algae (Symbiodinium) that grow in the mantle tissue and are able to photosynthesize resulting in food and pigment [1].
Ecologically, the clams are the biota that acts as bio-filtration and natural biodeposition. This ability is because they are a deposit feeder or suspension feeder. They are also able to filter out ammonia and soluble nitrate in seawater for the need for zooxanthellae to nitrogen for its growth process [4]. The presence of clams becomes a marker that the waters are still good and the absence of harmful pollutants.
Giant clams (Tridacnidae) have been an important traditional source of food and income for many coastal Indonesian people. Kondisi ini menyebabkan tingkat populasinya di alam semakin sedikit. This is also the basis why the giant clam is one of the marine animals that In recent years and along with these conditions, the existence of marine ecosystems feared to be degraded. This was also coupled with the use of a variety of marine life including the exploitation of tridacna for different economic commodities. The study of species diversity and abundance, or at least species richness, gives ecologists insights into the stability of communities (Walker, 1988 in [9]).
Information on giant clams collected from various locations has been described by [10], but specifically for Anambas waters do not yet exist. Therefore it is necessary to do this research which aims to know the diversity of species and abundance of clams in the waters of Anambas Islands. The results of this study are expected to be used as information for the government and other stakeholders involved in the protection, conservation and utilization of giant clams and the management of their habitats in a sustainable manner.

Survey
Field survey was conducted in August 2015. A total of 23 sites were surveyed spread over in the waters of Anambas Islands (Fig. 2). The data collecting was conducted on 42 sampling points ( Table 1).
Survey method used is a modification of Line Intercept Transect (LIT), namely Belt Transect and has been used previously by [11]. Each research station is only one transect line and pulled along the 70-meter transect line parallel to the coastline at a depth of 7-10 meters. Observations were made at every 1 meters to the right and the left of the transect line. Therefore, the area observed at each station is 140 m 2 [12].

Diversity Index
Measuring and analyzing of diversity based on Simpson's Index (D). Simpson's Diversity Index is a measure of diversity. In ecology, it is often used to quantify the biodiversity of a habitat. It takes into account the number of species present, as well as the abundance of each species. Simpson's diversity index (D 1 ) performed best when differentiating sites, but simpler indices were slightly preferable when detecting effects of land use intensity on diversity [13]. The formula to calculate species diversity according to Simpson is: where P i is the proportion of individuals found in species i. For a well-sampled community, we can estimate this proportion as P i = n i /N, where ni is the number of individuals in species i and N is the total number of individuals in the community. The range is from 0 to 1, where:  High scores (close to 1) indicate high diversity.  Low scores (close to 0) indicate low diversity.

Abundance
There are 2 abundances analyzed, the first is the abundance and the second is the relative abundance. Calculation of giant clam abundance found in each station is calculated based on [14]. (2) where A is abundance, N is total of Individu (indv.) and area is wide of sampling area (meter), meanwhile to calculate relative abundance using equation: (3) where RA is Relative abundance, ni is the number of individuals in species i and N is the total number of individuals in the community.

Species Diversity of Giant Clams
The  Table 1).
The number of species of clams found in this region is more than half the number of species of clams in Indonesia and even in the world. The comparison can also be seen with the waters of the Seribu Islands and Manado [2] which each has only 3 and 4 species. Even for the relatively close waters of Reef's Singapore, only 2 species are found [15]. Greater diversity of giant clam found in the central Indo-Pacific [16].
In Table 1 it can also be seen that Simpson's diversity index (D 1 ) at each site ranges from 0 -0.727. The highest D 1 value are in the Karang Beretih site, whereas the lowest D 1 value is in Karang Kuning and Karang Terumbu. Overall, the giant clam diversity index based on Simpson's diversity index (D 1 ) in Anambas Islands waters is 0.593 ( Table 2). The index value shows a relatively high diversity because the value is more than 0.5 on a scale of 0 to 1.

Abundance (A) and Relative Abundance (RA)
Overall, giant clams abundance from 5880 m 2 (42 transects) of total area surveyed in Anambas Islands is 0.144 indv.m -2 ( Table 3). This condition still higher then other places in western Area In Indonesia like Karimunjawa Islands were ranged only from 0.03 to 0.04 indv.m -2 [17]. But still lower than the survey results in several other places in eastern area of Indonesia like in Cenderawasih Bay found giant clams natural populations densities were from 0.6 to 0.7 indv.m -2 [18].
In Table 3 and also the graph in Fig. 3 shows that T. crocea is the highest abundance of 0.087 indv.m -2 . Tridacna. squamose and T. derasa have the same abundance of 0,013 indv.m -2 . Meanwhile, the smallest abundance of 0.003 indv.m -2 is T. gigas. The data presented here indicate that giant clam abundance are low. This is very different from the results obtained in Ashmore, Mermaid and Cartier Reefs, located within the Great Barrier Reef, host populations in excess of 100 indv.m -2 . Another example is on Tatakoto, French Polynesia, has an abundance of 90.9 indv.m -2 [19,20].  The results of the data analysis show that the RA of giant clam ranged between 2.36% to 60.54%. Species of T. crocea has the highest abundance and relative abundance of the highest species compared with the other species that is equal to 60.54%. Meanwhile, the smallest relative abundance is T. gigas which is only 2.36% (Fig. 4). Tridacna crocea, T. maxima, T squamosa and T. derasa belong to predominant species, whereas H. hippopus and T. gigas belong to subdominant species. This grouping is based on the approach presented by [21] which states that if RA> 5% belongs to predominant species, RA 2-5% belongs to subdominant species and RA <2% belongs to non-dominant species.

Fig. 3. Abundance of giant clam species
The composition of this abundance is similar to the observation of the population of giant clam in several places in Indonesian waters. The population is dominated by small species such as T. crocea and T. squamosa, while large species such as T. derasa and T. gigas are rarely found [22,23]. The high abundance of giant clam species in a waters is thought to be due to habitat factors suitable for the ability of the species to adapt and survive. The substrate type is one of the key factors in this regard [23].

Conclusions
The diversity of giant clam species found in Anambas Island waters shows 6 species from 2 generas namely Tridacna gigas, T. squamosa, T. derasa, T. maxima, T. crocea and Hippopus hippopus. The value of Simpson's diversity index (D 1 ) in each sites ranges from 0 -0.727. The highest biodiversity index value located in Karang Beretih site. Overall in Anambas waters, the diversity index value of 0,593 indicates that the area has a relatively high diversity of giant clam.
Overall, giant clam abundance in Anambas Islands is 0.144 indv.m -2 . The abundance dominated by T. crocea. The relative abundance T. crocea is the highest compared with the other species that is equal to 60.54%. Meanwhile, the smallest relative abundance is T. gigas which is only 2.36%