Giant Clam (Tridacna Gigas)

Introduction:

The Tridacna Gigas or the giant clam is the largest bivalve that has been discovered. This bivalve can attain lengths of 1.4 m and can weigh over 263 kg (578.6 lbs) (Klumpp, et al 1992). Like many bivalves, the giant clam possesses structures such as the shell, foot, mantle, reduced head, and gills. It is also a sessile organism who is found to inhabit the shallow water regions located from the Indian Ocean to the Pacific Ocean (Indo – pacific region) (Klumpp, et al 1992).

Figure 2: Picture provided by a-z animals: https://a-z-animals.com/animals/giant-clam/

Reproduction and Fertilization:

Giant clams are hermaphrodites that have been found to reproduce near or during a new moon. This suggest that giant clams have a lunar reproduction cycle (Beckvar 1981). To reproduce during this time, giant clams use a method called simultaneous spawning (Beckvar 1981). Simultaneous spawning is where all the giant clams, in an area, release their eggs and sperm into the water simultaneously. The gametes of one giant clam will eventually meet with the gametes of another, in the water, and external fertilization will occur. Sixteen hours after fertilization, the trochophore will hatch, and will begin assembling its shell (calcium carbonate). After twenty hours, the trochophore will develop into a veliger form, which has the velium. After a week of development, the veliger will transition into the pediveliger form, which now has a foot that the clam can use for locomotion. On day 10, the clam will settle, and metamorphosis will occur, which results in the motile muscle cells being changed. Once metamorphosis completes, the sessile juvenile will be present, which will be the final form of the clam (Beckvar 1981).

Figure 3: General life cycle of clams provided by the University of Connecticut: http://www.bcs.uconn.edu/Illustration/PenInk.php

Nutrition:

During the early stages of development, giant clams will only obtain their food through filter-feeding. As the clam develops, it will come to a point where it cannot sustain itself through filter-feeding alone, to remedy this, the clam needs to obtain zooxanthellae. To do this, the clam will need to be in its veliger form. After reaching this form, the clam will filter-feed as normal, and eventually through this process, the clam will obtain motile zooxanthellae who are present in the water (Fitt and Trench 1981). Rather than consuming the bacteria, the clam will allow for the bacteria to settle on its mantle. After settling, the bacteria will conduct photosynthesis, generating food, which will be absorbed by the clam. An interesting thing to note is that the giant clam’s size is mainly due to the ample amount of food that they obtain from their zooxanthellae and their filter system (Fitt and Trench 1981). Other clam species are unable to reach the giant clam's size because they do not have bacteria that provide them with extra food.

Figure 4: Taken by the Monterey Bay Aquarium: https://www.montereybayaquarium.org/animal-guide/invertebrates/giant-clam

Shell:

Figure 5: Taken by Naples Seashell Company: http://naplesseashellcompany.com/tridacna_gigas_clam_whole_sea_shell.html

The shell of the giant clam, as stated before, is composed of calcium carbonate. This shell can be quite large, as it can reach widths up to 1 m, and can weigh over 340 kg (Lin, A.U.M. et al 2006). In a study conducted by Lin, et al (2006), it was found that the shell has two different layers: the outer white layer and the inner translucent layer. Lin and his colleagues found that the outer layer is made of calcium carbonate and aragonite crystals, which makes the outer layer rigid. This allows for the outer shell to defend the clam against abiotic and biotic factors in its environment. The inner layer of the shell is also made from calcium carbonate, but in their study, they found it to be 10x weaker than the outer layer. The inner layer’s weakness is because it has many flaws that extend throughout the layer. Now, while this information about the shell is important, it is important to understand the overall strength of the shell.

Overall, the shell of the giant clam is very strong. In the study conducted by Lin, et al (2006), Lin and his colleagues exposed the giant clam shells to different pressures ranging from 80 – 500 MPa. It was found that the shell was able to withstand pressures that reached 123 MPa. This is impressive, because it means that the clam would be able to inhabit areas of the ocean that are 18,000 feet below the surface, but because of the giant clam’s dependency on its photosynthetic bacteria, this is not possible.

Endangerment:

Giant clams are a species that has been placed on the endangered species list. Their endangerment status is due to two main reasons, one reason being human harvesting. In the past, giant clams have constantly been harvested by humans, because they contained rich meat, and their giant shells were valuable (Junemie, M, et al 2010). This constant harvesting eventually exceeded the rate that giant clams were being produced, and as such, the population declined rapidly to the point of endangerment.

The other reason for why giant clams are endangered is due to pollutants. Pollutants are chemicals that increase the mortality rate of many species across the world, now while this is so, many humans still hold the belief that pollutants will not have any effect on the species across the world. This belief is wrong. In a study done by Blidberg (2004), Blidberg wanted to observe the effects of copper, a common pollutant; and salinity on giant clam larvae. In the study, Blidberg placed different giant clam larvae in different environments, which either had copper or no copper present, and different levels of salinity. Blidberg found that giant clam larvae would have a low survival rate if the salinity was low, and copper was present. This study shows that pollutants do affect animal species, and it also shows that pollutants, when combined with lower salinity, can have devastating effects on the giant clam population.

Table 1: From Effects of copper and decreased salinity on survival rate and development of Tridacna gigas larvae written by Blidberg: Shows the survival rate of giant clam larvae set in conditions with different salinity levels, and copper presence.

Giant clams may be endangered, but while this is so, it is still possible to get them out of endangerment. To do this, two main things need to be done. The first thing that needs to be done is that we need to end the harvesting of giant clams. In today’s world, there are laws, which prevent the harvesting of endangered species, but these laws seem to not be enough. We need to create conservation laws with more dire consequences in order to deter others from harvesting these clams. The second thing that needs to be done is that we need to lower the pollution of our oceans. In the world, many pollutants are dumped into the ocean by people who believe that there are no consequences. We need to educate these people about what they are doing, and hopefully through our efforts, these people will come to understand what they are doing, and cease their actions.

References:

Beckvar, N. 1981. Cultivation, spawning, and growth of the giant clams Tridacna gigas, T. derasa and T. squamosa in Palau, Caroline Islands. Aquaculture 24: 21-30.

Blidberg, E. 2004. Effects of copper and decreased salinity on survival rate and development of Tridacna gigas larvae. Marine Environmental Research 58: 793-797.

Fitt, W. K., Trench, R. K. 1981. Spawning, development, and acquisition of zooxanthellae by tridacna squamosa (Mollusca, Bivalvia) The Biological Bulletin 161: 213-235.

Junemie, M, et al. 2010. Growth and survival of hatchery-bred giant clams (Tridacna gigas) in an ocean nursery in Sagay Marine Reserve, Philippines. Aquacult Int 18: 19-33.

Klumpp, D.W., Bayne, B.L., Hawkins, A.J.S. 1992. Nutrition of the giant clam Tridacna gigas (L.). I. Contribution of filter feeding and photosynthates to respiration and growth. Australian Institute of Marine Science 155: 105-122.

Lin, A.U.M., Meyer, M.A., Vecchio, K.S. 2006. Mechanical properties and structure of Strombus gigas, Tridacna gigas, and Haliotis rufescens sea shells: A comparative study. Materials Science and Engineering 26: 1380-1389.

University of Connecticut. Pen and Ink Illustrations. [Internet]. University of Connecticut; [cited 2017, Nov. 28]. Available from http://www.bcs.uconn.edu/Illustration/PenInk.php