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You are here:   animal list > Tridacna maxima




Tridacna maxima Röding 1798    

Small Giant Clam

Boris Laffineur (2011)















Tridacna maxima


Small Giant Clam

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Research Project 

Are the Human activities impacting on the small giant clam inhabiting Heron Island ?


          The small giant clam (Tridacna maxima) is a protected species which is commonly found around the reef of Heron Island, QLD. While they were over-harvested in many places for food consumption, shell trade or aquarium trade, in the remote place which Heron Island is, no similar threats never occurred before. The different efforts of those last few decades to re-grow the population of clams shows some good results (Junemie, 2010). However, the increasing pressure from climate change and anthropogenic driven changes may have an impact on the population of clams. As clams are originally filter feeders, they filtrate huge amount of water everyday and thus may be impacted by some factors. Using Heron Island as a proxy for places non impacted by direct human impact (harvest and trade), we aimed to address the impact of other human activities.

The different areas of Heron Island are clearly not impacted in the same way by the different human activities associated with the University of Queensland Research Station and the Tourist Resort. Thus, we decided to have a look at the impact of the human activities on the Harbor area, compare to the Shark Bay area which was used as a control.

             Material and Methods

             We went snorkelling and reef walking for a week at the Shark Bay and the Harbor areas in the end of September 2011. The Tridacna maxima species was abundantly present but strictly forbidden to collect under the permit offered by the marine park management and thus we based our sampling method on in-situ measurements.

              Sampling method

              The first sampling method was to snorkel along 50m transect, to assess the abundance and distribution of the clams. However, this technique was not really efficient and time consuming, because of the patchy distribution of the clams. Thus, we choose to adopt a random sampling technique, in which each clam spotted would be sampled. We sampled two area, the Shark Bay area and the Harbor area. For each area we define two different sampling locations.


               We sampled 49 clams in the Harbor area and 45 in Shark Bay area. For each clam individuals, we measured the length of the shell from one end to another one, as shown on Figure 1, with a precision of ½ a centimeter. We also took a picture of each of the sampled clams and made comments if clams were particularly closed together or not.

Figure 1. How we measured the length of the shell.


                Before sampling we defined 3 different types of habitats. (1) The coral reef which was defined by a continuous reef of corals with clams growing on top or encrusted. (2) The reef, in the reef flat, which was defined by a more than 3x3m piece of reef in the reef flat. (3) The boulder, which was all of the boulder of living or dead coral inferior to 3x3m. In the Harbor area, the presence of clams on boulder habitat was not observed because, the boulder tend to be in the middle of the channel which is quite deep for a clam and maybe also for the diver, thus our studies may have been weaken by that.


             We identified 3 question that we could ask from the data set we obtained and thus we ran different analysis to respond to those 3 questions.

      1. Is there any differences in size of clams between Shark Bay and the Harbor areas ?

To answer this first question we ran an ANOVA (ANalysis Of VAriance) for which we obtained a p.value of 0.02657, which means that the two different set of data, from the Harbor and Shark Bay, are significantly different. In addition, we removed one of the sample of Shark Bay area because we believed that it was an individual from the Tridacna gigas species because of its size (44cm) which does not appear to be consistent with other observation around Shark Bay and especially because of differences in its morphology.

      1. Is there any differences in size of clams between the different habitats we defined ?

For this question, we sorted out the different habitat found. Thus we ran three ANOVA between each of the habitats and we found a p.value of 0.09213 for comparing The Coral Reef-The Reef habitats, a p.value of 0.13765 for comparing The Reef-The Boulder habitats and a p.value of 0.07561 for comparing The Coral Reef -The Boulder. Finally, it means that there is no significant difference of size of clams in function of those habitats.

      1. Is there any differences in clams colors between the two areas ?

Analysing colors from the picture was the most difficult task. Although Todd (2009) found 8 different morphs of color for T. crocea, it was absolutely not the case for T. maxima. We try to sort out the color pattern by approximate color and motifs but it was not consistent (34 different coloration patterns) and thus we decide to only sort it out by three criteria, dark, light or medium coloration. Thus we ran 3 ANOVA to compare Dark and Light, Light and Medium and finally Dark and Medium colorations. We obtained p.value respectively of 0.08942 ; 0.11034 and 0.07936. Thus, there is also no significance among those data sets.


               The differences observed in the size between the two areas were significantly different after being compared using an ANOVA. The size of the shell is related to the growth and the age in Tridacnidae species. However, since we do not know neither the age nor the growth rate of those clams, we cannot infers anything about the human activities and their potential impacts in the Harbor area compare to the Shark Bay area. We can only say that there is a differences in size which may be due to different factors observed in the field such as the water turbidity which is definitely more important in the Harbor area, as well as a potential pollution from boats. Smith (1984) had identified that the Heron Island Harbor was significantly polluted by some hydrocarbons, those hydrocarbons may have an effect on the size of the clams but it need to be demonstrated. Further research to identify the exact cause of this difference would be great, especially because there is no study about it and it may become important in the next few decades.

               For the different habitats, the study may be biased by two fact, the first one is that the definition of The Boulder habitats was not adapted to the Harbor area and thus difficult to compare. The second one was the definition of the two other habitat which may have not been consistent to be compared. The non significance of the data may also reflect that clams are evenly distributed across those two habitats and that they do not show preferences for settlement in one or the other habitat. One of the observation made on the field was that most of the clams are living on dead corals surrounded by living one, or not. This observation was significant because about 73% of the clams we found were living on dead corals. From that fact, two different question can be arise: (1) Are the clams killing the corals around them for any benefit or are the coral killed passively by clams for a reason that we do not know yet and (2) Is there any advantages for clams to settle on dead corals instead of living one ? As Neo (2009) already shows, some algae can improve settlement of clams, are dead corals playing a similar role ?

              Working on the color patterns of Tridacna maxima was a good idea, but in practice it is extremely difficult to adapt due to the high diversity of color that clams are exhibiting. What we did to simplify the data was maybe not a good idea, because we finally lost the accuracy of the research. However, if the non significance is true, it may shows that the potential for photosynthesis in the two area is the same and that clams are not really impacted by human activities in term of photosynthesis requirements. Just as an observation, one of the individual of the Harbor area appeared to be bleached which means that human may have an impact on photosynthetic potential or, the clams may have been bleached because of a disease.

              As a conclusion, our study was weak on some points but strong on some other one. The major problem is that we do not know exactly what we found because the data we have cannot really determine a human impact on clams species. However, we clearly identified some weaknesses in the litterature and we arose some interesting question that may worth to be answered.