One of the discoveries that we made was that giant clams could walk across the seafloor. Yes, you heard me right: they can walk!” – TED Talk on The fascinating secret lives of giant clams

Walking in giant clam is only the first part of the story. The ultimate goal of these baby giant clam was to reach each other, as seen in the last frame of the video. What motivates them to aggregate in groups?

Common-mussel-bed
Figure 1. An example of a dense mussel bed.

Gregariousness, defined as the tendency of an animal to aggregate with others of the same species, is frequently observed in other shellfish species such as mussels (Fig. 1) and oysters. This aggregative behaviour in bivalves can be beneficial such as reduced predation risk and increased reproductive success; on the other hand, there could be associated costs to living in groups such as increased transmission of pathogens, increased detection risk by predators, and mainly, competition for resources among each other.

In the past, I would be very frustrated with these clams of mine! I would clean them and arrange them in neat rows and columns to only discover that they jumbled up my sequence by clumping together the next day! Argh!

Then we begin to wonder the reasons behind this clumpy preference for these clams. Years later, a student of mine led the experiments to find out the trade-offs of this behaviour for the giant clam.

Aggregate
Often, we would see our young clams group together in clusters. What motivates them to do so?

He performed two separate experiments to quantify some benefits and costs of aggregation in the juvenile fluted giant clam, Tridacna squamosa.

Expt 1: We grew giant clams in either dispersed or aggregated treatments, and monitored their growth rate, survival, and parasite load

Expt 2: We introduced giant clams in either dispersed or aggregated treatments to a crab predator, and measured its predation rates (Fig. 2)

Sim et al - aggregation
Figure 2. Paired study design for predation experiment. a) Stone crab predator, Myomenippe hardwickii. b) Each predator presented either treatments in two separate sessions. c) The process of successful predation observed during the trials. Source: Sim et al. (2018) Marine Biology.

The results?

Group living for giant clam may not be too beneficial when it comes to growth, as it gives rise to competition for space and food. In the case of giant clams, they may not be able to fully extend their colourful outer mantle for light capture. Aggregation also increases the load of parasites, which may also contribute to the overall lower growth.

OLYMPUS DIGITAL CAMERA
Normally, the giant clam would fully extend its outer mantle tissue to capture light for photosynthesis. However, when in tight groups, this may become compromised.

On the bright side, clams within an aggregation are more resistant to predation by stone crab, because the crab had a more difficult time trying to pry a clam from a group compared to individual standalone clam. Perhaps when in groups, a lower growth rate for the giant clam isn’t a bad thing, compared to death by a crab!

The article can be found at: Sim et al., Trade-offs between defence and competition in gregarious juvenile fluted giant clams (Tridacna squamosa L.). Marine Biology, 165: 103.

If you would like a copy of our paper, drop me an email!

Further reading list:

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