“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?
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.
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)
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.
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:
- Huang D, PA Todd & JR Guest (2007) Movement and aggregation in the fluted giant clam (Tridacna squamosa L.). Journal of Experimental Marine Biology and Ecology, 342: 269-281.
- What’s in the Lab? 23 October 2017.
- Why did the giant clam cross the road? 2 May 2016.