Why did the giant clam cross the road?

Wait a minute – Giant clams walk?!

It is surprising for most people to imagine how giant clams could ‘walk’ around on the reef. When you see a giant clam on coral reefs, you would think that such a large animal like this cannot move very fast on its own. In fact, most people have usually described the adult giant clams as sedentary (or sessile) for the rest of its life!

B

Mama Jong Clam has remained in her same spot for the last 10 years of monitoring. Photos by Neo ML.

Anecdotal observations of clam walking were first noted by Yonge (1936) who was interested in the burrowing nature of Tridacna crocea. He wrote, “At the same time as the animals became larger (possibly owing to their now largely sessile life), the foot became smaller. Even at the present time, however, young T. crocea possess a well-developed foot by means of which they can move as actively as young Mytilus [mussel].”

Yonge (1936) further summarised his findings, “Young T. crocea (1-2cm long) attach themselves by means of a byssus in holes on the surface of boulders or beach limestone. They possess an extensible foot and can move actively, crawling up surfaces with the additional aid of temporary byssus threads.”

So how do they walk it?

Giant clams, in fact have the ability to ‘walk’ throughout their life. They have two main types of movement (Huang et al. 2007):

  1. Rotation movement – a change of clam orientation without the approximate center of the clam being shifted from initial position.
  2. Translation movement – a lateral movement away from the original position.
Larval locomotion

“Twist, twist, twist!”

This locomotion behaviour however reduces with age and size, and this is an expected ontogenetic change as the size of foot becomes reduced and the size of shell valves becomes larger and heavier. Therefore, it becomes more difficult for a large clam to do a translation movement compared to a small clam. Rotation movement, on the other hand, is possible throughout its lifetime.

Giant clam growth

Juvenile Tridacna squamosa: (A) 66 days old; (B) 6 months old; (C) 48 months old. Photo by Neo ML.

What makes the clams want to walk?

Aggregate

Baby clams stick together. Photo by Neo ML.

Initially, we did not know what could trigger clam walking, but we do know that they like to stick to each other. Therefore, Huang et al. (2007) designed an experiment to test the hypothesis of conspecific clam attractiveness with respecting to walking. Results showed that test clams were significantly attracted towards clam effluent (or juice!) compared to remaining in a neutral or away position.

Huang et al. 2007-01

Huang et al. (2007)’s experiment to test if clam individuals are attracted to groups of clams. Figures taken from the paper.

Using the same concept of attractiveness, Dumas et al. (2014) presented a choice experiment to juvenile Tridacna maxima, with treatments of either conspecific clams, favourable reef signals or unfavourable reef signals. Again, clam individuals were attracted to each other, and more interestingly, they were also more attracted to favourable reef signals. Results suggest that giant clams could differentiate its environments through chemosensory.

Dumas et al. 2014-01

Dumas et al. (2014)’s experiment to test if clam individuals can detect environment cues. Figures taken from the paper.

Overall, clams appear to perceive the presence of other conspecific clams and/or favourable reefs as positive cues, and actively move towards them. These studies have downstream implications for post-settlement habitat selection, which have further consequences on its growth, survival and (future) reproduction.

Aggregation of juvenile giant clams

Ultimately, clam walking leads to aggregation of individuals, either next to each other or in close proximity. Soo & Todd (2012) also found that juvenile clams appear to be nocturnally most active in walking, a possible adaptive behaviour to avoid diurnal predators. Watch the video to see what happened when you put a group of clams together.

There has been numerous studies on the costs and benefits of aggregating bivalves, but not well-documented for the giant clams. Drawing from previous studies, here are some of the possible costs and benefits of aggregation (Krause & Ruxton, 2002; Jackson, 1977; Parrish & Edelstein-Keshet, 1999).

Cost-benefit table

Table shows the possible costs-benefits analyses of aggregating giant clams.

Because the giant clam wants to get onto the other side of the road.  😉

Pediveliger

Reference List:

Dumas et al. (2014) Evidence of early chemotaxis contributing to active habitat selection by the sessile giant clam Tridacna maxima. Journal of Experimental Marine Biology and Ecology 452: 63-69.

Huang et al. (2007) Movement and aggregation in the fluted giant clam (Tridacna squamosa L.). Journal of Experimental Marine Biology and Ecology 342: 269-281.

Jackson (1977) Competition on marine hard substrata: the adaptive significance of solitary and colonial strategies. American Naturalist 111: 743-767.

Krause & Ruxton (2002) Living in Groups. Oxford University Press, New York.

Parrish & Edelstein-Keshet (1999) Complexity, pattern, and evolutionary trade-offs in animal aggregation. Science 284: 99-101.

Soo & Todd (2012) Nocturnal movement and possible geotaxis in the fluted giant clam (Tridacna squamosa). Contributions to Marine Science 2012: 159-162.

Soo & Todd (2014) The behaviour of giant clams (Bivalvia: Cardiidae: Tridacninae). Marine Biology 161: 2699-2717.

Suzuki (1998) Preliminary studies on locomotion and burrowing by juvenile boring clam, Tridacna crocea. Naga, The ICLARM Quarterly. Pp. 31-35.

Yonge (1936) Mode of life, feeding, digestion and symbiosis with zooxanthellae in the Tridacnidae. Great Barrier Reef Expedition: 283-321.

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