Commonly called the boring giant clam, Tridacna crocea isn’t ‘boring’ in that sense. A recent study confirmed that this species truly builds their boreholes with acid!

In my opinion, the boring giant clam (Tridacna crocea) is one of the most unique species of the giant clams. Unlike its relatives, this species is the smallest in size with a maximum shell length of 15 cm (still very big!) and you can hardly see its shells as it is completely seated within a borehole!

There remains a mystery that surrounds this particular species of giant clam for decades – How does it bore into the reef substrata?

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Brightly coloured, the burrowing giant clam is almost always found boring into the reef substrata.

Sir Charles Maurice Yonge (1899-1986), a notable malacologist, was one of the early scientists pondering over the mechanism of boring by this giant clam. As the coral rock is made up of limestone – subject to acid dissolution, boring by acid secretion has been often hypothesised as one of the mechanism. Yonge had conducted pH experiments but found no conclusive evidence for acid secretion. Since his initial experiments, Yonge was still unable to pinpoint the actual mechanisms, but hypothesised that T. crocea bores by both mechanical grinding and non-acid chemical boring.

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Burrowing clams, Queensland from the Great Barrier Reef Expedition, ca. 1928 by C.M. Yonge. Source: National Library of Australia. URL: http://nla.gov.au/nla.obj-145103942

At long last, this mystery has been solved by a team of researchers that utilised new technology of measuring pH, as well as careful consideration of the animal’s anatomy and behaviour during the experiments.

They first retrieved the animals from their boreholes (Fig. 1a), and upon removal, you can see the probable boring organ that extends out of the byssal gape (bottom) (Fig. 1c). This organ is also called the pedal mantle, which is enlarged in this species but not for other giant clam species. Researchers then carefully sit the relaxed clam onto pH-sensitive foils to measure the surface pH of pedal mantle and compared to surrounding seawater (Fig. 1b).

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Figure 1. (aTridacna crocea in the wild, viewed from above (courtesy of James Fatherree). The siphonal mantle is expanded above the surrounding coral rock. (b) Clam in open-topped study aquarium. (c) Clam suddenly removed from coral rubble and photographed immediately, showing the expanded pedal mantle protruding from the byssal opening (compare b(ii)). Source: Hill et al. (2018) Biology Letters.

The results?

The surface pH of the pedal mantle was significantly lower than the surrounding seawater pH! Right under the organ, the minimum pH measured was between 4.65 – 5.36, and this is approximately 2 units lower than seawater pH at 8.2! To further confirm this finding, the research team found an abundance of a particular enzyme (Vacuolar-type H+-ATPase) that facilitates the release of H+ ions, hence causing the lowering of pH and the mechanism of acidification. Therefore, this shows that bioerosion by T. crocea is in part chemically mediated by acid secretion!

We had previously kept T. crocea and T. maxima (Fig. 2) in our labs, and noticed that they had made shallow bore marks on the concrete substrata provided! Now we know how they did it!

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Figure 2. Tridacna maxima, or commonly called the small giant clam, also partially bore into the substrata. They left shallow boreholes on the concrete substrata that we keep them on.

The article can be found at: Hill et al., Acid secretion by the boring organ of the burrowing giant clam, Tridacna crocea. Biology Letters, June 13, 2018. DOI: 10.1098/rsbl.2018.0047 [This paper is Open Access!]

Further reading list:

  • ScienceNews, 22 June 2018: How a squishy clam conquers a rock
  • The New York Times, 14 June 2018: This burrowing clam is not boring. It uses acid to make its home
  • Neo ML, W Eckman, K Vicentuan-Cabaitan, SL-M Teo & PA Todd (2015) The ecological significance of giant clams in coral reef ecosystems. Biological Conservation 181: 111–123.
  • Yonge CM (1928) Burrowing clams, Queensland, ca. 1928. Retrieved July 22, 2018, from http://nla.gov.au/nla.obj-145103942
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