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Polymorphism and crypsis in the boring giant clam (Tridacna crocea): potential strategies against visual predators

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Abstract

The giant clam, Tridacna crocea, is a sessile bivalve that bores into rocks and dead corals found on shallow Indo-Pacific coral reefs. Clams are a valuable prey item and T. crocea have developed at least two effective strategies to avoid predation: borrowing, and simultaneous rapid mantle withdrawal and valve contraction. Tridacna crocea obtain their food via photosynthetic products of symbiotic zooxanthellae and from filter-feeding. When they retract their mantles in response to predators, both photosynthesis and feeding are disrupted. Visible polymorphism and crypsis should reduce the frequency of attack and corresponding mantle retraction. In order to determine whether T. crocea is polymorphic and/or cryptic, samples (n = 573) from Tioman Island, Malaysia, were surveyed and photographed. Classification of images into categories of size and mantle colour/pattern demonstrates that T. crocea is colour/pattern polymorphic and morph frequency is size-dependent. Eight morphs were identified, but there also existed considerable intra-morph variation. Morphs with brown and/or and green pigments dominated the larger clam size category (>8 cm). Furthermore, red/blue/green (RGB) values extracted from a subset (n = 93) of digital images of T. crocea showed significant positive correlations between mantle and substrate colours (blue, r = 0.641; green, r = 0.540; red, r = 0.528), indicating background matching (crypsis).

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Acknowledgements

We thank the members of Marine Biology Laboratory, NUS, who assisted with fieldwork and ideas, and Ow Yan Xiang for help with the figures and formatting. This research is supported by Singapore’s Ministry of Education’s AcRF Tier 1 funding: Grant Number R-154-000-280-112.

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  1. Marine Biology Laboratory, Department of Biological Sciences, National University of Singapore, 14 Science Drive 4, Blk S1 #02-05, Singapore, 117543, Singapore

    P. A. Todd, J. H. Lee & L. M. Chou

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  1. P. A. Todd
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  2. J. H. Lee
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  3. L. M. Chou
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Correspondence to P. A. Todd.

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Todd, P.A., Lee, J.H. & Chou, L.M. Polymorphism and crypsis in the boring giant clam (Tridacna crocea): potential strategies against visual predators. Hydrobiologia 635, 37–43 (2009). https://doi.org/10.1007/s10750-009-9859-9

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  • DOI: https://doi.org/10.1007/s10750-009-9859-9

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