The forgotten giants of the Western Indian Ocean reefs

Giant clams have long fascinated adventurers and naturalists. These large shallow-water molluscs certainly are among the most colourful, conspicuous and emblematic species of the Indo-Pacific coral reefs. They have been exploited for thousands of years for their flesh and shell. Giant clam conservation is also an increasingly concerning issue because of the vulnerability of giant clams to overharvesting. Surprisingly, up to recent years, giant clams have remained incompletely known and described, and their evolutionary history was poorly understood.

Image: C. Fauvelot (IRD) is doing a biopsy on a giant clam in Juan de Nova. Photo credit: T.B. Hoareau / TAAF-Iles Eparses research consortium.

FROM THE COVER: read the article on which this post is based …
Fauvelot, C, Zuccon, D, Borsa, P, et al. 2020. Phylogeographical patterns and a cryptic species provide new insights into Western Indian Ocean giant clams phylogenetic relationships and colonization history. J Biogeogr. 47:1086– 1105.

As Tridacna giant clams exclusively occur in coral reefs of the Indo-West Pacific (IWP), we believed that studying the mode and tempo of their speciation would provide us with clues on the evolutionary history of modern coral reef communities in the IWP. We addressed this objective by combining molecular phylogenies with the geographic distribution of Tridacna lineages across the IWP. With well-dated, albeit rare fossil records, we had a model of choice to link phylogenetic patterns to past geological events.

When we started our phylogeographic research in the late 2000s, little was known from the Indian Ocean although a robust phylogeography of Coral-Triangle and Pacific Tridacna lineages was already partly available. A distinctive T. maxima lineage and a newly rediscovered species (T. squamosina) had been reported from the Red Sea, but no phylogeographic information was then available from the western Indian Ocean (WIO). Hence our focus on Tridacna giant clams from that part of the tropical IWP.

During field work, several participants in this study – then working as separate teams – independently noticed giant clams initially identified as T. maxima but presenting somewhat distinctive features.

We noticed the sharply pointed triangular interstices between folds, and the remarkable emerald-green colour of the mantle edge.

Tridacna elongatissima from Etang Salé at Reunion Island. Photo credit: L. Bigot / Université de La Réunion.

Nucleotide sequences at the COI locus confirmed this giant clam was distinct from T. maxima, and from all other known Tridacna spp. then documented in public sequence databases. With an endemic species in the Red Sea (T. squamosina), two unverified rare species endemic to the Mascarene Basin (T. rosewateri and T. lorenzi), and now a new cryptic lineage in the WIO, we felt that we had an increasingly interesting subject to investigate. Our different teams merged efforts and datasets and pursued the phylogeographic work all together.

We managed to extract DNA from dried muscle tissue and ligament from the type material of T. rosewateri and from other specimens from the WIO region preserved in museum collections. Morphological and molecular analyses enabled us to identify the distinct Tridacna lineage present in the WIO as T. elongatissima, a long- forgotten species from Mozambique then synonymised with T. maxima, thereby adding a taxonomic hue to our primarily phylogeographic study. Meanwhile, T. lorenzi and T. rosewateri were found to be a single and same, distinct species. 

This newly resurrected WIO-endemic Tridacna elongatissima turned out to be the sister species of T. squamosina! These two species had evolved independently in, respectively, the WIO and the Red Sea (or perhaps an adjacent northwestern Indian Ocean refuge), revealing a geographic barrier between the two regions. The T. elongatissimaT. squamosina pair was itself sister to T. rosewateri, highlighting this part of the world as an hotspot of endemism for giant clams. Lineage diversification patterns within the widespread T. maxima mirrored those of T. elongatissima, T. rosewateri and T. squamosina with two unrelated lineages in the WIO, one of which was sister to a third lineage endemic to the Red Sea. Thus, the same geographic barriers and speciation processes may have acted repeatedly at different periods in the Pleistocene. 

We are aware, though, that no uniform explanation holds for the evolutionary history of species in the tropical IWP. At least we were able to refine our understanding of lineage diversification and endemism of Tridacna giant clams in the WIO and Red Sea region. Beyond the specific case of giant clams, our results emphasize the interest of sampling understudied regions of the tropical IWP, such as the WIO, to refine the evolutionary puzzle of this vast and complex geographic ensemble. Further investigations in the future may add to the story.

Written by:
Philippe Borsa and Cécile Fauvelot – Researchers – Institut de recherche pour le développement (IRD), UMR ENTROPIE, France.

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