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| Tridacna elongatissima Bianconi, 1856 in Fauvelot, Zuccon, Borsa, Grulois, ... et Bouchet, 2020.
DOI: 10.1111/jbi.13797
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Abstract
Aim:
The unique biodiversity in the Red Sea is the result of complex ecological and evolutionary processes driven by Pleistocene climatic change. Here we investigate the species diversity, phylogenetic relationships and phylogeographical patterns of giant clams in the Western Indian Ocean (WIO) and the Red Sea to explore scenarios of marine speciation in this under‐studied region.
Location: Mozambique Channel, Madagascar, and the Mascarene Islands (WIO); the Farasan Islands (Red Sea).
Taxon: Giant clams of the genus Tridacna (Cardiidae: Tridacninae).
Methods:
Giant clams were sampled as complete organisms or through underwater biopsies. Nuclear (28S ) and mitochondrial (16S and COI ) DNA sequences were subjected to Bayesian and maximum likelihood analyses to generate a phylogenetic hypothesis for all known species within the genus Tridacna . Bayesian inference with molecular and fossil calibration was used to infer their colonization history.
Results:
From the 10 genetically distinct clades recovered from the analyses of 253 sequenced specimens, five distinct Tridacna lineages were sampled, three of which were endemics of the WIO and the Red Sea. Each lineage corresponded to a distinct species, except one grouping the two formerly known WIO endemics, T. lorenzi and T. rosewateri. This lineage clustered with two other well‐supported lineages: the Red Sea endemic T. squamosina, and a previously unrecognized lineage, restricted to the WIO, for which we resurrect the long‐forgotten name T. elongatissima Bianconi, 1856. For the two other species sampled (T. maxima and T. squamosa), contrasting phylogeographical patterns were observed.
Main conclusions:
Our data confirm the validity of the species T. rosewateri, a WIO endemic genetically indistinguishable from T. lorenzi, which should be considered a junior synonym. The phylogenetic placement of the newly resurrected T. elongatissima provides insights into the probable origin of T. squamosina, which split from its sister species no later than 2 Ma, likely during Middle Pleistocene glacial periods. Two scenarios are discussed. Our results also suggest that T. maxima invaded the Red Sea more recently and that contemporary gene flow between Red Sea and WIO T. maxima is highly restricted. The deep Red Sea partition observed in T. squamosa suggests a much older divergence and raises the question of a possible cryptic lineage.
Keywords: biodiversity, biogeographical barriers, cardiidae, population genetics, Red Sea, species distribution, systematics
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| Figure 6: Shells of endemic WIO Tridacna species. (a‐d) Tridacna rosewateri Sirenko & Scarlato, 1991, Saya de Malha Bank, including a‐b. Paratype ZISP N10569, L 191 mm and c‐d. Holotype ZISP N10568, L 149 mm. (e,f) Tridacna lorenzi K. Monsecour, 2016, Mauritius, Cargados Carajos Shoals, St. Brandon, holotype MNHN‐IM‐2000–30800, L 225 mm. (g‐l) Tridacna elongatissima Bianconi, 1856. (g‐i) Mozambique, lectotype MZB 009876, L 198 mm (photo: Antonio Bonfitto). (j‐l) South Madagascar, Atimo Vatae stn TF20, MNHN‐IM‐2009–33486, L 175 mm |
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| Tridacna elongatissima from Masoala Cape, northeast Madagascar.
Giant clam endemic of the south Western Indian Ocean.
(Photo by Serge Andréfouët)
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| Tridacna elongatissima Bianconi, 1856. (g‐i) Mozambique, lectotype MZB 009876, L 198 mm (photo: Antonio Bonfitto). (j‐l) South Madagascar, Atimo Vatae stn TF20, MNHN‐IM‐2009–33486, L 175 mm |
Cécile Fauvelot, Dario Zuccon, Philippe Borsa, Daphné Grulois, Hélène Magalon, Florentine Riquet, Serge Andréfouët, Michael L. Berumen, Tane H. Sinclair‐Taylor, Pauline Gélin, Faustinato Behivoke, Jan Johan ter Poorten, Ellen E. Strong and Philippe Bouchet. 2020. Phylogeographical Patterns and A Cryptic Species provide New Insights Into Western Indian Ocean Giant Clams Phylogenetic Relationships and Colonization History. Journal of Biogeography. DOI: 10.1111/jbi.13797
The forgotten giants of the Western Indian Ocean reefs
