Bhagooli, Ranjeet

Kaullysing, Deepeeka

Ramah, Sundy

English

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193WIO Journal of Marine Science  Special Issue 2/ 2021 193-196Giant clams play an important ecological role in the structure and ecology of coral reef communities (Neo et al., 2015) and conducting research on these species is as important as the overwhelming attention being given to coral reefs worldwide. Giant clams (family: Cardiidae; sub family: Tridacninae) are the largest liv-ing bivalves; some species can live for a hundred years and reach a length over 1.5 m (Yonge, 1975; Nuryanto et al., 2007). Giant clams have a feeding behaviour sim-ilar to that of corals, exploiting nutrients in both het-erotrophic and autotrophic conditions, thanks to their obligate symbiotic association with photosynthetic dinoflagellates ( Jantzen et al., 2008), and are able to concentrate most of their energy into growth (Griffiths and Klumpp, 1994). They usually grow in the warm waters of the Pacific and Indian Oceans (Purchon, 1977) and use substrates such as coral, rock, rubble or sand to attach themselves (Ramah et al., 2017). Most species are found within shallow lagoons and on reef flats, usually not deeper than 30 m (Neo et al., 2017) due to their relationship with symbionts requiring light. Presently, there are 12 recognized extant species of giant clams worldwide, comprising two species of the Hippopus genus and ten species of the Tridacna genus (Fauvelot et al., 2020; Tan et al., 2021). Limited research on these invertebrates is available from the Western Indian Ocean (WIO) region, nota-bly from the Comoros Islands (Mohamed et al., 2016) and from several other WIO countries (Fauvelot et al., 2020). Among the 12 species, only two are known to be widely distributed in the WIO (Othman et al., 2010); namely Tridacna squamosa (Lamarck, 1819) and T. maxima (Röding, 1798). Two other species, endemic to the WIO, T. elongatissima (Bianconi, 1856) and T. rosewateri (Sirenko and Scarlato, 1991), have narrow distribution ranges (Fauvelot et al., 2020). The two widespread species within the waters of the Republic of Mauritius, T. maxima and T. squamosa, have been the subject of ecological studies in Mauritius and Rodri-gues Islands (Ramah et al., 2019). In addition, Mau-ritian waters are known to host the endemic species T. rosewateri, only known from the shallow bank of Saya de Malha (Sirenko and Scarlato, 1991) and Carga-dos Carajos archipelago (St Brandon) (Fauvelot et al., 2020). The only living specimens of T. rosewateri were sampled from Cargados Carajos archipelago and were described as “Tridacna lorenzi” (Monsecour, 2016), and recently synonymized with T. rosewateri (Fauvelot et al., 2020). The EAF-Nansen research survey expedition of 2018 provided an excellent opportunity to carry out some exploratory work within the waters of the Saya de Malha and Nazareth Banks in order to refine the geographic distribution of T. rosewateri. With the aid of the Argus Remotely Operated Vehicle (ROV), it was possible to take stock of the different ben-thic communities of the Saya de Malha and Nazareth Banks. In this note, the in-situ observation of two indi-viduals of the WIO endemic giant clam T. rosewateri in their natural habitat at depths of 38.60 m and 39.66 m (Fig. 1 B-H) within the Nazareth Bank are reported. Morphological identification of giant clams in situ can be challenging, especially when organisms are young, have different mantle colour, shells that are covered by epibionts, and when relevant morphological traits have not yet been fully developed (Ramah et al., 2017; 2019). Well-developed identification guides (Knop, 1996) are nevertheless available to easily identify spe-cies using morphological traits such as by observing First in-situ observation of the endemic giant clam Tridacna rosewateri from the Nazareth Bank, Mascarene PlateauSundy Ramah 1,2*, Deepeeka Kaullysing 2, Ranjeet Bhagooli 2Field Note1 Albion Fisheries Research Centre,  Ministry of Blue Economy, Marine Resources, Fisheries and Shipping, Albion, Petite Rivière 91001, Republic of Mauritius2 Department of Biosciences and Ocean Studies, Faculty of Science & Pole of Research Excellence in Sustainable Marine Biodiversity, University of Mauritius, Réduit 80837, Republic of Mauritius* Corresponding author: sundy.ramah@gmail.com http://dx.doi.org/10.4314/wiojms.si2021.2.16194 WIO Journal of Marine Science  Special Issue 2/ 2021 193-196  |  S. Ramah et al. Figure 1.  Figure 1. T. rosewateri from Musée national d’Histoire naturelle (MNHN) and in situ from the Nazareth Bank. A and E: Specimen photos from MNHN (photos adapted to show salient features only). B-D: Individual 1 recorded from Nazareth Bank at 38.60 m encrusted in coral. F-H: Individual 2 recorded from Nazareth Bank at depth at 39.66 m encrusted in coral and green coralline algae. Red circles indicate the shape of the valve margins, and the yellow arrows show the scutes projection on the primary folds. 195S. Ramah et al.  |  WIO Journal of Marine Science  Special Issue 2/ 2021 193-196the lateral, back and upper view of the organism’s shell, the rib interstices and the byssal orifice (Nor-ton and Jones, 1992). Photographs available from the Musée national d’Histoire naturelle (MNHN) of one T. rosewateri paratype (MNHN-IM-2013-49129, Fig 1A, E) were used for morphological comparison with the two individuals observed in situ (Fig. 1 B-D and F-H). Based on the Sirenko and Scarlato (1991) morphologi-cal description, a high number of similarities in mor-phological traits were easily observed between the two individuals recorded from the Nazareth Bank and that from the Saya de Malha Bank kept at the MNHN. Both specimens from the Nazareth Bank have an elongated shape, and the valve margins are large and sharp in a triangular shape (red circle in Fig. 1. A and C). The scutes at the primary folds are large and widely spaced (yel-low arrow in Fig. 1 A, B, E and F) very similar to that of T. squamosa. However, as compared to the latter, T. rosewateri was found to be semi-encrusted in coral (Fig. 1 D and H) which is not a trait of T. squamosa in Mauritian waters (Ramah et al., 2017). T. rosewateri have 4 to 9 dense convex rib-like folds which become more concentrated on the ventral slope. This can be seen from the lateral view of the specimen observed in Fig. 1 F and G. The shells of giant clams are often used as a substrate and shelter by other organisms such as sponges, corals, molluscs, and coralline algae, among others (Neo et al., 2015). As recorded previously by Sirenko and Scarlato (1991), these were also observed for the two individuals recorded from the Nazareth Bank (Fig. 1 B, C, F and G). This observation backed up by morphological evi-dence presents new data on the occurrence of T. rosewateri in the waters of the Republic of Mauri-tius, especially on its fishing banks. T. rosewateri that was previously thought to occur only at the Saya de Malha Bank and more recently in St Brandon is more regionally spread. It is noteworthy that no individ-uals of giant clams were observed in the surveyed water of the Saya de Malha Bank during this expe-dition, while two individuals of T. rosewateri were observed on the Nazareth Bank at a depth of about 40 m, which is quite unusual as giant clams prefer shallow waters for their symbiotic zooxanthellae to photosynthesize. The occurrence of T. rosewateri in the waters of Nazareth Bank is not improbable. Saya de Malha and Nazareth Banks and St Brandon are dominated by the South Equatorial Current (SEC) (New et al., 2013) which may play a role in the  con-nectivity of the marine flora and fauna of the Banks and St Brandon. With this new observation, it can be confirmed that the distribution of this endemic spe-cies of giant clam likely covers the entire Mascarene Plateau and not only the Saya de Malha Bank. AcknowledgementsThe underlying work was made possible with the support of the EAF-Nansen Programme “Supporting the Application of the Ecosystem Approach to Fish-eries Management considering Climate Change and Pollution Impacts” executed by Food and Agriculture Organization of the United Nations (FAO) and funded by the Norwegian Agency for Development Cooper-ation (Norad). The authors are thankful to FAO for funding and supporting the Indian Ocean research expedition 2018 on the Saya de Malha Bank and Naza-reth Bank with the R/V Dr Fridtjof Nansen, the Depart-ment of Continental Shelf, Maritime Zones Admin-istration & Exploration of Mauritius for co-leading and coordinating the scientific expedition, the Mau-ritius-Seychelles Joint Commission of the Extended Continental Shelf for their support and assistance and granting the necessary authorisations, the Min-istry of Blue Economy, Marine Resources, Fisheries & Shipping for hosting and spearheading the Habi-tat Mapping Workshop in Mauritius and for grant-ing necessary authorization to carry out research in the Nazareth Bank; the Institute of Marine Research, Norway for leading the expedition and providing the technical and logistic support. 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First in-situ observation of the endemic giant clam Tridacna rosewateri from the Nazareth Bank, Mascarene Plateau

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First in-situ observation of the endemic giant clam Tridacna rosewateri from the Nazareth Bank, Mascarene Plateau

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