Contents lists available at ScienceDirect Aquatic Botany journal homepage: www.elsevier.com/locate/aquabot An uncommon intertidal record of Halophila decipiens Ostenfeld in the vicinity of mangroves Mohammad Rozaimi*, Natasha Arina, Nur Farah Ain Zainee, Chandran Raynusha, Nur Hidayah Centre for Earth Sciences and Environment, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia ARTICLE INFO Keywords: Seagrass Estuary Subtidal ITS sequence Phylogenetics Morphology Voucher specimens Population Malaysia ABSTRACT In this paper, we report the occurrence of Halophila decipiens Ostenfeld found growing in an intertidal en- vironment. At the study site of Sungai Bahan estuary located along the Straits of Johor (Malaysia), the seagrass grew sparsely in the lower littoral zone of the mangrove coastline. Forming discontinuous patches, the seagrass was fully exposed on the mudflats during the lowest spring tides. Predictable times of exposure during low tides coincide with specific diel periods of lower light availability, which enabled avoidance of exposure to excessive irradiance. Hand-collected specimens of H. decipiens were initially defined through morphological and in- florescence characteristics. The identity of the species was confirmed based on analysis of the internal tran- scribed spacer sequence of its ribosomal DNA. Further analysis of this sequence demonstrated that among global populations, H. decipiens formed a sub-clade with populations from Malaysia and Australia. The colonisation by H. decipiens at the study site demonstrates its opportunistic acclimation to local environmental conditions. This discovery opens opportunities to further explore its ecology, as an instance of a less commonly studied seagrass within the region and to further investigate the unusual intertidal niche associated with the species. 1. Introduction Seagrasses are marine angiosperms that have fully adapted to in- habit the marine environment. Their distribution spans globally but is limited to the photic zone of estuaries and coasts that allows the maximum depth limits for photosynthetic production. Few species, such as members of the Halophila genus, are known to tolerate very low-light environments. Furthermore, Halophila species are among the most widely represented seagrass genus globally. They are very adaptable in inhabiting shallow to deep waters and marine to brackish systems (Green and Short, 2003). Halophila decipiens is a monoecious seagrass that normally resides in subtidal environments. Intertidal populations of H. decipiens are not known other than the population that occurs just below the intertidal water depths of 0.3–1.5 m from Panama (Schubert and Demes, 2017; McMillan, 1988). It has been suggested that H. decipiens avoids in- habiting intertidal areas because of a lack of ultra-violet absorbing pigments, which makes it susceptible to photoinhibition when exposed to relatively low irradiances (Durako et al., 2003). Other studies have suggested that populations at shallow water depths may tolerate higher light intensities through efficient energy dissipation and chloroplast movements (Schubert and Demes, 2017). If populations were to exist in the intertidal zone, it would point to site-specific acclimation of this species in such environments. Estuaries in Malaysia are commonly influenced by mangrove ve- getation (Faridah‐Hanum et al., 2014). Physiological tolerances to brackish conditions (< 30 practical salinity unit, PSU; Lalli and Parsons, 2006) allow the co-occurrence of the seagrasses Halophila ovalis, Halophila beccarii and Halodule pinifolia in the vicinity of man- groves (Japar Sidik et al., 2016). In lower estuaries with decreasing freshwater influence, such as in the Sungai Pulai estuary, typical marine seagrass species form meadows within the intertidal to subtidal depths that supports the local biodiversity (e.g. Ara et al., 2016; Japar Sidik et al., 2016; Fairoz et al., 2018). Seagrasses exposed during low tidal periods are prone to physiological stresses such as irradiance over- saturation and desiccation (e.g. Durako et al., 2003; Kahn and Durako, 2009). Only a specific group of organisms can tolerate the rapidly changing environmental fluctuations in the intertidal zone of estuaries. In this regard, certain members of the Halophila genus such as H. bec- carii and H. ovalis tolerate a range of salinity in intertidal waters (Green and Short, 2003), whereas intertidal-estuarine populations of H. deci- piens have not been reported so far. Among Halophila seagrasses, Halophila decipiens Ostenfeld is dis- tributed in waters spanning from Asia to the central Americas (Green and Short, 2003). Most of the research into H. decipiens has been per- formed on samples from the meadows of the Atlantic basin (e.g., https://doi.org/10.1016/j.aquabot.2019.103168 Received 22 July 2019; Received in revised form 23 October 2019; Accepted 23 October 2019 ⁎ Corresponding author. E-mail address: mdrozaimi@ukm.edu.my (M. Rozaimi). Aquatic Botany 160 (2020) 103168 Available online 30 October 2019 0304-3770/ © 2019 Elsevier B.V. All rights reserved. T