Halophila beccarii was first collected in 1867 by Italian botanist Odoardo Beccari from the Bintulu River (now Bintulu), Sarawak, East Malaysia (Beccari, 1904; den Hartog, 1970). This type specimen, later described by Ascherson in 1871, laid the foundation for recognizing H. beccarii, commonly known as Beccari’s seagrass or Ocean’s Turf seagrass, as a member of the Hydrocharitaceae family. Characterized by its rosette-forming, leafy shoots and short, creeping rhizomes, the species is well-adapted to colonize unstable substrates in brackish and estuarine environments (Figure 1).

The storyline of the discovery of H. beccarii in Malaysia (Figure 2) starts with the first collection made by Beccari in 1867 in Sarawak, East Malaysia. In Peninsular Malaysia, subsequent records of H. beccarii collection were noted by den Hartog (1970) from Sungai Tebrau and Jalan Skudai, Johor, in 1965. Our surveys confirmed the species’ presence along the east coast starting in 1992 at the mudflats of Sungai Kemaman, Terengganu. This was followed by successive discoveries in 1996 at Paka Lagoon, Paka River, Paka Shoal, and Chukai, and in 1997 at Telaga Simpul. During this period, field investigations also extended to Kelantan, where H. beccarii was observed at Pengkalan Nangka Lagoon, Pengkalan Nangka Shoal, Kampung Baru Nelayan, and Pantai Baru Lagoon. By 1999, its distribution expanded northward in Penang, observed at Seberang Prai and Korea Island. Rediscovery in Sarawak occurred much later, with documentation in Lawas after a 141-year gap from the original Bintulu collection. In Johor, the species reappeared at Sungai Pendas in 2015 and at Tanjung Surat, Kampung Nyior by 2021 (Figure 3), where it was found in both monospecific and mixed stands with Halophila ovalis.

Halophila beccarii is mainly found along the east coast of Peninsular Malaysia, with scattered populations also along the southeast, south, and west coasts. This species has been observed in 15 areas, as shown in Figure 4. It is found as either monospecies or two species in the lagoon, subtidal island, intertidal mangrove association, intertidal sandy beach, subtidal shoal, and intertidal areas associated with coral. It occupies the middle to upper intertidal zone, shallow intertidal in the mangrove area, and intertidal mudflat in front of the mangrove.

Table 1 shows the changes in distribution and area coverage before 2011 and from 2015 to 2025, highlighting significant population declines or complete losses across many sites. The largest historical populations were recorded in Kemaman (17 ha), Telaga Simpul (28 ha), Paka Shoal (43 ha), and Pengkalan Nangka Lagoon (35 ha). However, many of these sites now show significantly reduced sizes or are completely lost. For example, sites such as Chukai, Paka River, and Kampung Baru Nelayan no longer have any H. beccarii present. Field visits between 2011 and 2025 confirmed the ongoing decline and disappearance of H. beccarii meadows at several locations, while only a few sites, such as Paka Shoal, Telaga Simpul, Pengkalan Nangka Lagoon, Korea Island, and Sungai Pendas, still support small patches.

The decline results from both human and natural disturbances. Human pressures include riverbank reclamation at Chukai, mangrove clearing at Kemaman, sand mining or dredging for flood control projects at Paka Shoal (Figures 5A, B). In Pengkalan Nangka, dredging replaced muddy substrate with sand, allowing H. pinifolia to colonize the area previously occupied by H. beccarii (Figure 5C). Additionally, deepening the shoals and road construction damaged H. beccarii in parts of Pengkalan Nangka and Kg Baru Nelayan (Figure 5D). Shellfish harvesting for Ostrea sp., Meretrix meretrix, Gari elongata (Figures 5E, F), and urban development in Seberang Prai also destroy the habitat of H. beccarii. Natural factors such as freshwater input during monsoons, wind-driven waves, and sediment movement also contribute to habitat instability in shallow estuarine areas. These findings, gathered over decades of monitoring and field visits, indicate a significant reduction in the range of H. beccarii in Malaysia.

In Peninsular Malaysia, H. beccarii is distributed across various shallow intertidal zones with differing environmental and substrate conditions. The species thrives in areas influenced by the northeast monsoon, including the east coast sites of Terengganu (Kemaman River, Paka Lagoon) and Kelantan (Pengkalan Nangka Lagoon), where it shows notable seasonal changes in rainfall, salinity, and water temperature.

Table 2 presents the habitat characteristics, physical and chemical parameters of H. beccarii in various locations. This species inhabits a variety of intertidal environments, from freshwater zones during low tide to brackish waters at high tide. Environmental monitoring at key sites revealed that during the monsoon season, heavy rainfall, ranging from 300 to over 700 mm/month, causes reduced salinity, often approaching freshwater levels, especially during low tides. Temperatures remain warm, generally between 27 and 31°C, and total suspended solids (TSS) tend to rise, indicating increased turbidity. In contrast, during the dry season, rainfall is minimal, and salinity increases significantly, with peak measurements ranging from 30 to 38 psu, depending on the site and tide phase. Despite these changes, H. beccarii remains present, demonstrating its ecological adaptability and wide environmental tolerance.

Environmental variability is closely related to habitat type. In mangrove riverbank and mudflat systems such as Kemaman River and Sungai Paka, H. beccarii coexists with mangrove pneumatophores in warm, low-salinity environments with muddy or sandy-loam substrates. In these habitats, the species is usually found at depths ranging from 0.8 to 2.5 meters below the water surface (ELWS). The intertidal seagrass habitats are not entirely submerged but are exposed twice daily depending on the tides.

In protected coastal lagoons and estuaries, such as Pengkalan Nangka Lagoon, the species often forms pure stands or coexists with emergent freshwater species, including Fimbristylis dichotoma and Cryptocoryne ciliata. These areas exhibit fluctuating salinity levels ranging from 0 to 24 psu, warm waters with temperatures from 28 to 38°C, and sand to sandy-mud substrates.

Open shallow coastal areas, such as Telaga Simpul and Seberang Prai, have slightly higher salinity levels, ranging from 15 to 31 psu, and host H. beccarii on sandy-mud substrates at depths of 1.0 to 2.0 meters ELWS, often in association with other seagrasses like H. pinifolia. In contrast, shoals and reclaimed island habitats, including Korea Island and Paka Shoal, show evidence of colonization by H. beccarii in more disturbed or human-altered environments. Here, the species is found at greater depths of 2.7 to 3.5 meters ELWS, on calcareous or sandy mud substrates, and coexists with H. pinifolia. These areas exhibit high environmental variability, especially in salinity (13 to 32 psu) and temperature (30 to 35°C), yet still support healthy populations. The range of environmental conditions observed across these habitats demonstrates the adaptability of H. beccarii, especially in estuarine and monsoon-influenced ecosystems. This habitat-based classification, combined with seasonal environmental data, addresses reviewer concerns by showing how H. beccarii’s distribution is more influenced by its tolerance to fluctuations in salinity, temperature, and turbidity than by strict environmental thresholds. Additionally, the analysis of site-specific monsoon impacts highlights the importance of considering seasonal variability and hydrodynamic protection when interpreting ecological patterns of this vulnerable seagrass species.

Halophila beccarii is a small seagrass species characterized by its monoecious reproductive system and creeping rhizome with a single root per node that bears numerous root hairs. Morphometric measurement of vegetative morphology of H. beccarii from ten locations is illustrated in Table 3. The species features two protective scales: an upper transparent scale that covers the upright shoot and a lower scale that shields the rhizome. Each shoot produces 4 to 12 rosette leaves that are oblong to elliptical. Petiole lengths range from 3.36 to 17.18 mm, leaf lengths from 3.27 to 13.17 mm, and leaf widths from 0.20 to 3.14 mm across different locations in Malaysia. Leaf morphology varies across sites, likely due to local environmental factors and hydrodynamic exposure. The longest petioles were recorded at Sungai Pendas (12.52 ± 2.38 mm), and the widest leaf blades (2.55 ± 0.29 mm). These differences highlight H. beccarii’s ability to adapt to various hydrological and sedimentation conditions, especially in highly dynamic coastal and intertidal zones. The male flowers arise sub-terminally from short lateral branches within the rosette and have three pale brown tepals (1.82–2.91 mm × 0.55–2.18 mm) and three anthers (1.00–2.30 mm long). Female flowers emerge laterally, consisting of an ovoid ovary (0.55–1.82 mm × 0.18–0.86 mm) and 2–3 styles that can reach up to 13.01 mm in length. Typically, one fruit is produced per reproductive shoot, and it is ellipsoid to ovoid in shape, transparent, and contains 1 to 7 seeds. Each seed is globose, reticulate, and reddish-brown, measuring 0.64–1.40 mm × 0.45–1.09 mm.

Field observations and seedling studies reveal that H. beccarii exhibits both annual and perennial life cycles, which are affected by local hydrology and climate. At dynamic, monsoon-driven sites like Pengkalan Nangka Lagoon, Kelantan, and Paka Shoal, Terengganu, H. beccarii functions as an annual species, as illustrated in Figure 6. Plant cover completely disappears during the northeast monsoon from November to January, entering dormancy. Recolonization begins in February with seed germination (Figure 6A). The germinating seeds (Figure 6B) develop into seedlings (Figures 6C, D) from March to April, forming a network of rhizomes. Juvenile plants (Figure 6E) continue vegetative propagation, growing into mature plants that produce male (Figure 6F), female (Figure 6G) flowers, fruit (Figure 6H) and seeds (Figure 6I) from April to September. Between October and November, plant density decreases again, signaling another dormancy phase. From December to January, the meadow recolonizes through the germination of seeds and seedlings, ensuring its continued presence in the natural environment.

Conversely, at more stable estuarine sites such as Sungai Kemaman (Terengganu), H. beccarii exhibits perennial traits, with populations existing year-round. Flowering and fruiting activities were observed almost every month except June for male flowers and May–June for female flowers. This persistence suggests a more stable and resilient population structure, supported by both vegetative spread and seed production. This dual strategy, perennial in stable estuarine habitats and annual in monsoon-affected shoals, reflects H. beccarii’s ecological flexibility. It allows the species to survive disturbances such as sediment shifts, freshwater influx, and seasonal dieback, although its long-term survival remains vulnerable to increased human activity and habitat deterioration.

Halophila beccarii is currently listed as Vulnerable under the IUCN Red List Categories and Criteria Version 3.1 (Short et al., 2011; IUCN, 2012), with an estimated global distribution of less than 2,000 km² (Geng et al., 2022; Chen et al., 2024). Although den Hartog (1970) described it as rare yet widely distributed across the Indo-West Pacific, recent evidence suggests its range has contracted significantly. The species is still reported in India (Jagtap and Untawale, 1981; Untawale and Jagtap, 1977; Jagtap, 1991; Parthasarathy et al., 1991), Thailand (Poovachiranon et al., 1994), Vietnam (Van Tien, 1998), and Hong Kong (Hodgkiss and Morton, 1978), but it has been declared locally extinct in the Philippines (Liao and Geraldino, 2020) and Endangered in Sri Lanka (Udagedara et al., 2017). Notably, the first record from Bangladesh was recently confirmed in the Andharmanik River estuary (Haque et al., 2024), extending the known distribution along the Bay of Bengal.

In Malaysia, H. beccarii shows a patchy and declining distribution, with recent occurrences primarily along the east coast and limited observations on the west coast. It tolerates a wide range of salinity (0–32 psu) and temperature ranges (25–38 °C), thriving in substrates ranging from sandy mud, silty clay, and calcareous substrates. Its physiological flexibility allows it to inhabit estuarine environments, where it coexists with seagrasses such as Halophila ovalis and Halodule pinifolia, macroalgae like Ulva intestinalis, and emergent species like Fimbristylis dichotoma and Cryptocoryne ciliata (den Hartog, 1970; Abhijith et al., 2019). However, despite this flexibility, H. beccarii typically persists only as scattered patches or monospecific meadows, indicating ecological vulnerability when disturbed. Experimental studies confirm its salinity tolerance in Malaysian estuaries (Fakhrulddin et al., 2019), but regional evidence shows limited resilience: low clonal diversity in Vietnam (Phan et al., 2017), strong habitat specialization in Hainan (Geng et al., 2022), and small, recently reported intertidal populations in Bangladesh (Haque et al., 2024).

Monsoonal flooding, coastal erosion, and activities such as land reclamation and pollution directly threaten its population stability. For example, severe flooding in Paka Shoal in 2014 resulted in the complete loss of seagrass due to sediment burial (Japar Sidik et al., 2018; Hossain et al., 2015). A recent visit to Pengkalan Nangka Lagoon reveals that, following the 2024 flooding event, the H. beccarii area has decreased. Seasonal changes in salinity and turbidity, especially during the northeast monsoon, further threaten meadow persistence along Malaysia’s east coast. These patterns emphasize that H. beccarii’s survival depends not only on its physiological tolerance but also on minimizing human impacts, protecting seed bank health, and improving adaptive monitoring to observe short-term ecological changes.

Human activities have also contributed to the decline of H. beccarii in Peninsular Malaysia. In areas such as Chukai, Paka Lagoon, Paka River, Terengganu, and Pengkalan Nangka shoal, as well as Kampung Baru Nelayan, Kelantan, the species is now absent. The habitat is affected by large-scale disturbances, including land reclamation, dredging to deepen boat channels, and sand mining for construction. Dredging naturally removes bed material and seagrasses, causing increased sedimentation that smothers seagrasses over large areas and nearby regions. This sedimentation displaces H. beccarii, allowing more resilient species such as H. pinifolia to grow and dominate the area.

In disturbed or sediment-rich habitats, H. pinifolia often becomes the dominant species due to its higher adaptability and resilience. For example, Lamit and Tanaka (2019) reported that H. pinifolia thrived across sites with different sediment types and nutrient conditions, showing the highest biomass and leaf length in areas with increased silt and clay content related to ongoing coastal development. This adaptability is further supported by its exceptional tolerance to low-light conditions. Longstaff and Dennison (1999) demonstrated that H. pinifolia could survive extended periods of near darkness with minimal biomass loss, a trait not observed in other seagrass species such as Heterozostera tasmanica (Bulthuis, 1983) and Posidonia sinuosa (Gordon et al., 1994), which declined rapidly under similar stress. Additionally, the life cycle of H. pinifolia, characterized by quick colonization through seed banks, provides an extra advantage in post-disturbance environments. As Unsworth et al. (2015) observed, these early successional species can stabilize sediments and promote recovery, often outcompeting more sensitive taxa like H. beccarii.

This vulnerability differs from more resilient taxa like E. acoroides and follows similar patterns observed in other small seagrass species, Halodule pinifolia and H. ovalis, which are also vulnerable to sedimentation and habitat alteration (Short et al., 2011; Yaakub et al., 2014). The ecological impacts are significant, as H. beccarii meadows serve as habitat for juvenile fish, invertebrate nurseries, and support coastal food webs (Waycott et al., 2009). Moreover, their ability to sequester carbon is reduced under low-light and turbid conditions (Premarathne et al., 2021; Mishra et al., 2021). Recovery at these disturbed sites appears slow and likely depends on seed banks, a pattern also observed in Bangladesh (Rakib et al., 2022).

On the west coast of Peninsular Malaysia, the absence of H. beccarii from previously recorded sites, such as Sungai Tebrau and Jalan Skudai, Johor (den Hartog, 1970), suggests possible extirpation. Recent reappearances in 2021 in Sungai Pendas, Johor, suggest that populations may persist in less-disturbed habitats. These findings highlight the importance of adaptive and site-specific monitoring strategies, particularly for cryptic or small-bodied seagrass species that are easily overlooked during conventional surveys. Using rotational monitoring combined with more frequent or community-based assessments could enhance detection and provide early warning signs of biodiversity changes. Although this study benefited from long-term revisit data, it also highlights the need for consistent, frequent monitoring to capture local dynamics and support effective conservation efforts.