Pteroptyx malaccae, is an iconic species of bent-winged firefly in Southeast Asia. It is known for its synchronous flashing displays while congregating on riverine trees in intertidal mangrove forests. It is found in Cambodia, East Malaysia, Peninsular Malaysia, Thailand, Indonesia, and Singapore. The population size and trend for this species are unknown, though it is thought to be in decline due to habitat loss and degradation caused by residential and commercial development, light pollution, sea level rise and increased storm surges and flooding due to climate change, and entomo-tourist activities. The area of occupancy (AOO) could be as low as 320 km², based on the known extant occurrences. Continuing decline in quality, area and extent of habitat, and the number of mature individuals has been observed, as mangrove habitats are being lost and degraded across the region and the species has declined in areas such as Bangkok, where surveys are being carried out. This species is considered to occur in 8–9 locations due to gradual rise in sea level, which is projected to impact all suitable habitats by 2050 (Saintilan et al. 2020). Therefore, this species is assessed as Vulnerable under criterion B2ab(iii,v).

Pteroptyx malaccae inhabits mangrove forests in estuaries and brackish water ecosystems in Southeast Asia. It is known to occur in Borneo (Sandakan Bay and Kalimantan) (Ballantyne and McLean 1970, Ballantyne 2001, Ballantyne et al. 2011, Jusoh et al. 2018), Cambodia (Checko), East Malaysia (Sarawak and Sabah), Peninsular Malaysia (numerous localities), Thailand (Chao Phraya riverbank in Bangkok, Krabi, Chanthaburi, Chachoengsao, Chon Buri, Chumphon, Trat, Nakhon Sri Thammarat, and Prachuap Khiri Khan) (Buck and Buck 1966, Ballantyne and McLean 1970, Ballantyne 2001, Sartsanga et al. 2018, Jaikla et al. 2020, Ballantyne et al. 2011, Jusoh et al. 2018), Indonesia, and Singapore (Pulau Semakau and Pasir Ris Park).

The extent of occurrence (EOO) of this species, based on known occurrences, is over 2 million km². The area of occupancy (AOO) could be as small as 320 km², based on the known extant occurrences. While it is likely this species occurs in additional unknown localities, it is unlikely the AOO exceeds 500 km² (the criterion B2 threshold for Endangered) because this species is restricted to mangrove forests within estuaries, habitats that are limited in their distribution. This species is considered to occur in 8–9 locations due to gradual rise in sea level, which is projected to impact all suitable habitats by 2050 (Saintilan et al. 2020).

The population size for Pteroptyx malaccae is not known, although it is reportedly the most abundant and widespread Pteroptyx species found in Thailand (Jaikla et al. 2020). However, there is some evidence of localized population declines. In several monitoring studies, abundance was seasonally estimated in the Bangkrachao area of Bangkok, Thailand (Prasertkul 2018, Jaikla et al. 2020) and has continuously been monitored monthly by local communities since 2017. The long-term population trend suggests a declining population, resulting in smaller, more fragmented occurrences. In addition, at one site along the Chao Phraya riverbank, Buck and Buck (1966) reported P. malaccae in much higher numbers than the sympatric species P. valida (70 and 6 individuals respectively, or 8% P. valida). In 2013, species composition was estimated by Prasertkul (2018), and P. valida made up 10–25% of the individuals observed. In 2016, the percentage of P. valida increased up to 77% (Jaikla et al. 2020). Consistently, the results of community surveys (2017-present) displayed a very low percentage of P. malaccae compared to P. valida in inland mangrove areas (Bang Krasob and Song Khanong). In addition, where this species is found along the coast of Thailand, where it coexists with other Pteroptyx species (Sartsanga et al. 2018, Jaikla et al. 2020), the population appears to have declined. In 1965, mass synchrony of P. malaccae, that perched on every leaf of Sonneratia caseolaris, was continuously observed along the edges of Chao Phraya riverbank (Buck and Buck 1966). Over fifty years, the riverbank habitats at this site have been destroyed, modified, and fragmented, primarily due to urban development. More than 90% of the species along Chao Phraya riverbank have been lost and are now restricted to two meandering-river floodplain areas of 16 km² in the Bangkrachao estuary, which is designated as an environmental protection area, and on Koh Kret, a small island in the river. The population has not been closely monitored in other portions of the range, though declines are inferred due to widespread habitat losses. In Singapore, P. malaccae was thought to occur only in Pulau Semakau until in 2017, DNA barcoding work confirmed the presence of P. malaccae in Pasir Ris Park. Still, its population is observed to be small and scarce (<10 individuals) (W.F.A. Jusoh, pers. obs.). In Peninsular Malaysia, only very small colonies of this species are found amongst large populations of P. tener at most sites, including along the Perak River near Teluk Intan and along the Selangor River (C.H. Wong pers. obs.).

This species is found only in riverine mangrove forest habitats, slightly inland from estuaries. It is commonly observed flashing synchronously in the canopy of true mangrove tree species, such as Sonneratia alba, Sonneratia caseolaris and Rhizophora apiculata. Sometimes it can be found on other plant species adjacent to mangrove habitats, such as mango and coconut. Display trees in open areas seem to be favored by courting males, and other factors such as density, size, and arrangement of leaves may also be important in tree selection (Prasertkul 2018, Obha and Wong 2004). Mangrove vegetation is important not only for adult display trees but for larval and snail habitat as well. Larvae of this species have been observed feeding on a variety of snail species, in two families (Assimineidae and Ellobiidae), including Assiminea nitida nitida, Paludinella thonburi, Laemodonta punctigera, Melampus nucleolus, Melampus siamensis, and Cylindrotis siamensis. Larvae may also prey on other snail species, though this has not yet been observed. Tide level, sedimentation rate, rainfall, drought, salinity, water quality and chemicals influence both larvae and adults.

Globally, mangrove ecosystems have been heavily impacted by degradation and deforestation over the last 50 years, with some estimates suggesting up to 35% total losses (Valiela et al. 2001). In Southeast Asia, mangrove losses have slowed since the 1990s, though from 2000 to 2012, mangroves losses continued at a rate of 0.18% per year (Richards and Friess 2016). The majority of loss in Southeast Asia (82%) is attributed to anthropogenic threats such as conversion for aquaculture and agriculture, pollution from mining, and urban expansion (Goldberg et al. 2020). From 2000 to 2016, around 92% of global loss of mangroves due to commodities, such as rice, shrimp, and oil palm cultivation, occurred in Southeast Asia. Other drivers of loss in Southeast Asia include erosion and extreme weather events (Goldberg et al. 2020).

Population declines in Pteroptyx malaccae can be attributed to mangrove habitat destruction due to firefly tourism, residential and commercial development, light pollution, sea level rise and increased storm surges and flooding due to climate change, water pollution, pesticide run off from agriculture, and construction of concrete flood embankments and water gates. Urbanization is the main driver of mangrove loss in Bangkok and southern Malaysia, where this species is found (Richards and Friess 2016). In cases of firefly occurrence in urban areas, impacts of urbanization on firefly habitat quality and their populations are obvious compared with in conserved areas. For example, in Thailand, construction of concrete flood embankments along riverbanks with water gates to control water circulation from the main river and canals, resulted in observed declines in localized abundance at P. malaccae sites (A. Thancharoen pers. obs.). Over the past ten years, these firefly habitats and abundance recovered, because the concrete flood embankments degraded and water once again flowed freely across. However, saltwater intrusion then impacted agricultural areas and led to severe crop losses, so all flood control structures were restored. In addition to the aforementioned threats, light pollution could be a minor threat resulting from development. Increased artificial light has been shown to interfere with the production and reception of courtship signals in some firefly species, leading to decreases in mating success (Lewis et al. 2020, Owens and Lewis 2018). However, there is some evidence of behavioural adaptation to artificial light, as individuals from this species have been observed remaining in proximity to light pollution areas (Thancharoen and Masoh 2019).

The primary future threat to this species and its mangrove habitat, is sea level rise, increased storm surges and flooding, and increased variability in tidal inundation and salinity in mangrove soils due to climate change. From 1995 to 2015, the Southeast Asian region experienced rising sea level twice the rate of the global mean (Strassburg et al. 2015). This trend has since slowed, as it is influenced by a long-lived weather pattern which causes climate variability, called the Pacific Decadal Oscillation, but further rise is predicted long-term. Mangroves are halophytic woody tree species, and their distribution is limited by soil moisture, salinity, anoxia and nutrient availability, which in turn is maintained by tidal and freshwater inputs (Ball 1988). Changes to tidal and freshwater inputs will cause changes to these abiotic features of mangrove ecosystems. Sea-level rise will cause mangrove losses in areas where mangroves are not able to build surface elevations to keep pace with rising sea-level or where landward migration is not a possibility (Krauss et al. 2014, Gilman et al. 2008). By 2050, under high emissions scenarios, it is predicted that most tropical coastlines will experience sea level rise above the threshold that mangrove accretion will take place (Saintilan et al. 2020). Climate change is also predicted to increase the severity and frequency of tropical storms, increase the frequency of high water events, and alter precipitation patterns, all of which will impact abiotic parameters within mangrove ecosystems, causing localized or widespread degradation (Gilman et al. 2008). In addition to the impacts these threats will have on mangroves, tide level and sedimentation rate may have direct and indirect effects on the larvae of this species, as well as its prey, due to soil moisture level and the accumulation of organic and inorganic materials in the mangrove areas. For example, evidence of tide gate construction to control water level in the Lumpoo Bang Krasop Firefly Conservation Area, in Thailand, resulted in a decline in abundance of larvae (A. Thancharoen pers. obs.). Rainfall, drought, salinity, water quality and chemicals also influence on both larvae and adults.

This species is one of the main synchronous fireflies that attracts hundreds of thousands of tourists to Thailand every year. It can also be found as some firefly tourism sites in Malaysia, such as along the Selangor River (Wong 2022), though it is not the main attraction, as it is far out numbered by Pterotyx tener. Popular firefly sites in Thailand include Amphawa, in Samut Songkram province, and Bang Baimai, in Surat Thani province. In Amphawa, a decline in firefly abundance was observed a few years after the site was popularized as a destination to see fireflies (Lewis et al. 2021, Thancharoen 2012).While firefly ecotourism is generally good for local economies, care must be taken to protect firefly habitats at these heavily trafficked sites.

There is no range wide conservation plan for Pteroptyx malaccae, though there are a few localized conservation efforts targeted at this species and other co-occurring fireflies, such as P. valida. For example, an ongoing habitat enhancement project at Pasir Ris Park in Singapore, is underway, and the conservation status of this species has been assessed as Critically Endangered under the Singapore Red Data Book, Third Edition (W.F.A. Jusoh pers. obs.). In Thailand, where this species is most numerous, there are several efforts to protect and restore specific habitats. One example, is the OUR Kung BangKrachao project, which is working to zone a conservation area for long term preservation activities in the Kung BangKrachao area in Samut Prokarn province. The Kung BangKrachao is a large green space on the periphery of Bangkok, and is important for tourism and environmental, economic, social, and cultural reasons. The area is at risk due to the urban expansion of Bangkok, and therefor efforts are underway to protect it and restore any damage that has already been done to the biodiversity it houses. With fireflies as a flagship species, the OUR Khung BangKachao project was conceived as a social collaboration among 34 leading organizations in Thailand (as of September 2018) with an emphasis on community involvement. The project endeavors to promote economic development and improve the quality of life for those in the Khung BangKachao, as part of the Sufficiency Economic Philosophy enacted by His Late Majesty King Rama 9. The activities of the project include waste management, green development, youth education, cultural development, income augmentation, and tourism. In another example, there is an ongoing restoration initiative at the Sirinart Rajini Ecosystem Learning Park, at Nam Pran, Pran Buri District, Prachuap Khiri Khan. The project, which is supported by the The PTT Public Company Limited (PTT), is the first effort in Thailand to reforest mangroves lost to shrimp farming. Replanting efforts started in 1997, and fireflies were observed for the first time in 2020. Replanting will continue, with plans to plant 9,000 mangrove trees in the next few years. Visitors and local communities are involved in the effort, and contribute by planting mangrove seeds, which will be out planted in future years. Once fireflies are found in abundance at this site, it will become a firefly tourism area. This species may also be benefitting from site protections for other species. For example, conservation measures have been enacted on the Selangor River to protect Pteroptyx tener, which is the target of ecotourism interest at the site. A reserve was created in 1999 to protect firefly habitat. This reserve contains 1,108 ha of land across 40 km of river, including 150–400 meters of habitat on either side of the river, centred on the Kampung Kuantan firefly jetty (Wong 2022).

Additional site protections will be important for conserving this species. One site along the Tapee River, at Bang Bai Mai, Surat Thani province, contains high abundance of Pteroptyx fireflies, and may be good candidate for protection. There is firefly tourism at the site and it is the only site in Thailand where all four Pteroptyx species that occur in the country can be found. Another site, the Bang Pakong river, not too far form Bangkok, also boasts high abundance of Pteroptyx fireflies. This may be a good candidate for further protections, to prevent encroaching urban development from degrading mangrove habitats. While ecosystem level management is needed to mitigate many of the threats to this species, there are a few simple actions, in addition to site protections, that could bolster firefly habitats. For example, providing a protected buffer zone within the high tide flood line around the rivers edge could preserve vital larval habitat, reduce runoff from adjacent agricultural lands and aquaculture ponds, and prevent further encroachment of development into mangrove forests. Near urban areas, light pollution could be limited by planting tall trees adjacent to firefly habitats, or encouraging municipalities to limit the use of artificial light at night. Lastly, an effort to distribute educational materials to firefly tourism providers might help the general public better understand firefly conservation issues and encourage tourism operators to adapt sustainable practices.

There are also several surveys efforts to monitor populations of this species, including a nationwide survey to assess firefly species diversity in Singapore, which includes a series of field observations in nature reserves and parks to understand P. malaccae's distribution. Localized monitoring also takes in Thailand, both in Bangkok and at the Sirinart Rajini Ecosystem Learning Park. More widespread monitoring efforts might help us better understand the population trend of this species across its range.