Contemporary mangrove forest areas took shape historically and their genetic connectivity depends on sea-faring propagules, subsequent settlement, and persistence in suitable environments. Mangrove species world-wide may experience genetic breaks caused by major land barriers or opposing ocean currents influencing their population genetic structure. For Malay Peninsula, several aquatic species showed strong genetic differentiation between East and West coast regions due to the Sunda shelf flooding since the Last Glacial Maximum. In this study genetic diversity and structure of Avicennia marina populations in Malay Peninsula were assessed using nuclear microsatellite markers and chloroplast sequences. Even though all populations showed identical morphological features of A. marina, three evolutionary significant units were obtained with nuclear and cytoplasmic markers. Avicennia marina along a 586 km stretch of the West coast differed strongly from populations along an 80 km stretch of the East coast featuring chloroplast capture of Avicennia alba in an introgressive A. marina. Over and above this expected East-West division, an intra-regional subdivision was detected among A. marina populations in the narrowest region of the Strait of Malacca. The latter genetic break was supported by an AMOVA, STRUCTURE and BARRIER analysis whereas RST>FST indicated an evolutionary signal of long-lasting divergence. Two different haplotypes along the Western coast showed phylogeographic relationship with either a northern or a putative southern lineage, thereby assuming two Avicennia sources facing each other during Holocene occupation with prolonged separation in the Strait of Malacca. MIGRATE-n model testing supported a northward unidirectional stepping-stone migration route, although with an unclear directionality at the genetic break position, most likely due to weak oceanic currents. Low levels of genetic diversity and southward connectivity was detected for East coast Avicennia populations. We compared the fine-scale spatial genetic structure (FSGS) of Avicennia populations along the exposed coast in the East versus the sheltered coast in the West. A majority of transects from both coastlines revealed no within-site kinship-based FSGS, although the remoteness of the open sea is important for Avicennia patches to maintain a neighborhood. The results provide new insights for mangrove researchers and managers for future in-depth ecological-genetic-based species conservation efforts in Malay Peninsula.

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Barrier to Gene Flow of Grey Mangrove Avicennia marina Populations in the Malay Peninsula as Revealed From Nuclear Microsatellites and Chloroplast Haplotypes

Historical processes of long-distance migration and ocean-wide expansion feature the global biogeographic pattern of Rhizophora species. Throughout the Indian Ocean, R. stylosa and R. mucronata appear as a young phylogenetic group with expansion of R. mucronata towards the Western Indian Ocean (WIO) driven by the South Equatorial Current. Nuclear microsatellites revealed genetic patterns and breaks, however, estimating propagule dispersal routes requires maternally inherited cytoplasmic markers. Here, we examine the phylogeography of 21 R. mucronata provenances across a >4,200 km coastal stretch in the WIO using R. stylosa as outgroup. Full length chloroplast genome (164,474 bp) and nuclear ribosomal RNA cistron (8,033 bp) sequences were assembled. Boundaries, junction point, sequence orientation and stretch between LSC/IRb/SSC/IRa/LSC showed no differences with the R. stylosa chloroplast genome. A total of 58 mutations in R. mucronata encompassing transitions/transversions, insertion-deletions and mononucleotide repeats revealed three major haplogroups. Haplonetwork, Bayesian ML and Approximate Bayesian Computation (ABC) analyses supported discrete historical migration events. An ancient haplogroup A in the Seychelles and eastern Madagascar was as divergent from other R. mucronata haplogroups as it was from R. stylosa. A star-like haplonetwork referred to recent range expansion of haplogroup B from northern Madagascar towards the African mainland coastline, including a single variant spanning >1,800 km across the Mozambique Channel Area. Populations south of Delagoa Bight contained haplogroup C and originate from a unique bottleneck dispersal event. Divergence estimates of pre- and post-Last Glacial Maximum illustrated a recent emergence of WIO Rhizophora mangroves compared to other oceans. Connectivity patterns could be aligned with directionality of major ocean currents. Madagascar and the Seychelles each harbored haplogroups A and B, albeit among spatially separated populations, explained from a different migration era. Likewise, the Aldabra Atoll harbored spatially distinct haplotypes. Nuclear ribosomal cistron (8,033bp) variants corresponded to haplogroups and confirmed admixtures in the Seychelles and Aldabra. These findings shed new light on the origins and dispersal routes of R. mucronata lineages that have shaped their contemporary populations in large regions of the WIO, which may be important information for defining marine conservation units, both at ocean scale and at level of small islands.

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Complete Chloroplast Genome Variants Reveal Discrete Long-Distance Dispersal Routes of Rhizophora in the Western Indian Ocean

Dispersal plays a crucial role in the connectivity of established mangrove populations and in species range dynamics. As species ranges shift in response to climate change, range expansions can occur from incremental short-distance dispersal events and from stochastic long-distance dispersal events. Most population genetic research dealt with historically accumulated events though evidence of actual propagule dispersal allows to estimate genotypic features and origin of founders. In this study we aim to disentangle a contemporary dispersal event. Using microsatellite markers, we genotyped 60 Rhizophora racemosa drift propagules obtained on a bare unforested coastal area in southern Cameroon, estimated their relationship to 109 adult trees from most proximate sites (which were 3 to 85 km away), and assessed their relative difference with 873 trees of major mangrove areas (> 300 km) along the Cameroonian coastline. Proximate mangrove populations were considered as potential source populations in assignment tests. However, drift propagules could not be assigned to any of the Cameroonian mangrove sites and were genetically isolated from Cameroonian populations. Drift propagules showed higher levels of genetic diversity and private alleles giving a higher relatedness to each other than to any putative source population. Chloroplast sequences were used to confirm the identity of drift propagules as R. racemosa. We postulate that a complex interaction of ocean currents, estuarine geomorphology, and tidal patterns explain drift propagule dispersal to an area. Most likely the investigated cohort of propagules originated from more southern mangrove areas of the West African range beyond the Cameroonian border. This study unraveled the allelic, genetic, and genotypic features of stranded propagules following a stochastic long-distance dispersal. Transboundary dispersal of these propagules highlights the need for intergovernmental efforts in the management of biodiversity.

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Genotypes of Rhizophora Propagules From a Non-mangrove Beach Provide Evidence of Recent Long-Distance Dispersal

Mangrove forests provide many ecosystem services to coastal communities and are essential in addressing climate change and coastal erosion. Unfortunately, physical pressures, including timber extraction, firewood, and land conversion to agriculture and aquaculture have threatened this ecosystem. Recognizing the reduction in mangrove coverage, mangrove plantations are widely being utilized in many countries to restore ecosystem services, including capturing and storing atmospheric carbon. However, it is still being determined whether mangrove plantations can sequester carbon as much as natural mangroves. This study investigated the carbon storage potential of the planted mangrove in the Ayeyarwady Region, Myanmar. Field data: the diameter at breast height (DBH) ≥ 5 cm and the total tree height (H) ≥ 1 m of all standing trees within each plot were measured and recorded according to species and were used to calculate biomass and carbon storage. The findings of the present study described that the overall average above- and belowground carbon storage of the mangrove plantation was 100.34 ± 50.70 Mg C ha−1 and 34.76 ± 16.59 Mg C ha−1, respectively. Biomass and carbon storage were closely related to the stand basal area. Among species, the Avicennia officinalis species contributed the highest total biomass carbon accumulation. The average amount of carbon sequestration by the planted mangroves was 495.85 MgCO2-eq ha−1. According to the findings, mangrove plantations could achieve benefits in terms of carbon storage and sequestration in biomass with suitable species selection and management. This finding can be applied to mangrove plantation management at the regional and global levels.

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Assessing the Carbon Storage Potential of a Young Mangrove Plantation in Myanmar

Studies of mangrove population connectivity have focused primarily on global to regional scales and have suggested potential for long-distance connectivity, with archipelagos serving as stepping stones for trans-oceanic dispersal. However, the contribution of propagule dispersal to connectivity is still largely unknown, especially at local-scale. Identifying fine-scale propagule dispersal patterns unique to individual island systems is important to understand their contribution to global species distributions, and to select appropriate sizes and locations for mangrove conservation in archipelagos. Using population genetic methods and a release-recapture method employing GPS drifting buoys, we investigated the spatiotemporal scale of propagule dispersal of Rhizophora stylosa, one of the widely distributed mangrove species in the Indo-West Pacific. This study sought to quantify intra- and inter-island connectivity and to assess their contributions to oceanic scale dispersal of R. stylosa from the Ryukyu Archipelago, which spans over 545 km in southwestern Japan. Using 7 microsatellite markers, we tested 354 samples collected from 16 fringing populations on 4 islands. We identified 3 genetic populations, indicating distinct genetic structures comprising 3 distinguishable bioregions (genetic clusters). The western end of the archipelago receives relatively frequent migration (m > 0.1), but is genetically isolated from other sites. Based on genetic migration rates, we found that the central area of the archipelago serves as a stepping stone for southwestward, but not northeastward dispersal. On the other hand, with in-situ drifting buoys, we did not confirm prevailing dispersal directionality within the archipelago, instead confirming local eddies. Some buoys trapped in those eddies demonstrated potential for successful beaching from another island. A large portion of buoys were carried predominantly northeastward by the Kuroshio Current and drifted away from the coastal areas into the Pacific, contrary to local migrations. We found that the spatiotemporal scale of propagule dispersal is limited by the distance between islands (< 200km), propagule viability duration, and fecundity. Over all, recruitment does not occur frequently enough to unify the genetic structure in the archipelago, and the Ryukyu Archipelago is isolated in the center of the global mangrove distribution.

Extremely Stochastic Connectivity of Island Mangroves

Avicennia dominated mangrove forests occur from seaward to landward sites and hence are subject to different dynamics within estuarine ecosystems. Regeneration of mangrove forests primarily depends on the extent of propagule spread and subsequent establishment in suitable habitats. The complex nature of estuarine systems induces a wide variety of local conditions for within-site propagule retention and settlement thereby allowing spontaneous regeneration of mangroves. In this study, we estimated the fine-scale spatial genetic structure (FSGS) of Avicennia populations and examined whether their position relative to the seaside or the size of mangrove patches could have influenced the extant local population genetic structure. A kinship-based FSGS was performed using microsatellite markers in 523 A. marina, 189 A. rumphiana and 60 A. alba adult trees of 24 sites in The Philippines. Transects within each estuary were taken both parallel and perpendicular to the coastline or tidal river edge. The extent of local mangrove areas and various human-induced encroachments as such did not show any trend in allele diversity, heterozygosity values or inbreeding levels. However, farther inland situated mangrove patches showed a larger FSGS extent across the neighborhood (up to 75 m) though less diversity along with inbreeding, most likely due to retention of related propagules and lowered chance of external propagule input. Estimation of connectivity along a same coastline stretch supported a unidirectional steppingstone or adjacent migration model for populations of either A. marina, A. alba or A. rumphiana. These were congruent with ocean currents across mangrove estuaries of the Tablas Strait and along Western Leyte, thereby emphasizing the relevance of coastal connectivity for long term persistence. From this study, we conclude that both proximity to open water and narrowness of mangrove patches may affect their captured diversity, inbreeding and fine-scale structure caused by propagule movement within or beyond a local mangrove fragment during recent generations. Higher levels of allele diversity for seaward sites and highest likelihood of migration for adjacent mangroves both add to the importance of coastal connectivity that is the only natural cohesive force on longer term and necessary to counteract short term effects of increasingly encroached mangrove environments.

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Avicennia Genetic Diversity and Fine-Scaled Structure Influenced by Coastal Proximity of Mangrove Fragments

The Philippines is a hotspot for biological variety and a hub for endemism on a worldwide scale. Despite this, most of the research has focused on terrestrial and marine biodiversity, with little knowledge of freshwater fish diversity and status. There has been no systematic research on these species in Romblon. As a result, this research was conducted. This study aims to evaluate the freshwater fishes of Tablas Island, Romblon, utilizing modified hoop nets, gill nets, scoop nets, and hook and line. A total of 44 species belonging to 36 genera and 19 families were identified. They are often found in rivers, and streams, while most of them are native to the Philippines, where they are being eaten by natives. The IUCN Red List placed the majority of these species in the Least Concern category. The dominating species was Gambusia affinis, with Giuris margaritaceus being the most frequent across all sites. A new locality record of an indigenous minnow species (Barbodes hemictenus) was found. The ichthyofauna of Tablas Island is diverse, with local species dominating. Only two introduced species were identified as threatened (Vulnerable). On the other hand, the rest were classified as Least Concern, Unknown, Not Evaluated, and Data Deficient.

Vincent Jay Gado

Papers

Avicennia Genetic Diversity and Fine-Scaled Structure Influenced by Coastal Proximity of Mangrove Fragments

Freshwater ichthyofauna of wetlands in Tablas Island, Romblon, The Philippines