Source: SEAMEO BIOTROP's Research Grant | 2020
Gene flow cases often occurs in marine organism that have a dispersal across wide geographic ranges (Palumbi, 1994). Gene flow often precludes genetic subdivision so sampling species with high or intermediate dispersal abilities must be extensive (Lessios et al., 1998). Otherwise, structure of population can be can be separated by reason of genetic drift, strong post-settlement selection (Hedgecock, 1986) and spatial-landscape patterns (Johnson & Black, 1998; Watts & Johnson, 2004) as well as to a limited dispersal capability (Collin, 2001). Species with a limited dispersal capability are often composed of highly genetically structured populations with small geographic ranges, thus providing more opportunities to compare the depths and positions of intraspecific genetic with the locations as extrinsic factors (Bernardi & Talley, 2000).
Horsehoe crabs are one of the interesting groups of marine organism maintaining ther genetic structure virtually unchange over millions of years. These are an exotic aquatic biota and lives almost 500 million years. Horseshoe crab are also known as living fossil animals (Eldredge & Stanley, 1984). Horseshoe crabs are ancient marine arthropods exhibiting lifehistory and habitat preferences that might indicate a restricted dispersal capability (Sekiguchi, 1988). Generally, horseshoe crab classified as Atlantic horseshoe crab (Limulus Polyphemus) the only Atlantic species, inhabits the eastern coast of North America from Maine to Mexico (Rutecki et al., 2004; Walls et al., 2002). Three asian horseshoe crabs including Carcinoscorpius rotundicauda, Tachypleus gigas, and Tachypleus tridentatus (Lee & Morton, 2005; Sekiguchi & Shuster, 2009) are distributed sporadically from Southeast Asia to Japan. These can be found in Indonesian coastal waters, dispersion from Sumatra, Java, Kalimantan and Sulawesi (Rubiyanto, 2012; Mashar et al., 2017).
Throughout its life cycle, the horseshoe crab is highly dependent on environmental conditions in its coastal habitats. Adults spawn on the coarse sand near the high-tide zone and external fertilization during the breeding period. Juveniles inhabit the adjacent intertidal mudflats and gradually migrate to the deeper subtidal zone for maturation then come back to the natal beach for spawning (Sekiguchi, 1988; Chiu & Morton, 1999). The hatched trilobite larvae of L. polyphemus swim freely for a short period and settle to the bottom in shallow 2 waters in intertidal zone near their natal beaches (Shuster, 1982). In contrast, C. rotundicauda nests at the high-tide level of mangrove-penetrating tidal creeks just beyond the edges of terrestrial land (Cartwright-Taylor & Hsu, 2012). In addition, C. rotundicauda called mangrove horseshoe crab, is known to spend its life within mangrove swamps or sometimes moves to nearby deeper water but does not migrate to the sea (Davidson et al., 2008, Cartwright-Taylor et al., 2012). These life-history characteristics and habitat preferences suggest that the dispersal capability of horseshoe crabs might be restricted (Pierce et al., 2000).
Population genetic studies of horseshoe crabs have generally focused on Limulus polyphemus along the eastern coast of North America (Pierce et al., 2000; King et al., 2004). The existence of a genetic division between the Gulf of Mexico and Atlantic populations, a pattern observed for a variety of species (Avise, 2004) and microsatellites analysis (Saunders et al., 1986; King et al., 2004). In contrast, limited gene flow on small scales was reported for sequence variants of cytochrome oxidase I (COI) in the Chesapeake Bay and Delaware Bay (Pierce et al., 2000).
The Indian horseshoe crab was occupied in Southeast Asia to Japan (Sekiguchi, 1988). Most researchers suggest that the Asian horseshoe crabs are declining both locally and regionally. It is due to loss of suitable spawning grounds because of overharvesting for food and biomedical purposes and coastal development (Itow, 1993; Chiu & Morton, 1999; Botton, 2001; Chen et al., 2004). Tachypleus gigas was once relatively profuse along the northern of Java sea. It is now thought that the population of Indian horseshoe crab are unidentified based on the conservation status of these, data deficient (IUCN, 2015). Identification spesies (Meilana et al., 2016), dispersal analysis (Mashar et al., 2017), and population structure of T. tridentatus (Erwyansyah, 2018) have been done. This study examines population structure and gene flow in Indian horseshoe crab, T. gigas using mtDNA AT-rich region, which has proven to be a useful marker in intraspecific studies of some other arthropods (Brehm et al., 2001) in order to facilitate conservation efforts for this species.
In the present study, a nested hierarchical analysis was used to investigate the genetic structure of indian horseshoe crab, Tachypleus gigas in the coast of northern Java. A maternally inherited molecular marker, the mitochondrial AT-rich region, was used. Historical and recent perspectives on biogeography and population demography would support this 3 research. Genetic structure, gene flow, and the potential mechanisms (sea surface currents, demography, and others) creating any population division will be examined, allowing for recommendations of effective marine reserves establishment in the future for horseshoe crab conservation in Indonesia.
Research objectives according to the project:
1. Contribute to update IUCN status of Tachypleus gigas, now IUCN status of this animal based on the IUCN red list is data deficient
2. Identify the haplotype diversity as the basic data to crosscheck Indonesia horseshoe crab biodiversity with other horseshoe crab from several countries
3. Determine the structure population and gene flow of horseshoe crab in each location
4. Arrange effective management for Indonesian horseshoe crab conservation
This study has observed an overall high genetic diversity within populations of
horseshoe crab T. gigas and has a low lavel of genetic differentiation which indicates a
population connectivity and a single stock population. With regard to the limited movement
potential of coastal horseshoe crabs, it should be noted that historical demography was part of
the population expansion during the last glacial period. Therefore, local-based conservation is
the preferred management method, which can be one of the precautionary approaches to
conserving the Indonesian coastal horseshoe crab. Additionally, an advanced analysis based
on male and female horseshoe crab needs to be elucidated with characteristic of population
subdivision from the nuclear genome (e.g., microsatellites) and requires the expansion of the
number in geographic range around Indonesia.
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