Low genetic diversity and identification of two management units for tri-spine horseshoe crab Tachypleus tridentatus (Leach, 1819) in Palawan, Philippines

Abstract

Tachypleus tridentatus, locally known as "barangkas" in Palawan, Philippines, was classified globally as "Endangered" in 2019 in the International Union for Conservation of Nature (IUCN) Red List of Threatened Species. To aid future conservation efforts for this species, we investigated the genetic diversity and population structure of T. tridentatus in Palawan using mitochondrial cytochrome c oxidase subunit I (COI) sequence data. A total of 72 horseshoe crabs were randomly collected from Barangay San Pedro (n = 30) and Barangay San Jose (n = 30) in Puerto Princesa City, and Barangay Punta Baja, Rizal (n = 6), and Barangay San Juan, Dumaran (n = 6) on Palawan Island, Philippines. The samples varied in size, with prosomal width ranging from 6.4 to 34.19 cm and total length ranging from 12.64 to 74.3 cm. Those from San Pedro were mostly juveniles, while those from the other sites were adults. All samples were morphologically identified as T. tridentatus (n = 72). This identification was confirmed by mtDNA sequence data (n = 52), with percent identity ranging from 98.9 % to 99.6 % with the T. tridentatus sequence in the GenBank database. Eight haplotypes were identified among 52 DNA sequences for the 541 bp mtDNA COI fragment, with three haplotypes shared among locations. Overall sequence variation was relatively low, with only ten polymorphic sites observed. The average haplotype diversity was 0.426 ± 0.159, while the average nucleotide diversity was 0.0015 ± 0.0013. Analysis of molecular variance showed genetic differentiation among the samples, with a fixation index (F_ST) of 0.37 (p-value = 0.00 ± 0.00). Pairwise F_STs revealed that the collection from Punta Baja, Rizal was genetically distinct from those in other locations, suggesting the utility of two management units or stocks: respectively on the western (Rizal) and the eastern (Puerto Princesa and Dumaran) sides of the island. The low abundance and genetic diversity observed among horseshoe crabs in Palawan suggest an urgent need for measures to protect this species. Moreover, our results suggest the need for coordinated management strategies among sites that share a common stock.