Historical and contemporary population genetic connectivity of the European short‐snouted seahorse Hippocampus hippocampus and implications for management
Abstract:This first genetic study of Hippocampus hippocampus covers the species' entire geographic range and employs two mtDNA markers (control region and cytochrome b) to establish patterns of population structuring. A total of 255 specimens from 21 locations were used to obtain 89 concatenated haplotypes. The common haplotype was present in all but one population, however, most haplotypes were unique. The haplotype network had a star-like construction, suggesting expansion from a bottleneck event. F(ST) and AMOVA rev… Show more
“…The range expansion was a recent phenomenon and may not have obtained the migration-drift equilibrium, as shown by the lack of phylogeographical structure [38]. A similar star-like pattern of genetic relatedness among haplotypes was seen in other seahorses, such as H. hippocampus (L. 1758) [49].…”
Population genetic of seahorses is confidently influenced by their species-specific ecological requirements and life-history traits. In the present study, partial sequences of mitochondrial cytochrome b (cytb) and control region (CR) were obtained from 50 Hippocampus mohnikei and 92 H. trimaculatus from four zoogeographical zones. A total of 780 base pairs of cytb gene were sequenced to characterize mitochondrial DNA (mtDNA) diversity. The mtDNA marker revealed high haplotype diversity, low nucleotide diversity, and a lack of population structure across both populations of H. mohnikei and H. trimaculatus. A neighbour-joining (NJ) tree of cytb gene sequences showed that H. mohnikei haplotypes formed one cluster. A maximum likelihood (ML) tree of cytb gene sequences showed that H. trimaculatus belonged to one lineage. The star-like pattern median-joining network of cytb and CR markers indicated a previous demographic expansion of H. mohnikei and H. trimaculatus. The cytb and CR data sets exhibited a unimodal mismatch distribution, which may have resulted from population expansion. Mismatch analysis suggested that the expansion was initiated about 276,000 years ago for H. mohnikei and about 230,000 years ago for H. trimaculatus during the middle Pleistocene period. This study indicates a possible signature of genetic variation and population expansion in two seahorses under complex marine environments.
“…The range expansion was a recent phenomenon and may not have obtained the migration-drift equilibrium, as shown by the lack of phylogeographical structure [38]. A similar star-like pattern of genetic relatedness among haplotypes was seen in other seahorses, such as H. hippocampus (L. 1758) [49].…”
Population genetic of seahorses is confidently influenced by their species-specific ecological requirements and life-history traits. In the present study, partial sequences of mitochondrial cytochrome b (cytb) and control region (CR) were obtained from 50 Hippocampus mohnikei and 92 H. trimaculatus from four zoogeographical zones. A total of 780 base pairs of cytb gene were sequenced to characterize mitochondrial DNA (mtDNA) diversity. The mtDNA marker revealed high haplotype diversity, low nucleotide diversity, and a lack of population structure across both populations of H. mohnikei and H. trimaculatus. A neighbour-joining (NJ) tree of cytb gene sequences showed that H. mohnikei haplotypes formed one cluster. A maximum likelihood (ML) tree of cytb gene sequences showed that H. trimaculatus belonged to one lineage. The star-like pattern median-joining network of cytb and CR markers indicated a previous demographic expansion of H. mohnikei and H. trimaculatus. The cytb and CR data sets exhibited a unimodal mismatch distribution, which may have resulted from population expansion. Mismatch analysis suggested that the expansion was initiated about 276,000 years ago for H. mohnikei and about 230,000 years ago for H. trimaculatus during the middle Pleistocene period. This study indicates a possible signature of genetic variation and population expansion in two seahorses under complex marine environments.
“…These findings are consistent with Woodall et al . (), who concluded that these expansions were probably affected by interglacial cycles in the late Pleistocene (190 to 21 ka). In addition, the Cape Verde frontal zone, where the southward‐flowing Canary Current begins to shift westwards, has been observed as a barrier to gene flow in several eastern Atlantic species, and may have contributed to the observed divergence (Vangriesheim et al ., ; Woodall et al ., ).…”
Section: Discussionmentioning
confidence: 99%
“…In addition, our results also support those of Woodall et al . (), who concluded that the West African population of H. hippocampus is genetically distinct from the European population. Like H. patagonicus , the West African population of H. hippocampus should also be considered a priority for conservation.…”
Aim
To investigate how marine barriers shaped the demographic history of Atlantic seahorses (Syngnathidae: Hippocampus).
Location
Atlantic Ocean.
Methods
Range‐wide sampling (n = 390) at mitochondrial and up to five nuclear DNA loci was carried out across the Hippocampus erectus species complex (H. erectus from the Caribbean/North America, H. patagonicus from South America and H. hippocampus from Europe and West Africa). Multi‐species coalescent and approximate Bayesian computation (ABC) frameworks were used to estimate support of competing biogeographical hypotheses and demographic parameters, including lineage divergence times, effective population sizes and magnitudes of population size change.
Results
We identified four distinct lineages within the H. erectus complex. A posterior probability of 0.626 and corresponding Bayes factors ranging from 3.68 to 11.38 gave moderate to strong support for a basal divergence between South American populations of H. patagonicus and Caribbean/North American populations of H. erectus coincident with the inter‐regional freshwater outflow of the Amazon River Barrier (ARB). Estimates of historical effective population sizes and divergence times indicate that European and West African populations of H. hippocampus expanded after colonization from a more demographically stable Caribbean/North American H. erectus.
Main conclusions
Our findings of trans‐Atlantic colonization followed by isolation across a deep oceanic divide, and isolation across a freshwater barrier, may demonstrate a contrast in marine divide permeability for this group of rafters. Demographic inference supports the establishment of an ancestral population of the H. erectus complex in the Americas, followed by the ARB splitting it into Caribbean/North and South American lineages at a time of increased sedimentation and outflow. Our estimates suggest that following this split, colonization occurred across the Atlantic via the Gulf Stream currents with subsequent trans‐Atlantic isolation. These results illustrate that rafting can be a means of range expansion over large distances, but may be insufficient for sustaining genetic connectivity across major barriers, thereby resulting in lineage divergence.
“…This is concerning given that CITES documents annual exports of around 700 000 individuals (2004–2010; World Conservation Monitoring Centre, http://www.unep-wcmc.org), making this one of the most traded seahorse species. The short‐snouted seahorse Hippocampus hippocampus (L. 1758) also occurs in this region (Lourie et al , ; West, ), yet the only specimens attributed to West Africa (Senegal) in known analyses were obtained in Hong Kong markets (Woodall et al , ).…”
Section: Summary Statistics For Hippocampus Algiricus Hippocampus Himentioning
This study provides the first assessment of a heavily traded West African seahorse species, Hippocampus algiricus, and the first information on short-snouted seahorse Hippocampus hippocampus biology in Africa. A total of 219 seahorses were sampled from fisher catch in Senegal and The Gambia, with estimated height at reproductive activity for H. algiricus (161 mm) larger than mean ± S.D. catch height (150 ± 31 mm). Catch composition, height at reproductive activity and potential biases in fishery retention are discussed with regard to the current Convention on International Trade of Endangered Species (CITES) guidelines.
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