Genetic analysis provides insights into species distribution and population structure in East Atlantic horse mackerel (Trachurus trachurus and T. capensis)

Abstract

Two sister species of horse mackerel (Trachurus trachurus and T. capensis) are described that are intensively harvested in East Atlantic waters. To address long‐standing uncertainties as to their respective geographical ranges, overlap and intraspecific population structure this study combined genetic (mitochondrial DNA and microsatellite) analysis and targeted sampling of the hitherto understudied West African coast. mtDNA revealed two reciprocally monophyletic clades corresponding to each species with interspecies nuclear differentiation supported by FST values. The T. trachurus clade was found across the north‐east Atlantic down to Ghana but was absent from Angolan and South African samples. The T. capensis clade was found only in South Africa, Angola and a single Ghanaian individual. This pattern suggests that both species may overlap in the waters around Ghana. The potential for cryptic hybridization and/or indiscriminate harvesting of both species in the region is discussed. For T. capensis mtDNA supports high gene flow across the Benguela upwelling system, which fits with the species' ecology. The data add to evidence of a lack of significant genetic structure throughout the range of T. trachurus though the assumption of demographic panmixia is cautioned against. For both species, resolution of stock recruitment heterogeneity relevant to fishery management, as well as potential hybridization, will require more powerful genomic analyses.

The genomic basis and environmental correlates of local adaptation in the Atlantic horse mackerel (Trachurus trachurus)

“…Our genomic analysis indicated low genetic differentiation among horse mackerel populations inhabiting the area from the North Sea to north Africa (Figure 1A), more than 4,000 km of coastline (global mean pool- F ST =0.007, range: 0.001-0.012). This result is in agreement with previous studies using few genetic markers (Cimmaruta et al, 2008; Comesaña et al, 2008; Healey et al, 2020; Kasapidis & Magoulas, 2008), and implies that gene flow is pervasive. We infer that in the horse mackerel, since the larval stage lasts about one month (Rusell, 1976), gene flow could predominantly occur between neighbouring areas through passive transport of pelagic eggs and larvae by ocean currents.…”

“…Our genomic analysis indicated that very few genetic differences exist between horse mackerel populations inhabiting the vast area from the North Sea to north Africa (Figure 1A), more than 4,000 km of coastline (global mean pool- F ST =0.007, range: 0.001-0.012). This result is in agreement with previous studies using a few genetic markers (Cimmaruta et al, 2008; Comesaña et al, 2008; Healey et al, 2020; Kasapidis & Magoulas, 2008), and implies that gene flow is pervasive. Low genetic differentiation can be explained by minimal barriers to gene flow and large population sizes, commonly observed in migratory marine fish.…”

“…This separation was already proposed in earlier studies using body morphometrics, otolith shape, and parasitofauna (Abaunza et al, 2008). However, this is the first time that genetic data supports it, as previous studies using a few markers were inconclusive (Cimmaruta et al, 2008; Comesaña et al, 2008; Healey et al, 2020; Kasapidis & Magoulas, 2008). The Mediterranean-Atlantic divide has also been described in several marine species but not in all (Patarnello, Volckaert, & Castilho, 2007), likely because of differences in species’ life history or low resolution of the genetic markers used.…”

“…The discreteness of the stocks, as well as the location and levels of mixing between them, is unknown, which leads to uncertainty in the input data for stock assessments. Previous genetic studies have indicated low genetic differentiation between populations, and have provided inconclusive results in regards to population sub-structuring beyond the three main stock divisions identified using traditional methods (Brunel et al, 2016;Cimmaruta, Bondanelli, Ruggi, & Nascetti, 2008;Comesaña, Martínez-Areal, & Sanjuan, 2008;Farrell & Carlsson, 2018;Healey et al, 2020;Kasapidis & Magoulas, 2008;Mariani, 2012;Sala Bozano et al, 2015).…”

Ecological adaptation in European eels is based on phenotypic plasticity

“…Previous studies on the European eel using low-density marker sets ( 6 , 11 – 15 ) or reduced representation sequencing ( 15 ) found little to no differentiation between geographic areas, consistent with a single panmictic population. However, low genetic differentiation at selectively neutral markers is a common observation in marine species with large geographical ranges and gene flow between subpopulations ( 16 , 17 ), but does not necessarily capture patterns of local adaptation. For example, an early study based on 13 allozyme loci failed to identify genetic differentiation among Atlantic and Baltic herring ( Clupea harengus ) from diverse ecological conditions, and a single panmictic population could not be excluded ( 18 ).…”