Microphis brachyurus (Bleeker, 1854)

4.1 | M. brachyurus View in CoL , a widespread species

M. brachyurus View in CoL haplotype network revealed two distinct mitochondrial haplogroups: one in the CPO ( French Polynesia) and the other in the WPO (Figure 2a,b). There are no haplotypes shared by individuals from these two areas, suggesting reduced connectivity among maternal lineages of the French Polynesia and the WPO, a hypothesis confirmed by the high and significant Φst value between these two areas (0.914), showing a deep genetic structuring. The isolation of the mitochondrial lineages of the French Polynesian populations was previously observed in other widespread amphidromous species in the Indo-Pacific, such as the fish species Sicyopterus lagocephalus (Pallas, 1770) ( Lord et al., 2012) View in CoL and Eleotris fusca (Forster, 1798) ( Mennesson et al., 2018) View in CoL or the prawn Macrobrachium lar (Fabricius, 1798) ( Castelin et al., 2013) View in CoL . The duration of the marine phase in teleosts has traditionally been used as a proxy of species’ dispersal abilities; a species with high dispersal capacities (i.e., long marine phase) tends to have a wider geographic range than those with low dispersal potential ( Kinlan & Gaines, 2003). For instance, Stenogobius genivittatus (Valenciennes, 1937) View in CoL widely distributed in the Indo-Pacific, Awaous guamensis (Valenciennes, 1937) View in CoL largely distributed over the Pacific Ocean, or S. lagocephalus (Pallas, 1770) View in CoL widely distributed in the Indo-Pacific exhibit marine phase durations of 135 ± 9.2 days ( Radtke et al., 1988), 161 ± 5.7 days ( Radtke et al., 1988), and 199 ± 33 days ( Hoareau et al., 2007), respectively. In the case of M. brachyurus View in CoL , this duration was estimated to be 38.6 ± 13.1 days ( Haÿ et al., 2023b) and may be considered relatively short in comparison to other amphidromous species in the Indo-Pacific region. However, the duration of this marine phase is not always an accurate proxy of their dispersal capacity, which can vary between species with the marine stage of similar duration. For instance, Sicyopterus sarasini (De Beaufort, 1915) View in CoL spends 76.9 ± 3.9 days at sea, but it is endemic to New Caledonia ( Lord et al., 2010). Along the same line, Sicyopterus japonicus (Tanaka, 1909) View in CoL is endemic to Taiwan and southern Japan despite a very long marine phase duration of 163.7 ± 12.8 days ( Shen & Tzeng, 2008). However, S. japonicus View in CoL is the only temperate species of Sicydiinae goby, its life cycle is also controlled by seasonality as opposed to tropical species. Therefore, abiotic factors can also explain this isolation of the Polynesian mitochondrial lineage from the rest of the Pacific. The different currents encountered by planktonic individuals during the marine dispersal phase may explain this particular distribution of populations ( Abdou et al., 2015). Indeed, the Southern Equatorial Current (SEC), the Southern Equatorial Countercurrent (SECC), and the Marquesas Countercurrent (MCC) disperse organisms in different ways; SEC tends to transport individuals sporadically from the Polynesian zone to other West Pacific islands, whereas SECC and MCC tend to limit this transport ( Gaither et al., 2010) (Figure 6). Seabed topography can also act as a barrier to dispersal between these two geographical areas ( Planes & Fauvelot, 2002). The presence of the Tonga and Kermadec trenches in northern New Zealand over 10,000 m deep could limit the movement of larvae. The combined action of these different factors, biotic (short marine phase, migratory behavior of individuals at sea) and abiotic (bathymetry, large geographic distance between EPO and WPO) would therefore have caused this lack of connectivity between populations of M. brachyurus View in CoL from the WPO and EPO ( French Polynesia), a trend previously observed among coral reef fishes ( Hubert et al., 2017).

This isolation of the mitochondrial lineages reflects a relatively recent event of colonization of French Polynesia, as the most recent common ancestor between WPO and CPO haplotypes is dated around 0.3 MYA (Figure 3). Indeed, there is only an average of 1% of genetic divergence between these two populations for the partial COI. This genetic divergence is below the 3% threshold defined by some authors ( Dettaï et al., 2011) to consider two different species based on the COI barcode fragment. Therefore, we consider M. brachyurus View in CoL as a unique species in the Pacific zone, with distinct populations in West and Central Pacific. However, if the geographic isolation of French Polynesia persists, these divergences may lead to geographic isolation and evolutionary divergence between these two sets of populations. Dawson (1979) considered M. brachyurus View in CoL as a species complex for which the global distribution could allow it to be divided into four subspecies: M. brachyurus brachyurus View in CoL distributed from Sumatra to the Society Islands; M. brachyurus millepunctatus View in CoL distributed from Sri Lanka to the coasts of East Africa; M. brachyurus aculeatus View in CoL distributed from the coasts of West Africa; and M. brachyurus lineatus View in CoL distributed from the Caribbean islands on the western coasts of South America from Panama to Brazil. Although no morphological differences were identified by Dawson between these different species/subspecies, deep genetic divergences (> 20%) were observed within this complex ( Stiller et al., 2022). A large-scale phylogeographic study would then allow to highlight phylogeographic patterns, infer

NPO

East Indonesia

West Indonesia

N

5

15

10

Fst Hd h π Fu's F Tajima's D

0.941 0.6 2 3 3.526 1.685

0.949 0.8 7 1143 0.216 0.301

0.951 0.69 14 0,836 0.116 0.627

Note: Significant values ( p -value <0.05) are indicated in bold.

Abbreviations: N, number of individuals sampled; NPO, North Pacific Ocean; Fst, intra-zone differentiation parameter; Hd, haplotype diversity; h, number of haplotypes; π, nucleotide divers; F and D, neutrality tests.

NPO East Indonesia West Indonesia

NPO

East Indonesia 0.906

West Indonesia 0.952 0.964 GoogleMaps

Note: Significant values ( p -value <0.05) are presented in bold. Abbreviation: NPO, North Pacific Ocean.

East Indonesia West Indonesia NPO

East Indonesia 0.4

West Indonesia 4.44 0.2

NPO 3.3 5.1 0.4

Note: Intra-zone divergences are presented in bold.

connectivity of these different groups on a global scale, and define more precisely their taxonomic status. We therefore corroborate Dawson's (1979) findings.