Patterns of genetic structuring at the northern limits of the Australian smelt (Retropinna semoni) cryptic species complex

Abstract

Freshwater fishes often exhibit high genetic population structure due to the prevalence of dispersal barriers (e.g., waterfalls) whereas population structure in diadromous fishes tends to be weaker and driven by natal homing behaviour and/or isolation by distance. The Australian smelt (Retropinninae: Retropinna semoni) is a facultatively diadromous fish with a broad distribution spanning inland and coastal drainages of south-eastern Australia. Previous studies have demonstrated variability in population genetic structure and movement behaviour (potamodromy, facultative diadromy, estuarine residence) across the southern part of its geographic range. Some of this variability may be explained by the existence of multiple cryptic species. Here, we examined genetic structure of populations at the northern extent of the species’ distribution, using ten microsatellite loci and sequences of the mitochondrial cyt b gene. We tested the hypothesis that connectivity among rivers should be low due to a lack of dispersal via the marine environment, but high within rivers due to potamodromous behaviour. We investigated populations corresponding with two putative cryptic species, the South East Queensland (SEQ), and Central East Queensland (CEQ) lineages. In agreement with our hypothesis, highly significant overall FST values suggested that both groups exhibit very low dispersal among rivers (SEQ FST = 0.13; CEQ FST = 0.30). The two putative cryptic species, formed monophyletic clades in the mtDNA gene tree and among river phylogeographic structure was also evident within clades. Microsatellite data indicated that connectivity among sites within rivers was also limited, suggesting potamodromous behaviour does not homogenise populations at the within-river scale. Overall, northern groups in the smelt cryptic species exhibit higher among-river population structure and smaller geographic ranges than southern groups. These properties make northern Australian smelt populations potentially susceptible to future conservation threats, and we define eight genetically distinct management units to guide future conservation management.

Improved genetic markers for monitoring recruitment dynamics in the endangered Mary River cod (Maccullochella mariensis)

“…A range of other freshwater taxa are known to exhibit a parallel pattern of genetic divergence between Mary River and Tinana creek sub‐catchments. These include crustaceans (Bentley, Schmidt, & Hughes, ; Sharma & Hughes, ), fishes (Bishop, Hughes, & Schmidt, ; Hughes et al., ; Islam, Schmidt, Crook, & Hughes, ) and a turtle endemic to the Mary River (Schmidt, Espinoza, Connell, & Hughes, ). Together these findings highlight the significance of Tinana Creek as an important reservoir of intraspecific diversity in Mary River cod and numerous other freshwater taxa.…”