Are local extinctions and recolonizations continuing at the colder limits of marine fish distributions? Halobatrachus didactylus (Bloch & Schneider, 1801), a possible candidate

Abstract

In this paper, we compare the populations of the Lusitanian toadfish Halobatrachus didactylus along the Portuguese shore, using a putative fragment of the mitochondrial control region and the first intron of the S7 ribosomal protein gene. This demersal species ranges from the tropical West African coast to the Iberian Peninsula and the Mediterranean, having its northern limit in the Tagus River. For the putative control region, a single haplotype occurs in all fish from Tagus and Sado (the location immediately to the south) with a clear increase in diversity to the South, peaking at Algarve (south Portugal). The data seem to point to a very recent Holocenic colonization of Tagus and Sado from the South (possibly Algarve). We suggest that even small oscillations in sea surface temperature (SST) may cause local extinctions and subsequent recolonizations in populations of thermophilic fish that are at the cold limit of their distributions. IntroductionMany phylogeographic studies of marine organisms with larval dispersal have detected little structure along stretches of coastlines that span hundreds of kilometres or more (e.g. . This is assumed to result from the presence of planktonic eggs and/or larvae capable of dispersing over distances of kilometres or hundreds of kilometres per generation (however, see Ayre et al. 2009). Most studies assume that the Pleistocene and specially the Last Glacial Maximum (LGM) were critical times when populations were absent from the frozen north and suffered severe bottlenecks in their refugial habitats. In temperate areas like the west coast of Portugal and most of the south-west European shore, however, the conditions seem to have been more complicated. Although sea surface temperatures (SSTs) during the LGM along the coast of Portugal were not significantly different from what they are today (Abreu et al. 2003), several cold episodes towards the end of the Last glaciation considerably lowered mean summer SST (as much as 5°C).During the Holocene the oscillations in SSTs continued to be a salient feature of this geographic area, although their amplitudes were smaller. Some cold pulses such as the 8,400 years ago cold event and the Little Ice Age (LIA) caused temperature drops of one or a few degrees (Casper 2010) and may have affected the distributions of fish populations. A temperature anomaly need not to last very long to extirpate a population, it suffices that the population does not breed during a period that surpasses the lifespan of all its members. So, even short pulses that last for a few fish generations can cause a very substantial local effect. Even in present conditions small interannual oscillations can substantially affect the fish fauna, at least for the populations that are at or near the higher latitude limits of the Communicated by M. I. Taylor.

Development and characterization of novel microsatellite loci for Lusitanian toadfish,Halobatrachus didactylus

“…Only five of the 12 tested loci were considered polymorphic for this population (polymorphism rate of 41.7%), which is in agreement with the extremely low levels of genetic diversity obtained for this species by other authors (e.g., Marques et al, 2006 ; Robalo et al, 2013 ). The fact that our study was focused on a single population of the target species might have also contributed for the low number of polymorphic loci.…”

Genetic hypervariability of a Northeastern Atlantic venomous rockfish

“…This study is in line with previous research in the pursuit for patterns and processes involved in the phylogeography of the species from the North-East Atlantic (e.g., Bargelloni et al, 2005;Debes, Zachos & Hanel, 2008;Francisco et al, 2011;Robalo et al, 2013a). These previous studies used the same set of markers, allowing across species comparisons and multi-species approaches (e.g., Robalo et al, 2012;Robalo et al, 2013b;Francisco et al, 2014;Almada et al, 2017;Castilho et al, 2017) while revealing very distinct patterns.…”

Reserve effect of a small North‐East Atlantic marine protected area (Arrábida, Portugal) on soft‐sediment fish species

“…Either prevailing southward currents or colder waters could affect the populations of these species (Henriques, Gonçalves & Almada, 2007), which could explain the depletion of this group. It is noteworthy that H. didactylus , the species with the greatest contribution to the decreasing trend of the non‐commercial group, and also related to the abundance decrease registered in shallow bottoms, has its northern distribution limit in the study region and is known to have abundance fluctuations related with temperature (Robalo et al, 2013).…”