Cottus gobio

Alexander, Timothy & Seehausen, Ole, 2021, Diversity, distribution and community composition of fish in perialpine lakes – “ Projet Lac ” synthesis report, Eawag: Swiss Federal Institute of Aquatic Science and Technology : 142-143

The sculpin,

Cottus gobio

is a taxon with wide and irregular distribution across Europe and considerable phenotypic and genetic variation between populations that may well comprise of several species. Cottus was caught in the benthic zone of many lakes across a wide range of depths from the littoral zone to the deepest point. Along the depth gradient, two peaks in abundance were evident in many lakes, in the shallow littoral and in the profundal, with a gap at intermediate depths. Cottus were common in the Rhine catchment, naturally absent only from the higher-altitude Jura lakes Brenet, Joux and Rousses, and were notably missing from Projet Lac catches in the lakes Zug, Morat and Constance Untersee. In the Po catchment, Cottus were caught in Poschiavo, Como, Mezzola, Garda and Maggiore, while in the Rhone catchment, this species was only caught in Geneva, Annecy and Chalain.

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Genetic differences among catchments, and between lake and stream populations in the Aare-Rhine

Analysis of Cottus gobio collected in Projet Lac and an earlier Eawag project “BioChange” identified that the populations of Cottus gobio in the Rhine (Aare, but also including Geneva), Rhone (Doubs, but not Geneva) and Po catchments showed substantial genetic differentiation ( Figure 55 ) [41]. The similarity of the C. gobio in Lake Geneva to populations in the Rhine catchment had already been shown in earlier work [184] and is attributed to fish crossing from the Upper Rhine to Rhone through ephemeral waterways formed at the retreating edge of the Rhone glacier in the early Holocene (c. 11,000 years ago).

In addition, lake populations of Cottus within the Aare-Rhine catchment were genetically distinct from stream populations, whereas lake populations were more similar to each other than to geographically intervening stream populations. Lake and stream Cottus in the Aare catchment thus seem to belong to two distinct evolutionary lineages, possibly representing two separate colonisations (with lakes Constance and Geneva belonging to the stream lineage).This suggested that recolonization of Switzerland after deglaciation occurred in two waves, with the stream lineage representing the first wave of colonization. The lake lineage only arrived in the Aare after lakes Constance and Geneva had become inaccessible for colonisation of fish from the Aare; respectively by the Rhine falls in Constance and the retreat of the Rhone glacier further into the Alps for Geneva [41].

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Phenotypic and genetic differences between littoral and profundal forms

Phenotypically distinct forms of Cottus were recorded in the profundal zone of lakes in the Aare and Po-Adriatic catchments to almost the deepest point of several lakes. In the Aare catchment, Cottus were caught to 209 m deep in Lake Thun (lake max depth Zmax = 217 m), to 214 m deep in Lake Lucerne (Zmax = 214 m) and to 145 m in Lake Walen (Zmax = 151 m). Also in the Po-Adriatic catchment, Cottus were caught to 125 m deep in Lake Maggiore (Zmax = 372 m) and to 290 m deep in Lake Garda (Zmax = 350 m).The deep-caught fish were paler in colour and tended to have flatter heads compared to the fish caught in the littoral zone of the same lakes ( Figure 53 ). In the Po-Adriatic lakes the profundal fish corresponded to Cottus ferrugineus (described by Heckel & Kner in 1858), currently considered a synonym of C. gobio ( Figure 54 ).

Parts of the genomes of profundal and littoral individuals from lakes Thun, Walen and Lucerne were analysed in detail. Significant genomic differentiation between littoral and profundal Cottus existed in Lake Walen, but not in lakes Lucerne or Thun. However, several genetic loci showed substantial genetic differentiation between the profundal and littoral populations, especially in Lake Thun, suggesting that the very small number of profundal fish that were available for analysis (e.g. n = 5 in Lake Thun) may have limited the ability to detect genomewide differentiation. Alternatively, it is possible that the phenotypic differences are the result of plasticity (i.e. the ability of one genotype to produce different phenotypes when exposed to different environments) and further research with a greater number profundal fish is required to properly understand the situation. Sufficient numbers of profundal fish from lakes Maggiore and Garda were not available to test for genomic differentiation. However, DNA barcoding revealed that three profundal fish from Maggiore and Garda belonged to a different mitochondrial lineage than the one barcoded littoral fish from Lake Maggiore and two stream fish from the Maggia (collected by Progetto Fiumi).

Figures

Figure 53: Littoral (left) and profundal (right) Cottus were phenotypically distinct in lakes Thun (shown in photos), Walen and Lucerne in the Rhine catchment and lakes Garda and Maggiore in the Po catchment.

Figure 54: Phenotypic differences between littoral and profundal Cottus were consistent with the distinction between Cottus gobio (top) and Cottus ferrugineus (bottom) in Seeley [185].

Figure 55: Phylogenetic relationships of Cottus from perialpine lakes (green labels) and streams/rivers (blue labels). RAxML tree with 100 bootstrap replicates. Grey dots on nodes indicate>50% bootstrap support. Adapted from Lucek et al. [41].