A Novel Resource Polymorphism in Fish, Driven by Differential Bottom Environments: An Example from an Ancient Lake in Japan

Divergent natural selection rooted in differential resource use can generate and maintain intraspecific eco-morphological divergence (i.e., resource polymorphism), ultimately leading to population splitting and speciation. Differing bottom environments create lake habitats with different benthos communities, which may cause selection in benthivorous fishes. Here, we document the nature of eco-morphological and genetic divergence among local populations of the Japanese gudgeon Sarcocheilichthys (Cyprinidae), which inhabits contrasting habitats in the littoral zones (rocky vs. pebbly habitats) in Lake Biwa, a representative ancient lake in East Asia. Eco-morphological analyses revealed that Sarcocheilichthys variegatus microoculus from rocky and pebbly zones differed in morphology and diet, and that populations from rocky environments had longer heads and deeper bodies, which are expected to be advantageous for capturing cryptic and/or attached prey in structurally complex, rocky habitats. Sarcocheilichthys biwaensis, a rock-dwelling specialist, exhibited similar morphologies to the sympatric congener, S. v. microoculus, except for body/fin coloration. Genetic analyses based on mitochondrial and nuclear microsatellite DNA data revealed no clear genetic differentiation among local populations within/between the gudgeon species. Although the morphogenetic factors that contribute to morphological divergence remain unclear, our results suggest that the gudgeon populations in Lake Biwa show a state of resource polymorphism associated with differences in the bottom environment. This is a novel example of resource polymorphism in fish within an Asian ancient lake, emphasizing the importance and generality of feeding adaptation as an evolutionary mechanism that generates morphological diversification

Distinctive insular forms of threespine stickleback (Gasterosteus aculeatus) from western Mediterranean islands

Neutral and adaptive variation among populations within a species is a major component of biological diversity and may be pronounced among insular populations due to geographical isolation and island specific evolutionary forces at work. Detecting and preserving potential evolutionary significant units below the species rank has become a crucial task for conservation biology. Combining genetic, phenotypic and ecological data, we investigated evolutionary patterns among the enigmatic threespine stickleback populations from western Mediterranean islands, all of which are threatened by habitat deterioration and climate change. We find indications that these populations derive from different genetic lineages, being genetically highly distinct from the stickleback of mainland Europe and the northern Atlantic as well as from each other. Mediterranean island stickleback populations are also phenotypically distinct from mainland populations but interestingly stickleback from Iceland have converged on a similar phenotype. This distinctive island stickleback phenotype seems to be driven by distinct selective regimes on islands versus continents. Overall, our results reveal the status of western Mediterranean island stickleback as evolutionarily distinct units, important for conservation of biodiversity

Diver-operated manual suction pump sampler: a reliable method for sampling benthos on rock substrates

Although numerous methods have been developed for SCUBA diver-operated sampling, few are suitable for sampling complex physical substrates. Sites such as groundwater springs sometimes have narrow sections along with uneven vertical and horizontal rock substrates at various depths. Here we developed a SCUBA diver-operated suction pump sampling system, designed for the sampling of benthic material on morphologically complex substrates. The sampler was designed to be simple to use in an already gear-heavy operation. The device is attached to the diver, making it a suitable tool for work on vertical substrates, and can be operated by 1 person. We compared the macrozoobenthic sampling yield and usability of the suction pump sampler to a Surber sampler that was modified for use while diving. Samples taken with the suction pump sampler had an overall greater macrozoobenthic density and species richness than samples taken with the modified Surber sampler. Sampling was more efficient with the suction pump sampler, as sampling containers were easily replaced underwater, whereas the modified Surber needed to be transported to the surface and its sample fixed before the next sample could be taken. Various modifications of the suction pump sampler are possible, depending on the objective of the sampling process, and it can be constructed with inexpensive and readily available materials. The suction pump sampler presented here further allows researchers in the field of marine and freshwater ecology to accurately sample the benthic habitat, including habitats where physical complexity may previously have prevented sampling

Rapid evolution of muscle fibre number in post-glacial populations of Arctic charr <em>Salvelinus alpinus</em>

Thingvallavatn, the largest and one of the oldest lakes in Iceland, contains four morphs of Arctic charr Salvelinus alpinus. Dwarf benthic (DB), large benthic (LB), planktivorous (PL) and piscivorous (PI) morphs can be distinguished and differ markedly in head morphology, colouration and maximum fork length (FLmax,), reflecting their different resource specialisations within the lake. The four morphs in Thingvallavatn are thought to have been isolated for approximately 10 000 years, since shortly after the end of the last Ice Age.We tested the null hypothesis that the pattern of muscle fibre recruitment was the same in all morphs, reflecting their recent diversification. The cross-sectional areas of fast and slow muscle fibres were measured at 0.7 FL in 46 DB morphs, 23 LB morphs, 24 PL morphs and 22 PI morphs, and the ages of the charr were estimated using sacculus otoliths. In fish larger than 10 g, the maximum fibre diameter scaled with body mass (M-b)(0.18) for both fibre types in all morphs. The number of myonuclei per cm fibre length increased with fibre diameter, but was similar between morphs. On average, at 60 pm diameter, there were 2264 nuclei cm(-1) in slow fibres and 1126 nuclei cm(-1) in fast fibres. The absence of fibres of diameter 4-10 mum was used to determine the FL at which muscle fibre recruitment stopped. Slow fibre number increased with body length in all morphs, scaling with b. In contrast, the recruitment of fast muscle fibres continued until a clearly identifiable FL, corresponding to 18-19 cm in the dwarf morph, 24-26 cm in the pelagic morph, 32-33 cm in the large benthic morph and 34-35 cm in the piscivorous morph. The maximum fast fibre number (FNmax) in the dwarf morph (6.97X 10(4)) was 56.5 % of-that found in the LB and PI morphs combined (1.23X10(5)) (P&lt;0.001). Muscle fibre recruitment continued until a threshold body size and occurred at a range of ages, starting at 4+ years in the DB morph and 7+ years in the LB and PI morphs. Our null hypothesis was therefore rejected for fast muscle and it was concluded that the dwarf condition was associated with a reduction in fibre number.We then investigated whether variations in development temperature associated with different spawning sites and periods were responsible for the observed differences in muscle cellularity between morphs. Embryos from the DB. LB and PL morphs were incubated at temperature regimes simulating cold subterranean spring-fed sites (2.2-3.2degreesC) and the general lakebed (4-7degreesC). Myogenic progenitor cells (MPCs) were identified using specific antibodies to Paired box protein 7 (Pax 7), Forkhead box protein K1-alpha (FoxK1-alpha), MyoD and Myf-5. The progeny showed no evidence of developmental plasticity in the numbers of either MPCs or muscle fibres. Juveniles and adult stages of the DB and LB morphs coexist and have a similar diet. We therefore conclude that the reduction in FNmax, in the dwarf morph probably has a genetic basis and that gene networks regulating myotube production are under high selection pressure. To explain these findings we propose that there is an optimal fibre size. and hence number, which varies with maximum body size and reflects a trade-off between diffusional constraints on fibre diameter and the energy costs of maintaining ionic gradients. The predictions of the optimal fibre size hypothesis and its consequences for the adaptive evolution of muscle architecture in fishes are briefly discussed.</p