No abstract

SUMMARY The fish fauna of the continental shelf of the Southern Ocean is dominated by a single sub-order of Perciformes, the Notothenioidei, which have unusually large diameter skeletal muscle fibres. We tested the hypothesis that in fast myotomal muscle a high maximum fibre diameter (FDmax) was related to a reduction in the number of muscle fibres present at the end of the recruitment phase of growth. We also hypothesized that the maximum fibre number (FNmax) would be negatively related to body size,and that both body size and size-corrected FNmax would show phylogenetic signal (tendency for related species to resemble each other). Finally, we estimated ancestral values for body size and FNmax. A molecular phylogeny was constructed using 12S mitochondrial rRNA sequences. A total of 16 species were studied from the Beagle Channel, Tierra del Fuego (5-11°C), Shag Rocks, South Georgia(0.5-4°C), and Adelaide Island, Antarctic Peninsula (-1.5 to 0.5°C). The absence of muscle fibres of less than 10 μm diameter was used as the criterion for the cessation of fibre recruitment. FDmaxincreased linearly with standard length (SL), reaching 500-650 μm in most species. Maximum body size was a highly significant predictor of species variation in FNmax, and both body size and size-corrected FNmax showed highly significant phylogenetic signal (P<0.001). Estimates of trait values at nodes of the maximum likelihood phylogenetic tree were consistent with a progressive reduction in fibre number during part of the notothenioid radiation, perhaps serving to reduce basal energy requirements to compensate for the additional energetic costs of antifreeze production. For example, FNmax in Chaenocephalus aceratus (12 700±300, mean ± s.e.m., N=18) was only 7.7% of the value found in Eleginops maclovinus (164 000±4100, N=17), which reaches a similar maximum length (85 cm). Postembryonic muscle fibre recruitment in teleost fish normally involves stratified followed by mosaic hyperplasia. No evidence for this final phase of growth was found in two of the most derived families(Channichthyidae and Harpagiferidae). The divergence of the notothenioids in Antarctica after the formation of the Antarctic Polar Front and more recent dispersal north would explain the high maximum diameter and low fibre number in the derived sub-Antarctic notothenioids. These characteristics of notothenioids may well restrict their upper thermal tolerance, particularly for Champsocephalus esox and similar Channichthyids that lack respiratory pigments.

SUMMARY 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(Mb)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 μm 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 μm was used to determine the FL at which muscle fibre recruitment stopped. Slow fibre number increased with body length in all morphs, scaling with Mb0.45. 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 (6.97×104) was 56.5% of that found in the LB and PI morphs combined(1.23×105) (P<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 (2.2–3.2°C) and the general lakebed(4–7°C). Myogenic progenitor cells (MPCs) were identified using specific antibodies to Paired box protein 7 (Pax 7), Forkhead box protein K1-α (FoxK1-α), 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.

A 38.5 kDa alkaline protease from pyloric caeca of tambaqui (Colossoma macropomumj, a tropical freshwater fish, was partially purified in three steps: thermal treatment (45Cfor 30 min), salting‐out (ammonium sulfate at 40–80% of saturation) and gel filtration (Sephadex G‐75), The purification and yield were 51.2‐fold and 40%, respectively. The effects of pH, temperature, inhibitors, and substrates on proteolytic activities of partially purified enzyme were investigated. The optimum pH was 9.5, while the optimum temperature was 60C. This alkaline proteolytic activity remained unaltered after 30 min incubation at 55C. Active site inhibition provided additional evidence that this activity is attributed to a trypsin‐like enzyme.

The development of the axial muscles was investigated in Atlantic herring Clupea harengus from 2 spring-spawning (Clyde and Blackwater) and 1 autumn-spawning (Manx) populations in 2 successive years. The results indicate significant developmental differences between stocks. Myofibril synthesis and the development of acetylcholinesterase activity at the neuromuscular junctions occurred later with respect to embryonic stage at 5°C than at 8 and 12°C in Clyde herring, with much less pronounced differences between temperatures for the other populations. In a second series of experiments, Clyde herring were incubated at 5 and 12°C until shortly after hatching, and then reared at ambient temperature in triplicate tanks. The 5°C group had a faster growth rate than the 12°C group with respect to effective day-degrees from hatch. However, flexion of the notochord and the development of the dorsal and anal fin ray muscles occurred at shorter body lengths in the 12 than 5°C groups. The maximum escape velocity of larvae during fast-starts was determined using a high-speed video. Over the size range 11.2 to 18.5 mm total length, the adjusted mean maximum velocity was 24% higher in larvae hatched from 12 than 5°C eggs. Cruising swimming behaviour also differed between groups with the 12°C larvae showing reduced yaw relative to the 5°C larvae, indicating a more developmentally advanced sub-carangiform style of locomotion.