Muscle structure and differentiation in pelagic and demersal stages of the Antarctic teleost Notothenia neglecta

The distribution and fine structure of muscle-fibre types has been determined for the pelagic fingerling and demersal adult stages of the antarctic teleost Notothenia neglecta Nybelin, collected from Signy Island, Antarctica, between January and February 1984. In both stages, the pectoral fin adductor muscle (m.ad.p) is largely composed of slow fibres which contain abundant mitochondria (34 to 36%). During development, the ratio of capillaries to fibres increases less than does fibre diameter, so that capillary density is significantly lower in the m.ad.p of adults (498 mm^-2) than fingerlings (1 727 mm^-2). The secondary metamorphosis from a pelagic to a demersal mode of life is associated with the disappearance of subcutaneous lipid sacs and major changes in the distribution and structure of muscle fibres in the myotomes. The trunk cross-section of adult fish is almost entirely composed of poorly vascularised fast-muscle fibres (100 capillaries mm^-2), which contain densely packed myofibrils (86.3%), and have few mitochondria (1.4%). Slow-muscle fibres in adults are restricted to a thin wedge adjacent to the lateral line canal. In contrast, slow fibres occur around the entire circumference of the trunk in fingerlings representing 24% of the total cross-sectional area at the post-anal level. Volume densities (%) of mitochondria, intracellular lipid and myofibrils in this tissue are respectively 37.0, 7.9, 38.6 for fingerlings and 13.1, 0, 70.3 for adults. Slow-muscle fibres in adult fish are of unusually large diameter (50 to 120 m) and have relatively low capillary densities (266 mm^-2). These morphological changes reflect a general decrease in activity and a shift from a sub-carangiform to a labriform mode of swimming following transition from the fingerling to adult stage of the life cycle. The results are briefly discussed in relation to the physiology and ecology of antarctic fish.Please address all correspondence and requests for reprints to Dr. I.A. Johnston     

Use of the bomb radiocarbon chronometer for age validation in the blue grenadier Macruronus novaezelandiae

Accelerator mass-spectrometry was used to measure radiocarbon in the earliest formed portions of selected blue grenadier, Macruronus novaezelandiae, otoliths to provide a validation of fish-age estimates based on the quantification of opaque and translucent zones in otolith thin-sections. Δ¹⁴C data from blue grenadier otoliths were compared with previous estimates of Δ¹⁴C in seawater-dissolved inorganic carbon at similar latitutes, longitudes, and depths to link variation in otolith Δ¹⁴C to time. Minimum otolith Δ¹⁴C was −76.9 ± 7.7‰, indicative of pre-bomb radiocarbon levels below the surface mixed-layer at latitudes where juvenile blue grenadier are found. When plotted versus fish age estimated from otolith sections, the majority of the Δ¹⁴C data combined to define a curve reflecting the increase in bomb radiocarbon in temperate oceans of the Southern Hemisphere, indicating that age-estimation procedures based on otolith thin-sections are satisfactory for determining blue grenadier age. If otolith-section age estimates were correct, peak otolith Δ¹⁴C of 106.8 ± 7.9‰ occurred during the late 1960s, i.e. earlier than expected. This may be a manifestation of an increase in the mixed-layer depth associated with increased frequency of zonal westerly winds at this time. Received: 3 December 1996 / Accepted: 17 December 1996     

Influence of development and rearing temperature on the distribution,ultrastructure and myosin sub-unit composition of myotomal muscle-fibre types in the plaice Pleuronectes platessa

Eggs of the plaice Pleuronectes platessa L. were incubated at temperatures of 5, 8, 10, 12 and 15°C in March 1990, 1991 and 1992. The myotomes of yolk-sac larvae contain a single superficial layer of small-diameter muscle fibres which stain intensely for succinic dehydrogenase activity, surrounding 390 to 500 weakly staining inner-muscle fibres of larger diameter. Larvae reared at 15°C only survived for a few days and had significantly more inner-muscle fibres of larger average cross-sectional area than those hatching at 5 to 10°C. Myofibrils occupied 61% of the volume of inner-muscle fibres in 15°C larvae compared with 35 and 36% in larvae hatching at 5 and 10°C, respectively (P(0.01). Following metamorphosis, which occurs between 7 and 10 wk, the myotomes retain the single layer of superficial-muscle fibres characteristic of larvae. A thickening of the superficial-muscle layer is first evident in 4 to 5 mo-old laboratory-reared fish of 20 mm total length (TL) and in 0-group fish caught in June and July. On the basis of the histochemical staining reactions for myofibrillar ATPase and succinic dehydrogenase activities, the myotomes of 1-group (104 mm TL) and adult (280 mm TL) plaice were found to contain a minimum of six distinct muscle-fibre types. Two-dimensional gel electrophoresis and peptide mapping were used to investigate changes in myosin subunit composition during development. Myosin from the inner muscle of larvae contains two isoforms of the phosphorylatable light-chain 2 (LC2_L1 and LC2_L2). Following metamorphosis and during the first year, inner-muscle fibres co-express LC2 isoforms characteristic of the superficial fast-muscle fibres of adult plaice (LC2_F1 and LC2_F2) in addition to the larval isoforms. Fast-muscle fibres isolated from deep layers of the myotomes in adult plaice only contain LC2_F2. In contrast, myosin from larval muscle and adult fast muscle contain apparently identical alkali light chains (LC1 and LC3). Peptide maps of myosin heavy chains (MHCs) from 6 wk-old larvae and 10 wk-old fish that had completed metamorphosis are similar, but distinct from those of 1-group plaice. Further changes in white-muscle MHC composition are evident between 1-group fish of 104 mm TL and adults of 280 mm TL.     

The response of encrusting coralline algae to canopy loss: an independent test of predictions on an Antarctic coast

We assessed whether published observations of the ecology of encrusting coralline algae (Rhodophyta) from tropical and temperate coasts could be used to predict patterns and responses on a polar coast where such knowledge does not exist. On subtidal rocky coasts near Casey, East Antarctica, we detected a strong positive association of understorey encrusting coralline algae with canopies formed by the endemic alga Himantothallus grandifolius. The experimental removal of H. grandifolius caused corallines to bleach from red to pink/white concomitant with a decline in their photosynthetic activity. The magnitude of this decline (mean ± SE = 56.85±8.43%) was remarkably similar to that observed on temperate coasts (45.98±5.91%). Positive effects of nutrient enrichment of the surrounding water, hypothesized to alleviate the negative effects of canopy loss on encrusting corallines, were not detected. Removing H. grandifolius increased the intensity of photosynthetically active radiation and ultra-violet radiation reaching the substratum by three orders of magnitude, providing the basis for models invoking enhanced irradiance as the primary cause of the negative effects of canopy loss. Striking similarities among our results and those from tropical and temperate coasts suggest that responses of encrusting corallines to loss of canopies may have predictive properties across large distances and environmental gradients (tropical–temperate–polar).     

Resource acquisition in two intertidal fucoid seaweeds, Fucus serratus and Himanthalia elongata : seasonal variation and effects of reproductive development

Seasonal variations and the effect of reproductive development on resource acquisition by two intertidal fucoid species, the iteroparous Fucus serratus L. and the semelparous Himanthalia elongata (L.) S. F. Gray were examined. The oxygen-exchange characteristics of vegetative apical tissue of both non-fertile and fertile plants and receptacle tissue were compared at monthly intervals throughout reproductive development. Respiratory rates in non-fertile F. serratus varied seasonally between 1.5 and 8.0 μmol g⁻¹ fresh wt h⁻¹; in fertile plants the receptacle had a significantly lower respiratory rate than the vegetative tissue. The respiratory rate of the vegetative button of fertile H. elongata displayed less seasonal variation and was lower than that of the receptacle, which varied from a maximum of 9.5 μmol g⁻¹ fresh wt h⁻¹ at receptacle initiation in October to a minimum of 2.0 μmol g⁻¹ fresh wt h⁻¹ in February. The maximum photosynthetic rate (P _max) of non-fertile plants of both species did not vary in a distinct seasonal manner (∼60 μmol g⁻¹ fresh wt h⁻¹ for F. serratus and ∼12 μmol g⁻¹ fresh wt h⁻¹ for H. elongata). In fertile plants, the P _max of the receptacle tissue was (∼50% lower in F. serratus, and at its peak three times higher in H. elongata, than that of vegetative tissue. The stable carbon-isotope ratio (δ¹³C) did not differ between different tissue types in F. serratus, but values did vary seasonally, being less negative in the summer than in the winter (−13.5‰ compared to −18‰). The receptacle tissue of H. elongata also displayed a distinct seasonal variation in δ¹³C values (−12‰ in summer, −16‰ in winter), whilst the δ¹³C of the vegetative button did not vary seasonally. The rate of uptake of inorganic nitrogen by the vegetative thallus was lower in H. elongata than in F. serratus. The receptacle tissue of F. serratus had lower uptake rates than the vegetative tissue, whilst the uptake rate by H. elongata receptacle tissue was higher than that of the vegetative button. Received: 14 March 1997 / Accepted: 22 April 1997     

The Role of Mineral Weathering Rate Determinations in Generating Uncertainties in the Calculation of Critical Loads of Acidity and their Exceedance

Current legislation within Europe aimed at limitingecosystem damage resulting from inputs of atmosphericpollution is based on the critical load concept. Mineralweathering rates are central to the calculation ofcritical loads (acceptable levels) of acid deposition.The authors have undertaken a number of studies whichillustrate the complications and limitations inherent inpredicting mineral weathering rates and the implicationswhich these have for critical loads calculations andmapping. Calculated weathering rates and critical loadsfor two acid-sensitive parent materials (greywackes andgranites) are presented and are used to illustrate theimpact that uncertainty can have on critical loadexceedances. The results have obvious implications forportraying the uncertainties of critical loads to policy makers.     

Diversity and cover of a sessile animal assemblage does not predict its associated mobile fauna

Habitat-forming organisms often determine the structural properties and food resources available to a wide diversity of associated mobile species. Sessile invertebrate assemblages on marine hard substrates support an abundant fauna of mobile invertebrates whose associations with traits of their host assemblages are poorly known. To assess how changes to habitat-forming species are likely to affect their associated mobile fauna, the relationships between abundance, diversity and composition of mobile invertebrates and the diversity, cover and composition of the sessile assemblages they use as habitat were quantified in Sydney Harbour, Australia (33°50′S, 151°16′E). Similar compositions of sessile species were more likely to share a similar composition of mobile species, but univariate measures of the habitat (percent cover, species and functional diversity, prevalence of non-indigenous species) did not predict variation in associated mobile assemblages. These results demonstrate that in this habitat it is difficult to predict the diversity of marine assemblages based on common surrogate measures of biodiversity.