New insights into temperature-induced white muscle growth plasticity during Dicentrarchus labrax early life: a developmental and allometric study

Dicentrarchus labrax is a major finfish of interest in Mediterranean aquaculture. As the development of its hatchery production had gone with an increase of its larval rearing temperature, we studied the effect of a constant high (20°C) and two lower (13 and 15°C) temperatures on its early white muscle growth, with developmental and allometric approaches. D. labrax, sampled at hatching and at three developmental stages corresponding to main events in fish early life (first exogenous feeding, notochord flexion and completion of fin ray counts), were histologically processed in order to follow changes in the white myotomal muscle size, cellularity and localisation of proliferative nuclei. Morphometric analyses showed that, for 13, 15 and 20°C incubated/reared D. labrax, the total cross-sectional area of white muscle increased slowly in eleutheroembryos shorter than 5.2 mm, and four times faster in longer fish (free-swimming larvae). White muscle growth occurred both by hypertrophy (increase in muscle fibre size) and hyperplasia (increase in total number of muscle fibres) in all sampled fish, but both processes were higher in free-swimming larvae than in eleutheroembryos. The morphometric establishment of a higher hyperplastic growth of white muscle in free-swimming larvae was confirmed at immunological level by a high occurrence of proliferative nuclei. Early thermal environment was demonstrated to affect the growth process of white muscle on a different way in eleutheroembryos and free-swimming larvae: in the former, white muscle hypertrophy was stimulated at 20°C and hyperplasia at lower temperature, whereas in the latter, both white muscle hypertrophy and hyperplasia were more stimulated at 20°C than at lower temperatures. This was verified at once when the fish length and the white muscle total cross-sectional area were used as explicative variables.     

Effect of temperature on the egg and yolk-sac larval development of common pandora,<Emphasis Type="Italic"> Pagellus erythrinus</Emphasis>

Temperature is one of the most critical environmental factors for fish ontogeny, affecting the developmental rate, survival and phenotypic plasticity in both a species- and stage-specific way. In the present paper we studied the egg and yolk-sac larval development of Pagellus erythrinus under different water temperature conditions, 15°C, 18°C and 21°C for the egg stage and 16°C, 18°C and 21°C for the yolk-sac larval stage. The temperature-independent thermal sum of development was estimated as 555.6 degree-hours above the threshold temperature (the temperature below which development is arrested), i.e. 7°C for the egg and 12.1°C for the yolk-sac larval stage. Higher hatching and survival rates occurred at 18–21°C. At the end of the yolk-sac larval stage, body morphometry differed significantly (p<0.05) between the temperatures tested. The growth rate of the total length increased as temperature rose from 16°C to 18°C, while in the range of 18–21°C it stabilized and was independent of water temperature. The estimated Gompertz growth curve for the yolk-sac larvae of P. erythrinus was (r²=0.992) for the 16°C, (r²=0.991) for the 18°C and (r²=0.981) for the 21°C treatment. The efficiency of vitelline utilization during the yolk-sac larval stage was higher at 18°C.Communicated by O. Kinne, Oldendorf/Luhe     

Combined effects of dissolved oxygen concentration and water temperature on embryonic development and larval shell secretion in the marine snail <Emphasis Type="Italic">Chorus</Emphasis><Emphasis Type="Italic">giganteus</Emphasis> (Gastropoda: Muricidae)

The present study was undertaken to determine the effects of both extracapsular oxygen concentration and temperature on embryonic development in Chorus giganteus. In normoxia increasing water temperature from 12°C to 18°C reduced by 15 days the median time required for the capsules to hatch. Hypoxia (oxygen content at 50% of air saturation) generated a low development rate and totally prevented both shell secretion and larval hatching from the egg capsule. Experimental transfer at weekly intervals, from normoxia to hypoxia and vice versa, induced a decrease and increase in the embryonic ash content, respectively, but did not affect the number of hatched larvae. Such an effect was more pronounced at 12°C than at 15°C or 18°C. The embryonic inability to produce a shell under hypoxia is likely to be a result of the low intracapsular oxygen concentration (IPO₂) generated as the combined effect of a low extracapsular oxygen concentration (environmental) added to the intracapsular embryonic oxygen demands, which lowers the IPO₂ still further. Under such conditions, a decrease in intracapsular pH is likely to take place, and, if so, embryos might divert carbonates away from shell calcification to balance such changes in pH.     

Early life history of tropical Anguilla leptocephali in the western Pacific Ocean

In order to examine the early life-history characteristics of tropical eels, otolith microstructure and microchemistry were examined in leptocephali of Anguilla bicolor pacifica (27.6-54.1 mm TL, n=20) and A. marmorata (22.0-47.3 mm TL, n=8) collected during a cruise in the western Pacific. A. bicolor pacifica occurred between 10°N and 15°N in the west and between 5°S and 10°N farther to the east. A. marmorata also occurred in two different latitudinal ranges in the Northern (15-16°N) and Southern Hemispheres (3-15°S) of the western Pacific. The increment widths in the otoliths of these leptocephali increased between the hatch check (0 days) and about an age of 30 days in both species, and then gradually decreased toward the otolith edge. Otolith Sr:Ca ratios showed a gradual increase from the otolith center to the edge. The ages of A. bicolor pacifica and A. marmorata leptocephali ranged from 40 to 128 days and from 38 to 99 days, respectively. Growth rates of A. bicolor pacifica and A. marmorata leptocephali ranged from 0.33 to 0.71 mm day^-1 and from 0.45 to 0.63 mm day^-1, respectively. These leptocephali had estimated growth rates that were spread out throughout most of the reported range of growth rates of the leptocephali of the temperate species, the Japanese eel and the Atlantic eels. Differences in the spatial distribution in relation to current systems, and the age and size compositions of the leptocephali of A. bicolor pacifica and A. marmorata suggested different spawning locations for these two species.     

Osteological development of the vertebral column and of the fins in Diplodus sargus (Teleostei: Perciformes: Sparidae)

The osteological development of vertebral column and fins in white sea bream, Diplodus sargus (Linnaeus, 1758) was studied. Vertebral ontogeny started at 5.7 mm total length (TL, post-fixation), with the formation of the first cartilaginous neural and haemal arches, and was completed by the full attainment of dorsal ribs (20.8 mm TL). The formation of vertebral centra occurred between 7.3 and 9.4 mm TL, but was fully completed only after the development of post- and pre-zygapophyses (26.0 mm TL). Pectoral supports were the first fin elements that started to develop (3.8 mm TL), followed by those of the caudal fin (5.2 mm TL), dorsal and anal fins (6.7 mm TL), and then by those of the pelvic fin (9.5 mm TL). The caudal fin of D. sargus was the first to develop fin rays and attain the full count of lepidotrichia (5.6-7.8 mm TL), but the last to attain the full count of dermatotrichia (7.9-16.0 mm TL). The next fins starting to present rays were the pectoral (6.5 mm TL) and the dorsal and anal fins (7.5 mm TL), while the pelvic fins were the last (11.5 mm TL). Following the caudal lepidotrichia (7.8 mm TL), the anal (10.5 mm TL), dorsal (12.0 mm TL), pectoral (11.4 mm TL) and pelvic fins (12.5 mm TL) were the next with fully completed ray counts. Finally, fin meristics were fully developed with the caudal dermatotrichia. The results are discussed in comparison with the fin and vertebral ontogeny of other teleosts.     

Characterization of urease activity in three marine phytoplankton species,<Emphasis Type="Italic"> Aureococcus anophagefferens</Emphasis>,<Emphasis Type="Italic"> Prorocentrum minimum</Emphasis>, and<Emphasis Type="Italic"> Thalassiosira weissflogii</Emphasis>

The availability of different forms of nitrogen in coastal and estuarine waters may be important in determining the abundance and productivity of different phytoplankton species. Although urea has been shown to contribute as much as 50% of the nitrogen for phytoplankton nutrition, relatively little is known of the activity and expression of urease in phytoplankton. Using an in vitro enzyme assay, urease activities were examined in laboratory cultures of three species: Aureococcus anophagefferens Hargraves et Sieburth, Prorocentrum minimum (Pavillard) Schiller, and Thalassiosira weissflogii (Grunow) Fryxell et Hasle. Cultures of P. minimum and T. weissflogii were grown on three nitrogen sources (NO₃^m, NH₄⁺, and urea), while A. anophagefferens was grown only on NO₃^m and urea. Urease was found to be constitutive in all cultures, but activity varied with growth rate and assay temperature for the different cultures. For A. anophagefferens, urease activity varied positively with growth rate regardless of the N source, while for P. minimum, urease activity varied positively with growth rate only for cultures grown on urea and NH₄⁺. In contrast, for T. weissflogii, activity did not vary with growth rate for any of the N sources. For all species, urease activity increased with assay temperature, but with different apparent temperature optima. For A. anophagefferens, in vitro activity increased from near 0-30°C, and remained stable to 50°C, while for P. minimum, increased in vitro activity was noted from near 0-20°C, but constant activity was observed between 20°C and 50°C. For T. weissfloggii, while activity also increased from 0°C to 20°C, subsequent decreases were noted when temperature was elevated above 20°C. Urease activity had a half-saturation constant of 120-165 wg atom N l^у in all three species. On both an hourly and daily basis, urease activity in A. anophagefferens exceeded nitrogen demand for growth. In P. minimum, urease activity on an hourly basis matched the nitrogen demand, but was less than the demand on a daily basis. For T. weissflogii, urease activity was always less than the nitrogen demand. These patterns in urease activity in three different species demonstrate that while apparently constitutive, the regulation of activity was substantially different in the diatom. These differences in the physiological regulation of urease activity, as well as other enzymes, may play a role in their ecological success in different environments.     

Development of the skull in Dentex dentex (Osteichthyes: Sparidae)

The development of the skull skeleton in common dentex, Dentex dentex (Linnaeus, 1758), was studied under extensive rearing conditions. Skull ontogeny started after hatching and went through three stages: (a) the yolk-sac (during the phase of no growth) and the early larval stage (up to 6.9 mm in vivo total length, TL) in which the majority of the skull elements formed; (b) the late larval stage (up to 11.3 mm TL) in which the basisphenoid, infraorbital 1, parethmoids, nasals, ethmoids, pleurosphenoids, opisthotics and orbitosphenoids developed; and (c) the metamorphosis stage (up to 24.0 mm TL) during which the remaining infraorbitals formed, the cartilaginous areas of the neu- roskull regressed and the sclerotics started to ossify. Teeth first appeared at 5.2 mm TL, and gill filaments formed between 5.7 and 6.9 mm TL. The results are discussed in view of the ontogeny of functions and in comparison with the skull ontogeny of other teleosts. Received: 1 July 1998 / Accepted: 1 September 1999     

Morphological, reproductive, and genetic variability among three populations of Crucibulum quiriquinae (Gastropoda: Calyptraeidae) in northern Chile

Crucibulum quiriquinae (Lesson, 1830) is the only species of Crucibulum currently recognized in northern Chile. Recent analysis of three Crucibulum populations obtained in northern Chile demonstrates the existence of morphological, genetic, and reproductive differences among populations. Two populations present in Bahía Tongoy (30°15'S), one inhabiting the shells of the snail Turritella cingulata and the other shells of the pectinid Argopecten purpuratus, showed morphological differences. However, both had planktonic larval development and show low genetic divergence (D=0.002). A third population from Bahía La Herradura (29°58'S), which also inhabits the shells of T. cingulata, did not show morphological differences compared with its counterpart from Bahía Tongoy. However the Bahía La Herradura population had intracapsular development and metamorphosis, and a larger genetic distance (D=0.06) from both Tongoy populations. The results of the reproductive and genetic analyses strongly suggested that the two Tongoy populations, although showing morphological differences, are biologically the same species, Crucibulum quiriquinae, whereas the La Herradura population is a new species.     

Osteological development of Dentex dentex (Osteichthyes: Sparidae): dorsal, anal, paired fins and squamation

The osteological development of dorsal, anal and paired fins and the squamation in common dentex, Dentexdentex (Linnaeus, 1758), was studied under extensive larval rearing conditions. The ontogeny of the fins was determined in three stages: (1) the yolk-sac and the early larval stage (up to 6.7 mm in vivo total length, TL), in which the majority of the elements of the pectoral fins and the predorsals, dorsal and anal proximal pterygiophores formed; (2) the late larval stage (up to 11.6 mm TL), in which the majority of the pectoral lepidotrichia, the lower extrascapulae, all the elements of the pelvic fins, as well as the lepidotrichia, spines and the distal radials of the rest fins developed; and (3) the metamorphosis stage (up to 24.0 mm TL), when the upper extrascapulae formed and the pectoral lepidotrichia attained their full count. The ontogeny of squamation was carried out during the metamorphosis stage; it started at 13.4 mm TL with laterally symmetric points on the middle of each side of the body, followed by one ventral centre (15.6 mm TL) which was located anterior to the pelvic fins, and two bilateral pairs of centres which appeared on the head (17.8 and 21.2 mm TL). The results are discussed from a functional viewpoint and in comparison with the fin and scale ontogeny of other teleosts. Received: 1 March 2000 / Accepted: 20 September 2000     

Elevated temperature elicits greater effects than decreased pH on the development, feeding and metabolism of northern shrimp (Pandalus borealis) larvae

Climate models predict that the average temperature in the North Sea could increase 3–5 °C and surface-waters pH could decrease 0.3–0.5 pH units by the end of this century. Consequently, we investigated the combined effect of decreased pH (control pH 8.1; decreased pH 7.6) and temperature (control 6.7 °C; elevated 9.5 °C) on the hatching timing and success, and the zoeal development, survival, feeding, respiration and growth (up to stage IV zoea) of the northern shrimp, Pandalus borealis. At elevated temperature, embryos hatched 3 days earlier, but experienced 2–4 % reduced survival. Larvae developed 9 days faster until stage IV zoea under elevated temperature and exhibited an increase in metabolic rates (ca 20 %) and an increase in feeding rates (ca 15–20 %). Decreased pH increased the development time, but only at the low temperature. We conclude that warming will likely exert a greater effect on shrimp larval development than ocean acidification manifesting itself as accelerated developmental rates with greater maintenance costs and decreased recruitment in terms of number and size.     

Environmental controls on shell growth rates and δ<Superscript>18</Superscript>O of the shallow-marine bivalve mollusk <Emphasis Type="Italic">Phacosoma</Emphasis><Emphasis Type="Italic">japonicum</Emphasis> in Japan

Microgrowth patterns and the oxygen isotope composition of juvenile, shallow-marine bivalve mollusk shells of Phacosoma japonicum (Reeve) in Japan were analyzed and cross-calibrated with environmental parameters. Mark-and-recovery experiments indicate that a pair of two microgrowth lines and two microgrowth increments is produced every lunar day. This finding makes it possible to assign exact calendar dates to each portion of the shell. Average daily growth rates decrease by 61% from age two to three and 55% from age three to four. The length of the growing season and the growth rate are mainly controlled by temperature: shell growth ceases below 14.2°C (age two) and 16.8°C (age four) and is most rapid between 24.6°C and 27.2°C. Based on local temperature cycles, the growing season is longest in Seto Inland Sea, central Honshu (from May to November) and shortest at Hakodate Bay, North Japan (from June to October). The annual oxygen isotope profiles of the shells reflect the temperature cycle and the varying amounts of freshwater added to the seawater by precipitation. The most negative '¹⁸O values of -3.15‰ occur during the rainy season, i.e. during the monsoon and typhoon seasons. Growth rates are only slightly affected by salinity changes. Strongly reduced growth rates during the second half of the year at Seto Inland Sea and to a lesser extent at Tokyo Bay are explained by nutrient deprivation. Our study provides the basis for the use of P. japonicum in high-resolution ecological studies and environmental reconstructions.     

Being young in a changing world: how temperature and salinity changes interactively modify the performance of larval stages of the barnacle <Emphasis Type="Italic">Amphibalanus improvisus</Emphasis>

The fate of key species, such as the barnacle Amphibalanus improvisus, in the course of global change is of particular interest since any change in their abundance and/or performance may entail community-wide effects. In the fluctuating Western Baltic, species typically experience a broad range of environmental conditions, which may preselect them to better cope with climate change. In this study, we examined the sensitivity of two crucial ontogenetic phases (naupliar, cypris) of the barnacle toward a range of temperature (12, 20, and 28°C) and salinity (5, 15, and 30 psu) combinations. Under all salinity treatments, nauplii developed faster at intermediate and high temperatures. Cyprid metamorphosis success, in contrast, was interactively impacted by temperature and salinity. Survival of nauplii decreased with increasing salinity under all temperature treatments. Highest settlement rates occurred at the intermediate temperature and salinity combination, i.e., 20°C and 15 psu. Settlement success of “naive” cyprids, i.e., when nauplii were raised in the absence of stress (20°C/15 psu), was less impacted by stressful temperature/salinity combinations than that of cyprids with a stress history. Here, settlement success was highest at 30 psu particularly at low and high temperatures. Surprisingly, larval survival was not highest under the conditions typical for the Kiel Fjord at the season of peak settlement (20°C/15 psu). The proportion of nauplii that ultimately transformed to attached juveniles was, however, highest under these “home” conditions. Overall, only particularly stressful combinations of temperature and salinity substantially reduced larval performance and development. Given more time for adaptation, the relatively smooth climate shifts predicted will probably not dramatically affect this species.     

The influence of temperature on the development of Baltic Sea sprat (Sprattus sprattus) eggs and yolk sac larvae

In spring 2004 and 2005 we performed two sets of experiments with Baltic sprat (Sprattus sprattus balticus Schneider) eggs and larvae from the Bornholm Basin simulating ten different temperature scenarios. The goal of the present study was to analyse and parameterise temperature effects on the duration of developmental stages, on the timing of important ontogenetic transitions, growth during the yolk sac phase as well as on the survival success of eggs and early larval stages. Egg development and hatching showed exponential temperature dependence. No hatching was observed above 14.7°C and hatching success was significantly reduced below 3.4°C. Time to eye pigmentation, as a proxy for mouth gape opening, decreased with increasing temperatures from 17 days post hatch at 3.4°C to 7 days at 13°C whereas the larval yolk sac phase was shortened from 20 to 10 days at 3.8 and 10°C respectively. Maximum survival duration of non-fed larvae was 25 days at 6.8°C. Comparing the experimental results of Baltic sprat with existing information on sprat from the English Channel and North Sea differences were detected in egg development rate, thermal adaptation and in yolk sac depletion rate (YSDR). Sprat eggs from the English Channel showed significantly faster development and the potential to develop at temperatures higher than 14.7°C. North Sea sprat larvae were found to have a lower YSDR compared to larvae from the Baltic Sea. In light of the predictions for global warming, Baltic sprat stocks could experience improved conditions for egg development and survival.     

The effects of feeding or addition of dissolved inorganic nutrients in maintaining the symbiosis between dinoflagellates and a tropical marine cnidarian

The availability of solid food (Artemia nauplii) and dissolved inorganic nutrients (ammonium, nitrate, phosphate) to the shallow-water marine hydroid Myrionema amboinense was manipulated for 1-8 days in order to investigate their role in the growth of intracellular symbiotic dinoflagellates (zooxanthellae) of the genus Symbiodinium. Symbionts from hydroids collected from the field or maintained under laboratory conditions (25°C, 12 h:12 h light:dark cycle, 80 µE m^-2 s^-1 fluorescent lighting) always exhibited a single peak in mitotic index (MI) at dawn. Symbionts in freshly collected field animals had an MI peak of about 15%. Symbiotic dinoflagellates in hydroids fed Artemia nauplii twice daily in the laboratory maintained this dawn peak of MI between 10% and 15%, but in the absence of feeding or added inorganic nutrients, this peak declined to less than 1% within 2-4 days. In contrast, when hydroids were placed in solutions containing ammonium (20 µM NH₄Cl), nitrate (10 µM NaNO₃), and a combination of ammonium and phosphate (2 µM Na₂HPO₄) immediately after collection, the algal MI remained between 5% and 15% for 4-7 days; the addition of 2 µM phosphate did not increase MI relative to unfed rates. When unfed animals were placed in dissolved nitrogen or fed Artemia, the symbiont MI increased from <1% to 10-17% within 2-3 days; P alone had no effect. However, the increase resulting from added inorganic nutrients was temporary, lasting only 5-7 days. These observations suggest that algal division in the host is maintained indefinitely in the field or by feeding particulate foods twice daily in the laboratory, but the addition of inorganic nutrients alone (ammonium, nitrate and ammonium/phosphate) appeared to support the completion of a maximum of one additional round of cell division. Nutrients required for continued growth and division of symbiotic dinoflagellates are linked to host feeding and host growth; without external food, neither host nor symbiont continue to grow. The same phenomenon is seen in zooxanthellate anemones, clams and corals, where total numbers of symbionts appear to be linked to changes in host-tissue biomass (protein), achieving relatively stable densities in M. amboinense, corals and other cnidarian symbioses, depending on their local environmental conditions. The results of the present study help explain the cellular responses of algal symbionts in reef-dwelling invertebrates to additions of dissolved inorganic nutrients to coral-reef ecosystems.     

Burrowing behaviour and sediment reworking in the heart urchin Brissopsis lyrifera Forbes (Spatangoida)

Sediment reworking due to burrowing and feeding was studied in the spatangoid Brissopsis lyrifera, at two different temperatures (7°C and 13°C). Spine activity and burrowing behaviour were recorded with a real-time video camera. Reworked sediment volume was calculated from tracks produced by the heart urchin. Ingestion rates were measured by feeding the heart urchins with luminophores. Temperature had a significant effect on the bioturbation activity of B. lyrifera. At 13°C reworked sediment volume due to burrowing was 22 ml sediment h^-1 and at 7°C 14 ml sediment h^-1. The ingested amount of sediment was 0.08 and 0.02 g dry sediment h^-1 in 13 and 7°C, respectively. Reworked sediment volume due to burrowing was 60-150 times higher than the volume ingested. The large reworked volume is a consequence of B. lyrifera moving with a rocking motion through the sediment. The spines were continuously transporting sediment around the test with 5-min metachronal wave cycles.     

Oxygen and carbon stable isotopic profiles of the fan mussel, Pinna nobilis, and reconstruction of sea surface temperatures in the Mediterranean

Stable oxygen and carbon isotope profiles ('¹⁸O_skeletal and '¹³C_skeletal), taken along the direction of growth from the umbo to the shell margin in shells of the pinnid Pinna nobilis, were used to reconstruct sea surface temperatures (SST) in the south-east Mediterranean and ontogenetic records of metabolic CO₂ incorporation. Comparison of the seasonal cycle of SST, predicted from the '¹⁸O_skeletal record of a small (young) rapidly growing pinnid and temperature measured with a continuous in situ recorder showed that P. nobilis calcifies under isotopic equilibrium with surrounding seawater, thus indicating that P. nobilis shells can be used as a reliable predictor of SST. A 10-year SST record for the south-east Mediterranean was reconstructed from the shell profiles of four pinnid shells of different sizes and ages collected in 1995 and 1996. Reliable resolution of the seasonal SST could only be achieved during the first 4 years of shell growth. As the pinnids grew older, the temperature record was poorly resolved because the shell growth had diminished with age, resulting in time-averaging of the record. The amplitude of the generated seasonal temperature cycle compared favourably (DŽ°C) with a long-term temperature record from northern Mediterranean waters. Clear seasonal cycles in '¹³C_skeletal were observed with an amplitude of ~1.0‰, similar to the calculated seasonal changes in '¹³C of seawater (0.6‰) overlying seagrass meadows. An ontogenetic trend towards less positive '¹³C_skeletal values was too large to be attributed to any decrease in '¹³C in seawater resulting from the invasion of anthropogenic CO₂. It is suggested that the temporal changes of '¹³C_skeletal are due to incorporation of respiratory CO₂ into the extrapallial fluid and reflect changes in the metabolic activity of the pinnid rather than changes in the isotopic composition of dissolved inorganic carbon within the surrounding seawater.     

Temperature and developmental plasticity during embryogenesis in the Atlantic cod<Emphasis Type="Italic"> Gadus morhua</Emphasis> L.

Atlantic cod (Gadus morhua L.) embryos were reared at 4 °C, 7 °C, and 10 °C, and the relative timing of developmental events was characterized, with particular reference to myotomal muscle. Embryos started to feed at an apparently equivalent stage of development, so comparisons were made between temperature groups on the basis of percentage of time to first feeding and somite stage. No differences were found in the time of hatching or timing of appearance of the otic placode, unpaired median fin fold, gut lumen, otic vesicle, lens of the eye, otoliths, first muscular contractions, swim bladder, and hindgut, or in the rate of development of somites, myotubes, myofibrils, and acetylcholinesterase activity over the temperature range studied. In contrast, closure of the blastopore occurred late with respect to segmentation at higher temperatures, at the 3-somite, 10-somite, and 12-somite stages at 4 °C, 7 °C, and 10 °C respectively. Muscle cellularity was also markedly altered in the 10 °C group relative to the 4 °C and 7 °C groups. Larvae reared at 10 °C had significantly more (+14%) deep white fibers at hatch (P <0.001), whereas numbers of superficial red fibers remained unchanged. It is suggested that differences in muscle cellularity might be related to changes in the relative timing of epiboly, through differential proliferation of presomitic myogenic cells and/or their relative exposure to inductive signals.     

RNA-DNA ratio: an index of larval fish growth in the sea

Data on water temperature, RNA-DNA ratio, and growth of eight species of temperate marine fish larvae reared in the laboratory were fit to the equation: $$G_{pi} = 0.93{\text{ }}\operatorname{T} + 4.75{\text{ RNA - DNA}} - 18.18$$ where G_pi is the protein growth rate in % d^-1 and T is the water temperature. Water temperature and larval RNA-DNA ratio explained 92% of the variability in growth rate of laboratory-reared larvae. The model is useful over the entire range of feeding levels (starvation to excess), temperatures (2° to 20°C) and fish species studied. Estimates of recent growth of larval cod, haddock, and sand lance caught at sea based on water temperature and RNA-DNA ratio ranged from negative to 26% d^-1. These data demonstrate the importance of food availability in larval fish mortality and suggest that short-term growth under favorable conditions may be considerably higher than expected from long-term indicators. RNA-DNA ratio analysis offers new possibilities for understanding larval growth and mortality, and their relation to environmental variability.     

Ontogeny and allometric plasticity of Dentex dentex (Osteichthyes: Sparidae) in rearing conditions

The ontogeny of Dentex dentex (Linnaeus, 1758) was studied comparatively under extensive and semi-extensive rearing conditions. The exotrophic phase was divided into three distinct developmental stages (a) larval, from feeding onset up to 10 mm total length (TL); (b) metamorphosis, from 10 mm TL up to 24 mm TL; and (c) further juveniles. The majority of all ontogenic changes was expressed during the larval stage and integrated during the metamorphosis stage. The juvenile stage was characterized by developmental stability. The normal development of D. dentex was seriously affected by the rearing conditions applied during the early exotrophic phase. The two differently reared populations followed similar developmental patterns of general morphology, pigmentation and behaviour, but they presented high developmental plasticity where allometric growth was concerned. Rearing conditions influenced both the boundaries of the sequential stages (in 50% of the morphometric ratios) and the allometry coefficient of each morphometric character in the various developmental stages (in 80% of the characters). The results are discussed with respect to the ontogeny of the functional morphology under both ecological and aquaculture considerations. Received: 1 August 1998 / Accepted: 1 September 1999     

Biomass, photosynthesis and growth of Laminaria saccharina in a high-arctic fjord, NE Greenland

Biomass, photosynthesis and growth of the large, perennial brown alga Laminaria saccharina (L.) Lamour. were examined along a depth gradient in a high-arctic fjord, Young Sound, NE Greenland (74°18'N; 20°14'W), in order to evaluate how well the species is adapted to the extreme climatic conditions. The area is covered by up to 1.6-m-thick ice during 10 months of the year, and bottom water temperature is <0°C all year round. L. saccharina occurred from 2.5 m depth to a lower depth limit of about 20 m receiving 0.7% of surface irradiance. Specimen density and biomass were low, likely, because of heavy ice scouring in shallow water and intensive feeding activity from walruses in deeper areas. The largest specimens were >4 m long and older than 4 years. In contrast to temperate stands of L. saccharina, old leaf blades (2-3 years old) remained attached to the new blades. The old tissues maintained their photosynthetic capacity thereby contributing importantly to algal carbon balance. The photosynthetic characteristics of new tissues reflected a high capacity for adaptation to different light regimes. At low light under ice, or in deep water, the chlorophyll a content and photosynthetic efficiency (!) were high, while light compensation (E_c) and saturation (E_k) points were low. An E_c of 2.0 µmol photons m^-2 s^-1 under ice allowed photosynthesis to almost balance, and sometimes exceed, respiratory costs during the period with thick ice cover but high surface irradiance, from April through July. Rates of respiration were lower than usually found for macroalgae. Annual elongation rates of leaf blades (70-90 cm) were only slightly lower than for temperate L. saccharina, but specific growth rates (0.48-0.58 year^-1) were substantially lower, because the old blades remained attached. L. saccharina comprised between 5% and 10% of total macroalgal biomass in the area, and the annual contribution to primary production was only between 0.1 and 1.6 g C m^-2 year^-1.