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Marine teleosts inhabiting the brackish Baltic Sea have adapted to the less saline water with activation of spermatozoa at low salinity hypo‐osmotic conditions but with shorter longevity and lower swimming speed that affect the fertilization capacity. Aiming to elucidate if the fertilization capacity may be maintained by increasing the number of spermatozoa produced, testis size for the euryhaline flounder Platichthys flesus with external fertilization was assessed along a salinity gradient; with spawning at a salinity of c. 7, 10–18 and 30–35. Fulton's condition factor K = 0.881 ± 0.085 (mean ± S.D.), 0.833 ± 0.096 and 0.851 ± 0.086, for fish spawning at salinities of c. 7, 10–18 and 30–35, respectively, with no difference between areas, i.e. analysed fish were in similar nutritional condition. A general linear model, with testes dry mass as the dependent variable and somatic mass as covariate resulted in a significant difference between areas–populations with larger testes for P. flesus spawning at a salinity of c. 7 but no difference between fish spawning at a salinity of 10–18 and 30–35. The result suggests that adaptation by increasing the number of spermatozoa produced may be a key mechanism for marine teleosts spawning in areas with low salinities to sustain the fertilization capacity as shown here for the euryhaline P. flesus. [ABSTRACT FROM AUTHOR]

This study examined the relationship of seascape structure, prey availability and sex on the post-spawning distribution and diet of European flounder Platichthys flesus in the northern Baltic Sea. The objectives were to determine whether: (1) wave exposure and substratum affect abundance and distribution of P. flesus, (2) diet reflects the benthic prey composition and (3) sex affects the distribution or diet of P. flesus. The results showed that P. flesus was evenly spread in the archipelago with no correlation to wave exposure. The distribution was, however, sex specific; reproductive males dominated the exposed zone and mainly post-reproductive females dominated the intermediate and sheltered zones. Platichthys flesus fed mainly on two bivalve prey species: blue mussels Mytilus edulis and Baltic tellins Macoma balthica. Hard substratum invertebrates dominated the diet in all habitats and apart from some typical soft substratum species, there was no clear link between fish feeding and the dominance structure of benthic prey. Diet was further sex specific, with females showing a broader range of diet than males. Results suggest that P. flesus is a specialist molluscivore found commonly and equally in soft- and hard-substratum habitats throughout the archipelago area. Previous studies on P. flesus in the Baltic Sea have yielded inconsistent results regarding diet and it has commonly been believed that the distribution of Baltic Sea P. flesus is linked to sand and soft substrata. The present findings emphasize the importance of including the entire range of habitats when diet and regional species distributions are assessed. [ABSTRACT FROM AUTHOR]

Selection of substrata representing habitats with or without filamentous vegetation by juvenile flounder Platichthys flesus was investigated in laboratory experiments and a field study to assess the effect of the increasing occurrence of ephemeral filamentous macroalgae in shallow nursery areas and its potential effect on flounder recruitment. In the laboratory experiments, 79% of juvenile flounder (24–99 mm total length, LT) preferred bare sand substrata in comparison to substrata with simulated filamentous vegetation. Substratum preference did not change with fish size. Field sampling using a drop trap (1 m2) in a vegetated (>50% coverage of ephemeral macroalgae) habitat in June, July and August showed small flounder (mean LT= 37·3 mm) were caught more often in samples with less vegetation. In contrast, larger flounder (mean LT= 57·2 and 79·7 mm in July and August, respectively) were more evenly distributed in relation to the distribution of vegetation. The results of this study indicate that the increase in ephemeral macroalgae observed in recent years potentially reduces the quantity and quality of suitable nursery habitats for juvenile flounder, which could have detrimental consequences to the overall recruitment of flounder in coastal areas. [ABSTRACT FROM AUTHOR]

The GENIPOL cDNA microarray, comprising some 14 000 elements representing 3336 unique expressed sequence tag clusters, was developed for studies on pollutant chemical effects on hepatic gene expression in the sentinel flatfish species Platichthys flesus, the European flounder. This array derived from probes obtained by targeted cloning of xenobiotic metabolising and responsive genes, subtractive suppressive hybridization of pollutant-treated fish and subtracted normalized cDNA libraries of treated fish has been successfully utilized for diagnosing effects of prototypical pollutants, endocrine disruptors and analysis of environmental effects. In the latter context, it has been recommended by the International Council for Exploration of the Sea that the array is tested in European monitoring programmes for assessment of the biological effects of contaminants. Sequence homology is sufficiently great between P. flesus and other pleuronectid flatfish to enable satisfactory use with other species, including American winter flounder and Japanese flounder, plaice, Atlantic halibut and sole. Elucidation of the effects of pollutants requires knowledge of other affectors of gene expression, and in this context the observed effects of a bacterial infection have been studied. The ‘infection’ was experimentally applied by challenge with a vaccine containing Aeromonas salmonicida, the bacterial agent causing furunculosis in flounder and salmonids. These results show that, despite its currently limited gene coverage, application of this array is not confined to toxicology. [ABSTRACT FROM AUTHOR]

Freezing of adult flounder Platichthys flesus and turbot Psetta maxima for 24 h resulted in mean decrease of total length ( LT) of 2·1 and 1·3% and a reduced body mass ( M) of 3·1 and 1·3%, respectively. Increasing the freezing time to 25 days increased the shrinkage in turbot but not in flounder. The LT and MT conversion equations for the different species and time spent frozen are given. The differences between species could be explained by the higher volume to surface area relationship in turbot compared with flounder. [ABSTRACT FROM AUTHOR]

The effects of cyclic and constant hydrostatic pressure on the norepinephrine and epinephrine content in discrete regions of the brain of European flounder Platichthys flesus were studied. After a 14 day exposure to cyclic hydrostatic pressure with a tidal period of 12·4 h and with a maximum peak of 800 kPa (range 200–800 kPa of absolute hydrostatic pressure), fish showed a highly significant decrease in norepinephrine content when compared to control animals held at constant atmospheric pressure. No changes were detected in fish submitted to a constant hydrostatic pressure of 800 kPa. [ABSTRACT FROM AUTHOR]

A study was carried out to investigate the diet and feeding strategies of age 0 year juvenile flounderPlatichthys flesusin two different micro-tidal habitats in the nutrient enriched Mariager Fjord on the Danish east coast. Juvenile flounder and benthic macrofauna were sampled monthly from June to October 1999 in a bare sand habitat and a habitat covered by filamentous and mat forming macroalgae. The presence of the‘opportunistic’ macroalgae created a shift in the dominance of surface dwelling prey such as epifaunal amphipods to more infaunal groups such as oligochaetes and polychaetes. The diet of the flounder varied considerably between the two habitats mainly reflecting prey availability relative to their abundance, prey spatial distribution, habitat structure and ontogenetic prey shifts as a function of total length. Flounder in the vegetated site fed on a diverse diet of copepods, polychaetes and oligochaetes, whereas those caught in the bare sand site fed primarily on the amphipodCorophium volutatorwhich was numerically dominant at this site. During the growth season, two diet shifts were observed: from copepods early in the season to macrofauna organisms and, later in the season, the inclusion of more hyperbenthic prey such asMysideaspp;Idoteaspp. and the common gobyPomatoschistus microps. [ABSTRACT FROM AUTHOR]