Chemical nature of feeding stimulants for the juvenile Dover sole, Solea solea (L.)

Juvenile Dover sole, Solea solea , were weaned from a live food, Artemia salina nauplii, on to a casein-based particulate diet flavoured with flesh of the mussel, Mytilus edulis . These fish would not accept an unflavoured casein-based diet but readily ate the diet flavoured with either mussel flesh or a mixture of pure chemicals whose composition was based on an analysis of the low-molecular weight fraction of mussel flesh. The rate of growth and survival over a seventy-seven day period was essentially the same on either of the flavoured diets. The active constituent in the mixture of pure chemicals was identified as glycine betaine for fish of wet weight exceeding 50g while glycine betaine with certain L-amino-acids was required for fish of about 2.5 g wet weight. These results are discussed in relation to the known feeding behaviour and food preferences of the wild Dover sole.     

An electron microscopic examination of the gastrointestinal epithelium in Dover sole, Solea solea (L.)

The gastrointestinal tracts of adult and juvenile Dover sole, Solea solea (L.), were examined using scanning (SEM) and transmission electron microscopy (TEM). SEM showed little differentiation of the internal morphology of the gastrointestinal tract in adult fish, with longitudinally arranged mucosal folds present in all gut regions. Mucosal folds had a similar arrangement to that in the goldfish. Goblet cells were identified in the mucosal epithelium in all regions of the gut while microscopic ducts/pores of possible pancreatic origin were observed in the foregut region. SEM of juvenile gut samples showed a similar arrangement of longitudinal mucosal folds to that found in the adult fish. There was no visible evidence of goblet cells or secretory ducts/pores in any region of the juvenile gastrointestinal tract.
In TEM it was observed that apical microvilli were a feature of epithelial cells from all regions of the gastrointestinal tract in adult and juvenile Dover sole. Cells from the distal regions of the adult and juvenile gut showed invaginations and vacuolation of the apical cytoplasm. A high degree of vacuolation in cells from the juvenile hindgut-rectum indicated the possible occurrence of intracellular digestion of absorbed nutrients in this gut region.     

Metabolism in marine flatfish. II. Protein digestion in Dover sole (Solea solea L.)

The digestive tracts of adult and juvenile Dover sole were examined for protease activities. A pepsin-like protease with an optimal pH value of 1.7 predominated in the stomach region, but the main endoprotease action in the foregut, midgut and hindgut regions was optimal in the range of pH 9.5-10.5 and showed good activity towards elastin orcein. Experiments using synthetic substrates suggested the presence of chymotrypsin- and trypsin-like activities optimal between pH 7 and 8. Collagenase activity was also shown to exist in this pH region. The presence of enzymes corresponding to carboxypeptidases a and b and leucine aminopeptidase was indicated. The possible significance of these results to the farming of Dover sole is discussed.     

Parasitic copepods of the common sole, Solea solea (L.), from the Eastern Mediterranean coast of Turkey

There is increasing interest in the common sole, Solea solea (Linnaeus), as an alternative fish species in aquaculture in the Mediterranean region, and parasitic copepods are a potential hazard for farmed finfish. This paper provides taxonomic information on two species of sea lice (family Caligidae) collected from S. solea in eastern Mediterranean waters off the Turkish coast. Caligus brevicaudatus A. Scott, 1901 and Caligus apodus (Brian, 1924) were both found and this is the first report of C. brevicaudatus in Turkish waters. The discovery of C. apodus on S. solea is a new host record. Key diagnostic characters of both species are reported, supported by light and scanning electron microscopy observations. During a 12-month survey a prevalence of 28% was recorded for C. brevicaudatus, whereas for C. apodus peak prevalence was much lower (3%).     

Metabolism in marine flatfish--III. Measurement of elastase activity in the digestive tract of Dover sole (Solea solea L.)

A critical assessment of different methods for measuring elastase activity in crude preparations has been made using whole intestinal homogenates of Dover sole. The use of the natural substrate elastin or its dyed derivatives gave optimal pH values in the alkaline region (pH 9.4-9.8) whereas artificial substrates showed optimal hydrolysis nearer neutrality in the region pH 8.1-8.2. Exoproteases may interfere with certain assay procedures. The properties of Dover sole elastase have been further investigated using chromatographic techniques which indicated that the main elastase activity has a molecular weight of approximately 19,500 and an isoelectric point in the region of pH 5.7.     

Bacterial microflora in the gastro-intestinal tract of Dover sole (Solea solea L.), with emphasis on the possible role of bacteria in the nutrition of the host

Abstract There was a progressive increase in the size of the aerobic heterotrophic bacterial populations along the gastro-intestinal tract of farmed Dover sole. Moreover, higher counts were recorded in juvenile than in adult animals. Thus, in juvenile fish, 5.2 × 10⁵, 8.0 × 10⁵ and 9.8 × 10⁶ aerobic heterotrophs/g were recovered from the stomach/foregut, midgut and hindgut/rectum, respectively. In adult fish, comparative samples revealed the presence of only 3.0 × 10⁴, 7.0 × 10⁴ and 2.3 × 10⁵ bacteria/g, respectively. There bacteria were equated with Acinetobacter, Alcaligenes , Enterobacteriaceae representatives, Flavobacterium, Micrococcus, Photobacterium, Staphylococcus and Vibrio . Of the compounds tested, many isolates, particularly those recovered from the hindgut/rectum, degraded p -nitrophenyl- β - N -acetylglucosaminide, chitin and collagen. Consequently, it is likely that such organisms may contribute to nutritional processes within Dover sole.     

A histopathological report of some epidermal anomalies of Dover sole, Solea solea L., and other flatfish species in coastal waters off south-east England

The gross pathological and histological changes of typical examples of epidermal anomalies of Dover sole, Solea solea L., and other flatfish from the sewage dumping areas of the Thames Estuary and two 'control' areas are described. Five per cent of flatfish showed anomalies including trawl damage abrasions, fin-rot, ulcers, nodules and tumours, lymphocystis and skeletal anomalies. Many of the anomalies suggested a traumatic aetiology and were often chronic. There was no significant difference in disease incidence between the areas sampled, except more evidence of lymphocystis in one of the control areas.     

The influence of sand on the estimation of resting metabolic rate of juvenile sole, Solea solea (L.)

The influence of a sandy substrate on the estimation of the resting metabolic rate of juvenile sole was determined using a static type of respirometer. Oxygen consumption of fish deprived of sand was almost double that of fish provided with sand.     

The intestinal microflora of farmed Dover sole (Solea solea) at different stages of fish development

The bacterial flora in the intestines of farmed Dover sole ( Solea solea L.) was investigated at different stages of fish development in relation to different diets fed under farming conditions. The flora of water and diets were also analyzed. The dominant generic group isolated from all water samples examined was Pseudomonas Gp IV/ Alcaligenes. The same generic group was dominant in homogenized pelagic larvae indicating a relationship between the microflora of those young fish and that of tank water. In older fish a steady increase in the percentage incidence of Vibrio / anaerogenic Aeromonas was observed in the intestines as fish grew and diets changed. The Vibrio /anaerogenic Aeromonas group was dominant in Lumbricillus rivalis used as an intermediate diet and the increased incidence of this group in fish intestines occurred after fish were fed this diet. The composite pellet used for ongrowing carried a high percentage of Gram positive micro-organisms and there was no relationship between the pellet microflora and that found in the fish intestines.     

Lipid‐based indicators of nutritional condition in juvenile sole Solea solea

Effects of the feeding level on juvenile Solea solea (Linnaeus, 1758) lipid‐related indicators of nutritional condition were assessed in a 25‐day trial experiment. Morphometric indices (K and HSI), liver lipid content and fatty acids (FA) clusters were good indicators of the feeding level, while individual FA showed highly dynamic variation patterns. Additionally, liver indices were more sensitive to feeding stress than were muscle measurements. HSI determination (and K) is fairly straightforward, with lower costs and simple procedures. Nevertheless, liver lipid content is also an important indicator of the feeding level and constitutes a direct measurement of food utilization and energy storage capacity. The individual variability and complex dynamics of FA with the feeding level are disadvantageous for use in FA analysis even for early‐warning signals of feed limitation. Overall, S. solea nutritional condition can be suitably assessed by morphometric indices and lipid content, especially considering liver measurements (HSI and liver lipid content), provided that the fish development stage is accounted for (i.e. gonad development and reproductive cycle).     

Effect of temperature and salinity on the gastric evacuation of juvenile sole Solea solea and Solea senegalensis

The juveniles of Senegal sole, Solea senegalensis, Kaup 1858, and common sole, Solea solea (Linnaeus 1758) concentrate in estuarine and coastal nurseries of widely differing temperatures and salinities. Yet, little is known about the effect of these physiologically important variables on the gastric evacuation rates of these species. Gastric evacuation experiments were performed on juveniles of S. senegalensis and S. solea. Three temperatures were tested, 26, 20 and 14°C at a salinity of 35‰. A low salinity experiment was also carried out at 15‰, at 26°C. Experimental conditions intended to reflect conditions in estuarine and coastal nurseries where juveniles of these species spend their first years of life. The relation between stomach contents and time was best described by exponential regression models for both species. An analysis of covariance (ancova ) was performed in order to test differences in evacuation rate due to temperature and salinity (slope of evacuation time against stomach contents) for each species. While increasing temperature increased evacuation rates in both species (although not at 26°C in S. solea), the effect of low salinity differed among species, leading to a decrease in gastric evacuation rate in that of S. senegalensis and an increase in S. solea. Differences in gastric evacuation rate between species were related to its metabolic optimums and to its distribution in the nursery area where fish were captured. Implications for the habitat use of estuarine and coastal nurseries are discussed.     

Coupling of heart rate and locomotor activity in sole, Solea solea (L.), and bass, Dicentrarchus labrax (L.), in their natural environment by using ultrasonic telemetry

Field measurements of distance moved and heart rate in sole, Solea solea (L.) and bass, Dicentrarchus labrax (L.) using ultrasonic telemetry revealed two different strategies. Heart rate in the sole increases during activity, accurately reflecting fluctuation in metabolic rate and so can be used as a measure of metabolic rate in the field. In contrast, the relatively stable value of the heart rate in bass during the whole tracking period whatever the activity level suggests that in this species heart rate in the field cannot be associated with metabolic rate determination.     

Cross-shelf dispersion of Dover sole (Solea solea) eggs and larvae in Biscay Bay and recruitment to inshore nurseries

We studied the cross-shelf dispersion of Dover sole (Solea solea)eggs and larvae from the offshore spawning grounds to the coastalnurseries of Biscay Bay in 1987. Eggs and larvae were retainedover the spawning grounds from late February to early April,consistent with the limited movement of satellite-tracked Lagrangiandrifters at different depths in the water column. The distributionof the larvae coincided with maximum zooplankton abundance.In mid April, both sole larvae and drifters moved northwardin response to wind forcing but the advection rate of the larvaewas about one-third that of the drifters. No significant onshoreadvection of the pelagic stages was observed. No evidence wasfound for a behavioural selection of tidal currents by the pelagiclarvae that could lead to onshore transport Unless that behaviourdeveloped after settlement to the bottom, cross-shelf diffusionof the pelagic stages would represent the main avenue of transportto the coastal nursery grounds. This dispersion strategy wouldimply that the vast majority of sole larvae fail to recruitto the bays and estuaries and arc lost to the population.     

Seasonal distribution and duration of the planktonic stage of Dover sole, Solea solea, larvae in the Bay of Biscay: an hypothesis

The possibility of deriving a prediction about the effect of seasonal variations in the duration of the planktonic larval phase on the dispersal of larval Dover sole was investigated. During six cruises, from February to May 1992, the distribution of sole larvae was studied along a 100-km transect, from the offshore spawning grounds to the coastal nurseries of the Bay of Biscay (France). Samples ( n = 189) were collected with a suprabenthic sampler, and vertical profiles of water temperature and salinity were recorded simultaneously. Counts of otolith increments of larval stage 4b (onset of metamorphosis) were used to estimate the duration of the planktonic life. Duration of the larval phase decreases by about 15 days (37%) with water temperature increase (between 8° in February and 11.2° C in May). Sole larvae occur from the coastal area to 100 km offshore. Within the same cruise, no difference in the duration of the planktonic life was observed between the larvae caught in the onshore and the offshore area. In spite of seasonal differences in abundance, the extent and the shape of the larval distributions during the period of study suggest that the seasonal variations in the duration of the planktonic life did not affect the larval distribution.     

Interaction between exopolysaccharide and oxygenation levels on habitat selection in the sole Solea solea (L.)

A habitat selection experiment was conducted to examine the behavioural response of sole Solea solea to a combination of sediment quality (exopolysaccharides, EPS-free, i.e. 0 mg l⁻¹ and EPS-rich, i.e. 4 mg l⁻¹) and water oxygenation level (100 and 35% air saturation). The distribution of sole was influenced differently by the type of substratum depending on the water oxygenation level. In normoxia, sole settled preferentially on sand whereas under hypoxic conditions, sole settled preferentially on the muddy substratum. In order to explain these apparently counter-intuitive observations, it is proposed that, via cutaneous respiration, young sole are able to take advantage of the large quantities of oxygen produced by microphytobenthic organisms present in the upper few millimetres of muddy substratum.     

The influence of prey size,sediment thickness and fish size on consumption in common sole (Solea solea L.)

This study determined prey consumption in common sole as a function of prey size (0–0.5, 1–1.5, 2–2.5 and 4–5 g), sediment thickness (20 cm and 2 cm) and fish size (50 g, 125 g or 300 g). Prey consumption (in numbers of prey eaten per fish per day) was reduced with increasing prey size and sediment thickness, and was increased with increasing fish size (p < .001 for all factors). All 3 factors showed significant two way interactions (p < .001) when expressed in numbers of prey eaten. Prey consumption decreased with prey size when prey could not escape by burying (2 cm of sediment thickness) irrespective of fish size. We suggest that increasing effort to ingest and handle larger prey played a role. Prey consumption increased with fish size when prey could not bury (2 cm of sediment thickness). However, when prey was able to bury (at 20 cm sediment thickness) prey consumption was similar irrespective of fish size (p < .001 for interaction fish size × sediment). This interaction suggests that with increasing fish size there is an increasing mismatch between foraging adaptation and prey burial depth. This may explain the dominance of crustaceans in the diet of adult common sole in nature, despite the high abundance of polychaetes.     

Differential Use of Nursery Areas Within the Tagus Estuary by Sympatric Soles, Solea solea and Solea senegalensis

The spatio-temporal pattern of estuarine use by both Solea solea and Solea senegalensis was investigated between April 1995 and November 1996 by beam trawl surveys. Spatial and seasonal distribution and abundance were different for each species. The highest densities of S. solea were recorded in deeper, warmer, low salinity areas and where the sediment consisted of a high proportion of fine sand and presented a high abundance of amphipods. The 0-group individuals of this species first occurred on the nursery ground in April. Numbers peaked in June and July, and from October to April abundance was low. S. senegalensis had a wider distribution and its abundance was related to food availability. The 0-group individuals of this species occurred in the nursery areas later than S. solea, from June to August.