Effect of salinity on hypoxia tolerance of resting green crabs, Carcinus maenas, after feeding

Mechanisms that can influence the tolerance of hypoxia in brackish waters were studied in resting and fed crabs, Carcinus maenas, at 15?°C. Mortality, blood oxygenation, acid-base status and lactate concentration were analysed in fed crabs held in full-strength normoxic seawater (32.5‰?S) and then transferred for 24?h to a partial pressure of oxygen (P_o2) of 3?kPa (1.4?mg?l^?1) and various salinities (17, 12.5, 10, 8‰?S). At salinity levels >10‰, fed crabs tolerated P_o2 values as low as 3?kPa in the ambient water and 0.5?kPa in their arterial blood for 24?h without switching to anaerobic metabolism. Only below 10‰?S did their blood-lactate content rise, leading to their death despite the fact that their blood O₂-content was twice the control value measured in full-strength normoxic seawater and their blood P_o2 did not decrease below values recorded at higher salinity levels. Addition of CO₂ to 8‰?S water (CO₂ partial pressure increasing from 0.1 to 0.3?kPa) decreased blood-lactate production and mortality, suggesting that at 10‰?S impairment of the O₂ supply is limited by an excessive blood O₂-affinity. The results are discussed in terms of the distribution (?10‰?S) of C. maenas along salinity gradients in estuaries and bays.     

Predator-inducible changes in blue mussels from the predator-free Baltic Sea

Blue mussels, Mytilus spp., have inhabited the brackish Baltic Sea, an environment lacking predatory crabs and starfish, for several thousand years. In this paper we examined whether Baltic Mytilus that were transplanted to the North Sea showed predator-inducible plasticity like their "marine" conspecifics. Our experiments showed that native North Sea Mytilus changed their morphology when exposed to waterborne scents from shore-crabs and starfish. These predators induced different kinds of changes, with emphasis on shell thickness and adductor muscle size, respectively. Baltic Mytilus responded in a similar way to crab scents, whereas starfish scents had a relatively weak effect on the morphology. Crab and starfish scent induced strengthening of the byssal attachment in North Sea Mytilus, with crabs providing more stimulation than starfish. Baltic mussels also improved the byssal attachment when exposed to either of the predators, but the attachment strength, as well as the response to crabs, were relatively weaker than that of North Sea mussels. We conclude that inducible plasticity still is present in Baltic Mytilus, despite their recent evolution in a predator-free environment. There is probably no strong selective pressure against inducible plasticity, but it could also be maintained in the population by gene flow from Mytilus in the adjoining North Sea. The question whether Baltic Mytilus are M. edulis or M. trossulus may also be relevant for the present results.     

Are there two species of the polychaete genus Marenzelleria in Europe?

North Sea and Baltic Sea populations of the introduced polychaete Marenzelleria viridis (Verrill, 1873) reproduce at different times (spring and autumn, respectively). Enzyme separation by starch gel electrophoresis revealed major differences between specimens from the Baltic Sea and those from the North Sea (collected in 1992 and 1993) but a high degree of homogeneity among populations from the same sea. Three enzyme loci, glucose-6-phosphate isomerase (GPI-A, GPI-B) and malate dehydrogenase (MDH), were fixed to 100% by different alleles in the North and Baltic Sea populations, respectively. Different alleles are dominant for mitochondrial aspartate aminotransferase (mAAT) with allele frequencies of ca. 0.97 in all sampled populations from the North Sea and Baltic Sea, respectively, but heterozygotes were found in all populations. These genetic differences could be due to environmentally induced selection or genetically different origins of the populations, suggesting that populations of the genus Marenzelleria in the North and Baltic Seas may be two different species.     

Survival and chloride ion regulation of the porcelain crab Petrolisthes armatus exposed to mercury

Acute toxicity bioassays conducted at various salinities demonstrated that mercury (as mercuric chloride) at low concentrations was lethal to Petrolisthes armatus. Ninety-six hour LC₅₀ values varied from 50 to 64 parts per billion (ppb) of mercury, depending on test salinities. Lower salinities. decreased the time to death of mercuryexposed crabs. Differences in survival after 96 h due to salinity were not statistically significant. Blood chloride concentrations were regulated hyperchloride to the medium at low salinities and hypochloride at high salinities by acclimated crabs. The salinity isochloride to blood was 20 S. Transfer of crabs from 15 S to salinities ranging from 7 to 35 S resulted in new steadystate chloride levels within 12 h. Exposure to 50 ppb mercury did not alter chloride ion regulation of either acclimated crabs or crabs adjusting to new salinities.     

Experimentelle Untersuchungen zur Ökologie der marinen Kieselalge Thalassiosira rotula. II. Der Einfluß des Salzgehaltes

The effects of salinity (8 to 38‰) on the marine plankton diatom Thalassiosira rotula Meunier were investigated with respect to different temperature and light conditions. Two ecological aspects were examined separately: (1) the effect of various salinities, constant over a long period, was studied using well adapted cultures; (2) mixing processes were simulated by transferring cell chains from water of 33.9 or 20‰ S into lower salinities, as well as from 33.9 into 38‰ S. At 12° and 17°C, growth occurred at salinities from 12 to 38‰. In cold water (6°C), T. rotula did not grow below 16‰ S. Generation time was not influenced by salinities within the tolerance range if adapted cultures were grown under optimal illumination (1000 lux, light-dark cycle of 14:10 h; 12°C) or temperature (17°C; 2000 lux, light-dark cycle of 14:10 h). With increasing illumination or decreasing temperature, the other factors remaining constant, maximum growth rates were obtained with a salinity range of 20 to 32‰. Various salinities affected division rates most obviously if cultures were grown under continuous illumination; the optimum for cell division then ranged from 24 to 28‰ S only. Using adapted cultures again, the effect of different salinities on final cell yield is presumably more intense than their influence on generation time. T. rotula responded to sudden salinity changes only if transferred from water of 20‰ S into lower concentrations and from 33.9 into 38‰ S. Particularly at 12 and 38‰ S, growth rate and colony size (cells per chain) were reduced during the first few days. Increase of generation time and decrease of chain size occur coincidentally. The influence of different salinities on the occurrence of T. rotula in the sea is discussed in conjunction with experimental results on other ecological factors, and the present results are compared with the few available data obtained in nature by other authors. Accordingly, in the sea, a salinity range of 20 to 33‰ should prove optimal for T. rotula, 34 to 38‰ S would presumably represent still adequate conditions, while below 20‰ S T. rotula would probably rarely or never occur.     

Colonization of Europe by two American genetic types or species of the genus Marenzelleria (Polychaeta: Spionidae). An electrophoretic analysis of allozymes

Allozyme electrophoresis was conducted in an attempt to identify the origin ofMarenzelleria sp. found in the North Sea and Baltic Sea. The analysis covered eight enzymes with ten loti from nine populations found on the North American Atlantic toast, these populations in the North Sea and five populations in the Baltic. The North Sea spionids correspond to the Type IMarenzelleria from North American coastal waters between Barnstable Harbor (Massachusetts) and Cape Henlopen (Delaware). Nei's genetic distance between these North American populations and those from the North Sea wasD = 0.010 to 0.020. TheMarenzelleria sp. found in the Baltic Sea very probably stems from North American populations of Type II found from the region of Chesapeake Bay (Trippe Bay) south to the Currituck Sound (North Carolina). The genetic distance between these North American populations and the Baltic populations isD = 0.000 to 0.001. The invaders appear to have lost little of their genetic variation while colonizing the North and Baltic Seas. Probably, both colonizing events tan be attributed to large numbers of individuals reaching Europe simultaneously on one or more occasions. In addition, aMarenzelleria Type III was found by electrophoresis among specimens from Currituck Sound (North Carolina), rohere it is sympatric withMarenzelleria Type II. Salinity is discussed as an important factor for the establishment ofMarenzelleria Type I in the North Sea and Type II in the Baltic Sea.     

Influence of environmental salinity on oxygen consumption and ammonia excretion of the arctic under-ice amphipodOnisimus glacialis

Changes in salinity affect the metabolic rate of the sympagic amphipodOnisimus glacialis collected from the Barents Sea in 1986 and 1988. When transferred from 35 to 5 ppt S, oxygen consumption and ammonia excretion both increase three-fold during the first 5 h of exposure, and they remain high throughout the rest of the experimental period (26 h). During 24-h acclimation to various salinities (5 to 45 ppt), the amphipods exhibit a respiratory and excretory response to hyper- and hypoosmotic stress; however, a rather constant O:N atomic ratio (around 15) was obtained at the experimental salinities, indicating protein/lipids as metabolic substrate. Both rates of oxygen consumption and ammonia excretion increased with an increasing osmotic difference (0 to 650 mOsm) between the haemolymph and the environmental medium, indicating higher energy requirements for osmotic and ionic regulation at low salinities. In amphipods abruptly transferred from 35 to 5 ppt, a minor decrease of the haemolymph sodium concentrations together with an increased ammonia excretion output indicate a counter-ion regulation of NH ₄ ⁺ and Na⁺ during hyposmotic stress.     

Factors affecting buoyancy in the eggs of Bairdiella icistia (Pisces: Sciaenidae)

Several aspects of buoyancy were studied in the normally pelagic eggs of the sciaenid fish Bairdiella icistia (Jordan and Gilbert). Spawning fish acclimated to a low salinity (15‰) produced larger, more buoyant eggs which had a higher water content than eggs produced by fish living in ordinary sea water (33‰ S). Eggs fertilized in lower salinities were larger and more buoyant than eggs fertilized in higher salinities. The salinity of the medium during the first 5 to 7 min after fertilization had a lasting effect on egg buoyancy, but subsequent transfer to a different salinity also influenced buoyancy. Although egg buoyancy in this species can be influenced by the ambient salinity both before and after spawning, the capacity for adjusting buoyancy is limited, and eggs spawned in salinities lower than 30‰ would probably sink.     

Enzyme activities and allozyme polymorphism in two genetic types (or sibling species) of the genus Marenzelleria (Polychaeta: Spionidae) in Europe

Two populations of Marenzelleria spp. from the North Sea and the Baltic Sea that had already been characterized by allozyme electrophoresis and found genetically different in 1994 were again examined in 1997. The genetic identity (I = 1.0) examined in 1994 and 1997 shows that the structures of the populations of genetic Type I in the North Sea (Ringkobing Fjord) and genetic Type II in the Baltic Sea (Darss–Zingst Bodden chain) are stable. Three diagnostic gene loci that were determined additionally (citrate synthase, one locus; alanine aminotransferase, two loci) support the hypothesis of the existence of two sibling species of the genus Marenzelleria in Europe. In addition, the activities of ten electrophoretically separated enzyme systems were measured in the worms immediately after removal from their habitat and after they had been kept in the laboratory for 2 months. Under both conditions four enzymes showed significant differences between genetic types. The higher activity of the alanine aminotransferase of the North Sea worms (Type I) is discussed in the context of the different preferred salinities of the two Marenzelleria sibling species. Received: 21 January 1999 / Accepted: 13 July 1999     

Egg buoyancy of Baltic cod (Gadus morhua) and its implications for cod stock fluctuations in the Baltic

Successful spawning of Baltic cod (Gadus morhua L.) is restricted to the deep basins of the Baltic Sea and is dependent on abiotic conditions such as salinity and water oxygen content. Due to irregular inflows of saline water from the North Sea, the conditions for spawning fluctuate and consequently so does the cod stock. In May and June 1990 eggs were obtained from spawning cod caught off northern Gotland, Sweden. Our investigation revealed neutral egg buoyancy for Baltic cod at a salinity of 14.4±1.1 ppt, with a slight decrease some days before hatching. Today salinities of this magnitude occur only in the Bornholm basin, whereas in the Gdansk and Gotland basins the eggs sink toward the bottom and are exposed to lethal oxygen conditions.     

Conductive sodium entry in gill cells of the shore crab,Carcinus maenas

Isolated posterior gills of shore crabs, Carcinus maenas, collected from the Baltic Sea, were perfused and bathed with sea water and solutions of alkali chlorides. The preparation was used to measure fluxes of sodium from the external medium across the gills into the hemolymph and to determine transepithelial potential differences (PDs). Internally negative active transport PDs resulted from perfusion and bathing the gills symmetrically with the same medium (50% sea water). Passive (asymmetry) PDs following employment of 100% sea water as internal and 50% sea water as external medium were — in contrast to symmetry PDs — insensitive to cyanide and ouabain. This result indicates that the gill recognized the desired hyperosmotic state and responded by switching off the active transport component observed under symmetry conditions. Diffusional potential differences and fluxes of Na⁺ were inhibited by the externally applied diuretic amiloride. Gradients of pure alkali chlorides between medium and blood were accompanied by cation specific PDs. These PDs and their amiloride sensitivity were inversely related to the diameter of the unhydrated cation and allowed the calculation of the permeability sequence P: Li⁺>Na⁺>K⁺>Rb⁺>Cs⁺>Cl^-. The results obtained show that the permeability of the gills to cations greatly exceeds that to anions. In addition, these findings indicate that the initial amiloride-sensitive step in Na⁺ transport across the gill is not represented by an electroneutral sodium/proton exchange but by a conductive mechanism.     

Introgression and mitochondrial DNA heteroplasmy in the Baltic populations of mussels Mytilus trossulus and M. edulis

A strong clinal change in salinity occurs between the Baltic Sea and the North Sea, Atlantic Ocean, in the Danish Straits, where hybridization zone between mussels Mytilus edulis and M. trossulus has been reported. Eleven samples of mussels were studied from the Danish Straits and the inner Baltic Sea. Extensive introgression of M. edulis alleles from the North Sea into populations throughout the Baltic was ascertained for mitochondrial DNA (mtDNA) and two nuclear markers (ME15–16 and ITS). In the opposite direction, introgression of M. trossulus alleles into the M. edulis background was observed at the EFbis nuclear marker in populations from Kattegat (Danish Straits). While only M. edulis F (female) mtDNA was present in the Baltic, there were still strong differences in frequencies in the control region length variants between the Danish Straits and the inner Baltic samples, and weaker variation in coding region ND2–COIII haplotype frequencies. In the assays of the two mtDNA regions, various patterns of heteroplasmy were detected in 32% of all the studied individual mussels; this includes the presence of distinct, independently inherited M and F mitochondria in males, as well as the presence of two different distinguishable F genomes. The male-inherited M mtDNA genomes are quite common in the mussels from the Danish Straits, but very rare in males from the inner Baltic. Instead, a recombined control region variant (1r), which seems to have taken over the role of the M genome, was present in a number of specimens in the Baltic. Observations of heteroplasmy for two F genomes in some females and males confirm disruptions of the doubly uniparental inheritance mechanism in the hybrid Baltic Mytilus.     

Effects of salinity on spermatozoa motility,percentage of fertilized eggs and egg development of Baltic cod (Gadus morhua), and implications for cod stock fluctuations in the Baltic

Variation in cod stock strength in the Baltic Sea is considered to be dependent on abiotic conditions such as salinity and water oxygen content in the spawning areas (the Baltic deep basins). Spawning cod were caught off northern Gotland, Sweden, from April to June in 1989 and 1990. Our investigation revealed a reduction in spermatozoan motility and a subsequent reduction in the percentage of fertilized eggs in salinities of 10 to 12. Normal egg development required a minimum salinity of 11. This coincides with the osmolality of the seminal plasma and egg yolk; i.e., the Baltic cod is adapted to hyperosmotic conditions for spawning and is thus totally dependent on periodical inflows of saline water from the North Sea.     

Mytilus trossulus in Northern Europe

From data on allozyme, nuclear DNA and mitochondrial DNA markers, we show that the originally North Pacific/Northwest Atlantic mussel Mytilus trossulus is widespread on North European coasts, earliM. trossuluser thought to be inhabited only by Mytilus edulis. Several local occurrences of , interspersed with a dominant M. edulis, were recorded on the North Sea, Norwegian Sea and Barents Sea coasts of Norway and the Barents and White Sea coasts of Kola Peninsula in Russia. The proportion of M. trossulus genetic background observed at any one site varied from 0 to 95%. These new occurrences are not related to the previously known, introgressed M. trossulus population that occupies the Baltic Sea. The new northern occurrences retain both the F and M M. trossulus mitochondria, which have been lost from the Baltic stock. While hybridization takes place wherever M. trossulus and M. edulis meet, the extent of hybrization varies between the different contact areas. Hybrids are rare, and the hybrid zones are bimodal in the northern areas; more interbreeding has taken place further south in Norway, but even there genotypic disequilibria are higher than those in the steep transition zone between the Baltic mussel and M. edulis: there is no evidence of a collapse toward a hybrid swarm unlike in the Baltic. The Barents and White Sea M. trossulus are genetically slightly closer to the NW Atlantic than NE Pacific populations, while the Baltic mussel has unique features distinguishing it from the others. We postulate that the presence of M. trossulus in Northern Europe is a result of repeated independent inter- or transoceanic cryptic invasions of various ages, up to recent times.     

Chemical composition of Leptocythere psammophila (Crustacea: Ostracoda) as influenced by winter metabolism and summer supplies

Leptocythere psammophila is well known for the ecophenotypic variations of the ornamentation of its carapace. To test the respective influences of the environmental parameters on its ornamentation, 41 living specimens were collected in the Baltic Sea, the North Sea and in the English Channel during spring, summer and winter (April 1987, September and December 1988). The carbonate carapaces of these specimens were analyzed by means of electron microprobe. Thirteen elements were detected. Statistical comparisons of means and variances were performed using classical F and t-tests. Data were submitted to Normalized Principal Component Analysis and Discriminant Analysis. There are no significant differences between chemical composition of the carapaces from the different stations. However, the variability of Baltic Sea specimens is lower than that of other stations. Nonetheless, strong and significant differences appear when samples are gathered by season. The chemical composition of summer individuals is controlled by variations in water salinity and in fine grain terrigenous sediment supply. The composition of winter samples is related to the incorporation of Mg. In the case of spring specimens, both these factors have to be considered. Discriminant analysis between winter and non-winter samples sets correctly 84% of the individuals. The wrongly classified specimens are interpreted as ostracods that did not molt during the season of sampling. By attributing them to their possible season of molting, the result of the Normalized Principal Component Analysis improves. Differences in ornamentation are slight. They occur between the samples from the Baltic Sea and those from the other stations. The former have larger and less numerous punctations and lack the smooth surface in the anteroventral part of the carapace. The punctation diameter is more variable during summer than during spring. In the range of environments investigated, ornamentation of the carapace of L. psammophila seems unaffected by the seasonal environmental variations whereas its composition exhibits strong differences.     

Control of populations of the coral-feeding nudibranch Phestilla sibogae by fish and crustacean predators

Juvenile lesser blue crabs, Callinectes similis Williams, were exposed to a range of salinities for measurement of survival and bioenergetics. Effects of salinity on survival were determined by exposing juvenile crabs to salinity treatments ranging from 0 to 74‰. All crabs survived 21 d of exposure to 5 and 45‰S. The 21 d LC₅₀ values for salinity tolerance (calculated from survival data) were 2.6 and 60.8‰S at low and high salinities, respectively. Energy-budget components and scope for growth were determined for crabs exposed to 2.5, 10, 25, 35 and 50‰S. Energetic absorption rates were highest at 2.5 and 35‰S. Energetic expenditure rates (energy lost to respiration and excretion) were greatest at 2.5‰S, and decreased as salinity increased. Respiration constituted the majority of energetic expenditure at all salinities (92.3% average). Scope for growth was significantly affected by salinity and was highest in crabs exposed to 35‰S. Increased respiration at low salinity may indicate that C. similis incurs greater costs due to osmoregulation. The results of this study indicate that C. similis is capable of surviving and growing in waters with salinities as low as 10‰. Received: 10 January 1997 / Accepted: 11 February 1997     

Use of seaweeds for monitoring trace elements in coastal waters

Concentrations of a wide range of trace elements: arsenic, cadmium, cobalt, chromium, hafnium, nickel, thorium, uranium, zinc and the rare earth elements, cerium, europium, samarium, terbium and ytterbium were determined by instrumental neutron activation analysis in the brown alga,Fucus vesiculosus from Eckwarder Hörne, North Sea and from Rügen, Baltic Sea. Another brown alga,Sargassum filipendula from Sri Lanka, Indian ocean (representing an unpolluted control station) was similarly investigated. Cobalt, chromium and nickel concentrations were highest inF. vesiculosus from the North Sea while zinc was highest in samples from the Baltic Sea, reflecting high levels of these elements in coastal waters of the North and the Baltic sea. Cadmium, cobalt, nickel and zinc levels were lowest inS. filipendula from Sri Lanka, probably demonstrating lower levels of those elements in coastal waters. Concentration levels of hafnium, thorium, uranium, and the rare earth elements were highest inS. filipendula. Two years later in 1994,S. filipendula along withUlva sp. (green alga) was resampled from the same sampling site, and in addition to the above elements, six other trace elements (Ag, Ba, Br, Rb, Se and Sr) were determined.Sargassium filipendula showed a particular affinity for Ag, As, Br and Sr. For the other elements, marginal concentration differences were observed betweenS. filipendula andUlva sp., probably reflecting the regional background levels. Substantially higher concentrations of Hf, Th, U, and the rare earths were found again in the 1994Sargassum andUlva samples, reflecting the effect of a substrate rich in rare earth elements. The brown algae used in this study may be used to monitor trace elements in coastal waters.     

Resource degradation: a subtle effect of bottom fishing

Populations of hermit crabs are critically limited by the availability of suitable gastropod shells that they utilise to reduce their risk of predation and environmental stress. Common whelks are the main source of shells for large hermit crabs in the northern Atlantic but are vulnerable to direct and indirect effects of fishing activity. This study examined the potential consequences of degrading shell resources for common hermit crabs. Laboratory trials demonstrated that hermit crabs avoid low-quality damaged shells throughout their life history. This laboratory preference was corroborated by direct field observations of shells preferentially occupied by hermit crabs, compared with shells available for occupation. In the field, 8 times as many empty shells had holes compared to shells occupied by hermit crabs. In the North Sea, the abundance and biomass of live whelks and hermit crabs collected at sites where they co-occurred were significantly related. However, whelks occurred at far fewer sites overall and were more patchily distributed at high abundance than hermit crabs, which were more widespread. At a subset of sites, whelks of the same body-mass range occurred in the Irish and North Sea. However, at these sites, hermit crabs sampled from the North Sea had a significantly lower biomass. This suggests that the shells available for occupation at the North Sea sites would not support crabs of a body mass comparable to that found in the Irish Sea. Using published data, we calculated that in some of the intensively fished areas of the North Sea, 24% of the available shell resource will be damaged each year. The reduction in shell quality in the North Sea may impose a physical constraint on the upper size limit currently attainable by hermit crabs and hence may have implications for population viability.Communicated by J. P. Thorpe, Port Erin     

Na-K-ATPase generates an active transport potential in the gills of the hyperregulating shore crab Carcinus maenas

Perfusing and bathing isolated gills of shore crabs Carcinus maenas with artificial saline or sea (brackish) water enabled us to determine potential differences (PDs) between ambient bathing medium and perfusion solution. Establishment of diffusional PDs was avoided by employment of the same solution on the internal and external side. The PDs measured were therefore of an active nature. We compared the properties of the PDs with the well-known properties of the Na-K-ATPase: dependence on biological energy (ATP), on salinity and sodium concentration, susceptibility of PD to depletion of internal K and to the addition of 5 mM internal ouabain. Considering also the magnitude of the PDs measured, the results obtained indicate that it is the Na-K-ATPase that generates an active transport potential for Na in the gills of shore crabs. This PD represents the driving force for the active uptake of Na in crabs that hyperregulate their body fluids when they inhabit environments of reduced salinity regimes or fluctuating salinities in tidal estuaries. This process counteracts diffusional losses of Na in crabs exposed to dilute media.