Effects of salinity on the growth and morphology of the invasive,euryhaline hydroid Cordylophora (Phylum Cnidaria,Class Hydrozoa)

The invasive, euryhaline hydroid Cordylophora sp. is a colonial cnidarian present in both freshwater and brackish water habitats. Individuals contend with osmotic stress at the tissue and cellular level. It has been suggested that this hydroid's ability to expand its range of distribution by invading new habitats is due in large part to an ability to acclimate to new salinities. The purpose of this study was to assess colony growth and morphological changes at various salinities in freshwater and brackish genotypes of Cordylophora sp. Single genotypes from a known freshwater clade (0.5 psu; Des Plaines River) and a known brackish clade (16 psu; Napa River) were cultured and gradually transitioned to 12 different salinities ranging 0.5–22 psu, and we characterized the growth rates and hydranth morphological features at each salinity. Colony growth was optimal at 0.5 psu for the freshwater genotype and 10 psu for the brackish genotype. Changes in hydranth morphology in the freshwater genotype were primarily observed at higher salinities, while morphological changes in the brackish genotype primarily occurred at lower salinities. Our results for the brackish genotype generally concur with previous work, but this study is the first to document the response of a freshwater genotype of Cordylophora sp. to various salinities. Differences in growth between these two genotypes strongly support the previously proposed existence of multiple cryptic species. Furthermore, because this hydroid is quite prevalent in freshwater and brackish systems as a fouling organism, understanding the effects of various salinities on the successful establishment of Cordylophora sp. is an important contribution to the understanding of the ecophysiology and management of this invasive hydroid.     

Effect of age,size and digestive tract development on weaning effectiveness in crucian carp,Carassius carassius (Linnaeus, 1758)

The study aim was to determine the optimum age, wet body weight (WBW) and total length (TL) of the crucian carp, Carassius carassius (L.), to ensure the effectiveness of weaning directly without a gradual transfer from live food to a compound feed. Moreover, the state of development of the digestive tract was analyzed histologically based on the height of enterocytes. Experimental rearing was conducted between days 5 and 45 post hatch (DPH). Initial WBW of fish was 2.2 ± 0.6 (n = 30) mg and TL 6.1 ± 0.1 (n = 30) mm. Rearing was carried out at 27 ± 0.5°C, with fish divided into six groups: one control (C) fed with Artemia sp. nauplii, and five groups initially fed with Artemia sp. but later replaced by a compound feed. Weaning with the compound diet started at 15, 20, 25, 30 and 35 DPH in groups labeled F15, F20, F25, F30, F35, respectively. Larvae were fed three times per day (08.00 h, 13.00 h, 18.00 h) in equal portions (4% of larvae biomass per day, converted to the dry matter of the feed). Daily biomass growth was adopted as 15%. Each group was triplicated (n = 50 individuals per replicate). Highest values of TL 42.1 ± 0.7 (n = 30) mm and WBW 905.3 ± 50.3 (n = 30) mg were recorded in the control group at 45 DPH; lowest survival rate of 45 DPH was in group F15 (90.7 ± 1.2%, n = 30). The highest value of the enterocyte epithelial length was observed in individuals within groups F30, 34.8 ± 1.2 μm (n = 30) and F35, 35.4 ± 3.6 μm (n = 30), respectively, 30 and 35 DPH; highest percentage of deformations on the final day of the experiment was in group F15 (100 ± 0.0%, n = 30). The results indicate that an effective direct transfer from live food to prepared diets (with no gradual transfer) cannot be performed with crucian carp larvae before 30 DPH at 27°C, when the fish have reached TL = 31.1 ± 0.4 mm (n = 30) and WBW = 436.9 ± 13.7 mg (n = 30).     

Larval salinity tolerance of the South American salt-marsh crab, Neohelice (Chasmagnathus) granulata: physiological constraints to estuarine retention, export and reimmigration

The semiterrestrial crab Neohelice (=Chasmagnathus) granulata (Dana 1851) is a predominant species in brackish salt marshes, mangroves and estuaries. Its larvae are exported towards coastal marine waters. In order to estimate the limits of salinity tolerance constraining larval retention in estuarine habitats, we exposed in laboratory experiments freshly hatched zoeae to six different salinities (5–32‰). At 5‰, the larvae survived for a maximum of 2 weeks, reaching only exceptionally the second zoeal stage, while 38% survived to the megalopa stage at 10‰. Shortest development and negligible mortality occurred at all higher salt concentrations. These observations show that the larvae of N. granulata can tolerate a retention in the mesohaline reaches of estuaries, with a lower limit of ca. 10–15‰. Maximum survival at 25‰ suggests that polyhaline conditions rather than an export to oceanic waters are optimal for successful larval development of this species. In another experiment, we tested the capability of the last zoeal stage (IV) for reimmigration from coastal marine into brackish waters. Stepwise reductions of salinity during this stage allowed for moulting to the megalopa at 4–10‰. Although survival was at these conditions reduced and development delayed, these results suggest that already the zoea-IV stage is able to initiate the reimmigration into estuaries. After further salinity reduction, megalopae survived in this experiment for up to >3 weeks in freshwater, without moulting to juvenile crabs. In a similar experiment starting from the megalopa stage, successful metamorphosis occurred at 4–10‰, and juvenile growth continued in freshwater. Although these juvenile crabs showed significantly enhanced mortality and smaller carapace width compared to a seawater control, our results show that the late larval and early juvenile stages of N. granulata are well adapted for successful recruitment in brackish and even limnetic habitats.     

Effects of salinity on the survival and aggression of the invasive Rio Grande cichlid (Herichthys cyanoguttatus)

The spread of non-native Rio Grande cichlids (Herichthys cyanoguttatus) in southeast Louisiana includes brackish habitats. We studied the effects of three different salinity levels on the biology of juvenile H. cyanoguttatus for 13 months to determine the potential of this species to spread through local estuaries. The highest salinity tolerated was 30.0 psu, and these fish did not survive acclimation to the 32 psu treatment. Fish in brackish conditions grew slower than fish in freshwater conditions, indicating a potential long-term detriment to juvenile fish living in brackish conditions. Aggression levels were notably higher for fish in brackish (15 psu) vs. freshwater conditions. This persisted through the entire experiment even after acclimation back to freshwater. This study indicates that higher salinity habitats in Louisiana can be tolerated by this species. It also raises a question about the effect of higher salinities on aggressive behavior.     

Growth,maturation and survival in a laboratory-reared Rhabdocoelid Turbellarian,Macrostomum orthostylum (BRAUN 1885)

Growth, maturation and survival of a free living turbellarian Macrostomum orthostylum (BRAUN), from a brackish water fish-farm, were studied in the laboratory under a constant temperature of 24 °C. The worms tolerated a wide range of salinity (1 to 30‰). Maximum growth (total length) of 1000 μm was attained in 56 days with a mean growth rate of 15.7 μm d^-1. Minimum maturation time (7 days) and highest longevity (112 days) were recorded in 9%. salinity. Survival period was considerably longer at lower salinities (1 to 10‰) and showed negative relationship with higher salinities (11 to 30‰).     

The Round Goby, Neogobius melanostomus, a Fish Invader on both sides of the Atlantic Ocean

During the past decade, a bottom-dwelling, aggressive, multiple-spawning fish, the round goby (Gobiidae: Neogobius melanostomus), has spread from its native region in the Ponto-Caspian throughout Europe and to the Laurentian Great Lakes in North America. An international workshop, held at the Hel Marine Station, Poland, was organized to summarize population features of the round goby. Common fish predators of round gobies in the Great Lakes and in native regions are obligate and facultative benthic fishes and occasionally, pelagic fishes. In contrast, the main predator of the round goby in the Gulf of Gdansk is the Great Cormorant (Phalacrocorax carbo). In the Great Lakes, round gobies have lead to the decline of mottled sculpin (Cottus bairdi) and logperch (Percina caprodes) and reduced the hatching success of native fishes by feeding on their eggs. In the Gulf of Gdansk, round gobies have increased in abundance, while three-spined sticklebacks (Gasterosteus aculeatus) have declined. Round gobies have a broad diet throughout their range; larger specimens are molluscivores. There are fewer species of parasites and lower infection rates of round gobies in recently colonized areas than in native areas. Overall, newly colonized round gobies in brackish waters and lakes are smaller, mature earlier, have a male biased operational sex ratio and are more short-lived compared with round gobies from marine (native) habitats.     

The growth of juvenile flounders (Platichthys flesus L.) at salinities of 0, 5, 15 and 35%%

Growth and food intake of juvenile flounders from the Kiel Bight were measured experimentally at different salinities (0, 5, 15 and 35 %). The growth at the middle levels (5 and 15 %) was faster then at 0 and 35 %. The reason for lower rates of increase in the freshwater group is the lower food intake. In the 35 % group the slower growth is caused by the worse food conversion. The variance of the individual growth rates in the 15 % salinity group remains far below the values of all other groups.     

Estimating salinity stress via hsp70 expression in the invasive round goby (Neogobius melanostomus): implications for further range expansion

Species invasions often occur on coasts and estuaries where abiotic conditions vary, e.g. salinity, temperature, runoff etc. Successful establishment and dispersal of non-indigenous species in many such systems are poorly understood, partially since the species tend to show genetic and ecological plasticity at population level towards many abiotic conditions, including salinity tolerance. Plasticity may be driven by shifting expression of heat shock proteins such as Hsp70, which is widely recognized as indicator of physical stress. In this study, we developed a qPCR assay for expression of the hsp70 gene in the invasive round goby (Neogobius melanostomus) and tested the expression response of fish collected from a brackish environment in the western Baltic Sea to three different salinities, 0, 10 and 30. hsp70 expression was highest in fresh water, indicating higher stress, and lower at brackish (ambient condition for the sampled population) and oceanic salinities, suggestive of low stress response to salinities above the population’s current distribution. The highest stress in fresh water was surprising since populations in fresh water exist, e.g. large European rivers and Laurentian Great Lakes. The results have implications to predictions for the species’ plasticity potential and possible range expansion of the species into other salinity regimes.

     

Effect of temperature and salinity on the energetics of juvenile gray snapper (Lutjanus griseus): implications for nursery habitat value

Gray snapper (Lutjanus griseus) encounter a wide range of temperatures and salinities in nearshore and estuarine juvenile habitats. The energetic response of juvenile gray snapper to temperature and salinity was measured in laboratory experiments to determine the influence of these physicochemical factors on the potential value of different juvenile nurseries. Maximum consumption and growth rates of juvenile (25-50 mm SL) gray snapper were determined in 12-day trials at 20 temperature/salinity combinations representing conditions in juvenile habitats. Ad libitum feeding level of individual fish was measured daily. Maximum weight specific feeding rate increased significantly with temperature and salinity; however, the effect of salinity was much less than that of temperature. Linear growth rate and specific growth rate both increased with temperature, and salinity did not have a significant effect on either. Gross growth efficiency (K₁, growth×consumption⁻¹*100) increased with temperature and was significantly lower at high salinities, indicating increased energetic costs. The higher K₁ at lower salinities has several implications for juvenile gray snapper in low salinity habitats: (1) they would need less food to achieve the same somatic growth as juveniles in high salinity habitats; (2) they would have higher growth at limited ration levels as compared to high salinity habitats; and (3) they would have less impact on prey populations than higher salinity habitats assuming similar gray snapper densities.     

A multilevel trait-based approach to the ecological performance of Microcystis aeruginosa complex from headwaters to the ocean

The Microcystis aeruginosa complex (MAC) clusters cosmopolitan and conspicuous harmful bloom-forming cyanobacteria able to produce cyanotoxins. It is hypothesized that low temperatures and brackish salinities are the main barriers to MAC proliferation. Here, patterns at multiple levels of organization irrespective of taxonomic identity (i.e. a trait-based approach) were analyzed. MAC responses from the intracellular (e.g. respiratory activity) to the ecosystem level (e.g. blooms) were evaluated in wide environmental gradients. Experimental results on buoyancy and respiratory activity in response to increased salinity (0–35) and a literature review of maximum growth rates under different temperatures and salinities were combined with field sampling from headwaters (800 km upstream) to the marine end of the Rio de la Plata estuary (Uruguay-South America). Salinity and temperature were the major variables affecting MAC responses. Experimentally, freshwater MAC cells remained active for 24 h in brackish waters (salinity = 15) while colonies increased their flotation velocity. At the population level, maximum growth rate decreased with salinity and presented a unimodal exponential response with temperature, showing an optimum at 27.5 °C and a rapid decrease thereafter. At the community and ecosystem levels, MAC occurred from fresh to marine waters (salinity 30) with a sustained relative increase of large mucilaginous colonies biovolume with respect to individual cells. Similarly, total biomass and, specific and morphological richness decreased with salinity while blooms were only detected in freshwater both at high (33 °C) and low (11 °C) temperatures. In brackish waters, large mucilaginous colonies presented advantages under osmotic restrictive conditions. These traits values have also been associated with higher toxicity potential. This suggest salinity or low temperatures would not represent effective barriers for the survival and transport of potentially toxic MAC under likely near future scenarios of increasing human impacts (i.e. eutrophication, dam construction and climate change).     

Changes in plasma osmolality and Na+/K+ ATPase activity of juvenile obscure puffer Takifugu obscurus following salinity challenge

The osmoregulatory capabilities of 6-month-old juvenile obscure puffer Takifugu obscurus, transferred directly from fresh water to different salinities (0‰, freshwater control; 10‰; 20‰ and 30‰), were studied over an 8-day period. After transfer, plasma osmolality of the fish at 30‰ was significantly higher than those at all other salinities throughout the experiment. The Na⁺/K⁺ ATPase activity in the gills of the fish treated with various salinities increased significantly, peaking at 48 h, then decreased gradually to the control level at 192 h. Similar fluctuation trends of the Na⁺/K⁺ ATPase activity were observed in the kidneys. Modified Gaussian model provided accurate fits for the time-course changes in the Na⁺/K⁺ ATPase activities after abrupt salinity challenge. The results demonstrated that obscure puffer has strong capacity to tolerate abrupt salinity changes and can osmoregulate well over a wide range of salinities even in juvenile stage.     

Rapid dispersal and establishment of a benthic Ponto-Caspian goby in Lake Erie: diel vertical migration of early juvenile round goby

The round goby, Apollonia melanostoma, a molluscivore specialist, was introduced to the Great Lakes in the early 1990s and rapidly expanded its distribution, especially in Lake Erie. Adult round goby morphology suggests low dispersal and migration potential due to the lack of a swim bladder and benthic life style. Given that the larval stage occurs inside the benthic egg, and juveniles have adult morphologies, it has been suspected that dispersal and invasion potential is low for early life stages also. However, we identified early juvenile round gobies in the nocturnal pelagic in Lake Erie and thus we conducted a sampling study to determine the extent to which this life stage uses the nocturnal pelagic. Replicate ichthyoplankton samples were collected at 3-h intervals (1900–0700 h) at three depths (2 m, 5 m, 8 m) in western Lake Erie (water depth = 10 m) in July and August 2002 and June 2006. Early juvenile round gobies (6–23 mm TL) were present almost exclusively in the nocturnal samples (2200 h, 0100 h, 0400 h) with peak densities approaching 60 individuals per 100 m³ of water sampled. Nocturnal density was also significantly greater at 8-m depth versus 2-m and only the smallest fish (6–8 mm TL) migrated to the surface (2-m). Analyses of diet clearly demonstrated that these fish are foraging on plankton at night and thus may not be light limited for foraging in ship ballast tanks. In ships that take on thousands of tonnes of water for ballast, nocturnal ballasting could easily result in transport of thousands of young round gobies at a time. Additionally, within-lake dispersal at this lifestage is likely common and may facilitate downstream passage across barriers designed to limit range expansion.     

Reproductive isolation in Chara aspera populations

Possible reproductive isolation between freshwater and brackish water populations of the dioecious charophyte Chara aspera was studied by means of cross-fertilization experiments and AFLP (Amplified Fragment Length Polymorphism). Three Swedish freshwater populations and three (German and Swedish) Baltic Sea populations of C. aspera were sampled. Cross-fertilization experiments were performed in a full combination setup of all populations and with two different salinities (0 and 10 PSU). Both freshwater and brackish water females formed about 70% more gametangia at 0 than at 10 PSU. Male individuals collected from freshwater had higher fertility than brackish water males at both salinities. 57% of all gametangia of females from freshwater developed into oospores compared to only 8% of gametangia of brackish water females. 42% of all oospores were fertilized in crosses between habitats (freshwater–brackish water) compared to 36% in crosses within habitats, the difference was not significant.Oospore and bulbil germination was investigated using propagules from freshwater and brackish water populations and incubation salinities of 0, 5, 10 and 20 PSU. None of the oospores collected from brackish water germinated. Germination of oospores and bulbils from freshwater was higher at 0 and 5 PSU than at higher salinities. Only around 40% of bulbils from brackish water germinated at 20 PSU compared to around 70% at the other three salinities. Germination of all bulbils was delayed at 20 PSU compared to other salinities.Genetic similarities (Jaccard indices of AFLP data) were higher within than between populations, but comparisons within habitat (freshwater–freshwater and brackish water–brackish water) were not different from comparisons between habitats.Our results did not identify any reproductive isolation between freshwater and brackish water populations, but indicate low gene flow between the two habitats. Oospore and bulbil germination success were highest at salinities corresponding to the conditions of their original habitat, suggesting genetic adaptation to their environmental conditions and indicating that propagules transported from freshwater to brackish water or vice versa will hardly develop into fertile plants. Additionally, brackish water plants perform poorer in all aspects of sexual reproduction than freshwater plants. Possibly, successful dispersal of oospores is not subjected to high selective pressure within the Baltic Sea where new sites easily can be colonized by means of vegetative reproduction. We assume that these adaptations will favour speciation within C. aspera and support the idea of the geologically young Baltic Sea as a “cradle of plant evolution”.     

Elongation and mat formation of <Emphasis Type="Italic">Chara aspera</Emphasis> under different light and salinity conditions

Former laboratory results indicate that shoot elongation at low light intensities of Chara aspera is absent already at 10 psu which is within the physiologically optimal salinity range for brackish water populations. To investigate if similar restrictions occur in the field, density and morphology of C. aspera were compared between three freshwater and three brackish water sites along its depth range. The lower depth limit of C. aspera varied considerably among sites (30–600 cm) related to turbidity. Light availability at the lower depth limit corresponded to about 15% of surface irradiance in freshwater and brackish water with lower salinity (3.4 psu). Total length increased and fresh weight:length ratio decreased with depth at these sites indicating shoot elongation related to lower light availability. Due to shoot elongation, light availability was far higher at the upper parts of the shoot than at the bottom in the turbid sites. Light availability at the lower depth limit was higher (about 40%) at two sites with higher salinity (7–8 psu), where no shoot elongation was observed at the lower depth limit. Instead, the plants were stunted and often covered with filamentous algae or shaded by other rooted submerged macrophytes indicating competitive disadvantages of C. aspera at higher salinities. As growth in high densities (mat formation) exposes the plants to severe self-shading, it is suggested that shoot elongation is a prerequisite to mat formation. Dense vegetation of C. aspera was found only in freshwater and brackish water with lower salinity. Single, richly branched plants occurred in clearwater sites with higher salinity. C. aspera was not found in “double stress” environments with both high turbidity and high salinity: We asume that the species is a poor competitor under these conditions. Our results indicate that morphological differences between freshwater and brackish water populations of C. aspera are at least partly explained by salinity rather than genetic differences.     

Toxic effects of mercury on the hard clam,Meretrix lusoria,in various salinities

1. The 96-hr lc₅₀ values for juvenile hard clams, Meretrix lusoria, were 328, 392 and 194 μg/l Hg in 10, 20 and 30 ppt salinities at 25 ± 1°C, respectively; for adult hard clams 341 and 140 μg/l Hg in 20 and 30 ppt salinities, respectively.2. Acclimatizing the adult clams to low salinity of 10 ppt lessened the toxicity of mercury. However, juvenile animals appeared to be more sensitive to mercury poisoning after 96 hr exposure in 10 ppt salinity.3. All embryos exposed to 40 μg/l Hg and above died within 30 hr. In the control, 44% of hatched embryos had developed into D-stage larvae, while those exposed to 20 μg/l Hg were still in the trochophore stage. Most of the retarded larvae developed into abnormal forms within 30 hr at 28°C in 15 ppt salinity.4. In order to maintain water quality and protect natural resources, the recommended safe level of mercury is 0.046 (0.039–0.053) μg/l Hg, based on the estimated 30-hr EC₅₀ for the clam embryos, with an application factor of 0.01.     

Effect of salinity on oxygen consumption in fishes: a review

The effect of salinity on resting oxygen uptake was measured in the perch Perca fluviatilis and available information on oxygen uptake in teleost species at a variety of salinities was reviewed. Trans‐epithelial ion transport against a concentration gradient requires energy and exposure to salinities osmotically different from the body fluids therefore imposes an energetic demand that is expected to be lowest in brackish water compared to fresh and sea water. Across species, there is no clear trend between oxygen uptake and salinity, and estimates of cost of osmotic and ionic regulation vary from a few per cent to >30% of standard metabolism.     

Determining the optimum temperature and salinity for larval culture,and describing a culture protocol for the conservation aquaculture for European smelt Osmerus eperlanus (L.)

Populations of anadromous European smelt Osmerus eperlanus (L.) are declining across its geographical range in northern Europe, but no practical culture techniques exist to develop stock enhancement programmes for this species. In this study, a culture protocol is described to rear fish from fertilised eggs to mature adults in 2 years involving the use of ‘green water’, live feed and artificial diets. The sequence of embryonic development for eggs incubated at 10°C/0 ppt was described and photographed. To determine the optimum conditions for larval culture, fertilised eggs were reared at a range of salinities (0–20 ppt) and temperatures (5–18°C) until first feeding. Best hatching success (ca. 97%), size at hatch (ca. 0.8 mm) and survival to first feeding (ca. 96%) of larvae were achieved under combined conditions of low salinity (0–0 ppt) and temperature (5–10°C). No larvae survived a salinity of 20 ppt. The time taken from fertilisation to hatch (FtH) and hatching duration (HD) were temperature-dependent ranging from 42 days FtH and 10 days HD at 5ºC, to 10 days FtH and 2 days HD at 18°C irrespective of salinity. The results indicate that conservation programmes could utilise existing salmonid hatchery facilities (i.e. freshwater, ≤10°C water temperature) for stock enhancement. Since on-growing of smelt involves the logistical and technical problems of live feed production, it is recommended that smelt enhancement programme utilise freshwater hatchery facilities to rear fish until hatching, and then stock out onto known spawning grounds in rivers allowing hatched larvae to drift into estuaries to complete the larval and juvenile phases. This approach would minimise the time spent in the hatchery post-hatching, eliminate the need for live food production, prevent the development of predator-naïve fish, and hence would mimic the natural life cycle of the species as closely as possible.     

Influences of Salinity on Growth and Survival of Juvenile Pinfish <Emphasis Type="Italic">Lagodon rhomboides</Emphasis> (Linnaeus)

We investigated the effect of salinity on growth, survival, and condition of pinfish Lagodon rhomboides juveniles (36–80 mm standard length) in two laboratory experiments in July 2003 and June/July 2004. Our results show that juvenile pinfish grown in laboratory conditions under a range of salinities experience rapid growth and high survival in typical estuarine-like salinities (15–30 ppt). We also found that relative weight as an index of condition corroborates the idea that pinfish are well adapted to survive and grow in a wide range of salinities. Such rapid growth and high survival in a dynamic environment may afford juvenile pinfish potential ecological advantages over other estuarine-dependent fish species that are relatively more constrained by changes in salinity regime. Because coastal development is wide-spread throughout Gulf of Mexico and Atlantic estuaries, insights concerning the impacts of human-induced changes to estuarine environments are essential for effective management practices.     

Growth,production, and demography of Moina micrura in brackish tropical fishponds (Layo,Ivory Coast)

Sampling and experiments were performed in brackish fishponds, located near the Ebrie Lagoon, Ivory Coast. Moina developed here at salinities up to about 4 g l^-1. Mean embryonic and juvenile development times in experimental conditions were 0.82 and 0.80 day, respectively, with 2 juvenile stages at 30 °C (salinity of 3 g l^-1; 29 µg chlorophyll-a l^-1) and 1.22 and 1.37, with 2 stages for most individuals and 3 or 4 stages for some of them at 26 °C (2 g l^-1 and 36 µg l^-1). The corresponding Q₁₀ for embryonic development was relatively high (2.7). Growth rates in weight at 30 ° C were 2.11 and 1.37 µg µg^-1 d^-1, respectively, for the embryonic and juvenile stages, 0.21 for mean somatic growth of the five first female adult stages, and 1.00 for primipara, summed for somatic growth, production of eggs, and embryonic growth. Consequently, differences in growth rates between young and adults were small. M. micrura also was a high fecundity species according to its size, fecundity in nature varying from 2.2 to 7.7 eggs per adult female. Rates of population increase evaluated for some sequences of parthenogenetic growing periods, reached up to 0.9. Most of the daily P/B evaluated by the cumulative growth method reached values above 1.0 (28 to 31 °C), minima being about 0.7 for lower temperatures (approximatively 26 °C). Populations were also characterized by low juvenile to adult ratios (down to 0.9) and high daily birth rates (up to 1.2). Overall, M. micrura is a highly productive, opportunistic species, well adapted to the low salinities that occur during part of the year in the ponds.Biological and population characteristics of this species, and literature data on regulation mechanisms, possible use for aquaculture, and on size in tropical species are discussed.     

Effect of salinity change on cardiac activity in Hiatella arctica and Modiolus modiolus, in the White Sea

A great majority of salinity studies have dealt with intertidal species. Little is known about the way subtidal animals respond to salinity fluctuations. Even less details are available on invertebrates from the White Sea, which salinity is ca. 25. The heart rate of two subtidal Bivalvia—Hiatella arctica and Modiolus modiolus—exposed to different salinities was recorded. Changes in cardiac activity were monitored for 9 days of the animals’ acclimation to salinities of 15, 20, 30 and 35, and for 4 days of reacclimation (return to the initial salinity of 25). The initial response to salinity change was a significant heart rate reduction. On the other hand, cardiac activity in M. modiolus intensified at salinities of 30 and 35. Reacclimation induced different HR responses: from a decrease to a rise, depending on the species and the salinity applied in the experiment. The differences in responses to salinity are discussed with respect to the morphological and ecological characteristics of the species.