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”.     

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.     

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.     

Diet,stable isotopes and morphology of Eurasian perch (Perca fluviatilis) in littoral and pelagic habitats in the northern Baltic Proper

We present morphology, stable isotope signals and stomach contents of Eurasian perch (Perca fluviatilis) from littoral and pelagic habitats in a brackish water embayment in the northern Baltic Proper. Studies conducted in freshwater habitats repeatedly state that littoral perch have deeper bodies than the pelagic ones. In this study we observed the opposite; the perch from the pelagic study site had deeper bodies than the littoral ones, indicating that more factors than habitat structure affect the perch morphology. A possible explanation to this discrepancy is the diet choice; the pelagic perch in this study were more benthivorous than freshwater pelagic perch. Our results on stable isotope signals combined with the stomach contents also shed new light on the dietary preferences of perch. Perch is known to be a generalist predatory fish, but our results indicate that perch have individual diet preferences. Based on our results, it seems that at some point in their lives the perch in brackish water choose between the littoral and pelagic habitats and also specialise in a certain diet. This study shows that the perch morphology and diet in the Baltic Sea coast differ among habitats, but the patterns are not similar to those observed in freshwater studies.     

Changes in some nitrogenous compounds in the blood and tissues of freshwater pikeperch (Sander lucioperca) during salinity acclimation

The effect of ambient salinity changes (0.9, 6 and 12 psu) on the levels of dissolved ammonia (DA), ninhydrin positive substances (NPS), trimethylamine (TMA) and trimethylamine oxide (TMAO) in the blood and tissue of medium-acclimated Sander lucioperca L. (also Stizostedion lucioperca) were investigated. In freshwater, blood and tissue total free amino acid levels (measured as NPS) were 3.62 mM and 60.61 mM, respectively. The NPS content increased significantly (P<0.05) in the tissue and blood on acclimation to 6 and 12 psu salinities. The mass-specific tissue TMAO concentration of pikeperch acclimated to normal freshwater was 0.413+/-0.084 micromol TMAO g(-1). Results reveal that TMAO levels are positively influenced by the external salinity medium where significant differences in mean levels occurred between the groups (P<0.05). The calculated p[NH(3)] and [NH(4)(+)] gradients reveal that the [NH(3)] gradient was consistently low (cf. the [NH(4)(+)] gradient). The gradient of p[NH(3)] decreased with the medium increased salinities. The results suggest that freshwater pikeperch may be able to resist salinity changes by manipulation of nitrogen metabolism. Free amino acids and TMAO are involved in mediating response to salinity exposure in freshwater pikeperch.     

Plasma osmolality and oxygen consumption of perch Perca fluviatilis in response to different salinities and temperatures

The present study determined the blood plasma osmolality and oxygen consumption of the perch Perca fluviatilis at different salinities (0, 10 and 15) and temperatures (5, 10 and 20° C). Blood plasma osmolality increased with salinity at all temperatures. Standard metabolic rate (SMR) increased with salinity at 10 and 20° C. Maximum metabolic rate (MMR) and aerobic scope was lowest at salinity of 15 at 5° C, yet at 20° C, they were lowest at a salinity of 0. A cost of osmoregulation (SMR at a salinity of 0 and 15 compared with SMR at a salinity of 10) could only be detected at a salinity of 15 at 20° C, where it was 28%. The results show that P. fluviatilis have capacity to osmoregulate in hyper‐osmotic environments. This contradicts previous studies and indicates intraspecific variability in osmoregulatory capabilities among P. fluviatilis populations or habitat origins. An apparent cost of osmoregulation (28%) at a salinity of 15 at 20° C indicates that the cost of osmoregulation in P. fluviatilis increases with temperature under hyperosmotic conditions and a power analysis showed that the cost of osmoregulation could be lower than 12·5% under other environmental conditions. The effect of salinity on MMR is possibly due to a reduction in gill permeability, initiated to reduce osmotic stress. An interaction between salinity and temperature on aerobic scope shows that high salinity habitats are energetically beneficial during warm periods (summer), whereas low salinity habitats are energetically beneficial during cold periods (winter). It is suggested, therefore, that the seasonal migrations of P. fluviatilis between brackish and fresh water is to select an environment that is optimal for metabolism and aerobic scope.     

Variation in habitat use along the freshwater–marine continuum by grey mullet Mugil cephalus at the southern limits of its distribution

In this study, habitat use by Mugil cephalus was investigated in the waters of the west coast of the North Island of New Zealand by analysing microchemical composition of otoliths (laser‐ablation inductively coupled plasma mass spectrometry) obtained from individuals from commercial fish stocks and research surveys. Results of this study show that M. cephalus at the southern limits of its distribution display highly flexible migratory behaviour with extensive use of freshwater and brackish habitats, potentially enabling them to maximize foraging opportunities. Mugil cephalus can tolerate a wide range of salinities and can therefore utilize higher productivity areas, such as estuaries and eutrophic riverine lakes. Finally, M. cephalus populations across a range of climates and latitudes appear to differ in the extent to which they utilize freshwater and brackish habitats, possibly with increasing penetration of fresh waters with increasing latitude.     

Diverse migration strategy between freshwater and seawater habitats in the freshwater eel genus Anguilla

The freshwater eels of the genus Anguilla, which are catadromous, migrate between freshwater growth habitats and offshore spawning areas. A number of recent studies, however, found examples of the temperate species Anguilla anguilla, Anguilla rostrata, Anguilla japonica, Anguilla australis and Anguilla dieffenbachii that have never migrated into fresh water, spending their entire life history in the ocean. Furthermore, those studies found an intermediate type between marine and freshwater residents, which appear to frequently move between different environments during their growth phase. The discovery of marine and brackish-water residents Anguilla spp. suggests that they do not all have to be catadromous, and it calls into question the generalized classification of diadromous fishes. There has been little available information, however, concerning migration in tropical Anguilla spp. Anguilla marmorata, shows three fluctuation patterns: (1) continuous residence in fresh water, (2) continuous residence in brackish water and (3) residence in fresh water after recruitment, while returning to brackish water. Such migratory patterns were found in other tropical species, Anguilla bicolor bicolor and Anguilla bicolor pacifica. In A. b. bicolor collected in a coastal lagoon of Indonesia, two further patterns of habitat use were found: (1) constantly living in either brackish water or sea water with no freshwater life and (2) habitat shift from fresh water to brackish water or sea water. The wide range of environmental habitat use indicates that migratory behaviour of tropical Anguilla spp. is facultative among fresh, brackish and marine waters during their growth phases after recruitment to the coastal areas. Further, the migratory behaviours of tropical Anguilla spp. appear to differ in each habitat in response to inter and intra-specific competition. The results suggest that tropical Anguilla spp. have a flexible pattern of migration, with an ability to adapt to various habitats and salinities. The ability of anguillids to reside in environments of various salinities would be a common feature between tropical and temperate species without a latitudinal cline. Thus, the migration of Anguilla spp. into fresh water is clearly not an obligatory behaviour. This evidence of geographical variability among Anguilla spp. suggests that habitat use is determined by environmental conditions in each site.     

Oxygen consumption of the aquatic leaf beetles Macroplea mutica and Macroplea appendiculata is low and not influenced by salinity

Leaf beetles of the genus Macroplea live permanently under water. Species‐specific preferences for either freshwater or brackish water are available in the literature. To detect potential physiological differences, the oxygen consumption of Macroplea mutica and Macroplea appendiculata from habitats with differing salinities is measured at two different salinities (0 and 10). The specific oxygen consumption does not depend on oxygen saturation of the water (values in the approximate range of 25–100% occur during the experiments). There is no difference between species or sampling locations. Similarly, the salinity during the measurements does not affect the oxygen consumption of the beetles, either when compared as salinity per se (0 versus 10), or when classified as home salinity or atypical salinity. Comparisons with other chrysomelid beetles and aquatic insects (using available published data) reveal that the two Macroplea species have relatively low metabolic rates. This finding is discussed in the light of activity patterns and morphology, especially the reduction of flight muscles, which comprise a highly metabolically active tissue.     

Comparing salinity tolerance in embryonic and larval development of two species of water strider,Aquarius paludum and Gerris latiabdominis (Hemiptera: Gerridae)

Water strider Aquarius paludum (Fabricius) is a cosmopolitan species colonizes mainly freshwater but occasionally brackish habitats throughout the Palearctic and Oriental regions. Water strider Gerris latiabdominis (Miyamoto) is a common species in Japan lives in temporary habitats as freshwater paddy fields. These two species often occur syntopically. We investigated differences in the developmental response to brackish water during embryonic and larval stages between the two species. Eggs were exposed to 0–1.8% NaCl solutions within 24 h of oviposition. Larvae of G. latiabdominis were exposed to salinities of 0, 0.5%, and 0.9% from the first instar until adult emergence. Limits of NaCl concentration for hatching were 1.3% and 1.0% for A. paludum and G. latiabdominis, respectively. The hatching rate of G. latiabdominis was lower than that of A. paludum at salinities ≥0.9%. The period of embryonic development of G. latiabdominis was more prolonged than that of A. paludum at a given salinity. Although the salinity tolerance of G. latiabdominis was lower than that of A. paludum, our results suggest G. latiabdominis has the physiological capacity to expand into brackish waters. High and low salinity tolerances of A. paludum and G. latiabdominis, respectively, reflect the relatively wide range of habitat salinities utilized by A. paludum and the relatively restricted habitats preferred by G. latiabdominis. The high salinity tolerance of A. paludum could be an important factor contributing to their cosmopolitan distribution because high tolerance to salinity means the possibility of them to be dispersed via ocean or sea to other continents and islands.     

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.     

Molecular and morphological divergence in the inland silverside (<Emphasis Type="Italic">Menidia beryllina</Emphasis>) along a freshwater-estuarine interface

Populations that vary across ecological gradients or that have invaded novel habitats are important to elucidate the association between adaptive divergence and gene flow, factors that may play an important role in speciation of silverside fishes. The inland silverside, Menidia beryllina, is an ideal organism for this kind of research, displaying a great diversity in morphology among freshwater and coastal brackish habitats. Using a combination of geometric morphometrics and mitochondrial (mt) DNA, we investigated patterns of variation within and among the nominal freshwater Menidia audens and coastal M. beryllina, spanning the transition from freshwater to tidally influenced semi-brackish waters of the lower Mississippi River to brackish waters of the Lake Pontchartrain estuary. Although we found no evidence for a phylogenetic split between M. audens and M. beryllina, our results indicate that significant genetic divergence corresponds with body shape differences among the two, with a clear distinction at the interface of freshwater and brackish water. Patterns in mtDNA suggest that freshwater populations referred to as M. audens are of recent origin with evidence for habitat-based divergence compared to coastal populations referred to as M. beryllina. Our findings add to a growing body of evidence that ecological shifts, following colonization of novel habitats, may promote rapid adaptive divergence and reduced gene flow among silverside populations in adjacent environmental regimes.     

Flexible Life History Strategies of Ninespine Sticklebacks,Genus <Emphasis Type="BoldItalic">Pungitius</Emphasis>

Synopsis We studied the life histories of the ninespine sticklebacks, Pungitius pungitius and Pungitius tymensis, collected from Japanese freshwater and brackish (sea) water habitats by examining the strontium (Sr) and calcium (Ca) concentrations in their otoliths. The Sr:Ca ratios in the otoliths changed with the salinity of the habitat regardless of identification as freshwater or brackish water type based on morphological characteristics. The ninespine sticklebacks living in a freshwater environment showed consistently low Sr:Ca ratios throughout the otolith. These samples were identified as a standard freshwater type. In contrast, all freshwater-type fishes collected from the intertidal zone showed higher otolith Sr:Ca ratios than those in the standard freshwater type, and the ratios fluctuated with the growth phase. All brackish water-type fishes collected in the intertidal zone showed the highest otolith Sr:Ca ratio throughout the otolith. In the present study, besides the two representative life history types of P. pungitius, i.e., freshwater and brackish water life history types, other sticklebacks had an anadromous life history type. These findings clearly indicate that the ninespine stickleback has a flexible migration strategy with a high degree of behavioral plasticity and an ability to utilize the full range of salinity in its life history.     

Dispersal of yellow phase Japanese eels <Emphasis Type="Italic">Anguilla japonica</Emphasis> after recruitment in the Kojima Bay-Asahi River system,Japan

The density, size and age distribution were investigated for 233 eels, Anguilla japonica, sampled in fresh and brackish water areas of the Kojima Bay-Asahi River system, Okayama, Japan, to evaluate the possible patterns of dispersal of eels that recruit to this area. Migratory histories of 183 eels were categorized into 5 types depending on the Sr and Ca concentrations in their otoliths: (1) brackish water residents (74 fish, 40.4%), which settled in saline water and remained until capture; (2) freshwater residents (46 fish, 25.1%), which settled in freshwater and remained until capture; (3) upstream shifters (3 fish, 1.6%), which settled in saline water and moved upstream into freshwater; (4) downstream shifters (53 fish, 29.0%), which settled in freshwater and moved downstream into saline water; (5) multiple habitat shifters (7 fish, 3.8%), which shifted their habitats between freshwater and saline water more than twice. For eels captured in the brackish water area, fish density decreased with distance in the downstream direction, while the size and age of eels increased. For eels captured in the freshwater area, size and age were greater than those in the upper-most brackish site. These observations suggest that eels in this system initially accumulate in the lower reaches of the river and then disperse in both upstream and downstream directions following their growth.     

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.     

Salinity Tolerance in Some Selected Aquatic Oligochaetes

Salt tolerance of the freshwater tubificid Limnodrilus hoffmeisteri varied from an LD 50 of 10 ppt to 10.5 ppt for mature and immature worms without sediment, 9.4 to 10.5 in sand, and 14.3 for immatures acclimated to 5 ppt. Ilyodrilus templetoni was similarly intolerant to higher salinities. The estuarine Tubificoides gabriellae tolerated salt water, but was susceptible to freshwater, with LD 50's of 2 and 3.5 when acclimated to 5 and 10 ppt sea water. It survived immersion in freshwater for 3 hours. The estuarine naidids Paranais litoralis and frici experienced mortalities at both high and low salinities but showed a wide tolerance range. The coastal tubificid Monopylephorus irroratus showed almost complete tolerance to the salinity range employed. The findings reflected field distributions especially when interstitial salinities were measured, but worms did not occupy the full range of habitats possible in terms of salinity alone.     

Adaptive patterns of osmotic and ionic regulation, and the invasion of fresh water by the palaemonid shrimps

To evaluate trends in the osmoregulatory behavior of neotropical, palaemonid shrimps, we investigated osmotic and ionic regulatory patterns in five species of Palaemon or Macrobrachium. The species' life histories depend on saline water to differing degrees, their habitats ranging from the marine/intertidal (P. northropi), through estuaries (P. pandaliformis) to coastal, freshwater streams (M. olfersii, M. potiuna) and inland, continental river systems (M. brasiliense). Hemolymph osmolality, chloride, sodium and magnesium concentrations were measured in shrimps exposed to experimental media ranging from fresh water (<0.5 per thousand ) to concentrated seawater (42 per thousand ) for up to 10 days. The marine and estuarine Palaemon species exhibit well-developed hyper/hypo-osmotic, sodium and chloride regulatory capabilities in mid-range salinities, tending to hyperconform in low salinities. The freshwater Macrobrachium species show variable hyperosmotic, sodium and chloride regulatory capacities, tending to hypoconform or unable to survive at higher salinities. All species hyper-regulate magnesium in fresh water, but hyporegulate strongly in saline media. Palaemonids from the saline habitats show the strongest osmoregulatory capabilities, and fresh water may have been gradually invaded by ancestral species with similar regulatory capacity. However, this regulatory plasticity has been lost to varying degrees in extant freshwater species.     

Na,K-ATPase activity and epithelial interfaces in gills of the freshwater shrimp Macrobrachium amazonicum (Decapoda,Palaemonidae)

Diadromous freshwater shrimps are exposed to brackish water both as an obligatory part of their larval life cycle and during adult reproductive migration; their well-developed osmoregulatory ability is crucial to survival in such habitats. This study examines gill microsomal Na,K-ATPase (K-phosphatase activity) kinetics and protein profiles in the freshwater shrimp Macrobrachium amazonicum when in fresh water and after 10-days of acclimation to brackish water (21‰ salinity), as well as potential routes of Na⁺ uptake across the gill epithelium in fresh water. On acclimation, K-phosphatase activity decreases 2.5-fold, Na,K-ATPase α-subunit expression declines, total protein expression pattern is markedly altered, and enzyme activity becomes redistributed into different density membrane fractions, possibly reflecting altered vesicle trafficking between the plasma membrane and intracellular compartments. Ultrastructural analysis reveals an intimately coupled pillar cell-septal cell architecture and shows that the cell membrane interfaces between the external medium and the hemolymph are greatly augmented by apical pillar cell evaginations and septal cell invaginations, respectively. These findings are discussed regarding the putative movement of Na⁺ across the pillar cell interfaces and into the hemolymph via the septal cells, powered by the Na,K-ATPase located in their invaginations.