Variation in predation pressure as a mechanism underlying differences in numerical abundance between populations of the poeciliid fish Heterandria formosa

We explored whether a variation in predation and habitat complexity between conspecific populations can drive qualitatively different numerical dynamics in those populations. We considered two disjunct populations of the least killifish, Heterandria formosa, that exhibit long-term differences in density, top fish predator species, and dominant aquatic vegetation. Monthly censuses over a 3-year period found that in the higher density population, changes in H. formosa density exhibited a strong negative autocorrelation structure: increases (decreases) at one census tended to be followed by decreases (increases) at the next one. However, no such correlation was present in the lower density population. Monthly census data also revealed that predators, especially Lepomis sp., were considerably more abundant at the site with lower H. formosa densities. Experimental studies showed that the predation by Lepomis gulosus occurred at a much higher rate than predation by two other fish and two dragonfly species, although L. gulosus and L. punctatus had similar predation rates when the amount of vegetative cover was high. The most effective predator, L. gulosus, did not discriminate among life stages (males, females, and juveniles) of H. formosa. Increased predation rates by L. gulosus could keep H. formosa low in one population, thereby eliminating strong negative density-dependent regulation. In support of this, changes in H. formosa density were positively correlated with changes in vegetative cover for the population with a history of lower density, but not for the population with a history of higher density. Our results are consistent with the hypothesis that the observed differences among natural populations in numerical abundance and dynamics are caused in part by the differences in habitat complexity and the predator community.     

Diurnal behavioural patterns and spread of the Ponto‐Caspian invader Hemimysis anomala G. O. Sars, 1907 (Crustacea,Mysidacea) in the Elbe River,Czech Republic

The Ponto‐Caspian invader Hemimysis anomala was studied in the river Elbe, Czech Republic. The study confirmed further spread of the species: the presence of juveniles and gravid females indicated that H. anomala is established within the Elbe River. Bottom, mid‐water and surface drift samples were collected during 24‐h cycles from July to September 2007. A tendency to remain hidden was the main behavioural strategy of H. anomala in the riverine environment. The mysid was found mainly in the bottom and mid‐water layers, and observed density and mean length of drifting individuals increased with decreasing light levels, being highest at night. Similarly, density and mean length of drifting individuals increased with increasing turbidity. The effect of turbidity on density was particularly apparent during the night samples, suggesting further advantage to night‐drifting individuals. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The changing ecosystem of a shallow, brackish lake, Hickling Broad, Norfolk, U.K. I. Trophic relationships with special reference to the role of Neomysis integer

• 1 Hickling Broad, a shallow, brackish lake in England, changed from dominance by submerged aquatic plants to dominance by phytoplankton in the 1970s. These changes were ascribed to the effects of guanotrophicatioh by black-headed gulls, and increased salinity which together resulted in fish kills caused by a toxic alga, Prymnesium parvum. A mysid, Neomysis integer, was believed to be important in switching the system to plankton dominance through its presumed selective feeding on Cladocera and increasing population size as fish predation decreased.
• 2 Studies on laboratory predation rates of the mysid on Cladocera and on the population dynamics, predation rates and diet of the mysid and the populations of the major zooplankter, Eurytemora affinis in the Broad in the 1980s, have shown that the former explanations were incomplete. The mysid could have markedly reduced the cladoceran community, for it has potentially high predation rates, but Cladocera were probably lost through salinization. Neomysis feeds efficiently on Eurytentora affinis, but the latter reproduces rapidly and its populations are unlikely to be controlled by mysid predation. Nauplii and small copepodites are selectively taken. Alternative and probably major food sources for the mysid are periphyton and benthic algae and detritus.
• 3 Roach and bream readily take the mysid but the present fish stock in the Broad is very low and can now exert minimal pressure on the mysid population. An invertebrate predator on the mysid, Palaemometes varians, is also unlikely to have major effects. The former model of the operation of the Broad's ecosystem needs reconsideration in view of the findings.

     

Ecological differences influence the thermal sensitivity of swimming performance in two co-occurring mysid shrimp species with climate change implications

Temperature strongly affects performance in ectotherms. As ocean warming continues, performance of marine species will be impacted. Many studies have focused on how warming will impact physiology, life history, and behavior, but few studies have investigated how ecological and behavioral traits of organisms will affect their response to changing thermal environments. Here, we assessed the thermal tolerances and thermal sensitivity of swimming performance of two sympatric mysid shrimp species of the Northwest Atlantic. Neomysis americana and Heteromysis formosa overlap in habitat and many aspects of their ecological niche, but only N. americana exhibits vertical migration. In temperate coastal ecosystems, temperature stratification of the water column exposes vertical migrators to a wider range of temperatures on a daily basis. We found that N. americana had a significantly lower critical thermal minimum (CT_min) and critical thermal maximum (CT_max). However, both mysid species had a buffer of at least 4 °C between their CT_max and the 100-year projection for mean summer water temperatures of 28 °C. Swimming performance of the vertically migrating species was more sensitive to temperature variation, and this species exhibited faster burst swimming speeds. The generalist performance curve of H. formosa and specialist curve of N. americana are consistent with predictions based on the exposure of each species to temperature variation such that higher within-generation variability promotes specialization. However, these species violate the assumption of the specialist-generalist tradeoff in that the area under their performance curves is not constant. Our results highlight the importance of incorporating species-specific responses to temperature based on the ecology and behavior of organisms into climate change prediction models.     

Deep impact: in situ functional responses reveal context‐dependent interactions between vertically migrating invasive and native mesopredators and shared prey

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Seasonal and Diel Changes in Behaviour,Microhabitat use and Preferences by Young Pool-dwelling Atlantic Salmon,Salmo salar,and Brown Trout,Salmo trutta

There was a pronounced decline in activity of young pool-dwelling Atlantic salmon, Salmo salar, and brown trout, Salmo trutta, as the water temperatures dropped in the autumn and early winter, and the fish switched from a predominantly diurnal towards a nocturnal activity pattern. Such a switch in activity pattern has previously been observed in young brown trout, but the present study is the first documentation for juvenile Atlantic salmon under natural conditions. Juvenile fish fed actively even when water temperatures were below 0°C, although foraging behaviour at near-freezing temperatures was recorded exclusively during night surveys. This indicates that other proximate factors, in addition to water temperature, affect the activity of young salmon and trout in rivers. Trout kept feeding positions significantly higher above bottom than salmon in August and September, but both species reduced the height above bottom at the onset of winter, possibly due to reduced swimming performance and lowered food availability in the upper part of the water column.     

Application of Perturbation Simulations in Population Risk Assessment for Different Life History Strategies and Elasticity Patterns

Population structure and life history strategies are determinants of how populations respond to stressor-induced impairments in organism-level responses. Effects on population growth rate were modeled using seven theoretical constructs that represented the life history strategies and elasticity patterns of a broad range of species. Simulations of low to high ranges of simultaneous reductions in survival and reproduction were conducted and results indicated that stressor impacts on population growth rate were a function of population characteristics and the magnitude of the stress. Species that had high reproductive elasticity had greater population-level impacts than species with high survival elasticity for the same organism-level effects. Perturbation simulations were performed to assess the extinction risk in two species with similar elasticity patterns but different life history strategies: mysid shrimp, Americamysis bahia, and the gypsy moth, Lymantria dispar, which can be related to classical K- and r-strategists, respectively. Deterministic extinction risk was greater for the K-strategist, which indicated that population level risks were dependent on life history strategies, and toxicity values (e.g., LC50s) should be interpreted with caution. Ecological risk assessments should consider both population structure and life history strategy of an ecological receptor, in addition to the intrinsic sensitivity of the species to contaminant stressors.     

Phylogeography of an estuarine mysid, Neomysis integer (Crustacea, Mysida), along the north-east Atlantic coasts

Aim The brackish water mysid, Neomysis integer, is one of the most common mysid species along the coasts of the north‐east Atlantic. In the present study, the phylogeographical patterns were examined throughout the distribution range of N. integer. In particular, the latitudinal trends in genetic diversity and the distribution of genetic variation were examined in order to elucidate the imprints of the Pleistocene glaciations. Location North‐east Atlantic coasts from the Baltic Sea to the south of Spain. Methods A total of 461 specimens from 11 populations were analysed by means of single‐stranded conformation polymorphism analysis combined with DNA sequencing of a fragment of the mitochondrial cytochrome c oxidase I gene. The genetic structure was examined by using a progression of phylogenetic, demographic and population genetic analyses to elucidate not only the geographical structure, but also the evolutionary history producing that structure. Results The levels of genetic diversity were relatively uniform throughout the distribution range, with the exception of a decline at the northern and southern edges of distribution. A high heterogeneity was observed between the populations analysed (global Φ_ST = 0.787). This is caused by the disparate distribution of the cytochrome oxidase I haplotypes, with several population‐specific haplotypes. A clear genetic break (2.4% sequence divergence) occurred between the southernmost Guadalquivir population and all other populations. Main conclusions The present study corroborates the expectations of the genetic patterns typically observed in an estuarine species. The within‐population variability was low, whereas a significant (moderate to high) divergence was observed between populations. Phylogeographical analysis revealed that northern populations within the English Channel, North Sea and Baltic Sea are characterized by several widespread haplotypes, while the Irish population and all sites south of the Bay of Biscay consist solely of unique haplotypes. This pattern, combined with the relative high levels of genetic diversity, could be indicative for the presence of a glacial refugium in the English Channel region. Under this scenario N. integer must have survived the Last Glacial Maximum in the palaeoriver system present in that region.     

Morphological Differentiation Between Geographically Separated Populations of <Emphasis Type="Italic">Neomysis integer</Emphasis> and <Emphasis Type="Italic">Mesopodopsis slabberi</Emphasis> (Crustacea,Mysida)

Morphological variation was examined in Neomysis integer and Mesopodopsis slabberi, two abundant, low dispersal mysid species (Crustacea, Mysida) along the European coasts. Both species dominate the hyperbenthic communities in the northeast Atlantic, and M. slabberi is also widely distributed in the Mediterranean and Black Sea. Three populations of these species were sampled throughout their distribution range; samples of N. integer were collected in the northeast Atlantic Eems-Dollard, Gironde and Guadalquivir estuaries; in the case of M. slabberi, mysids were sampled in two northeast Atlantic estuaries (Eems-Dollard and Guadalquivir) and one Mediterranean site (Ebro Delta). A total of 12 morphometric and 2 meristic characters were measured from 30–64 mysids per sample. Multivariate analysis showed clear morphometric differences between populations of both species. The morphological differentiation within M. slabberi was highly concordant with the available genetic data from mitochondrial loci, pointing to a large divergence between the Atlantic and Mediterranean populations. However, due to some overlap of individuals between the different populations, the present morphometric analysis does not suffice to assign the different populations to a separate (sub)species status. In the case of N. integer, the morphometric patterns showed a divergence of the Gironde population. Differentiation of populations within this mysid, as in M. slabberi, were mainly related to eye and telson morphology. Potential interactions of the mysid morphology and environmental conditions are discussed.     

The possibility of predator avoidance by Lake Michigan zooplankton

Low densities of Diaptomus ashlandi, Diacyclops thomasi, and Daphnia galeata mendotae were measured at depths where Mysis relicta formed nighttime aggregations. Calculations suggest that mysid predation can not account for the rarity of prey animals at these depths, which further suggests that the prey avoided the mysids. Unlike D. galeata mendotae, Daphnia pulicaria was common in mysid aggregations. The somewhat larger size of D. pulicaria may reduce its vulnerability to mysid predation, and consequently may explain the vertical distribution differences between the two congeners. Vertical distributions of Limnocalanus macrurus and copepod nauplii showed no obvious relationships to the mysid distributions. These were the only two taxa with distributions that were correlated with chlorophyll a concentrations. All crustacean taxa were rare in the epilimnion at night when sonar recorded a dense fish school.     

The foregut of the Mysida (Crustacea,Peracarida) and its phylogenetic relevance

The foreguts of the mysids Antarctomysis maxima, A. ohlinii, Hansenomysis antarctica, Heteromysis formosa, Mesopodopsis slabberi, Neomysis integer, Paramysis kessleri, Praunus flexuosus, and Siriella jaltensis were examined by maceration methods, histological techniques, and scanning electron microscopy. Their morphology, their connection with the midgut glands, and probable function are described and summarized. Previous stomach investigations on mysids and the results of the present study are tabulated; a list of foregut characters, common to all Mysida, is presented. The phylogenetic relevance of these characters within the Malacostraca, especially within the Peracarida, is discussed. Most features are inherited from the ground pattern of the Malacostraca or Eumalacostraca. The bulbous cardia with its dorsal fold, the armature of the lateralia, and the construction of the funnel region are apomorphies for the Mysida. The results suggest that characters of mysidan and other peracaridan foreguts might also be useful in the elucidation of the phylogeny of the Mysida and Peracarida, respectively.     

Decline of the Migratory Form in Bull Charr, Salvelinus Confluentus, and Implications for Conservation

Large-bodied, migratory life history forms of bull charr, Salvelinus confluentus, were historically abundant in northwestern North America, but many remaining populations of this now-threatened species presently persist as small-bodied residents isolated in headwater streams. We examined whether the migratory form has been lost from headwater populations of bull charr and their potential for re-establishment. Upstream and downstream movement of bull charr and other salmonids from three tributary populations in the Bitterroot River drainage, Montana, was measured with weirs over a 17-month period. The migratory life history was rare or absent in two tributaries but still present at a low level in a third. In contrast, substantial numbers (n = 1745) of juvenile and adults of other salmonids (brown trout, Salmo trutta, cutthroat trout, Oncorhynchus clarki, and mountain whitefish, Prosopium williamsoni) were captured near tributary mouths, indicating a migratory life history was common in other species. Apparent decline of the migratory life history in bull charr was not directly related to damming suggesting other downstream mortality factors (predation, temperature) also are involved. Isolated, nonmigratory forms have increased risk of extinction, and restoration of the population connectivity via the re-establishment of migratory stocks is an important conservation goal for bull charr recovery. However, the factors governing migratory tendency remain unclear.     

In situ feeding rates and grazing impact of Mesopodopsis africana O. Tattersall in the St Lucia Estuary, South Africa

Mesopodopsis africana is an important mysid in southern African coastal zooplankton and a key species in the St Lucia estuarine lake, which is currently undergoing severe desiccation owing to freshwater deprivation. M. africana populations through much of the system are consequently under severe environmental stress. This study investigates the grazing dynamics of this mysid species, in relation to autotrophic food availability and other environmental constraints in two contrasting areas of the St Lucia Estuary, Charters Creek, heavily affected by the desiccation process and the Mouth, virtually under unchanged conditions. Gut evacuation experiments were conducted once each during the day and the night. Evacuation rates were consistently higher during the night, ranging from 0.27 to 0.33 h^-1 at Charters Creek and from 1.13 to 1.24 h^-1 at the Mouth. Ingestion rates were, therefore, higher at the Mouth resulting in population grazing impacts of 2.5% of the total microalgal biomass, while the grazing impact at Charters Creek was only 0.5%. The spatial variation in ingestion rates could be attributed to seasonal differences in gut evacuation rates, differences in the mean size of mysids used, or the physicochemical conditions present at the two stations. It is suggested that mysid populations at Charters Creek are predominantly driven by bottom-up forces, initiated by the harsh environmental conditions. Despite the lower ingestion rates exhibited at Charters Creek, results indicate that these mysids are capable of meeting all their energetic requirements from a microalgal diet alone, although they may also utilise a heterotrophic diet.     

Distribution and salinity tolerance of marine mysids from a subtropical estuary,Brazil

We studied the spatial and temporal distribution pattern of three marine mysids (Chlamydopleon dissimile, Metamysidopsis elongata atlantica and Mysidopsis coelhoi) from the Cananeia-Iguape Coastal System, southeast Brazil, in relation to environmental variables and zooplankton abundance. We also analysed mysid reproductive traits, and tested their tolerance to salinity gradients in laboratory experiments. Salinity was identified as a major factor influencing mysid spatial and temporal distribution, selectively limiting their distribution within the estuary. Chlamydopleon dissimile and M. coelhoi were common, but occurred at low abundances throughout the year, whereas the numerically dominant M. elongata atlantica had abundance peaks in spring, when thermohaline conditions were more favourable. Mysid brood size varied interspecifically; the occurrence of juveniles throughout the year indicated that mysids breed continuously in the region. The high biomass of M. elongata atlantica observed in spring confirms the importance of mysids in coastal regions' food webs.     

Suprabenthic euphausiids and mysids from the South Shetland Islands and the Bransfield Strait, Southern ocean (BENTART-95 cruise)

During the austral summer 1995, the suprabenthic assemblages from 24 stations (45–649 m water depth) located around the South Shetland Islands and in the Bransfield Strait were sampled with a Macer-GIROQ sled. The purpose of this paper is to describe the structure of two main crustacean groups (euphausiids and mysids) in the near-bottom water layers sampled by the sled and to focus on the life history of two dominant mysid species from these areas (Mysidetes posthon and Antarctomysis maxima). The distribution and the main factor structuring populations in the investigated area are discussed.     

Predicting the distribution of the crested tinamous, Eudromia spp. (Aves, Tinamiformes)

A bioclimatic analysis of the crested tinamous was conducted to explore climatic factors underpinning the distribution of both Eudromia elegans and E. formosa and to evaluate its potential application in paleontological studies. The study utilized records throughout the entire known range of Eudromia spp. in southern South America. Relationships between 20 environmental parameters and the presence of Eudromia species were established, mapping and characterizing their spatial distribution in a geographic information system using BIOCLIM and MAXENT algorithms. The MAXENT prediction map shows a more homogeneous pattern while BIOCLIM showed a patchier pattern. The models applied here generated maps that adjust to the well-known previous distributions of both species. Nevertheless, for Eudromia elegans, the distribution predicted by MAXENT includes areas where it is actually considered absent, and the BIOCLIM prediction does not include some areas where it is presumed present. Eudromia formosa were found in warmer and wetter sites than E. elegans. Low precipitation areas were identified as suitable for Eudromia elegans. Strong differences between the climatic profiles for both Eudromia spp distributions occurred, with the precipitation the most important influence. E. formosa tolerates the highest maximum temperatures, whereas E. elegans supports the lowest temperatures.     

Spectral sensitivity differences in two Mysis sibling species (Crustacea, Mysida): Adaptation or phylogenetic constraints?

The variation in eye spectral sensitivities of the closely related mysid species Mysis relicta Lovén, 1862 and Mysis salemaai Audzijonyt? and Väinölä, 2005 was studied in sympatric and allopatric populations from the brackish Baltic Sea and from two lakes representing different light environments. In the Baltic Sea the maximum spectral sensitivity of M. relicta, measured by the electroretinogram (ERG) technique, was shifted by ca 20 nm to longer wavelengths than in M. salemaai (564 and 545 nm, respectively). The spectral sensitivity of M. salemaai was closer to that of marine mysid species, which is consistent with its broader euryhalinity and the presumed longer brackish-water history. The species-specific sensitivities in the Baltic Sea were not affected by regional differences in light environments. In two lake populations of M. relicta, the spectral sensitivity was further shifted by ca 28 nm towards the longer wavelengths compared with the conspecific Baltic Sea populations. The spectral sensitivities in the four M. relicta populations were not correlated to the current light conditions, but rather to the phylogeographic histories and fresh- vs. brackish-water environments. A framework to further explore factors affecting spectral sensitivities in Mysis is suggested.     

GENETIC VARIATION IN ASTERIONELLA FORMOSA (BACILLARIOPHYCEAE): IS IT LINKED TO FREQUENT EPIDEMICS OF HOST‐SPECIFIC PARASITIC FUNGI?1

Understanding of the genetic basis for susceptibility and resistance is still lacking for most aquatic host–parasite systems, for instance, for phytoplankton and their fungal parasites. Fungal parasites can have significant effects on phytoplankton populations, mainly through their ability to decimate algal host populations during epidemics. We used random amplified polymorphic DNA (RAPD) and amplified fragment length polymorphism (AFLP) analysis to study levels of genetic variation within a population of the freshwater diatom Asterionella formosa Hassall in relation to parasitism by the obligate, host‐specific, fungal parasite Zygorhizidium planktonicum Canter. The level of genetic variation within the A. formosa population in Lake Maarsseveen, The Netherlands was found to be high despite the presumed absence or very low frequency of sexual reproduction in this species, the limited gene flow, and the severity of parasite attack that would purge the population from susceptible genotypes. RAPD analysis revealed four distinct banding patterns, with 3 of 21 markers (14%) being polymorphic. In AFLP analysis, every single isolate of A. formosa showed a unique banding pattern, and 120 of the 210 AFLP markers (57%) were found to be polymorphic. Furthermore, character compatibility analysis revealed that sexual reproduction may be one of the mechanisms that generates and maintains genetic variation in the A. formosa population in Lake Maarsseveen. The presence of genetic variation in A. formosa was reflected in infection experiments, which showed that genetically different A. formosa strains differed in their susceptibility to various Z. planktonicum strains and that parasite strains differed in their ability to infect particular host strains.     

Do density‐driven mating system differences explain reproductive incompatibilities between populations of a placental fish?

Matrotrophy, the provisioning of embryos between fertilization and birth, creates the potential for conflict between mothers and embryos over the level of maternal investment. This conflict is predicted to drive the evolution of reproductive isolation between populations with different mating systems. In this study, we examine whether density‐driven mating system differences explain the patterns of asymmetric reproductive isolation observed in previous studies involving four populations of the matrotrophic least killifish, Heterandria formosa. Minimum sire number reconstructions suggested that two populations characterized by low densities had lower levels of concurrent multiple paternity than two populations characterized by high densities. However, low levels of genetic variation in the low‐density populations greatly reduced our probability of detecting multiple mating in them. Once we took the lower level of genetic variation into account in our estimations, high levels of multiple paternity appeared the rule in all four populations. In the population where we had the greatest power of detecting multiple mating, we found that multiple paternity in H. formosa typically involves multiple sires contributing to offspring within the same brood instead of different fathers contributing to distinct, simultaneously provisioned broods. Paternity was often skewed towards one sire. Our results suggest that differences between H. formosa populations in the levels of multiple paternity are not sufficient to explain the reproductive isolation seen in previous studies. We suggest that other influences on maternal–foetal conflict may contribute to the pattern of reproductive isolation observed previously. Alternatively, the asymmetric reproductive isolation seen in previous studies might reflect the disruption of maternal–foetal coadaptation.     

Predator-dependent diel migration by <Emphasis Type="Italic">Halocaridina rubra</Emphasis> shrimp (Malacostraca: Atyidae) in Hawaiian anchialine pools

Diel migration is a common predator avoidance mechanism commonly found in temperate water bodies and increasingly in tropical systems. Previous research with only single day and night samples suggested that the endemic shrimp, Halocaridina rubra, may exhibit diel migration in Hawaiian anchialine pools to avoid predation by introduced mosquito fish, Gambusia affinis, and perhaps reverse migration to avoid the predatory invasive Tahitian prawn, Macrobrachium lar. To examine this phenomenon in greater detail, we conducted a diel study of H. rubra relative abundance and size at 2-h intervals in three anchialine pools that varied in predation regime on the Kona-Kohala Coast of Hawai‘i Island. We found two distinct patterns of diel migration. In two pools dominated by visually feeding G. affinis, the abundance of H. rubra present on the pool bottom or swimming in the water column was very low during the day, increased markedly at sunset and remained high until dawn. In contrast, in a pool dominated by the nocturnal predator M. lar, H. rubra density was significantly lower during the night than during the day (i.e., a pattern opposite to that of shrimp in pools containing fish). In addition, we observed that the mean body size of the shrimp populations varied among pools depending upon predator type and abundance, but did not vary between day and night in any pools. Our results are consistent with the hypothesis that H. rubra diel migratory behavior and size distributions are influenced by predation regime and suggest that diel migration may be a flexible strategy for predator avoidance in tropical pools where it may be a significant adaptive response of endemic species to introduced predators.