Diversification, Sympatric Speciation, and Trophic Polymorphism of Arctic Charr, Salvelinus Alpinus Complex, in Transbaikalia

In northern Transbaikalia, independently evolving landlocked populations of Arctic charr are found in mountain lakes. To assess the diversity of charr in this region, speciation modes involved in the evolution of charr forms, and the role of trophic polymorphism in their divergence, we studied the morphology and feeding of dwarf, small, and large forms of Arctic charr from a number of Transbaikalian lakes. Meristic data on charr from five lakes support the earlier conclusion that the three forms do not represent separate lineages but have independently diverged in sympatry in each of the lakes. In 10 lakes, the dwarf form showed varying degrees of differentiation from normal (small and large) charr in meristic characters (up to morphologically distinct and presumably reproductively isolated groupings), which is viewed as various levels of sympatric divergence. Indexes of gill raker length in fish from 20 lakes vary among populations of both dwarf and normal charr, with forms having short and long rakers being sympatric in some of these lakes. However, the index can be used only for comparing charr of different forms up to about 32cm fork length (FL) because it is strongly negatively correlated with size in larger fish. The study of charr diets in 21 lakes indicates that large charr are piscivorous whereas dwarf and small charr feed on a wide range of invertebrates, partitioning these resources in different ways. Planktivores, including very specialized ones, and non-planktivores (benthic feeders, insectivores) can be identified within the small and dwarf forms. The proportion of plankton in the diets of dwarf and small charr is positively correlated with the number and length of gill rakers while the proportion of benthos is negatively correlated. Allopatric planktivorous and non-planktivorous small charr differ in body proportions; parallel emergence of such morphotypes in different parts of the range is a characteristic feature of the Salvelinus alpinus complex.     

Atypical swimbladders of lake charr, Salvelinus namaycush, from Great Slave Lake, Northwest Territories, Canada

Swimbladder walls of lake charr, Salvelinus namaycush, from Great Slave Lake (GSL), Northwest Territories, Canada, were unusually thick for the species. The thinnest sections of the GSL bladders (mean = 2.44mm, range = 1.1–4.4mm) were significantly thicker (P = 0.001) than lake charr swimbladders collected from two small Province of Ontario lakes (means = 0.65 and 0.92mm), whose populations were assumed to be representative of the species. Variance in wall thickness was also greater in GSL lake charr than in charr from two small lakes (P < 0.02). Within individuals, some of the GSL bladder walls were markedly irregular in thickness, but whether these anomalies exist in situ or were artifacts of preservation remains uncertain. The bulk of the tissue in the thickest sections of the GSL swimbladders was in the tunica serosa (outer layer). The extent of the modification of the GSL swimbladders is extraordinary for northern fishes in postglacial lakes.     

A Sex-linked Microsatellite Locus Isolated from the Y Chromosome of Lake Charr, Salvelinus Namaycush

A small-insert library was constructed after microdissection of the short arm of the Y chromosome of lake charr, Salvelinus namaycush. Clones from this library were sequenced and two dinucleotide CA-repeat microsatellite sequences were recovered. Oligonucleotide primers for one locus were designed and optimized for amplification by PCR. This locus (designated Yp136) was tested for sex linkage in twelve lake charr families and found to be a distance of 37 centimorgans (cM) from the sex-determining locus. This microsatellite locus was also examined in three informative, gynogenetic/diploid, lake charr crosses for linkage to the centromere on the X chromosome. Results from these families show that this locus is 19cM from the centromere. The combination of linkage data and microdissection information places the sex-determining locus near the telomere of the Y chromosome in lake charr. This is consistent with studies of several other fish species including some salmonids that place the sex locus near the telomere.     

Genetic analysis of four sympatric morphs of Arctic charr,Salvelinus alpinus,from Thingvallavatn,Iceland

Synopsis The degree of genetic differentiation among four morphs of Arctic charr (small benthivorous, large benthivorous, piscivorous, and planktivorous) from Thingvallavatn, Iceland, was determined electrophoretically. Five of 36 enzyme loci were found to be polymorphic (Est2, Gpi3, Ldh4, Mdh4, 5 and Pgm2). However, only Est2 and Mdh4,5 showed enough variability to permit statistical analysis of divergence among morphs. All four morphs are very closely related; the values of Nei's (D) range from 0.00004 to 0.00126. These morphs are conspecific and do not represent different evolutionary lineages. There is significant genetic differentiation between the small benthivorous charr and the other three morphs. The relative relatedness of morphs based on gene frequency data is only partially concordant with that based on morphology and ecological specialization. The biological significance of this result is unclear because of the limited number of polymorphic loci upon which the genetic analysis is based and the high degree of relatedness among morphs.     

Assessment of the Feasibility of Preventing Reproduction of Lake Charr, Salvelinus Namaycush, in Shallow Areas of Reservoirs Affected by Drawdowns

Lake charr, Salvelinus namaycush, spawn almost exclusively in lakes, in the fall. Spawning sites in inland lakes are generally located close to the shore in areas with coarse-textured substrate, and in depths of less than 2m. Because of the type of water management system applied in reservoirs where water levels are lowered during the winter, eggs deposited during the fall around the shallow shoreline of reservoirs can be exposed. The depth of spawning sites relative to drawdowns may be a limiting factor for lake charr survival in reservoirs. In 1995 and 1996, we evaluated the ability to induce lake charr reproduction in deeper water, below the range of fluctuating water levels by the sequential construction of artificial reefs and limiting access to natural spawning habitat. The creation of artificial deeper water spawning areas, adjacent to natural sites, produced a partial transfer of reproduction. However when natural spawning sites were covered with tarpaulins almost all the eggs were laid in the artificial deeper water spawning areas. Complete or partial transfer of reproduction activities to sites below the depth not affected by water level fluctuations is therefore a potential way of alleviating the effects of hydroelectric reservoir management.     

Abundance, Annual Survival, and Recruitment of Unexploited and Exploited Lake Charr, Salvelinus Namaycush, Populations at the Experimental Lakes Area, Northwestern Ontario

We studied abundance, annual survival, and recruitment of nine lake charr, Salvelinus namaycush, populations at the Experimental Lakes Area, Ontario, for periods of 9–24 years. We used the Jolly–Seber mark-recapture method to evaluate abundance and annual survival in all populations, and total catches of individual year classes to evaluate recruitment. Seven populations were unexploited and unaffected by whole-lake experiments. One population was exploited prior to mark-recapture study and another was affected by experimental acidification. Abundance ranged from 8 to 24 fish ha^–1 in the unaffected populations. Annual survival ranged from 78% to 93%yr^–1 in all populations except the exploited population. This population may have been additionally affected by northern pike, Esox lucius, predation. Yearly recruitment was relatively constant in all populations and related to lake charr abundance. Lake charr abundance did not recover in populations affected by exploitation or acidification. Unexploited lake charr populations were characterized by relatively constant abundance, high annual survival, and relatively constant annual recruitment.     

Limnological Characteristics of 70 Lakes Spanning Arctic Treeline from Coronation Gulf to Great Slave Lake in the Central Northwest Territories,Canada

Latitudinal transects across subpolar ecozones display striking changes in lakewater chemistry reflecting steep gradients in vegetation, climate, and other variables. This paper explores the relationships among chemical and physical lakewater characteristics of 70 lakes spanning arctic treeline in Canada's Central Northwest Territories. Principal components analysis (PCA) was used to examine trends and relationships among environmental variables and these 70 sites. In general, lakes in this data set were dilute, slightly acidic to slightly alkaline, and nutrient-poor. However, a strong trend toward more concentrated lakewater conditions in densely forested areas was observed relative to tundra regions. Interrelationships among measured limnological variables appear to be strongly influenced by catchment characteristics associated with proximity of sites to treeline.     

Comparative Morphometrics and Associated Growth Trends of Two Benthophagous Cichlid Species from Lake Malawi (Pisces, Perciformes)

Body shape differences between two benthophagous, sympatric cichlid species, Ctenopharynx pictus and Otopharynx sp. “heterodon nankhumba” were examined for 14 morphometric characters using measurements obtained from 11 morphological landmarks and additional 6 characters from the head region using principal component and regression analyses. The two species, though not closely related, superficially resemble each other and, being benthic feeders, directly compete for food resources unless morphological differences lead to trophic specialization. Hence, the present study explores the presence of such morphological variations. Principal component analysis (PCA), revealed that only four characters loaded highly on PC II and, therefore, account for most of the variations in the external morphology between the two species. C. pictus has longer lower jaws reflecting longer gape, and a shorter abdominal cavity than O. sp. “heterodon nakhumba”. The regression analyses showed significant differences in the allometric coefficients of the lower jaw and abdominal cavity, which might have contributed to differences in overall shape detected between the two species. The difference in gape size between the two species may be related to different feeding habits employed by each species, and such specializations are suggested to reduce interspecific competition for food resources. We suggest that the ensuing ecological separation promotes sympatry of these two species.     

Variation in body size and trophic morphology within and among genetically differentiated populations of the cichlid fish, Metriaclima zebra, from Lake Malawi

1. The cichlid fish Metriaclima zebra , common in Lake Malawi, feeds by filtering plankton from the water and by brushing items from sediment covered substrata. It inhabits isolated rocky reefs among which community structure, resource availability and gene pools are likely to differ. We speculated that body size and trophic morphology of M. zebra might vary concomitantly.
2. We quantified the extent of genetic, body size and trophic variation within and between populations of M. zebra from southern Lake Malawi. Specifically, we tested the hypotheses that: (i) local populations are genetically differentiated, (ii) local populations differ in jaw morphology, dentition and standard length (SL), and (iii) variation in size is correlated with variation in trophic morphology.
3. Local populations of M. zebra differed in mean SL and were genetically differentiated. Moreover, populations exhibited dissimilar oral jaw morphologies and dentitions, perhaps related to differences in feeding biology. Variation in jaw shape was largely restricted to the curvature of the distal tip of the dentary. Populations were characterised by individuals with oblique, upward or downward directed gapes. Dental patterns differed in the proportion of unicuspid teeth in all rows of each jaw (dentaries and premaxillae) and the spacing of teeth in affected rows.
4. Within populations, jaw and tooth shapes were correlated with body size. Smaller individuals possessed upward curving jaws and closely packed multicusped teeth, while larger individuals exhibited relatively downward-directed jaws with increasing numbers of widely spaced unicuspid teeth.
5. Metriaclima zebra populations have increased in mean SL over the last decade, in contrast to a decline among Lake Malawi pelagic cichlids. Differences in size may contribute to variation in trophic morphology and may track local environmental dynamics in this lacustrine system.     

Morphology and ultrastructure of teratological forms of the diatoms Stephanodiscus niagarae and S. parvus (Bacillariophyceae) from Hamilton Harbour (Lake Ontario,Canada)

Teratological forms of Stephanodiscus niagarae Ehrenb. and S. parvus Stoermer & Håkansson were observed during a study of diatoms preserved in a radiometrically dated core from Hamilton Harbour (Lake Ontario, Canada). Morphological features and ultrastructures of both species were studied under the light and scanning electron microscope. The valve structure of abnormal forms of S. niagarae appears to be weakly silicified, especially in the central area. The shape of satellite pores are very irregular in comparison with the round shape of the normal specimen. The central fultoportulae are characterized as small tubes extending out of the external valve. Two types of abnormal frustules are present in the population of S. parvus studied. In one type valves are underdeveloped and only the siliceous layer and ribs are present, and in the other type the valves are strongly silicified and the areolae are almost completely occluded.The teratological forms of both species appeared in the core sediments after 1911, and both became the dominant components of diatom assemblages after 1970. Their occurrence and increased abundance coincides with records of heavy metal pollution in the harbour.     

Evidence for Late Glacial and Holocene environmental changes from subfossil assemblages in sediments of Lake Neuchâtel, Switzerland

Ostracod and diatom assemblages, mollusc remains, plus pollen and sediment stratigraphy from three cores taken at sites 40, 80 and 135 m deep in Lake Neuchâtel, Switzerland, show distinct changes since deglaciation. Shifts are interpreted in terms both of climatic change and major changes in the catchment input caused by the Aar River entering or bypassing Lake Neuchâtel. Three palaeoenvironmental phases are recognized: (1) the Late Glacial, with the inflow of the river before 10,000 yr B.P.; (2) the early to mid-Holocene, with temporary bypassing of the river between 10,000 and 4850 yr B.P.; and (3) the mid- to late Holocene, without river input after approx. 4850 yr B.P. During periods with river input, rhythmites were deposited. Ostracods indicate a well oxygenated hypolimnion, suggesting deep ventilation enhanced by the river inflow. The diatom assemblage indicates eutrophic to mesotrophic conditions and is dominated by generally well preserved periphytic species. The high number of unidentifiable pollen suggests oxidation during transport in the river. During periods without river input, a massive calcareous silt with abundant authigenic calcite was deposited, diatoms indicate a change to mesotrophic and oligotrophic conditions, and decreased sedimentation rates and higher alkalinity enhanced the dissolution of diatoms. Terrestrial and littoral molluscs as well as periphytic diatoms indicate erosion of the shore when the lake level rose or fell with onset or offset of the river inflow. Before 12,600 yr B.P., the patchy vegetation cover led to intensive soil erosion and high sedimentation rates. The lake was still frozen during the winter months. After approx. 12,600 yr B.P. the sediment changed from clastic rhythmites to thinly bedded, non-glacial rhythmites, the vegetation cover became denser, the abundance of ostracods increased, and molluscs started to colonize the profundal zone of the lake. The Younger Dryas (approx. 10,700–10,000 yr B.P.) displays maximum abundance of ostracods, indicating deep ventilation. The transition to the Holocene is characterized by a change in the ostracod assemblage and the river bypassed the lake between approximately 10,000 and 8900 yr B.P. When the river reentered the lake after approximately 8900 yr B.P., a new ostracod species appeared. After two short periods of river bypassing between approximately 7200 and 6950 yr B.P., diatoms indicate a higher trophic level about 6000–5000 yr B.P. associated with higher erosion rates and increased nutrient input due to Neolithic agricultural activities. After approximately 4850 yr B.P. the river bypassed the lake again and the trophic level decreased. In contrast to the early to mid-Holocene periods of decreased oxygen supply, the mid- to late Holocene is characterized by deep ventilation. This is attributed to the northward retreat of the polarfront and enhanced westerlies consequent to decreased insolation after 6000 yr B.P. The change of the river flow pattern correlates with glacier oscillations and may suggest a climatic control. The final cessation of the inflow is attributed to diversion of the river consequent to isostatic uplift and late Neolithic clearance activity.