Seasonal grazing of Pseudocalanus minutus on particles

Grazing by Pseudocalanus minutus on naturally occurring partieles has been investigated over a 2 year period at 5 m depth, in a small coastal embayment in Nova Scotia, Canada. Large variations in the standing stock of particulate matter in the water from 5 m depth were associated with seasonal changes in phytoplankton, and at times with runoff from rivers. Each size group displayed seasonal changes, with high amplitudes occurring in the large-particle size ranges. P. minutus consumption was associated with seasonal changes in total particle concentration as well as with the concentration in each size group. The food uptake was correlated (P>0.01) with the particle concentration in all particle categories, except in the size range below 3.57 . Frequency of positive electivity indices increased with particle sizes up to 57 and then decreased. Observations on particle spectra revealed considerable seasonal variability in both particle sizes and copepod feeding patterns. P. minutus adapted to seasonal variations within the particle spectrum by shifting its grazing pressure from one size range to another. By taking advantage of every particle peak concentration, P. minutus revealed a strong opportunistic feeding behaviour, and a very efficient utilization of the standing stock. Maximum consumption was recorded in early spring, when P. minutus fed on large-size particles. Feeding took place in the medium and small-size ranges during the summer and part of fall. Food uptake was rarely less than 2.26% of body weight during the winter, and reached up to 55% and sometimes more in spring. The unselective feeding patterus demonstrated by P. minutus suggest certain ecological implications of feeding pressure on standing stock.     

Distribution and relative abundance of Pseudocalanus moultoni and P. newmani (Copepoda: Calanoida) on Georges Bank using molecular identification of sibling species

The sibling species, Pseudocalanus moultoni (Frost, 1989) and P. newmani (Frost, 1989), occur sympatrically on Georges Bank. Taxonomic discrimination of the species relies on subtle morphological characteristics, making routine identification of the species very difficult. DNA sequence variation of two mitochondrial genes, 16S rRNA and cytochrome oxidase I (COI), reliably discriminated P. moultoni and P.␣newmani. Levels of DNA sequence variation for both genes were consistent with those between species of calanoid copepods. A molecular systematic protocol (based on allele-specific PCR amplification) was designed from the COI sequences and used to discriminate females of the two species. The distributions and relative abundances of the two species were mapped for April 1996 based on samples of 15 to 30 females from 12 collections across Georges Bank. The results of this study indicated that P. moultoni females predominated along the northern flank of Georges Bank, while P. newmani females were common on the southern flank, deeper than the 60-m isobath. Received: 16 May 1997 / Accepted: 26 April 1998     

Muscle esterase variation and size variation in the sand launce Ammodytes dubius

Esterase variation detected in homogenates of muscle tissue taken from the sand launce Ammodytes dubius in April, 1970 and 1971 is described. Eight alleles were found segregating at a single locus, and the frequencies of these alleles were homogenous in population samples taken on the Emerald Bank, off Nova Scotia, Canada, in successive years. When ordered by their electrophoretic mobility, the profile of the frequencies of these alleles was unimodal and symmetric. In both of the population samples, there were significant excesses of homozygotes and deficencies of heterozygotes. There was an association between genotype and size of individuals among homozygous genotypes; individual homozygous for the most common, intermediately-migrating allele were larger than homozygotes bearing alleles with relatively fast or slow mobilities.     

Changes in the ratio of the sulcus acusticus area to the sagitta area of Pomatoschistus minutus and P. lozanoi (Pisces,Gobiidae)

The Pomatoschistus minutus (Pallas) and P. lozanoi (de Buen) stock of the ebb-tidal delta of the former Grevelingen estuary was sampled monthly from September 1992 to October 1993. Sagittae were extracted and the areas of the sagitta and sulcus acusticus were measured with a digitizer. The frequency response and auditory threshold of these two sympatric goby species were determined from the ratio of the sulcus acusticus area to the sagitta area (S:O ratio). The S:O ratio of P. minutus increased while in P. lozanoi the S:O ratio was constant with increasing standard length. The average S:O ratio of P. lozanoi was significantly higher than that of P. minutus of the same mean length, which makes the former more sensitive to sound frequency. The S:O ratios of these two sympatric fishes were lower than the S:O ratios of demersal and pelagic fishes available from the literature. An interspecific growth difference of sagitta and sulcus acusticus was observed. The sagitta and sulcus acusticus of P. minutus are larger than those of P. lozanoi of the same length.     

Trophic ecology and fecundity of Fundulus heteroclitus in Chezzetcook Inlet,Nova Scotia

The ontogenetic and seasonal dietary shifts of the mummichog Fundulus heteroclitus (L.), from eelgrass (Zostera marina L.) beds off the shore of Halifax County, Nova Scotia, Canada were examined over a 5 mo period in 1980. Switches to abundant, stationary, or high-calorie prey types were observed in the foraging period. Smaller fish generally took less diverse prey. The indices of foregut fullness varied according to body length and season. The successive 10 mm total-length classes of the mummichog (14 to 101 mm TL) consumed whole prey of increasing size. In comparison with populations living further south, the Nova Scotia fish had a shorter spawning period, lower fecundity, and smaller eggs.     

Seasonal variation in lipid class and fatty acid composition of two small copepods in Balsfjorden,northern Norway

Zooplankton were collected at Svartnes, northern Norway between 1985 and 1986. The lipid class and fatty acid composition of a subarctic population ofPseudocalanus acuspes (Giesbrecht, 1881) andAcartia longiremis Lilljeborg are described for four seasons of the year. The results are discussed in relation to the seasonal variation in abundance and dry weight of the predominant developmental stages. The lipid composition of these small copepod species resembles that of omnivorous or carnivorous species.P. acuspes andA. longiremis overwinter in a state of arrested development, but seem to feed actively throughout the year. This behaviour resembles that of the omnivoreMetridia longa (Lubbock), but is in contrast to that of the herbivoreCalanus finmarchicus (Gunnerus) from the same area, which spends the winter in diapause.     

Continuous plankton records: Geographical variations in numerical abundance,biomass and production of euphausiids in the North Atlantic Ocean and the North Sea

Geographical variations in the numbers, biomass and production of euphausiids and the contribution of common species to the total are described from samples taken during 1966 and 1967 in the North Atlantic Ocean and the North Sea by the Continuous Plankton Recorder at 10 m depth. Euphausiids were most abundant in the central and western North Atlantic Ocean and the Norwegian Sea. Thysanoessa longicaudata (Krøyer) was numerically dominant. Biomass was greatest in the Norwegian Sea and the north-eastern North Sea where Meganyctiphanes norvegica (M. Sars) accounted for 81 and 59%, respectively, of the total biomass. Production was highest off Nova Scotia and in Iberian coastal waters; the dominant species were T. raschi (M. Sars) in the former area and Nyctiphanes couchi (Bell) in the latter. The mean P:B ratios were correlated with temperature.     

Trophodynamics and seasonal cycle of the copepod Pseudocalanus acuspes in the Central Baltic Sea (Bornholm Basin): evidence from lipid composition

Seasonal lipid dynamics of the copepod Pseudocalanus acuspes were studied in the Bornholm Basin (Central Baltic Sea) on a monthly basis from March 2002 until March 2003 and were interpreted in light of life cycle strategies and diet selection. The individual total lipid content of females ranged from 0.9 to 1.8 μg, with relative wax ester content reaching a significant maximum in May (44% of total lipids) and minimum (17% of total lipids) in April and November. Significant changes in size, lipid content, lipid classes and fatty acid composition of structural as well as storage lipids suggested five characteristic seasonal phases that were induced by different feeding histories and environmental conditions. Storage lipids were characterized by 18:1(n−9) as major component, which ranged between 44% of total fatty acids in June and 23% in February. The strong coherence between 18:1(n−9) in the seston lipids and the occurrence of ciliates emphasized the importance of ciliates in the diet of P. acuspes. As indicated by changes in the amounts of fatty acid markers, other food sources varied over the year, suggesting an opportunistic feeding behavior. The spring period was characterized by an increase in typical diatom and dinoflagellate markers, whereas other sources, potentially cyanobacteria, became more important during summer. The life cycle strategy is discussed with respect to extant adaptations to high latitudinal habitats.     

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

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

Polymorphism of mannose phosphate isomerase in North Sea and Baltic Sea populations of the amphipods Gammarus zaddachi and G. salinus

Genetic variation at the mannosephosphate isomerase (MPI) locus was examined in the euryhaline sibling species Gammarus zaddachi Sexton and G. salinus Spooner. Both crustacean amphipods share identical enzyme mobilities, following electrophoresis on vertical starch gels. The MPI locus turned out to be highly polymorphic; it is encoded by 6 alleles in G. zaddachi and 7 alleles in G. salinus. Geographic variation of allelic diversity was studied in samples from 9 G. zaddachi and 10 G. salinus populations, primarily obtained from Baltic Sea and North Sea sites. Patterns of inter- and intraspecific heterogeneity are described. Differences in allelic composition exist between Baltic and North Sea samples of G. salinus. In G. zaddachi, levels of polymorphism are higher in North Sea populations than in those from Baltic Sea areas. The significance of these findings is discussed in the light of previous biochemical genetic investigations on the population structure of the two amphipods considered.     

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

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

Life cycle of <Emphasis Type="Italic">Pseudocalanus acuspes</Emphasis> Giesbrecht (Copepoda,Calanoida) in the Central Baltic Sea: II. Reproduction,growth and secondary production

The population dynamics of Pseudocalanus acuspes in the Central Baltic Sea were studied from March 2002 to May 2003 on a monthly basis. All stages were present year round with a stage shift from nauplii to older copepodite stages over the course of the year. Biomass, estimated from prosome length, peaked between May and September with maximum recorded values of 594 and 855 mg C m⁻² in May 2002 and 2003, respectively. Differences in biomass between stations up to a factor of 20 were observed especially in April/May and October. Mean egg production rate (EPR) showed a seasonal course and was highest in April 2002 and 2003 with 3.6 and 2.1 eggs f⁻¹ day⁻¹, respectively, corresponding to a mean weight-specific egg production rate (SEPR) of 0.13 and 0.04. Egg production seems to be limited by food from May on. Stage durations determined from moulting experiments turned out to be extremely long. Maximum growth rates based on stage durations of 15–25 days at 4°C in May and July 2003 amounted for 0.03–0.05 day⁻¹ in CI-CIV. Comparing these rates with rates derived from temperature–development relationships for P. acuspes from the literature resulted in five times higher growth rates for the latter case. Secondary production reached values up to 9.1 mg C m⁻² day⁻¹ (method for continuously reproducing populations) and 10.5 mg C m⁻² day⁻¹ (increment summation).     

Feeding preferences of gammarid amphipods among four species ofFucus

Each of fourFucus species occurring on the Atlantic coast of Canada was presented to two species of gammarid amphipods in no-choice and multiple-choice laboratory experiments. We also measured concentrations of anti-herbivore phlorotannins in each of theFucus species. The study site was Cranberry Cove in Nova Scotia, Canada, and experiments were conducted in 1988. In a multiple-choice experiment,F. vesiculosus was very heavily grazed. Other species were much less affected. In a no-choice experiment,F. evanescens, F. spiralis andF. vesiculosus were eaten at similar rates, whileF. distichus was barely grazed. These results correlate with measurements of phorotannin concentrations in each of the species.F. distichus had much higher concentrations than did the other species (which were not significantly different among themselves). There is no general clear-cut relationship between grazer deterrence and competitive capability within the fucoid guild studied.     

Changing lobster abundance and the destruction of kelp beds by sea urchins

In a study area in Nova Scotia, Canada, abundance of the lobster Homarus americanus decreased by nearly 50% in 14 years. The lobster is a major predator of sea urchins, and during the past 6 years the sea urchin Strongylocentrotus droebachiensis has destroyed 70% of the beds of Laminaria spp. in the area. Implications for management are discussed.     

In situ feeding rhythms of herbivorous copepods,and the effect of starvation

In situ diel variations in gut pigment contents of neritic (Acartia omorii andPseudocalanus minutus) and oceanic copepods (Calanus plumchrus andC. cristatus) were analyzed.A. omorii andP. minutus were sampled in Onagawa Bay on the east coast of Japan in May and August 1987, andC. plumchrus andC. cristatus were sampled in the Bering Sea in June 1986. Gut pigments were generally high at night, and bimodal feeding rhythms were observed in all species. The first peak of gut pigments occurred between sunset and midnight and was followed by a midnight decrease in gut pigment levels, resulting in eventual evacuation of the gut. The second peak was observed a few hours after sunrise. Incubation experiments indicated that ingestion rates of starved copepods were higher than those of acclimated copepods. This phenomenon was most notable at high food concentrations. Gut pigments of starved copepods rapidly increased after exposure to high concentrations of culturedThalassiosira decipiens. These findings suggest that in situ feeding behavior of herbivorous copepods includes periods of cessation or reduction in feeding during the night, and consequently, feeding activity is periodically enhanced with starvation. Starvation enhanced feeding behavior is most obvious in the large oceanic species,C. plumchrus andC. cristatus and is not distinct in small coastal species such asA. omorii.     

Autecology of Fucus distichus ssp. distichus (Phaeophyceae: Fucales) in Nova Scotia,Canada

The autecology of Fucus distichus L. ssp. distichus was investigated at several sites on the Atlantic coast of Nova Scotia, Canada, throughout a 2-year period. This species is confined to high-littoral rock pools at exposed sites, where there appears to be little competition from other organisms. Water temperature, salinity, dissolved oxygen, and hydrogen-ion concentrations underwent both considerable seasonal and short-term variation. Pools with extremes of salinity did not contain F. distichus ssp. distichus. Receptacles are formed during winter, and by late spring they are cast. Sporelings were first visible during September; their development was slow, and these plants did not become reproductive until the second year. During the period February to May, rapid growth of new fronds occurred; an increase in both length and bulk of the plants resulted from this growth. Fronds of mature plants continued to elongate slowly throughout the summer and the early autumn with the formation of receptacles in winter. Plants were frequently damaged, which resulted in regeneration of new fronds from the wounded surface.Issued as NRCC No. 14510     

Biogeographical,biochemical and genetic differentiation among North American saccoglossids (Hemichordata; Enteropneusta; Harrimaniidae)

Populations of two common enteropneusts, Saccoglossus bromophenolosus King et al., 1994 and S. kowalevskii (Agassiz 1873) were sampled between 1991 and 1993 from the southern and northern extremes of their respective ranges: Nova Scotia/Maine and Padilla Bay, Washington for S. bromophenolosus and Maine/South Carolina for S. kowalevskii. Though previously considered a single species, the two taxa were clearly distinct biochemically and genetically. Four of five allozyme loci were diagnostic and indicative of differentiation at the species level. Sequence divergence (27%) of a portion of the mitochondrial 16S rRNA gene suggests that the two taxa have been genetically isolated for a considerable time; hybridization was not evident in sympatric populations. Both taxa contained high concentrations of bromoorganics. The constant association of bromophenols and bromoindoles with S. bromophenolosus and bromopyrroles with S. kowalevskii when they occur sympatrically indicates that bromoorganic contents were genetically and not environmentally determined. Consistent associations between external morphology and bromoorganic contents for additional saccoglossid species support the use of bromoorganics as indices of evolutionary clades in the Enteropneusta.     

Reproduction under predation risk in the sand goby,Pomatoschistus minutes,and the black goby,Gobius niger: the effect of age and longevity

Summary In aquarium experiments, the two marine gobiid fish species Pomatoschistus minutus and Gobius niger were allowed to build nests and to spawn in the presence and absence of a predator (cod, Gadus morhua); behind a glass wall the predator was kept where it could be clearly seen by the gobies and vice versa. P. minutus showed no difference in number of nests or number of spawnings in the different treatments; approximately half of the males built nests, and the females spawned in half of those nests. G. niger, on the other hand, responded differently to the simulated predation risk. No nests were built in sight of the predator, whereas in the absence of predators, half of the males built nests and received eggs. The G. niger individuals in this experiment were 2–3 years old. However, when comparing the reproduction of G. niger of different age in the presence of a predator, older individuals (4–5 years) spawned, whereas younger (2–3 years) did not. No difference in vulnerability towards predators was found between equal-sized P. minutus and G. niger. The optimal behavior during the breeding season must depend on prospects of survival, based both on maximal lifespan and vulnerability to predation.     

Solitary and eusocial nests in a population of Augochlorella striata (Provancher) (Hymenoptera; Halictidae) at the northern edge of its range

Summary Augochlorella striata was studied at the northern limit of its range. The study population contained a mixture of solitary and social nest foundresses. Eusocial foundresses produced 1 or 2 workers before switching to a male biased brood. Solitary foundresses produced males first. Cells vacated by eclosed offspring were reused late in summer. A female biased brood resulted from cell reuse in both solitary and eusocial nests. Workers were slightly smaller than their mothers and were sterile although most of them mated. In comparison to published data from a Kansas population of this species, the Nova Scotia population had i) a lower proportion of multiple foundress nests, ii) a smaller worker brood and iii) a briefer period of foraging activity but iv) comparable overall nest productivity.     

Comparison of chlorophyll a and carotenoid pigments as predictors of phytoplankton biomass

Chlorophyll a concentration was compared with carotenoid concentration as a predictor of seasonal changes in phytoplankton biomass within Bedford Basin, Nova Scotia, Canada (1976–1977). For all seasons, predictions of biomass from different measures of chlorophyll a were poor and were not improved when chlorophyll a was measured accurately by chromatography. Chlorophyll a and a carotenoid (fucoxanthin) were highly correlated and equally good predictors of total biomass, but neither was related to changes in peridinin concentration. Correlations between specific carotenoids and diatom or dinoflagellate biomass indicate that carotenoids may be useful to describe changes in biomass composition. For all pigments measured, predictions of biomass were hampered when large dinoflagellate cells were present, which biased estimates of total cell volume. Regardless of species composition or cell density, dinoflagellate biomass contributed on the average 68% of the total cell volume measured each day compared with only 14% for diatoms and 17% for flagellates, the most abundant taxa.