Genetic structure of asexually reproducing Enteromorpha linza (Ulvales: Chlorophyta) in Long Island Sound

Heterotrophic bacterial uptake and turnover of glycolate were measured in the water column of the New York Bight apex during four seasonal cruises over almost a one-year period between May 1977 and March 1978. Glycolate turnover was most rapid in May and July, when primary productivity and estuarine runoff were high. Extremely rapid glycolate turnover times (<1 h) were detected at some stations during these months. Increased bacterial glycolate tunover did not accompany increased primary productivity in March, when water temperatures were lowest. Glycolate flux calculations indicate that the measured rates of glycolate consumption by bacteria always exceeded estimated glycolate production by phytoplankton, except in March. This excess may reflect an underestimation of phytoplankton production or the input of glycolate from other sources, such as estuarine runoff. Glycolate utilization appears to be seasonally important to bacteria in the New York Bight apex, coinciding with fluctuations in phytoplankton primary productivity.     

A Possible Relationship Between Enteromorpha (Chlorophyta) and Melosira nummuloides (Chrysophyta: Bacillariales)

While investigating the effect of fluoride on the green alga, Enteromorpha, healthy chains of the centric diatom, Melosira nummuloides appeared in cultures containing a small piece of Enteromorpha, nutrient medium and all concentrations of fluoride, as Na₂SiF₆ to 160 p.p.m. Both the Melosira alone and the Enteromorpha, accompanied by Melosira, were transferred into a culture medium (germ intercepting filtrate, S=30g./kg. to which 0.4 ml./1. of the plant nutrient 'Kerigrow' was added) with a normal fluoride content; after three weeks the Melosira alone was present only in short, broken chains, whereas that in the presence of Enteromorpha had formed long, healthy chains.     

Adaptive phenotypic differentiation across the intertidal gradient in the alga Silvetia compressa

Populations of intertidal species span a steep environmental gradient driven by differences in emersion time. In spite of strong differential selection on traits related to this gradient, the small spatial scale over which differences occur may prevent local adaptation, and instead may favor a single intermediate phenotype, or nongenetic mechanisms of differentiation. Here I examine whether a common macroalga, Silvetia compressa, exhibits phenotypic differentiation across the intertidal gradient and evaluate how local adaptation, developmental plasticity, and maternal effects may interact to shape individual phenotypes. Reciprocal transplants of both adults and embryos showed a "home-height advantage" in two of the three populations tested. In laboratory trials, the progeny of upper-limit individuals survived exposure to air significantly better than lower-limit progeny from the same population. I compared the emersion tolerance of full-sib families generated from gametes produced in the field to those produced under common garden conditions. The relative advantage of upper-limit lineages was robust to maternal environment during gametogenesis; this pattern is consistent with genetic differentiation. The possible role of local adaptation has historically been ignored in studies of intertidal zonation. In S. compressa, phenotypic differentiation may have important consequences for vertical range, both within and among sites.     

Effect of temperature on nitrogen-limited growth rate and chemical composition of Ulva curvata (Ulvales: Chlorophyta)

Ulva curvata (Kutz.) de Toni growing in shallow estuaries experiences a highly variable supply of dissolved inorganic N, which can limit growth rates. The effects of N supply variability and annual temperature and light variation on growth rates and chemical composition were assessed in plants grown in outdoor tanks supplied with running seawater and either pulsed or continuous N additions for 8 d. Environmental variables were measured every 12 h, growth rates every 2 d, and plant nitrogenous constituents every 4 d. The experiment was repeated 7 times over the course of a year. The slopes of growth rate-tissue N and growth rate-N supply plots increased with temperature and varied at periods at least as short as 2 d. Temperature explained 44% of variation in growth rate, and temperature, light, and N supply together explained 53% of variation in growth rate. N-limitation occurred only above 20°C, the approximate temperature for the maximum growth rate. Growth rate and tissue N were not affected by N supply frequency. Tissue N, ribulose-bisphosphate carboxylase activity, and chlorophyll contents were correlated with each other and with environmental variables but not with growth rate. U. curvata thus does not maintain balanced growth in the strict sense, but rather stores nutrients supplied as pulses for use when they become growth-limiting. Since the relative importance of any single growth-limiting factor is highly dynamic, assessing the importance of multiple limiting factors requires longterm, high-frequency sampling of principal limiting factors and seaweed growth rates.     

Genetic differentiation of bicolor damselfish (Eupomacentrus partitus) populations in the Florida Keys

Electrophoretic variation in proteins encoded by 23 loci revealed substantial genetic differentiation among populations of bicolor damselfish (Eupomacentrus partitus) collected from four coral reefs in the Florida Keys, USA, during 1986–1988. Genetic differentiation was concentrated between a sample collected from Little Grecian Rocks Reef (LGR) and the remaining samples, including fish from a reef only 600 m distant (Grecian Rocks Reef). Genetic distinction of the LGR sample derived from significantly heterogenous allelic frequencies at six of eight polymorphic loci. Aco-1 (aconitase); Ada (adenosine deaminase); Gpi-2 (glucosephosphate isomerase); Ldh-2 and Ldh-3 (lactate dehydrogenase); and Me-1 (malic enzyme); nevertheless, differentiation at cytosolic aconitase (Aco-1) far exceeded that observed for other loci (fixation index, F _ST=0.482), and differences in Aco-1 allele frequencies were largely responsible for large genetic distances (0.20) between LGR and the other reefs. Paradoxically, estimates of numbers of migrants exchanged between reefs per generation (mN _e=17.47) indicated the potential for extensive gene flow. The extent of genetic differentiation among these populations is evaluated relative to models of population genetic structure based on equilibrium between gene flow and natural selection or genetic drift.     

Genetic differentiation independent of intertidal gradients in the pulmonate limpet Siphonaria kurracheensis

The intertidal limpet Siphonaria kurracheensis (Reeve, 1856) has a bimodal vertical distribution of abundance on rocky shores at Rottnest Island, Western Australia. An electrophoretic study of 5 polymorphic enzymes revealed no consistent genetic differences between adults high and low on the shore. Contrasting with this absence of a detectable genetic response to the steep environmental gradients in the intertidal zone, there were genetic differences among low-shore adults from different sites, and between adults and recruits. This genetic differentiation could be due to either localized selection or temporal variation in the genetic makeup of recruits.Communicated by G. F. Humphrey, Sydney     

Genetic differentiation of populations of the planktonic copepod Labidocera aestiva

Planktonic populations of the calanoid copepod Labidocera aestiva show significant biochemical genetic heterogeneity along the Atlantic coast of the USA. In summer, 1981, copepods were collected by surface tows at Beaufort Inlet, North Carolina; Fort Pierce Inlet, Florida; and Vineyard Sound, Massachusetts. Genetic variation within each population and genetic differentiation among the three populations were studied by micro-acrylamide gel electrophoresis of six loci encoding four enzymes. All six enzyme loci were polymorphic when all populations were considered together, but the North Carolina population was monomorphic at two loci. High genetic variability was indicated by the following: (1) the number of alleles per locus averaged over all loci was 2.57±0.26 SD; (2) the average proportion of loci for which the frequency of the most common allele was not greater than 0.95 was 0.78±0.10; (3) the frequency of heterozygous individuals was 0.25±0.07. Genetic differentiation among population samples in the three regions was demonstrated in several ways: allele frequencies at one aminopeptidase-I locus, Lap-1, differed significantly among samples of the three populations, and there were unique alleles of high frequency at this locus in two population samples. Values of the statistic of genetic distance (D) averaged 0.20±0.08 for pairwise comparisons between all samples. Compared to expected heterozygosity if individuals across the entire range sampled mated at random, there were highly significant heterozygote deficiencies at five of the six loci. Genetic differentiation of populations of L. aestiva may result from (1) differential selection on populations in the three regions, or (2) restricted gene flow between the populations. Gene flow may be limited by geographic separation or differences in life history, such as seasonal presence in the plankton and diapause egg production.Contribution No. 5810 of Woods Hole Oceanographic Institution     

Genetic differentiation in Littorina saxatilis (Gastropoda)

Littorina saxatilis, an ovoviviparous gastropod, displays considerable morphological variability, perhaps due to limited dispersal abilities. Gene and genotype frequencies of 2 polymorphic enzymatic loci were determined electrophoretically for 7 populations of L. saxatilis, to estimate degree of population differentiation. Results show that significant population differentiation may occur over distances of as little as 2 km, while widely separated populations may be nearly identical. An index of probability of genotypic identity calculated for populations of L. saxatilis shows variable and heterogeneous values. Comparison of this index for L. saxatilis with an index for 12 populations of Nassarius obsoletus —which possesses strong dispersal abilities due to long-term veliger larva and high, homogeneous indices of probability of genotypic identity between populations — indicates a strong correlation between population differentiation and dispersal ability of the organism. Possible factors influencing population differentiation in L. saxatilis are discussed.     

Genetic divergence between colour morphs in populations of the common intertidal sea anemones Actinia equina and A. prasina (Anthozoa: Actiniaria) in the Isle of Man

A distinctive morph ofActinia equina (L.) is found at low frequency among populations on intertidal hard substrata on some shores on the Isle of Man. This morph is red with rows of green oval spots or elongate markings running longitudinally down the column. Starch gel electrophoresis of 21 allozyme loci was used to compare samples of this morph from two localities with other sympatricActinia spp. Collections were made in early summer 1986. For one sampling site the comparison was with redA. equina and greenA. prasina Gosse, whilst at the other site with redA. equina only. At both sites significant genetic differences in allele frequencies at several loci were found between the new morph and sympatricA. equina andA. prasina. The results also confirm the reproductive isolation ofA. prasina. As with much previous work on sea anemones, levels of mean heterozygosity per locus (H) were found to be high (H=0.157 to 0.342). A surprising feature of the results, although bearing out earlier unpublished data, is that high levels of genetic differentiation are found between populations of what appear to be the same morphs collected from shores only a few km apart. Among theActinia spp. studied, differentiation between populations of the same morph on different shores was of the same order as between sympatric, reproductively isolated populations.     

Release of dissolved organic carbon from the estuarine intertidal macroalga Enteromorpha prolifera

The estuarine macroalga Enteromorpha prolifera was collected from Coos Bay, Oregon, USA during 1981, and its release of photosynthate as dissolved organic carbon (DOC) was studied using ¹⁴C as a tracer. During photosynthesis in 30 S sea water, with a fixation rate averaging 7.37 mg C g^-1 dry wt h^-1, release ranged from 0.13 to 0.57 mg C g^-1 dry wt h^-1 and from 1.65 to 6.23% of total fixed carbon. Release of DOC appears to be linear with time over 3 h. As exposed algae become increasingly desiccated, their photosynthetic rates decline dramatically, but upon reimmersion the highly desiccated algae lose a larger fraction of their fixed carbon than the slightly desiccated algae. This loss comes in a pulse release of DOC over the initial 15 min, followed by declining release rates. The pulse loss due to rainfall is 5 times greater than that due to tidal resubmergence, and may briefly exceed the prior photosynthetic rate. Although lowering the salinity from 30 to 5 does not substantially alter photosynthetic rates, it does increase the DOC release range up to 1.02 mg C g^-1 dry wt h^-1 and 16.10% of fixed carbon. Heterotrophic microbes from the algal habitat readily use the available DOC at about 15% h^-1.     

Genetic variability and differentiation of sporophytes and gametophytes in populations of Gelidium arbuscula (Gelidiaceae: Rhodophyta) determined by isozyme electrophoresis

Genetic differentiation and genetic variability of sporophytic and gametophytic populations of Gelidium arbuscula (Bory) from three localities sampled in 1989 and 1990 in the Canary Islands (Spain) were examined by isozyme electrophoresis. Twenty-three to 29 putative alleles corresponding to 22 gene loci, were compared. High deviations in Hardy-Weinberg equilibrium, and significant differences between allelic frequencies of sporophytic and gametophytic subpopulations at the same locality were found, suggesting a predominant asexual reproduction of G. arbuscula. The genetic variability (percentage of polymorphic loci, mean number of alleles per locus and average gene diversity) of haploid subpopulations was lower than that of diploid subpopulations at all three localities, being the lowest described for seaweeds. No correlation between genetic and geographical distance was found. The high genetic differentiation coefficient between all subpopulations suggests a very reduced genetic flow between subpopulations of the same and of different localities. These results suggest that the genetic structure of the populations of G. arbuscula from the Canary Islands is due to a founder-effect combined with a predominance of asexual reproduction. This is the first report comparing allelic frequencies between sporophytic and gametophytic subpopulations of seaweeds.     

Photosynthesis and amelioration of desiccation in the intertidal saccate alga Colpomenia peregrina

Rates of gross photosynthesis for the intertidal saccate alga Colpomenia peregrina (Sauv.) Hamel were determined under submersed and emersed conditions. Maximal photosynthetic rates were lower than for most seaweeds but comparable with other saccate members of the genus. By fitting the data to a hyperbolic tangent function, maximal photosynthetic rates were estimated to be 5.29 mmol CO₂ m^-2 h^-1 under submersed conditions and 2.06 mmol CO₂ m^-2 h^-1 under emersed conditions. I_k for submersed thalli was 69.1 E m^-2 s^-1, wherea for emersed thalli it was 149.0 E m^-2 s^-1, or 2.2 times higher. At low tide in the field and under saturating irradiance, carbon from seawater retained within the thallus cavity was assimilated at 0.9 mmol CO₂ m^-2 h^-1. In the laboratory under emersed conditions, carbon from this source was taken up at 0.6 mmol CO₂ m^-2 h^-1 at 20°C and at 0.34 mmol CO₂ m^-2 h^-1. Retained seawater also greatly reduced drying under desiccating conditions. Experimental thalli from which seawater had been removed lost thallus water continuously throughout the drying period (120 min). On the other hand, control, thalli lost water for the first 15 min, after which no further water loss occurred. At the termination of the experiment, control thalli had lost 7.2% of their water, whereas experimental thalli had lost 39.2%. Desiccation affected the emersed photosynthetic rate of experimental and control thalli. Emersed photosynthetic rates for thalli dried for 15 min were higher than for fullyhydrated thalli. However, emersed photosynthesis of thalli dried for longer than 15 min was lower than fully-hydrated rates and was directly related to percent water loss. Utilizing data from this study, a model was constructed to determine total photosynthetic production of C. peregrina over a single daylight period. From these calculations it was determined that emersed photosynthesis can increase daily photosynthetic production of C. peregrina by 50%.     

Genetic differentiation between populations of Talitrus saltator and Talorchestia deshayesii (Crustacea: Amphipoda) from coastal areas of the north-western European continent

Electrophoretic studies of gene-enzyme variation in the littoral talitrids Talitrus saltator (Montagu) and Talorchestia deshayesii (Audouin) were undertaken to estimate the amount of divergence among geographically separated populations. Samples of both species were taken from sandy beaches over a transect of approximately 3 500 km along the coast of the European continent including Baltic, North Sea and Atlantic locations. A total of 22 T. saltator and 15 T. deshayesii populations were analysed for genetic variation at various enzyme loci. Both amphipods revealed relatively low levels of polymorphism and heterozygosity. Among the loci studied, phosphoglucose isomerase (Pgi) and phosphoglucomutase (Pgm) were highly polymorphic. Patterns of micro- and macrogeographic variation in terms of distributions of allele frequencies at these particular loci are compared. Interpopulation allozymic variation was shown to be lower in T. deshayesii than in T. saltator. As demonstrated by T. saltator populations sampled in coastal sites ranging from Denmark to western France, clinal variation in frequencies of two alleles became evident at the PGI locus, exhibiting a steady increase in the level of polymorphism with decreasing latitudes. It is argued that limited gene flow and, to some extent, random genetic drift may account for the gene pool structure of the talitrid species investigated.     

Genetic diversity and genetic structure in the brown alga Halidrys dioica (Fucales: Cystoseiraceae) in Southern California

Genetic diversity and genetic structure in a population of the brown seaweed Halidrys dioica Gardner were evaluated in five sites in southern California, USA, in 1991, using isozyme electrophoresis. H. dioica is relatively long-lived and has an outcrossing mating system and floating reproductive fronds with the potential for longdistance gamete dispersal. Because these characteristics are hypothetically important in determining genetic diversity and structure, we predicted that genetic diversity would be high and genetic structure would be exhibited only at relatively large geographic scales in H. dioica populations. The data were consistent with the prediction: genetic diversity (% polymorphic loci, no. of alleles/locus, average expected heterozygosity) was high compared to that of other seaweed species. Genetic structure (Wright's F statistics, Nei's genetic distance, point-pattern analysis of alleles) was low within and among distinct rocky reefs over 4 km of coast but high in subpopulations separated by 90 km. Life-history characteristics may be useful predictors of genetic diversity and structure in seaweed populations, but information on selection regimes, long-distance dispersal, and the extent of clonal propagation, for example, are critically lacking.     

Genetic relatedness of worker nestmates in Myrmica ruginodis (Hymenoptera : Formicidae) populations

Summary Genetic relatedness and mating structure of the red ant, Myrmica ruginodis, were studied from a large data set in several natural populations, one of which was a substructured archipelago population. Within-colony relatedness of worker nestmates was measured as genotypic correlation and mating structure as deviation from random mating; both were calculated from the genotype frequency data. The average relatedness of worker nestmates was rather high across populations (0.43–0.73), and random mating was the rule within populations. The data were used to deduce the social structure of populations. With one possible exception, populations turned out to be weakly polygynous, implying that they consisted of the macrogyne form of the species. The exceptional population was relatively more polygynous and inbred, suggesting that it might include the microgyne form. This exceptional population lived in a habitat not noticeably different from that of the other populations, challenging previous suggestions of the habitat specialization of the two forms. The colonies were facultatively polygynous, and a comparison with earlier studies shows that polygyny is functional. Relatedness of worker nestmates varied slightly among populations, indicating that the social structure of Myrmica colonies is flexible.     

Demography of the intertidal brown alga Pelvetia fastigiata in Southern California,USA

The demography of Pelvetia fastigiata was studied from 1973 through 1977 in the intertidal zone near La Jolla, California, USA. Populations were usually small and relatively stable as new recruits replaced older individuals that were lost; however, in 1977 high recruitment substantially increased total abundances. The survivorship of recruits was similar regardless of the season or year of recruitment. Mortality rates decreased with age, and the mean life expectancy continued to increase for at least 1 yr. Losses of juveniles were high; 50% lived 80 d and only 9% survived to the age of first reproduction (about 1.5 yr). Plants present at the start of the study had higher survivorship rates than new recruits, 19 and 7%, respectively, living more than 3 yr. Individuals within aggregations of large plants lived longer than most dispersed individuals, and aggregations remained intact throughout the study in spite of low recruitment. Established individuals had low mortality rates and populations probably relied on these persistent individuals for reproduction.     

Genetic variability in bloodworm (Glycera dibranchiata) populations in the Gulf of Maine

Genetic structure of eight Northwest Atlantic populations of the marine polychaeteGlycera dibranchiata Ehlers was examined with starch gel electrophoresis. Samples were collected during summer and fall 1981, and seven polymorphic and four monomorphic loci were consistently scored. Average heterozygosity (0.126) and percent polymorphic loci (59.3) were comparable to the averages reported for marine invertebrates. Minimum genetic distances between populations ranged from 0.003 to 0.093, levels typically associated with local populations of the same species in other taxa. Based on these data, inter- and intra-estuarine migration and gene flow appear to be low. Only two populations, separated by 13 km along the same river in New Brunswick, Canada, were not genetically different from each other. These findings may have relevance for management strategies in bloodworms.     

Evidence for seasonal mate limitation in populations of an intertidal amphipod, Corophium volutator (Pallas)

Potential rates of reproduction (PRR) differ between the sexes of many animal species. Adult sex ratios together with PRR are expected to determine the operational sex ratio (OSR) defined as the ratio of fertilizable females to sexually active males at any given time. OSR is expected to determine the degree to which one sex competes for another—the limiting sex. We explored the potential for mate limitation in an intertidal amphipod, Corophium volutator (Pallas). Males have higher PRR than females, but males may be limiting because of extreme female-biased sex ratios observed in this species. Consistent with this idea, late season females were less likely to be ovigerous and had smaller size-specific clutches, both of which were associated with seasonal declines in availability of males of reproductive size. Seasonal changes in ovigery could not be explained by seasonal changes across sites in other factors (e.g., female body size or phenology of breeding). Smaller females were less likely to become ovigerous later in the season at three of four sites. Seasonal reductions in clutch size also occurred among small females expected to be reproducing for their first time. In complimentary laboratory experiments, reduced likelihood of ovigery and reduced fecundity occurred when the number of receptive females was increased relative to availability of a reproductively active male. Our results suggest male mate limitation can occur seasonally in this species and that male limitation is regionally widespread and may affect recruitment.     

Effects of wave-induced lightflecks in the intertidal zone on photosynthesis in the macroalgae Postelsia palmaeformis and Hedophyllum sessile (Phaeophyceae)

Intertidal algae are exposed to a highly variable photic regime because of crashing waves. We measured photosynthetic rate of whole fronds of the seaweeds Postelsia palmaeformis (Ruprecht) (at Bodega Marine Laboratory, 1991) and Hedophyllum sessile (Setchell) (Phaeophyceae) (at Friday Harbor Laboratories, 1990) in flashing and steady-state photosynthetically active radiation (PAR) of equal irradiance, using a recirculating metabolism chamber designed to minimize the thickness of diffusional boundary layers on the surfaces of algal tissues. The dimensionless ratio of photosynthetic rate under flecking PAR (P_fleck) and the sum of photosynthetic rate under steady state PAR (P_s) of high and low irradiance was computed for lightfleck periods from 0.2 to 100 s. P_fleck:P_s is a measure of the light-flash utilization-efficiency in flashing light, and was greater than unity at periods between lighflecks of 2 to 30 s, with a peak at 10 s. We used a novel optical fiber irradiance meter to measure PAR incident on fronds of P. palmaeformis as they were washed about by waves breaking in the intertidal zone, and compared the light records to that obtained by a stationary sensor under the canopy. PAR flashing period was closely correlated with the period of breaking waves in stands of P. palmaeformis. We estimated the seasonal spectrum of period between light flashes in stands of this species by examining the spectral density of ocean waves at Bodega Marine Laboratory, Bodega Bay, California. The wave spectrum peaks at a period (10 s) where light-flashes may theoretically enhance the light-flash utilization-efficiency of seaweeds. We calculate that the enhanced light-flash utilization-efficiency wrought by wave-induced light-flashes may contribute to significant gains in primary productivity of these macroalgae under some conditions.