Genetic protein variation in red mullet (Mullus barbatus) and striped red mullet (M. surmuletus) populations from the Mediterranean Sea

Starch-gel electrophoresis of allozymes was used to differentiate the two red mullet species (Mullus barbatus L. and M. surmuletus L.) in the Mediterranean Sea and, further, to investigate the genetic stock structure of M. barbatus in the eastern Mediterranean area. Twenty putative enzyme-coding loci were examined in eight M. barbatus samples caught in the Aegean and Ionian Seas (Greece) and in the Gulf of Lion (France), and two M. surmuletus samples caught in the Aegean and Gulf of Lion. A high degree of genetic polymorphism was found in both species. Species-specific electrophoretic patterns were found in PGI* and PGM*. Estimates of variance of allele frequencies among samples (F _ST) and ² analyses both revealed significant differences (P < 0.05) among the M. barbatus samples. Most of the genetic variation was among samples regardless of region. The mean value of Nei's genetic distance between the two species was 0.329. Genetic distance among M. barbatus samples was low (maximum Nei's D = 0.012), with the sample from Platania differing most from other M. barbatus samples. This is probably be due to founder effects existing at this area. These results suggest that allozyme analysis may provide important information on the genetic structure of the red mullet to ensure sustainable management of this species. Received: 7 May 1997 / Accepted: 13 October 1997     

Polycyclic aromatic hydrocarbons in red mullet (Mullus bavbatus) and sediments from the Eastern Aegean Sea

The distribution of polycyclic aromatic hydrocarbons (PAHs) was investigated in red mullet (Mullus barbatus) and sediment samples in the eastern Aegean Sea. The data were collected during cruises in July and November 1994, in the framework of National Marine Measurement and Monitoring Programme in the Aegean Sea. PAHs were detected in the tissue of Mullus barbatus of different sex and size from nine sampling sites throughout the study area ranging from 0.03 to 0.46 ug/g (fresh weight). The observations show the high levels of PAHs were found in tissues of females. The average concentration of PAHs in Aegean Sea sediments ranged from 0.20 to 1.8 ug/g (dry weight). No significant correlation was observed between PAH and sedimentary organic carbon.     

Life history of the red mullet Mullus surmuletus from the bottom-trawl fishery off the Island of Majorca (north-west Mediterranean)

The red mullet Mullus surmuletus is one of the main target species of the trawling fishery along the continental shelf off the Island of Majorca. The size distribution of the catches, and the reproduction, age and growth of this species have been studied based on sampling carried out from 1990 to 1992. The length range of the catches was between 10 and 32 cm, with a main distribution between 15 and 20 cm, but this included small specimens (recruits of 10 to 11 cm) from August. In the >19 cm length-class, females clearly dominated. Monthly variations in the gonadosomatic index (GSI) and in the percentage of mature specimens showed that males spawn from December to June, whereas the reproductive activity of the females centres around spring. Fifty percent of males and females mature at 15 and 16.8 cm, respectively, corresponding to 1 yr of age. Otolith age-readings indicate that the population exploited in the trawl fishery consists of six age-groups, including a very high proportion of individuals between 0 and 4 yr old. Population growth curves revealed that females grow comparatively slowly over a longer period of time and attain greater asymptotic sizes than males. The growth parameters for the whole population are: asymptotic length, L =31.28 cm; growth coefficient, K=0.211 yr^-1; theoretical age when length is zero, t ₀=-2.348 yr.     

Assessment of genetic variation in lucerne (Medicago sativa L.) using protease inhibitor activities and RAPD markers

The aim of the present study is to identify and characterize lucerne lines resistance to weevil infestation. After three years of field screening for resistance to weevil infestation, 13 lines of lucerne were selected to assess the genotypic variations for lucerne weevil (Hypera postica Gyll.) at biochemical and molecular levels. Total phenols varied from 0.15 to 0.91 mg g (DM) in these genotypes. The highest trypsin (11.11 unit mg(-1) protein) and chymotrypsin (93.0 unit mg(-1) protein) inhibitors activities were recorded in G-1-02 and B-4-03 lines respectively, whereas highest alpha-amylases inhibitor activity (14.2 unit mg(-1) protein) in C-6-01. Zymogram patterns for trypsin inhibitor activity showed quantitative variations among the lines. In total 262 DNA fragments were generated when 45 deca-mer random primers were employed. Genetic variation in terms of genetic distance ranged from 0.65 to 0.85. Sequential Agglomerative Hierarchical and Nested (SAHN) clustering using the Un-weighted Pair Group Method with Arithmetic mean (UPGMA) algorithm yielded two clusters (cluster I and II) which converged at 72% similarity level. Cluster I contained most of the lines having low level of weevil infestation. High bootstrap values (>40) indicated the significance of nodes embodied in these two clusters. However, SDS-PAGE analysis of the leaf proteins of these 13 lines showed no major variations except minor difference in the protein bands of molecular weights between 14 to 20 kD.     

Genetic variation in Symbiodinium isolates from giant clams based on random-amplified-polymorphic DNA (RAPD) patterns

We have compared the random-amplified-polymorphic DNA (RAPD) patterns of Symbiodinium isolates from seven species of giant clams to investigate the large genetic variation that we previously reported for this group of dinoflagellate symbionts using allozyme analysis. Comparisons of 163 RAPD characters by unweighted pair-group arithmetic-average cluster analysis (UPGMA) corroborate our previous findings that giant clams associate with a large number of genetically distinguishable algal symbionts, and that the isolates from a single Tridacna gigas individual form a group of closely related algae. However, the overall topology of the UPGMA tree constructed from RAPD data differs from that of the previous allozyme data, indicating that the combined data we have collected to date are insufficient to accurately infer phylogenetic affiliations between the isolates studied. Comparisons of our data set with those published for strains of Gymnodinium catenatum, a toxic dinoflagellate with a sexual life stage, shows that our isolates are even more diverse. Algal isolates from giant clams have a level of RAPD variation comparable to organisms that are able to undergo sexual recombination. This study demonstrates the sensitivity of the RAPD technique in detecting genetic diversity in this group of algae, and highlights the need for more comparative data for the major clades of Symbiodinium. Received: 24 August 1999 / Accepted: 8 March 2000     

Analysis of genetic structure of blacklip abalone (Haliotis rubra) populations using RAPD, minisatellite and microsatellite markers

 We investigated the utility of three polymerase chain-reaction (PCR)-based DNA molecular markers in analysing genetic structure of the populations of the blacklip abalone Haliotis rubra (Leach) of Victoria, Australia. The DNA markers included 84 randomly amplified polymorphic DNA (RAPD) bands amplified using six random primers, two minisatellites, GHR (putative growth-hormone-gene-repeat) and MIPR (putative mollusca-insulin-like peptide-gene-repeat), and three microsatellites, RUBGT1 [containing (GT)_n repeats], RUBCA1 [containing (CA)_n repeats] and RUBGACA1 [containing (GACA)_n repeats]. All three types of DNA markers revealed significant subdivision in the H. rubra populations along the coastline. This is postulated as being related to the abalone's relatively short pelagic period and limited dispersion. Further analysis revealed that a Point Cook population sampled from within the semi-enclosed Port Phillip Bay was distinct from two other central zone populations (Apollo Bay and Cape Schanck). The genotypes of microsatellites indicated excessive homozygotes across all the populations at all three microsatellite loci, and possible causes such as larval recruitment pattern and asynchronous spawning are discussed. The excessive homozygotes recorded for the three microsatellite loci contrast with those observed in the minisatellite loci GHR and MIPR, the heterozygosities of which were at Hardy–Weinberg equilibrium. Received: 17 March 1999 / Accepted: 24 November 1999     

Allozyme variation in global populations of striped mullet, Mugil cephalus (Pisces: Mugilidae)

The striped mullet, Mugil cephalus Linnaeus, 1758, is one of the few species of marine shore fish with a worldwide circumtropical distribution. Because of this distribution and the dependency of M. cephalus on coastal waters during various phases of its life cycle, as well as nearshore living habits, questions have been raised regarding levels of genetic divergence and gene flow among transoceanic populations. To cast more light on this, allozyme variation at 27 presumptive gene loci was investigated in ten globally diverse populations. The observed heterozygosity ranged from 0.018 (Hawaii) to 0.081 (Florida), averaging 0.050. The proportion of polymorphic loci showed a similar trend. Several populations were characterised by fixed allelic differences. Estimated gene diversities were very high, the allele frequency variation among populations was found to be 68%; genetic distances reached 0.242, with an average of 0.117. Estimated rates of gene flow were high among Mediterranean populations (Nm = 7.26), and between Mediterranean and East Atlantic populations (Nm= 2.86), but extremely low between non-contiguous populations within the Indian, Pacific and Atlantic Oceans, where Nm ranged from 0.03 to 0.05. Received: 7 April 1997 / Accepted: 4 February 1998     

Analysis of the genetic structure of European eel (Anguilla anguilla) using microsatellite DNA and mtDNA markers

The spawning population of European eel (Anguilla anguilla L.) has been considered panmictic on the basis of genetic markers and morphometric studies. This hypothesis was tested by screening glass eel from five locations (Ireland, Italy, Morocco, Sweden and U.K.), belonging to two cohorts at the cytochrome b (cyt b) locus (392 bp) of the mitochondrion and at five nuclear microsatellite loci. Seventeen cyt b haplotypes were detected, of which ten were singletons; the most common haplotype occurred in 47% of all fish. Haplotype number increased significantly with latitude. Phylogeographical structure based on the cytoplasmic marker was weak (F_ST=0.014) and non-significant. Close similarity was revealed between British and Irish glass eel populations, and weak differentiation among the British/Irish, Atlantic Moroccan, Italian and Swedish Baltic populations, respectively. No hierarchical genetic structure was obvious. Levels of genetic variation detected with five microsatellites were much higher levels than found with allozymes in previous studies (mean number of alleles per locus=11.1; mean expected heterozygosity=0.68). Overall among-population microsatellite variance was low but significant (F_ST=0.004), and caused by the linked microsatellite loci Aan03 and Aan04. The Hardy-Weinberg-Castle equilibrium and the absence of gametic disequilibria at these loci in the Moroccan population might point to its genetic isolation, although the impact of just two out of five loci is puzzling. Given the weak differentiation typical for marine species and the limitations of our data, the results should be interpreted with caution. However, combined with recent evidence from a related study, the paradigm that the European eel constitutes a panmictic population becomes difficult to maintain.     

Mitochondrial DNA variation in Eritrean hamadryas baboons (Papio hamadryas hamadryas): life history influences population genetic structure

The hamadryas baboon, Papio hamadryas hamadryas, represents a rare exception from the pattern of female philopatry and male-biased dispersal predominant in mammals including primates. To elucidate the possible consequences of the dispersal pattern on the population genetic structure of hamadryas baboons, we sequenced the maternally transmitted mitochondrial hypervariable region I of 74 individuals from ten sampling locations in different ecogeographic zones of Eritrea. To this end, individual fecal samples were collected at sleeping cliffs. Upon comparing the individual sequences by means of phylogenetic tree reconstructions and AMOVA, we could not detect a population genetic structure corresponding to a geographic pattern. Tree reconstructions revealed the existence of two profoundly different lineages both present at most of the sampling locations. These findings and Mantel correlations of genetic distances and the frequency of shared haplotypes to geographic distances point to the presence of female dispersal. Female-mediated gene flow is detectable over geographic distances exceeding those between neighboring subpopulations. Our study therefore corroborates local behavioral observations on a broad geographic scale. After inclusion of geographically closely situated olive baboons, P. h. anubis, in the analyses, all anubis sequences fell within one hamadryas clade. Possible scenarios leading to this situation including long-term hybridization processes are discussed.     

Transferability of STS markers in studying genetic relationships of marvel grass (Dichanthium annulatum)

Transferability of sequence-tagged-sites (STS) markers was assessed for genetic relationships study among accessions of marvel grass (Dichanthium annulatum Forsk.). In total, 17 STS primers of Stylosanthes origin were tested for their reactivity with thirty accessions of Dichanthium annulatum. Of these, 14 (82.4%) reacted and a total 106 (84 polymorphic) bands were scored. The number of bands generated by individual primer pairs ranged from 4 to 11 with an average of 7.57 bands, whereas polymorphic bands ranged from 4 to 9 with an average of 6.0 bands accounts to an average polymorphism of 80.1%. Polymorphic information content (PIC) ranged from 0.222 to 0.499 and marker index (MI) from 1.33 to 4.49. Utilizing Dice coefficient of genetic similarity dendrogram was generated through un-weighted pairgroup method with arithmetic mean (UPGMA) algorithm. Further, clustering through sequential agglomerative hierarchical and nested (SAHN) method resulted three main clusters constituted all accessions except IGBANG-D-2. Though there was intermixing of few accessions of one agro-climatic region to another, largely groupings of accessions were with their regions of collections. Bootstrap analysis at 1000 scale also showed large number of nodes (11 to 17) having strong clustering (> 50). Thus, results demonstrate the utility of STS markers of Stylosanthes in studying the genetic relationships among accessions of Dichanthium.     

Temporal genetic variation in subpopulations of bicolor damselfish (Stegastes partitus) inhabiting coral reefs in the Florida Keys

In 1986 we observed significant genetic heterogeneity among samples of bicolor damselfish (Stegastes partitus) from geographically proximate coral reefs in the Florida Keys. This result was in contrast to results of virtually all previous studies of coral reef fishes. To assess temporal stability of localized genetic differentiation, we resampled reefs in 1989. Within approximately two generations, allele frequencies at the most differentiated locus,ACO-1 ^*, had changed by as much as 0.65, resulting in almost complete homogeneity between previously differentiated subpopulations. Estimates of 10.78 migrants per generation suggested that high gene flow is the most likely factor responsible for the significant change in allele frequencies. We attribute the original genetic differentiation to a population bottleneck, possibly caused by environmental perturbations such as hurricanes and consequent genetic drift. For reef fishes with pelagic egg and/or larval stages, a growing body of data suggests that: (1) populations are geographically extensive gene pools, and (2) rates of dispersal appear to be high enough to allow for continual repopulation of isolated and/or perturbed coral reef communities.     

Population genetic structure of the prosobranch Nassarius reticulatus (L.) in a ria seascape (NW Iberian Peninsula) as revealed by RAPD analysis

Randomly amplified polymorphic DNA (RAPD) banding patterns were compared between samples of the netted dogwhelk Nassarius reticulatus from 11 locations along the NW Iberian Peninsula coast. To detect if rias (estuaries formed by drowned river valleys) might promote genetic differentiation, five sampling sites were located within a ria (ria of Muros) and the remaining six were scattered along open-coast areas at increasing distances from the ria mouth. Population differentiation statistics (Φ-values) were estimated using a hierarchical analysis of molecular variance (AMOVA) with samples sorted into two groups: open-coast and ria populations. Despite a high potential to disperse, AMOVA demonstrated a modest, statistically significant genetic heterogeneity among N. reticulatus populations. Most of the genetic structure resided in differences among open-coast populations; ria populations were genetically homogeneous. No obvious geographical pattern was detected for the pairwise genetic distances (non-metric multidimensional scaling; UPGMA tree; Mantel test). Unlike previous studies with other species at a variety of estuarine systems other than rias, there was no evidence that the ria of Muros may enhance the genetic divergence of N. reticulatus populations. This discrepancy is discussed in relation to the biological features of the species (high dispersal potential and a preference for mid-low estuarine habitat) and the strong hydrographic connectivity between ria and neighbouring off-shore waters.     

Genetic structure of natural populations of California red abalone (<Emphasis Type="Italic">Haliotis rufescens</Emphasis>) using multiple genetic markers

Mitochondrial cytochrome oxidase subunit one (COI) sequence, nuclear microsatellites, and amplified fragment length polymorphisms (AFLPs) were used to evaluate connectivity among nine red abalone (Haliotis rufescens) populations sampled between August 1998 and November 2003 along approximately 1,300 km of California coastline from Crescent City (41°46′N, 124°12′W) to San Miguel Island (34°02′N, 120°22′W). COI sequences and microsatellite genotypes did not show significant genetic divergence among nine sampled populations. A subset of five populations spanning the geographic range of the study was scored for 163 polymorphic AFLP markers. Of these, 41 loci showed significant divergence (P < 0.001) among populations. Still, no AFLP markers were diagnostic for any of the study populations, and assignment tests did not consistently assign individuals to the correct population. Although the AFLP data are the first to suggest there is significant genetic differentiation among California red abalone populations, the discordance between the different genetic markers needs further study before unambiguous conclusions can be drawn with respect to connectivity among the populations. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Allozyme and mitochondrial DNA variation in Cuban populations of the shrimp Farfantepenaeus notialis (Crustacea: Decapoda)

We investigated the genetic diversity among populations of the shrimp Farfantepenaeus notialis, the most abundant penaeid species around Cuba. A total of 25 allozyme loci were analyzed in samples of shrimps from seven localities at the south central platform of the island (Ana María Gulf). Samples from three of these localities and from Batabanó Gulf and Guacanayabo Gulf at the south west and south east platforms of the island, respectively, were also characterized at the mtDNA level through sequence variation of a 2027 bp segment including part of the COI and COIII genes. Of the 25 allozyme loci studied 9 were polymorphic: Akp2, Akp3, AmyB, Est3, Gdh, GP7, and Per1, 2 and 3. In contrast to mtDNA, the pattern of allozyme variation among localities revealed strong population structuring at Ana María Gulf, with significant F _st in all pairwise comparisons. The magnitude of F _st estimates as well as the grouping pattern obtained by a UPGMA analysis based on a distance matrix indicated that the level of differentiation was concordant with the geographical position of the localities and the hydrographic regime. Homogeneity of mtDNA suggested that differentiation of allozyme loci might be due to more recent events rather than historical isolation of the sampled populations. Ana María and Guacanayabo Gulf populations were differentiated by mtDNA from Batabanó Gulf, at the southwestern end of the island. The analysis showed three restriction site differences among them, suggesting genetic isolation of the two regions. The present results also suggest that an artificial introduction of larvae from Tunas de Zaza into Batabanó Gulf, in an effort to repopulate this fishing region, may have been ineffective. Received: 13 December 1999 / Accepted: 2 October 2000     

Total and bioavailable metals in two contrasting mining regions (Sudbury in Canada and Lubumbashi in DR-Congo): relation to genetic variation in plant populations

Knowledge of total and bioavailable metal contents in soil is important for regional risk assessment and management. The main objective of the present study was to analyse the concentrations of metals in soils in two contrasting mining regions (Sudbury in Canada and Lubumbashi in DR-Congo). Genetic variation of plant populations was investigated to assess the potential impact of metal contamination on forest sustainability. The levels of copper, cobalt, zinc, arsenic, and lead were significantly higher (up to 200-fold) in mining sites in the Lubumbashi compared to the most highly contaminated Sudbury sites. The nickel content in soil samples from Lubumbashi was much lower compared to Sudbury region samples. Only 3.5% and 4 % of total copper and nickel, respectively, were phytoavailable, with values of 6%, 5.7%, 3.6%, and 5.4% for cobalt, magnesium, manganese, and zinc, respectively. There were significant positive correlations between total metal and phytoavailable metal concentrations for copper (r=0.99), nickel (r=0.86), cobalt (r=0.72), strontium (r=0.71), and zinc (r=0.66). Although genetic variation was high in Picea glauca populations from the Sudbury region, no association was found between metal contamination levels and genetic variation within and among the P. glauca populations.     

Population structure of red drum (Sciaenops ocellatus) in the northern Gulf of Mexico, as inferred from variation in nuclear-encoded microsatellites

Allelic variation at eight nuclear-encoded microsatellites was assayed among 967 red drum (Sciaenops ocellatus) sampled from four consecutive cohorts at seven geographic localities (=28 samples total) in the northern Gulf of Mexico (Gulf). Number of alleles per microsatellite ranged from 6 to 21; average direct-count heterozygosity values per sample (-SE) ranged from 0.560ǂ.018 to 0.903ǂ.009. Tests of Hardy-Weinberg equilibrium revealed significant departures from expected genotype proportions at one microsatellite, which was omitted from further analysis. Tests of genotypic equilibrium indicated that genotypes between pairs of microsatellites were randomly associated. Homogeneity tests of allele distributions across cohorts within localities were non-significant following correction for multiple tests executed simultaneously, and results from molecular analysis of variance indicated that the genetic variance component attributable to variation among cohorts did not differ significantly from zero. Homogeneity tests of allele distributions among localities (cohorts pooled) revealed significant differences both before and after correction for multiple tests. Neighbor-joining clustering of a pairwise matrix of Š values (an unbiased estimator of F_ST), spatial autocorrelations, and regression analysis revealed a pattern of isolation by distance, where genetic divergence among geographic samples increases with geographic distance between sample localities. The pattern and degree of temporal and spatial divergence in the nuclear-encoded microsatellites paralleled almost exactly those of mitochondrial (mt) DNA, as determined in a prior study. Stability of both microsatellite and mtDNA allele distributions within localities indicates that the small but significant genetic divergence among geographic samples represents true signal and that overlapping populations of red drum in the northern Gulf may be influenced by independent population dynamics. The degree of genetic divergence in microsatellites and mtDNA is virtually identical, indicating that genetic effective size of microsatellites and mtDNA in red drum are the same. This, in turn, suggests either that gene flow in red drum in the northern Gulf could be biased sexually or that red drum populations may not be in equilibrium between genetic drift and migration. If a sexual bias exists, the observation that divergence in mtDNA is considerably less than 4 times that of microsatellites could suggest female-mediated dispersal and/or male philopatry. The observed isolation-by-distance effect indicates a practical limit to dispersal. Approximate estimates of geographic neighborhood size suggest the limit is in the range 700-900 km. Although the genetic studies of red drum indicate significant genetic divergence across the northern Gulf, the genetic differences do not delimit specific populations or stocks with fixed geographic boundaries.     

Mitochondrial DNA variation in king mackerel (Scomberomorus cavalla) from the western Atlantic Ocean and Gulf of Mexico

King mackerel (Scomberomorus cavalla Cuvier) collected in 1992 and 1993 from 13 localities along the Atlantic coast of the southeastern USA and in the northern Gulf of Mexico were surveyed for variation in mitochondrial (mt)DNA and a nuclear-encoded dipeptidase locus (PEPA-2). Both polymorphic and fixed mtDNA restriction sites were identified and mapped using conventional and polymerase chain-reaction (PCR)-based methods. Heterogeneity in mtDNA haplotype frequencies was found only in comparisons of pooled haplotypes from Atlantic localities versus pooled haplotypes from Gulf localities. This finding indicates weak genetic divergence between king mackerel from the Atlantic and those from the Gulf. Frequencies of two PEPA-2 alleles essentially paralleled previous findings: one allele (PEPA-2a) was common among samples from western Gulf localities, whereas the other allele (PEPA-2b) was common among samples from Atlantic and eastern Gulf localities. There was considerable variation in PEPA-2 allele frequencies within broadly-defined regions. Variation in mtDNA haplotypes and PEPA-2 genotypes was independent, as was variation in mtDNA haplotypes with sex or age of individuals. Variation in PEPA-2 genotypes was not independent of sex or age of individuals. The latter result suggests that frequencies of PEPA-2 alleles in samples of king mackerel may stem, in part, from sex and age distributions of individuals within samples, and indicates that caution should be exercised in using allelic variation at PEPA-2 as a measure of population (stock) structure in king mackerel. The discordance in spatial patterning of mtDNA haplotypes versus PEPA-2 alleles across the Gulf (i.e. homogeneity in mtDNA haplotype frequencies versus heterogeneity in PEPA-2 allele frequencies) may be due to either female excess at several localities, sex-biased migration, or both. Observed patterns of genetic variation also are consistent with the hypothesis that king mackerel in the western Atlantic may have been subdivided during Pleistocene glaciation, and that the current distribution of PEPA-2 alleles may be a historical artefact. Received: 17 December 1996 / Accepted: 2 April 1997     

Mytilus galloprovincialis as bioindicator in embryotoxicity testing to evaluate sediment quality in the lagoon of Venice (Italy)

The possibility of using Mytilus galloprovincialis from natural populations of the lagoon of Venice in toxicity bioassays based on embryo development, according to international standard methods, was investigated. In order to valuate this method for the lagoon, iterative steps including evaluation of reproducibility, sensitivity and discriminatory capacity towards some pure substances and environmental samples were performed. Evaluation of sensitivity towards a reference toxicant (copper) evidenced good replicability and repeatability. Results showed that natural population is better than hatchery population for performing bioassays. Evaluation of applicability to elutriates revealed that the method is able to discriminate among sediments of the lagoon of Venice at different typologies and levels of contamination, showing an important effect for almost all study sites.     

Diel variation of bacterial productivity in seagrass (Zostera capricorni) beds measured by rate of thymidine incorporation into DNA

Diurnal variations occurred in bacterial growth rates in the sediment and water column associated with seagrass (mainly Zostera capricorni Aschers) beds in Moreton Bay, Queensland, Australia. Studies were carried out in March and June 1981. Cell production rates increased by 5- to 10-fold during the morning and decreased during the afternoon. No nocturnal variation was observed. Daily bacterial cell production rate in the aerobic zones of the seagrass bed was estimated to be 43 mg C m^-2. A minimum of 100 mg C m^-2 d^-1 would be required to support the bacterial production. This represents about 10% of net primary production. The incorporation of tritiated thymidine into DNA was used to measure bacterial growth. The validity of the method is discussed.