Influence of temperature changes on oxygen uptake and ammonia and phosphate excretion,in relation to body size and weight,in Oikopleura dioica (Appendicularia)

The relationship between the rates of oxygen consumption, ammonia and phosphate excretion of a pelagic tunicate, the larvacean Oikopleura dioica Fol, 1872 were assessed as a function of size, dry weight and ash-free dry weight at 15°, 20° and 24°C. O. dioica has higher respiration and excretion rates than copepods of similar weight, but the weight exponent of the allometric power function: Y=aX ^b is similar to that of other poikilotherms. Temperatures above 20°C have a depressing effect on respiration and ammonia excretion. 90% of the variance in metabolic rates is explainable by body mass and temperatures Q₁₀ values for oxygen consumption, ammonia and phosphate excretion, respectively, are 2.45, 1.86 and 1.75 between 15° and 20°C, and 3.75, 2.90 and 3.60 between 20° and 24°C. Metabolic quotients (O:N, O:P, N:P) indicate a protein-oriented diet. The results of this study suggest weak metabolic regulation in O. dioica, an energetic strategy which allows an immediate response to favourable changes in feeding conditions.     

Sites of bioluminescence in the appendicularians Oikopleura dioica and O. labradoriensis (Urochordata: Larvacea)

Although bioluminescence is known in larvacean tunicates, its origin has not been reported. Two species, Oikopleura dioica and O. labradoriensis, from the northeastern Pacific Ocean, were examined during 1979–1982 to determine the sites of luminescence. An appendicularian lives within a secreted, mucous house, and its body is tightly invested by a house rudiment. Mechanical stimulation elicited multiple, summated blue-green flashes from free individuals invested by house rudiments and from empty houses that were virtually free of contamination by exogenous luminescent bacteria or dinoflagellates. Direct microscopic observations showed that the light is produced by clusters of 1 to 2 m fluorescent granules that form intricate, species-specific patterns of inclusions in the house rudiment. These granular inclusions are probably present in the expanded house, where they account for the multiple, point-sources of light observed in flashing houses. Neither fluorescence nor luminescence were observed in any other parts of the house rudiments, expanded houses, or free appendicularians (including the oral glands, which were previously suspected of producing bioluminescent secretions).     

Species-specific grazing rates of heterotrophic dinoflagellates in oceanic waters,measured with a dual-label radioisotope technique

A dual-isotope method was developed to measure grazing rates and food preferences of individual species of heterotrophic dinoflagellates from natural populations, collected from the Slope, Gulf Stream, and Sargasso Sea and from a transect from Iceland to New England, in 1983. The isotope method measures the grazing rates of microzooplankton which cannot be separated in natural populations on the basis of size. Tritiated-thymidine and ¹⁴C-bicarbonate were used to label natural heterotrophic and autotrophic food, respectively. Nine oceanic dinoflagellate species in the genera Protoperidinium, Podolampas, and Diplopsalis fed on both heterotrophic and autotrophic food particles with clearance rates of 0.4 to 8.0 l cell^-1 h^-1, based on ³H incorporation, and 0.0 to 28.3 l cell^-1 h^-1, based on ¹⁴C incorporation. Two dinoflagellate species, Protoperidinium ovatum and Podolampas palmipes, fed only on ³H-labelled food particles. Several species of dinoflagellates fed on bacteria (<1 m) which had been prelabelled with ³H-thymidine. The clearance rates of heterotrophic dinoflagellates and ciliates were similar and within the range of tintinnid ciliate clearance rates reported in the literature. As heterotrophic dinoflagellates and ciliates can have comparable abundances in oceanic waters, we conclude that heterotrophic dinoflagellates may have an equally important impact as microheterotrophic grazers of phytoplankton and bacteria in oceanic waters.Partially supported by a grant from the National Science Foundation, OCE-81-17744     

Temporal and spatial variation in bacterial production in the water column over a coral reef

Production and doubling times of the bacterial populations in the water around and over the reefs at Lizard Island, Great Barrier Reef were measured during summer and winter, 1982 and 1983. Bacterial productivity, determined from the rate of tritiated thymidine incorporation into DNA, was high over the reef flats and a Thalassia hemprichii sand flat (28 to 58 g Cl^-1 d^-1). Bacterial growth rates increased during the day and fell at night over the reef flats and seagrass bed. Growth rates were slower over the reef front and in open water. Doubling times ranged from about 2 d in the open water to about 3 h over the reef flat in summer. As numbers did not increase, grazing was probably intense on the reef flats. Growth rates were much slower in winter. The main source of organic nutrient used by the bacteria was probably mucus released following photosynthesis in the corals. The cyanobacterium Synechococcus sp. was sometimes very numerous, especially in summer when 2×10⁸ cells l^-1 were recorded in one water mass. The number of bacteria was also very high in summer, with values ranging from 1×10⁹ to 2.5×10⁹l^-1.     

Seasonal variations in the densities of fecal pellets produced by Oikopleura vanhoeffeni (C. Larvacea) and Calanus finmarchicus (C. Copepoda)

Two abundant macrozooplankters, Oikopleura vanhoeffeni (Lohmann) and Calanus finmarchicus (Gunnerus) were collected from the coastal waters off Newfoundland in different seasons during 1990–1991 and incubated in natural seawater to collect freshly egested, field produced, fecal pellets. The densities of fecal pellets from O. vanhoeffeni and C. finmarchicus were measured in an isosmotic density gradient. These are the first reported seasonal measurements of fecal pellet densities from two different types of macrozooplankters, O. vanhoeffeni, a larvacean, filter feeder and C. finmarchicus, a crustacean, suspension feeder. Pellet density ranges and medians were significantly different among seasons for both species, depending primarily on the type of phytoplankton ingested and its ability to be compacted. Winter O. vanhoeffeni and fall C. finmarchicus feces filled with nanoplankters and soft bodied organisms had less open space [packing index (% open area) = 3.5 and 4% for O. vanhoeffeni and C. finmarchicus, respectively] and were more dense (1.23 and 1.19 g cm^-3) than spring feces filled with diatoms (packing index = 15 and 23%, density = 1.13 and 1.11 gcm^-3). For copepods, these results contrast with previously published density values and with the predicted copepod fecal pellet density calculated, in the present study, using the conventional mass/volume relationship. Copepod spring and summer diatom-filled feces had a calculated density of 1.12 and 1.24 gcm^-3 vs a measured median density of 1.11 gcm^-3 for both spring and summer feces; the fall feces containing nanoplankters had a calculated density of 1.08 gcm^-3 vs a measured median density of 1.19 gcm^-3. Knowledge of the seasonal variations in fecal pellet densities is important for the development of flux models.     

Saving by freezing? Metabolic rates of<Emphasis Type="Italic"> Adamussium colbecki</Emphasis> in a latitudinal context

Standard metabolic rates of the endemic Antarctic scallop, Adamussium colbecki (Smith, 1902), were measured in austral summer and under simulated winter conditions. Average mass-specific metabolic rates were significantly different between "summer" (151.17±45.06 µl O ₂ g ^-1 h ^-1) and "winter" (106.52±39.65 µl O ₂ g ^-1 h ^-1) animals. The overall metabolic rates of A. colbecki are comparable to those of other Antarctic bivalve species, but well below those of temperate scallop species. Data for 24 scallop populations (13 species) from different latitudes give no evidence for elevated metabolic rates in A. colbecki as suggested by the concept of "metabolic cold adaptation". A world-wide comparison of metabolic rate and overall growth performance of scallops indicates that in the Antarctic scallop the energetic advantage of low basal metabolism does not counterbalance the disadvantage of the prolonged seasonal period of food shortage.     

Degradation of bacteria by Mytilus edulis

The uptake of several species of bacteria by the common mussel Mytilus edulis (L.) and the subsequent fate of some polymers of the bacteria have been investigated in a study carried out during 1981. Bacteria (Escherichia coli, Micrococcus luteus, M. roseus, Bacillus cereus, Staphylococcus aureus and a marine pseudomonad, 1-1-1) were radiolabelled by growth in medium containing ³H-thymidine and the uptake of bacteria by Mytilus edulis was monitored. Labelled and unlabelled bacteria, at initial concentrations of 0.5 to 1x10⁷ bacteria ml^-1, were cleared at similar, exponential rates with no significant difference in the rates for different bacteria: 90% of bacteria were cleared in a mean time of 1.93±0.12 h (SEM, n=63). Those bacteria with cell walls which were sensitive to M. edulis lysozyme were rapidly degraded by the mussel and ³H-labelled DNA was released in a form not precipitable by 10% trichloroacetic acid. Lysozyme-resistant bacteria (Micrococcus roseus and S. aureus) were cleared from suspension by Mytilus edulis but most were rejected intact. By measuring the rate of release of ³H-thymidine-labelled material from the mussel the rate of degradation of lysozyme-sensitive bacteria by M. edulis was found. For different bacteria the degradation rate varied from approx 2x10⁸ to 27x10⁸ bacteria h^-1 with an overall mean of 10x10⁸ bacteria h^-1. A thymidine- and diaminopimelicacid-requiring auxotroph of E. coli was radiolabelled with ³H-thymidine, ³H-diaminopimelic acid or ¹⁴C-glucose and fed to M. edulis. Bacteria were cleared and degraded by the mussel; ³H-diaminopimelic acid-labelled or ¹⁴C-glucose-labelled polymers were retained, whereas ³H-thymidine-labelled polymers were released into the surrounding water. Extracts of the digestive gland of M. edulis degraded lysozyme-sensitive bacteria to release ³H-thymidine-labelled material, but did not release ³H-thymidine-labelled material from lysozyme-resistant bacteria. It is concluded that M. edulis can select lysozymesensitive bacteria for subsequent processing and discriminate between bacterial polymers to reject DNA. Also, bacteria could provide a substantial fraction of the carbon requirement of the mussel.     

Effects of Olisthodiscus luteus on the growth and abundance of Tintinnids

The effects of the red tide flagellate Olisthodiscus luteus Carter on the growth of two tintinnid species, Tintinnopsis lubulosoides Meunier and Favella sp. (Clap. & Lach.) Jorg., were measured in batch culture. T. tubulosoides and Favella sp. grew at rates equivalent to 1.2 (10°C) and 2.0 (20°C) population doublings per day, respectively, when offered nutritionally adequate phytoplankton species. The growth rates of both tintinnid species were reduced in the presence of 10²–10³ O. luteus cells · ml^-1 in multialgal treatments. Growth rate inhibition was proportionately greater at higher O. luteus densities. Lethal effects were observed for both tintinnid species at O. luteus concentrations of 5x10³ cells · ml^-1 in multi-algal treatments. T. tubulosoides mortality occurred at all O. luteus concentrations in unialgal culture. O. luteus-conditioned medium did not substantially inhibit tintinnid growth when combined with acceptable food species, suggesting that toxicity is induced by ingestion or direct contact with O. luteus cells, or by exposure to a short-lived exudate. In agreement with these results, an inverse relationship between O. luteus concentration and tintinnid abundance was observed in Narragansett Bay, Rhode Island, over a two year period. The small lorica diameter of the species apparently inhibited by these O. luteus blooms suggests a detrimental effect independent of cell ingestion. In addition to the absolute concentration of O. luteus cells, the availability of nutritionally adequate algal food may be an important factor determining the impact of O. luteus blooms on tintinnid populations.Contribution no. 5048 from the Woods Hole Oceanographic Institution     

Filter feeding by Oikopleura vanhoeffeni: grazing impact on suspended particles in cold ocean waters

Because of the abundance and size of Oikopleura vanhoeffeni its quantitative role as a suspension feeder in cold ocean waters needs to be defined. To minimize the effect of manipulation and containment, and to assess the effect of naturally occurring factors on clearance rate, I used an in situ latex microbead technique in Logy Bay, Newfoundland, from February 1985 to June 1986. Individual clearance rates ranged from 8–944 ml h^-1, increasing exponentially with increasing trunk length. Partial correlation and principal components analysis indicated that trunk length and the concentration of ingestible chlorophyll a accounted for a majority of the variation in clearance rate. At densities of 4–110 m^-3, O. vanhoeffeni populations removed from >1 to 13% of the standing stock of ingestible food particles each day. Grazing by near-surface populations was lowest during the spring diatom bloom (>1.4% of daily particle production removed per day), and was highest in June during the post-bloom crash (4 to 10% of daily production removed). Some populations in mid-depth waters had much higher population clearance rates (ca. 50% of daily production removed) because of a greater proportion of large animals. The median percentage daily ration (g Cxg C^-1xd^-1x100%) of 64% accounted for observed house production rates (1 to 2 d^-1, with each house=23% of body carbon).     

Daily feeding rates in herbivorous labroid fishes

Estimates of daily feeding rates were obtained for two groups of herbivorous labroid fishes, one confined to cold water and the other to tropical reef environments. These were the family Odacidae, represented by Odax pullus from New Zealand waters, (Goat Island Bay: Latitude 36° South; on the northeastern coast of New Zealand) and the family Scaridae, represented by Scarus rivulatus, S. schlegeli and S. sordidus from the northern Great Barrier Reef (Lizard Island; a mid-shelf reef at 14° South latitude). Observations on the odacid were made in 1984 and in 1992, and on the scarids in 1984 and 1988. O. pullus displayed a diurnal feeding pattern in which the rates (expressed as bites min^-1) are greatest early in the day. The mean combined feeding rate for three size groups (juveniles, subadults and adults) peaked (average of 2.9 bites) from 06.00 to 08.00 hrs and declined fourfold to a combined average of 0.7 bites min^-1 by midday. The greatest mean feeding rate recorded was 3.7 bites min^-1, with an overall mean of 1.8 bites min^-1. For subadults and adults there were consistent trends in feeding, with subadults feeding at a greater rate than adults and both groups displaying a decline in feeding rate during the day. The change in feeding rate with time of day was statistically significant in both groups. The pattern for juvenile O. pullus was different from that in the two larger size groups in that juveniles did not show a uniform decline in feeding with time of day. For scarids, the daily feeding rate varied by site, but the pattern was similar for all species, characterised by initial low rates increasing to higher but variable levels by midday. The influence of both site of feeding and time of day on feeding rate was confirmed by analysis. The overall mean values for each species were 20.1 bites min^-1 for S. rivulatus, 19.7 bites min^-1 for S. schlegeli and 14.9 bites min^-1 for S. sordidus. For scarids, the peak feeding rates varied from 19.3 to 32.8 bites min^-1, with overall rates from 14.9 to 21.1 bites min^-1. Estimates of activity and movement patterns during feeding were obtained for O. pullus. Distance moved per unit time was highly variable, 0.1 to 47.5 m min^-1, with a mean of 8.5 m min^-1 (SD=9.9). Trends in movement among sexes and size classes were obscured by the variable movement patterns of individual fishes.     

Surface-film microbial populations: diel amino acid metabolism,carbon utilization,and growth rates

Microheterotrophic dissolved free amino acid (DFAA) utilization, and microbial community and bacterial community carbon production and growth were studied using ³H-labeled organics as tracers in marine surface-film and subsurface (10 cm) waters off Baja California in November 1983. DFAA utilization was generally more rapid during the day (0.14 to 0.38 nM h^-1) than at night (0.04 to 0.14 nM h^-1) in surface-film and subsurface waters, but the percent of utilized amino acid which was respired was always greater during the night (22 to 57%) compared to the day (14 to 18%). Utilization of DFAA-carbon was estimated to range from 0.3 to 5.3 g C l^-1 d^-1 for all stations studied. In six of the 8 samples examined, the percentage of microbial carbon accounted for by the bacterial component of the population (1.4 to 5.9%) was strikingly similar to the percentage of microbial carbon production accounted for by bacterial carbon production (1.9 to 5.1%). In all of these six samples, total microbial specific-growth rates and bacterial specific-growth rates were approximately equivalent (0.9 to 2.2 d^-1 for the microbial community; 0.7 to 1.9 d^-1 for bacteria). The two exceptions were samples apparently influenced by transient flagellate populations migrating into the surface or subsurface waters at night. These observations support the conclusion that surface films contain unique and highly active microbial populations.     

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.     

Otolith ring deposition in relation to growth rate in herring (Clupea harengus) and turbot (Scophthalmus maximus) larvae

Larvae of Clyde spring-spawning Clupea harengus L. and hatchery-produced Scophthalmus maximus (L.) were reared from hatching through metamorphosis in 1980 and 1981 in laboratory tanks and in large enclosures under various light, temperature, and feeding regimes in order to study otolith ring deposition and growth under different conditions. Ring deposition and growth rates were significantly affected by rearing conditions in both species. The ring deposition rates observed under the conditions tested ranged from 0.34 to 0.92 rings d^-1 in herring larvae, and from 0.07 to 1.0 rings d^-1 in turbot larvae. Growth rates ranged from 0.11 to 0.42 mm d^-1 in herring and from 0.05 to 0.27 mm d^-1 in turbot. The number of otolith rings was dependent on the growth rate of the individual larva. At the population level, higher ring deposition rates were observed in faster growing populations. In herring larvae, the relationship between average growth rate and average ring deposition rate was logarthmic, reaching an asymptote at 1 ring d^-1 for growth rates approaching 0.40 mm d^-1. The relationship was linear for turbot larvae for the range of growth rates observed.     

Marine diatoms grown in chemostats under silicate or ammonium limitation. I. Cellular chemical composition and steady-state growth kinetics of Skeletonema costatum

Skeletonema costatum was grown in chemostats under ammonium or silicate limitation to examine its growth kinetics and changes in cellular chemical composition at different steady-state growth rates. When the relationship between the effluent limiting substrate concentration and steady-state growth rates was examined, deviations from the simple hyperbolic form of the Monod growth equation were noted at low and high dilution rates. The data from the plot of growth rate and substrate concentration were divided into 4 regions and the relationship of these region to cell quota is discussed. Two physiological states were identified. All populations grown at D<0.05 h^-1, regardless of the size of the cells or the magnitude of Q, exhibited a maximal growth rate of approximately 0.05 h^-1, while populations grown at higher dilution rates (D>0.06 h^-1 to 0.14 h^-1). The maximal value of growth rate is obtained only in cultures grown at very high dilution rates where nutrient shift-up appears to occur, the cell quota approaches a maximum and the heterogeneous cell population becomes more homogeneous.Contribution No. 881 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA. This paper represents a portion of two dissertations submitted by P.J.H. and H.L.C. to the Department of Oceanography, University of Washington, Seattle, in partial fulfillment of the requirements for the Ph.D. degree.     

Metabolism and elemental composition of zooplankton from the Barents Sea during early Arctic summer

Rates of oxygen consumption, ammonia excretion and phosphate excretion were measured on a hydromedusae (Aglantha digitale), pteropods (Limacia helicina, Clione limacina), copepods (Calanus finmarchicus, C. glacialis, C. hyperboreus, Metridia longa), an amphipod (Parathemisto libellula), a euphausiid (Thysanoessa inermis) and a chaetognath (Sagitta elegans), all of which were dominant species in the Barents Sea during early summer 1987. Water and ash contents and elemental composition (C and N) were also analysed on the specimens used in these metabolic experiments. Between species variations were 67.8% to 94.7% of wet weight in water content, 6.4% to 56.5% of dry weight in ash content, 16.7% to 61.0% of dry weight in carbon content, and 4.3% to 11.2% of dry weight in nitrogen content. Oxygen consumption rates ranged from 0.33 to 13.8 l O₂ individual^-1 h^-1, ammonia excretion rates, from 0.0072 to 0.885 gN individual^-1 h^-1 and phosphate excretion rates, from 0.0036 to 0.33 g P individual^-1 h^-1. In general, higher rates were associated with larger species, but considerable differences were also seen between species. The ratios between the rates (O : N, N : P, O : P) exhibited a wide species-specific variation, indicating differences in dominant metabolic substrates. Typical protein oriented metabolism was identified only in S. elegans. From the results of metabolic rate measurements and elemental analyses, daily losses of body carbon and nitrogen were estimated to be 0.50 to 4.15% and 0.084 to 1.87%, respectively, showing faster turnover rates of carbon than that of nitrogen. Comparison of daily loss of body carbon of the Barents Sea zooplankton with that of the Antarctic zooplankton indicated reduced rates of the former (63% on average).     

Trophodynamic function of copepods,appendicularians and protozooplankton in the late summer zooplankton community in the Skagerrak

The study was carried out in the Skagerrak during late summer when population development in the pelagic cycle culminated in the yearly maximum in zooplankton biomass. The cyclonic circulation of surface water masses created the characteristic dome-shaped pycnocline across the Skagerrak. The large dinoflagellate Ceratium furca dominated the phytoplankton biomass. Ciliates and heterotrophic dinoflagellates were the major grazers and, potentially, consumed 43–166% of daily primary production. The grazing impact of copepods was estimated from specific egg production rates and grazing experiments. The degree of herbivory differed between species (14–85%), but coprophagy (e.g. feeding on fecal pellets) and ingestion of microzooplankton were also important. The appendicularian Oikopleura dioica was present in lower numbers than copepods, but cleared a large volume of water. The grazing impact of copepods and O. dioica was estimated to 57±24% and 12±12% of daily primary production, respectively. Sedimentation of organic material (30 m) varied between 169 and 708 mg C m^–2 day^–1, and the contribution from the mesozooplankton (copepod fecal pellets and mucus houses with attached phytodetritus of O. dioica) was 5–33% of this sedimentation. Recycling of fecal pellets and mucus houses in the euphotic zone was 59% and 36%, respectively. However, there was a high respiration of organic material by microorganisms in the mid-water column, and 34% of the sedimenting material actually reached the benthic community in the deep, central part of the Skagerrak.     

Quantitative feeding ecology of the hydromedusan Nemopsis bachei in Chesapeake Bay

We determined feeding rates of the hydromedusan Nemopsis bachei L. Agassiz in the mesohaline region of Chesapeake Bay, USA during the spring of 1989 and 1990 from gut contents, digestion rates and abundances of medusae and zooplankton. The medusae consumed primarily copepodites of Acartia tonsa, selecting against naupliar stages. The peak abundance of N. bachei medusae was in April to May, when densities averaged more than 10 m^-3. Medusa densities were similar in both years, but were greatest (maximum of 132 medusae m^-3) along a southern transect sampled only in 1990. At peak densities, N. bachei medusae consumed 30% d^-1 of the copepodite standing stocks, but they consumed <1% d^-1 at the lower densities typical of late May or early June. The predation effects were generally greater than those reported for other hydromedusan species. But even at peak predation, N. bachei medusae could not have controlled or reduced A. tonsa copepod populations, which had a production rate of 85% d^-1 at that time. Medusa feeding rates were highest at nighttime, and were correlated with prey density in the field, but not in the laboratory.Communicated by J. Grassle, New Brunswick     

Bacteria as food for marine harpacticoid copepods

Feeding experiments were carried out on the harpacticoid copepods Tisbe holothuriae and Paramphiascella vararensis, using various species of marine and brackish water bacteria as food. Short-term experiments with ³H-labelled bacteria are described, as well as experiments covering longer periods of time using non-labelled, dried bacteria. Results show that for both T. holothuriae and P. vararensis, the amount of dry bacteria consumed ranged from 2.06 to 7.07 g copepod^-1 day^-1, indicating a carbon requirement of 1 to 3.5 g copepod^-1 day^-1. The food value of the bacteria strains tested is given in terms of the generation time of the copepods raised on these bacteria.     

High recovery of culturable bacteria from the surfaces of marine algae

The culturability of heterotrophic marine bacteria obtained from the surfaces of two species of marine algae (Lobophora variegata andHalimeda copiosa) was assessed by comparing total DAPI-stained cell counts to colony-forming bacterial counts on two agar media. The colony-forming bacterial counts on a low-nutrient medium (LN) consisting of seawater and agar were significantly greater for both algal species than counts obtained on a high-nutrient medium (HN) similar in composition to that typically used for the isolation of heterotrophic marine bacteria. On average, 14 and 58%, respectively, of the total bacteria fromL. variegata andH. copiosa were culturable on LN. These recovery rates far exceed those typically reported for marine bacteria. Of 119 LN strains obtained in pure culture, 55% failed to grow on HN. The yeast extract component of HN (1.5 gl^-1) was responsible for the majority of the observed inhibition, suggesting that this nutrient can be highly toxic to marine bacteria. Eighty-nine percent of the strains inhibited by HN were capable of growth when the nutrients in this medium were diluted by a factor of 100 with seawater. Of 66 epiphytic strains, 30 (45%) initially inhibited by HN showed the ability to adapt to this medium after a period of laboratory handling. The initial inability of low-nutrient-adapted bacteria to grow on high-nutrient media may be due to nutrient shock. The results presented here indicate that the culturability of specific populations of marine bacteria can be dramatically improved by the use of low-nutrient media. Further, the importance of developing new medium formulations that eliminate traditional nutrients, some of which are clearly toxic to bacteria, is demonstrated.