Predator-prey interactions between the larvacean Oikopleura dioica and bacterioplankton in enclosed water columns

The larvacean Oikopleura dioica Fol was fed ³H-labeled natural assemblages of marine bacterioplankton. Grazing rates ranged from <1 to >100 ml day^-1 individual^-1 and were highly dependent onlarvacean body size. These rates were combined with estimates of abundance of O. dioica in large floating enclosures with semi-natural populations (Controlled Ecosystems Populations Experiment, CEPEX) to determine the impact of the larvacean on the bacterial populations and to estimate the amount of bacteria ingested by the larvaceans. Apparently, O. dioica has minimal influence on the population dynamics of the free-living bacteria, although bacteria may form a substantial portion of the larvacean's diet.     

Multifarious activities of gut epithelium in an appendicularian (Oikopleura dioica: Tunicata)

The functions of the various gut regions of Oikopleura dioica (oesophagus, left and right gastric lobes, vertical intestine, mid-intestine and rectum) were investigated by means of histochemical, histoenzymatic and immunohistochemical techniques at light and electron microscopes. Ciliary food progression is evidenced by the presence of ATPases on cilia, along the entire gut, with the exception of the cardiac valve, a passive device controlling food direction. Absorptive processes (alkaline phosphatase), active transport (ATPases) and nitrogen excretion (D-amino acid oxidase) occur along the entire gut, in both ciliated microvillar and globular cells. The latter, typical of the left gastric lobe and rectum, are also involved in endocytotic processes (exogenous peroxidase as tracer) and intracellular digestion (!-amylase, aminopeptidase M, acid phosphatase, 5'-nucleotidase, non-specific esterase). The giant cells of the gastric band participate in extracellular digestion; they contain secretory granules positive to various hydrolytic enzymes, the activity of which is also recognisable in faecal pellets inside the intestinal lumen. Lipid storage occurs mainly in the right gastric lobe and vertical intestine, whereas protein storage takes place in the rectal granular cells. Epithelial transport and possible osmoregulation occur along the entire gut, especially at the level of diffuse baso-lateral interdigitations, which increase the plasmalemma surface enormously, are often associated with mitochondria and possess numerous ATPase pumps. Data extend previous histological observations and hypotheses on the physiological role of the various gut regions. The remarkable and specific location of enzymatic activities and nutrient storage are in agreement with the high capacity of O. dioica to process a great quantity of food very rapidly and efficiently.     

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).     

The decline and recovery of four predatory fishes from the Southern California Bight

What to do about fisheries collapse and the decline of large fishes in marine ecosystems is a critical debate on a global scale. To address one aspect of this debate, a major fisheries management action, the removal of gill nets in 1994 from the nearshore arena in the Southern California Bight (34°26′30″N, 120°27′09″W to 33°32′03″N, 117°07′28″W) was analyzed. First, the impetus for the gill net ban was the crash of the commercial fishery for white seabass (Atractoscion nobilis; Sciaenidae) in the early 1980s. From 1982 to 1997 catch remained at a historically low level (47.8 ± 3.0 mt) when compared to landings from 1936–1981, but increased significantly from 1995–2004 (r = 0.89, P < 0.01) to within the 95% confidence limit of the historic California landings. After the white seabass fishery crashed in the early 1980s, landings of soupfin (Galeorhinus galeus; Triakidae) and leopard shark (Triakis semifasciata; Triakidae) also significantly declined (r = 0.95, P < 0.01 and r = 0.91, P < 0.01, respectively) until the gill net closure. After the closure both soupfin and leopard shark significantly increased in CPUE (r = 0.72, P = 0.02 and r = 0.87, P < 0.01, respectively). Finally, giant sea bass (Stereolepis gigas; Polyprionidae) the apex predatory fish in this ecosystem, which was protected from commercial and recreational fishing in 1981, were not observed in a quarterly scientific SCUBA monitoring program from 1974 to 2001 but reappeared in 2002–2004. In addition, CPUE of giant seabass increased significantly from 1995 to 2004 (r = 0.82, P < 0.01) in the gill net monitoring program. The trends in abundance of these fishes return were not correlated with sea surface temperature (SST), the Pacific Decadal Oscillation (PDO) or the El Niño/Southern Oscillation (ENSO). All four species increased significantly in either commercial catch, CPUE, or in the SCUBA monitoring program after the 1994 gill net closure, whereas they had declined significantly, crashed, or were absent prior to this action. This suggests that removing gill nets from coastal ecosystems has a positive impact on large marine fishes.     

Effects of ocean acidification, temperature and nutrient regimes on the appendicularian Oikopleura dioica: a mesocosm study

Increasing pCO₂ is hypothesized to induce shifts in plankton communities toward smaller cells, reduced carbon export rates and increased roles of gelatinous zooplankton. Appendicularians, among the most numerous pan-global “gelatinous” zooplankton, continuously produce filter-feeding houses, shortcutting marine food webs by ingesting submicron particles, and their discarded houses contribute significantly to carbon fluxes. We present a first mesocosm-scale study on the effects of temperature, pCO₂ and bloom structures on the appendicularian, Oikopleura dioica. There were effects of temperature and nutrients on phytoplankton communities. No shifts in functional phytoplankton groups, nor changes in particle sizes/morphotypes, known to impact appendicularian feeding, were observed under manipulated pCO₂ conditions. However, appendicularian abundance was positively correlated with increased pCO₂, temperature and nutrient levels, consistent with hypotheses concerning gelatinous zooplankton in future oceans. This suggests appendicularians will play more important roles in marine pelagic communities and vertical carbon transport under projected ocean acidification and elevated temperature scenarios.     

Triacylglycerols and wax esters in the lipids of deep midwater teleost fishes of the Southern California Bight

Total lipid contents and the proportions of triacylglycerols and wax esters were examined in 23 species of deep-sea fishes collected between 1977 and 1980 in the Santa Catalina, San Clemente and San Pedro Basins off Southern California, USA. Mean total lipid content ranged from 1 to 22% of wet weight. Triacylglycerols accounted for 1 to 91% of total lipids present and were more abundant than wax esters in the lipids of all but a single species. Wax ester contents ranged from 0 to 24% of total lipids but in 20 species were 10% of the lipids. These results do not substantiate the previously suggested idea that wax esters are important constituents of the lipids of deep-sea fishes. Moreover, the hypothesis that wax ester content increases with increasing depth of occurrence is not supported. To assess the importance of the lipids present in bouyancy regulation, the water content and swimbladder inflation of these species were also examined. Neither total lipids nor wax esters appear to play an important role in buoyancy regulation in the majority of these fishes. Eight species have gas-filled swimbladders. Of the species lacking inflated swimbladders, 7 have watery bodies (87 to 95% H₂O) and 6 with low lipid (8.3%) and water (85%) contents may need to swim continuously to maintain their position in the water column. Lipids, largely in the form of triacylglycerols, may be important in the buoyancy of three species with lipid contents>10% of wet weight. Two of these species also possess inflated swimbladders. The reasons for the production of large quantitites of was esters by certain deep-sea teleosts, especially gonostomatids and myctophids, remain unknown.     

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.     

The annual cycle of protozooplankton in the Kiel Bight

Protozooplankton (heterotrophic dinoflagellates and ciliates) composition and biomass was studied in a 20-m water column in the Kiel Bight on 44 occasions between January 1973 and April 1974. Both groups attained comparable biomass maxima during spring and autumn (0.3 to 0.7 g C m^-2 in the 20-m water column) and biomass levels were much lower in summer and lowest in winter. The spring protozooplankton maximum coincided with that of phytoplankton and during the rest of the year, protozooplankton stocks did not appear to be food limited as phytoplankton stocks were large throughout; many protozoans with ingested microplankton cells were observed, indicating that their potential food supply is not restricted to nanoplankton. Non-loricate organisms dominated biomass of the ciliates and tintinnids were of little importance. Tintinnids predominated in plankton samples concentrated by 20 m gauze indicating that most non-loricate ciliates, irrespective of size, were not retained. When phytoplankton sotcks were large (>3 g C m^-2) but those of metazooplankton small, as in spring and autumn, protozooplankton were the major herbivores with biomass levels comparable to those attained in summer by metazooplankton ( 0.5 g C m^-2). A highly significant negative correlation was found between protozooplankton and metazooplankton during the plankton growth season. Predation by the latter is thus an important factor regulating size of the protozooplankton population, although other factors also appear to be in operation. Loss rates of the pelagic system through sedimentation are highest in spring and autumn when protozooplankton dominate the grazing community and loss rates are much lower in summer when metazooplankton are the dominant herbivores. Apparently, the impact of protozooplankton grazing on the pelagic system is quite different to that of the metazooplankton.Publication No. 268 of the Joint Research Programme (SFB 95), Kiel University     

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.     

Studies on the functional role of tintinnids in the Southern California Bight. I. Grazing and growth rates in laboratory cultures

Five species of tintinnids, Amphorella quadrilineata (Claparède and Lachmann, 1858), Tintinnopsis cf. beroidea Stein (1867), T. cf. acuminata Daday (1887), Eutintinnus pectinis (Kofoid and Campbell, 1929), and Helicostomella subulata (Ehrenberg, 1833), were isolated from Southern California coastal waters and maintained in laboratory cultures which were used to investigate, several aspects of the feeding dynamics and population growth rates of this group of planktonic ciliates. Data are most complete for the latter three species. Both E. pectinis and H. subulata displayed ingestion and growth rates which increased with increasing food concentration until a maximum rate was obtained which then remained essentially constant as food levels increased further. Maximum hourly ingestion was equivalent to approximately 10 to 20% of the body weight of the tintinnids. T. cf. acuminata showed no such maximum ingestion rate, as ingestion rates increased throughout the entire range of food concentrations used. The dependence of growth rate of T. cf. acuminata on food concentration also differed from the other species, being characterized by a broad region of maximum growth at intermediate food levels with reduced growth at both lower and higher food concentrations. Maximum observed growth rates represented doubling times of approximately 12 h for both E. pectinis and T. cf. acuminata and 24 h for H. subulata. The data suggested gross growth efficiencies exceeding 50% over much of the range of food concentrations used. No strong evidence supporting the existence of feeding thresholds or switching behavior was collected during these experiments, although apparently selective feeding was observed in one experiment with E. pectinis.     

Studies on the functional role of tintinnids in the Southern California Bight. II. Grazing rates of field populations

The feeding behavior of relatively undisturbed natural populations of tintinnids was studied in a series of experiments which utilized water samples collected in Southern California coastal waters. Dilute suspensions of corn starch were presented to the tintinnids and the rates of ingestion of this visual tracer determined after short (5 to 20 min) incubations followed by fixation in Lugol's iodine fixative. Tintinnids were observed to ingest particles of diameters up to approximately 43% of their lorica oral diameter and at rates proportional to their oral diameters. Grazing rates observed in these experiments ranged up to approximately 10 l tintinnid^-1 h^-1, and generally agreed well with the rates observed in experiments utilizing laboratory cultures of tintinnids. No significant diel periodicity in feeding rates was observed in the one study extending over a 24 h period.     

Feeding mechanism and house of the appendicularian Oikopleura vanhoeffeni

Although the feeding apparatus of oikopleurid Appendicularia has been described in general, functional details of the feeding mechanism and fluid mechanical constraints on the feeding process remain unknown. My goals were to determine the number of mucous-net layers in the feeding filter of Oikopleura vanhoeffeni collected from March 1985 to February 1986, and to describe the pathway of water flow through the filter. Marker particles (i.e. Isochrysis galbana, carmine, charcoal powder, and starch), and rhodamine dye were added to the natural food suspension to help in visualizing structure and flow. The feeding filter was composed of three layers. Water flowed into the filter through two large, lateral openings at the base of each wing, and along the open distal margins of the filter. Under hydrostatic pressure generated by the tail, water moved through the filter in a one-way bulk flow and was forced through the mucous mesh of both the dorsal and ventral layers. Thus, the feeding filter concentrated the food suspension by sieving most of the incoming water. The filter did not collect or trap food particles. Because of its function, I propose calling the feeding filter the food concentrating filter.     

Rearing Pandalus borealis larvae in the laboratory

Larvae of the northern shrimp Pandalus borealis (Krøyer) are pelagic. In the Estuary and Gulf of St. Lawrence, Canada, the early stages are found in the upper 25-m of the water column in spring and early summer and are expected to experience a range of water temperatures from as low as 0°C to as high at 6°C. Little is known of the impact of water temperature on metabolic requirements of northern shrimp larvae. In this study, routine respiration (VO₂), maximum respiration (electron transport system activity, ETSA) and metabolic scope for growth (MS, ETSA–VO₂) of northern shrimp larvae were measured as a function of temperature (3, 5 and 8°C), developmental stage (I–V at 3°C, I–VII at 5°C and 8°C) and growth rate in dry mass. After logarithmic transformation, all three metabolic variables were linearly related to dry mass. The increase in VO₂ with body mass was faster at 5°C than at 3 or 8°C, whereas with ETSA this increase was slower. As a result, MS increased more slowly with dry mass at 5°C than at 3 and 8°C. However, MS did not limit growth in this study, since it explained only 39% of the variability in growth. All three metabolic variables as well as growth varied together as a function of temperature and ontogeny. Q₁₀ of all three metabolic variables ranged from 1.6 and 2.2 for stages I–V larvae, except for VO₂ at stage I (3.9) and stage III (2.9).     

Lack of population genetic structuring in the marine planktonic diatom Pseudo-nitzschia pungens (Bacillariophyceae) in a heterogeneous area in the Southern Bight of the North Sea

Several marine holoplanktonic organisms show a high degree of geographically structured diversity for which it often remains unclear to what extent this differentiation is due to the presence of cryptic taxa. For the genetically distinct diatom Pseudo-nitzschia pungens var. pungens, we used six microsatellite markers to investigate the spatial and temporal genetic composition in the heterogeneous Southern Bight of the North Sea. Although our sampling area (ca. 100 km) comprised water bodies with different environmental conditions (marine, estuary, saline lake) and different degrees of connectivity (from complete isolation to supposedly free exchange between environments), no evidence of genetic differentiation was found. Expanding our sampling area (ca. 650 km), suggested a homogenous population structure over even larger areas in the North Sea. Our results suggest that the population structure of this diatom is mainly shaped by strong homogenizing effects of gene flow preventing genetic drift, even in water bodies with limited connectivity. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Biology of the conger eel Conger oceanicus in the Mid-Atlantic Bight

In the Mid-Atlantic Bight, conger eels (Conger oceanicus) occur from the coastal portions of estuaries to the edge of the continental shelf. In deeper waters they occupy burrows of the tilefish (Lopholatilus chamaeleonticeps). Between 1972 and 1974 we examined the stomachs and intestines of conger eels from inshore New Jersey (USA) waters (n=35, with a total length: TL of 21 to 49 cm) and between 1980 and 1983 offshore (n=295, 50 to 125 cm TL)_collections. Eels from both areas fed primarily on decapod crustaceans and fish. The specific identity of prey items within these groups generally differed from inshore to offshore areas, probably reflecting the differences in prey availability. Foods of specimens collected offshore varied with size: smaller eels (<80 cm TL) fed most heavily on decapod crustaceans, whereas larger eels (>80 cm) consumed more fishes. The presence of some nocturnally active prey items in the gut, primarily the eel Lepophidium cervinum, suggests that conger eels are nocturnal feeders. This is supported by in situ observations that conger eels are present in some tilefish burrows during the day and are presumably out of burrows and foraging at night.     

Use Impairments and Ecosystem Impacts of the New York Bight

The New York Bight is perhaps one of the most used and abused coastal areas in the world as a consequence of urbanization and the disposal of the waste of some 20 million people who reside by its shores and surrounding bays and estuaries. A variety of sources, including those associated with sewage wastes, industrial wastes, contaminated dredged material, urban runoff, and atmospheric fallout contaminate these coastal waters. Many of the stresses of excess population and industrialization as measured by pollutant loadings and ecosystem impacts can be crudely quantified in terms of use impairments-use impairments that have measurable social and economic relevance. Five broad categories of impairment attributed to pollution in the Bight that are causing significant losses of ecological, economic, or social values are: beach closures, unsafe seafoods, hazards to commercial and recreational navigation, loss of commercial and recreational fisheries, and declines in birds, mammals and turtles. These impairments are generally caused by floatable wastes, nutrients, toxicants, pathogens and habitat loss. Measures of such impairments are not standard, nor in many cases totally quantifiable. We have examined specific subsets of these impairments in terms of their spatial and temporal changes and as a first approximation determined the economic and social significance of these changes. the cost of these impaired uses of the Bight are measured in terms of billions of dollars annually for New York and New Jersey.     

Use Impairments and Ecosystem Impacts of the New York Bight

The New York Bight is perhaps one of the most used and abused coastal areas in the world as a consequence of urbanization and the disposal of the waste of some 20 million people who reside by its shores and surrounding bays and estuaries. A variety of sources, including those associated with sewage wastes, industrial wastes, contaminated dredged material, urban runoff, and atmospheric fallout contaminate these coastal waters. Many of the stresses of excess population and industrialization as measured by pollutant loadings and ecosystem impacts can be crudely quantified in terms of use impairments-use impairments that have measurable social and economic relevance. Five broad categories of impairment attributed to pollution in the Bight that are causing significant losses of ecological, economic, or social values are: beach closures, unsafe seafoods, hazards to commercial and recreational navigation, loss of commercial and recreational fisheries, and declines in birds, mammals and turtles. These impairments are generally caused by floatable wastes, nutrients, toxicants, pathogens and habitat loss. Measures of such impairments are not standard, nor in many cases totally quantifiable. We have examined specific subsets of these impairments in terms of their spatial and temporal changes and as a first approximation determined the economic and social significance of these changes. the cost of these impaired uses of the Bight are measured in terms of billions of dollars annually for New York and New Jersey.     

Flow rate and particle concentration within the house of the pelagic tunicate Oikopleura vanhoeffeni

The food-concentrating filter within the house of Oikopleura vanhoeffeni Lohmann consists of three layers, upper and lower fine-mesh layers and an intermediate coarse-mesh layer. The filter is a natural analogue of man-made tangential, or cross-flow, filters that are used to concentrate fine particulate and colloidal material by the exclusion of water. Flow analysis using video microscopy of O. vanhoeffeni, collected in 1989 and 1990 at Logy Bay, insular Newfoundland, indicates concentration of particles of 74 to 1089 times that of ambient seawater ( ,n=23), for individuals 1.8 to 5.4 mm long. The smallest individuals had the highest concentration factors. Flow through the house is predominately affected by viscous forces and values of Reynolds numbers are 1. Flow rates through the house of O. vanhoeffeni indicate a maximum filtration capacity of ca. 1.4 to 26 liters d^-1 and clearance rates (assuming ca. 50% of the time is spent not actively pumping) of 0.7 to 13 liters d^-1, depending on body size. The estimated thickness of the boundary layer around individual fibers of the food-concentrating filter is 0.08 m, which is less than the pore width. The estimated concentration factors suggest that colloidal and fine particulate material may aggregate into larger particles within the food-concentrating filter and food tube during feeding.     

Seasonal and annual variability in an epifaunal community in the German Bight

While the benthic infauna of the North Sea has been studied intensively over the past decades, few studies have focused on the larger mobile epifauna. Studies carried out to date have described the distribution of epifaunal communities over the whole of the North Sea, but variability within the identified communities, which occurs on a much smaller scale, has so far remained unstudied. This is the first study to describe seasonal and annual variability of an epifaunal assemblage in the German Bight area, where environmental conditions are highly variable. The benthic community was sampled with a 2 m beam trawl from 1998 to 2001. The echinoderms Ophiura albida and Asterias rubens and the crustacean Pagurus bernhardus were the dominant species caught throughout the study period. Overall the species composition of the catches was relatively consistent, while abundances of dominant species fluctuated considerably between sampling periods. Differences between sampling periods were not only influenced by the abundances of dominant species, but also by less dominant species such as Ophiura ophiura, Astropecten irregularis, Corystes cassivelaunus, Crangon crangon and Aphorrahis pespelicani. The abundances of these species varied annually and seasonally in the assemblage. Clear differences between summer and winter in the species composition, abundance and biomass were identified. Annual and seasonal changes were most likely linked to migratory movements of epifauna into and out of the area under investigation. Temporal changes in species composition and abundance correlated best with water temperatures, while the spatial distribution of the total biomass over the whole sampling period was correlated with sediment characteristics. Anthropogenic influences such as fishing activity and chronic large-scale eutrophication are thought to have influenced the community on a long-term basis, but have been considered unlikely causes for the short-term variability described by this study.Communicated by J.P. Thorpe, Port Erin