Raymond J. Thompson

Fatty acids as trophic markers of phytoplankton blooms in the Bahía Blanca estuary (Buenos Aires, Argentina) and in Trinity Bay (Newfoundland, Canada)

The fatty acid compositions of phytoplankton and major primary consumers were analyzed during the development of seasonal algal blooms in the Bahia Blanca estuary, situated on the southern coast of the province of Buenos Aires (Argentina), and Trinity Bay, at Sunnyside, on the eastern coast of Newfoundland (Canada). Primary consumers in the Bahia Blanca estuary were zooplankton dominated by the calanoid copepod Acartia tonsa. At Sunnyside, the primary consumers were the sea scallop Placopecten magellanicus, an ecological and economical important benthic bivalve. The study shows that in spite of obvious differences between the two environments and the analytical approaches employed in each case, the analyses of fatty acid biomarkers can provide relevant ecological information. The fatty acid composition of the lipids of Bahia Blanca phytoplankton (high concentrations of the fatty acids 14:0, 16:4ω1, and 20:5ω3) reflected the presence of diatoms as a major component throughout the bloom. Fatty acid markers of the post-bloom phytoplankton in Bahia Blanca indicated a decline of phytoplankton biomass, and a relatively high input of detritus and terrestrial plant materials to the particulate organic matter of the estuary. Linoleic acid (18:2ω6), a typical “terrestrial” fatty acid, was conspicuous in the lipids of the post-bloom particulate matter of the Bahia Blanca estuary; 18:2ω2 was subsequently incorporated into zooplankton lipids diatom markers were also prominent in the lipids of pre-bloom and bloom phytoplankton at Sunnyside; post-bloom phytoplankton showed higher proportions of 18:0, 18:1ω9, and 18:4ω3, characteristic and often major fatty acids of dinoflagellates. The fatty acids of the digestive gland of P. magellanicus reflected the fatty acid composition of the phytoplankton, whereas those of the adductor muscle were practically unaffected by the composition of the food. This organ-specific response of an animal to the fatty acid composition of the diet is examined in terms of different applications of the fatty acid marker concept.

Eicosapentaenoic Acid Regulates Scallop (Placopecten magellanicus) Membrane Fluidity in Response to Cold

The lipid core of a biological membrane requires a certain degree of structural rigidity, but it must also be sufficiently fluid to permit lateral movement of the constituent lipids and embedded proteins. Ectotherms can counteract the ordering effects of reduced temperature by changing the structure of their membranes, a process known as homeoviscous adaptation (1). Although the content of unsaturated fatty acids in the membranes of ectothermic animals is generally known to increase in response to cold (2), no clear and direct relationship between unsaturated fatty acids and membrane fluidity has been established in marine organisms. For example, phospholipid molecular species containing docosahexaenoic acid (22:6 3) are believed to be important in controlling finfish membrane fluidity (3–6), but a direct correlation between 22:6 3 and membrane fluidity has not been found (4, 5, 7, 8). In contrast, we show here a simple but very strong relationship between fluidity and a single polyunsaturated fatty acid, eicosapentaenoic acid (20:5 3), in gill membranes from a marine bivalve mollusc, the sea scallop Placopecten magellanicus. Phospholipids are the main structural elements of biological membranes, and their physical characteristics are key determinants of membrane structure and function. Many vital cell activities that depend on the optimal functioning of membranes are therefore sensitive to the chemistry of the membrane lipids (9) and to environmental conditions, such as temperature and pressure, that perturb the phase behavior and dynamics of lipids in membranes (10). Under extreme or variable conditions, organisms can exploit the tremendous chemical diversity among membrane lipids to defend the physical properties of the membrane (10). Thus in ectotherms, where changes in temperature cause important membrane perturbations, the usual adaptive response includes a modification of lipid composition (11). Sessile animals living in Newfoundland waters must maintain membrane structure and function in the face of extreme cold in deep waters (as low as 1.4°C) or seasonally highly variable conditions in surface waters (as much as 22°C in 6 months) (12). In the present study, we exposed sea scallops to a 10°C decrease in temperature for up to 3 weeks and then examined the relationship between the fatty acid composition of branchial phospholipids and membrane fluidity. Vesicles were prepared from the gills of scallops acclimated to temperatures of 15 and 5°C. After three weeks of thermal acclimation, the structural order of the phospholipids was measured by electron spin resonance (ESR) spectroscopy at five temperatures (0–20°C) that span the physiological range of Placopecten magellanicus (Fig. 1). The vesicles prepared from gills of 5°C-acclimated scallops were significantly (ANCOVA, P 0.03) less ordered than vesicles from 15°C-acclimated scallops. Temperature acclimation had shifted the order parameter curve 1–2°C toward lower assay temperatures, giving a homeoviscous efficacy (13) of 14%. Such a partial adjustment towards an ideal or complete homeoviscous response has also been found in crabs (14) and crayfish (15). In these invertebrates, the costs of perfect compensation may be too high, or the benefits too low. On the other hand, the ESR measurements in this study were made with the spin probe 5-doxyl stearic acid, reflecting the homeoviscous response in the outer region of the purified lipid bilayer. It is possible that the response deeper in the bilayer, in the actual region of alkenyl chain unsaturation, would have been greater (16). Received 30 November 2001; accepted 22 March 2002. 1 Present address: Department of Physiology and Experimental Medicine, The George Washington University Medical Center, Ross Hall Room 402, 2300 Eye Street, NW, Washington, DC 20037. 2 To whom correspondence should be addressed. E-mail: cparrish@ mun.ca Reference: Biol. Bull. 202: 201–203. (June 2002)

The reproductive cycle and physiological ecology of the mussel Mytilus edulis in a subarctic, non-estuarine environment

Mytilus edulis L. occupies a habitat in eastern Newfoundland, Canada, which is characterised by low temperatures and a lower concentration of particles in suspension than is found in estuarine locations. Gametogenesis occurs in the spring, and energy reserves from the previous year are not utilised in the synthesis of gametes. Oxygen uptake is not elevated during the winter, in contrast to most European populations, which undergo gamete development at that time. There is little seasonal variation in clearance rate. This may be attributable in part to the absence of the high particulate load which has been shown to depress feeding activity in some estuarine populations. Energy balance is positive at all times of the year, and conversion efficiency is high, suggesting that conditions are favourable. A number of physiological comparisons are made between Newfoundland mussels and those from European waters.

Gill function and particle transport in Placopecten magellanicus (Mollusca: Bivalvia) as revealed using video endoscopy

The technique of endoscopic video observation was used to study feeding processes of Placopecten magellanicus (Gmelin), collected from Bull Arm, Newfoundland in August 1991 and 1992, under near-natural feeding conditions. The fate of captured particles depended on the extent of ingestive or handling capacity saturation. Under low (1 to 10 particles μl-1) to medium (10 to 20 particles μl-1) particle concentrations, most particles were incorporated in continuous anteriorly directed slurries in the dorsal ciliated tracts of the gill arch and dorsal bends. As particle concentration or exposure time to the lower particle concentrations increased, four endogenous mechanisms of ingestion volume control were increasingly observed: (1) rejection of dense mucus-particle masses from the principal filament troughs onto the ventrally beating cilia and associated currents of the ordinary filament plicae, counter to and below the incoming pallial current maintained by the principal filament cilia; (2) intermittent stopping of the anteriorward flow in the dorsal ciliated tracts; (3) reduction or cessation of input from the principal filaments to the dorsal ciliated tracts; (4) detachment of the dorsal bends from the mantle to establish a shunt from the infrabranchial to the suprabranchial cavity. Chemical and histochemical tests of purified fluid withdrawn from the dorsal ciliated tracts indicate that mucus is present at all particle concentrations. Mucus therefore participates both in normal feeding and in ingestion volume control on the bivalve gill, although different mechanisms, and types of mucus, effect transport of material in the dorsal (feeding) and ventral (cleaning) ciliated tracts.

Lipid composition of eggs and adductor muscle in giant scallops (Placopecten magellanicus) from different habitats

Scallops (Placopecten magellanicus Gmelin) were collected during August 1989 from shallow water (10 m) and deep water (31 m) habitats at Sunnyside, Trinity Bay, Newfoundland, to compare the lipid composition of eggs and adductor muscle tissue. Less favorable food levels and lower temperature conditions associated with deeper water have previously been shown to produce slower growth and reduced fecundity in individuals from this habitat. Triacylglycerol reserves consistently accounted for 60% of the total lipids present in both groups. The total lipid content of the eggs and the composition of their triacylglycerol fatty acid pools were similar in shallow water and deep water scallops, indicating very little if any nutritional difference between the two groups. Relative to their counterparts from shallow water, individuals from deeper water contained higher proportions of docosahexaenoic acid [22:6(n-3)] in the egg phospholipids and higher levels of 24-methylenecholesterol (a phytosterol commonly found in diatoms) in the adductor muscle. Differences in fatty acid composition are interpreted as biochemical adjustments of cell membranes to increase membrane fluidity, thereby compensating for the lower temperatures prevailing at the greater depth.

In Vivo Studies of Suspension-Feeding Processes in the Eastern Oyster, Crassostrea virginica (Gmelin)

Suspension-feeding processes in the eastern oyster Crassostrea virginica (Gmelin, 1791) were examined, in vivo, with an endoscope linked to a video image-analysis system. We found that many of the previously published concepts of particle transport and processing in this species, obtained using surgically altered specimens or isolated organs, are incomplete or inaccurate. In particular, our observations demonstrate that (1) captured particles are transported along the gills by both mucociliary (marginal grooves) and hydrodynamic (basal tracts) processes; (2) the labial palps accept material from the gills both in mucus-bound particle strings (transported in marginal grooves), and suspended in particle slurries (transported in basal tracts); (3) the labial palps reduce the cohesive integrity of the mucous strings and disperse and sort the entrapped particles; (4) particles are ingested in the form of a slurry; and (5) ciliary activity on the labial palps is independent of that on the lips, allowing the oyster to filter particles from suspension and produce pseudofeces without ingesting any particulate matter. Because many ostreids have the same plicate gill structure, we believe that our conclusions are applicable to other oyster species. In addition, the present observations are consistent with other endoscopic examinations recently made on bivalves in different families. We conclude that accepted theories of particle handling in suspension-feeding bivalve mollusks must be modified to accommodate observations made with the endoscope.

Direct observations of feeding structures and mechanisms in bivalve molluscs using endoscopic examination and video image analysis

A new technique is described for observing the structures and mechanisms of suspension feeding in bivalves using endoscopic examination and video image analysis. This method permits direct in vivo observations of whole, intact structures of relatively undisturbed specimens. No surgical alterations of shell or tissue are required for most species. Pallial organ activity can be recorded for future observations and analysis. Using this technique we examined three bivalve species, each with different degrees of mantle fusion:Mya arenaria L.Mytilus edulis L., andPlacopecten magellanicus (Gmelin). The specimens were collected between April and September 1990 at various locations in Trinity Bay, Newfoundland, Canada. Particle retention by the gill and transport of material to the palps was observed, and velocity of particles moving on the gill was determined. We demonstrate that the endoscope-video-analysis system is an efficient and affordable technique suitable for studies of pallial organ function and mechanisms of feeding.

Biodeposition by the horse mussel Modiolus modiolus (Dillwyn) during the spring diatom bloom

The production and nutritional quality of faeces and pseudofaeces from the horse mussel Modiolus modiolus (Dillwyn) was measured during the spring diatom bloom in Logy Bay, Newfoundland. Chloropigments, organic carbon, organic nitrogen and biogenic silica were determined in faeces and pseudofaeces, and compared with values from seston samples. The biodeposition rate of a horse mussel (faeces and pseudofaeces) of 5 g dry meat weight varied from 40.9 mg dry weight · day−1 at the peak of the bloom to 4–8 mg · day−1 at the end. The concentrations of organic carbon and organic nitrogen were lower in the faeces and pseudofaeces than in the seston, but chlorophyll a levels were greater in the pseudofaeces than in the seston or faeces. In terms of dry weight, faeces production was five or more times greater than pseudofaeces production, but the latter were more important in recycling chlorophyll a, especially during the peak of the diatom bloom. Microscopic analysis of the biodeposits showed that chains and large diatoms were very abundant in the pseudofaeces, whereas the faeces contained only frustules of small cells and of short chains, indicating that M. modiolus concentrates the large diatoms contained in the seston and rejects them in the pseudofaeces. The rejection of organic-rich diatoms by M. modiolus during the spring bloom provides a mechanism for the recycling of nutrients to suspension-feeders and deposit-feeders.

Aerobic Metabolism Octopine Production And Phosphoarginine As Sources Of Energy In The Phasic And Catch Adductor Muscles Of The Giant Scallop Placopecten-Magellanicus During Swimming And The Subsequent Recovery Period

1. 1. The energy contributions of aerobic metabolism, phosphoarginine, ATP and octopine in the adductor muscles of P. magellanicus were examined during swimming and recovery. 2. 2. A linear relationship was observed between the size of the phosphoarginine pool and the number of valve snaps. A linear increase in arginine occurred during the same period. 3. 3. Octopine was formed during the first few hours of recovery, particularly in the phasic muscle. 4. 4. The restoration of the phosphoarginine pool appeared to be by aerobic metabolism. 5. 5. It is concluded that the role of octopine formation is to supply energy when the tissues are anoxic and to operate at such a rate as to maintain the basal rate of energy production.

The effects of coastal and estuarine conditions on the physiology and survivorship of the mussels Mytilus edulis, M. trossulus and their hybrids

Among the three taxa of smooth-shelled blue mussels, it is widely believed that Mytilus trossulus is the best adapted to conditions of low and/or fluctuating salinity. We examined the affect of two contrasting environmental regimes on aspects of the biology of M. edulis, M. trossulus and their hybrids from Newfoundland, Atlantic Canada. We employed two experimental treatments, “coastal” (natural, unfiltered coastal seawater pumped directly into the laboratory) and “estuarine” (natural, unfiltered freshwater from a nearby pond added to ambient sea water), and for both treatments we measured temperature, salinity, total particulate matter (TPM mg l−1), and particulate organic matter (POM mg l−1). We compared the mortality rate, energy acquisition, biochemical composition and specific growth rate (SGR) of M. edulis, M. trossulus and their hybrids under both sets of experimental conditions. Under coastal conditions, mussels from two sites (Bellevue, an open coast, non-estuarine site: Traytown, an estuarine site) exhibited mortality rates which were so low that they precluded meaningful statistical comparison. Under estuarine conditions, mortality occurred among M. trossulus mussels from both sites before it occurred among the M. edulis mussels. Consistent with the mortality data we also observed greater utilisation of non-protein energy reserves in M. trossulus than in M. edulis. Our data therefore do not support the hypothesis that M. trossulus is better adapted to estuarine conditions than is M. edulis. Although there was some limited evidence of taxon-specific effects for the other variables investigated (SGR, soft body C:N ratio, absolute C and N values (μg per mg dry body weight), weight standardised clearance rate (CRS), faecal production rate (mg dry weight day−1 and mg ash-free dry weight (AFDW) day−1), and absorption efficiency (AE)), these were almost completely obscured by the effects of the experimental treatment. We conclude that the lack of taxon-specific differences in physiological responses (SGR, CRS, faecal production rates, AE) indicates that the higher mortality rate of M. trossulus in the estuarine regime did not result from a reduction in energy intake. This interpretation is consistent with findings for the same taxa from northern Europe, and suggests that other factors at the cellular or biochemical levels (for example, a failure to control intracellular ionic balance) warrant attention as possible explanations for ecological differences between the two hybridising taxa. We suggest that detailed study of the micro-scale distributions of these mussels is required to examine further the influence of low salinity, and associated environmental variability, on the geographic distributions of both taxa and their hybrids in Atlantic Canada.