The concentration and composition of suspended particulate food available to estuarine suspension-feeding bivalve molluscs varies temporarily and spatially. Non-algal food sources may be important to suspension-feeders when algal concentrations are seasonally low or where there are high concentrations of suspended detrital material and bacteria, as found withln marshes. We carried out a series of laboratory experiments and field measurements to determine to what extent 2 common estuarine bivalve molluscs, the oyster Crassostrea virgin~ca and the ribbed mussel Geukensia demissa, could utilize cellulose and bacteria from Canary Creek marsh. Delaware, USA. Endogenously produced extracellular cellulases of the oyster depolymerized ingested cellulose to soluble oligomers. Subsequent intracellular cleavage of the oligomers to glucose was limited. The oyster absorbed carbon from refractory cellulosic material with an efficiency of only 3 %. In contrast, the ribbed mussel absorbed carbon from the same cellulosic material with an efficiency of 9 % and thls increased to 14 % if mussels were subjected to a 6 h exposure/6 h submergence cycle, a typical exposure regime for this intertidal species. We estimated that suspended cellulosic carbon in Canary Creek marsh during summer could supply 0.7 O/O and 8.6 % of the respiratory carbon requirements of subtidal oysters and intertidal mussels, respectively. In laboratory feeding experiments, colonization of refractory cellulosic food material by cellulolytic bacteria isolated from the marsh resulted in the oyster indirectly assimilating cellulosic carbon with an efficiency of 10 %. The oyster was able to filter free, unattached bacteria from suspension with an efficiency of only 5.0 %, compared with an efficiency of 15.8 % for the ribbed mussel. We estimated that both unattached and attached bacteria combined in Canary Creek marsh during summer provide only 5.5 % of the oysters' metabolic carbon requirements but could provide 31.0 % of an intertidal mussel's metabolic carbon requirements. Experiments with 1 5~ labelled bacteria indicated that attached bacteria associated with the breakdown of cellulosic material could mediate the flow of dissolved inorganic nitrogen from seawater to the oyster We estimated that unattached and attached bacteria in Canary Creek marsh during summer could contribute 26.7 % and 70.6 % of the metabolic nitrogen requirements of subtidal oysters and intertidal mussels, respectively. These results indicate that in thls marsh, utilization of bacteria as a food source could make a significant contribution during the summer to the nitrogen requirements of the oyster and to the carbon and nitrogen requirements of the mussel. However, cellulosic detritus and bacteria do not appear to fully meet the requirements of these bivalve species for carbon and nitrogen and utilization of other food sources is required, such as phytoplankton, nanozooplankton or non-cellulosic particulate and dissolved organic matter O Inter-Research/

The ab~lity of the ribbed mussel Geukensia demissa (Dillwyn) to ingest and absorb refractory lignocellulosic carbon was evaluated using 14C-radiotracer techniques. Mussels were maintained in a simulated tidal cycle of 6 h submergence and 6 h exposure and were fed on [14C]lignocellulose during the first submergence period. The mean (f SD) absorption efficiency for 14C was 13.3 + 2.5 % in an experiment performed in January 1985, and 14.2 f 1.6 % in a second experiment conducted in March 1985. Approximately 29 % of absorbed I4C was respired and 31 % incorporated into the proteidlipid fraction of the mussel. Mean (k SDI cellulose concentration in seston samples collected in the summer from a small creek dralmng a Delaware salt marsh was 66 + 28 pg I-'. It was estimated that adult ribbed mussels in this marsh could obtain 15 % of their total carbon requirements in the summer by utilization of refractory cellulosic material.

The vascular plant Spartina alterniflora Loisel., grown in an atmosphere containing '"CO2, was chem~cally treated to produce a lignocellulosic, crude f~b e r material containing 0 1 ' 10 lipid, no measurable protein, 4.4 Yo starch, 85.4 % cellulose and 7.7 YO lignin. This material was introduced. via syringe through the mouth, into the stomachs of oysters Crassostrea virginjca; these were then held for 24 h in 13 ppt filter-sterilized seawater at 25 "C. A second group of C, virginica was treated identically, except for the addition of 5 mg I-' of the antibiotics chlorarnphenlcol and rifampicin to both food material and seawater. Direct enumeration (DAPI) of bacteria demonstrated that treatment with antibiotics eliminated bacteria from the oysters' stomach fluid. The I4C specific activities of cell-free hemolymph and tissue samples taken 24 h after feeding indicated that oysters were able to digest and absorb carbon from the S. alterniflora mater~al with a mean efficiency of 1.3 %. There were no significant differences (ANOVA, p > 0.05) in the digestion and absorption of I4C material between antibiotic treated and untreated oysters. Furthermore, in vjtro cellulolytic activities of tissue homogenates of antibiotic treated oysters were not significantly different (ANOVA, p > 0.05) from those of untreated oysters. These results indicate that oysters are able to digest only small amounts of refractory cellulosic material and that this process is not enhanced by bacterla present in the stomach. In vitro characterization of the cellulolytic enzymes of the crystalline style of C. virginica under aseptic conditions indicated the presence of P-1,4-glucanase (Cx cellulase) activity that released oligosaccharides from the S. alterniflora material. Neither C, cellulase, capable of degrading crystalline cellulose, nor $-glucosidase activities were detected, using cotton fibre and cellobiose as substrates, respectively. These results suggest that the principal function of style cellulase activity is the partial depolymerisation of refractory amorphous cellulose, perhaps aiding the digestion of algal cells and detritus.

DIFFICULT to machine, high temperature materials have been introduced at an ever increasing rate into aircraft propulsion systems over the last twenty five years. Over the same period, the aircraft industry has had an ‘on‐off’ affair with electrochemical machining (ECM).