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<?xml version="1.0" ?> <tei> <teiHeader> <fileDesc xml:id="0"/> </teiHeader> <text xml:lang="en"> <head>INTRODUCTION<lb/></head> <p>Suspension feeding bivalves often occur in<lb/> shallow coastal waters and estuaries, an environ-<lb/>ment characterized by large variations in the concen-<lb/>tration of suspended particulate matter. A major part<lb/> of the suspended matter consists of inorganic silt or<lb/> refractory organic matter, leading to a 'dilution' of the<lb/> food particles in the seston (e.g. <ref type="biblio">W~ODOWS etaL, 1979;<lb/> BERG and NEWELL, 1986; SMAAL et aL, 1986</ref>).<lb/></p> <p>Most species of suspension feeding bivalves<lb/> produce high amounts of pseudofaeces when parti-<lb/>culate matter concentrations are above a threshold<lb/> level <ref type="biblio">(FOSTER-SMITH, 1975a)</ref>. A number of authors<lb/> <ref type="biblio">(J~RGENSEN, 1966; OWEN, 1966; BERNARO, 1974)</ref> stated<lb/> that this process of rejection of material through the<lb/> pseudofaeces was not just an overflow mechanism,<lb/> with rejection of the surplus material when the<lb/> filtration by the animal exceeds the maximum inges-<lb/>tion rate. They suggested that large and heavy<lb/> particles were sorted by the gills or the labial palps<lb/> and egested through the pseudofaeces. First ex-<lb/>perimental results were obtained by <ref type="biblio">FOSTER-SMi TH<lb/> (1975b)</ref>, who used mixtures of algae and alumina.<lb/> This author did not observe selection between<lb/> organic and inorganic particles. However, experi-<lb/>ments with a number or bivalve species fed with<lb/> mixtures of silt and phytoplankton have shown a<lb/> reduction in the amount of chlorophyll or fluorescent<lb/> particles present in the pseudofaeces, compared with<lb/> the food <ref type="biblio">(KIEIRBOE et al., 1980; KIs<lb/> and M~HLEN-<lb/>BERG, 1981; NEWELL and JORDAN, 1983; BRICELJ and<lb/> MALOUF, 1984)</ref>. The selective ingestion of food<lb/> particles may help the animals to maintain the food<lb/> uptake at high level, in spite of dilution of the food<lb/> items with indigestible material.<lb/></p> <p>The mechanism of selective feeding is still<lb/> unclear. It is unlikely that the density of the particles<lb/> plays a role in the selection <ref type="biblio">(KIORBOE and MOHLENBERG,<lb/> 1981)</ref>. Cell shape, electrical charge or chemical<lb/> characteristics may be important factors <ref type="biblio">(TEN WlNKEL<lb/> and OAVeDS, 1982; NEWELL et aL, 1989)</ref>. <ref type="biblio">Kl i ~RGOE and<lb/> MOHLENBERG (1981)</ref> have shown a positive correlation<lb/> between the size of the labial palps and the efficiency<lb/> of selective ingestion, suggesting the labial palps to<lb/> be the site of selection. Myti/us edulis from the<lb/> Wadden Sea, a site with high suspended matter<lb/> concentrations, had larger palps and a higher selec-<lb/>tion efficiency than mussels from the Oresund, where<lb/> particle concentrations are lower. <ref type="biblio">ESSINK eta/. (1989)</ref><lb/> have shown mussels to be able to adapt the relative<lb/> size of their gills and palps, after transplantation to<lb/> sites with high particulate matter concentrations. The<lb/> morphological adaptations may help to adapt the<lb/> selective feeding ability to the in situ concentrations.<lb/> In conclusion it is hypothesized that the efficiency of<lb/> selective ingestion may show adaptation to the<lb/> environmental seston concentrations (cf. <ref type="biblio" >KIeRBOE<lb/> and MOHLENBERG, 1981</ref>).<lb/></p> <p>In the Oosterschelde estuary (SW Netherlands)<lb/> suspended particulate matter concentrations are<lb/> generally between 2-70 mg.1-1 <ref type="biblio">(WETSTEYN et aL,<lb/> 1990)</ref>. The seston is mainly composed of inorganic<lb/> particles and refractory organic matter and phyto-<lb/>plankton forms only a minor fraction of the seston<lb/> (ca. 2%) <ref type="biblio">(SMAAL et al., 1986)</ref>. Earlier experimental<lb/> results <ref type="biblio">(PRJNS and SMAAL, 1989)</ref> suggest that selec-<lb/>tion efficiencies of mussels from the Oosterschelde<lb/> are comparable to the efficiencies observed by<lb/> <ref type="biblio">KIEIRBOE et al. (1981)</ref>. In this paper, experimental<lb/> results on the selective ingestion of algae by Mytilus<lb/> edulis and Cerastoderma edule from the Ooster-<lb/>schelde estuary are reported. Experiments were<lb/> carried out to establish the relation between the<lb/> concentration of suspended particulate matter and<lb/> the selection efficiency. The impact of the selective<lb/> ingestion on the food budget of the bivalves was<lb/> quantified.<lb/></p> <head>MATERIALS AND METHODS<lb/></head> <p>Experiments with the blue mussel Mytilus<lb/> edulis were carried out from January to March 1988.<lb/> Experiments with the cockle Cerastoderma edule<lb/> were carried out from March to April 1988. The<lb/> animals were collected in the western part of the<lb/> Oosterschelde, from sites near the low water tidal<lb/> level. They were transported to the field station of the<lb/> Tidal Waters Division, and stored in flowing sea<lb/> water. The mussels were cleaned of epizoa and<lb/> allowed to attach to individual trays. On this trays a<lb/> small partition was fixed wich separated the mantle<lb/> edge from the ouffiow siphon. This simplified the<lb/> separate collection of faeces and pseudofaeces. The<lb/> cockles were allowed to burrow in small cups filled<lb/> with sand.<lb/></p> <p>Mussels with a shell length of 57-64 mm were<lb/> selected. The ash free dry weight of mussels was<lb/> 1.133 + 0.069 g (n=15; mean + S.E.). The shell length<lb/> of the cockles ranged from 31-36 ram, and the ash<lb/> free dry weight of the cockles was 0.304 + 0.014 g<lb/> (n=20).<lb/></p> <p>The alga Phaeodactylum tricomutum was cultu-<lb/>red in large outdoor ponds. The carbon:chlorophyll<lb/> ratio of the algae was 131 _+ 3.9, and the C:N ratio<lb/> 7.9.<lb/></p> <p>Silt was collected by scraping off the upper<lb/> 0.5 cm of the sediment of a creek in the eastern<lb/> part of the Oosterschelde. The sediment was passed<lb/> through a 125 I.Lm sieve, and kept in stock suspen-<lb/>sions of 10-20 g j-1 The carbon content of the silt<lb/> was 2.7 + 1.7%, the C:N ratio was 19.6.<lb/></p> <head>Selection experiments<lb/></head> <p>The animals were kept in raceways, and fed with<lb/> a mixture of the diatom Phaeodactylum tricomutum<lb/> (20.103 cells.m1-1) and silt. The algae and silt were<lb/> continuouly added to a flow of filtered natural sea<lb/> water. The concentrations of suspended particulate<lb/> matter (SPM) were set at a range from 5 to 90 mg.I -t<lb/> in the mussel experiments, and from 20 to 120<lb/> mg.1-1 in the cockle experiments. The experiments<lb/> were carried out at in situ water temperatures, wich<lb/> varied between 5.1 and 7.9~ The current velocity in<lb/> the raceways was low (<1 cm.s -1) to prevent<lb/> resuspension of faeces and pseudofaeces.<lb/></p> <p>Before the start of the experiment animals were<lb/> stored in filtered sea water for 24 hours. In each<lb/> experiment 5 animals were fed with an experimental<lb/> diet for a period of 20-24 hours. At the end of that<lb/> period faeces and pseudofaeces of each individual<lb/> were collected separately. If necessary, faeces (and<lb/> sand in the experiments with cockles) were sorted<lb/> out of the pseudofaeces using the differences in<lb/> density. Faeces and pseudofaeces were subsampled<lb/> for various chemical analyses.<lb/></p> <p>The suspended particulate matter in the diets,<lb/> and pseudofaeces and faeces were analyzed for total<lb/> dry weight, inorganic matter content, chlorophyll-a<lb/> and phaeophytin-a. Dry weight was analyzed after<lb/> drying at 70~ for 48 hours. Inorganic matter content<lb/> was measured by determining ash content after<lb/> ignition at 520~ for 4 hours. Chlorophyll-a and<lb/> phaeophytin-a were measured by HPLC analysis.<lb/></p> <head>Calculation of chlorophyll budget<lb/></head> <p>Rates of faeces and pseudofaeces production<lb/> were recalculated to rates per hour. As the animals do<lb/> not absorb inorganic matter, it was assumed that the<lb/> total weight of inorganic matter (PIM) in the biode-<lb/>posits (faeces + pseudofaeces) equalled the amount<lb/> of PIM filtered by the animals. From the rate of<lb/> biodeposition of PIM (rag.h-I), and the PIM concen-<lb/>tration (mg.l-1) in the diets the clearance rate<lb/> (volume of water swept clear by the animals per time<lb/> unit) was calculated:<lb/></p> <formula>CLR = PIM biodeposition in I.h -1<lb/> [PIM]diet<lb/></formula> <p>Chlorophyll-a filtration (FIl_chl) was calculated as the<lb/> product of clearance rate and chlorophyll-a concen-<lb/>tration 9 (l~g.l-1) in the diets:<lb/></p> <formula>FILchl = CLR. [chl]diet in p.g.h -f<lb/></formula> <p>The rate of ingestion of chlorophyll-a (INGchJ) was<lb/> estimated from the difference between filtration and<lb/> rejection trough the pseudofaeces:<lb/></p> <formula>INGchl = FILchl-PSFchl in I~g.h -1<lb/></formula> <p> The amount of chlorophyll-a digested (DIGchl) was<lb/> estimated from the difference between the ingestion<lb/> and the amount of chlorophyll-a in the faeces:<lb/></p> <formula>DIGchi = INGchi-Fchl in i~g.h -1<lb/></formula> <p> The dry weight of the ingestion was estimated<lb/> from the dry weight of the faeces (mg.h-1), and the<lb/> ash content (mg.mg -1) of the food and faeces <ref type="biblio" >(CRtSP,<lb/> 1984)</ref>:<lb/></p> <formula>INGdrywt = Fdrywt [PIM/dry wt]faeces in mg.h -1<lb/> [PIM/dry Wt]diet<lb/></formula> <p>If no selection would occur, the composition of the<lb/> ingested material should be the same as the compo-<lb/>sition of the diet. The theoretical ingestion of chlo-<lb/>rophyll-a without selection could be estimated from<lb/> the dry weight of the ingestion (mg.h -1) and the<lb/> amount of chlorophyll-a in the diet (pg.mg-1):<lb/></p> <formula>INGno sel. = INGdry wt. [chl/dry Wt]diet in p.g.h -1<lb/></formula> <p>The rate of filtration of chlorophyll-a in the<lb/> experiments is dependent on both the concentration<lb/> in the diets and the clearance rate of the animals. In<lb/> order to get an estimate of the chlorophyll-a inges-<lb/>tion without the variance added by differences in<lb/> chlorophyll-a concentrations between experiments,<lb/> the percentage of the chlorophyll-a filtration that was<lb/> ingested was calculated.<lb/></p> <p>The observed chlorophyll-a ingestion was com-<lb/>pared to the (theoretical) ingestion without selection<lb/> in order to calculate a selection coefficient:<lb/></p> <formula>INGchJ<lb/> selection coefficient-<lb/>INGno sel.<lb/></formula> <p>This selection coefficient is equivalent to the factor of<lb/> increase of the chlorophyll-a ingestion, due to<lb/> selection. In case of non-selective ingestion the<lb/> coefficient is 1, if preferential ingestion occurs the<lb/> coefficient is higher than 1.<lb/></p> <p>The digestive efficiency was calculated from the<lb/> difference between the amount of chlorophyll-a<lb/> ingested, and the amount of chlorophyl-a in the<lb/> faeces <ref type="biblio">(ROBINSON et aL, 1984)</ref>:<lb/></p> <formula>digestive efficiency = (1 -Fchl/INGchl). 100%<lb/></formula> <figure type="table">Table 1. Chlorophyll budgets of Mytilus edu/is and Cerastoderma edul~, rates per animal (mean • S.E.).<lb/> SPM<lb/> ChLa<lb/> Clearance<lb/> Pseudo-<lb/>Chl.a<lb/> Chl.a<lb/> Cbl.a in<lb/> rate<lb/> faeces<lb/> filtration<lb/> ingestion<lb/> faeces<lb/> (mg.1-1 )<lb/> (p.g.1-1 )<lb/> (l.h -1 )<lb/> (mg.h -1 )<lb/> (p.g.h -1 )<lb/> (Hg.h -1 )<lb/> (iJg.h -1)<lb/> mussel<lb/> 4.8<lb/> 7.26<lb/> 0.64 -s<lb/> 1.8 •<lb/> 4.65 10.15<lb/> 3.10 _+0.30<lb/> 8.8<lb/> 7.46<lb/> 0.38 •<lb/> 2.1 !-0.3<lb/> 2.48 _+0.26<lb/> 1.86 10.15<lb/> 18.9<lb/> 5.91<lb/> 0.47 10.04<lb/> 6.7 10.7<lb/> 2.79 10.22<lb/> 1.28 •<lb/> 23.4<lb/> 6.28<lb/> 0.68 10.06<lb/> 13.2 •<lb/> 4.25 10.40<lb/> 2.71 •<lb/> 74.9<lb/> 5.11<lb/> 0.28 •<lb/> 19.8 •<lb/> 1.44 _+0.16<lb/> 0.23 10.08<lb/> 86.1<lb/> 5.14<lb/> 0.30 !-0.04<lb/> 24.3 •<lb/> 1.53 10.20<lb/> 0.06 10.06<lb/> cockle<lb/> 23.7<lb/> 4.20<lb/> 0.63 •<lb/> 12.0 •<lb/> 2.66 t0.26<lb/> 1.73 10.21<lb/> 24.99<lb/> 0.76<lb/> 0.55 •<lb/> 9.4 +1.3<lb/> 0.42 •<lb/> 0.36 !0.04<lb/> 28.51<lb/> 3.68<lb/> 0.75 _-+0.06<lb/> 12.4 •<lb/> 2.10 10.22<lb/> 1.19 •<lb/> 29.27<lb/> 1.18<lb/> 0.39 t-0.08<lb/> 8.1 •<lb/> 0.46 10.09<lb/> 0.40 10.11<lb/> 31.97<lb/> 2.78<lb/> 0.26 •<lb/> 3.7 •<lb/> 0.73 10.09<lb/> 0.61 10.08<lb/> 35.66<lb/> 3.08<lb/> 0.23 •<lb/> 3.2 •<lb/> 0.71 •<lb/> 0.58 10.11<lb/> 60.88<lb/> 5.03<lb/> 0.18 •<lb/> 9.2 •<lb/> 0.89 +0.15<lb/> 0.45 •<lb/> 92<lb/> 6.27<lb/> 0.23 10.03<lb/> 19.3 •<lb/> 1.46 10.20<lb/> 0.89 •<lb/> 114<lb/> 4.47<lb/> 0.34 _-_+0.05<lb/> 34.1 •<lb/> 1.51 10.24<lb/> 0.41 t-0.15<lb/> 121<lb/> 4.66<lb/> 0.36 •<lb/> 38.4 •<lb/> 1.66 ---4-0.15<lb/> 0.40 10.09<lb/> 0.43 •<lb/> 0.15 •<lb/> 0.24 !-0.05<lb/> 0.30 !0.22<lb/> 0.06 _+0.00<lb/> 0.06 10.00<lb/> 0.24 •<lb/> 0.05 10.01<lb/> 0.31 •<lb/> 0.10 •<lb/> 0.23 10.04<lb/> 0.24 •<lb/> 0.19 10.04<lb/> 0.18 10.04<lb/> 0.16 10.02<lb/> 0.19 10.03<lb/> Ing.<lb/> .100%<lb/> Fil.<lb/> 66+4.9<lb/> 66 + 2.4<lb/> 45 • 3.9<lb/> 63 + 4.5<lb/> 18+5.9<lb/> 4+4.2<lb/> 65 • 5.3<lb/> 81 + 3.6<lb/> 56 -+ 4.7<lb/> 83 -+ 2.7<lb/> 83 + 3.6<lb/> 82 • 3.0<lb/> 48 +13.3<lb/> 59 + 6.8<lb/> 26 + 7.6<lb/> 26 • 7.2<lb/></figure> <head>RESULTS<lb/></head> <head>Clearance rate and pseudofaeces production<lb/></head> <p>The calculated clearance rates are shown in<lb/> <ref type="table">Table 1</ref>. The clearance rates showed a negative<lb/> correlation with SPM concentrations (mussel:<lb/> r=-0.64, n=28, p<0.001; cockle: r=-0.34, n=46,<lb/> p<0.05). The amount of pseudofaeces produced<lb/> showed a strong increase with increasing SPM<lb/> concentrations in the diets (mussel: r=0.89, n=28,<lb/> p<0.001; cockle: r=0.84, n=46, p<O.100).<lb/></p> <head>Composition of pseudofaeces<lb/></head> <p>The chlorophyll-a content of the pseudofaeces<lb/> was compared to the composition of the diets <ref type="figure">(Fig.<lb/> 1)</ref>. The chlorophyll-a content of both mussel and<lb/> cockle pseudofaeces was significantly reduced com-<lb/>pared to the food. (Wilcoxon-test, p<O.O01).<lb/></p> <p>Phaeophytin-a concentrations in the pseudo-<lb/>faeces were often below the detection limit. If<lb/> phaeophytin-a was detected in the pseudofaeces, the<lb/> relative concentrations were not significantly diffe-<lb/>rent from the concentrations in the food (p>0.100).<lb/></p> <figure>~.<lb/> 0.3<lb/> ..............<lb/> .........<lb/> : $<lb/> o.1<lb/> ~'i' ........... * 4<lb/> ~D. 0.03<lb/> =~ o.oi<lb/> +<lb/> 0.003<lb/> .<lb/> . , ....<lb/> ,<lb/> ,<lb/> 9<lb/> 9 , ....<lb/> o.o3<lb/> 0.06 o.1 o.=<lb/> 0.5<lb/> ;<lb/> chl In food (pg/mg dry wt.)<lb/> Fig. 1. Clorophyll-a content of pseudofaeces compared to the<lb/> compositJon of the diets. Symbols are means • S.E. Squares:<lb/> Myti/us edulis, triangles: Cerastoderma edule.<lb/></figure> <head>Chlorophyll budget<lb/></head> <p>In <ref type="table">Table 1</ref> the components of the chlorophyll-a<lb/> budget are shown. The filtration of chlorophyll-a<lb/> showed some variation between diets due to dif-<lb/>ferences in chlorophyll-a concentrations between<lb/> diets, and decreased at high SPM concentrations<lb/> as a consequence of lower clearance rates. Multiple<lb/> regression showed that the ingestion of chlorop-<lb/>hyll-a was positively correlated with chlorophyll-a<lb/> filtration, but decreased with increasing SPM con-<lb/>centrations <ref type="table">(Table 2)</ref>.<lb/></p> <p>Due the negative effect of SPM on the chlo-<lb/>rophyll-a ingestion, the proportion of filtered chlo-<lb/>rophyll-a that was ingested decreased with increa-<lb/>sing SPM concentrations. In <ref type="figure">Fig.2</ref> the chlorophytl-a<lb/> ingestion as a percentage of the filtration is shown.<lb/> The curves represent the estimated chlorophyll-a<lb/> ingestion if no selection had occured, wich is an<lb/> inverse function of SPM.<lb/></p> <figure type="table">Table 2. Regression statistics of chlorophyll-a ingestion (INGchl'<lb/> pg.h -1 ) versus suspended particulate matter concentrations (SPM,<lb/> mg.1-1) and ehlorophyll-a filtration rate (FILchl" pg.h -1)<lb/> mussel<lb/> INGchl = 0.661+O.024*FILchl -0.011+0.001 *SPM<lb/> n328, F3415.3, p<O.O01, r2=0.97<lb/> cockle<lb/> INGchl = O.672_+O.040*FILchl -0.004+0.001 *SPM<lb/> n=44, F=214.5, p<O.O01, r230.91<lb/></figure> <p>On average, the selection coefficient of the<lb/> mussel was 2.0 + 0.3 (n=28; mean + st.error). The<lb/> selection coefficient of the cockle was 2.8 + 0.3<lb/> (n=44). The selection coefficients were not correlated<lb/> with SPM concentration. The difference between<lb/> mussel and cockle was not significant (p>0.05).<lb/></p> <figure>(A)<lb/> loo<lb/> 80<lb/> so<lb/> ~ zo<lb/> 100<lb/> 8O<lb/> eo<lb/> ==<lb/> c<lb/> ~ 4o<lb/> ~ ~ 20<lb/> '.. 1<lb/> ': ....... : ........ : ....... : ........ :.+: ........ :<lb/> Z 'O<lb/> 40<lb/> $0<lb/> 80<lb/> 100<lb/> SPM In mg/I<lb/> (s)<lb/> \;++ t<lb/> 20<lb/> 40<lb/> 60<lb/> 80<lb/> 100<lb/> 120<lb/> 140<lb/> SPM in mg/I<lb/> Fig. 2. Chlorophyll-a ingestion in % of chlorophyll-a filtration for<lb/> Mytilus edulis (A) and Cerastoderma edule (B). Symbols are means<lb/> • S.E. The curves represent the estimated chlorophyll-a ingestion (in<lb/> % of filtration) without selection.<lb/></figure> <head>Composition of faeces<lb/></head> <p>Due to the selective ingestion of chlorophyll-a<lb/> the ingested material contained relatively more chto-<lb/>rophyll-a than the diets. Compared to the com-<lb/>position of the diets however, mussel and cockle<lb/> ]=<lb/> faeces contained less chlorophyll-a (Wilcoxon-test,<lb/> ~.<lb/> p<O.O01), whereas phaeophytin-a concentrations<lb/> had increased both in mussel faeces (p<O.010) and in<lb/> $<lb/> cockle faeces (p<O.O01) <ref type="figure">(Rg.3)</ref>.<lb/> .=.<lb/></p> <p>The digestive efficiencies <ref type="table">(Table 3)</ref> varied be-<lb/>, =-<lb/>tween 60-90% in most diets, and decreased with<lb/> _ -=<lb/> increasing SPM concentrations (mussel: r=-0.45,<lb/> p<O.05; cockle: r=-0.41, p<O.01).<lb/></p> <figure type="table">Table 3. Digestive efficiencies of Mytilusedu/isand Cerastoderma<lb/> edule (mean :!: S.E.).<lb/> SPM<lb/> Chlorophyll<lb/> Oigestive<lb/> efficiency<lb/> (mg.1-1 )<lb/> (l~g.1-1 )<lb/> (%)<lb/> mussel<lb/> 4.8<lb/> 7.26<lb/> 85 + 2.9<lb/> 8.8<lb/> 7.46<lb/> 92 • 0,5<lb/> 18.9<lb/> 5.91<lb/> 80 + 4;5<lb/> 23.4<lb/> 6.28<lb/> 90 + 6.2<lb/> 74.9<lb/> 5.11<lb/> 73 + 8.3<lb/> 86.1<lb/> 5.14<lb/> 79 +15.8<lb/> cockle<lb/> 23.7<lb/> 4.20<lb/> 85 + 2.6<lb/> 24.99<lb/> 0.76<lb/> 84 • 0.9<lb/> 28.51<lb/> 3.68<lb/> 71 • 7.1<lb/> 29.27<lb/> 1.18<lb/> 75 + 7.7<lb/> 31.97<lb/> 2.78<lb/> 61 • 6.8<lb/> 35.66<lb/> 3.08<lb/> 59 + 3.9<lb/> 60.88<lb/> 5.03<lb/> 36 •<lb/> 92<lb/> 6.27<lb/> 77 • 5.8<lb/> 114<lb/> 4.47<lb/> 50 •<lb/> 121<lb/> 4.66<lb/> 45 +10.5<lb/></figure> <head>DISCUSSION<lb/></head> <p>The experiments were carried out with concen-<lb/>trations of algae and silt that are reprensentative of<lb/> the natural situation <ref type="biblio">(SMAAL et aL, 1986)</ref>. The<lb/> concentrations used were all higher than the thres-<lb/>hold for pseudofaeces formation (between 1-6.5<lb/> rag.l-l; <ref type="biblio">BAYNE and NEWELL, 1983</ref>).<lb/></p> <p>The clearence rates of mussel and cockle<lb/> showed a decrease at high SPM concentrations. In<lb/> experiments wih pure algal cultures several bivalve<lb/> species have been observed to regulate their clearan-<lb/>ce rate in response to food concentrations. When the<lb/> ingestion rate has reached its maximum, clearance<lb/> rates are reduced in reponse to increasing food<lb/> concentrations, thus maintaining ingestion rates at a<lb/> constant level without producing pseudofaeces <ref type="biblio">(WIN-<lb/>TER, 1973; GERDES, 1983; WINTER etaL, 1984; SPRUNG<lb/></ref></p> <figure> 0.3<lb/> 0.1<lb/> 0.03<lb/> 0.01<lb/> 0.003<lb/> o.o= " o.;~<lb/> o:1<lb/> 0:=<lb/> (A)<lb/> ............. ~ *$<lb/> ~<lb/> f ~<lb/> t'<lb/> 0:6 ;<lb/> I<lb/> ohl In food (pg/mg dry wt.)<lb/> 0.2<lb/> 0.1'<lb/> 0.05<lb/> 0.01<lb/> 0.005 '<lb/> 0.001<lb/> 0.001<lb/> (s)<lb/> §<lb/> ....<lb/> 9<lb/> II<lb/> i<lb/> 0.005<lb/> 0.01<lb/> 0.05<lb/> 0.1<lb/> phaeophytln In food (pg/mg dry wt.)<lb/>o:=<lb/> Fig. 3. Chlorophyll-a (A) and phaeophytin-a (R) content of faeces<lb/> compared to the composition otthe diets. Symbols are means • S.E.<lb/> Squares: Myti/us edu/is, triangles: Cerastoderma edu/e.<lb/></figure> <p><ref type="biblio">and ROSE, 1988)</ref>. Experiments with pure algal cultu-<lb/>res should be interpreted with care, however <ref type="biblio">(BAYNE<lb/> and NEWELL, 1983; JBRGENSEN, 1990)</ref>. In experiments<lb/> with diets more closely resembling the natural<lb/> situation, the reduction in clearance rates as a<lb/> consequence of high particle concentrations is much<lb/> less pronounced <ref type="biblio">(WINTER, 1978; W~ODOWS etaL, 1979;<lb/> KIORBOE et aL, 1981)</ref>. Increasing particle concentra-<lb/>tions cause a slight reduction of the clearance rate,<lb/> according to <ref type="biblio">BAYNE and NEWELL (1983)</ref>. The amount of<lb/> material filtered and the production of pseudofaeces<lb/> increase, and ingestion is maintained at a constant<lb/> level. Our results, showing a reduction in clearance<lb/> rate and a strong increase in pseudofaeces produc-<lb/>tion at high SPM concentrations, agree with this<lb/> pattern.<lb/></p> <p>The results show that chlorophyll-a concentra-<lb/>tions in the pseudofaeces are reduced, compared to<lb/> the food. As the pseudofaeces were collected over a<lb/> period of 20-24 hours the loss of chlorophyll-a from<lb/> the pseudofaeces might have been caused by degra-<lb/>dation of the pigment. The reduced chlorophyll-a<lb/> concentrations did not coincide with an increase in<lb/> the amount of phaeophytin-a, indicating that no<lb/> substantial breakdown of the chlorophyll-a molecule<lb/> within the period of 24 hours occured. <ref type="biblio">ROBINSON etaL<lb/> (1984)</ref> observed no changes in chlorophyll-a con-<lb/>centrations in faeces from Spisu/a so/idissima, kept<lb/> in seawater at 12~ during a period of 52 hours. It is<lb/> therefore concluded that the chlorophyll-a in the<lb/> pseudofaeces remained undegraded during the 24-<lb/>hour period. The relatively low chlorophyll-a concen-<lb/>trations in the pseudofaeces are explained by selecti-<lb/>ve ingestion of phytoplankton. The ability of Myti/us<lb/> edulis and Cerastoderma edule to selectively ingest<lb/> algae agrees with observations from other authors<lb/> <ref type="biblio">(KIORBOE etaL, 1980; KIORBOE and MOHLENBERG, 1981;<lb/> CUCCl et al., 1981 )</ref>.<lb/></p> <p> The calculated chlorophyll-a budgets show the<lb/> effective selection by the bivalves. The clearance<lb/> rates of the bivalves in our experiments were low<lb/> compared to other observations <ref type="biblio">(SMAAL et aL, 1986)</ref>.<lb/> The rates of ingestion were comparable to values<lb/> reported by <ref type="biblio">BAYNE et al. (1989)</ref>. As a consequence,<lb/> pseudofaeces formation may have been depressed in<lb/> our experiments, wich means that our estimates of<lb/> the selection coefficients may have been too low. In<lb/> most of the diets the animals were able to ingest<lb/> 40-90% of the filtered chlorophyll. The proportion of<lb/> filtered algae that was ingested decreased when SPM<lb/> concentrations increased. Similar estimates made by<lb/> <ref type="biblio">BRICELJ and MALOUF (1984)</ref> for Mercenaria mercenaria<lb/> showed that this species ingested at least 82% of the<lb/> filtered algae. The lowest percentage was observed at<lb/> the highest SPM concentrations. <ref type="biblio" >KtORBOE and M~H-<lb/>LENBERG (1981)</ref> used the ratio between chlorophyll-a<lb/> content of food and pseudofaeces as a measure of<lb/> selection efficiency. This measure has several disad-<lb/>vantages. It will decrease at high seston concentra-<lb/>tions, and does not give information on the quantita-<lb/>tive effect of the selective ingestion on the food<lb/> budget of the animals. Nevertheless, if we use our<lb/> results obtained under similar experimental condi-<lb/>tions (SPM<50 rag.l-l), to calculate selection effi-<lb/>ciencies as defined by <ref type="biblio">KIORBOE and MOHLENBERG<lb/> (1981)</ref>, average values of 2.5 for the mussel and 3.0<lb/> for the cockle are derived. These values are slightly<lb/> lower than the selection efficiencies reported by them<lb/> (2.9 for mussels from the Oresund, 4.3 for coc-<lb/>kles).<lb/></p> <p>If no selection had occured the proportion of<lb/> filtered chlorophyll-a that was ingested would have<lb/> shown a sharp decrease at high SPM concentrations.<lb/> The observed ingestion of chlorophyll-a by mussel<lb/> and cockle was 2.0 and 2.8 times higher, respective-<lb/>ly. These results show both bivalve species to be able<lb/> to increase the ingestion of chlorophyll at SPM<lb/> concentrations above the pseudofaeces threshold.<lb/> The selection coefficients were not affected by SPM<lb/> concentration. Still, a negative effect of the SPM<lb/> concentration on the rate of chlorophyll-a ingestion<lb/> was observed. This observation shows that the<lb/> animals can partly counteract the diluting effect of<lb/> high SPM concentrations, by a 2-3 times increase of<lb/> the ingestion of food particles. At the highest SPM<lb/> concentrations Mytilus edulis seemed not to be able<lb/> to effectively select phytoplankton anymore, possibly<lb/> as a consequence of the increased dilution of the<lb/> algae with silt. On the other hand, this result may<lb/> have been an artefact since differences in the very<lb/> tow relative concentrations of chlorophyll-a in food<lb/> and pseudofaeces at the highest SPM concentrations<lb/> were hard to detect. The cockle had a slightly higher<lb/> selection coefficient than the mussel, and there was<lb/> no comparable decrease of the ingestion rate at high<lb/> SPM concentrations. This result agrees with the<lb/> results of <ref type="biblio">KIORBOE and MOHLENBERG (1981)</ref>, and may<lb/> indicate a better adaption of the cockle to high SPM<lb/> concentrations.<lb/></p> <p>Due to the preferential ingestion of algae the<lb/> relative amount of chlorophyll-a in the ingested<lb/> material is higher than in the food. This phenomenon<lb/> may lead to higher concentrations in the faeces than<lb/> in the food, even if the material is digested <ref type="biblio">(NEWELL<lb/> and JORDAN, 1983)</ref>. Still, in the faeces the chlorophyll-<lb/>a content was lower than the concentrations in the<lb/> diets. This fact, combined with the simultaneous<lb/> strong increase in the amount of phaeophytin-a in the<lb/> faeces, indicate that a large part of ingested chloro-<lb/>phyll-a was digested in the alimentary tract of the<lb/> bivalves. The digestive efficiencies calculated for<lb/> chlorophyll-a were high in most diets, and in the<lb/> same range as reported by other authors <ref type="biblio">(ROBINSON et<lb/> aL, 1984; HAWKINS eta/., 1986; BAYNE et aL, 1987)</ref>.<lb/> In diets with a high SPM concentration the chloro-<lb/>phyll-a digestion decreased. A decrease in digestive<lb/> efficiencies at high SPM concentrations was also<lb/> observed by <ref type="biblio">ROBINSON et aL (1984)</ref>. <ref type="biblio">BAYNE et aL<lb/> (1987)</ref> observed the lowest digestive efficiencies in<lb/> diets with a high food quality (high amount of algae),<lb/> wich is in contrast to the decrease at high SPM<lb/> concentrations in our study. In their experiments<lb/> BAYNE eta/. (1987) used particle concentrations<lb/> below the pseudofaeces threshold, wich influenced<lb/> the maximum ingestion rate and may have affected<lb/> physiological mechanisms like gut passage time and<lb/> digestion in a way different from our experiments.<lb/></p> <head>Concluding remarks<lb/></head> <p> In our experiments Mytilus edu/is and Cerasto-<lb/>derma edu/e from the Oosterschelde were offered<lb/> diets with concentrations of algae and SPM that are<lb/> representative of the conditions in this estuary. Both<lb/> species showed an ability to selectively ingest<lb/> phytoplankton, and were able to increase the inges-<lb/>tion rate of chlorophyll-a 2 to 3 times compared to<lb/> the estimated ingestion without selection.<lb/></p> <p>Selective ingestion is most efficient when filtra-<lb/>tion rates are not reduced at high SPM concentra-<lb/>tions, and pseudofaeces production is high <ref type="biblio" >(BRICELJ<lb/> and MALOUF, 1984)</ref>. Animals which control ingestion<lb/> primarily by reducing clearance rates are probably<lb/> less succesful at exploiting a turbid environment than<lb/> animals which combine high filtration rates which<lb/> pseudofaeces production and pre-ingestive selection<lb/> <ref type="biblio">(BRICELJ and MALOUF, 1984)</ref>. The increasing metabolic<lb/> costs of filtration, pseudofaeces formation (e.g.<lb/> mucus excretion) and digestion, at high silt concen-<lb/>trations <ref type="biblio">(BAYNE eta/., 1987; 1989)</ref>, however, set an<lb/> upper limit to the SPM concentrations at which the<lb/> animals are able to maintain a positive net energy<lb/> budget.<lb/></p> <p>In addition to selective ingestion bivalves may<lb/> compensate for unfavourable food conditions by<lb/> other physiological processes, like adaption of the<lb/> clearance rate and modification of the digestion<lb/> process <ref type="biblio">(BAYNE and NEWELL, 1983; BRICELJ and MA-<lb/>LOUF, 1984; CUCC~ et al., 1985; SHUMWAY et aL, 1985;<lb/> NEWELL et al., 1989)</ref>, and by morphological adaptions<lb/> of the feeding apparatus <ref type="biblio">(ESSINK eta/., 1989)</ref>. The<lb/> various physiological and morphological compensa-<lb/>tory mechanisms increase the range of SPM concen-<lb/>trations to which the bivalves can adapt.</p> </text> </tei>