From Spartina Alterniflora by the Ribbed Mussel Geukensia Demissa

MARINE ECOLOGY PROGRESS SERIES Vol. 42: 171-179, 1988 - Published February 18 Mar. Ecol. Prog. Ser.

Utilization of refractory cellulosic carbon derived from Spartina alterniflora by the ribbed mussel Geukensia demissa

Daniel A. ~reeger",Christopher J. ~an~don~,Roger I. E. ~ewell~

l College of Marine Studies, University of Delaware, 700 Pilottown Road, Lewes, Delaware 19971, USA Hatfield Marine Science Center, Oregon State University, Newport. Oregon 97365. USA Center for Environmental and Estuarine Studies, Horn Point Laboratories. University of Maryland, PO Box 775, Cambridge, Maryland 21613, USA

ABSTRACT: The ab~lityof 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.

INTRODUCTION Maccubbin & Hodson 1980). Very little S. alterniflora is grazed by herbivores prior to senescence (Pomeroy & The ribbed mussel Geukensia demissa (Dillwyn) has Wiegert 1981). Following senescence, non-structural been reported to be the dominant animal in salt mar- components are rapidly removed from S. alterniflora by shes of the eastern United States (Kuenzler 1961a, bacteria and fungi (Benner et al. 1984) or by natural Jordan & Valiela 1982). Filtration and biodeposition by leaching processes (Haines & Hanson 1979, Wilson et G. demissa can be important agents in the cycling of al. 1986). Approximately 20 % of post-senescent plant energy (Kuenzler 1961a), phosphorus (Kuenzler material may be exported from the marsh during 1961b), and nitrogen (Jordan & Valiela 1982) in the unusually high tides (Pickral & Odum 1976) or by ice marsh. Despite the obvious importance of G. demissa in rafting (Heinle & Flemer 1976). However, most of the salt marsh ecosystems (Jordan & Valiela 1982), few structural lignocellulose remains in the marsh and studies have examined the mussel's ability to utilize slowly decomposes in situ (Maccubbin & Hodson 1980, refractory carbon from the dominant vascular plant, Valiela et al. 1985). Spartina alterniflora (Teal 1962). Filter-feeding bivalves typically assimilate phyto- Salt marshes are detritus-based systems and most of plankton with an efficiency greater than 75 % (Winter the detrital material is derived from Spartina alterni- 1978, Bayne & Newell 1983). In contrast, bivalves flora (Teal 1962, Odum & de la Cruz 1967, Marinucci apparently are unable to efficiently assimilate refrac- 1982, Valiela et al. 1985). Approximately 70 to 82 % of tory compounds such as lignocellulose. Growth studies the dry weight of live S. alterniflora is carbohydrate indicate that salt marsh grass detritus is a poor food for (Squiers & Good 1974, Smith et al. 1979), of which 90 % Argopecten irradians (Kirby-Smith 1976), Mytilus is structural lignocellulose (McIntire & Dunstan 1976, edulis (Williams 198l), and Crassostrea virginica (Pennock 1981). Stuart et al. (1982) reported that the Present address: Hatfield Manne Science Center, Oregon mussel Aulacomya ater assimilated kelp detritus with State University, Newport, Oregon 97365, USA an efficiency of 50 %; however, detritus from kelp is

O Inter-Research/Printed in F. R. Germany 172 Mar. Ecol. Prog. Ser.

more easily degraded than detritus derived from salt ing. The distribuhon of 14C among cellulose, lignin, marsh vascular plants (Findlay & Tenore 1982). Newel1 starch, lipid and protein frachons was determined & Langdon (1986) and Crosby (1987) found that the according to the methods described by Newell & Lang- oyster Crassostrea virginica has a digestion efficiency don (1986). of less than 3 O/O for ['4C]lignocellulose prepared from Feeding experiments with ['4C]lignocellulose. Col- Spartina alterniflora. lection and acclimation of Geukensia demissa. Adult In contrast, stable isotope studies (Haines & Monta- ribbed mussels were collected for experiments in Janu- gue 1979, Hughes & Sherr 1983) indicate that material ary (mean & SD dry tissue weight [DTW] = 0.36 & derived from Spartina alterniflora may be important in 0.10 g) and March (mean -t SD DTW = 0.28 + 0.03 g), the nutrition of intertidal fauna such as Geukensia from the Canary Creek marsh. The mussels were col- demissa. Peterson et al. (1985) measured the stable lected from a marsh elevation with an approximate isotope ratios of carbon, nitrogen, and sulfur in tissues tidal penodicity of 6 h submergence and 6 h exposure. of G. demissa collected from the Great Sippewissett Mussels were maintained in the laboratory under sub- Marsh. Mussels from the marsh interior had an isotopic mergence/exposure conditions which were in syn- signature indicating that 80 % of the assimilated diet chrony with natural tidal periodicity during both accli- was derived from S. alterniflora whereas the diet of mation and experimentation in order to avoid dsturb- mussels collected from seaward creeks was estimated ing their natural digestive rhythm (Gillmor 1982, Mor- to consist of 70 % phytoplankton. ton 1983). Mussels were acclimated to laboratory tem- In thls study, the ability of Geukensia demissa to perature (21 "C) for 2 to 3 wk before experimentation. drectly uhlize lignocellulosic carbon prepared from Unfiltered marsh water was added to the culture sys- Spartina alterniflora was quantified using tracer tem to provide the mussels with a diet of natural seston. techniques, under conditions simulating their natural, Experimental design. Water was collected from intertidal habitat. Canary Creek prior to each experiment and warmed to 21 "C. Prefiltered (38 pm mesh) marsh water was con- bnuously delivered at a mean (& SD) flow rate of 0.43 METHODS (& 0.11) l h-' to mussels held individually in 1 1 beakers. Water drained from each beaker through an over- Production of [14C]lignocellulose from Spartina flow tube positioned at a level which maintained alterniflora. Dormant short-form S. alterniflora plants 700 m1 of water in each beaker. The beakers were with associated soil were collected from the Canary gently mixed with magnetic stirrers to prevent particle Creek marsh, Delaware, USA (38"47'N, 75" 10'W) in settlement. Each beaker contained either one live March 1985. They were transplanted into 4 l plastic pots Geukensia demissa or a 'control mussel' consisting of a and held at 5OC until use. All above-ground biomass pair of empty mussel shells glued together. Each was removed. In June 1985,4 pots of plants were labeled mussel or control was positioned on a plastic Petri dish with 14C02, according to the methods described by suspended above the bottom of the beaker. Newel1 & Langdon (1986). The 14C-labeled plants were The experimental mussels were subjected to a 6 h harvested, dried and the lipid and protein fractions were submergence:6 h exposure cycle in the 2 feeding chemically removed. The residue was then treated with experiments as shown in Fig. 1. The ['4~]lignocellulose hot acid and hot alkali to remove non-lignocellulosic was suspended homogeneously in marsh water and material (Newell & Langdon 1986). delivered to the mussels during the first 6 h period of Newel1 & Langdon (1986) produced a refractory submergence only. A total of approximately 1.35 pCi material with 88 O/O of the 14C label distributed in the (9.64 mg cellulosic material) was delivered to each cellulose fraction, 8 % in the lignin fraction and 4 % in chamber over this 6 h feeding period. the starch fraction. Starch content was reduced further Clearance of ['4C]lignocellulose. Water samples in this study by incubating the extracted plant matenal (20 ml) were collected in rapid succession from the with a-amylase for 24 h. After incubabon, the extracted inflow, outflow, and water immediately above each material was separated from the digest by centrifuga- mussel every 2 h during the first submergence period. tion (1400 X g, 10 min) and washed twice with distilled The time required to collect each sample was recorded water. The product was ground, sieved with a 20 pm and used to calculate flow rates of seawater through sieve and lyophhzed. the expenmental chambers. Samples were preserved Triplicate subsamples of the 14C-labeled matenal with 1 O/O W/V (final concentration) formalin and were were weighed and suspended homogeneously in a gel stored at 5 "C. The instantaneous filtration rates (FR) of formed with 10 m1 Aquasol (New England Nuclear mussels fed on the [14C]lignocellulose particles were Research Products) and 3 m1 distiled water, and the calculated using a formula adapted from Hildreth & 14C-specific activity determined by scintdlation count- Cnsp (1976). Kreeger et al.. Utilization of refractory carbon by mussels 173

Unlabeled 14~-laboled Unlabelad Unlabeled Diet Diet H Diet H Diet

t-' t-' I f 100- - >

OmP X W il 4',I 4 1l Fig. 1. Protocol for submergence and ex- * o-:==-===-< 11 I: - Expl 1 Expt 2 posure of Geukensiademissa prior to n (dashed line) and during (solid line) I I I I I I I EXP~S1 and 2 -12 -6 0 6 12 18 24 30

I - 0 ured in the air above the water surface of the beakers, FR (m1 min-l) = X flow rate (m1 min-l) probably because the seawater was alkaline, at pH 7.9, M (1) and CO2 solubility was high. Respired 14C02from sub- where I, 0 and M refer to the respective radioactivities merged mussels was detected in the effluent water of (dpm ml-l) of inflow, outflow, and chamber samples. the beakers. Effluent from each beaker was collected in This formula corrects for refiltration of water by the 2 l stoppered flasks containing 1 O/O w/v (final concen- mussels in the experimental chambers (Hildreth & tration) formalin. At the end of each submergence Crisp 1976). Filtration rates were corrected for particle period, the volume of water collected in each 2 l flask settlement by subtraction of settlement rates measured was recorded, and a 100 m1 sub-sample was transferred in control beakers. Instantaneous mussel clearance to a 150 m1 flask. A 5 m1 glass test tube containing 600 rates (dpm min-') were determined every 2 h by multi- p1 38 % w/v KOH was then added to each flask. Each plying filtration rates (m1 min-') by the respective 150 m1 water sample was acidified to pH <3.0 with specific radioactivities of the water in the chambers 1 m1 75 O/O v/v H2S04 to liberate CO2 which was then (dpm ml-l). The total amount of [14C]lignocellulose trapped by KOH in the test tubes. After 1 wk, 200 yl of cleared per mussel during the first period of submer- KOH from each test tube was added to 10 m1 Aquasol gence was estimated by integration for each 2 h period together with 1.8 m1 methanol (to solubilize KOH) for between samples. 14C-counting. Radioactivity measurements of samples Biodeposition of l4C. Feces and pseudofeces were were corrected for CO? trapping efficiency, measured collected at hourly intervals during each experiment. as 79.9 % with [14C]NaHC03 standards. The average Pseudofeces were distinguished from feces by their total '*CO2 activity of marsh water collected from con- production from the posterior end of the bivalve, trol chambers (microbial respiration) was subtracted whereas feces were produced along the dorso-posterior from the total 14C02 activity of each experimental margin. Pseudofeces only contained 14C when [14C]lig- beaker to estimate 14C-respiration by each submerged nocellulose was present in the surrounding water; mussel. therefore, pseudofeces were only collected during the Following each 6 h period of submergence, live first submergence period. Biodeposits were collected, Geukensia demissa and controls were gently rinsed diluted to 20 m1 with 1 % w/v (final concentration) with fresh marsh water and individually placed in formalin, and sonicated to disperse clumps. A subsam- empty stoppered 2 l flasks for a 6 h period of exposure. ple of 3 m1 was added to 10 m1 Aquasol for determina- A humid atmosphere was created in each flask by tion of I4C-activity. Hourly measurements of 14C in addition of 1 m1 distilled water. A vial containing 600 pl feces and pseudofeces were summed to calculate total 38 % w/v KOH was suspended in each flask to trap production of 14C in both types of biodeposits. The respired 14C02 during exposure. After exposure, depuration period after feeding mussels 14C-material mussels and controls were gently returned to clean (12 to 24 h) was sufficient for all undigested 14C to be beakers and a 200 v1 aliquot of KOH from each CO2 voided from the gut of Geukensia demissa (Kreeger trap was added to 10 m1 Aquasol and 1.8 m1 methanol 1986). forX4c-counting. The radioactivity of each sample was Respiration of l4C. Respiration of 14C by each ribbed corrected for the trapping efficiency of this technique, mussel was measured for each 6 h time period. During measured as 65.9 O/O with ['4C]NaHC03 standards. The periods of submergence, vials containing 600 p1 of 38 % mean total 14C02 activity of control flasks (microbial w/v KOH were suspended above the water surface of respiration) was subtracted from the total 14C02activity the beakers to absorb 14C02 and the beakers were of each flask holding a live mussel to determine the sealed with 'Parafilm'. However, no 14C02was meas- mussel's aerial production of 14C02. Total respiration 174 Mar Ecol. Prog. Ser.

by each mussel over the experimental period was the negligible in unstressed Geukensia demissa (Bayne sum of 14C02 respired during the periods of submer- 1976) and was not measured in this study. Total absorp- gence and exposure. tion (A) of I4C was determined by addition of respired Body absorption and incorporation of 14c. Dry ''C and absorbed body burden of 14C: tissue weights were determined for each mussel by A=R+B, (4 ) lyophilizing (VirTis model 10-010) the body tissue in Absorption efficiency (AE) was then calculated using pre-weighed vials. Three control mussels not used in Eq. (5): the feeding experiments were treated similarly. Each AE = (A / I,) X 100 % (5) tissue sample was ground in a ball mill (Crescent 'Wig- Means and standard deviations of percentages were L-Bug') and suspended in 10 m1 sterile seawater. The calculated after arc sine transformation of individual 14C-specific activity of each tissue sample was deter- values. Statistical differences between experiments mined. were examined by t-tests (Sokal & Rohlf 1969). Undigested [14C]lignocellulose that may have Concentration of cellulose in seston. Water was col- remained in the mussels was separated from digested lected from the Canary Creek marsh on the high spring and absorbed 14C by alkali and acid extraction (Newel1 tide at monthly intervals for 1 yr (March 1985 to Febru- & Langdon 1986). Subsamples of 2 m1 mussel tissue ary 1986). Triplicate subsamples were measured vol- suspension were each added to 8 m1 0.9 O/O v/v H2SO4, umetrically, sieved through a 149 blm sieve to remove mixed for 30 S, heated at 95OC for 30 min, centrifuged large particles, and the remaining particulate matter at 1400 X g for 10 min, and the supernatant removed. was collected on baked (450 "C) glass-fiber filters The pellet of each sample was washed with 10 m1 (Whatman GF/C). Filters were incubated for 36 h in a distilled water, resuspended in 10 m1 12.5 % w/v solution of cellulases prepared from Trichoderma vinde NaOH, mixed for 30 S, heated at 95°C for 30 min, and (Sigma Type V) and Penicillium funiculosum (Sigma centrifuged at 1400 X g for 10 min. Each pellet was Type VII). The antibiotics thimerosal and toluene were again washed with 10 m1 distilled water and resus- added at a concentration of 5 mg 1-' each to prevent pended in a gel of 10 m1 Aquasol and 3 m1 distilled microbial activity during incubation (Newell & Lang- water for determination of 14C-activity. Pellet 14C rep- don 1986). The release of glucose was measured spec- resented ['4C]lignocellulose not dj.gested by the trophotometrically using a hexokinase enzymatic test mussel. The solubilized "C tissue fraction of each sam- lut (Sigma 16 UV) with correction for non-cellulosic ple was corrected for solubilization of [14C]lignocellu- contaminants. The amount of digestible cellulose col- lose by the acid/alkali extraction, which was found to lected on each filter was calculated on the basis of the be 31 %, based on results from [14C]lignocellulosecon- amount of glucose released from each digest and the trols which were similarly treated with acid and alkali. concentration of cellulose in the seston determined The proportions of incorporated 14C present in the (Newel1 & Langdon 1986). lipid and protein fractions of Geukensia demissa were determined in the second (March) feeding experiment. Lipid was extracted with the chloroform/methanol RESULTS method described by Holland & Gabbott (1971) and the Distribution of 14c in the lignocellulosic fraction percent of tissue in the form of [14C]Liptdcalculated. prepared from Spartina alterniflora Protein was extracted with a technique modified from Holland & Gabbott (1971) where each tissue sample The mean 14C-specific activity of the labeled Spar- was heated at 95OC for 30 min in a solution of 5 O/O tina alterniflora material was 0.14 pCi mg-' dry weight. trichloroacetic acid (TCA).The 14C-speciflc activity of A modified Klason hydrolysis (Benner et al. 1984) indi- each pellet was expressed as a percent of the total cated that 7.4 + 1.6 "6 (mean f SD) of the '"C-label incorporated 14C to calculate the proportion of tissue was distributed in the lignin fraction. The [14C]lipid and

I4C present as [l4CIprotein. [14C]starch fractions accounted for 1.6 f 1.5 ':I (mean Calculations. Ingestion (I)of the 14C-labeled diet by t SD) and 1.7 k 1.6 "b (mean -C SD) of the '"C of the Geukensia demissa was calculated indirectly by 2 Lignocellulose, respectively. There was no measurable methods: protein. The remaining portion of the ['4C]polysac- I1 = (C - P) (2) charide fraction, 89.3 f 4.6 "0 (mean + SD). was and I, = (F + R +B, +B,) (3) assumed to be [14C]cellulose. The only d~fference where C = clearance; P = pseudofecal deposition; F = between the [14C]lignocelluloseproduced in thls study fecal deposition; R = respiration; B, = body absorp- and the material prepared by Newell & Langdon (1986) tion; B, = non-absorbed gut material. Eq. (3) 1s valid if was in the [14C]starchcontent, which was s~gnificantly non-fecal excretion of I4C is insignificant. Absorbed (t-test, p = 0.045) lower in this study because of pre- energy lost as excreted amino acids was considered treatment with R-amylase. Kreeger et al.: Utilization of refractory carbon by mussels 175

[14C]lignocellulose feeding experiments the proportion of ingested I4C that was defecated, approximately 86 %, was not significantly different (p The average filtration rate of Geukensia demissa > 0.10) between experiments (Table 1). Production of during the flrst submergence period was 4.3 l h-' (g dry fecal 14C in each experiment is shown in Fig. 2 as a tissue weight [DTW])-' in Expt 1 (January) and 11.0 1 cumulative percentage loss of the quantity ingested. h-' (g DTW)-' in Expt 2 (March). Ribbed mussels in Defecation patterns were similar in each experiment. Expt 2 were more active, clearing more than twice the Fecal I4C appeared within the first hour from com- amount of ['4C]lignocellulose (7.33 x 10"pnm g mencement of feeding on the ['4C]lignocellulose and DTW-l) as those in Expt 1 (3.03 X 106 dpm g DTW-l) the rate of defecation increased throughout the first (Table 1). Ingestion (Eq. 2) of 14C by mussels in Expt 1 submergence period. Defecation of 14C continued at was 2.81 x 106 dpm g DTW-' and 6.28 X 106 dpm g the onset of the second submergence period; however, DTW-' in Expt 2 (Table 1). Ingestion of I4C by G. the n~usselshad apparently cleared their guts of I4C by demissa, when calculated by summation of l4C-absorp- the 18th hour. tion and 14C-defecation (Eq. 3) (Table l), was not sig- Approximately 4 % of ingested I4C was respired, nificantly different (t-test, p >0.10) from 14C-ingestion mainly when the mussels were submerged (Table 1). determined by Eq. (2) in both experiments. Little respiration of I4CO2 occurred when Geukensia Although absolute amounts of l4C measured in each demissa were exposed to air in either experiment component of the mussel's I4C budget varied greatly (Table 1). Absolute respiration of 14C was significantly among individuals, the proportional relationships higher (t-test, p = 0.020) in the March experiment than between those components varied little. Therefore, the in the January experiment (Table 1); however, the pro- different components of the ribbed mussel's 14C budget portion of ingested I4C that was respired did not differ were expressed as percentages of the absolute quantity significantly (t-test, p > 0.10) between experiments. ingested. Absorption of I4C into body tissue was 3 times greater Ribbed mussels defecated significantly less (p = in the March experiment than in the January experi- 0.025) 14C in Expt 1 than in Expt 2 (Table 1). However, ment (Table 1). However, the proportion of ingested 14C

Table 1. Radioactivity (mean dpm X 103 [g dry tissue weight]-' + SD) of each component of the energy budget of Geukensia demissaafter being fed [14C]lignocellulose in Expts 1 and 2. Values in parentheses represent the mean (f SD) percentage of ingested 14C (I2)distributed among each 14C-budget component. Differences between experiments were analyzed with t-tests

14C-budget component Expt l Expt 2 t-test Mean f SD (n = 4) Mean 2 SD (n = 5) p-value

Clearance (C) Pseudofecal deposition (P) Ingestion (I) (I, = C-P) (Iz = F+A)

Fecal deposition (F)

Respiration (R) Exposed (K)

Submerged (R,)

Body (B) Unabsorbed gut material (B,)

Body absorption (B,)

Body incorporation (B,)

Total absorption (A = R, + R, + B,)

nm: not measured; ns: not significant (p>0.10) Mar Ecol. Prog. Ser. 42: 171-179, 1988

0 Experiment 1 Experiment 2

Fig. 2. Geukensia demissa. Average (f SD) hourly defecation of 14C in Expts 1 and 2 expressed as a cumulative percentage of ingested I4C. Percentages were arc sine transformed before means and I S 1 E I S 1 E I' S ,, 1 standard devlabons were calculated that was absorbed into tissue only increased from 8.7 % [14C]lignocellulose can be incorporated into tissue in Expt 1 (January) to 10.5 % in Expt 2 (March), an (Table 1). Furthermore, in both experiments the pro- insignificant difference (t-test, p >0.10) (Table 1). Neg- portional utilization of ingested 14C was consistent ligible I4C was found undigested in the mussel's gut at among individual mussels, despite highly variable the end of each experiment (Table 1). The proportion of absolute amounts ingested by individuals (Table 1). absorbed I4Cincorporatedin the [14C]lipid and [14C]pro- The average filtration rates of Geukensia demissa tein fractions of the mussel's tissue (measured only in were 4.31 h-' (g dry tissue weight [DTW])-l in Expt 1 the second experiment) totaled 42 %, representing and 11.0 1 h-' g DTW-' in Expt 2, which are similar to 4.4 O/O of ingested 14C (Table 1). rates reported by other workers for ribbed mussels of Respiration and absorption of 14C by Geukensia similar size. Jordan K Vahela (1982) observed a mean demissa were summed to determine overall absorption summer filtration rate of 3.5 1 h-' g DTW-' and Kuenz- of 14C (Table 1). The absolute amount absorbed was ler (1961b) documented an average rate of 6.8 1 h-' g again significantly higher (p = 0.0085) in the second DTW-' at temperatures from 10 to 20°C. The filtration experiment (March); however, proportional absorption rate and resulting clearance of ['4C]lignocellulose by values were similar (Table 1). mussels in the second experiment was significantly greater than that of mussels in Expt l (Table l), perhaps because the marsh water used in the January Seston concentration of cellulose experiment differed in seston concentration from that of the March experiment. Alternatively, the physiologi- Seasonal mean (f SD) cellulose concentrations mea- cal condition of the mussels may have changed from sured in seston collected from the Canary Creek marsh, January to March, resulting in different rates of activity Delaware, are presented in Table 2. The concentration of cellulosic material was greatest during winter when water temperatures were lowest (December to April). Table 2. Seasonal mean (+ SD) cellulose concentrations in averaging 127 pg 1-' in contrast to a mean of 54 pg 1-' seston collected from the Canary Creek study site at high from late spring through autumn (May to November). spring tide over 1 yr The highest cellulose concentrations occurred in samples collected on 3 Mar 1985, which averaged 165 Season (collection dates) Cellulose concentration pg 1-l. Mean +_ SD (n = 6) (ccg I-')

Winter (27 Dec, 4 Feb. 3 Mar) 122 f 42 DISCUSSION Spring (7 Apr, 5 May, 3 Jun) 57 +75 Summer (4 Jul. 2 Aug. 29 Aug) 66 +28 Geukensia demissa can absorb lignocellulosic car- Fall (l Oct, 29 Oct, 26 Nov) 68 f 3 bon derived from Spartina alterniflora with 14 56 effi- Annual mean (+ SD, n = 12) 78 k 47 ciency and at least 4.4 % of the ingested ration of Kreeger et al.. Utilization of refractory carbon by mussels 177

between the 2 groups (Bayne & Newel1 1983).Absolute (Haines & Montague 1979, Hughes & Sherr 1983, I4C-activities of the major components of the I4C- Peterson et al. 1985, 1986). Assin~ilation of vascular budget were higher in the second experiment due to marsh plant detritus by bivalves has only recently been increased clearance of ['4C]lignocellulose (Table 1). quantified (Newel1 & Langdon 1986, Crosby 1987), and Rejection of 14C in pseudofeces accounted for less it remains unclear whether utilization occurs directly or than 15 O/O of "'C cleared from suspension. Agreement indirectly via heterotrophic microorganisms. Stuart et between both indirect calculations of 14C-ingestion al. (1982) reported assimilation efficiencies of up to (Table 1) supports the assun~ption that negligible 14C 50 % for Aulacomya ater fed on detritus formed from was lost in non-fecal excretion. Defecation of 14C kelp; however, structural materials of seaweed are less accounted for 86 % of ingested (Table 1). The refractory than those of salt marsh plants, such as hourly pattern of 14C production in feces was charac- Spartina alterniflora (Findlay & Tenore 1982). terized by rapid deposition of unabsorbed material Absorption efficiency of detritus by the oyster (Fig. 2). Negligible 14C was defecated after the third Crassostrea virginica was investigated by Newell & hour of the second period of submergence (9th hour of Langdon (1986), using a [14C]lignocellulose prepara- submergence) suggesting a short gut residence time. A tion similar to that used in this study. As only 1.3 % of similar time-course of defecation was reported by the 14C was absorbed over a 24 h incubation period, Kuenzler (1961b) who fed continuously submerged Newell & Langdon (1986) concluded that lignocellulose Geukensia demissa 32P-labeled diatoms. Only 8 % of was unimportant as a source of carbon. The observa- ingested 32P remained in the gut of the mussels 12 h tion that Ceukensia demissa can absorb 14 % of a after feeding (Kuenzler 1961b). [14C]lignocellulose diet quantitatively supports previ- Respiration of 14C by Geukensia demissa rep- ous suggestions that the ribbed mussel has a greater resented only 30 % of '4C-absorption (Table l). ability to utilize marsh grass detritus than the oyster Metabolic costs typically demand 70 % of carbon (Haines & Montague 1979, Hughes & Sherr 1983, assimilated by ribbed mussels (Kuenzler 1961a). The Peterson et al. 1986). respiration rate of G. dernissa in this study may have The contribution of cellulosic carbon to the energy been depressed because of a change in food availabil- requirements of Geukensia demissa during summer ity or because the physiological condition of the can be estimated by considering the mussel's ability to mussels may have differed from those of animals ingest and absorb [14C]lignocellulose, the mussel's studied elsewhere (Bayne & Newel1 1983). The ab- summer carbon requirement, and summer concen- sence of '4C-respiration while mussels were exposed to trations of seston cellulose. Present data can validly be air (Table l) suggests that 14C-labeled products of used in this estimation, heca~~sesummer seston tem- cellulose digestion may not have been utilized for peratures most closely approximate the laboratory aerobic respiration during the limited duration of these temperature of this study (21 "C). Widdows et al. (1979) experiments. It is unlikely that the mussels were reported submerged and exposed respiration rates for metabolizing anaerobically, since G. demissa typically G. demissa to be 0.41 m1 h-' g DTW-' and 0.23 m1 h-' g respire aerobically while exposed to air (Widdows et al. DTW-l, respectively, at 20°C. Assuming that Canary 1979, Nicchitta & Ellington 1983). Creek mussels are exposed to air 50 O/O of the time, their Although '4C-absorption efficiencies were similar overall respiration rate would be 0.32 m1 h-' g DTW-', between experiments, the proportion of absorbed I4C which is equivalent to 0.17 mg C h-' g DTW-'. Assum- used for respiration and assimilated into body tissues ing that 30 % of assimilated energy is utilized for differed significantly between experiments (Table 1). growth (Kuenzler 1961a), the total carbon requirement Mussels were possibly directing more energy into of C. demissa for growth and respiration would be 0.24 gonadal tissue during the March experiment when mg C h-' g DTW-'. It is assumed that a mean carbon gonad development naturally occurs (Lind 1975). demand of 0.24 mg C h-' g DTW-' and a mean (Expt 1 Reports of high absorption efficiencies for bivalves and 2, n = 9) filtration rate of 7.8 1 h-' g DTW-' for fed on phytoplankton are numerous (Winter 1978, mussels are representative of Canary Creek mussels Bayne & Newel1 1983). In contrast, little information is during summer temperatures. available concerning the quantitative ability of bi- The summer (1985) mean concentration of cellulose valves to assimilate refractory compounds. Most in seston from the Canary Creek marsh was 66 yg 1-' studies have either evaluated the growth of bivalves (Table 2). Ribbed mussels filtering at a rate of 7.8 1 h-' fed detritus (Kirby-Smith 1976, Pennock 1981, Williams g DTW-l could, therefore, clear 0.51 mg h-' g DTW-' 1981), estimated detrital digestibility in bivalves using while submerged, which is equivalent to 0.26 mg h-' g in Mtro gut enzyme assays (Crosby & Reid 1971, Lucas DTW-' per tidal cycle. With a 14 % absorption effi- & Newel1 1984), or estimated the origin of dietary ciency for lignocellulosic carbon (Table l), Geukensia material from stable isotope ratios of bivalve tissues demissa could absorb cellulosic carbon at a rate of Mar. Ecol. Prog. Ser. 42: 171-179, 1988

0.036 mg C h-' g DTW-l. Absorption of seston cellu- Hughes, E. H., Sherr, E. B. (1983). Subtidal food webs in a

lose could therefore satisfy 15 O/O of the ribbed mussel's Georgia estuary: bI3c analysis. J. exp. mar. Biol. Ecol. 67: 227-242 summer total carbon demand. Refractory cellulosic car- Jordan, T E., VaIieIa, I. (1982). A nitrogen budget of the bon may be a more important resource for ribbed nbbed mussel, Geukensia demissa, and its significance In mussels at other t~mesof the year, however, when nitrogen flow in a New England salt marsh. Limnol metabolic energy demands are reduced (Kuenzler Oceanogr. 27: 75-90 1961a, Hilbish 1987) and when concentrations of seston Irby-Smith, W. W. (1976). The detritus problem and the feeding and digestion of estuarine organisms. In: Wiley. cellulose are higher (Table 2). IM. (ed.) Estuarine processes. Vol. I. Academic Press, New York, p. 469-479 Acknowledgements. We thank MS hnda Franklin for determi- Kreeger, D. A. (1986). Utilization of refractory cellulosic nation of cellulose concentrations from seston samples. 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This article was submitted to the editor; it was accepted for printing on November 19, 1987