Egestion Rates of the Estuarine Mysid Neomysis Integer (Peracarida: Mysidacea) in Relation to a Variable Environment

Journal of Experimental Marine Biology and Ecology L 245 (2000) 69±81 www.elsevier.nl/locate/jembe

Egestion rates of the estuarine mysid Neomysis integer (Peracarida: Mysidacea) in relation to a variable environment

S.D. Roastaba, , J. Widdows , M.B. Jones * a Plymouth Environmental Research Centre (Department of Biological Sciences), University of Plymouth, Plymouth, Devon PL48AA, UK bCentre for Coastal and Marine Sciences, Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth, Devon PL13DH, UK Received 21 August 1998; received in revised form 23 March 1999; accepted 7 October 1999

Abstract

The hyperbenthic, estuarine mysid Neomysis integer (Crustacea: Mysidacea) is exposed to wide ¯uctuations of temperature and salinity on tidal and seasonal cycles. Using sieved sediment as an environmentally relevant food source and egestion rates as a measure of ingestion, the feeding rates of N. integer have been quanti®ed at temperatures (5, 10 and 158C) and salinities (1, 10, 20 and 30½) experienced in the ®eld. Egestion rates (0.017±0.049 mg faeces mg21 dry wt. mysid 21 h ) increased with increasing temperature (Q10 values ranged from ¯ 1.9±2.4) and with increasing salinity. There was a signi®cant interaction between temperature and salinity such that egestion rates were suppressed at high temperature ( $ 108C) in combination with high salinity (30½). Male egestion rates were not signi®cantly different from those of females at any temperature/salinity combination. Absorption ef®ciency (¯ 0.35) was unaffected by temperature or salinity, con®rming that egestion rates are representative of energy acquisition by N. integer.In the estuarine environment, mysid feeding rates are predicted to be low for much of the tidal cycle as the sites occupied by N. integer are dominated by low salinity, cold river water.  2000 Elsevier Science B.V. All rights reserved.

Keywords: Estuarine mysids; Sediment; Egestion rates; Feeding; Temperature; Salinity

1. Introduction

Mysids (Crustacea: Peracarida) contribute signi®cantly to the secondary production of estuaries. The hyperbenthic mysid Neomysis integer dominates the upper regions of

*Corresponding author. Tel.: 144-175-223-2911; fax: 144-175-223-2970. E-mail address: [email protected] (M.B. Jones)

European estuaries and has an estimated productivity of 300 mg ash-free dry weight m22 year21 in the Westerschelde Estuary (Netherlands) (Mees et al., 1994). As many mysids are hyperbenthic, they are thought to provide a signi®cant link in the exchange of organic matter between the benthic and pelagic systems of estuaries, however, published data on the contribution of mysids to such food ¯uxes are limited (Moffat, 1996; Mees and Jones, 1998; Roast et al., 1998a). While it is well established that the feeding rates of many crustaceans are in¯uenced by various factors including temperature, salinity, weight, gender and food density (Kinne, 1970, 1971; Newell and Branch, 1980; Toda et al., 1987; Guerin and Stickle, 1995), few of these factors have been investigated for mysids. Previous investigations of mysid feeding have concentrated on ®lter feeding and predatory feeding (Cooper and Goldman, 1982; Fulton, 1982; Webb et al., 1987; Chigbu and Sibley, 1994). In laboratory feeding experiments, mysids are generally fed brine shrimp (Artemia sp.) nauplii (Astthorsson, 1980; Collins et al., 1991) or Daphnia magna (Irvine et al., 1993), food items not representative of their normal diet. Stomach content analyses have indicated that mysids feed on a wide variety of foods including detritus (Mauchline, 1980). For N. integer, amorphous material from sediment ¯ocs has been identi®ed as an important food item (Fockedey and Mees, 1999). The aims of the present study were to establish the effects of temperature and salinity on the feeding rates of Neomysis integer using an environmentally relevant food source, and to interpret the implications of these laboratory ®ndings to mysids in the natural environment. To achieve the latter, mysids were collected from the East Looe River Estuary, Cornwall (UK), where details of seasonal and tidal ¯uctuations of water temperature, salinity and current velocity are available (Roast et al., 1998b; 1999).

2. Materials and methods

2.1. Animal collection and maintenance

During spring 1996, adult Neomysis integer were collected from Terras Bridge, East Looe River Estuary, Cornwall, UK (National grid reference SX 532256) by sweeping a Freshwater Biological Association (FBA) dip net (1 mm mesh) along the water's edge at low tide. Mysids were returned to the laboratory in habitat water (salinity ¯ 1½), placed in holding tanks (1061½,15618C, ambient lighting from ¯uorescent lights) and fed ad libitum on , 48 h old Artemia sp. (Great Lakes, Utah) hatched from cysts in the laboratory.

2.2. Measurement of egestion rate

Although sediment is a natural dietary item of Neomysis integer (Fockedey and Mees, 1999), there are experimental dif®culties in quantifying its consumption in feeding rate investigations. When food is limited, mysids feed coprophagously (pers. obs.) and, to prevent this, an excess of sediment was used in these experiments. The amount of sediment consumed, however, was extremely small compared with the amount of sediment supplied, making gravimetric analysis of ingested sediment dif®cult. Therefore, S.D. Roast et al. / J. Exp. Mar. Biol. Ecol. 245 (2000) 69 ±81 71 the more readily quanti®ed rate of egestion was used as a surrogate measure of mysid feeding rate. Although gut residence times of crustaceans are variable (Murtaugh, 1984), egestion rates have been used previously to calculate feeding rates of crustaceans (Gaudy, 1974; Reeve et al., 1977) including mysids (Gaudy et al., 1991). For mysids in particular, egestion rates are highly positively correlated with ingestion rates (Murtaugh, 1984), validating their use as a measure of feeding rate. Sediment was collected from the intertidal region at Terras Bridge, where mysids swarmed, by scraping off the top 10 mm of surface sediment. Granulometric analysis showed the sediment in this part of the estuary consisted mainly of mud [particles , 100 mm accounted for more than 75% by weight of the sediment (Roast et al., 1998b)]. The sediment was returned to the laboratory in water of ¯ 1½, stored in the dark in a refrigerator (¯ 28C) and used within 7 days. Immediately prior to each experiment, the sediment was passed through a 63 mm sieve into a plastic aquarium, using water of 10½ to rinse the sediment through the sieve. After standing for 1 h, when most sediment particles had dropped out of suspension, the supernatant was decanted off to leave a concentrated slurry of sediment ( , 63 mm diameter size). The slurry was mixed vigorously to ensure a homogenous sediment suspension immediately prior to injecting approximately 100 ml of slurry into 500 ml plastic containers (110 mm diameter) using a 50 ml plastic syringe. The containers were left for 1 h to consolidate the sediment. Exposure water was decanted carefully into each vessel so that the sediment was undisturbed and a single mysid was placed in each vessel. After feeding for 16 h, each mysid was removed, freeze-dried and weighed (60.01 mg) using a Sartorius R200-D balance. Following mysid removal, the water in each test chamber was shaken gently to re-suspend the sediment and the resultant slurry was sieved through a 128 mm sieve (the larger sieve being used to allow sediment ¯ocs, which formed during the course of the experiment, to pass through the sieve). Neomysis integer faecal material (¯ 1.5 mm long and cylindrical) was retained on the sieve while the loose sediment passed through. The former was washed gently with distilled water and collected onto pre-ashed, weighed Whatman GF/F ®lter papers. Filter papers and faeces were freeze-dried and weighed (60.01 mg). Egestion rates were calculated as mg dry weight of faecal material mg21 mysid dry weight h21 .

2.3. Measurement of food absorption ef®ciency

Food absorption ef®ciency was calculated using the ash-ratio method (Conover, 1966). Dried and weighed faecal material was placed in pre-ashed, weighed aluminium containers, and ashed at 4508Cfor2htoensure that all organic matter was combusted fully. The aluminium containers were re-weighed (60.01 mg) to establish the ash-free content. For each experiment, three vessels containing sediment alone (i.e. no mysid) were exposed to the corresponding temperature/salinity combination, and sediment samples from these chambers were dried, weighed and ashed in the same manner as the faecal pellets. Due to the extremely low dry weight of faeces produced by individual mysids, replicate material from each temperature/salinity combination was combined. At all weighing stages, blank aluminium containers were also weighed to allow 72 S.D. Roast et al. / J. Exp. Mar. Biol. Ecol. 245 (2000) 69 ±81 correction for any residual weight change. Absorption ef®ciency was calculated using the equation: A 5 (F 2 E) 4 [(1 2 E) 3 F] where: A5absorption ef®ciency, F5ash-free fraction of food source, and E5ash-free fraction of faeces (Conover, 1966).

2.4. Experimental protocol

Egestion rates and absorption ef®ciencies were investigated at salinities (1, 10, 20 and 30½) and temperatures (5, 10 and 158C) within the range experienced by N. integer in the estuarine environment (Roast et al., 1998b). Salinities were prepared by diluting ®ltered (10 mm) seawater with tap water (de-chlorinated by aeration for 24 h). All experiments were carried out in a Sanyo MLR-350HT growth cabinet with pro- grammable temperature (60.18C) and photoperiod. Test vessels were placed in the cabinet 2 h prior to the addition of mysids to allow the water temperature to equilibrate with cabinet temperature. Experimental vessels were aerated constantly with ®ltered, compressed air. Experiments were run for 16 h (overnight), with the cabinet lighting programmed to synchronize with the natural photoperiod [16 h light/8 h dark with dawn and dusk sequence (i.e. gradual increase and decrease of light intensity)]. During the experiment, mysids were, therefore, exposed to 4 h light/8 h dark/4 h light. All mysids were adults of similar size (1261 mm from the anterior margin of the rostrum to the tip of the telson); ovigerous females were excluded. A total of 18 experiments was run at each temperature/salinity combination (using nine mysids of each gender).

2.5. Statistical treatment of results

Two-way analysis of variance (ANOVA) was applied to egestion rate data to determine the signi®cance of temperature, salinity or gender effects and to establish factor interactions. Multiple linear regression analysis determined how egestion rates differed with temperature, salinity and gender.

3. Results

3.1. Effect of temperature on egestion rates

At most salinities, Neomysis integer egestion rates increased signi®cantly with increasing temperature (Fig. 1) [ANOVA, f5401 (male) and 328 (female), d.f.52, p,0.01]. The relationship was not simple, however, as temperature interacted signi®cantly with salinity to affect faecal production [ANOVA, f53.42 (male) and 5.94 (female), d.f.56, p,0.01]. At salinities between 1±20½, egestion rates were most sensitive to temperature change between 5 and 108C, compared with mysids at 30½ where egestion rates were affected most by temperature change between 10 and 158C.

Temperature coef®cients (Q10) indicated there were no obvious differences in tempera- S.D. Roast et al. / J. Exp. Mar. Biol. Ecol. 245 (2000) 69 ±81 73

Fig. 1. Effect of temperature on the egestion rate of Neomysis integer at various salinities (numbers correspond to salinity, ½). n59 for each temperature/salinity combination. Data are means with 95% con®dence intervals. ture sensitivities for egestion rates over a wide salinity range or between males and females (Table 1).

3.2. Effect of salinity

Generally, faecal production increased signi®cantly with increasing salinity [ANOVA, f539.3 (male) and 47.9 (female), d.f.53, p,0.01] (Fig. 2). As salinity interacted signi®cantly with temperature (see previous section), the relationship between these two variables was complex. At 58C, faecal production increased as salinity was increased from 20 to 30½, but at 10 and 158C, faecal production decreased at 30½; this effect

Table 1

Temperature coef®cients (Q10) for the faecal production of male and female Neomysis integer at various salinities (n59 for each temperature/salinity combination)

Gender Salinity (½) Q10 value Male 1 2.10 10 2.35 20 2.17 30 1.91 Female 1 1.88 10 2.06 20 2.09 30 1.91 74 S.D. Roast et al. / J. Exp. Mar. Biol. Ecol. 245 (2000) 69 ±81

Fig. 2. Effect of salinity on the egestion rate of Neomysis integer at various temperatures (numbers correspond to temperature, 8C). n59 for each temperature/salinity combination. Data are means with 95% con®dence intervals. was signi®cant only at 108C (Fig. 2) (95% con®dence intervals, p,0.05). Thus, egestion rates at 10 and 30½ were not signi®cantly different at 108C (or 158C for males) (95% con®dence intervals, p.0.05), whereas they were signi®cantly different at 58C (95% con®dence intervals, p,0.05).

3.3. Effect of gender

At each temperature/salinity combination, there was no signi®cant difference between male and female egestion rates (Figs. 1 and 2) (95% con®dence intervals, p.0.05), and no signi®cant interaction between gender and either temperature (ANOVA, f51.45, d.f.52, p.0.05) or salinity (ANOVA, f50.09, d.f.53, p .0.05).

3.4. Combined effects of temperature, salinity and gender

Multiple linear regression analysis revealed that temperature, salinity and gender affected the egestion rate of N. integer according to the equation: E 5 0.0061 1 0.0022T 1 0.0003S 2 0.0005G where: E5egestion rate (mg faeces mg21 dry wt. h21 ), T5temperature (8C), S5salinity (½) and G5score for gender (males50, females51). The positive coef®cients for temperature and salinity con®rm that faecal production increased with increasing temperature and salinity [multiple linear regression, t529.7 (temperature) and 10.5 (salinity), d.f.52 (temperature) and 3 (salinity), p,0.01]. The negative coef®cient value S.D. Roast et al. / J. Exp. Mar. Biol. Ecol. 245 (2000) 69 ±81 75

Table 2 Effect of temperature and salinity on the food absorption ef®ciency of male and female Neomysis integer a Gender Temperature Salinity Food value Faeces value Absorption (8C) (½)(F )(E) ef®ciency Male 5 1 0.080 0.054 0.344 10 0.089 0.060 0.347 20 0.079 0.053 0.343 30 0.091 0.061 0.346 10 1 0.079 0.053 0.347 10 0.083 0.056 0.341 20 0.082 0.055 0.344 30 0.082 0.055 0.349 15 1 0.084 0.056 0.349 10 0.091 0.061 0.352 20 0.083 0.056 0.347 30 0.080 0.054 0.350 Female 5 1 0.082 0.056 0.338 10 0.081 0.054 0.347 20 0.083 0.056 0.341 30 0.082 0.055 0.349 10 1 0.080 0.054 0.344 10 0.079 0.053 0.347 20 0.082 0.055 0.349 30 0.082 0.055 0.344 15 1 0.083 0.056 0.347 10 0.082 0.055 0.349 20 0.081 0.055 0.340 30 0.083 0.056 0.347 a Data for each temperature/salinity combination are pooled from nine replicates. F5ash-free fraction of sediment, E5ash-free fraction of faeces. for gender implies that male egestion rates were faster than those of females, however, this was not signi®cant (multiple linear regression, t520.91, d.f.51, p.0.05).

3.5. Absorption ef®ciencies

There were no obvious increases in absorption ef®ciency with increased temperature or salinity (Table 2). Absorption ef®ciency of females appeared to be more variable than males (Table 2), but this could not be con®rmed due to pooled data preventing statistical analyses.

4. Discussion

The novel method used here to measure mysid egestion rates was practical and robust as the faecal pellets separated well from the sediment slurry. As mysid egestion rates have been shown to be highly positively correlated with mysid ingestion rates (Murtaugh, 1984), the present egestion values are thought to represent a true measure of 76 S.D. Roast et al. / J. Exp. Mar. Biol. Ecol. 245 (2000) 69 ±81 mysid feeding. The use of sediment as an environmentally relevant food source is supported by ®eld observations in the Westerschelde Estuary (Netherlands), where large quantities of detritus were found in the stomachs of adult Neomysis integer (Fockedey and Mees, 1999). The majority of detrital particles in the stomachs of N. integer from the Westerschelde were of a similar size (,65 mm) to the sediment used in the present study, however, absence of meiobenthic fauna and micro-organisms (bacteria, fungi, etc.), normally associated with sediment detrital particles in mysid stomachs, led Fockedey and Mees (1999) to conclude that N. integer fed only from the water column and did not forage in the substratum. In the laboratory, N. integer has been observed to sink to the substratum and collect aggregations of sediment for processing prior to ingestion (Roast, 1997), supporting the use of surface sediment in its diet. Further work is required to establish the source of the sediment used as a food by N. integer. The relatively low organic content of the sediment used in the present study (¯8±9%), and the refractory nature of this material, was chosen to be representative of British estuaries (e.g. Widdows et al., 1979, 1998), and helps to explain the lower absorption ef®ciency for N. integer feeding from sediment (¯0.35, present study), compared with feeding on zooplankton (0.7±0.9) or phytoplankton (0.6±0.9) (Astthorsson, 1980). Comparatively low absorption ef®ciencies (¯0.15) have been recorded for the amphipod Hyalella azteca feeding on sediment (Hargrave, 1970). In general, if material of high nutritional value forms only a small proportion of the total organic fraction of the sediment (i.e. there is a high proportion of refractory material), the overall absorption will be low, even if the nutritional fraction is absorbed ef®ciently (Grahame, 1983). The data imply, therefore, that N. integer needs to consume sediment at a much faster rate than when feeding on phyto- or zooplankton to obtain the same amount of energy. The present study demonstrates that N. integer feeds rapidly on sediment, with mysids passing their own dry weight in faecal material within 24 h. Thus, while it may be more bene®cial energetically for N. integer to feed on meiofauna [e.g. calanoid copepods (Fockedey and Mees, 1999)], sediment may be an important food resource in certain estuarine areas when other food types are absent. Due to the paucity of relevant published data on the feeding rates of mysids, it is not possible to compare the feeding rate data from this study with those of previous investigations. Most publications on mysid feeding rates report the factors (usually food density) affecting the clearance rates for ®lter feeding (on phytoplankton) or predatory feeding (on small crustaceans) (Table 3), entirely different feeding behaviours from the sediment feeding response used in the present study (Cannon and Manton, 1927; Tattersall and Tattersall, 1951; Roast, 1997). Present results showed that N. integer feeding increased with increasing temperature, as anticipated from the general effect of temperature on most rates of physiological processes including the feeding responses of aquatic invertebrates (Kinne, 1970; Newell and Branch, 1980; Schmidt-Nielsen, 1997). Previous studies have shown that increases in temperature increase mysid predation rates (Astthorsson, 1980; Cooper and Goldman, 1982) and the feeding rates of mysids feeding on ¯ake ®sh food (Gaudy et al., 1991). The feeding rate of N. integer increased also with increasing salinity, however, the reason for increased feeding at high salinity is unknown. Salinity may affect sediment characteristics and/or mysid physiology. The effect of salinity on the sediment is unknown as the structural characteristics of the S.D. Roast et al. / J. Exp. Mar. Biol. Ecol. 245 (2000) 69 ±81 77

Table 3 Feeding responses of several mysid species under various experimental conditions

Species Food Condition/variable Feeding rate Reference

Leptomysis lingvura Tetramin ¯aked ®sh 148C; female 100 faecal pellets mysid21 h21 Gaudy et al. (1991) food 188C; female 180 faecal pellets mysid21 h21 Mysis mixtaa Artemia sp. Light 7.6 mg Artemia mysid21 h21 Gorokhova and Hansson (1997) Dark 14.8 mg Artemia mysid21 h21 Mesopodopsis Anaulus birostratus Male 1.8 million cells mysid21 h21 slaberri Female 2.8 million cells mysid21 h21 Neomysis americana Acartia tonsa 5 mm long mysids 8 copepods mysid21 day21 Fulton (1982) 10 mm long mysids 24 copepods mysid21 day21 Neomysis integer Nitzschia closterium .2000 cells ml21 1.3 million cells mysid21 day21 Lucas (1936) 1000±2000 cells ml21 0.7 million cells mysid21 day21 N. integer Eurytemora af®nis 9.0 mm long mysid 53 copepodites mysid21 day21 Irvine et al. (1993) 9.4 mm long mysid 34 copepodites mysid21 day21 Daphnia magna 8.9 mm long mysid 15 Daphnia mysid21 day21 9.5 mm long mysid 4.4 Daphnia mysid21 day21 N. integerb Sediment 58C(10½, males) 0.057 mg faeces mg21 dry wt. h21 This study 158C(10½, males) 0.132 mg faeces mg21 dry wt. h21 1½ (108C, males) 0.084 mg faeces mg21 dry wt. h21 30½ (108C, males) 0.096 mg faeces mg21 dry wt. h21 Neomysis mercedis Daphnia magna 108C29Daphnia mysid21 12 h21 Chigbu and Sibley (1994) 148C30Daphnia mysid21 12 h21 a Original data expressed g21 mysid. Representative dry weight of Mysis mixta taken as 4.0 mg (based on length data from Mauchline and Murano, 1977). b Calculated assuming mean dry weight of N. integer53.0 mg (own data).

sediment used in the experimental chambers were not evaluated in the present study. The sediment for each experimental vessel was prepared identically (e.g. all sediment was sieved at the same temperature and salinity), indicating that sediment characteristics should be consistent in all vessels. Furthermore, in the present study, ash-free dry weight: dry weight ratios of the sediment were similar at all salinities (`Food value', Table 2), suggesting that there was no salinity-related variation in the organic content of the sediment. With regard to the effects of salinity on physiology, N. integer is an extremely ef®cient hyper±hypo-osmoregulator and attains osmotic balance within 2 h of experiencing a change of salinity (Moffat, 1996). The respiratory physiology of N. integer, however, is susceptible to changes in salinity. In general, oxygen consumption rates are lowest at high salinity (Roast et al., 1999), indicating that some differences in energy budget at different salinities may occur. The energy budget of an organism represents an integration of the basic physiological responses such as feeding, food absorption, respiration, excretion and production (Widdows, 1993; Widdows and Salkeld, 1993). Energy is acquired through feeding and food absorption, and lost through respiration and excretion, with any surplus energy then available for growth and reproduction. In the present study, absorption ef®ciency was generally unaffected by temperature or salinity, so feeding rates may be considered to represent energy acquisition. Energy acquisition was found to be highest at 20½, suggesting that energy 78 S.D. Roast et al. / J. Exp. Mar. Biol. Ecol. 245 (2000) 69 ±81 available for growth is also highest at 20½ (Roast, 1997). However, although oxygen consumption by N. integer decreases with increasing salinity, the stronger in¯uence of temperature on mysid oxygen consumption suggests that oxygen consumption is greatest at high tide (Roast et al., 1999). The increased feeding rates predicted for N. integer at high tide may, therefore, relate to this increased rate of oxygen consumption and associated energetic costs. The availability of detailed descriptions of tidal and seasonal ¯uctuations of temperature and salinity at Terras Bridge (Roast et al., 1998b) enables discussion of the environmental implications of these feeding rate data to N. integer. Terras Bridge is close to the tidal limit and is dominated by freshwater ¯ow. The site is ¯ooded with seawater for ¯2 h either side of coastal high tide, with low salinity (or fresh) water present for the remainder of the tidal cycle. Temperature varies tidally (the fresh river water is ¯5-78C cooler than the seawater) and seasonally (summer high tide maxima are ¯158C, compared with winter high tide maxima of ¯88C). If temperature effects are considered in isolation, feeding rates of N. integer are predicted to be comparatively low for much of the tidal cycle due to the lower temperature of the freshwater ¯ow, and rise as the site ¯oods with warmer, incoming seawater (Fig. 3a). Similarly, if the effects of salinity on mysid feeding rates are considered in isolation, feeding rates are predicted to be comparatively low for most of the tidal cycle when the site is dominated by freshwater (Fig. 3b). As the salinity increases with the ¯ood tide, feeding rates are predicted to increase to a maximum at 20±30½ (Fig. 3b). When the effects of temperature and salinity are considered together, changes in mysid feeding rates are generally predicted to be even more pronounced than for either factor acting alone (Fig. 3c). At high tide, however, feeding rates are predicted to decrease slightly due to the interaction between temperature and salinity. Although increases in both temperature and salinity elevate mysid feeding rates, changes in temperature appear to have a more pronounced effect than do changes in salinity (Fig. 3a and b). At Terras Bridge, changes in these two variables are reciprocal, such that temperature and salinity increase and decrease together (Roast et al., 1998b, 1999). Seasonal temperature change is also predicted to cause seasonal changes in the feeding rate of N. integer, with increased rates of feeding occurring during the warmer summer months, the main reproductive period (Mauchline, 1971; Roast, 1997). In conclusion, the present study provides quantitative data on the sediment ingested by a dominant member of the hyperbenthic fauna of the upper reaches of European estuaries under different salinity and temperature conditions. Such data, together with details of seasonal population dynamics (Moffat and Jones, 1992), are required to identify the role played by hyperbenthic mysids in the detrital-based food ¯uxes of estuaries.

Acknowledgements

This work was conducted whilst SDR was in receipt of a Natural Environment S.D. Roast et al. / J. Exp. Mar. Biol. Ecol. 245 (2000) 69 ±81 79

Fig. 3. Predicted effects of tidally-based temperature and salinity changes on the egestion rate of Neomysis integer. (a) Effect of temperature only, (b) effect of salinity only, and (c) effect of temperature and salinity acting in concert on mysid metabolism. H.W.5time of high water. Temperature and salinity data taken from Roast (1997).

Research Council C.A.S.E. studentship with the Brixham Environmental Laboratory (Zeneca Limited) (Studentship No. GT4/94/399/A) for which we are grateful. The study was conducted in collaboration with the Plymouth Marine Laboratory. [SS] 80 S.D. Roast et al. / J. Exp. Mar. Biol. Ecol. 245 (2000) 69 ±81

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