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

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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 ø 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 (ø 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) 0022-0981/00/$ ± see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0022-0981(99)00152-5 70 S.D. Roast et al. / J. Exp. Mar. Biol. Ecol. 245 (2000) 69 ±81 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 ø 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 ø 1½, stored in the dark in a refrigerator (ø 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 (ø 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.
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