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.D. Roast et al. J. Exp. Mar. Biol. Ecol. 245 2000 69 –81
22
European estuaries and has an estimated productivity of 300 mg ash-free dry weight m
21
year in the Westerschelde Estuary Netherlands Mees et al., 1994. As many mysids
are hyperbenthic, they are thought to provide a significant 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 fluxes 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 influenced 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 filter 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 flocs has been identified 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 findings 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 fluctuations 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 ¯ 1‰, placed in holding tanks 1061‰, 15618C, ambient lighting from fluorescent 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 difficulties 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 difficult. Therefore,
S .D. Roast et al. J. Exp. Mar. Biol. Ecol. 245 2000 69 –81
71
the more readily quantified 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 ¯ 1‰, 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 10‰ 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 flocs, 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 filter papers. Filter papers and faeces were freeze-dried and weighed 60.01 mg. Egestion rates were calculated as mg dry weight of faecal material
21 21
mg mysid dry weight h
. 2.3. Measurement of food absorption efficiency
Food absorption efficiency 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 4508C for 2 h to ensure 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
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correction for any residual weight change. Absorption efficiency was calculated using the equation:
A 5 F 2 E 4 [1 2 E 3 F ] where: A5absorption efficiency, F 5ash-free fraction of food source, and E 5ash-free
fraction of faeces Conover, 1966. 2.4. Experimental protocol
Egestion rates and absorption efficiencies were investigated at salinities 1, 10, 20 and
30‰ 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
filtered 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 filtered, 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 significance 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
