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. Ejdung et al. J. Exp. Mar. Biol. Ecol. 253 2000 243 –251
cycle Thorson, 1950. After some time in the plankton the larvae settle at the bottom. The period immediately following settlement is generally considered to be the most
critical for a just settled postlarva e.g., Thorson, 1966. Both the newly settled postlarvae and their predators occupy the surface layer of the sediment, exposing the
postlarvae to a great risk of being eaten. The losses to predation in this temporary meiofauna sensu McIntyre, 1964 are size-dependent and change as the postlarvae
˚ outgrow some potential predators Segerstrale, 1962. Survival of postlarvae is greatly
influenced by inter- and intraspecific interactions among resident adult macro- and ´
meiofauna, and juvenile macrofauna Elmgren et al., 1986; Luckenbach, 1987; Olafsson et al., 1994 and references therein; Danovaro et al., 1995; Cummings et al., 1996;
Beukema et al., 1998; Ejdung and Elmgren, 1998. Predation, the killing and consump- tion of one organism by another, is thought to be the most important cause of
post-settlement juvenile mortality Thorson, 1966; Gosselin and Qian, 1997, influencing and regulating abundance, distribution and reproduction of benthic populations and the
composition of communities.
In the Baltic Sea, the bivalve Macoma balthica L. and the amphipods Monoporeia ¨
affinis Lindstrom syn. Pontoporeia affinis, see Bousfield, 1989 and Pontoporeia ¨
femorata Kroyer dominate large areas, both in abundance and biomass Ankar and Elmgren, 1976; Ankar, 1977; Cederwall, 1996; Rumohr et al., 1996; Laine et al., 1997;
Cederwall, 1999. In areas with dense amphipod populations, M . balthica is generally
less common. Hessle 1924 hypothesized that predation or competition for food caused poor recruitment of M
. balthica in areas densely populated by M. affinis. Elmgren et al. 1986 showed that newly settled , 400 mm M
. balthica plantigrade-stage postlarvae cf. Baker and Mann, 1997 are killed by adult M
. affinis due to a direct physical contact. Juvenile M
. affinis and adult P. femorata also kill the plantigrades, but their impact is less drastic than that of adult M
. affinis Ejdung and Elmgren, 1998. While showing that both adult and juvenile amphipods can kill the bivalves, the two last-
mentioned studies did not present conclusive evidence that the amphipods also ingest the killed bivalve postlarvae, although this was considered highly likely. Normally M
. affinis and P
. femorata in the Baltic Sea are food limited Elmgren, 1978; Uitto and Sarvala, 1990, except for short periods after sedimentation of algal blooms. The amphipods feed
mainly on surface sediment Lopez and Elmgren, 1989 and utilise settled phytoplankton and detrital organic matter as their main food source, but bacteria, and probably
meiofauna are also included in the diet Elmgren, 1978; Uitto and Sarvala, 1990; Goedkoop and Johnson, 1994; Lehtonen, 1996.
The aim of this study was to clarify whether the M . balthica postlarvae killed by M.
affinis are also eaten and thus used as food by the amphipod. We used two methods,
14
C-labelling and Rhodamine B fluorescent staining of plantigrade-stage postlarval M .
balthica.
2. Materials and methods
¨ The experiments were performed in July 1999 at the Asko Laboratory 588499 N,
G . Ejdung et al. J. Exp. Mar. Biol. Ecol. 253 2000 243 –251
245
178389 E, on the Swedish east coast, north-western Baltic Sea proper, in a room, with near natural light- and temperature conditions. The daily light dark cycle, regulated with
2
a faint green light, was set to 16 h 8 h. We used plastic 105-ml aquaria 13 cm sediment area; sediment depth, 1.9–2.1 cm Ejdung and Elmgren, 1998, each supplied
21
with 200 ml h cold 6.060.58C, mean6S.E.M., filtered seawater 5.960.1 PSU. A
few days before the experiment, sediment and animals were collected with a benthic sled Blomqvist and Lundgren, 1996, from 30 to 40 m amphipods and 4 to 16 m depth
postlarval M . balthica. Before the experiment, they were stored in a room with the
same light and temperature conditions as in the experiments. Sediment from the amphipod habitat with a loss on ignition of 4, and sieved through
a 100-mm mesh sieve, was used in the experiments. The postlarvae, concentrated
14
according to Elmgren et al. 1986, were picked in batches and labeled with C or
stained with Rhodamine B. In order to replace dead or injured individuals, all M . affinis
and M . balthica were checked under a stereo-microscope before being added to the
experimental aquaria. Amphipods were left without food for 5 days before the start of the experiments. Animals were measured with an image analyser. The length of the
straightened out amphipod was measured from the tip of the rostrum to the end of the last urosome segment, while for M
. balthica the maximum length and height of the shell were measured.
14
2.1. C-Labelled Macoma balthica postlarvae and Monoporeia affinis
A 9-day experiment with four treatments, 1 M . affinis control, 2 M. affinis and
14 14
C-labelled M . balthica, 3
C-labelled M . balthica, 4 unlabelled M. balthica
control, each replicated 8 times, was started on July 5. For experimental details see Table 1.
14
Newly settled M . balthica were
C-labelled by adding a 2-ml suspension of
14 21
C-labelled diatoms Skeletonema costatum, 760 000 dpm ml to small 90 ml
aquaria without sediment. Each aquarium contained 100 M . balthica, and the total
volume of sea water and added algae was 50 ml. The diatoms had been cultured in a
14
medium of artificial sea water and NaH CO Kester et al., 1967. After 5 days,
3 14
thoroughly rinsed C-labelled M
. balthica, and unmarked M. balthica were picked in
14
separate batches of 10. Five randomly chosen batches of M . balthica, either
C-labelled or unlabelled, were added to each aquarium Table 1. Thereafter five M
. affinis were
14
added to both the C-labelled M
. balthica and M. affinis treatment, and the M. affinis control.
At the end of the experiment, the content of each aquarium was sieved through a 100-mm sieve. Postlarvae, amphipods and amphipod fecal pellets, were retained on the
sieve, separated, and prepared for analysis of radioactivity. The numbers of surviving postlarvae and amphipods were counted, and all living individuals transferred to
scintillation jars one jar per species and replicate, containing 1 ml Lumasolve Lumac, as were the amphipod fecal pellets. After solubilisation, a 10-ml scintillation cocktail
Hionic-Fluor, Packard was added. All samples were counted on a LKB liquid scintillation counter 1214 RackBeta ‘Excel’, LKB Wallac, Finland.
246 G
. Ejdung et al. J. Exp. Mar. Biol. Ecol. 253 2000 243 –251 Table 1
14 a
Experimental set-up and results of the C-labelled Macoma balthica and Monoporeia affinis experiment
Parameter Treatments
14 14
M . balthica
C-Labelled C-Labelled
M . affinis
control M
. balthica M
. balthica control
1 M. affinis No. of M
. balthica per aquarium Final
4961 4861
3962 –
Eaten –
– 1162
– No. of M
. affinis per aquarium Initial
– –
5 5
Final –
– 5
5 Radioactivity dpm per aquarium
M . balthica
72006200 57006300
– M
. affinis –
– 7506120
M . affinis fecal pellets
– –
170650 461
Sediment depth cm 2.060.1
2.060.1 2.160.1
2.060.1
a
The initial number of M . balthica was 50 specimens per aquarium. The radioactivity was measured on
surviving specimens, pooled from each aquarium, and dpm values reported have been corrected by subtraction of the radioactivity of the scintillation fluid. The experiment lasted 5–14 July 1999. All treatments had eight
replicates. Mean6standard error of mean are shown.
2.2. Rhodamine B-stained Macoma balthica postlarvae and Monoporeia affinis A 5-ppm mixture of the vital fluorescent stain Rhodamine B and filtered brackish
water was used to stain M . balthica postlarvae. The postlarvae were exposed to the stain
for 3 days in 90 ml aquaria without sediment. They were then rinsed thoroughly in filtered cooled brackish water and picked out in batches of 10. Randomly selected
batches giving a total of 100 M . balthica were added to each of three 105-ml aquaria
with a sediment layer. Ten M . affinis were added to two of these. Another two aquaria,
each with 10 M . affinis in sieved sediment, served as amphipod controls. After 3 days
the content of each aquarium was sieved through a 100-mm sieve, and surviving bivalves and amphipods counted. The amphipod exoskeleton is transparent, allowing the gut and
parts of the gut content to be observed under a fluorescence microscope without dissection. Rhodamine B fluoresces in orange when exposed to UV-light and remains of
ingested stained bivalves could be detected as fluorescing spots in the amphipod gut. One control amphipod and one offered stained M
. balthica, were randomly placed in pairs on a microscope slide, and observed under the fluorescence microscope. Two
persons, neither of whom knew which of the amphipods had been offered stained M .
balthica, independently decided which individual in each pair showed the strongest fluorescence.
G . Ejdung et al. J. Exp. Mar. Biol. Ecol. 253 2000 243 –251
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2.3. Statistics The Mann–Whitney U-test, the sign test, and regression analysis were performed with
Statistica 5 for PC.
3. Results
