Journal of Experimental Marine Biology and Ecology 253 2000 243–251
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Benthic predator–prey interactions: evidence that adult Monoporeia affinis Amphipoda eat postlarval Macoma
balthica Bivalvia
´ Gunilla Ejdung , Lars Byren, Ragnar Elmgren
Department of Systems Ecology , Stockholm University, SE-106 91 Stockholm, Sweden
Received 28 April 2000; received in revised form 17 July 2000; accepted 24 July 2000
Abstract
Predation by adults of the amphipod Monoporeia affinis on the plantigrade postlarval stage of the bivalve Macoma balthica was studied in the laboratory. We confirmed that M
. affinis consumes
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small M . balthica. Amphipods offered
C-labelled postlarvae took up the radioactive tracer, while those presented Rhodamine B-stained postlarvae acquired gut contents fluorescing strongly in
orange, whereas control amphipods did not. Both labelling methods proved convenient to use in laboratory experiments, and are particularly useful when organisms lack structures that can be
easily identified after being ingested, or when cross-over reactions may bias the results of immunoassays. The results reported here support the conclusion from earlier studies that predation
by M
. affinis on M. balthica can affect population dynamics of M. balthica and is likely to be an important structuring factor in the low-diversity benthic macrofauna community of the Baltic Sea.
2000 Elsevier Science B.V. All rights reserved.
Keywords : Predator–Prey interaction; Baltic Sea; Macoma–Pontoporeia interaction; Post-larval bivalves;
Predation
1. Introduction
Marine soft bottoms are the habitat on earth with the greatest areal extension. The recruitment success of benthic invertebrates is of crucial importance for the structure of
the marine soft-bottom communities Bachelet, 1990. Post-settlement mortality is very high . 90 among benthic marine invertebrate taxa Thorson, 1966; Gosselin and
Qian, 1997, and references therein, a majority of which have a two-phase larval life
Corresponding author. Tel.: 1 46-8-161-744; fax: 1 46-8-158-417. E-mail address
: gunillasystem.ecology.su.se G. Ejdung. 0022-0981 00 – see front matter
2000 Elsevier Science B.V. All rights reserved.
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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,
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C-labelling and Rhodamine B fluorescent staining of plantigrade-stage postlarval M .
balthica.
2. Materials and methods
