Feeding strategies of the copepod Acartia clausi on single and mixed diets of toxic and non-toxic strains of the dinoflagellate Alexandrium minutum

Abstract

The role of mesozooplankton grazers in the development of monospecific algal blooms has often been examined in a scenario in which grazers, depending on their abilities of recognition, select against toxic species and increase grazing pressure on non-toxic species. Here, we present a different ecological scenario in which grazers may select between different strains (toxic and nontoxic) of the same species, which may coexist in similar densities in natural environments prior to bloom initiation. The calanoid copepod Acartia clausi was fed with single and mixed diets of 2 strains of the dinoflagellate Alexandrium minutum, a producer of paralytic shellfish poisoning (PSP) toxins. One strain produced high, and the other low, quantities of PSP toxins. We examined feeding strategies and estimated copepod responses based on their food selection abilities, toxic effects on maintenance physiology and fitness, and benefits produced by a toxin-dilution strategy in a mixed diet. Copepods were found to feed selectively on A. minutum strains. Diet composition had a strong effect on parameters such as food ingestion, mortality, egg hatching, and egg production. The effect on copepod mortality and egg production was greatly reduced when a mixed diet (toxic + non-toxic) was provided to the copepods. However, the negative effects on egg hatching were dose-dependent, and this parameter was not recovered by toxin-dilution mechanisms. We conclude that copepods did not effectively reject the toxic strain and that the effect of A. minutum on mortality and egg production, but not on egg hatching, is reduced by dilution mechanisms. Therefore, we suggest that feeding pressure by grazers does not appear to be an important mechanism that favors toxic over non-toxic strains prior to bloom initiation.

Grazer-induced toxin formation in dinoflagellates: a transcriptomic model study

“…Toxin levels can be manipulated with trace amounts of copepod cues, without the confounding effects of wounding (Selander et al, 2006;Selander, 2007). The results show that corresponding change in transcription is modest, suggesting that toxin induction is not accompanied by a massive amount of confounding factors, as is the case when toxicity is manipulated with, for example phosphate limitation (Barreiro et al, 2006) or the use of different species (Teegarden, 1999). A comparison of the gene expression response associated with grazer-induced elevated toxin formation with expression differences between toxin-producing and non-toxin-producing strains revealed two sequences with expression patterns suggesting involvement in the regulation of toxin content.…”

Ingestion and clearance rates of the red Noctiluca scintillans fed on the toxic dinoflagellate Alexandrium minutum (Halim)

“…compressum, Hansen et al (2004) also found that the green Noctiluca preyed actively on Pyrodinium, without exhibiting signs of reduced ingestion or satiation, at values of 540 P. bahamense cells ml À1 , close to the 466 A. minutum cells ml À1 observed in our study for condition 2 (complete darkness). Nevertheless, the ingestion rates of N. scintillans fed on A. minutum are considerably lower than those of the copepod Acartia clausi fed on the same species and the same strain (Frangó pulos et al, 2000;Guisande et al, 2002;Barreiro et al, 2006). As pointed out by Kiørboe and Titelman (1998), this is mainly because Noctiluca preys more efficiently on immobile (diatoms) than on mobile food (for example dinoflagellates).…”

Toxicity of Alexandrium lusitanicum to gastropod larvae is not caused by paralytic-shellfish-poisoning toxins

“…Many such studies have shown negative responses of grazers to PST-producers. Grazers may change their behavior (Hansen, 1989; Tillmann and John, 2002), reject toxic cells (Teegarden, 1999), or reduce clearance or ingestion rates (Ives 1985; Turriff et al, 1995; Barreiro et al, 2006; Teegarden et al, 2007). Grazer mortality (Hansen, 1989; Tillmann and John, 2002; Tillmann et al, 2007) or decreased reproduction (Dutz, 1998; Frangopulos et al, 2000; Barreiro et al, 2006) can also occur.…”

“…Grazers may change their behavior (Hansen, 1989; Tillmann and John, 2002), reject toxic cells (Teegarden, 1999), or reduce clearance or ingestion rates (Ives 1985; Turriff et al, 1995; Barreiro et al, 2006; Teegarden et al, 2007). Grazer mortality (Hansen, 1989; Tillmann and John, 2002; Tillmann et al, 2007) or decreased reproduction (Dutz, 1998; Frangopulos et al, 2000; Barreiro et al, 2006) can also occur. However, some studies have shown no effects of PST producers on at least some of the grazers tested (Teegarden et al, 2001; Colin and Dam, 2003; Kamiyama et al, 2005; Tillmann et al, 2007), or conversely found that only some (but not all) PST-producers had an effect on a particular grazer (Tillmann and John, 2002; Teegarden et al, 2007; Tillmann et al, 2007).…”