Grazing by copepods

Investigations on the effects of PSP on mesozooplankton grazers have primarily focused on copepods and have resulted in a scope of controversial results regarding both the ingestion or rejection of PSP-toxic Alexandrium spp. cells and the effect of the toxins on the copepods (Turriff et al. 1995, Teegarden & Cembella 1996, Turner et al. 1998, Cembella 2003, Colin & Dam 2003; and see Table 1.1).

Table 1.1: Summarized results of studies that compared effects of sole versus mixed diets of PST-containing Alexandrium spp. on copepods. Feeding rates: (-) reduced rates compared to mixed or control species; (+) higher rates compared to sole diet or equal to control species.

Prey selection: (+) species was selected for or against (-), but still ingested at low rates; + in both columns indicates no preference. Effects: (-) was worse compared to mixed or control diet; (+) effect was better than sole diet or indifferent from control diet. NJ: New Jersey; ME:

Maine (Adopted from Colin & Dam 2003).

The interaction between copepods and PSP-toxin producing Alexandrium spp.

is therefore highly variable depending on the Alexandrium strain investigated, and can vary greatly among zooplankton species. The assumption that natural co-occurrence of the copepod species and its toxic prey item could be an important determinant in the outcome of those interactions was previously raised by Runge (1992) and Ives (1985), however only explicitly shown by Colin and Dam (2003 &

2005) who compared the ingestion rates over time, as well as respiration rates as fitness parameter for historically exposed and non-exposed strains of Acartia hudsonica. They concluded that copepods that co-exist with PSP-toxic Alexandrium

spp. can become resistant to the toxins and such a resistance can include different mechanisms: behavioral avoidance or metabolic resistance that increases the rate at which toxins are broken down or a decreased sensitivity towards the toxins (Taylor 1986, Colin & Dam 2003). Behavioral avoidance of copepods against toxic prey was shown by i.e. Turriff et al. (1995) and Teegarden (1999); species specific differences in toxin accumulation by i.e. Turriff et al. (1995) and Teegarden et al. (2003);

differences in toxin retention by i.e. Teegarden & Cembella (1996); variation in depuration rates by i.e. Teegarden (1999) and Guisande et al. (2002) and copepod-species specific toxin transformations by i.e. Shimizu (1978) and Teegarden et al.

(2003). Behavioral avoidance strategies against toxic algae (i.e. Turner & Tester 1997, Teegarden 1999, Teegarden et al. 2008) can, however, not be linked definitively to PSTs because they are based on correlative evidence and may well be confounded by other traits, correlated to PST levels. More concrete examples of counter-adaptation are described by Avery and Dam (2007) and Chen (2010). Avery and Dam (2007) demonstrated that the resistance to PSP-toxic Alexandrium spp. by the copepod Acartia hudsonica carries a cost in the absence of these toxic algae. Their results suggest a heterozygote advantage in the resistant trait, and in turn, that this heterozygote advantage hampers the fixation of the trait within the population (Avery & Dam 2007). In addition, the frequency of heterozygotes appears to increase with the degree of historical exposure to toxic Alexandrium spp. (Chen 2010). A mutation at one of the isoforms of their voltage-gated sodium channels might be responsible for this resistance (Chen 2010, Dam & Haley 2011). This mutated sodium-channel seems to function as a kind of “saxitoxin scavenger” and is thought to be leaky if no saxitoxin is bound (Chen 2010). The counter-adaption in the case of A. hudsonica is therefore a double-edged sword and less elaborate than described adaptations within shellfish, where the sodium-channel mutation disables the binding of saxitoxin at the extracellular side at no apparent cost (Bricelj et al. 2005, Connell et al. 2006). These two examples illustrate that counter-adaptations to saxitoxin can evolve with different trade-offs and different levels of elaborateness, even when targeting the same functional gene. If more and more counter-adaptations are detected, further structural radiations or a decline or loss of saxitoxin production may occur since this metabolite would lose its selective advantage. Hence, we know that non-PST producers occur with PST producing species (Lilly et al. 2007) with a recent indication, that these non-producers still harbor the putative sxt-genes in their genome (Stüken et al. 2011, Hackett et al.

2012). This either indicates a purifying selection of PST production in A. tamarense

different populations/ribotypes. Generally speaking, PSTs cannot be said to have evolved with a particular purpose. They are present either because they confer a selective advantage, or they are biologically neutral with respect to evolutionary developmental processes. The latter constrains our attempt to clearly assign an ecological function to this group of secondary metabolites. In addition, further putative functions have been proposed for PSTs, including physiological (e.g., ion homeostasis or nitrogen storage (Cembella 1998, Pomati et al. 2004, Soto-Liebe et al.

2012) and autecological functions (pheromone activity (Wyatt & Jenkinson 1997) and impact on associated bacteria (Jasti et al. 2005).

The interaction between copepods and Alexandrium spp. becomes even more complex when considering that at least some strains of Alexandrium spp. are able to induce increased PST production in the proximity of copepods (Selander et al. 2006, Bergkvist et al. 2008): Alexandrium minutum increased its cellular PST content up to

> 25-fold due to the presence of naturally occurring concentrations of copepods which, in turn, is correlated with an increased resistance to copepod grazing (Selander et al. 2006). This response towards copepods was induced by “waterborne-cues” or “infochemicals” present in the water, and no direct contact between A.

minutum and the copepod Acartia tonsa was necessary to elicit the response (Selander et al. 2006). Further experiments showed that this induced response in Alexandrium minutum is not of a general nature against copepods and that mostly, also here, co-existence is necessary for the algal cell to recognize its predator (Bergkvist et al. 2008). In a recent study, Selander et al. (2011) added that A.

tamarense cells split off their cells from chains and swim mostly solitarily and more slowly in the presence of copepods. This behavioral response, or “stealth behavior”, led to a reduced encounter rate with the rheotactic copepod grazer Centropages typicus, which tracks its prey using hydrodynamic signals (Selander et al. 2011). This stealth behavior can also reduce the encounter rates with copepods using chemical prey detection, simply because more cells leak more chemical signals (Selander et al.

2011; Fig. 1.7).

Fig. 1.7: Comparison of hydrodynamic- and chemical signals generated from single cells and two or 4 cell-chains (model provided by Erik Selander, University of Gothenburg).