Prey selection and behavior of copepods fed on toxic and nontoxic algae

Copepods play a vital role in marine ecosystem linkage between primary producers and higher trophic levels and in controlling marine food web dynamics. They have an enormous diversity in taxonomy and spatial and temporal distributions, as well as phenotypic behavioral characteristics, or `traits´. Feeding behavior is one of the most important traits of copepods for regulating their fitness and trophic interactions.

With the increasing occurrence of global harmful algal blooms (HABs), various kinds of algal toxins are threatening both components of marine ecosystems and human consumers of seafood. How copepods, the main direct predator, selectively feed on toxic algae, and to what extent copepods can acclimate to these toxins and transmit them to higher trophic levels are, therefore, important questions in marine science.

This thesis aims to provide a clearer understanding of the feeding interactions between copepods and toxic algal prey by direct observations. High-speed camera and position tracking programs were introduced in our methods for quantifying copepods’ feeding behaviors, such as: ingestion of prey cells, rejection of prey cells, regurgitation of ingested cells, fraction of time beating feeding appendages, frequency of appendages’ beating, time budget of swimming, time budget of jump, and distance of jump.

We described five typical feeding behaviors of copepods when fed with toxic algae: (i) normal feeding behavior where the feeding-current feeding Temora longicornis beats its feeding appendages to create a feeding current for most of the time (>90%), or, for the ambush feeding Acartia tonsa, occasional swimming and jumping to capture motile prey ; (ii) an aberrant beating of feeding appendages of T. longicornis; (iii) an increasing rejection of captured prey cells by T. longicornis; (iv) a significant regurgitation of ingested prey cells T. longicornis; (v) an aberrant jump of A. tonsa.

We found that copepods respond to toxic algae in an extremely species- and strain-specific way. It is striking that there is no significant relationship between selective behavioral responses of copepods and toxicities as we generally known in prey cells, which may involve an extracellular aposematic signal.

It is also striking that there is no short-term acclimation (within five days) of copepods to toxic algae. We identified prey rejection as the dominant mode of copepod response to toxic algae, which is considered a true grazer deterrence strategy of the prey, but we also further illustrated a perplexing co-evolutionary mechanism between copepods and toxic algae.

In contrast with prey rejection, the other observed effects of toxic algae on the grazers would benefit both toxin producing cells as well as ‘cheaters’ and competing species and, thus, do not confer a fitness advantage over other present algal cells. We argue that such a diverse set of grazeralgae interactions may simply reflect different stages of an ever ongoing evolutionary `arms race´ between grazers and their prey that results in a sort of `red queen´ dynamics IIThe overall conclusion is that the selective feeding behaviors of copepods fed on toxic algae are highly species- and strain-specific between or within both copepod populations and algal strains, and this diversity can be considered as an ever ongoing co-evolutionary `arms race´ between copepods and toxic algae