When intraspecific exceeds interspecific variance: Effects of phytoplankton morphology and growth phase on copepod feeding and fitness

When different growth phases (exponential or stationary) or forms (solitary cells or colonies) of a single clone of Phaeocystis globosa were fed to three copepods, grazing, measured indirectly by fecal-pellet production, on different types of P. globosa differed by nearly two orders of magnitude, with differences on this clone sometimes exceeding differences between different phytoplankton species. The copepods Pseudodiaptomus pelagicus and Eucalanus pileatus fed more on colonies than solitary cells, with P. pelagicus also feeding more on exponential than on stationary cultures. Feeding by Acartia tonsa was complex and dramatically more variable. A. tonsa consumed 16–92 times more P. globosa when feeding on stationary-phase colonies than on any other P. globosa cell type. It fed five times more on stationary-phase colonies of P. globosa than on the palatable Rhodomonas baltica, but fed more on R. baltica than on other stages of P. globosa. Diet effects on copepod fitness were not related to amounts of foods consumed. Survivorship of A. tonsa and E. pileatus did not differ on any of the P. globosa cell types, but survivorship of P. pelagicus was suppressed on colonies (which they consumed more of) versus solitary cells. A. tonsa consumed 30 times more stationary-phase colonies than exponential-phase solitary cells, but produced two times more eggs on the lesser consumed food. Dramatic consumption of stationary-phase colonies may occur because this is a low-quality food and A. tonsa attempts to compensate by consuming more. The limited consumption of other P. globosa types is suggestive of chemical defenses that may be compromised when colonies enter stationary phase. Phytoplankton, seaweeds, higher plants, and invertebrates exhibit intraspecific variance in morphology and chemistry that alters how consumers use these resources as foods (Butler et al. 1989; Karban and Baldwin 1997; Tollrian and Harvell 1999). While many studies emphasize interspecific differences among plants and the effects of this variance on herbivore feeding and fitness, recent studies suggest that intraspecific variation within plants may be equally important to plant–herbivore interactions (Lawrence et al. 2003). Intraspecific variance in plant traits can strongly determine herbivore preference (Jones 1962) as well as herbivore growth, survivorship, and fitness (Barrett and Agrawal 2004). This intraspecific variance can lead to significant differences in natural rates of herbivory (Lawrence et al. 2003), herbivore population densities (Ylioja et al. 1999), and even community structure and ecosystem function (Bailey et al. 2004). Although relatively well described for terrestrial plants and a few seaweeds, the effects of variable phytoplankton traits on zooplankton herbivores are understudied despite the potential for cascading affects of this variance on food webs 1 Corresponding author ([email protected]).