Reproductive costs and hydrodynamic benefits of chemically induced defenses in Keratella testudo

Abstract

Food concentration differentially affected reproductive rates and hydrodynamic characteristics of spined and unspined phenotypes of the polymorphic rotifer Keratella testudo. The intrinsic rate of population growth (0.15 d−1) of the predator‐induced phenotype was less than half that of the unspined morph (0.39 d 1) at high food concentrations. The spined phenotype had considerably lower survivorship and fecundity than the unspined one after the first 3–4 d of life. At low food concentrations population growth rates were not statistically different; their age‐specific survivorship and fecundity schedules were nearly identical.Sinking rates and swimming speeds of both morphs were significantly and negatively correlated (r = −0.96). Attached eggs increased sinking rates regardless of food level. Changes in sinking rate occurred within 6 h of transferring animals acclimated to high food to conditions of starvation. Food concentration greatly influenced the center of gravity of the morphs and, consequently, affected orientation during sinking. They sank posterior‐end downward in high food conditions and in a more horizontal plane in low food treatments. The latter position should give maximal resistance to sinking, consistent with the slower sinking rates in low food conditions.The specific gravity of both phenotypes increased in high food concentrations—the spined morph being consistently higher than the unspined one at both food levels. Lorica densities of both phenotypes were >1.3 g cm−3 suggesting that this structure may contribute significantly to body mass and may be energetically costly to produce or maintain. The coefficient of form resistance (the ratio of the sinking rate of the rotifer to the sinking rate of a sphere having an equivalent volume and specific gravity) was ∼27% higher in the spined phenotype than in the unspined one in low food treatments. Greater form resistance may provide an energetic benefit which offsets the costs associated with the spined phenotype.