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

Abstract

Food concentration differentially affected reproductive rates and hydrodynamic characteristics of spincd and unspined phenotypes of the polymorphic rotifer Kerutella testudo. The intrinsic rate of population growth (0.15 d-I) of the predator-induced phenotype was less than half that of the unspined morph (0.39 d I) 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 ofgravity ofthe 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 concentrationsthe 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.

Effects of food availability on predator-induced morphological defence in the ciliate Euplotes octocarinatus (Protista)

“…(1991) observed a significant reduction in the fitness of spined Daphnia pulex under low food levels, compared to those on high food levels . Stemberger (1988) observed reduced population growth rates at high food levels, and no significant difference between defensive and typical morphs of Keratella testudo under low feeding . In some studies, a low morphological transformation in Daphnia was associated with low food availability (Hanazato, 1991 ;Parejko, 1992) .…”

“…The fitness costs of induced morphological defences have been the subject of numerous studies . In most cases some sort of demographic cost has been observed, and defensive phenotypes were usually reproductively inferior to typical morphs (Barry & Bayly, 1985 ;Black & Dodson, 1990 ;Dodson, 1984 ;Havel & Dodson, 1987 ;Kusch & Kuhlmann, 1994 ;Riessen & Sprules, 1990 ;Stemberger, 1988 ;Walls & Ketola, 1989 ;Wicklow, 1990) . In some instances, however, no such cost was found (e .g ., O'Brien et al, 1980 ;Gilbert, 1980) .…”

Hydromechanical mechanisms of colony organization and cost of hefense in an encrusting bryozoan, Membranipora membranacea

“…Stemberger (1988) observed drastically decreased growth rate and survivorship in the spined phenotype of the rotifer Keratella testudo relative to the unspined phenotype, but only at high food concentrations-at low food concentrations, these differences were absent. He attributed this to an energetic cost of spine construction or maintenance but also noted decreases in swimming and sinking speeds associated with spination due to an increase in fluid drag that may have a variety of fitness consequences.…”

Identification of microzooplankton seasonally using time series analysis

“…This non-consistency in the seasonal pattern of ciliates may be due to the great temporal and spatial heterogeneity of population density. Ciliates have high specific growth rates and short generation times when compared with other small animals such as nematodes and coelenterates (Stemberger, 1987(Stemberger, , 1988Pennak, 1989;Lay bourn-Parry, 1992;Christou and Verriopoulos, 1993). With rapid division rates it is easy for a large population of a given species to develop over a short period of time when suitable conditions and abundant food prevail.…”