John J. Gilbert

Body Size, Food Concentration, and Population Growth in Planktonic Rotifers

Population growth rates of eight species of planktonic rotifers were assessed for five to seven food concentrations using daily renewed batch cultures. The food concentration for which population growth rate was zero (the threshold food level) varied by a factor of 17 among species. The log of threshold food concentration was positively and significantly related to the log of body mass. Similarly, a strong positive log-log relationship was found between rotifer body mass and the food concentration supporting one-half the maximum population growth rate (rax/2); this food con- centration varied by a factor of 35 among species. There was a positive relationship between rotifer body mass and maximum population growth rate. Because the smallest species have the lowest threshold food levels and the food levels necessary for them to attain rm,,a/2 are lowest, they appear well adapted to living in food-poor environments. Large species appear to be restricted to food-rich environments but may thrive there because of their high reproductive potential. These conclusions are consistent with observations on species distribution and community structure of rotifers in nature.

Rotifer Ecology and Embryological Induction

The rotifer Asplanchna releases into its environment a water-soluble, nondialyzable, pronase-sensitive factor which causes uncleaved eggs of another rotifer, Brachionus calyciflorus, to develop into individuals with a pair of long, movable spines which neither their mothers nor the unaffected controls have. These appendages protect the Brachionus from Asplanchna predation.

Predator-prey behavior and its effect on rotifer survival in associations of Mesocyclops edax, Asplanchna girodi, Polyarthra vulgaris, and Keratella cochlearis

SummaryThe predatory copepod Mesocyclops edax preys effectively on the rotifers Asplanchna girodi and Polyarthra vulgaris but not on the rotifer Keratella cochlearis. It readily captures individuals of this latter species but usually releases them unharmed, being unable to remove the soft parts within their loricae. The predatory A. girodi regularly eats K. cochlearis but cannot catch P. vulgaris. When P. vulgaris is contacted by the corona of A. girodi, it immediately escapes by elevating its paddles and jumping away a distance up to about ten times its own body length. In experimental communities of these predator and prey species the survival of Polyarthra and Keratella is significantly affected by the species of predator present and by predator-prey interaction between the two predators when both are present.

Rotifers as predators on small ciliates

Clearance rates of Synchaeta pectinata, Brachionus calyciflorus and Asplanchna girodi on Tetrahymena pyriformis (46 µm in length) at a density of 10 cells ml−1, in the presence of algal food, were 2.5 to 6.1 ml rot.−1 day−1. Clearance rates of these rotifers were, respectively, about 2, 3, and 13 times lower on Strobilidium gyrans (58 µm in length) than on T. pyriformis, indicating that the saltations of S. gyrans are an effective escape response. Clearance rates of S. pectinata were considerably lower on Colpidium striatum (81 µm) than on S. gyrans, suggesting that S. pectinata may not be able to ingest ciliates of this size. S. pectinata had a clearance rate of 19 ml rot.−1 day−1 on S. gyrans at a density of 1.2 cells ml−1, in the absence of edible algal food. Rotifers may prey extensively on ciliates in natural plankton communities, ingesting 25 to 50 individuals in the 45–60 µm size range day−1.

Quantitative comparison of food niches in some freshwater zooplankton

SummaryThe abilities of some zooplankton (rotifers, cladocerans, copepods) to ingest different sizes and kinds of food cells were quantified by determining the relative efficiencies with which they ingested nine tracer-cell types, ranging from a coccoid bacterium (0.45 μm3) to the alga Cryptomonas erosa (800–920 μm3). These efficiencies were obtained by dividing the clearance rate of each zooplankton group (species population, developmental stage or size class of a species population) on each 32P-labeled cell type by that of a simultaneously-offered, 33P-labeled, standard cell type — Chlamydomonas reinhardtii. Similarities of efficiency patterns on these cell types (food niches) between all possible pairs of the 17 zooplankton groups from 4 ecosystems were determined by calculating correlation coefficients. Although the utilization of the tested cell types may vary greatly within a species, three feeding guilds could be distinguished — based primarily on the efficiencies with which the smallest cell types were ingested. Guild I (Poyarthra vulgaris, Keratella crassa, Diaptomus minutus nauplii) ate the smallest cells (<4 μm diameter) (bacterium, Synechococcus, Nannochloris) and Ankistrodesmus very ineffifently but the three Cryptomonas species very efficiently. Guild II (Bosmina longirostris, D. minutus copepodites and Adults) had higher efficiencies on Synechococcus, Nannochloris, Ankistrodesmus, Stichococcus, and Stephanodiscus than guild I but similarly low ones on the bacterium and high ones on the Cryptomonas species. Guild III (Conochilus inicornis, Keratella cochlearis, Ceriodaphnia quadrangula, Diaphanosoma leuchtembergianum) differed from guilds I and II in having uniformly high efficiencies on all the small cells as well ad the larger ones. Principal component analysis of the matrix of correlation coefficients provided objective confirmation of the three guilds and provided a visual representation of the food niches of the 17 zooplankton groups in 3-dimensional space.

Rotifer Threshold Food Concentrations and the Size‐Efficiency Hypothesis

Using published data, we develop a physiological explanation for the positive relationship between threshold food concentration (the food concentration at which the population growth rate is zero) and body mass in planktonic rotifers. The exponent de? scribing maximum clearance rate as a function of body mass is similar to the exponent expressing energy intake rates at low food concentrations. The former exponent (0.42) is considerably lower than the exponent for respiration (0.66). Consequently, the difference between energy intake and metabolism should decrease with increasing body size at low food concentrations. Rotifer swimming speeds are rather uniform for a wide range of body sizes and species. Consequently, length-specific swimming speeds (body lengths moved per unit time) and, particularly, mass-specific swimming speeds (distance moved per unit time per unit mass) decrease rapidly with increasing body size. We conclude that swimming speed, which is under the energy constraints of ciliary locomotion and which directly determines the rate of food encounters, provides the mechanism for the positive relationship between body size and threshold food concentration.

Discrimination between exploitative and interference competition between Cladocera and Keratella cochlearis

Rotifers and cladocerans are important components of most freshwater plankton communities. Large cladocerans can suppress rotifers through both mechanical interference competition (IC) and exploitative competition (EC) for shared food resources. We assessed the relative extent by which small- and large-bodied cladocerans suppressed the rotifer Keratella cochlearis by these mechanisms, using circulating algal suspensions in dual-chamber laboratory cultures and natural animal densities. The small-bodied cladoc- eran Ceriodaphnia dubia suppressed Keratella exclusively through EC, while a slightly larger species (Daphnia ambigua) did it primarily by this mechanism. Adults (Daphnia galeata mendotae and Daphnia pulex) and subadults (Daphnia pulex) of two much larger bodied cladocerans suppressed Keratella primarily through IC. The ratio of initial Keratella to adult Daphnia pulex density had little effect on the relative importance of the mecha- nisms-IC always greatly exceeded EC. Suppression by EC occurred in all treatments only after food was depleted below the concentration necessary to support maximal rotifer population growth, while suppression by IC occurred at both high and low food concen- trations. These two mechanisms by which cladocerans interact with rotifers should affect the composition of natural rotifer communities in markedly different ways; IC should favor invulnerable rotifer species, many of which are large bodied, while EC should favor rotifer species with low food requirements, which tend to be small bodied.

Observations of insect predation on rotifers

Interactions between rotifers and their insect predators have not received adequate attention, possibly due to the assumption that rotifers are too small for insects to eat. In laboratory experiments, we offered the rotifers Hexarthra mira, Plationus patulus and small and large Synchaeta pectinata to four common insect predators: the notonectids Notonecta lunata and Buenoa macrotibialis, the smaller hemipteran Neoplea striola and small (1.5 mm) aeschnid dragonfly larvae. Excepting Plationus offered to dragonflies, all rotifer preys were consumed to some degree. No size selectivity was apparent for predators that ate few rotifers, but small instar Buenoa ate significantly more large (420 µm) than small (300 µm) Synchaeta. Predator size appeared to be less important than predatory style and prey morphology in determining ingestion rates. Neoplea and dragonflies ate more Hexarthra than Plationus, while the pattern was reversed for Buenoa, possibly because Buenoa is able to manipulate the hard lorica of Plationus better. Insect predators are capable of direct suppression of rotifer populations, an interaction which may be particularly important in littoral zones and fishless ponds where macroinvertebrates are numerous.