Responses of the predatory rotifer Asplanchna intermedia to prey species differing in vulnerability: laboratory and field studies

1. We have studied the numerical and functional responses of campanulate morphs of Asplanchna intermedia fed five species of rotifer ( Brachionus rubens , B. patulus , B. calyciflorus , Hexarthra mira and Filinia longiseta ). The vulnerability of the prey varied with their morphology and mode of swimming.
2. To test the numerical and functional responses, prey species differing in their morphology and mode of swimming were provided. Responses were also tested with mixtures of evasive and non-evasive prey provided in three different ratios.
3. A. intermedia showed a type II functional response to all the prey species provided.
4. The population growth rate of A. intermedia on the various prey species provided ranged from a minimum of –0.24 to a maximum of 0.68. There was a significant correlation between the capturability of a prey species and the population growth rate of the predator feeding on it. The capturability of a prey species also has a significant influence on the maximal predator density but not on the time taken to reach it.
5. Observations from a field study undertaken over a 10-month period to study the prey preferences of A. intermedia in nature were corroborated by the laboratory findings.     

Effect of the cyclopoid copepod Mesocyclops thermocyclopoides on the interactions between the predatory rotifer Asplanchna intermedia and its prey Brachionus calyciflorus and B. angularis

In many shallow, eutrophic subtropical ponds, brachionid rotifers are common prey of the predatory copepod Mesocyclops thermocyclopoides. The predatory rotifer Asplanchna intermedia, which is itself a potential prey of the copepod, also feeds preferentially on brachionids. We studied in the laboratory the population dynamics of two mutually competing prey species, Brachionus angularis and B. calyciflorus, in the presence of the two predators A. intermedia and M. thermocyclopoides. The experimental design included separate population dynamics studies with one prey–one predator, two prey–one predator, one prey–two predator, and two prey–two predator systems. These combinations were compared with controls, in which both the prey species (B. angularis and B. calyciflorus) were grown separately and in combination with each other. In the absence of any predator, B. angularis generally eliminated the larger B. calyciflorus. Selective predation by the copepod allowed B. calyciflorus to persist longer in competition with B. angularis. Feeding by M. thermocyclopoides on A. intermedia reduced the predation pressure on B. calyciflorus. However, given enough time, the cyclopoid copepod was able to eliminate both the brachionids as well as the predatory Asplanchna.     

The effect of prey morphology on the feeding behaviour and population growth of the predatory rotifer Asplanchna sieboldi: a case study using five species of Brachionus (Rotifera)

1. We investigated the numerical response, functional response and prey preference of Asplanchna sieboldi to five different prey brachionids. We also analysed the feeding behaviour of the predator in terms of encounters, attacks, capture and prey ingested per unit time. 2. The five prey species (Brachionus havanaensis, B. rubens, B. patulus, B. macracanthus and B. calyciflorus) differed in their body size and spine length. 3. The population growth rates of A. sieboldi ranged from 0.074 ± 0.03 to 0.431 ± 0.02 depending on prey type and density. There was a significant impact of the spine length rather than body size per se on the population growth rates of the predator. 4. The maximum number of prey consumed depended on both body size and spine length. In the functional response analyses, the plateau was reached at a prey density of 4–8 ind. mL^?1. 5. There was a significant impact of prey density on the prey preference of the predator.     

Feeding responses of Asplanchna brightwelli (rotifera): laboratory and field studies

Laboratory-reared A. brightwelli were offered 16 different prey rotifer species from the genera Anuraeopsis, Brachionus, Epiphanes, Filinia, Hexarthra, Lecane, Lepadella, Pompholyx, Proalides and Trichocerca. Feeding responses (prey encounter, escape, capture, rejection and ingestion) were recorded. In general, the predator's ingestion time was positively correlated with prey length. B. falcatus and H. intermedia were never captured. Once captured, smaller rotifer species were ingested. Gut contents of field-collected A. brightwelli revealed increased occurrence of prey numbers when concentrated plankton samples were preserved after some delay.     

Differential vulnerability of prey to an invading top predator: integrating field surveys and laboratory experiments

Abstract 1. A new top predator, the dragonfly Cordulegaster boltonii, invaded Broadstone Stream (U.K.) in the mid‐1990s. This provided a rare opportunity to assess the impact of a new, large carnivore on a community that has been studied since the 1970s and has one of the most detailed food webs yet published. The vulnerability of the resident species to the invader was assessed by integrating experiments, which examined discrete stages in the predation sequence, with empirical survey data. 2. Although the new predator preyed on nearly every macro‐invertebrate in the food web, vulnerability varied considerably among prey species. Size‐related handling constraints initially set the predator's diet, resulting in strong ontogenetic shifts, with progressively larger prey being added while small prey were retained in the diet, as predators grew. Within the size range of vulnerable prey, encounter rate limited the strength of predation, with mobile, epibenthic species being most at risk. Contrary to most studies of interactions between freshwater predators (usually stoneflies) and prey (usually mayflies), the new predator did not elicit avoidance responses from its prey, probably because it combined a highly cryptic feeding posture with an extremely rapid attack response. 3. The invader exploited its prey heavily in experiments, even at prey densities orders of magnitude above ambient. In the field, electivity reflected prey availability, as determined by mobility and microhabitat use, rather than prey abundance or active predator choice. Consequently, the invader had skewed effects within the prey assemblage, with sedentary, interstitial species being far less vulnerable than more active, epibenthic species, some of which, including a previous top predator, have declined markedly since the invasion. 4. By examining the predation sequence in detail and integrating surveys with experiments, species traits and system characteristics that determine the strength of trophic interactions may be identified, and their potential importance in natural food webs assessed. In so doing, greater insight can be gained into which species (and systems) will be most vulnerable to invading or exotic predators, an imperative in both pure and applied ecology.     

Post-encounter vulnerability of some rotifer prey types to predation by the copepod Acanthocyclops robustus

Prey characteristics affecting vulnerability of a number of rotifer species to the predator Acanthocyclops robustus were investigated. Body volume was the main feature affecting the probability of attack after encounter and, with the exception of Polyarthra major, capture after attack. The lorica provided protection against predation in some species and a skipping escape response reduced the likelihood of capture in Polyarthra major.     

Studies of oogenesis in the rotifer,Asplanchna

Summary Oocyte development in Asplanchna brightwelli was studied by observation through the transparent body wall of living females and by electron microscopy. During oogenesis, which requires four to six hours, the oocyte increases in volume approximately 1000-fold. Most of its cytoplasm appears to be derived from the vitellarium by direct flow through the cytoplasmic bridge. This flow is rapid enough to be easily observed in the living female at low magnifications. Ribosomes, mitochondria, cortical granules, and lipid droplets were observed in the bridge area in electron micrographs.RNA synthesis during oogenesis was studied by means of autoradiography. Very little synthesis could be demonstrated in oocyte nuclei at any period of oogenesis, whereas the vitellarium nuclei show active incorporation of ³H-uridine throughout the reproductive life of the adult female. Most of this RNA is subsequently transferred to developing oocytes.This research was supported by USPHS Grant GM 121183 to Dr. C. W. Birky, Jr.     

Control of feeding behaviour and selective cannibalism in the rotifer Asplanchna

SUMMARY. Feeding responses in campanulate females of Asplanchna sieboldi were induced with filtrates of homogenized saccate female clonemates, showing that such behaviour can be triggered entirely by chemical stimuli. The failure of campanulates to attack campanulate clonemates, however, probably cannot be explained by the tissue chemistry of these females – filtered and unfiltered homogenates of campanulates were as active as those of saccates—and appears to be a function of some property limited to the campanulate body surface.
The roles of chemical and physical properties of food items in controlling feeding behaviour in Asplanchna and other rotifers are considered.     

Differences in predation among morphotypes of the rotifer Asplanchna silvestrii

1. Interactions were observed between three morphotypes of the predatory rotifer Asplanchna silvestrii and six different prey (Brachionus plicatilis, B. rotundiformis, B. pterodinoides, B. satanicus, Hexarthra jenkinae and copepod nauplii) to understand the differences in feeding abilities among morphotypes that may have led to the evolution of this predator polymorphism. The outcome of predation events was affected significantly, both by predator morphotype and prey type. Predator morphotypes also interacted differently with different prey types. 2. The two smaller morphotypes, the saccate and the cruciform, responded similarly to prey overall, except that the smallest morphotype (saccate) was unable to ingest the most mobile prey (nauplii) and less able to ingest relatively large prey (B. plicatilis). The largest morphotype, the campanulate, had the highest encounter rate with prey, but the lowest probability of attack after encounter, so that it consumed far fewer prey per feeding bout than did the smaller morphotypes. This may have been because campanulates prefer larger prey than used in this study. 3. Highly mobile prey (H. jenkinae and copepod nauplii) were much less susceptible to predation than the less mobile Brachionus species. While evasiveness reduced attacks by saccates and cruciforms, campanulates did not have a significantly lower attack rate on H. jenkinae and copepod nauplii than on less evasive prey. Large body size moderately defended B. plicatilis against ingestion by saccates only. The short-spined B. satanicus was the only prey that was rejected after capture, resulting in lower ingestion probabilities for B. satanicus than other brachionid prey.     

Different feeding behaviours in a single predatory mite species. 2. Responses of two populations of Phytoseiulus longipes (Acari: Phytoseiidae) to various prey species,prey stages and plant substrates

Tritrophic studies involving several populations of the predatory mite Phytoseiulus longipes showed distinct life history traits depending on the prey offered and/or the plant substrate. In order to better understand the biology of this predator, the response to several combinations of prey species (Tetranychus evansi and Tetranychus urticae), prey stages (eggs and mobile stages) and plant substrates (bean and tomato leaf discs) has been assessed for two populations of P. longipes. Unlike what was found for life history traits, both populations displayed similar behaviour: they went and stayed more on tomato than on bean, they preferred T. urticae over T. evansi and mobile stages over eggs. Combining the previous life history data with the present results suggests that P. longipes may display host-plant mediated specialization on a prey species, T. evansi. Possible underlying mechanisms are discussed, as well as directions for future work.     

Cannibalism and interspecific predation in two predatory ladybirds in relation to prey abundance in the field

Life tables for two predatory ladybirds,Coccinella septempunctata andHarmonia axyridis, were constructed in two years in which prey abundance differed. The prey aphid,Aphis gossypii, was abundant on its primary host from late May to late June. Females ofH. axyridis andC. septempumctata laid their eggs during the increase and peak in aphid abundance. The oviposition period ofC. septempunctata started before and was shorter than that ofH. axyridis. In both species, intraguild predation and/or cannibalism of the egg and fourth instar stages, but rarely of other developmental stages, were observed. Fourth instar larvae ofH. axyridis had to complete their development when aphids were scarce more frequently than those ofC. septempunctata. The highest frequency of intra- and interspecific predation was of fourth instarH. axyridis larvae. In terms of intraguild predation, the larvae ofH. axyridis preyed on larvae ofC. septempunctata, but the reverse was not observed. Percentage survival from egg to adult inH. axyridis was higher than inC. septempunctata in both years, and least in both species in the year of low aphid abundance. These results suggest that prey abundance influenced the frequency of cannibalism and intraguild predation, which were important in structuring this guild of ladybirds.     

Gigantism and the potential for interference competition in the rotifer genus Asplanchna

Summary Laboratory experiments showed that A. bright-welli could outreproduce, or coexist with, A. silvestrii only when the absence of dietary tocopherol prevented the latter from transforming to the giant cruciform and campanulate morphs. When tocopherol permitted polymorphic transformations, as often occurs in nature, the giant morphs of A. silvestrii ingested and rapidly excluded the much smaller, only slightly polymorphic A. brightwelli. Such interference (or encounter) competition from trimorphic Asplanchna species is known to occur in nature and must limit the distribution and abundance of monomorphic or only slightly polymorphic species. The ability to eat congeneric competitors may have provided some selective pressure for the evolution of gigantism in the genus.     

Spine development in the rotifer Keratella cochlearis: induction by cyclopoid copepods and Asplanchna

SUMMARY. 1. Spine development in a clone of unspined Keratella cochlearis was promoted by a water soluble factor released by the copepods Tropocyclops prasinus and Mesocyclops edax and by the predatory rotifer Asplanchna priodonta. Between 9 and 55% of K. cochlearis populations cultured in predator-conditioned media responded to the inducing chemical.
2. The K. cochlearis form possessing a posterior spine of medium length was much less susceptible to predation by small Tropocylops and Asplanchna than the form lacking the posterior spine. These predators consistently selected the unspined form over the spined form when offered equal densities of each. However, both spined and unspined forms were equally susceptible to predation by large Mesocyclops.
3. The phenotypic variation of offspring produced from induced unspined Keratella females encompassed much of the variation reported for the taxon in North America. These morphotypes are similar to the variation of forms reported for the K. cochlearis tecta series known from Europe. We suggest that the North American morphotypes be identified in studies of this species because the presence of the posterior spine can greatly affect predator selectivity.     

Foraging activity of two species of predatory leeches exposed to active and sedentary prey

Summary The foraging activity of the freshwater leeches Nephelopsis obscura and Erpobdella punctata during acclimation to active or sedentary prey, starvation and refeeding was examined by filming over a 24 h cycle. For both species, the time of activity of individual leeches and the total number of leeches active were significantly correlated. Activity in the dark was significantly higher than in the light with the activity of N. obscura in the dark 16.2 times higher in terms of time and 6.7 times higher in terms of numbers of individuals moving than E. punctata. Large N. obscura were significantly more active than medium or small individuals and encountered more sedentary prey. With lower levels of activity E. punctata encounters and feeds on more active prey, and in lotic ecosystems its lower exposure to fish predation and drift is, at least partially, responsible for its numerical abundance over N. obscura. There were no significant differences in the activity of N. obscura or E. punctata fed on active or sedentary prey during either subsequent starvation of 1 to 10 d, or after refeeding, suggesting that unchanged foraging activity provides the optimun net energy gain during periods of change in prey type and availability.     

Relative vulnerabilities of natural rotifer and ciliate communities to cladocerans: laboratory and field experiments

SUMMARY. 1. Two experiments with plankton communities from Storrs Pond (NH), one conducted in the laboratory and one in field enclosures, assessed the impact of different cladocerans on rotifers and ciliated protozoa.
2. The smallest cladoceran, Bosmina longirostris , did not depress rotifer or ciliate growth rates while the intermediate sized dadoceran, Daphnia galeata mendotae , reduced ciliate growth rates in the enclosure experiment but had only a marginal effect in the jar experiment. D. galeata mendotae had no effect on any of the rotifers in either experiment.
3. In both experiments the largest cladoceran, Daphniapulex , depressed the growth rates of ciliates and those rotifers known to be vulnerable to interference competition. Polyarthra vulgaris , previously shown to be resistant to cladoceran interference, was the only rotifer unaffected by D. pulex in the field experiment but was depressed by the much higher densities of this cladoceran in the laboratory experiment.
4. Cladocerans did not affect phytoplankton or bacterioplankton abundance in either experiment. Therefore the mechanism most likely to be responsible for the suppressive effect of cladocerans on rotifers and ciliates in these experiments is direct mechanical interference or predation, rather than exploitative competition.     

Sensory receptors involved in the feeding behaviour of the rotifer Asplanchna brightwelli

A study of the anterior sensory receptors of male and female Asplanchna brightwelli by scanning electron microscopy reveals some important differences in the region surrounding the mouth. In the male, the ventrolateral sensory bristles, the pseudotrochus, the inner and the outer buccal tufts and the mastax receptors are absent. The oral receptors are reduced. Transmission electron microscopy of these receptors shows that they consist of ciliated sensory cells surrounded by epithelial supporting cells. The distal ends of the cilia of the mastax receptors are modified; the cilia of the other receptors differ only in their length and rootlet structure from the locomotor cilia of the cingulum. A consideration of the feeding behaviour of Asplanchna leads us to suppose that these sensory cilia function in mechanoreception and in chemoreception.     

Selective feeding and its effect on polymorphism and sexuality in the rotifer Asplanchna sieboldi

SUMMARY. Campanulate females of A. sieboldi (clone 12C1) attack Asplanchna brightwelli and A. girodi much more readily than either the alga Volvox aureus or the rotifer Brachionus calyciflorus . Pre-feeding campanulates on B. calyciflorus prior to testing does not appreciably affect their response to this prey. Discrimination during feeding occurs immediately after the campanulate's corona contacts a potential food organism and is probably mediated by chemoreceptors. Once a food organism is attacked, the probabilities of it being captured and subsequently swallowed are high and similar for each of the four organisms tested.
Since food organisms other than Asplanchna . such as V. aureus and B. calyciflorus , induce campanulates in this clone to produce cruciform, and thus frequently sexual, females in succeeding generations, a tendency of these campanulates to select congeneric prey would favour the maintenance of the campanulate morphotype and thus the continuation of parthenogenetic reproduction in the ensuing population.