Chemical signals and habitat selection by three zooplankters in Andean Patagonian ponds

1. Zooplankton may react differently to chemical signals produced by macrophytes in shallow systems. They may be attracted by macrophytes, as the plants may be used as a refuge against predators, or the plants may have a repellent effect (e.g. when the plants are a habitat for numerous invertebrate predators or fish). In fishless Patagonian ponds, the structural complexity provided by macrophytes modulates the rate of predation on zooplankton by the invertebrate predator Mesostoma ehrenbergii (Turbellaria). 2. We performed a field study to analyse the coexistence of M. ehrenbergii and three of its prey (two copepods, the calanoid Boeckella gracilis and the cyclopoid Acanthocyclops robustus, and the cladoceran Ceriodaphnia dubia) in four ponds. In two of the ponds, we carried out day and night sampling to evaluate the influence of macrophytes on the distribution of these zooplankters. 3. In laboratory experiments, we analysed the response of the zooplankters to the chemical signals produced by macrophytes (the emergent Juncus pallescens and the submerged Myriophyllum quitense), the predator M. ehrenbergii and the ‘alarm signal’ provided by a homogenate of conspecifics. 4. Our field studies demonstrated the coexistence of M. ehrenbergii and the selected prey in different seasons and that A. robustus and C. dubia choose the vegetated area (a mixed bed of J. pallescens and M. quitense) over the non‐vegetated area. The habitat choice experiments indicated that the presence of M. ehrenbergii may directly affect the habitat selection of B. gracilis, because this zooplankter swam away from the predator. In addition, Mesostoma may indirectly affect the habitat selection of the cyclopoid copepod A. robustus and the cladoceran C. dubia as both zooplankters exhibited a negative response to the alarm signal produced by crushed conspecifics. 5. The presence of the submerged M. quitense did not affect the horizontal movements of any of the zooplankters studied. In contrast, the emergent macrophyte J. pallescens elicited a positive response of B. gracilis, suggesting that this aquatic plant may act as a predation refuge. 6. Our results suggest that predator avoidance behaviour can occur in fishless environments in response to a tactile invertebrate predator like Mesostoma. In addition, the refuge effect of emergent macrophytes, enhancing the survival of pelagic zooplankters, may act as a key factor in stabilizing predator–prey interactions in fishless Patagonian ponds, as has been widely recorded in northern temperate lakes with fish.     

Natural populations of the Closterium ehrenbergii (Desmidiales, Chlorophyta) species complex in Nepal

Several natural populations of the Closterium ehrenbergii Meneghini ex Ralfs species complex were collected in Nepal, in October–December 1982. Water temperature and pH were also recorded. Clonal isolates from these populations were identified to one of four mating groups (H, I, J and M) by test crossing with standard mating-type strains of known mating groups. Groups H and M have smooth walled zygospores, while Groups I and J have scrobiculated zygospore walls. Several undetermined isolates were found in some population samples. In contrast to the previously reported population samples from Nepal, especially from dried soil samples, some of these populations appeared to be rather heavily loaded with mutations that are deleterious to the sexual cycle (i.e. sexual compatibility, zygospore formation and germination). By genetic analysis, a zygote maturation-defective mutation (zym) was detected. One reason for such a heavy genetic load was suggested to be that most population samples had been maintained exclusively by asexual reproduction for a long period in large lakes and nearby ponds, or left-over vegetative populations in paddy fields after other members entered into dormancy through sexual reproduction. The significance of studying such mutations at sexual gene loci is discussed in the light of speciation problems in microalgae.     

STABLE DIPLOIDS FROM INTRAGROUP ZYGOSPORES OF CLOSTERIUM EHRENBERGII MENEGH. (CONJUGATOPHYCEAE)1

Two pairs of stable diploid clones were obtained as aberrant forms among F₁ progeny of an intragroup (intraspecific) cross between R-11-4 (mating type +) and M-16-4b (mating type -) of Group A of Closterium ehrenbergii Menegh. Each pair was derived from the two germination products of a single zygospore, and both clones were mating type minus. The cell size range of these four diploid minus clones was considerably above that of normal (haploid) Group A clones. Chromosome counts at the second meiotic metaphase indicated that these clones were diploid with approximately 200 chromosomes, which was double the number for normal Group A clones. Diploid minus clones conjugated normally with any haploid Group A plus clones, and yielded many triploid zygospores. Triploid zygospores germinated normally as did intragroup diploid zygospores. In metaphase I preparations, only bivalents were observed except on a few occasions where some uni- and multivalents were also detected. Viability of F₁ progeny from triploid zygospores (55–74%) was somewhat lower than from diploid zygospores of Japanese Group A populations (65–90%), but higher than intergroup (interspecific) hybrid zygospores from Groups A, B and H (0–12%). In addition to lower viability, some F₁ progeny from triploid zygospores exhibited slow vegetative growth. Almost all pairs of F₁ clones from single triploid zygospores were of opposite mating type, similar to normal diploid zygospores of the intragroup cross. Morphological variability of F₁ progeny of triploid zygospores was great. The apparently normal meiosis of triploid zygospores and the high viability of F₁ progeny suggested that the genome of Group A contains several sets of chromosome complements with mechanisms by which bivalents are regularly formed in the first meiotic division.     

DETECTION AND EVALUATION OF A NOVEL SEXUAL PHEROMONE THAT INDUCES SEXUAL CELL DMSION OF CLOSTERIUM EHRENBERGII (CHLOROPHYTA)1

Mating type-plus (mt⁺; NIES-228) cells of Closterium ehrenbergii undergo a division to form gamete-shaped cells. This cell division is induced by a substance produced by mating type-minus (mt^?; NIES-229) cells. Light and the presence of mt⁺ cells enhanced production of the substance. The active substance is heat labile and has an apparent molecular mass of 20 kDa. From these results, we conclude that the substance is a novel, proteinaceous sexual pheromone involved in reproduction of Closterium ehrenbergii.     

Predator–prey instability: individual-level mechanisms for population-level results

1. In previous work we established that increasing temperature led to a destabilization of the population dynamics of the invertebrate carnivore Mesostoma ehrenbergii and its prey Daphnia pulex , which ultimately resulted in the local extinction of Daphnia at higher temperatures. Two mechanisms are proposed to explain the population-level phenomena: (1) quantitative changes in carnivore vital rates with increasing temperature led to stronger functional and numerical response and (2) qualitative changes in the dynamic allocation of energy to reproduction by the predator with increasing temperature introduces inverse density dependence in the predator's response.
2. The growth of individual M. ehrenbergii was monitored under various food conditions to determine the effect of two temperatures (18 and 24 °C) and five food levels on rates of growth, prey consumption and reproduction and on reproductive allocation patterns.
3. The first mechanism was supported by both higher consumption rates (stronger functional response) and faster growth rates with earlier age at maturity and shorter generation time (stronger numerical response).
4. Evidence for mechanism two was also provided by an alteration of the reproductive allocation pattern with temperature. Viviparous (subitaneous) eggs were more likely to be produced by this carnivore at low food levels at 24 °C, while at 18 °C, high food levels were required before individuals made this switch. This shift actually introduces inverse density dependence in the predator's numerical response which is highly destabilizing.
5. Based on the results of this study, the differential effect of M. ehrenbergii on the dynamics and structure of its D. pulex prey populations can be attributed to changes in both physiological rates and reproductive allocation patterns with temperature.     

TIME-LAPSE ANALYSES OF SEXUAL ISOLATION BETWEEN TWO CLOSELY RELATED MATING GROUPS OF THE CLOSTERIUM EHRENBERG SPECIES COMPLEX (CHLOROPHYTA)1

Sexual isolation between Groups A and B of Closterium ehrenbergii, two closely related species, was studied by a multiple-choice mating method, as well as the nochoice mating method which has been used in previous work on microalgae. Time lapse photomicrographs and the difference in cell shape and size between the two mating groups allowed identification of a given cell in the mixture as either Group A or B, even when certain morphological changes occurred during the several day culture required for sexual induction. When plus and minus mating types of Group A were mixed with those of Group B (multiplechoice mating), no intergroup hybrid zygospores were formed. However, many intragroup zygospores of either Group A or B were formed. When one plus strain of Group A was mixed with one minus strain of Group B or when one plus strain of Group B was mixed with one minus strain of Group A (no-choice mating), intergroup sexual interactions took place resulting in a small number of hybrid zygospores; however, the process took much longer than intragroup sexual interactions. It was also shown that cell size difference itself hardly affects sexual interactions between haploid and autodiploid strains of Group A. It is suggested that sexual isolation between Groups A and B would be complete in nature, although they may interact sexually in the laboratory.     

The influence of flatworm predation on zooplankton inhabiting small ponds

Experiments were performed in 1977 to determine which large zooplankton in a series of high altitude ponds can be consumed by the predatory flatworm Mesostoma ehrenbergii. This predator consumes Daphnia at a high rate and the fairy shrimp Branchinecta at a low rate, but does not consume Diaptomus. Experiments were performed in 1978 and 1979 to determined the rate of predation on Daphnia in 30 liter tubs and to determine if predation rate is correlated with surface to volume ratio of experimental containers. There is a clear correlation between surface to volume ratio and predation rate. Determinations of Mesostoma and Daphnia densities were made in a series of eight high altitude ponds, and pond surface to volume ratios were determined. Examination of these parameters lends credence to the argument that Mesostoma predation affects Daphnia dynamics in some circumstances. The results suggest that benthic invertebrate predators may affect zooplankton dynamics, especially in shallow ponds.     

Morphological and cytogenetic characteristics of intergroup hybrids between closely related mating groups of the Closterium ehrenbergii species complex (Desmidiales, Chlorophyta)

Five F₁ hybrid strains were established from rare survivors in intergroup crosses between three closely related mating groups (A, B and H) of the Closterium ehrenbergii Meneghini ex Ralfs species complex. Cell sizes of these five strains studied under our standard culture conditions were compared to those of their parental stains and also to the total range of cell-size variation in each mating group. All five F₁ strains were larger in mean cell width than their parental strains. In cell length, three of them were larger than, one was the same as, and the other was intermediate between their parental strains. Their cell sizes were always larger than the range of their respective smaller parental mating group and three of them were larger than the range of their respective larger parental mating group.     

A new species of freshwater turbellarian from Africa,predatory on mosquitoes: Mesostoma zariae n. sp. (Typhloplanoida)

A new species of the genus Mesostoma Ehrenberg 1935, M. zariae n.sp. is described and its relationship discussed. The new species belongs to the M. lingua species-group and is characterised by the presence of two kinds of prostate secretions and a pear-shaped penis papilla. It occurs in small standing or slowly running waters at Zaria, Nigeria. It has previously been proven (Mead 1978) that this species is predatory on the aquatic stages of mosquitoes.     

Hidden deleterious genetic factors affecting post-meiotic viability in zygospore progeny of a unicellular haplontic alga, mating group A of the Closterium ehrenbergii species complex (Chlorophyta)

In a haplontic green alga, mating group A of the Closterium ehrenbergii Meneghini ex Ralfs species complex, viability of meiotic progeny was studied by isolating two gone cells from a single germinating zygospore. In F₁ progeny of a cross between mating-type plus M-16-4a and mating-type minus M-16-4b, studied in six independent experiments, percentage survivals varied little from 86 to 96 with a mean of 93 ± 1.4 SE. In F₂ progenies of crosses among eight mating-type plus and eight mating-type minus F₁ clones of the M-16-4a · M-16-4b cross, percentage survival varied considerably from 24 to 100, with a mean of 70.8 ± 2.2. In B₁ progenies of the above eight mating-type plus F₁ clones, survival values were significantly different between backcrosses to the recurrent M-16-4b (range = 32–83, mean ± SE = 58.3 ± 6.8) and backcrosses to a genetically unrelated mating-type minus, R-13-20, (85–97, 92 ± 1.6). Also in B₁ progenies of the above mating-type minus F₁ clones, survival values were significantly different between backcrosses to the recurrent M-16-4a (56–90, 68.3 ± 4,4) and backcrosses to a genetically unrelated mating-type plus, R-13-131 (78–93, 86.1 ± 1.6). Clearly, viabilities of meiotic progeny differed considerably between outbreedings (M-16-4a × M-16-4b, and F, clones × R-13-20 or R-13-131) and inbreedings (F₂ and F₁ clones × M-16-4a, or M-16-4b). These data suggest the presence of hidden deleterious genetic factors that may reduce viability of zygospore progeny if inbred between a pair of wild-type strains from the normally outbreeding mating group A of the C. ehrenbergii species complex.