S. Nandini

Life history strategies of cladocerans: comparisons of tropical and temperate taxa

We review recent works on different life tropical and temperate freshwater bodies, comparing the strategies that cladocerans have evolved to adapt to contrasting environmental conditions in the two geographical regions. These life-history parameters relate to age and size at maturity, survival, fecundity, life-expectancy at birth, lifespan, gross, and net reproductive rates, generation time, the rate of population increase, peak population density and day of peak abundance. We also discuss the role of photoperiod and temperature on some of these life history parameters.We found a general paucity of experimental work and field data in tropics on cladocerans. There is very limited information on the few Daphnia species found in the tropics. The misconception of low species diversity of cladocerans in the tropics arose due to several reasons including lack of extensive and intensive field collections. Higher water temperatures apparently promote permanent infestation of tropical waters with toxic cyanobacteria, which reduce the zooplankton diversity. In addition to higher temperatures in the tropics, the year-round high predation pressure of planktivorous fish probably causes the tropical species, particularly in pelagic habitats, to reach maturity earlier (<3 days) than in temperate regions. Species of Daphnia in temperate regions are particularly adapted to living at food concentrations that are much lower and seasonably more variable than those for tropical genera such as Diaphanosoma. This is further corroborated by the more than an order of magnitude higher threshold food concentration (TFC) for tropical Cladocera than for their temperate counterparts. Fecundity patterns differ between tropical and temperate cladoceran taxa: cultured under optimal temperature regimes, tropical taxa have fewer eggs than temperate species of a comparable body size. Predation pressure may act differently depending on the size of the cladoceran neonates and thus on their population size structure. Global warming and climate changes seem to affect the behaviour (migration), distribution, and abundance of cladocerans. Apparently, in direct response to these changes, the possibility of encountering the tropical cladocerans in the northern, temperate hemisphere (bioinvasions) is on the rise.

Population growth of some genera of cladocerans (Cladocera) in relation to algal food (Chlorella vulgaris) levels

We studied the patterns of population growth of 7 cladoceran species (Alona rectangula, Ceriodaphnia dubia, Daphnia laevis, Diaphanosoma brachyurum, Moina macrocopa, Scapholeberis kingi and Simocephalus vetulus) using 6 algal densities, viz. 0.05×106, 0.1×106, 0.2×106, 0.4×106, 0.8×106 and 1.6×106 cells ml−1, of Chlorella vulgaris for 18 – 30 days. In terms of carbon content these algal concentrations corresponded to 0.29, 0.58, 1.16, 2.33, 4.65 and 9.31 μg ml−1, respectively. Cladocerans in the tested range of algal levels responded similarly, in that increasing the food concentrations resulted in higher numerical abundance and population growth rates (r). The peak population densities were (mean±standard error) 71±5; 17.1±0.4, 3.6±0.3, 12.7±1.1, 18.2±2.7, 15.8±1.0 and 10.9±0.02 ind. ml−1, respectively for A. rectangula, C. dubia, D. laevis, D. brachyurum, M. macrocopa, S. kingi and S. vetulus. In general, the lowest r values were obtained for D. laevis (0.01±0.001) at 0.05×106 cells ml−1 food level while the highest was 0.283±0.004 for A. rectangula at 1.6×106 cells ml−1 of Chlorella. When the data of peak population density for each cladoceran species were plotted against the body length, we found an inverse relation, broadly curvilinear in shape. From regression equations between the food level and rate of population increase, we calculated the theoretical food quantity (the threshold level) required to maintain a zero population growth (r = 0) for each cladoceran species, which varied from 0.107 to 0.289 μg ml−1 d−1 depending on the body size. When we plotted the cladoceran body size against the corresponding threshold food levels, we obtained a normal distribution curve. From this it became evident that for up to 1300 μm body size, the threshold food level increased with increasing body size; however, beyond this, the threshold level decreased supporting earlier observations on rotifers and large cladocerans.

Review of Recent Ecotoxicological Studies on Cladocerans

Cladocerans have been widely used as the bioassay organisms in evaluating the impact of different toxic substances. Literature survey during the last 10 years revealed that cladoceran ecotoxicological research has been in an exponential phase constituting nearly 10% of publications on this group. Many studies have considered typically planktonic taxa such as Daphnia magna, D. pulex, Moina macrocopa, M. micrura and Ceriodaphania dubia. Experimental data on toxicity tests, to a lesser extent, are also available for littoral-benthic genera such as Simocephalus, Macrothrix and Alona. Most toxicity tests are limited to the derivation of median lethal concentrations of various durations but mostly at 24 or 48 h. Studies related to the evaluation of changes in the life history variables of cladocerans as a result of sublethal exposure to toxic substances are not many, but gaining importance. The common toxic substances used in the cladoceran toxicity tests appear to be heavy metals, pesticides and a few natural toxins such as cyanotoxins. We review here the effect of different toxic substances on cladocerans based on both the field and the laboratory studies from an ecotoxicology point of view. Suggestions for the future cladoceran ecotoxicology are also commented on.

Lifetable demography of four cladoceran species in relation to algal food (Chlorella vulgaris) density.

Algal food density is known to influence life history variables of cladoceran species. It is not, however, well established whether both littoral and planktonic cladocerans show similar trends when exposed to increasing food concentrations. In the present work, we studied the life table demography of four cladoceran species (Ceriodaphnia cornuta, Moina macrocopa, Pleuroxus aduncus and Simocephalus vetulus) in relation to three algal food concentrations (low: 0.5 × 106, medium: 1.5 × 106 and high: 4.5 × 106 cells ml−1 of Chlorella vulgaris) (in terms of carbon content, these were equivalent to 0.15, 0.45 and 1.35 μg ml−1, respectively) at 25 °C. In general, for all the tested cladoceran species, values of average lifespan, gross reproductive rate, net reproductive rate, generation time and the rate of population growth were higher at lower food concentrations. Furthermore, high food concentration resulted in a negative population growth rate (mean ± standard error: −0.091 ± 0.026) for P. aduncus. The highest population growth rate (0.602 ± 0.014) was recorded for M. macrocopa at low food density. S. vetulus had the longest average lifespan (40 ± 1 d) while M. macrocopa had the lowest (5 ± 1 d). C. cornuta showed better performance at medium food concentration. We conclude that among the algal concentrations used here, 0.5 × 106 – 1.5 × 106 was beneficial not only to the planktonic species but also to the littoral P. aduncus and S. vetulus while 4.5 × 106 cells ml−1 was unsuitable for all the cladocerans tested.

Seasonal variations of zooplankton abundance in the freshwater reservoir Valle de Bravo (Mexico)

Information on the density and diversity of zooplankton from drinking water reservoirs in Mexico is meagre. This is important not only from the point of view of lake management but also for providing clean drinking water for human populations. In the present work, we provide quantitative information on the seasonal variations of zooplankton and selected physico-chemical variables from Valle de Bravo, a large man-made reservoir in the State of Mexico. Based on the nutrient data, this reservoir can be regarded as mesotrophic. However, we found a high density of phytoplankton. Among Cyanophyceae, Anabaena, Microcystis, Nostoc and Oscillatoria were encountered, particularly during the warmer months. Microcystis blooms were observed from June to September. Diatoms dominated the phytoplankton during the remaining months of the year. Among zooplankton, Rotifera comprised the highest number of species. The most common species occurring throughout the year were Keratella chochlearis, Polyarthra vulgaris, Trichocerca capucina, Trichocerca similis, Asplanchna priodonta and Synchaeta pectinata. The highest abundance of K. cochlearis was observed in May at site 5 (340 ind 1−1 ). P. vulgaris occurred in high numbers throughout the year. Both Trichocerca capucina and T. similis were encountered in low numbers (mean annual average: 10 ind 1−1). Among the crustacean zooplankton, Bosmina longirostris occurred in higher numbers (up to 105 ind 1−1) regardless of the time of the year. Copepods were predominantly from the genera Mesocyclops and Megacylops; calanoids were rare. Naupliar stages were dominant during winter months (up to 670 ind 1−1). These results have been discussed from the point of view of Microcystis control in water bodies.

The efficacy of Scenedesmus morphology as a defense mechanism against grazing by selected species of rotifers and cladocerans

Phytoplankton often develop various defense mechanisms in response to zooplankton grazing, such as spines and colonies. While it is now known that increased spine length and cells in a colony of members of the genus Scenedesmus, when zooplankton grazing is intense, helps in reducing zooplankton filtering rates, the effect of these defense mechanisms at the population level has been observed in few studies. Here we present data on the growth rates of four zooplankton species, Brachionus calyciflorus, B. patulus, Ceriodaphnia dubia and Daphnia pulex at two food levels using two species of colony-forming Scenedesmus spp.: S. acutus (cell length = 18.2 ± 0.4 µm; width = 4.2 ± 0.1 µm; average colony length = 90 µm; width: 21 µm) and S. quadricauda (cell length: 21 ± 0.5 width 7.5 ± 0.3 µm; average colony length: 84 µm; width: 30 µm). Whereas S. acutus had no spines, S. quadricauda had spines of 6–10 µm. Population growth experiments of the test rotifers and cladocerans were conducted in 100 ml containers with 50 ml of the medium with test algae. Algae concentrations used were: 13 and 52 mg dw l–1 of each of the two algal species offered in colonial forms. We used an initial inoculation zooplankter density of 1 ind. ml–1 for either of the rotifer species and 0.2 ind. ml–1 for either of the cladoceran species. In all, we had 64 test containers (4 test species of zooplankton × 2 test species of algae × 2 algal densities × 4 replicates). We found a significant effect of algal size on the growth rates of all the four tested species of zooplankton. The population growth rates of zooplankton ranged from −0.58 to 0.66 and were significantly higher on diet of S. acutus than of S. quadricauda. Thus, our study confirms that the larger colony size and the formation of spines in S. quadricauda were effective defenses against grazing by both rotifers and smaller sized cladoceran Ceriodaphnia dubia but that larger-bodied Daphnia pulex could exploit both the algal populations equally.

Cyst ornamentation in aquatic invertebrates: a defence against egg-predation

Eggs, including encysted embryos (cysts) of aquatic invertebrates may not only be thick-walled, but also provided with various external ornamentations such as spines and honeycombings. We argue that these provide protection, additional to that of the walls themselves, against invertebrate predators, and test this idea by offering intact (honeycombed) and decapsulated cysts of Chirocephalus diaphanus to the carnivorous flatworm Mesostomasp. Except at the lowest density of cysts, when intact as well as decapsulated cysts seem to be poorly detected by the (tactile?) searching system of the flatworms, intact cysts were relatively immune from predation. Similar causes elicit similar responses: the defences that occur in eggs are essentially similar to those present in adults of rotifers and various branchiopod microcrustaceans, copepods (marine species), and ostracods. Circumstantial evidence suggests that egg defences may vary in intensity with predation pressure. In some cases, they might be predator-inducible.

Seasonal Variations in the Species Diversity of Planktonic Rotifers in Lake Xochimilco, Mexico

ABSTRACT We collected rotifers during one year (2002–2003) from four different sites in Lake Xochimilco and analyzed the species diversity. We found on an average 25–30 species co-existing at any one site, and rotifer species density was highest in the summer months. The common genera found throughout the year were Brachionus, Keratella, Polyarthra, Trichocerca, Filinia and Asplanchna, and we often found six to eight species of Brachionus in the same sample. Keratella cochlearis and K. tropica were often found together, as were Trichocerca similis, T. pusilla and T. cylindrica. The genus Asplanchna was represented almost exclusively by A. brightwelli.

Competition between Brachionus calyciflorus Pallas and Brachionus patulus (Müller) (Rotifera) in relation to algal food concentration and initial population density

Competitive laboratory experiments between Brachionus calyciflorus and B. patulus were conducted at low (1×106 cells ml−1) and high (3×106 cells ml−1) densities of Chlorella vulgaris and four initial inoculation densities (numerically, 100% B. calyciflorus; 75% B. calyciflorus + 25% B. patulus; 50% each of the two species; 25% B. calyciflorus + 75% B. patulus and 100% B. patulus). Population densities were enumerated and the medium was changed daily for 20 days. B. patulus was a superior competitor in low food density regardless of inoculation density. At high food density, B. calyciflorus showed higher population growth in the first week but thereafter was outcompeted by B. patulus regardless of initial density. When grown alone, B. calyciflorus reached peak abundances (mean ± standard error) of 31±3 and 81±7 individuals ml−1 at low and high food densities, respectively. The corresponding values for B. patulus were 130±2 and 306±13. The adverse effects of B. patulus on the peak abundances of B. calyciflorus were higher at low food concentration. Data on egg ratios (eggs female−1) revealed an inverse relation with population abundance of both tested rotifer species. Our results indicated that the rate of population increase of a species was not a good indicator of its competitive ability. Instead, the ability to reproduce under continuously diminishing food resources (until a threshold level) was responsible for the competitive edge of B. patulus over B. calyciflorus. This was further influenced by the relative inoculation densities of the tested rotifer species and the offered food densities.

Cost of reproduction in selected species of zooplankton (rotifers and cladocerans)

Reproduction is an energetically costly biological process. Among the freshwater zooplankton, rotifers and cladocerans reproduce parthenogenetically and the cost of reproduction can be estimated using the life table data from demographic studies. Reduced probability of future survival or future reproduction as a result of current investment in offspring production (trade-off) is the central theme of the cost hypothesis. Correlations using present reproduction vs. future reproduction (called the reproductive costs) or future survival (called the survival costs) can be used to evaluate the cost hypothesis. In this work sets of correlations were made: (1) between present reproduction (mx) vs. future survival (lx+1, lx+2, lx+3 etc. for the entire lifespan) (survival costs), and (2) present reproduction (mx) vs. future reproduction (mx+1, mx+2, mx+3 etc. for the entire reproductive span) (reproductive costs). These correlations were plotted against the cohort age-classes in order to quantify survival and reproductive costs in rotifers (Asplanchna girodi, Brachionus macracanthus, B. variabilis and Platyias quadricornis) and cladocerans (Ceriodaphnia cornuta – one strain maintained on Chlorella and another strain adapted to Microcystis), Daphnia carinata, D. laevis, Moina macrocopa, Pleuroxus aduncus, Scapholeberis kingi and Simocephalus vetulus). All the tested rotifer species showed negative tendency in correlation coefficients (when the data of current reproduction vs. future reproduction and future survival were plotted) for both reproductive and survival costs. However, from the total survival and reproductive costs derived, 84% of the former and 42% of the latter were statistically significant. In cladocerans about 80% of the costs (correlations between current reproduction vs. future survival or future reproduction) were negative suggesting that present reproduction had negatively affected both the further survival and reproduction of test populations. In terms of statistically significant survival costs, the cladocerans showed a trend slightly lower (72%) but comparable to rotifers. The reproductive costs were significant in 45% cases. In our study, the simple statistical correlations detected the trade-offs between reproduction and survival. Thus, in more than 60% cases of both survival and reproductive costs in zooplankton were negative, and our data supported the cost hypothesis, in the majority of cases where reproduction by zooplankton of a given age class caused reduced survival and reproduction of the next age class.