ZOOPLANCTON (CLADOCERA Y COPEPODA) DE LA LAGUNA DE CAJITITLÁN

Cladocerans and copepods are the main zooplankton formers in freshwaters. The actual state of pollution faced by the lagoon of Cajititlán restricts the amount of species able to habit this water body, and with it, the functionality of the ecosystem due to their role as phytoplankton consumers and as the principal food source for fishes and other organisms. Until now, there was very little knowledge about these groups in the lagoon, which is because of the lack of specialists in the occidental region of Mexico and the inherent difficulties with respect to the manipulation of these organisms (their size round between 0.2 to 4 mm, besides, manual dissections must be done in order to observe taxonomic characters). In this study, we analyze the composition and abundance of the zooplankton in Cajititlán Lagoon focused in both groups (Cladocera and Copepoda). To achieve this objective, five sites within the lagoon were chosen based on their position with respect to the principal human populations. Samples were collected using a plankton net of 63 μm mesh light with one horizontal tow-in each site monthly. The analyzed period comprehends from May 2015 to April 2016, completing an annual cycle. Each sample was fixed using 4 % formaldehyde. Organisms were first identified the species level and their abundance was calculated with manual counts using a Bogorov chamber. Six species were recorded: Daphnia ambigua and Diaphanosoma birgei (Cladocera) and Arctodiaptomus dorsalis, Leptodiaptomus siciloides, Mesocyclops edax and Neoergasilus japonicus (Copepoda). The most abundant and frequent species was A. dorsalis, followed by M. edax and D. ambigua. Leptodiaptomus siciloides and N. japonicus appeared to be rare species, but L. siciloides was very abundant in March and April. The most abundant species was A. dorsalis, occurring nearly in 77 % of the samples and an average of 5 organisms per liter (up to 60 org/l), while the most frequent was M. edax, with up to 87 % of occurrences and an average of 3 org/l (up to 80 org/l). The general abundance of both groups in this water body was below and the species composition corresponds to the expected in an eutrophicated waterbody.

STUDY ON ZOOPLANKTON DIVERSITY DURING SPRING SEASON IN CHASHMA LAKE, PAKISTAN

The present study was conducted to know the zooplankton species diversity in Chashma Lake during spring (March 2016 to May 2016). Various physico-chemical parameters like Temperature, PH, Salinity, conductivity, Total dissolved solids (TDS), Turbidity, Dissolved Oxygen, Alkalinity, Chloride and Carbon dioxide which effect the distribution of zooplankton were also analyzed during this study. Zooplankton community was dominated by different species of rotifers. Those rotifers were Asplanchna herricki, Asplanchna brightwelli, Polyarthra remata, Brachionus angularis, Keratella cochlearis, Keratella tecta and Keratella valga tropica. Among these Keratella valga tropica was most common. Some species of copepods were recorded such as, Diacyclops thomasi, Acanthocyclops robustus, Mesocyclops edax, Tropocyclops prasinus mexicanus and Orthocyclops modestus. Among these Diacyclops thomasi was most abundant. Among copepods some nauplii and copepodid stages were also observed. Different species of cladocerans such as, Bosmina longirostris, Daphnia pulex, Chydorus sphaericus, Alona bicolor, Alonella exigua and Simocephalus serrulatus were also recorded.

Deadly competition and life-saving predation: the potential for alternative stable states in a stage-structured predator–prey system

Predators often undergo complete ontogenetic diet shifts, engaging in resource competition with species that become their prey during later developmental stages. Theory posits that this mix of stage-specific competition and predation, termed life-history intraguild predation (LHIGP), can lead to alternative stable states. In one state, prey exclude predators through competition (i.e. juvenile competitive bottleneck), while in the alternative, adult predators control prey density to limit competition and foster coexistence. Nevertheless, the interactions leading to these states have not been demonstrated in an empirical LHIGP system. To address this gap, we manipulated densities of cannibalistic adult cyclopoid copepods ( Mesocyclops edax ) and their cladoceran prey ( Daphnia pulex ) in a response-surface design and measured the maturation and survival of juvenile copepods (nauplii). We found that Daphnia reduced and even precluded both nauplii maturation and survival through depletion of a shared food resource. As predicted, adult copepods enhanced nauplii maturation and survival through Daphnia consumption, yet this positive effect was dependent on the relative abundance of Daphnia as well as the absolute density of adult copepods. Adult copepods reduced nauplii survival through cannibalism at low Daphnia densities and at the highest copepod density. This work demonstrates that predation can relax a strong juvenile competitive bottleneck in freshwater zooplankton, though cannibalism can reduce predator recruitment. Thus, our results highlight a key role for cannibalism in LHIGP dynamics and provide evidence for the interactions that drive alternative stable states in such systems.

Chromatin diminution in the copepod Mesocyclops edax: elimination of both highly repetitive and nonhighly repetitive DNA

Chromatin diminution, a developmentally regulated process of DNA elimination, is found in numerous eukaryotic species. In the copepod Mesocyclops edax, some 90% of its genomic DNA is eliminated during the differentiation of embryonic cells into somatic cells. Previous studies have shown that the eliminated DNA contains highly repetitive sequences. Here, we sequenced DNA fragments from pre- and postdiminution cells to determine whether nonhighly repetitive sequences are also eliminated during the process of chromatin diminution. Comparative analyses of these sequences, as well as the sequences eliminated from the genome of the copepod Cyclops kolensis, show that they all share similar abundances of tandem repeats, dispersed repeats, transposable elements, and various coding and noncoding sequences. This suggests that, in the chromatin diminution observed in M. edax, both highly repetitive and nonhighly repetitive sequences are eliminated and that there is no bias in the type of nonhighly repetitive DNA being eliminated.

Chromatin diminution in the copepod Mesocyclops edax: diminution of tandemly repeated DNA families from somatic cells

Chromatin diminution, i.e., the loss of selected chromosomal regions during the differentiation of early embryonic cells into somatic cells, has been described in taxa as varied as ciliates, copepods, insects, nematodes, and hagfish. The nature of the eliminated DNA has been extensively studied in ciliate, nematode, and hagfish species. However, the small size of copepods, which makes it difficult to obtain enough DNA from early embryonic cells for cloning and sequencing, has limited such studies. Here, to identify the sequences eliminated from the somatic cells of a copepod species that undergoes chromatin diminution, we randomly amplified DNA fragments from germ line and somatic line cells of Mesocyclops edax, a freshwater cyclopoid copepod. Of 47 randomly amplified germ line clones, 45 (96%) contained short, tandemly repeated sequences composed of either 2 bp CA-repeats, 8 bp CAAATAGA-repeats, or 9 bp CAAATTAAA-repeats. In contrast, of 83 randomly amplified somatic line clones, only 47 (57%) contained such short, tandemly repeated sequences. As previously observed in some nematode species, our results therefore show that there is partial elimination of chromosomal regions containing (CAAATAGA and CAAATTAAA) repeated sequences during the chromatin diminution observed in the somatic cells of M. edax. We speculate that chromatin diminution might have evolved repeatedly by recruitment of RNAi-related mechanisms to eliminate nonfunctional tandemly repeated DNA sequences from the somatic genome of some species.Key words: chromatin diminution, Mesocyclops edax, copepod, satellite DNA, hetorochromatin.