Limnology (2+1)

Practical No. : 7

Collection and identification freshwater Zooplankton
Zooplankton, the microscopic free swimming animal components of aquatic systems, are represented by a wide array of taxonomic groups; of which the members belonging to protozoa, rotifer, cladocera and copepod are most common and often dominate the entire consumer communities. They are endowed with many remarkable features and are often armoured with spines, which hamper their predation by higher organisms. The ability of movement not only provide them an effective defense measure but also enable them to actively search and feed upon the phytoplankton. Their high and rapid rate of parthenogenetic reproduction usually overcomes the predation losses and enables them to exploit algal blooms. They constitute important link between primary producers and consumers of higher order in aquatic food webs. Therefore, the population dynamics of zooplankton with reference to system provides key information for the management practices.
Preparation of sample
• Collect known amount of water sample eg. 25 litre and filter through a plankton net of bolting silk of No. 25
• Transfer the net plankton in 50ml bottle and preserve in 5% formalin. Add few drops of glycerin to it.
• If further concentration is required allow the sample to stand for a day.
• Practically all the zooplankton will settle down at the bottom of the bottle.
• Remove supernatant plankton-free water with the help of pipette and reduce the sample to the designed volume.
Qualitative and quantitative analysis
Identify the zooplankton in the sample using keys and monographs.
1. Brachionus
Lorica dorso-ventrally flattened, anterior end with 2, 4 or 6 spines, posterior end angled, rounded or with 1 to 2 spines, foot opening posterior, foot long, worm like, wrinkled annulated, flexible, sharply marked off from body, 2 toes forked completely retracible within lorica, eye present, cosmopolitan, planktonic.
2. Keratella
Lorica thick, dorsally curved and ventrally flattened or concave, dorsal plate strong with polygonal facets, ventral plate delicate, anterior spines – 6, symmetric, posterior spines mostly present, foot and toes absent, cosmopolitan, planktonic.
Cladocera (Water fleas)
3. Daphnia
Body compressed, valve surface squarish or rhomboidal, dorsal and ventral margins rounding over towards each other, posterior part provided with a sharp caudal spine, head not separated from body by a dorsal notch, females with well marked and pointed rostrum, small antennules, 3-4 abdominal processes-anterior one bent forward, tongue shaped and long, males with large antennules and first leg with hook and long flagellum, lack rostrum.
4. Moina
Valves thin, somewhat rhomboidal, not covering wholly the thick, heavy body and post abdomen, posterior margin of carapace without spine, cervical sinus present, head large, thick, rounded in front and bent downwards, without a beak, fornix small, rostrum and ocellus lacking, in female abdominal projection horse-shoe shaped, post abdomen wide, bears ciliated spines and bident, abdominal setae long, claw small, in male antennules long and stout, denticulate, oval ephippium with 1 or 2 eggs.
Ostracoda
5. Cypris
Shells tumid, broad, length lesser than twice the width, shells covered with tubercles and hairs, left valve slightly larger than the right, valve margins without irregular canal system, natatory setae of second antenna long and at least reach the tips of the terminal claws, masticatory process with smooth spine like setae, second thoracic leg with bent denticulate claw and its second segment with single seta, furca well developed, long, elongate with subterminal and normal dorsal seta, whitish in colour.
Copepoda
6. Cyclops
Anterior part fatter and antennae comparatively shorter, first antennae 17 segmented, three distal segments with row of fine hyaline spines, second antennae 4-segmented, first 4 pair of legs 3-segmented, fifth pair of legs 2-segmented, the dorsal segment small, narrow and bears a long terminal bristle and short or moderately long inner lateral spine, receptaculum seminis round, eggs sacs two, caudal ramus usually with longitudinal dorsal ridge and inner margin armed with fine hairs.
7. Nauplius
Body oval unsegmented, smaller in size, do not resemble the parents, eye frontal, median, appendages incompletely developed, typically three pairs, first pair (antennules) uniramose, second and third (antennae, mandibles) biramose, furcal setae long, project on either side of the rear end, free swimming.
8. Diaptomus
Body slender, rami 3 segmented in first four pairs of thoracic legs except the first endopod which is 2-segmented, endopod of 1st leg 2-segmented, 3rd and 4th legs 3-segmented, 5th leg biramos, rami 1 to 2-segmeted with or without two apical setae and in male asymmetrical, endopod rudimentary, right leg ending in a single claw, furcal processes short, divided into many sub-genera, freshwater, planktonic.