Aquatic ecology and Biodiversity

Unit 12- Biodiversity and conservation

Practical 3- Study of meiobenthic organisms
Experiment No. 3
Collection
Subsamples are taken with core tubes from a grab or gravity corers. The craib corer with ball closing device in position, essentially a frame mounted core tubes, 5-7cm in diameter which is forced into the bottom by weights. Gently lower the core tube into the sediment. Remove the core tube from the bottom sediment and empty the core sediment into a container and transport to the laborartory for extraction of meiofauna.
Shore samples
Samples are usually collected by using core tubes of various diameters depending on the abundance of the meiofauna. Corer of about 10mm internal diameter suitable for estuarine muds sediments, and 22.5 mm internal diameter for sands where numbers are generally lower. Sediment samples are brought to the laboratory for extract meiofauna after preservation.
Preservation
Preservation and extraction depends on the degree to which taxa are to be identified. “Hard” meiofauna like nematodies, copepods, ostracods and kinorhynchs preserved in 4% formaldehyde. Soft meiofauna like gastrotrichs and turbellarions are difficult to recognize after preservation. When the soft meiofauna need to be examined, live extraction and examination are recommended. Preserved samples are stained with rose Bengal before extraction.
Extraction of meiofauna from sand
• Decantation technique (for living soft meiofauna)
• Elutriation technique
Decantation technique (for living soft meiofauna):
Wash the sample into one litre capacity stoppered measuring cylinder and make up to 800ml with tap water giving a sedimentation height of about 30cm. Invert the cylinder several times to suspend the sediment and leave it for 60 sec. for sand particles to settle. Decant the supernatant through a sieve with a pore size not more than 63um (45um and 37um may also be used). Repeat several times (3 times). Wash the material in the sieve into a petri-dish for counting.
Elutriation technique
Biosseau type apparatus is for elutriation of sand, which has closed system arrangement. Sand sample is elutriated. Elutriation carries over most of the light fauna and the same is collected in sieve mesh. The sediment should be examined for heavier organisms (mollusks, ostracods) which can be extracted by hand.
Sea-ice technique
It is used for extraction of interstitial fauna (Uhlig 1966, 1968). The sediment is placed in a tube on a nylon guaze which just dips into filtered sea water in a collection dish. (Fig 148)’ Add crushed sea-water ice to a layer of cotton wool on top of the sediment. As the ice melts, organisms move down into the collecting dish. This technique is well suited to soft meiofauna – ciliates and glagellates; however nematods and larger animals are extracted by elutriation or decantation.
Extraction from mud and detritus
Floatation technique using Ludox- TM:
• Remove any large particles, pieces of shell etc and the fine silt fraction from the sediment sample by decantation and sieve separation.
• Wash the light fraction with tap water into a centrifuge (50ml – 250 ml capacity)
• Add a teaspoon full of Kaolin powder to each container
• Shake vigorously for 7 min at 6000rpm in a high speed centrifuge.
• Pour off the water, the kaolin settles last and forms a plug over the sediment preventing it from being poured out.
• Add Ludox – TM until specific gravity is 1.15 and then resuspend the sediment by vigorous agitation.
• Stirring with spatula may be necessary to break up the Kaolin plug.
• Rebalance the pots and spin again for 7 min at 6000rpm.
• Pour off the supernatant through the meiofauna sieve and wash thoroughly with tap water before washing into the sorting dish.
• Repeat the extraction through the meiofauna sieve all but a few animals, virtually free of detritus and sediment.
Note: Centrifugation in a silica sol-sorb mixture is recommended if living material is extracted.
Examination and counting
The animals are sorted and counted under a stereoscopic microscope in petri-dishes. For identification to species, the animals can be picked out mounted, on slide using a fine pipette, needle etc and focus under high power microscope.
Mounting
Transfer formalin preserved specimens to a mixture of 9 parts of 70% alcohol to one part of glycerol by volume contained in a cavity block. Allow to evaporate slowly, leaving the animals cleared in pure glycerol. Mount on slides in anhydrous glycerol, supporting the cover slip with fine glass beads or rods of appropriate diameter. The mount can be made permanent by sealing the edges with glycerol.