A comparison of two extractors for separating meiobenthic nematodes from fine-grained sediments

Abstract

Two methods for the quantitative extraction of meiobenthic nematodes from fine-grained sediments were investigated and compared: (i) Oostenbrink-sugar flotation and (ii) Ludox flota-tion. The results show that Ludox flotation yielded between approximately 200 and 700% more nematodes. A conversion graph for nematode numbers of the two extracting methods was compiled. s. Afr. J. ZooI. 1982, 17: 41-43 Twee metodes vir die kwantitatiewe ekstrahering van meioben-tiese nematodes uit kleierige grond is ondersoek en vergelyk: (i) Oostenbrink-suikerflottasie en (ii) Ludox-flottasie. Resultate toon dat Ludox tussen ongeveer 200 en 700% meer nematodes ekstraheer. 'n Omsettingsgrafiek vir nematode-getalie van die twee tegnieke is saamgestel. Since the beginning of the century a number of methods have been developed for the extraction of nematodes from sediments. Techniques involving decantation and elutriation are useful in quantitative separation of nematodes from sandy sediments (Furstenberg, Dye & De Wet 1978). Huling & Gray (1971) recommended that for quantitative work with fine-grained sediments sieving and hand-sorting are necessary. This is an extremely laborious and time-consuming method when dealing with coastal and estuarine muds (Nichols 1979), in comparison with centrifugal flotation of soil suspensions. To date sugar (centrifugal) flotation or a combination of Oosten-brink and sugar flotation has been used by many nemato-logists for separating meiobenthic nematodes from fine-grained sediments. However, the efficiency of extraction with the popular sugar solutions can be variable (Heip, Smol & Hautekiet 1974) apart from the distortion which it can cause to the nematodes. This makes the technique less acceptable to taxonomists. A substitute for sugar with less harsh effects on animals in sediment samples is a colloidal sol called Ludox which to our knowledge has been used by nematologists for less than a decade. The aims of this study were to compare Ludox with Oostenbrink-sugar flotation and to compile a conversion graph for their nematode yields per sample. Material and Methods Sediment samples were taken at eleven sites selected in the Swartkops River, Port Elizabeth (34°SI26°E) over a distance of 24 km from the mouth. Each site consisted of two mid-tidal locations, ca. 5 m apart, and were sampled with a hand corer, 30 cm long and 3,57 cm in diameter. These sites were initially chosen for ecological studies on the meiobenthic fauna of the Swartkops River. For the purpose of this study all coarse sandy stations were excluded. Samples taken at each location were thoroughly mixed (Furstenberg et al. 1978) and subsequently divided into three subsamples of 200 ml each. Two were extracted by Oostenbrink-sugar flotation and Ludox respectively and the third analysed for particle size. Particle size distribution was measured using 50 g of oven-dried substrate by net-sieving through sieves conforming to the Wentworth scale at 1 phi intervals (Morgans 1956). 42 S.-Afr. Tydskr. Dierk. 1982, 17(1) 3,5 Table 1 Nematode yield efficiencies of Ludox vs. Oostenbrink-sugar Figure 1 Logarithm of Ludox nematode count versus logarithm of Oostenbrink-sugar nematode count together with the least-squares regression line and a 9011Jo confidence band. (n = 22; r 2 = 0,74), 3 950 417 184 Increased percent efficiency 95 417