Nemertina
Methods

Most of the earlier data on the vertical distribution of bathypelagic nemertines, based on the use of non-closing nets, were inaccurate, whereas closing nets such as modified Tucker trawls with thermally protecting cod ends (Childress et al., 1978) or a Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS) (Wiebe et al., 1985) provide the most accurate figures for depth and other hydrographic data for pelagic organisms as well as improving the condition of the specimens obtained. The small size of many pelagic nemertines, which particularly as juveniles may be less than 10 mm long, indicates that an appropriately sized mesh should be employed. P. Schalk (fide Van der Spoel, 1988) used a combined rectangular mid-water trawl RMT 1 + 8 with two nets, one with a mesh of 0.32 mm and the other of 4.5 mm, at depths of 0-500 m and obtained large numbers of meso- and bathypelagic species from several taxonomic groups, including nemertines. Recently Roe and Norenburg, in a paper presented at the Fourth International Conference on Nemertine Biology in 1995, demonstrated that the use of a slow net towing speed (of less than about 2 knots) was essential for the recovery of living pelagic nemertines in excellent condition.

As with benthic nemertines, the identification of pelagic species depends ultimately upon a study of their internal anatomy by means of histological sections, although specimens should be carefully examined alive prior to preservation. Certain external features, such as the tentacles of male Nectonemertes (Ne 1), are of taxonomic significance whilst particular internal structures, such as the gut diverticula, gonads or nervous system, can sometimes be distinguished in whole mount specimens (Ne 1) stained with borax or alum carmine and then cleared (Coe, 1954a; Van der Spoel, 1985); smaller sized and transparent worms can often be studied as whole mounts without the need for staining, or other appropriate stains, such as Mayer's haemalum, can be employed.

Histological studies should be made on carefully fixed specimens embedded in a suitable wax; harder waxes (e.g., 56ºC m.p. paraffin wax) are generally more suitable than lower melting point media, and serial sections of 5-8 µm are ideal. The choice of fixative is to some extent governed by which staining protocol will be followed; some authors have used formalin or alcohol fixation, with the sections subsequently stained in haematoxylin/eosin, but Bouin's solution, formol-calcium or Susa's fluid followed by staining with a trichrome method (e.g., Mallory or Masson) work well with benthic nemertines and normally yield a better differentiation between body organs and tissues; these techniques can equally be applied to pelagic species.