Hydromedusae
Collection of material

Hydromedusae can be caught by plankton nets towed very slowly (1 to 1.5 knots) behind large or small powered vessels, or even a rowboat, for about ten to twenty minutes, depending on the concentration of plankton. The mesh of the net should be about 200-250 µm. Larger meshes permit many small specimens to escape, whereas smaller ones clog easily and damage the specimens. In coastal waters the mouth-opening of the net should be from 30 cm to 1 m, depending on the power of the vessel. Largest mouths are used in the open sea and in deep water where the fauna is sparser. In areas very rich in plankton, a hand net or even a bucket may be used. The richest catches are generally obtained in the early morning or at dusk, and on rising tides. For qualitative surface sampling the plankton net is towed 50 m or more behind the vessel to minimise turbulence. For sampling a few meters below the surface a buoy can be attached with a rope of known length to one side of the mouth-ring and a weight on the other side. Sampling between fixed depths requires closing nets. Quantitative sampling is possible with plankton nets fitted a little behind the front with a calibrated flow-meter to measure the volume of water filtered (see UNESCO, 1968; Boltovskoy, ed., 1981; Omori and Ikeda, 1984; for further details on general zooplankton sampling techniques).

If it is impossible to sort the medusae immediately, the catch should be fixed as quickly as possible with 5% formaldehyde in sea water (see Fixation and preservation below). If the material can be brought rapidly to the laboratory, the plankton samples, shielded from direct sunlight and kept as cool as possible, should be examined under a stereo-microscope, and the medusae individually transferred with wide-mouthed pipettes to finger bowls of clear sea water. After observation they can either be kept for rearing, or anaesthetised and fixed as described below. This method is of course the most fruitful, allowing observation of characters of the living animal and perfect fixation.