Ctenophora
Methods

Neritic ctenophores can be collected with nets, preferably with large mouth areas. Species of Beroe, Hormiphora, Pleurobrachia and Mnemiopsis are sturdy enough to survive trawling action. A gentle plankton net vertical haul is the best method as it causes less damage to the ctenophore. Plankton nets towed horizontally or obliquely at low speed (1 knot) are also suitable. As with other gelatinous organisms (medusae, salps, etc.), it is preferable to collect at night, when the animals are more abundant near the sea surface.

Pelagic ctenophores should be collected by divers using jars to capture the specimens (Harbison et al., 1978), or by submarines (Harbison, 1985, 1986), as they are more delicate, and trawling destroys them. These collection methods are also suggested for studies of feeding, growth, and metabolism.

Volumetric measures, as those used for the abundant coastal species (Deason, 1982; Feigenbaum and Kelly, 1984), are recommended for quantitative studies. Purcell (1988) proposed the use of the tentacle bulbs, which persist intact after preservation in 5% formaldehyde, for quantification of Mnemiopsis.

Immediately after collection specimens should be separated from other organisms and carefully transferred to glass jars filled with fresh, clean seawater using a perforated spoon or a glass pipette. Examination of live specimens is recommended. If the specimens have to be preserved for later examination, photographs should first be taken with the aid of a microscope in order to record structures of taxonomic value which may disappear later due to damaged or improper preservation. It is also advisable to remove specimens of the genus Mnemiopsis from the plankton samples with a sieve (1 cm mesh size), as they rapidly disintegrate after fixation leaving a gelatinous, difficult-to-handle plankton sample, unfit for further zooplankton quantitative studies (Mianzan, 1990).

Ctenophores are extremely fragile animals, difficult to fix and preserve, improper fixation producing a mass of amorphous jelly. Even with great care some distortion and shrinkage of specimens is to be expected. Formalin solutions are inadequate to fix ctenophores. Early methods using solutions containing osmic or chromic acid caused discoloration and sometimes disintegration of the specimens. The following procedure (Adams et al., 1976; O’Sullivan, 1986) is recommended for adequate preservation. Specimens must be poured into a jar of fixative (1 g of trichloracetic acid or 1 g of p-toluenesulphonic acid in 99 ml of seawater) for thirty minutes. Slight changes in body color, from transparency to translucency, will be observed. Then, animals must be kept in 1 ml of stock preserving solution (0.5 ml propylene phenoxetol, 4.5 ml propylene glycol, 5.0 ml formaldehyde 40%, and 90 ml seawater) and 99 ml sea water, for five to seven days. For storage, 5 ml stock solution and 95 ml sea water must be used. Never store ctenophores with Crustacea as the sharp spines will cut them, leading to disintegration. Storage temperatures should be between 5 and 20ºC. Avoid knocking or shaking the jars.