Amphipoda
Methods

Pelagic amphipods can be collected with any plankton net, but collections not specifically designed for them may be poorly representative. Planktonic amphipods usually have well-developed eyes, are active swimmers, and can therefore dodge standard nets quite efficiently. Thus, for most species, large nets with mouth openings above 1 mö2 are required for quantitative faunal analysis. Large high-speed nets (Bongo, Isaacs-Kidd midwater trawls with a mouth area about 5 mö2, etc.) have made it possible to acquire representative materials, yielding a more complete picture of species compositions, including rare species (Shulenberger, 1977; G. Vinogradov, 1991, 1993b). The mesh of these nets, notably Isaacs-Kidd trawls, should preferably be small enough (e.g., 5 mm in the Samyshev-Aseev modification) to retain the animals efficiently; larger mesh-openings, while inadequate for quantitative purposes, can still retrieve qualitatively interesting samples. Despite avoidance problems, even smaller plankton nets may collect enough specimens to furnish a general picture of the diversity at a given site. It should be stressed that some amphipods often swarm, with the result that a single tow may strongly over- or underestimate their overall abundance. In addition, some hyperiids may enter a sampler with hosts (gelatinous planktonic animals), thus resulting in unrealistically high numbers of amphipods. Bentho-pelagic gammarid scavengers may be collected by means of baited traps. Of course trap samples are only qualitative.

Animals must be preserved in 70% alcohol or in 4% buffered formaldehyde.

Identification of amphipods usually requires dissection, or at least manipulation under a binocular microscope. Before detailed examination and dissection of the specimen, the characters which can be destroyed by subsequent handling should be observed: e.g. (1) form and comparative length of the head, (2) form and colour of the eyes (if any), (3) shape of the rostrum (when present) and of the epistome (in gammarids), (4) presence or absence of depressions or cairns and spines on the body, (5) presence or absence of fused segments in the pereon and the urosome (see General Morphology below). Subsequently, glycerine or glycerine-gelatine slides must be prepared with the pereopods (taking care not to lose the coxae), uropods, both antennae, telson, epimera (at least epimera III), and the mouth parts (it is preferable to place the last on a separate slide). In Hyperiidea epimera are very rarely used for diagnostic purposes, and in the hyperiid suborder Physocephalata mouth parts are taxonomically useful only at the level of families and some small species of Hyperiidae.

Dissections are performed with dissecting needles in a drop of water with glycerine on a glass plate or crystallising dish under the binocular microscope. The liquid is renewed at suitable intervals. For a detailed examination, the appendages from one side of the body should be removed (taking care not to disturb their order). Mouth parts are usually removed one by one, and it may be easier to isolate the entire buccal mass and dissect it separately, starting with the maxillipeds (see General Morphology below). If permanent slides are not necessary, the appendages may be placed in a cavity slide in glycerine and covered with a cover slip for subsequent analysis with a high-powered compound microscope.

Glycerine-gelatine slides are usually the most useful. The medium is prepared by soaking 7 g of gelatine in 42 cmö3 of distilled water for 2-3 hours, then adding 50 g of glycerine and 0.5 g of crystalline phenol (carbolic acid) or, preferably, thymol. The mixture is warmed in a water bath with continuous stirring, after which it is filtered and cooled, turning it into a dense gelatinous substance. Slides are prepared by placing amphipod appendages in a small drop of glycerine on the glass slide. A tiny amount of glycerine-gelatine is dropped on the cover slip and heated until it melts (care should be taken to avoid letting the mixture boil; if this happens the process must be started over). The cover slip is then turned and placed on the material on the slide. During this procedure the appendages may move. If some of them overlap or trap air bubbles, carefully pressing the cover slip with a needle under the dissecting microscope may correct the problem. If bubbles appear in the glycerine-gelatine near the margins of the cover slip, small amounts of glycerine-gelatine may be picked up with a dissection needle, melted, and forced under the cover glass. The slide is then allowed to cool, avoiding further manipulation until the medium has completely hardened. Such slides may be used for a long time; and if the cover glass was mounted on a "ridge" of water-proof varnish enclosing the medium with the appendages, they become practically permanent. Once mounted in the glycerine-gelatine slide, appendages cannot be removed without risking damage or loss. Canada balsam-mounted materials last even longer, but this technique is more laborious and time-consuming, and is not recommended for field conditions and/or with very abundant materials.