Descriptions of glandular structures of copepods have been a feature of the literature for many decades (Richards, 1891; Yonge, 1932; Clarke, Conover, David, & Nicol, 1962; Fahrenbach, 1962; Park, 1966; Mauchline, 1977; Briggs, 1978; Gharagozlou-van Ginneken, 1979; Arnaud, Brunet, & Mazza, 1988a; Arnaud, Brunet, & Mazza 1988b; Dumont & Silva-Briano, 1997). Unfortunately the few detailed studies which have been undertaken serve only to highlight our lack of understanding of these structures, their secretions and their significance to the biology and ecology of these animals.
Despite the fact that many different types of glands have been located in various parts of the copepod body, few of them have had their secretions characterised and their biological significance determined conclusively. Those best studied include the glands of the buccal cavity (Boxshall, 1982; Arnaud et al., 1988ab; Zeni & Zaffagnini, 1992; Vaupel Klein & Koomen, 1994) the luminous glands of the Calanoida (Clarke et al., 1962; Herring,1988; Bannister & Herring, 1989) and the mucous glands of the capsule-dwelling copepods (Hicks & Grahame, 1979). Glandular complexes in the urosome have been described previously by (Gharagozlou-van Ginneken, 1979) who suggested they produced chemicals involved in sexual activity. Fahrenbach (1962) and Boxshall (1982) also described glands in the urosomes of copepods but did not suggest a function for them. Glands in the thoracic limbs have also been widely reported (Fahrenbach, 1962; Park, 1966; Dumont & Silva-Briano, 1997).
Other glands are commonly found in close association with the integument (‘integumental glands’) but their role is still the subject of debate. Dennell (1947) and Stevenson (1961) suggested that they might be involved in cuticle tanning, while Rybakov & Dolmatov (1991) suggest a role in cuticle formation. Other functions such as predator deterrence (Pochon-Masson, Renaud-Mornant, & Cals, 1975) immunity to host nematocysts (Briggs, 1978), cuticle anti-fouling (Boxshall, 1982; Bannister, 1993), drag reduction (Bannister, 1993) ‘secretion-trap’ for water-borne chemical cues (Hipeau-Jacquotte, 1987) and production of pheromones for mate attraction (Gharagozlou-van Ginneken, 1979) have all been suggested as functions of these glands. It is likely that there is variation in integumental gland products and functions between species which probably represent evolutionary changes necessitated by the particular environment in which the animal lives.
In contrast to the widely accepted idea that glands are actively involved in secretion, Chapman (1981) showed that the ‘dermal’glands of Neocalanus plumchrus were involved in the uptake of dissolved glucose from the surrounding sea water.
Previous work (Bron, 1993; Andrade-Salas, 1997) has shown that the immunohistochemical stain 3’,3-diaminobenzidine tetrahydrochloride (DAB) highlighted glandular regions in L. salmonis larvae and adults. DAB is a chromogenic compound used routinely in immunohistochemical analyses. It is often used coupled to antibodies, as a marker, to highlight areas of antibody binding. Used on its own however it reacts, in the presence of hydrogen peroxide (H2O2) to produce a characteristic brown product which is insoluble in water, alcohol and xylene. This positive reaction to the DAB stain generally occurs with endogenous peroxidases but may also occur with catalase, cytochrome oxidase and possibly a number of other compounds. Peroxidase enzymes have been reported to be involved in many different functions within animal tissues, including: free radical neutralisation (Salin & Brown-Peterson, 1993), prostaglandin production (Bowman, Dillwith, & Sauer, 1996) and iodine binding as part of protothyroid activity in tunicates (Fredriksson, Öfverholm, & Ericson, 1988).
As part of a wider investigation into the functional significance of exocrine glands in L. salmonis we undertook a study to locate all the DAB-positive regions in all life-stages of that species and the related parasitic copepod Caligus elongatus. To determine whether DAB-positive regions are a widespread feature of the Copepoda, we stained several species of free-living copepod and also some other aquatic arthropods.