Microtubule systems associated with the septate junctions of the gill cells of four gammarid amphipods

The microtubular systems associated with the septate junctions of the gill epithelial cells of four species of gammarid amphipod are described. The four species examined included two relatively stenohaline marine forms, Chaetogammarus marinus and Gammarus locusta; a highly euryhaline species, Gammarus duebeni, and a stenohaline freshwater species, Gammarus pulex. Of these amphipods, G. locusta and C. marinus maintain only a limited osmotic gradient between their haemolymph and the medium and have a poorly developed junctional microtubular system; G. pulex has haemolymph which is some 300 mOsmol hypertonic to freshwater and has a well ordered system of microtubules on both sides of fairly long septate junctions; G. duebeni from brackish water tend to have a somewhat shorter length of septate junctions lined by one or occasionally by a double row of microtubules. The most complex junctional microtubular systems are shown by specimens of the freshwater race of G. duebeni celticus which have been acclimated to seawater. These can take the form of multiple arrays in which some microtubules are linked to the plasma membrane by dense strands. It is suggested that these findings are consistent with the hypothesis that one role of these microtubules is to provide mechanical stability to enable the integrity of the septate junctions to be maintained during osmotic stress.