Gill morphology of the intertidal estuarine crab Chasmagnathus granulatus Dana, 1851 (Decapoda, Grapsidae) in relation to habitat and respiratory habits

Histological and morphometric analyses were performed on the gills of the s emiterrestrial estuarine crab Chasmagnathus granulatus. Three different epi thelia were recognized: (1) A thin epithelium, 2.42 +/- 0.33 mu m thick, wh ich lines the whole lamellae in the three anterior gills and was assigned t o respiratory functions. (2) A thick cuboidal epithelium, 6-12.5 mu m thick , covering part of,of gills 4 and 5 and the better part of the three most p osterior gills. This tissue seems to be involved in ion-regulation, since i t is located in the same zones which are darkly stained with silver nitrate and possess large numbers of mitochondria, closely associated to basolater al interdigitations and abundant infoldings of the apical membrane. (3) An attenuated epithelium, 0.5-1.5 mu m thick, bordering the marginal channels of all the gills. This tissue is clearly of a respiratory type and probably plays an important role during air breathing when the branchial water stor es become reduced by evaporation, and the consequent gill collapse impairs both ventilation and perfusion of the central part of the lamellae. During exposure to humid air, C. granulatus is able to maintain its branchi al chambers almost completely filled with water, thus keeping its gills fun ctional in spite of little mechanical support. These adaptations for mainta ining gill respiration in air allow C. granulatus to sustain high metabolic rates during emergence, with little increase in venous partial pressure of carbon dioxide.