Structural adaptations of the branchial apparatus of Holthuisana (formerly Parathelphusa) transversa (Martens) for bimodal gas exchange and ion uptake are described using optical and electron microscopy.
A single epithelial cell type (besides pillar cells) in the gill lamellae fulfils the dual role of ion uptake from freshwater and aquatic gas exchange. The total blood/water distance is about 5–8 μm (cuticle 2–3 μm, cells 2–5 μrn). Apical cellular extensions delimit an extensive system of subcuticular spaces. Mitochondria are numerous in the sub-apical cytoplasm but do not form “mitochondrial pumps” with the weakly developed basal infoldings. Nephrocytes are present in the gill shaft but not in the lamellae. Each gill possesses two arteries termed epibranchial and hypobranchial arteries), which run parallel and medial to the main afferent and efferent vessels, and which branch extensively in each lamella (lamellar arteries). The multi-layered endothelial wall of the arteries forms a longitudinally pleated intima. Larger arteries possess an external collagenous sheath. Sheath and intima are absent from the terminal capillaries. It is suggested that the arteries supply oxygen to the metabolically active tissues of collapsed gills during aerial respiration.
The branchiostegites are expanded to form lungs which are ventilated by a unique tidal mechanism involving lateral movements of the thoracic wall. The inner cuticular lining of the branchiostegite is smooth and the epidermal cells extremely attenuated. The total blood/ gas distance is only 250–300 nm (cuticle 200–250 nm, epidermal cell extensions 20–50 nm, basal lamina 30–40 nm) and comparable with the lungs of higher vertebrates. Deoxygenated systemic blood is directed close to the gas exchange surface by newly-described fimbriated cells bordering the connective tissue partitions separating fine afferent and efferent blood sinuses.
The observations are discussed in detail in relation to general features of the structure of crustacean gills, branchiostegites and permeable cuticles and of crustacean circulatory and respiratory physiology.