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Hyperosmoregulation in the red freshwater crab Dilocarcinus pagei (Brachyura, Trichodactylidae): structural and functional asymmetries of the posterior gills

Horst Onken^* and John Campbell McNamara

Departamento de Biologia, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, Avenida Bandeirantes 3900, Ribeirão Preto 14040-901, São Paulo, Brasil

View larger version (120K): [in a new window]   Fig. 1. (A–F) Anatomical and histological features of the gills of the hololimnetic trichodactylid crab Dilocarcinus pagei. (A) Macroscopic view of the eight gills (g) in situ in the left branchial chamber after removal of the carapace. Scale bar, 5 mm. (B) Phyllobranchiate gills 1–8 (1–8) arranged according to insertion sequence along the antero-posterior body axis. Scale bar, 5 mm. (C,D) Epoxy-embedded whole mounts of paraformaldehyde/glutaraldehyde/OsO4-fixed lamellae from anterior gill 4 (C) and posterior gill 7 (D). The dense osmiophilic areas (*) in gill 7 reflect an underlying, thick transporting epithelium (see E,F). A clearly less-dense area is also seen in anterior gill 4. Afferent epibranchial (e) and efferent hypobranchial (h) vessels and haemolymph channels (c) originating on either side of the gill shaft (s) are visible. Scale bar, 1 mm. (E) Micrograph of a 0.5 µm thick epoxy section taken transversely across a lamella from posterior gill 7. An intralamellar septum (s) in the haemolymph space (h) separates the two thin epithelia (arrowheads) near the gill shaft (left side, not visible). Approximately 80 µm from the shaft, the epithelial layers become asymmetrical: 3–10 µm thick on the distal side (d) and 18–20 µm on the proximal side (p). Scale bar, 20 µm. (F) Micrograph of a 0.5 µm thick section taken transversely through the osmiophilic region of a lamella from posterior gill 7. The dense, thick proximal epithelium is characterised by numerous basal invaginations (i) and a few apical vesicles. The thin distal epithelium consists of the extensive apical flanges (f) of the pillar cells (pc), populated by numerous vesicles (v) and apical invaginations. h, haemolymph space. Scale bar, 20 µm.

View larger version (10K): [in a new window]   Fig. 2. (A–C) Time courses of the electrical potential difference (Vte) across three different, perfused, whole posterior gills, demonstrating examples of the different responses to ouabain (2 mmol l–1, black bars) added to the perfusate. Vte was measured as the external potential with respect to the internal medium. An identical NaCl saline was used in the bath and perfusate.

View larger version (18K): [in a new window]   Fig. 3. Representative time course of the short-circuit current (Isc) across a split distal lamella of a posterior gill with NaCl saline on both sides. The following manipulations were performed: 1, substitution of Cl– on both sides of the preparation; 2, substitution of Na+ on both sides of the preparation; 3, addition of ouabain (2 mmol l–1) to the internal bathing medium; 4, addition of dimethylsulphoxide (DMSO, 0.2 %) to the internal medium; 5, addition of diphenylamine-2-carboxylate (1 mmol l–1, dissolved in DMSO) to the internal medium; 6, addition of acetazolamide (0.2 mmol l–1) to the internal bathing medium. The vertical current deflections are due to short voltage pulses and reflect the preparation conductance (Gte; see lower right corner for scale).

View larger version (22K): [in a new window]   Fig. 4. Representative time course of the short-circuit current (Isc) across a proximal split lamella of a posterior gill with NaCl saline on both sides. The following manipulations are shown: 1, addition of ouabain (2 mmol l–1) to the internal bathing medium; 2, substitution of Na+ on both sides of the preparation. Substitution and readministration of Na+ resulted in fast, transient current overshoots (to approximately –40 and 60 µA cm–2, respectively). These Isc transients are due to concentration gradients resulting from the non-simultaneous replacement of the external and internal bathing media. The vertical current deflections are due to short voltage pulses and reflect the preparation conductance (Gte; see upper right corner for scale).

View larger version (10K): [in a new window]   Fig. 5. Diagram showing the electrical resistances ( cm2) of different posterior gill preparations: 1, whole gill lamellae (461±54  cm2, N=6); 2, distal split gill lamellae (284±38  cm2, N=6); 3, proximal split gill lamellae (81±20  cm2, N=7); 4, sum of distal and proximal split gill lamellae (365±43  cm2); 5, isolated cuticles (29±7  cm2, N=5). Values are means ± S.E.M.

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