Aerial respiration in the semaphore crab,Heloecius cordiformis,with or without branchial water

• 1.1. Semaphore crabs (Heloecius cordiformis) are active in air at low tide. Their branchial chambers are lined with a vascular epithelium and are expanded above the gills (five pairs) to form air cavities which could function as lungs. Water is continuously circulated over the gills.
• 2.2. The relative contribution made by the gills and lungs to gas exchange in semaphore crabs active in air and circulating branchial water, was determined by measuring oxygen consumption (at 25°C) in crabs with and without branchial water, and in crabs with their lungs subsequently occluded.
• 3.3. Activity levels and VO₂ were unaffected by the absence of branchial water.
• 4.4. With their lungs occluded, VO₂ dropped (on average) by 61% in crabs with branchial water (i.e. gills still functional) and by 81% in crabs without branchial water (gill function impaired).
• 5.5. It is concluded that semaphore crabs are obligate air breathers while active on land, despite carrying water within their branchial chambers. Lung development and gill reduction in land crabs is discussed briefly in relation to “terrestriality”.