Body armour and lateral‐plate reduction in freshwater three‐spined stickleback Gasterosteus aculeatus : adaptations to a different buoyancy regime?

Several factors related to buoyancy were compared between one marine and two freshwater populations of three‐spined stickleback Gasterosteus aculeatus . Fish from all three populations had buoyancy near to neutral to the ambient water. This showed that neither marine nor freshwater G. aculeatus used swimming and hydrodynamic lift to prevent sinking. Comparing the swimbladder volumes showed that freshwater completely plated G. aculeatus had a significantly larger swimbladder volume than both completely plated marine and low‐plated freshwater G. aculeatus . Furthermore, body tissue density was lower in low‐plated G. aculeatus than in the completely plated marine and freshwater fish. The results show that G. aculeatus either reduce tissue density or increase swimbladder volume to adapt to lower water density. Mass measurements of lateral plates and pelvis showed that loss of body armour in low‐plated G. aculeatus could explain the tissue density difference between low‐plated and completely plated G. aculeatus . This suggests that the common occurrence of plate and armour reduction in freshwater G. aculeatus populations can be an adaptation to a lower water density.