Gill development in sympatric morphs of Arctic charr from Loch Awe,Scotland: A hidden physiological cost of macrobenthos feeding?

The development of the respiratory surfaces was compared in two sympatric, lacustrine morphs of Arctic charr. A macrobenthic invertebrate feeding specialist, that forages in the littoral benthic zone, had a gill cavity that was 54% larger in volume and had 31% greater respiratory surface area than that of a zooplankton feeding morph that forages in the pelagic zone. The large respiratory surface area in the benthic‐feeding form was the result of longer gill arches, more and longer gill filaments and more numerous secondary lamellae. The difference in gill cavity volume and filament length appears to be the result of a larger head, but not body size, in the benthic‐feeding form. This suggests that differences in these characteristics may have arisen as a by‐product of the expression of larger head size commonly described in macrobenthos foraging specialist charr. The other differences, particularly the more numerous secondary lamellae and the length of the gill arches, were not the result of head size differences between morphs, and thus, these are most likely an adaptation to greater respiratory requirements. Benthic‐feeding fish may have a greater respiratory capacity to allow them to forage in areas with lower levels of dissolved oxygen and/or engage in a more active lifestyle compared to the pelagic‐feeding form. In any event, the strikingly larger respiratory surface is likely to impose an additional ionoregulatory stress on the benthic‐feeding and thus may represent a hidden physiological cost of specialisation for foraging on benthic prey.