Answer :C) to prevent loss of ions to the surrounding water
Explanation :Basically,by closing the lamellae it prevents the entry of water by OSMOSIS , into high solute(ions) potential plasma .if the lamellae were not shut, osmosis drives water in, the fish produced hypotonic urine with resultant loss of ions,
Therefore with the closure of the lamellae, it countered continuous plasma ion loss, and ensure re-establishment of the water-ion balance in cells of muscle tissues and blood. Therefore, the net loss of plasma ions was cut down, and stability of the ion concentration ensured,as a result of a drop in the passive ion efflux to the surrounding aquatic medium.
Completed Ques.
The crucian carp (Carassius carassius) is a Northern European freshwater fish often inhabiting ponds that become hypoxic (have reduced oxygen levels) and even anoxic (have no oxygen) when the surface freezes during the winter. Surprisingly, when oxygen levels are normal, these fish lack the lamellae that provide a large surface area for gas exchange between water and blood: their gills are smooth. Yet when the level of oxygen in the water falls, the gill morphology undergoes a change: packing cells stop dividing and programmed cell death is induced, exposing gill lamellae that were buried in other tissue. With lamellae exposed, the gills have increased surface area for gas exchange. These changes in gill lamellar profile are reversible: investigators observed that the gills return to their normal structure within seven days after returning the fish to well-oxygenated water. (Jørund Sollid, Paula De Angelis, Kristian Gundersen, and Göran E. Nilsson. 2003. Hypoxia induces adaptive and reversible gross morphological changes in crucian carp gills. Journal of Experimental Biology 206:3667-73.)
