Discussion
To adapt to the changing salinity of esturine environment crabs :
Answer: D
Estuarine crabs keep the salt concentration of their bodies more or less constant by active transport.
Estuaries swing between near-fresh and near-marine salinity with the tides and the seasons. Crabs living there, such as the shore crab Carcinus, are osmoregulators: their gills actively pump ions in when the water is dilute and out when it is salty, so the body fluid stays near a fixed concentration.
Letting the body fluid follow the water (osmoconforming) is the strategy of many open-sea invertebrates, which cannot survive the wide swings of an estuary. Moving towards the fresh or the sea-water side is not an adaptation to changing salinity but an escape from it, and the tide brings the change to the crab wherever it is.
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