All of the rock pool fish and most of the crabs will be resident in the deeper, warmer water.
Air temperature is the important factor.
1.87 o C the pools will actually freeze. This would be very rare in the south but could conceivably occur in upper shore pools on Scottish coasts. The average air temperature during January is about 2 o C on the eastern English Channel coast and at about 1.5 o C on the North Sea coast with annual fluctuations, but because of the lesser specific heat of air the temperature can vary much more quickly than the open sea. In Cornwall the mean air temperature in January has an average of 5 o C, so the fish and crabs may still remain on the shore during this month. Common Starfish, Asterias rubens .