Males and females of each of the seven species of sea turtles 

are the same size; there is no gender dimorphism.

In comparison to their terrestrial or freshwater relatives, 

sea turtles often have a more fusiform body pattern. 

In contrast to many other turtles and tortoises, 

sea turtles cannot withdraw their head and limbs into their shells for protection 

due to the tapering at both ends, which decreases volume. 

However, the streamlined body structure makes swimming easier 

and faster for sea turtles by reducing drag and friction in the water.