Destroyers delay the removal of sub casualties, allowing them to fire back in their normal step even if they are hit by a sub. However, both attacking and defending subs always fire in the Opening Fire step, so a defending sub will return fire even if it is hit, regardless of the presence of destroyers. This is because fire within a step is simultaneous. In effect, the presence of a destroyer only affects the ability of surface vessels to return fire when hit by subs.