SHUTTER SPEED PRIORITY: You set the shutter speed and the camera sets the aperture so enough light hits the sensor.  Shutter priority refers to the length of time the shutter stays open and is exposed to light. A fast shutter speed will let in less light than a slow shutter speed. A slow shutter speed will let in more light.  Since mushrooms are stationary you probably won’t be using the shutter speed priority feature on your camera too much – unless you are the source of movement.  If you decide to handhold your camera, you will want to minimize the problem of motion blur caused by shaky hands by increasing the shutter speed.  But be aware that increasing the shutter speed will narrow the DOF (because the camera will set the aperture to a larger opening to compensate for the loss of light due to the increased shutter speed).  Experiment to find out which shutter speeds work best for you.  Shutter speeds below 1/60th of a second are considered too slow to hold a camera steady without using support, like a tripod. Sometimes 1/60th of a second is still too slow for me.  You can always increase your odds of getting at least one in focus photo by taking lots of pictures of the same mushroom.  They might also shed light on the scene by using their flash with a diffuser.