So you can see drawing the cut off line for even lower animals could present considerable challenges. You have trouble reaching an adequate definition of "sentient". You are now facing how much awareness a creature has to perceive pain and pleasure along with joy from anticipation of future events to consider it morally significant. If a cat is significant, but not a fish, what makes the cat a moral patient while the fish is not? Where is there a difference? There is a problem of arbitrarily assigning moral value when actual feelings and emotions are beyond description.
To go a step further away from human sentience you would hold all living thing to be of moral value. This would then bring plants and non-sentient animals into the picture. This view holds life as the ultimate intrinsic value. Beings have moral value in just being alive. So life is viewed as an intrinsic good, and no verifying pleasures or pains being experienced are needed to allot this worth. Anything living is held with a reverence for that life .