I've seen very dumb animals, box turtles, express emotions such as mourning and sadness as well as excitement; so I imagine most animals have emotions with the capacity for the complexity of their emotions being based on the complexity of their brain.
The septal nuclei is the pleasure center in the brain. The amygdala is the part of the brain thatdeals with emotional process and processing memory. Both are found in humans AND complex animals. Yes, animals can in fact feel emotions.
We should regulate what we do to what animals though, but they should be in-line. As a hunter and fisherman, I know the importance of this.
I know that deer populations are healthier if they are hunted. It seems irrational at first but it makes sense when you think about it, think back to high school biology where you learned about the population rates of rabbits. When they got too high, they ended up dying off, due to diseases and lowered food supply.
Regulations are usually seasonal. Here in Florida, trout are illegal to keep in February because that is mating season, then they limit you to 1 trout above 20 inches and 5 above 15. The rest are too small they need to grow up and mate.
There is a circle, and we were designed to be a part of it. Rationally and not cruelly.
What is an emotion? It's just a survival mechanism. We "feel" things so that we can be motivated to take certain actions that will help us survive. Nothing that we have just came to be because we are "higher" than other animals. We follow the laws of nature, and so do other animals.
And animals do cry. Maybe not in the same way as humans, but they can grieve, feel sadness, and they do have tears.
And until we can come up with a real definition for love, love is not an emotion, it is a concept of extreme fondness. By that definition, animals can indeed love. Dogs will sacrifice themselves to protect their owners, and spend years sleeping by their owners' graves after they pass away. I haven't seen a single human that would be willing to do that.
if it has a nervous system, centralized or decentralized, then the potential for emotions exists somewhere in that matrix. It is possible the range of emotions may be less, but it is likely that as long as the creature's neural matter is at least the size of marble, some emotional connections probably exist, and if the size of a golf ball or larger, most definitely. Although you have to account for size - larger living things have larger grey matter dedicated to controlling the body, without necessarily existing for cognitive function. I believe like/dislike are among the most fundamental emotions, and then they grow more complex from there.
Most animals aren't evolved enough to feel full human emotion i know dogs can feel emotions to a degree and maybe a few other animals but i would have to say that most animals on this planet cannot express emotion.
Emotion is a form of intelligence so i guess to narrow it down only the smarter animals could feel emotions but animals like fish and bugs i highly doubt they have a care in the world about anything.
I'm not sure how credible this particular source is, but you should really look into the possibility of "lower" species having emotions. http://voices.yahoo.com/fish-feel-pain-emotions-personalities-2610743.html
I don't have a hard time believing that fish have something akin to emotion.