Translation:The turtles eat the rice and the tomato.
Nope, even on the plurals that don't change you still add an "-ene" for the definite plural: skoen to skoene, fisken to fiskene.. etc. In Danish, en ris would be more like a grain of rice as it is a countable word. The only way to be able to tell is the grammar and sentence structure (and normally in English, if you're eating en ris one would normally say "a grain/piece of rice").
Edit: A little correction to what I was saying, you can also say "risen" and it is accepted, but just for the example here it is in plural
I compare to Swedish, where rice does not really have a plural (unless you are talking about the grains/pieces just like you said).
In Swedish it is Jag äter ris = I eat rice Jag äter riset = I eat the rice
Is "risene" in Danish the same as "riset" in Swedish? Then, what is the difference between "risen" och "risene"? Sorry for comparing to Swedish if that does not really work here, but I am so confused :).
I found this Danish dictionary which shows risen as "the rice" and the plural as adding -ene: http://sproget.dk/lookup?SearchableText=ris I couldn't understand everything so I copied to Google translate out of curiosity of what it might say. https://translate.google.com/?hl=en#da/en/Ordb%C3%B8ger%20%20%20%20%20%0A%0A%0ARetskrivningsordbogen%0A%0A%20%0A1.%20ris%20sb.%2C%20-en%2C%20ris%2C%20-ene%20(en%20plante%3B%20riskorn)%2C%20i%20sms.%20ris-%2C%20fx%20rismark%2C%20og%20risen-%2C%20fx%20risengryn%0A%0A2.%20ris%20sb.%2C%20-et%2C%20ris%2C%20-ene%20(knippe%3B%20et%20redskab%3B%20en%20m%C3%A5leenhed%20for%20papir%3B%20negativ%20kritik)%3B%20ris%20og%20ros%0A%0ADen%20Danske%20Ordbog
The best is this Wikipedia article in Danish about rice: http://da.wikipedia.org/wiki/Ris
I am thinking they are eating more than one kind of rice, such as brown rice and regular long grain rice or short grain rice.