For thousands of years, philosophers have struggled with the concepts of language, communication and meaning. Turtles have not. One reason might be that there is no "Turtlestotle" to speak of. A more realistic explanation is that communication in turtle species differs vastly from that of humans. Whereas humans largely associate communication with speaking and listening, turtles communicate mostly through touch and sight.
Communication Through Sound
Until 2014, turtle experts have believed their subjects to be mostly deaf. This is because turtles' ears are internal and not sensitive to many sounds. That said, scientists have recently begun to research what may be sound communication occurring between turtles, especially around the time of egg-laying and hatching. The sounds that the turtles produce are low-frequency so they carry through water better. In fact, these are normally out of the range of human hearing, which is why the phenomenon is only recently being documented.
Another purpose communication serves in turtles is courtship. Specific turtle mating behaviors are often dependent on the specie of turtle, but these often involve both visual and tactile communication. In red-eared sliders, the most common turtle in the pet trade, courtship rituals occur underwater and involve the male touching the female's head and neck with his claws. Land turtles are known to have complex, head-bobbing mating "dances." These behaviors can last for hours or even days.
Communication with Humans
Turtles can't talk to us, and their courtship behaviors just look silly to people, but turtles nonetheless can certainly be said to communicate their emotions through a type of body language that pet turtle owners may come to know well. When threatened, most specimens will retract their heads, legs and tails into their shells to protect themselves. But some turtles, once comfortable with people, will not retract into their shells when handled and may even actively investigate things that make them curious. The more of a turtle's extremities you can see, the less threatened the animal feels.
Using This Information
Knowing how turtles communicate with one another and with you is an important part of being a good pet owner. By observing your animals, you can often judge whether a turtle feels like being handled and whether she is happy with her environment. A sudden change in communicative behavior in your turtle may indicate that she is sick, and knowing this can help you get her to a veterinarian before physical symptoms manifest themselves.