The sea apple is a common name for colorful, round sea cucumbers in the genera Pseudocolochirus and Paracucumaria. They are predominantly found in tropical Indo-Pacific waters, where they feed by filtering the water of plankton and other nutrient sources. Sea apples use their tentacles to attract and capture plankton, which they then glean off the tentacle using their mouth. Sea apples are well known for releasing toxins into the water when threatened or stressed, and they can even release internal organs into the water in order to distract predators and, thus, create a diversion in order to escape.
Sea apples have a distinctly ovate body, often about 8 inches long. The most distinguishing feature of this body is the central mouth cavity that is surrounded by feathery tentacles it uses to capture plankton and other seaborne nutrients in the water. On the other side of its body, the sea apple has several rows of tube feet that it uses to move over and attach itself to various structures like underwater rocks. Various species of sea apples carry different colorations, almost always bright and highly decorative, with bright purples and reds being very common.
When threatened, sea apples respond in one of a number of ways: their preferred reaction is to release a toxic saponin into the water. Holothurin, the name of the toxic substance, is highly poisonous to most fish and will deter many predators from attempting to consume the sea apple. Sea apples can also imbibe themselves with seawater, doubling their size and letting the current take them away at high speed. This is not as fast as many fish can swim, but will often serve to protect the sea apple from slower predators.
Sea Apples do not have a true brain. While a ring of tissue dedicated to neural activity exists, surrounding the oral cavity, this simple network of nerves does not constitute a real nervous system. The sea apple has been shown to function even when the nerves are damaged, indicating that the role played by this part of its physiognomy is not as important as, for instance, the mammalian brain. Sea apples also lack sensory organs, with no eyes, ears, olfactory organs or even touch-sensitive cells being present on any part of the creature’s largely featureless, rotund body.