The Editor says: The sea apple is renowned for its vivid appearance, with its many tube feet blooming like flowers, drawing countless divers in for a closer look. The more beautiful something is, the more toxic it tends to be — and the sea apple truly lives up to the name "poison apple." Under normal circumstances, however, it is completely harmless. So when exactly does the sea apple transform into a "poison apple" and release toxic substances to defend itself? Follow along with BlueTrend as we explore this most stunning of seafloor apples!
The Most Beautiful Sea Cucumber — An Introduction to the Sea Apple
The brilliantly coloured sea apple is formally known as the purple sea cucumber (scientific name: Pseudocolochirus violaceus), a member of the family Cucumariidae.
A sea apple that has not yet "bloomed." photo credit：李承錄。
The sea apple looks quite different from the typical sea cucumber we see every day. Its body is round, short, and plump, with both its mouth and anus oriented toward the dorsal side — a rather peculiar arrangement. Around the mouth are dozens of bright yellow, tree-like branching tentacles. Body colour varies greatly and is usually a vivid red or purple, dotted with yellow spots. When fully open, it resembles an apple, which is how it earned the nickname "sea apple."
A fully bloomed sea apple is one of the true stars of the seafloor! photo credit：林祐平。
Sea apples are typically found in subtidal rocky reef zones. They also occur in the waters surrounding Taiwan, and can be spotted in places such as Keelung Chao-Jing and Penghu.
The Sea Apple's Unique Filter-Feeding System
The sea apple has numerous prominent tube feet, with those on its ventral side being particularly well developed. These tube feet anchor it to rocky reef on the seafloor, and strong currents allow the ten tree-like tentacles around the mouth to more easily capture and trap suspended organisms. The tentacles, now covered in food, are then placed into the mouth one by one to be slowly savoured — making the sea apple a filter-feeder.
They use dozens of tentacles to trap plankton drifting in the water, then slowly draw them into the mouth to eat. photo credit：林祐平。
Although plankton is their primary food source, sea apples do not have large appetites. Due to limited digestive capacity, they consume relatively little food at a time.
The Poison Apple's Defence Strategy
Don't let some of the sensational headlines you may find online frighten you — sea apples will not actively attack humans. While their body wall and internal organs are indeed toxic, they only begin releasing toxic substances when under extreme threat or near death, and such situations are actually quite rare.
Sea apples add a splash of colour to an otherwise drab seafloor. photo credit：蔡佳均
What Should You Keep in Mind When You Spot a Sea Apple?
When you encounter a sea apple underwater, remember never to touch it with your hands, and never disturb it.
Most importantly, as a marine citizen scientist, once you've confirmed that the surrounding environment is safe, don't forget to pull out your camera and take a photo. After all, when you come across such a beautiful sea cucumber on the seafloor, you absolutely must document it and upload it to the database!
Editor-in-charge: Jenny Tsai
Further Reading
