17th edition of Marine Life monthly post. Marine Life post published every 19th of the month. It aims to share information on specific marine life species and to promote their conservation. All images are taken by Indah Susanti unless stated otherwise.

My first time seeing a flatworm on my scuba diving, I mistakenly thought it was a nudibranch. After several dives and seeing the nudibranch and flatworm, finally, I can see their differences. It is not easy to notice the differences because both species look alike and have beautiful vibrant color patterns.

The first fact about flatworm is, it has a flat body and thinner than nudibranch. It does not have the rhinophores, cerata or mantle like nudibranch.

Nudibranch Anatomy

Flatworm

Secondly, the flatworm does mimicry to protect itself from a potential predator. Some flatworm species are not toxic that make them as easy targets to the predator. They imitate the color patterns of nudibranch that known for its toxic. The following two images show a flatworm imitates a nudibranch. The nudibranch image is from Sea Slug Forum while the picture of the flatworm was taken by me when scuba diving in Indonesia. Can you tell which one is a flatworm?

Racing Flatworm

Thirdly, flatworm reproduction is the most bizarre part of all. The flatworm has an asexual reproduction that allows it to clone itself into two or more clones, and they are all identical clones. I wish there were a video of how the flatworm clones itself.

Besides of cloning itself, the flatworm which is a hermaphrodite has a unique mating behavior. The mating behavior called as “Penis Fencing.”  Two flatworms “(mating) fight” against each other to inseminate the mating opponent with its sperm. The first one who succeeded then become the father. This two penises fight could last for hours, and National Geographic has it on a video – a short one, thankfully! Seriously, if I were the flatworm, I just go cloning myself.