Today I learned about the sea slug Elysia marginata, which (I quote) can lose its head like no other: when this slug is overwhelmed by parasites, it doesn’t go on marathons to Instagram gurus but takes a radical approach — it detaches its head and starts a new life. (I quote Anna Fe above and below because I learned everything from her post) The slug’s neck has a special groove labeled “cut here.” The head separates along this line, not immediately, but over several hours. Having shed its fatty, annoying body, the head, giggling, crawls away, saying “goodbye” to its former self – along with all the parasites, and also with the heart, kidneys, intestines, and reproductive organs. The slug survives understandably – “like everyone else, using its head (it eats with it).”
And here I started thinking: is the definition of a head—where you eat from? How do biologists understand where the head is in some fantastical creature? For example, does a sea cucumber have a head?
I Googled it. Wow, interesting. No, the head is not where you eat from. It is what first encounters the surrounding environment when moving forward. Since this end needs to quickly understand what’s ahead (danger or food), during evolution, two things concentrate there:
1) Main sensory organs (eyes, antennae, locators, chemical receptors)
2) CNS (brain or the largest nerve nodes – ganglia), to instantly process the information received from these organs.
Therefore, they write, when a biologist looks at an unknown fantastical thing,” he looks not for the mouth, but for the main control point. Where there is the highest concentration of nerve cells and receptors, there is the head.
But let’s return to the independent head of the slug. How does it manage without a stomach?
These slugs feed on algae and are able to retain their chloroplasts (structures for photosynthesis) in their tissues. Thus, the head temporarily turns into a plant and gets energy directly from sunlight, while it regrows its new body. The wound on the neck heals within 2 days, a new heart takes about a week to grow, and a fully new torso with all organs—just 20 days.
The former body, meanwhile, just lives its life on the ocean floor. Its heart beats and nerves feel, but poor thing, it cannot eat and eventually dies from exhaustion.
Hi, I'm Rauf Aliev. 