Number of anemone arms depends on how much food they eat

A new study into the starlet sea anemone (Nematostella vectensis) has revealed the number of tentacle arms they can grow throughout their life is determined by how much food they eat, which opens up interesting questions about the genetics of developmental biology.  

An adult starlet sea anemone in all its glory. Most of its main column-like body is normally submerged beneath the seafloor

In almost all animal species in the natural kingdom, the number of limbs you can grow is defined and limited by specific genes that control body development. As a result humans have two arms and two legs, dogs have four legs and a tail, spiders have eight legs, birds have two wings and so on. However in certain marine invertebrates such as sea stars, jellyfish and in particular sea anemones, there appear to be fewer rules on how many limbs you can grow. The reason for this has long been a mystery amongst marine biologists, but new research from the European Molecular Biology Laboratory (EMBL) has now revealed that in sea anemones the number of arms an individual can grow is linked to the available food it has throughout its lifetime. This discovery changes what we know about the role genetic and environmental factors play in body development and opens up new research questions into morphology and cell differentiation.

Anemonenomones

Sea anemones are marine invertebrates that belong to the group known as cnidarians, containing over 11,000 species including jellyfish, corals and hydrozoans. Like other cnidarians, anemones lack a central nervous system and passively capture food using specialized stinging cells on their tentacle arms. Their life cycle is very similar to corals, where they spend their juvenile stage as free-floating larvae that eventually settle into sessile organisms on the sea floor or in rock pools or seagrass meadows. They are also similar to corals in that most anemones can create energy from photosynthesis via a symbiotic relationship with zooxanthellae algae in their tentacles.

An up-close look at the tentacle arms of the starlet sea anemone. Not only are the number of tentacles variable, but they also contain photosynthetic algae as well

Anemones can reproduce either sexually or asexually, via a process known as budding, and most can live long lives up to around 65 years old. How they live so long (especially for invertebrates) has long been something of a mystery, as has the number of tentacle arms they can grow. All anemone species have a variable number of tentacles which do not seem to be strictly governed by genes and can change throughout an individual’s life, making them something of an enigma in the animal kingdom and more similar to plants instead.

You are what you eat

In a new study researchers, from the EMBL in Heidelberg and the Stowers Institute for Medical Research in Kansas City, turned their attention to the starlet anemone (Nematostella vectensis) to see if they could understand what causes anemones to grow new tentacles. Starlet sea anemones can have anything between 10 to 20+ tentacles and when looking at 1000 polyps in the lab the researchers soon discovered why. It turns out that the amount of food they consume throughout their lives will determine how many tentacles they can grow. This makes their long lives more understandable, as it gives them flexibility and allows for them to make physiological changes over time.

When excess nutrients are taken in by the anemone it’s cells can start to differentiate into new tentacles as shown above (Image by EMBL)

In addition to discovering that nutrient intake was the limiting factor to the number of limbs an anemone can grow, the team also learnt how they were able to grow new ones as well. They discovered that specialized muscle cells pre-mark the location of new tentacles and that excess nutrients triggers these cells to differentiate into a new limb. So whereas humans and other animals turn excess energy into fat stores, it seems anemones can instead use their extra nutrients to grow new arms instead. The team recently released their findings in a new paper released in the journal Nature Communications.

Genetics & environment

This type of gene expression and molecular signalling shown by anemones is found in the cells of most living species, including humans, but is normally restricted to embryonic development and is rarely seen in adulthood. This opens up questions about how environmental changes can shape body development. “We can conclude that the number of tentacle arms must be determined by the interplay between genetic and environmental factors” explains lead author Aissam Ikmi, in a recent press release. The next step for the researchers is to better understand which nutrients best trigger the cell differentiation and learn more about how their cells express themselves to uncover more secrets about the anemones, as well as maybe ourselves.

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