This is our closest invertebrate relative - and it could help us grow new limbs!

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A species of sea squirt called the Star ascidian (Botryllus schlosseri) is believed to be the most closely-related invertebrate to humans, with 77% of human genes present.

This tiny marine animal that many see as invasive may hold the secret that could allow humans to grow back limbs they have lost or accept a transplanted organ from a donor without the risk of rejection.

The Star ascidian is thought to be the first invertebrate to have a vasculature heart system, similar to our own, with blood cells traveling through blood vessels. But what's really amazing about this animal is its ability to regrow itself just with its blood vessels.

"The whole body can regenerate from the vasculature alone: the heart, digestive system, sophisticated tissues," said Ayelet Voskoboynik, a scientist at Stanford's Stem Cell Institute and Hopkins Marine Station, and the lead author on the study. "And it can do this relatively fast, probably using stem cells."

Researchers also hope this little sea squirt could help with advances in transplant surgery. When two genetically distinct Botryllus colonies come into contact with one other, they either fuse their blood vessels to create a single organism, or reject one another and maintain individuality.

When the blood vessels fuse into one interconnected network, the stem cells from each partner colony begin to circulate throughout the other.

The stem cells compete and in many cases one partner's stem cells "win" – and any new or replacement tissue grown through the fused colony does so based on the "winner's" genetic code.

Scientists hope that if they can learn what makes a highly competitive stem cell a winner, and why others are rejected, it could shed new light on rejection of donor tissue or cells by some patients after transplant.

While its native range is the north eastern Atlantic Ocean, the Mediterranean Sea and the North Sea, B. schlosseri has spread over the last century and is now found almost worldwide. Its colonies coat rocks, molluscs and ship hulls and it's considered an invasive species.

The study was recently published in the journal eLIFE.