Science for All | What are tunicates?

The Hindu’s weekly Science for All newsletter explains all things Science, without the jargon.

July 12, 2023 02:14 pm | Updated July 13, 2023 09:09 am IST

Golden tunicate. Photo: Vikas Madhav Nagarajan

Golden tunicate. Photo: Vikas Madhav Nagarajan

(This article forms a part of the Science for All newsletter that takes the jargon out of science and puts the fun in! Subscribe now!)

They are a species of marine invertebrates with an evolutionary history from atleast 500 million years ago. Researchers are interested in them as they are the closest relatives of vertebrates, which includes fish, mammals, and people. Studying them is critical to understanding our evolutionary history. The limited studies on them reveal them to be weird creatures that come in an array of shapes and sizes. An adult tunicate typically looks like a barrel with two siphons projecting from its body. One of the siphons draws in water with food particles through suction, allowing the animal to feed using an internal basket-like filter device. After the animal feeds, the other siphon expels the water.

About 3,000 species of tunicate exist in the world’s oceans, living mostly in shallow water. There are two main tunicate lineages, ascidiaceans - often called “sea squirts and the most numerous -followed by appendicularias. Most ascidiaceans begin their lives looking like a tadpole and mobile, then metamorph into a barrel-shaped adult with two siphons. They live their adult life attached to the seafloor. In contrast, appendicularians retain the look of a tadpole as they grow to adults and swim freely in the upper waters. Tunicates seem extremely far removed from vertebrates but this tadpole-like link is what connects them. Researchers recently described a 500-million-year-old tunicate fossil species, which suggested that the modern tunicate body plan was already established soon after the Cambrian Explosion, the time when there was a dramatic rise in the number of animal species on earth.

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