How the Sea Squirt's Heart Is Like a Human's
Each of the pedal-like structures is an individual sea squirt, which has grouped together with others to form a system. Together these systems form colonies. The animals above are star sea squirts.
Credit: Jungho Ohn & Annette Hellbach

As plantlike as it may appear, the sea squirt, an invertebrate marine animal, is not heartless. In fact, scientists recently found that so-called star sea squirts have a pacemaker mechanism similar to that found in the human heart.

The discovery means this species' and other sea squirts' simple hearts might help scientists better understand the evolution of this organ and how it functions in other animals, including humans, said Annette Hellbach, study researcher and graduate student at the Max Planck Institute of Biochemistry in Germany.

Sea squirts are our unlikely relatives. In their larval stage, they resemble tadpoles, with a primitive version of a backbone, called a notochord, a feature that places them closer to humans in the family tree of life than, say a cockroach, a jellyfish or a sponge. [Our Bizarre Relatives: A Sea Squirts Family Album]

Tadpole sea squirts, also known as ascidians, attach to surfaces, and metamorphosize into adults. Some go on to form colonies. The adult sea squirt Hellbach worked on, Botryllus schlosseri, is scarcely recognizable as an animal. Individuals measure between 0.2 and 0.4 inches (0.5 and 1 centimeters) long, and these form flower- or starlike systems, which build up to form colonies. Each animal has nerve cells, intestines and a tubelike heart.

A human heart is made of beating muscle, kept in rhythm by pacemaker muscle cells that generate an electrical signal that tells the other muscle cells to contract.

When the electrical charge within a pacemaker cell becomes significantly more negative than its surroundings, channels open to allow positively charged ions into the cell. These channels are called HCN for hyperpolarization-activated, cyclic nucleotide-gated. The influx of positive ions generates an electrical signal that spreads to other muscle cells telling them to contract, and the heart beats. [10 Amazing Facts About the Heart]

A sea squirt heart has a simpler shape than our own. It contracts at one end, and the contraction spreads along the tube to the other end, then, after two to three minutes, the beating reverses direction, Hellbach said.

Hellbach and her colleagues began with a gene from a larger sea squirt that does not live in colonies, Ciona intestinalis. This sea squirt is known to contain a gene associated with these channels in human hearts. They then looked for the same gene in B. schlosseri. After finding it, they checked for expression of the HCN channel gene in the heart of B. schlosseri and indeed found the gene was expressed or "active."Next, they highlighted the channels using a dye, and found that they were located all along the tube, not at either end as they had expected.

And finally, the researchers exposed the sea squirts to the drugs cilobradine and zatebradine while recording their heartbeats. In humans, these drugs act directly on HCN channels and decrease heartbeat. They had the same effect on the sea squirts.

The results indicate that pacemaker cells containing HCN channels likely control a sea squirt heartbeat; therefore, these animals may be used to better understand how hearts keep pace, the researchers write online in July in the Journal of Experimental Zoology.

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