Symbiosis is defined as two organisms living together. In the sea, the best known symbioses are those in which both organisms benefit, and these are called mutualisms. The brittlestar that lives inside the gray tube sponge was thought to be a mutualism, with the brittlestar gaining a safe home, and the sponge getting cleaned. Strangely, the brittlestar was only found on the one tube sponge species, despite others being available, and this prompted experiments to better understand their relationship. Gray tube sponges that had brittlestars grew at the same rate as those without brittlestars, suggesting no benefit of cleaning. But, gray tube sponges released their babies from the inside walls of their tubes, unlike other tube sponge species, and it was discovered that brittlestars eat these babies. So, experiments revealed that brittlestars are mostly parasites of the gray tube sponge.