What do red lettuce bryozoans; flowerbed tunicates, yellow finger sponges and intestine sea squirts have in common? They all like to hang out at the oyster reef! Oyster reefs provide habitat for a myriad of sea creatures – a treasure trove of feathery, crusty, and squishy invertebrates. We’ll focus on just one group of these spineless friends today: the sea squirts.
Picture this: you’re walking along your favorite oyster reef when you notice something white and otherworldly on the rocks, moving softly in the current. As the tide lowers, the creature is exposed to the air and slumps into a gelatinous blob, all grace leaving with the water. You give in to the temptation to poke it gently, and it responds with a spray of water in your general direction! What is this thing? Here in the San Francisco Bay, it’s likely that you’ve stumbled across either the intestine sea squirt (Ciona intestinalis) or Savignyi’s sea squirt (Ciona savignyi).
The sea squirt lives a solitary life attached to a hard substrate such as a dock, a boat hull, or an oyster reef. It can grow as large as 6 inches in length, and its main features are two tubes–one to take in water, the other to expel it. Its body is soft, white-ish and translucent, and you can catch a glimpse of its insides: a pharyngeal basket, a stomach, perhaps a heart. The pharyngeal basket plays an important role by filtering algae and other food particles from the water, which is pumped in one tube and out the other, leaving its payload behind. When poked or prodded by the unwary observer, it will express its disapproval by forcibly squirting water through its tubing system.
Sea squirt larvae resemble small tadpoles and must swim away and find a place of their own to settle. They have only hours to complete their mission, and they are so single-purposed that they do not even have mouths to eat with. Once they find a suitable spot, they must commit and attach themselves to it head-first. They then begin the mysterious process of resorbing their now-useless tail and converting it to something more functional–say, a pharyngeal basket, so that they may eat.
It is in this larval stage that sea squirts reveal their unexpected secret–they have a nerve cord that runs down their backs. As simple as this feature sounds, it means that sea squirts are our (relatively) close relatives. Indeed, they are more closely related to vertebrates than to most other invertebrates. Sea squirts lose this nerve cord along with their tail when they transform into adults, and so it is only the larvae that show us the link.
Final note: The intestine sea squirt is so widely distributed throughout the world’s oceans that its original range is not clear; however, we do know that it is not native to San Francisco Bay. Like many invasive invertebrates, the sea squirt probably hitched a ride on a boat, either growing on the hull or taken on with ballast water. Sadly, as many as 70% of the invertebrate species in the bay are invasive. We can only hope that by doing our part to restore native oyster habitat, we can help the native invertebrates re-claim the bay.
So, if you ever find yourself poking a white-ish blobby thing on the oyster reef and it squirts water in your eye, remember: you share a common ancestor.
Interested in seeing sea squirts and other cool species that have made our Point Pinole Oyster Reef their home? Volunteer to monitor oysters and their roommates by contacting Helen Dickson.