Often passing unnoticed while snorkeling and diving, ascidians (sea squirts) should deserve more attention, as they are relatively evolved organisms belonging to the animal kingdom and to the subphylum of tunicates (animals having an external tunica). They include many species that are characterized by different shapes and colors.
The ascidians live anchored to a substrate (typically, a coral) and are constituted by a body having an oral siphon through which, thanks to the sea currents, sea water enters together with nutrients, before being filtered by a complex branchial system that retains micro-organisms and other nutrients. The sea water is then expelled through the exhaling siphon. The captured nutrients are digested by the stomach and used for energy balance, growth and reproduction of the animal. Sea squirts also possess a rudimental heart, although they lack a true circulatory system.
Pictures of sea squirts. Raja Ampat is a paradise for sea squirts (ascidians) and you can often see vertical walls of rock completely covered by these animals, especially among the most protected channels that still ensure constant ocean currents, but without the powerful waves of the Pacific Ocean. One of the most common species is the sea squirt belonging to the genus of Polycarpa (more precisely Polycarpa aurata).
|Picture of sea squirt showing the top syphon.|
|Pictures of ascidians. The Ascidiacea are defined "tunicates", since the body has a coating called tunica. In addition to the sea squirt, tunicates include other genres such as Atriolum and Didemnum, whose photos will be shown later.|
|Picture of ascidian taken during a snorkeling trip to Raja Ampat (West Papua, Indonesia). These Ascidiacea are characterized by a more complex shape, as they are actually compound ascidians, formed by the association of multiple individuals. In the photos, ascidians belonging to genus Lissoclinum.|
|Photos of sea squirts living on a vertical wall in Raja Ampat islands, together with soft corals and other invertebrates.|
|More photos of ascidians. Ascidians are colorful guys, living together with many other invertebrates and forming a kind of "underwater garden".
|Ascidians picture from Raja Ampat. They are often located close to the surface of the water, so they can be easily seen also during simple snorkeling trips without have to dive.|
|More sea squirts photos.|
|Sea squirts pictures showing walls fully covered by these creatures, in addition to sponges and other invertebrates. The ascidians prefer to live in protected channels where there is still a good water flow, but without the violent waves of the open sea.
|Photos of tunicates. The tunicates also include the genus Atriolum (in the pictures, Atriolum robustum), composed of small colonies of multiple individuals.|
|Close to Atriolum, is also the genus Didemnum (in the photos, Didemnum molle), another tunicate that is distinguished by the larger size of the body.|
|An ascidian composed of more individuals living in a single body.|