Often overlooked by divers coming in Lembeh in search of emblematic critters of the area, like mimic octopus, rhinoplasty or Lembeh sea dragons, ascidians are beautiful creatures. The kind of things we can see in each dive site around without realizing how pretty they are. Particularly here in Lembeh Strait, we can observe a huge diversity of these animals.
Ascidians, also commonly called sea squirt or sea pork, are largely unrecognized marine animals from the tunicates family. It is a large family of invertebrates considered as being at the origin of all the vertebrates as they feature a central nerve cord equivalent to the vertebrates´ dorsal nerve cord.
While some tunicates form a long chain of individuals swimming in the ocean, the ascidians are sessile (immobile) animals attached to the sea bottom. The smaller species are usually colonial, while the larger are usually solitary.
Although they seem very simple, they are complex animals with digestive, reproductive, nervous, circulatory and, possibly, endocrine systems.
Their shape, size, and color vary considerably between species. They look like tubes featuring two siphon openings, one oral siphon (the mouth), usually the big one at the top and one atrial siphon (the anus), generally the lower one. The seawater is sucked through the oral siphon by cilia beating, providing planktonic nutrients and oxygen absorbed by the gills. The seawater is later expelled by the atrial siphon. When a diver approaches an ascidian, it will usually close rapidly those openings by the action of muscles.
Ascidians are hermaphrodites; they will release sperm or eggs that will meet randomly in the water, generating a larva provided with a long muscular tail. After some time, the larva will attach to the sea bottom transforming in an adult ascidian.
Ascidians are natural prey for nudibranchs, flatworms, crustaceans or fish. They also have chemical defenses harmful for some of their predators. The nudibranchs, for instance, will concentrate them in order to become toxic. As they are filtering big quantities of seawater, some species of ascidians are a good indicator of marine pollution.
Chemicals found in some ascidians are used in medicines against cancer or as antivirals. Ascidians are consumed as food in some parts of the world and there are some studies to use them as biofuel.
Present in all our dives sites in Lembeh, where we can see all these incredible creatures, ascidians remind us to retain the capacity to be amazed by simple things that can be very sophisticated. Take the time to stop and to watch attentively their delicate structure. In Two Fish Divers Lembeh, we don´t forget that ascidians are beautiful!
Dive manager, Two Fish Divers Lembeh