A is for Ascidian

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Today marks the first day, or rather the first letter, of an A-Z Photographic Glossary of Biological Terms challenge! Each day for 26 days straight I’ll be posting an image that illustrates a biological term that may be unfamiliar to some but which hopefully will turn you on to what turns me on!

Today it is A for Ascidian, as illustrated by a colony of Bluebell Tunicates (Clavelina puerto-secensis), from Sanctuary Caye, Belize; depth 30′.

Ascidians are a class of marine invertebrate filter feeders, and in fact there are about 2,300 species of ascidians grouped into three main types: solitary ascidians, social ascidians that form clumped communities by attaching at their bases, and compound ascidians that consist of many small individuals (each individual is called a zooid) forming colonies up to several meters in diameter. Like all other ascidians, these Bluebell Tunicates have very specialized feeding systems employing a mucus film to capture prey that has been sucked in with the water; cillia then move the plankton or detritus that has been stirred up into the esophagus.

This genus, Clavelina, reproduces asexually through budding and then broods its larvae which are tadpole-like and mobile so they can disperse throughout the water column; at this stage they contain a notochord and a nerve cord but when the larvae settle, they lose these structures and become sessile (immobile and fixed).

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