Although it is an important member of the planktonic community, it is certainly one of the most overlooked by divers.  The reason is that frankly, there isn't much to look at.  It’s easy to bypass a tiny, wiggling, tadpole-shaped animal enveloped in a bubble of schmutz.  Enter the larvacean, a pelagic tunicate, also known as an appendicularian.

The larvacean body is a whopping 2  mm long, comprising no more than a trunk and a tail. The trunk houses both male and female sex organs together, glands for producing its mucous house, a mouth, and other standard-issue organs. The muscular tail has a flexible rod for support and a nerve cord.

Larvaceans secrete a spectacular feeding house, which comprises a complex mucous net composed of protein and cellulose.  Undulations of the tail draw water onto a coarse-mesh filter, which excludes large particles.  A second, bi-lobed, finer-mesh filter sieves and concentrates food particles smaller than 1/1000th of a millimeter (sub-micron).  A thin tube delivers them to the pharynx. 

Yet another version of the ocean’s many grazers, their job is to filter and consume phytoplankton, protists, bacteria and detritus -- perhaps including colloidal dissolved organic carbon.  Larvaceans periodically discard their feeding houses, abandoning anywhere from 3 to 12+ houses per day. Opinions differ as to the reason for this. It was commonly thought that houses are vacated only after clogging or wearing out, but studies show that the tiny animals are always in production mode. They have a spare house secreted and ready to go.  All they have to do is inflate it.

Microscopic images by Wim van Egmond

Recycled Houses

The continuous cycle of net building has a secondary impact on the marine ecosystem.  Larvacean feeding houses trap bacteria, cyanobacteria, ciliates, and flagellates. Particulates remain embedded in discarded structures, which provide a habitat or food source for other organisms. Billions and billions of cast-off houses contribute to marine snow and provide vertical transport of organic matter to the abyssal depths, where it can be utilized by other organisms.

65 species currently described

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