A very, very, very fine house.

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Posted by Merry on February 19, 2013 at 20:04:27:

This is certainly one of the most overlooked animals in the ocean, and the reason is that frankly, they arenít much to look at. Itís pretty easy to ignore vague, almost microscopic spheres of organized debris with wing-like structures in the center. However, thereís a tiny animal living inside this bubble of schmutz. Enter the larvacean, a tadpole-shaped tunicate also known as an appendicularian.

Yet another version of the oceanís many grazers, its job is to filter and consume phytoplankton, protists, bacteria, detritus, and perhaps even colloidal dissolved organic carbon.

How small can you get? Less than two millimeters long, a larvacean is 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.

A naked larvacean.
 photo Nakedlarvacean_zps19b5f3fe.jpg
Photo taken from http://palaeos.com/vertebrates/chordata/chordata.html#Oikopleuridae

From glands on its head, larvaceans secrete a feeding house of protein and cellulose, which contains a complex mucous net. As the tail undulates, water is drawn first onto mesh prefilters, which exclude large particles. Beyond the incurrent filters, an even finer mucus network sieves and concentrates food particles smaller than a micron, smaller than 1/1000th of a millimeter! Besides being a wonder of nature, the net is possibly the only attractive thing about the animal.

This drawing shows the direction of the water currents. The mouth of the animal is connected to the mucous net by a thin feeding tube.

 photo Wimsdrawingoflarvacean_zps12deeaac.jpg

Phil and I found ourselves in a storm of these last Sunday. There had to be millions. How wide was the swath? From where did they originate? Are they concentrated into dense swarms by currents and/or shear layers?

Note the blue larvacean trunk. Within the external mucous sheet are 2 sets of feeding filters.

 photo Larvacean690DSC_7436_zpsf57187a6.jpg

 photo LarvaceanmucusnetDSC_7479_zps00997d28.jpg

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 with water.

 photo Larvaceancomposite800bDSC_7478_zpsa6f4c220.jpg

Larvaceans have a tremendous impact on the marine environment. Their 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.

It's a sticky little house!
 photo Larvaceanlgnet690DSC_2646_zps1696596b.jpg

The shape of the feeding net appears to differ between species.

This is probably an Oikopleura that sports a festive yellow and blue trunk, and pale blue tail.

 photo OikopleuraDSC_7248_zps1c35cfd7.jpg

Fritillaria sp., perhaps.
 photo Larvacean552DSC_3068_zps9ffdf457.jpg

 photo Larvacean690closeDSC_2975_zps5f050d70.jpg

An ellipsoidal or ovoid house belonging to a different species.
 photo LarvaceanKowalevskiaDSC_5070_zps86b3caf0.jpg

Although larvaceans arenít the most glamorous photo subjects, scientific interest in them is keen because of their important role in the ocean carbon cycle.

This link will take you to larvaceans in action.

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