#appendicularia

(Sources - 1, 2, 3, 4)

Appendicularia (commonly called “Larvaceans”) are a class of tunicates that retain their solitary, free-swimming, tadpole-like larval forms throughout their entire life, with their notochord running through their tail.

Appendicularians are transparent planktonic animals, ranging from 2 mm to up to 10 cm (3.9 in) in length. They are known for the large houses of mucus and cellulose (image 3 and gif below) they build around their bodies. These structures assist in filter-feeding, often comprising several layers of filters and reaching up to ten times the larvacean’s body length. The house consists of external filters which stop food particles too big for the larvacean to eat, and internal filters which direct edible particles to the larvacean’s mouth. In some species, the house surrounds the animal like a bubble, in others it is merely attached to some part of their body, varying in shape. As the animal grows or if the filters become clogged, it will discard its house and build a new one. Some genera can deflate and reinflate their houses in an attempt to clean the filters.

Larvaceans reproduce sexually, with all but one species being hermaphroditic. They reproduce externally, releasing sperm and eggs into the water. Larvae resemble the tadpole-like larvae of ascidians, and go through a short metamorphosis involving their tail twisting into place. Their life cycle is short: the metamorphosis takes place less than one day after fertilization and they reproduce after 5-7 days. They will die shortly after, as their body wall ruptures during egg release, killing the animal.

Due to their soft and delicate bodies, there is no definite fossil record for appendicularians. The Cambrian animal Oesia disjuncta has been suggested as an early form of appendicularian.

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Propaganda under the cut:

GIANT LARVACEANS ARE TRANSPORTING MICROPLASTIC INTO THE DEEP SEA

Larvaceans are abundant components of global zooplankton assemblages, regularly build mucus “houses” to filter particulate matter from the surrounding water, and later abandon these structures when clogged. According to US researchers at Monterey Bay Aquarium, giant larvaceans (Bathochordaeus stygius) are able to filter a range of microplastic particles from the water column, ingest, and then package microplastics into their fecal pellets. The study is recently published in Science Advances.

-  During feeding experiments, microplastics were observed inside and attached to the inner house and inside the gut of giant larvaceans. Microplastic are represented by different colors, these plastic particles are smaller than a millimeter.

Microplastics also readily affix to their houses, which have been shown to sink quickly to the seafloor and deliver pulses of carbon to benthic ecosystems. According to Kakani Katija who leads the study, giant larvaceans can contribute to the vertical flux of microplastics through the rapid sinking of fecal pellets and discarded houses. Larvaceans, and potentially other abundant pelagic filter feeders, may thus comprise a novel biological transport mechanism delivering microplastics from surface waters, through the water column, and to the seafloor. 

GIF: Monterey Bay Aquarium

Oikopleura vanhoeffeni by Alexander Semenov