Beautiful, ethereal larvaceans play a central role in ocean ecology.
Resembling jellyfish, larvaceans are finger-shaped creatures that spin fragile webs of mucus around themselves. Their gossamer "houses" are usually the size of a walnut or smaller, but can, in rare cases, reach two meters in width. Like fine nets, the webs act as filters to trap smaller members of the plankton.
Larvaceans are found in the surface waters of the world's oceans, where phytoplankton are plentiful. The class Larvacea is the most specialized in the subphylum Urochordata, or Tunicata, and contains some 70 species. Larvaceans are so called because the adults have retained the larval characteristics of other tunicates, a tadpole-shaped body and a tail. Three main families of larvaceans are known: the Oikopleuridae, which spin bubble-shaped houses; the Fritillaridae, with their butterfly-shaped houses; and the rare Kowalevskiidae, with houses that resemble umbrellas.
Larvaceans drift in the sea in numbers almost too great to imagine. Where abundant, they often reach a density of several thousand per cubic meter of seawater. Along the Atlantic Coast in the waters of Delaware Bay, Oikopleura dioica can occur in numbers of 3,000 to 5,000 per cubic meter. In a single 10-minute net sampling of the bay's waters, more than 270,000 of another larvacean species, Fritillaria borealis, have been caught. Scuba diving among larvaceans is like swimming in a snowstorm, says larvacean expert Alice Alldredge of the University of California at Santa Barbara. Poetically inclined divers have compared floating among these ethereal creatures to "being brushed by thousands of fairy wings."
With their intricate passages and miniature nets, larvacean webs are one of the most complex external structures built by any organism. The animal continuously beats its tail to draw water into its hollow house, which is interlaced with a variety of filters for excluding particles too large to ingest. Once water has been sieved for plankton, it flows up both edges of the arched "wings," out another set of openings, and back into the sea. Most larvaceans discard their houses and secrete new ones every four hours or so, the amount of time it takes for the incoming-flow filter to become clogged with debris. Some species, however, manufacture new houses every few minutes.
Larvacean feeding nets have fascinated marine scientists for more than a century, and led to the discovery of the nanoplankton--extremely small drifting plants of the seas. These lilliputian plants were discovered in the 19th century by German biologist Hans Lohmann when he observed them in the filters of larvacean houses. "Man has not yet succeeded in devising suitable means for the capture of the tiny members of the plankton," wrote the British biologist Sir Alister Hardy in his 1965 classic The Open Sea: Its Natural History. "That larvaceans should have solved the problem so efficiently makes one marvel all the more at these unusual animals." Larvacean filters coccolithophores and noflagellates, small organisms that make up the nanoplankton; few animals but larvaceans are able to capture and feed on these dwarfs.
Larvaceans play a central role in ocean ecology. They are a vital link between the tiny plankton they fee and the larger animals oceanic food chain, who in turn feed on them. Major predators of larvaceans and their houses include the young of fish like herrings, sardines, and flounders. Young plaice, or flatfish, may consume 25 or 30 larvaceans in a day. Yellow-and-black-striped coral reef fish called sergeants major, and siphonophores, relatives of the Portuguese man-of-war jellyfish, also subsist on these ubiquitous creatures.
Larvaceans are responsible for yet one more important connection in the marine food web. When they die or spin new dwellings, their cast-off houses collapse and sink to the deep sea at rates as fast as 1,000 meters per day, contributing to marine snow (the flow of detritus from surface waters to the depths). Biologists Mary Silver of the University of California at Santa Cruz and Alice Alldredge have found that marine snow--in the form of abandoned larvacean houses--may be a primary source of food for animals living in the ocean's deepest regions. (Silver and Alldredge received the Woods Hole Oceanographic Institution's Bigelow Medal in 1992 for their work on marine snow.)
The darkness of the abyss is lit by these discarded larvacean houses. When they encounter another object, Oikopleura houses may glow for up to four hours--even after they've been abandoned by their occupants. Scientists once believed that this bioluminescence came from microscopic organisms such as dinoflagellates living on the houses' surfaces. But experiments have shown that Oikopleura houses with no light-emitting microorganisms on their surfaces can still produce light: The animals leave light-producing structures and chemicals behind in their discarded webs.
Not all larvacean houses found in the depths are empty; at least one rare larvacean dwells in the deep sea. More than 10 times larger than its shallow-water Oikopleurid relatives, Bathochordaeus charon spins a web that may reach the size of a pumpkin. This giant larvacean was discovered in the southeastern Atlantic Ocean by oceanographers using plankton nets during the 1898 German deep-sea Valdivia expedition. Between 1898 and 1991, only 17 specimens of Bathochordaeus were reported in the scientific literature, these from the North and South Atlantic, Indian, southwestern Pacific, and eastern Pacific oceans.
This past year with the help of Ventana, a remotely operated vehicle owned by California's Monterey Bay Aquarium Research Institute, marine scientists observed several of these larvaceans as deep as 700 meters in Monterey Bay. Their research shows that, rather than forming a house that completely encircles its body like other larvaceans, Bathochordaeus spins a fan-shaped web that floats above the animal like a huge balloon. Few of the "fiery pelagiae" glimpsed by the crew of the submarine Nautilus in Jules Verne's 20,000 Leagues Under the Sea could be more eerie than giant larvaceans as they slowly whirl through the ocean's nether regions.
Cheryl Lyn Dybas is a science writer who was "brushed by the fairy wings" of Oikopleura while scuba diving in Monterey Bay last summer. Her articles on the "underappreciated" creatures of the deeps have also appeared in Wildlife Conservation, National Wildlife, International Wildlife, and NSF's Directions magazines.
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|Author:||Dybas, Cheryl Lyn|
|Date:||Jun 22, 1993|
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