@Sea - Drifting Houses in the Gulf Stream

	FOCUS ON APPENDICULARIA Even now, at the dawn of the 21st Century, the world's oceans are full of mysteries. One of the more perplexing of these mysteries involves a planktonic drifter: how can a class of pelagic animals currently estimated to comprise more than one-tenth of all the marine biomass on our planet still remain largely unknown to most of us? The conundrum deepens when the fauna in question appear to be among the most vital links in the cycling of energy among various trophic levels of shallow and deep sea food webs. To top it off, these organisms are likely the single biggest piece of a puzzle that seeks to chart the probable evolution of the vertebrates - including humans - from invertebrate ancestors. HARBOR BRANCH Research scientist Marsh Youngbluth has a hypothesis as to why these animals - a pelagic class of tunicates called appendicularians - remain off the radar of public perception, even as they challenge foremost marine scientists to rethink long-held conceptions of how food webs function and how vertebrate life even came to be. "Scientists have not often translated their work in this field for a general audience, so we have as yet been unable to share important discoveries about these animals," suggests Youngbluth. "At the very least, we need to show people that the ocean is a home for animals other than turtles, fishes, and whales." True enough. But one suspects even the best PR outfit in town would have trouble selling appendicularians as the next wave of warm, fuzzy, feel-good marine life. Why? Two words: snot houses. Appendicularians (also called larvaceans) are unusually delicate and difficult to see or capture, and most are only a couple millimeters in size. They are a pelagic, tadpole-like class of zooplankton related to the more familiar sea squirts. And they have specialized glands on their head that secrete mucus which the animals inflate to serve as domiciles for themselves. More than a place to hang their hats, appendicularian houses are actually intricate filtering structures. After secreting a house and filling it with water to inflate it, an appendicularian will beat its muscular tail to create water currents that draw in suspended microbial particles such as phytoplankton, bacteria, and detritus. Appendicularians prefer microscopic food items in the nanoplankton range (one-billionth of a meter). In most species, seawater is initially pulled into feeding houses through course inlet filters that exclude relatively large particles. Particles of the preferred size class are then sieved from the water flow via a set of food concentrating filters with pore sizes in the submicron range. Afterwards, particles are consolidated and carried to the straw-like feeding tube and into the pharynx for ingestion. The food items trapped by appendicularian feeding houses are much smaller than those that can be collected by pelagic copepods and other zooplankton grazers which use fine hair-like structures called setae to filter food from the water. Moreover, Norwegian scientist, mission co-investigator, and appendicularian expert Dr. Per Flood has published research findings suggesting that the appendicularian feeding houses may be capable of concentrating food particles nearly a thousand-fold relative to ambient densities. The course inlet filters of the feeding houses can be rapidly clogged by larger particulate matter and macroplankton, greatly reducing filtering efficiency. Many appendicularian species are capable of altering the movement of their tails to temporarily reverse the direction of water flow, effectively backwashing the clogged inlet apparatus in much the same way one might backwash a swimming pool filter. But even this strategy only marginally extends the useful life of a feeding house; houses are worn out and abandoned at astonishing rates. Yet these industrious little animals remain undaunted. They inflate and take up residence in replacement houses, prefabricated by the animals and held in reserve, ready for service as soon as the previous abode is discarded. In colder waters, appendicularians may produce an average of six houses each day. But according to co-investigator Dr. Russ Hopcroft of the University of Alaska, Fairbanks, many of the small tropical representatives of the group generate upwards of a dozen houses a day. The result is that incalculable billions of discarded appendicularian feeding houses make a tremendous contribution to the "marine snow" - the endless blizzard of particulate organic matter raining down through the water column to supply food webs of the dark abyssal depths below. The contribution of appendicularians to productivity in ocean food webs reverberates through multiple trophic levels. Cast off houses and the particles that foul them are rapidly colonized by marine bacteria and grazing zooplankton. The amount of carbon associated with discarded feeding houses may at times surpass that of the phytoplankton resource. Even more remarkable is the accumulating body of evidence suggesting that, in addition to consuming submicron particulate matter, appendicularians seem capable of filtering and ingesting colloidal dissolved organic carbon (DOC). In fact, the research of Dr. Flood and his colleagues has indicated that DOC may be at least as important at particulate matter in appendicularian diets. The implications of these findings are profound, since it identifies a metazoan animal apparently capable of directly deriving nutrition from an organic carbon source previously thought available only to bacterial trophic levels. Thus, appendicularians may be able to bypass two levels of highly inefficient trophic energy transfer (bacterial and protistan), yielding a net metabolic transfer from DOC to metazoan consumer that is fully two orders of magnitude larger than possible via the traditional trophic pyramid. This bonus in metabolic yield directly benefits the larval and juvenile fish, copepods, euphausiid shrimp, chaetognaths, and varied gelatinous zooplankton that all prey on appendicularians and their feeding houses. The likely ability to exploit a pool of organic carbon unavailable to other members of the metazoan grazer feeding guild helps explain the phenomenal growth rates exhibited by the animals. Dr. Hopcroft estimates that appendicularian productivity roughly translates into a growth rate of nearly thirty times their body weight per day. That production value does not include the construction of the serially replaced feeding houses which may increase that total by an additional 50 percent.