Appendicularia (Larvaceans)

(Larvaceans)

Phylum: Chordata

Class: Appendicularia

Number of families: 3

Thumbnail description
Transparent, generally small organisms that produce a complex external mucous net (the house) to collect tiny planktonic food and that retain the larval tadpole stage as sexually mature adults

Evolution and systematics

The class Appendicularia is made up of one order, three families, and 64 species. Appendicularians are related to benthic tunicates and are commonly called larvaceans because they retain the larval tadpole stage as sexually mature adults. These animals are transparent and lack the outside covering, or tunic, of their benthic relatives. The body is composed of a trunk containing most of the internal organs and of a tail with a notochord running down the middle. The trunk secretes a mucous house, which may enclose the animal, as in Oikopleura and Bathochordaeus species, or enclose only the tail, as in Fritillaria species. The body and house appear jelly-like and are therefore grouped as gelatinous zooplankton. Larvaceans are small, ranging from a 0.04 in (1-mm) body length with a 0.2 in (4 mm) house to Oikopleura to 1 in (25 mm) body length with a house more than 6.6 ft (2 m) in diameter for the giant larvacean Bathochordaeus.

Physical characteristics

Body length ranges from 0.04 in (1 mm) for Oikopleura dioica to 1 in (25 mm) for Bathochordaeus charon.

Distribution

Cosmopolitan in ocean currents.

Habitat

Appendicularians are pelagic and live in oceanic and nearshore waters.

Behavior

Appendicularians move their tails rhythmically inside the house to produce a current that filters tiny food particles and to move the house through the water. If the filters become clogged or something bumps the house, the appendicularian abandons the house through a mucous trap door in the posterior of the house. The beginnings of a new house lie on the trunk of the body, and the animal inflates the new house and flips inside.

Some appendicularians have bioluminescent granules embedded in the house wall. It is thought that predators of the animal may eat an empty house that is flashing light while the original house builder swims away to make another house. Surface waters of some coastal bays and harbors can look brilliantly bioluminescent from large aggregations of appendicularians.

Feeding ecology and diet

All appendicularians are filter feeders with an amazing filtration apparatus. The mucous house has two prefilters embedded in the wall and an inner filter connected to the animal's mouth. The filters are made of strands of mucus that allow only the smallest food particles <0.0004 in (<1 µm) into the tube leading to the mouth.

Appendicularians can be important prey for many animals, including larval and adult fish. At least one fish important to humans, the anchovy, relies heavily on appendicularians for food.

Reproductive biology

Only sexual reproduction occurs in appendicularians. All except Oikopleura dioica are hermaphrodites. Gametes are shed directly into the surrounding water. Hermaphrodites release sperm, and the eggs burst out of the body wall, a process that results in the death of the animal. If phytoplankton numbers are high, production of a large number of eggs and a fast generation time result in rapid development of blooms of appendicularians, sometimes in a matter of a few days.

Conservation status

Although there is much to be learned about the ecology of appendicularians, none of these species is known to be in danger of extinction. No species is listed by the IUCN.

Significance to humans

Because of potentially high population numbers, fast generation times, and ability to clear waters of phytoplankton, appendicularians can have considerable effect on the food web of the pelagic environment and thus on the numbers of fish important to humans.

Species accounts

Resources

Bone, Q. Biology of Pelagic Tunicates. New York: Oxford University Press, 1997.

Esnal, G. B. "Appendicularia." In South Atlantic Zooplankton, Vol. 2, edited by D. Boltovskoy. Leiden, The Netherlands: Backhuys, 1999.

Wrobel, D., and C. Mills. Pacific Coast Pelagic Invertebrates. Monterey, CA: Sea Challengers and Monterey Bay Aquarium, 1999.

Yamaji, I. Illustrations of the Marine Plankton of Japan. Osaka: Hoikusha, 1976.

Hamner, W. M., and B. H. Robison. "In Situ Observations of Giant Appendicularians in Monterey Bay." Deep-Sea Research 39 (1992): 1299–1313.

Silver, M. W., S. L. Coale, C. H. Pilskaln, D. R. Steinberg. "Giant Aggregates: Importance as Microbial Centers and Agents of Material Flux in the Mesopelagic Zone." Limnology and Oceanography 43 (1998): 498–507.

"The JelliesZone." (15 July 2003). .

[Article by: Michael S. Schaadt, MS]

A class of marine planktonic animals in the subphylum Tunicata. This class is characterized by the persistence of a tail, notochord, gill slits, and dorsal nerve cord throughout life and by a unique feeding structure, the “house.” Appendicularians are free-swimming, solitary animals that are believed to have evolved through neoteny from the tadpole larvae of a bottom-dwelling, ascidianlike, ancestral tunicate. They resemble a bent tadpole with a flat, muscular tail and a trunk containing all major organs. Included are a complex digestive system, reproductive organs, two ciliated openings of the gill slits (the spiracles) leading to a mucus-lined pharynx, a mucus-producing gland called the endostyle, and a simple circulatory system with a single, muscular heart. See also Neoteny.

Appendicularians feed primarily on small particles from 0.1 to 30 micrometers in diameter. Larger individuals may filter up to several thousand milliliters of seawater per day. As one of the few metazoan groups capable of capturing bacteria and tiny phytoplankton, appendicularians are important marine grazers which may significantly reduce phytoplankton populations in the ocean. About 13 genera and 70 species of appendicularians are known. They are found in all oceans of the world and are a common component of plankton samples, particularly in coastal waters. The most abundant genera are Oikopleura and Fritillaria. See also Chordata; Tunicata.