Appendicularia

(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

List of Species

No common name

Fritillaria borealis

order

Copelata

family

Fritillaridae

taxonomy

Fritillaria borealis (Lohmann, 1896), Irminger Sea.

other common names

None known.

physical characteristics

Body length to 0.2 in (4.5 mm) (trunk, 0.06 in [1.5 mm]; tail, 0.1 in [3 mm]). Distinguished from other appendicularians by connection of the tail at the midpoint of the trunk. The tail is the only part that is entirely inside the very small house.

distribution

Cosmopolitan in epipelagic zone (less than 656 ft [200 m]). (Specific distribution map not available.)

habitat

One of the most common appendicularians worldwide.

behavior

The house is small and encloses only the tail and part of the trunk. When the tail stops moving, the house collapses on the body. Once the animal floats away, the tail starts moving again, inflating the house. Filter feeding continues in a new area.

feeding ecology and diet

Filters only the smallest particles out of the water (less than 0.0004 in [1 µm]).

reproductive biology

These hermaphrodites release sperm first then release eggs by rupturing their body wall, a process that results in the death of the animal.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Giant larvacean

Bathochordaeus charon

order

Copelata

family

Oikopleuridae

taxonomy

Bathochordaeus charon (Chun, 1900), Cape of Good Hope.

other common names

None known.

physical characteristics

Largest of all appendicularians, reaching a length of 1 in (25 mm). Short, broad tail with clearly visible notochord embedded. The gut and gonads are easily seen in the large conspicuous trunk. The house is a series of elaborate tubes and filters used for feeding. A roughly ovoid sheet of mucus that can reach 6.6 ft (2 m) in diameter is continuously secreted above the house for protection and for catching sinking food particles.

distribution

Cosmopolitan in mesopelagic zone (328–1,640 ft [100–500 m]) (Specific distribution map not available.)

habitat

Pelagic in deeper waters (328–1,640 ft [100–500 m]).

behavior

The tail beats slowly inside the house producing a current that moves water and food particles into the filters. The giant larvacean discards its mucous sheet and house when disturbed or when food particles clog the filters. Because abandoned mucous sheets and houses are laden with food, detritus-feeding zooplankton, and smaller microbes, they become important contributors of organic materials to the deep ocean as they sink to the botoom.

feeding ecology and diet

The large mucous sheet secreted above the house catches sinking particles, which are strained and directed to the inner filter of the house. The house filters further strain food particles of the proper size onto a mucous sheet, which is eaten.

reproductive biology

Little is known except that these animals are hermaphrodites.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Oikopleura dioica

order

Copelata

family

Oikopleuridae

taxonomy

Oikopleura dioica (Fol, 1872), Mediterranean Sea.

other common names

None known.

physical characteristics

Smallest of all appendicularians with a body length to 0.1 in (3 mm) (trunk, 0.04 in [1 mm]; tail, 0.08 in [2 mm]). House approximately 0.2 in (4 mm) long.

distribution

Cosmopolitan in tropical and temperate waters in epipelagic zone (less than 656 ft [200 m]). (Specific distribution map not available.)

habitat

Can be abundant in coastal surface waters and is one of the most common appendicularians worldwide.

behavior

When the filters become clogged, the house is abandoned, and a new house is inflated. Studies have shown that on average a new house can be made every 4 hours.

feeding ecology and diet

Filters only the smallest particles out of the water (less than 0.0004 in [1 µm]). Some fish, such as the blacksmith Chromis bipunctata in California kelp forests, are known for feeding almost exclusively on Oikopleura dioica.

reproductive biology

Separate male and female forms release their sperm and eggs into the surrounding water.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Oikopleura labradoriensis

order

Copelata

family

Oikopleuridae

taxonomy

Oikopleura labradoriensis (Lohmann, 1892), Labrador Current.

other common names

None known.

physical characteristics

Body length to 0.24 in (6 mm) (trunk, 0.08 in [2 mm]; tail, 0.16 [4 mm]). House is approximately 0.4 in (9 mm).

distribution

Cosmopolitan in temperate to cold waters, in epipelagic zone (less than 656 ft [200 m]).

habitat

Can be abundant in cooler offshore and coastal surface waters.

behavior

These animals have bioluminescent granules embedded in the walls of the house that may help confuse predators, who eat abandoned houses rather than animals that have abandoned the houses.

feeding ecology and diet

Filters only the smallest particles out of the water (less than 0.0004 in [1 µm]).

reproductive biology

These hermaphrodites release sperm first then release eggs by rupturing the body wall, a process that results in the death of the animal.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Resources

Books

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.

Periodicals

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.

Other

"The JelliesZone." (15 July 2003). <www.jellieszone.com>. <http://bonita.mbnms.nos.noaa.gov/sitechar/pelagic.html>

Michael S. Schaadt, MS