Thaliacea (Salps)

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Thaliacea

(Salps)

Phylum Chordata

Class Thaliacea

Number of families 5

Thumbnail description
Marine, holoplanktonic organisms with cylindrical bodies that have openings on both ends

Evolution and systematics

The class Thaliacea is made up of three orders, five families, and 73 species. The three orders are Pyrosomatida, with one family and 10 species; Doliolida, with three families and 23 species; and Salpida, with one family and 40 species. Recent work indicates the thaliaceans are an artificial group because each of the orders evidently arose from a different group of benthic tunicates (class Ascidiacea). Evidence for this polyphyly is in comparative embryology and anatomy, where each of the orders show their ascidiacean ancestry to greater or lesser degrees. Their similar body forms are an example of convergent evolution in which animals look alike despite their differing phylogeny. Molecular work being done on thaliaceans should help determine relationships more precisely; consequently, this group may be dismantled and reorganized into separate groups in the future. Salps are related to the benthic tunicate class Ascidiacea.

Physical characteristics

Salps have a cylindrical body, called the test, that is mostly clear with openings called siphons on both ends. One opening is the oral and the other the cloacal siphon. Their body walls appear jelly-like and are therefore grouped as gelatinous zooplankton. Varying numbers of muscle bands are embedded within the gelatinous wall of the test. They range in size from 0.2–7.8 in (5 mm–20 cm), with some colonial forms many feet (meters) in length

Distribution

Thaliaceans are found in tropical and temperate seas. They tend to be found in the epipelagic zone from the surface to 656 ft (200 m), but some have been found in the mesopelagic zone from 656 to 3,280 ft (200 to 1,000 m).

Habitat

Thaliaceans tend to be more abundant in phytoplankton-rich surface waters of tropical and temperate seas. They are free swimming and holoplanktonic (at the mercy of ocean currents during their whole lives). Thaliaceans are often found with hyperiid amphipods living on the outside or even on the inside of their bodies in a symbiotic relationship.

Behavior

They move through the water by either ciliary action, as in the pyrosomatids, or by contraction of muscles embedded in the tunic, as in the doliolids and salps moving water into the oral siphon and out the cloacal siphon.

At times, because of their fast generation time, large numbers of thaliaceans can make up the largest proportion of gelatinous zooplankton.

Feeding ecology and diet

Thaliaceans are all filter feeders upon phytoplankton. Water flows into the oral siphon through a mucous sheet covering a basket and out the cloacal siphon. Phytoplankton and other small organisms are caught on the mucus. The mucus is then moved to the mouth and eaten. Because of their large numbers at certain times of the year, thaliaceans are important consumers of phytoplankton and make a large impact on the flux of carbon because of their production of massive amounts of fecal material.

Reproductive biology

Alternation of generations enables thaliaceans to colonize seas quickly, taking advantage of phytoplankton blooms resulting in large swarms. One of the generations is asexual with vegetative growth resulting in the sexual stage. The other generation is sexual, which produces the egg and sperm that fuse to make the asexual stage.

Thaliaceans show different strategies to ensure successful reproduction. Pyrosomatids develop eggs that, when fertilized, develop to hatching inside the zooid. Doliolids retain the tadpole larval stage of their benthic tunicate ancestors during development. Salpids retain both asexual and sexual stages and true embryonic connection (via a placenta) to the parent during development.

Conservation status

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

Significance to humans

At times, some thaliaceans can become so abundant that they can outcompete other herbivores at the base of the oceanic food web, such as copepods. Copepods are important in the diet of young fish species that humans rely on for food. In this way, the thaliaceans have a negative impact on human fishing activities.

Species accounts

List of Species

Doliolid
Pyrosome
Cyclosalpa affinis
Salpa fusiformis
Thalia democratica
Thetys vagina

Doliolid

Dolioletta gegenbauri

order

Doliolida

family

Doliolidae

taxonomy

Dolioletta gegenbauri (Uljanin, 1884).

other common names

None known.

physical characteristics

Barrel-shaped, clear, gelatinous body about 0.35 in (9 mm) in length with eight prominent muscle bands in the asexual stage and the 0.47-in (12-mm) sexual stage has nine muscle bands.

distribution

Semi-cosmopolitan in cool waters of both hemispheres. (Specific distribution map not available.)

habitat

Little is known about vertical distribution, but commonly found in oceanic surface waters.

behavior

Known for their jumpy swimming pattern when disturbed. Swims by contracting muscle bands present in the gelatinous body.

feeding ecology and diet

Can feed on a wide variety of food particle sizes ranging from bacteria 0.0002 in (5 µm) to large diatoms >0.00394 in (>100 µm). Can be very important herbivores, completely clearing the waters they inhabit in about two days.

reproductive biology

The complicated lifecycle includes an asexual stage (sometimes called a nurse) during which a tail will grow. Many feeding stages bud on the tail, when released they grow a tail on which the sexual stage grows. The hermaphroditic sexual stage releases fertilized eggs that develop into tadpole larvae lacking the characteristic ascidiacean tail; the larvae quickly grows into the asexual nurse stage. Although complicated, the lifecycle helps doliolids respond to blooms of phytoplankton, resulting in blooms of their own, reported to be hundreds of square miles (kilometers) in the open ocean.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Pyrosome

Pyrosoma atlanticum

order

Pyrosomatida

family

Pyrosomatidae

taxonomy

Pyrosoma atlanticum (Peron, 1804), Mediterranean Sea.

other common names

None known.

physical characteristics

Individuals called zooids are about 0.33 in (8.5 mm) and embedded in a thick, clear tubular test that can reach 23.6 in (60 cm) in length. Colonies are pink or yellowish pink. The oral siphons of the zooids are on the outer surface of the tube and the cloacal siphon point to the inside. Water comes in the oral siphon and empties into the common opening inside of the tube. One end is closed and the water exits out a common opening at the other end of the colony, propelling it through the water. Each of the zooids is brilliantly bioluminescent and this is the reason for the scientific name of pyrosoma, "fire body."

distribution

Semi-cosmopolitan in temperate to tropical waters, it is the most common pyrosome. (Specific distribution map not available.)

habitat

Commonly seen in surface waters, but daily vertically migrates more than 2,460 ft (750 m).

behavior

Known for the brilliant luminescence of each zooid, communication among zooids results in waves of light along the entire colony.

feeding ecology and diet

Water passes into each zooid through the oral siphon, into a mucous filter where phytoplankton is filtered out for food. Pyrosomes can form huge swarms that can result in significant quantities of fecal pellets. The sinking pellets can be very important in carbon input into the depths.

reproductive biology

Zooids are hermaphrodites, possessing both testis and ovary. In each zooid of the colony a single egg is fertilized, which grows to a four-zooid stage that leaves the parent to start a new colony asexually by budding. This form of reproduction can result in huge swarms of pyrosome colonies that are dependent upon abundant phytoplankton for food.

conservation status

Not listed by the IUCN.

significance to humans

The brilliant light displays given off from colonies of pyrosomes have bewildered and fascinated sailors for generations.

Salp

Cyclosalpa affinis

order

Salpida

family

Salpidae

taxonomy

Cyclosalpa affinis (Chamisso, 1819).

other common names

None known.

physical characteristics

Both stages possess a clear, barrel-shaped, gelatinous body with circular muscle bands embedded in the body wall. Solitary (asexual) stage 2.9 in (74 mm) in length with seven muscle bands and a thick test. Aggregate form (sexual) is irregularly shaped, about 1.7 in (4.5 cm) long with four muscle bands. The aggregates are connected to form a chain of linked circles.

distribution

Temperate and tropical oceanic waters. (Specific distribution map not available.)

habitat

Most commonly found in surface of offshore waters.

behavior

The only salp to form chains of linked circles.

feeding ecology and diet

Known for the ability to filter small food particles like phytoplankton with 100% efficiency.

reproductive biology

The fertilized egg grows into the asexual stage inside the sexual stage, connected to the parent by a placenta used for norishment. Once released the sexual stage grows a tail where the sexual stage buds in chains of linked circles. Each sexual stage contains testis and ovary, which produce sperm and egg to produce the next generation of the asexual stage. They lack the tadpole larval stage.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Salp

Salpa fusiformis

order

Salpida

family

Salpidae

taxonomy

Salpa fusiformis (Cuvier, 1804).

other common names

None known.

physical characteristics

Both stages possess a clear, barrel-shaped, gelatinous body with circular muscle bands embedded in the body wall. Solitary (asexual) stage 0.39–1.9 in (1–5 cm) in length with nine muscle bands. Aggregate form (sexual) 0.39–1.5 in (1–4 cm) long with six muscle bands and can be found solitary after breaking off long chain. The body shape of the solitary (asexual) stage is symmetrical; it is asymmetrical for the aggregate (sexual) stage, with short outerior and posterior projections. Gut is the only prominently colored part of the body.

distribution

Semi-cosmopolitan in temperate to tropical waters, mostly oceanic with occasional near-shore swarms. (Specific distribution map not available.)

habitat

Most commonly found in surface waters at night. Can make daily vertical migrations of up to 1,640 ft (500 m).

behavior

Long chains of aggregrate forms can move through the water surprisingly quickly, up to 1.85 in (4.7 cm) per second.

feeding ecology and diet

Important filter feeders on phytoplankton. Known for fast growth rates.

reproductive biology

Asexual stage produces a tail onto which bud two rows of the sexual stage that stay connected together, forming long chains several feet (meters) in length. The sexual stage can break away from the chain and swim along independently. The sexual stage bears fertilized eggs that are connected to a kind of placenta.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Salp

Thalia democratica

order

Salpida

family

Salpidae

taxonomy

Thalia democratica (Forskal, 1775).

other common names

None known.

physical characteristics

The asexual stage is about 0.47 in (12 mm) long with a pair of lateral posterior projections. The sexual stage is about 0.23 in (6 mm). The test is thick and has five prominent muscle bands.

distribution

Semi-cosmopolitan in temperate to tropical waters. (Specific distribution map not available.)

habitat

Usually found in surface waters.

behavior

Swim actively through the water by contraction of the muscles in the test.

feeding ecology and diet

Important filter feeders on phytoplankton. Recorded growth rates among the fastest of any multicellular organism (up to 70% length increase per hour).

reproductive biology

Asexual stage produces a tail onto which bud rows of the sexual stage that stay connected together, forming long chains several feet (meters) in length. They lack the tadpole larval stage, the egg develops directly into the asexual stage. Generation times can be as fast as a few days, resulting in huge swarms covering hundreds of square miles (kilometers) of open ocean.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Salp

Thetys vagina

order

Salpida

family

Salpidae

taxonomy

Thetys vagina (Tilesius, 1802).

other common names

None known.

physical characteristics

Solitary stage has a very thick test, 20 weakly developed muscle bands, two prominent posterior green lateral projections, and can reach up to 13 in (33 cm) in length, making it the largest of all salps. Aggregate stage up to 10 in (25 cm) and five muscle bands. Prominent gut mass is often red-pigmented in both stages.

distribution

Semi-cosmopolitan in temperate to tropical waters. (Specific distribution map not available.)

habitat

Commonly found in oceanic surface waters down to 490 ft (150 m).

behavior

Can be very prominent member of oceanic and near-shore surface waters. The large aggregate stage stays connected, forming very long chains many feet in length.

feeding ecology and diet

Feed on phytoplankton.

reproductive biology

The fertilized egg is connected to the inside of the sexual stage by a placenta; it swims out only when it has developed. It grows into the asexual stage, which grows a tail where the sexual stages bud in two rows. Each sexual stage contains testis and ovary, which produce sperm and egg to produce the next generation of asexual stage.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Resources

Books

Berrill, N. J. The Tunicata: With an Account of the British Species. London: The Ray Society, 1950.

Brusca, R. C., and G. L. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, Inc., 1990.

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

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

Esnal, G. B., and M. C. Daponte. "Doliolida." In South Atlantic Zooplankton, volume 2, edited by D. Boltovskoy. Leiden, The Netherlands: Backhuys, 1999.

——. "Salpida." In South Atlantic Zooplankton, volume 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 Publishing Co., 1976.

Periodicals

Madin, L. P., and G. R. Harbison. "The Associations of Amphipoda Hyperiidea with Gelatinous Zooplankton. I. Associations with Salpidae." Deep-Sea Research 24 (1977): 449–463.