Sorberacea

(Sorberaceans)

Phylum Chordata

Class Sorberacea

Number of families 1

Thumbnail description
Small group of exclusively deep-water Tunicata comprising solitary benthic species with a strongly reduced branchial sac

Evolution and systematics

The class Sorberacea was erected in 1975 by the French ascidiologists Claude and Françoise Monniot and Françoise Gail for several highly adapted deepwater species belonging to four genera (Oligotrema, Hexacrobylus, Gasterascidia, and Sorbera) united in the family Hexacrobylidae. The characters used to distinguish the Sorberacea from class Ascidiacea were the absence of the branchial sac, the presence of dorsal nervous cord, a very superficial position of the nervous ganglion, the histology of the digestive tract, and some other features.

The taxonomic position of Hexacrobylidae was not certain, but the family was most often considered as related to the ascidian family Molgulidae, or placed into a separate order, Aspiraculata, within the Ascidiacea. However, other taxonomists, including the Australian ascidiologist Patricia Kott, doubted whether the Hexacrobylidae should be removed from the class Ascidiacea. Kott considered that the characters used to distinguish Sorberacea and Ascidiacea could not be confirmed as true differences between Hexacrobylidae and the rest of the Ascidiacea. In particular, although the branchial sac is strongly reduced in Hexacrobylidae, it is not absent, and the neural complex in Hexacrobylidae is in the same position beneath the epidermis as in other ascidians. In addition, the structure and histology of the digestive tract varies in other ascidiaceans.

There are also problems in defining genera and species in the group. Of the four nominal genera mentioned above, Gasterascidia, Sorbera, and Hexacrobylus are now treated as synonyms of Oligotrema, and all species assigned previously to Hexacrobylus (apart from the type species of this genus) now belong to Asajirus. Thus the family Hexacrobylidae contains only two valid genera: Oligotrema and Asajirus. Oligotrema contains five species, and Asajirus (formerly known as Hexacrobylus), contains 12 nominal species, six of which are most probably synonyms of widely distributed Asajirus indicus.

Physical characteristics

All species of sorberaceans are solitary. Body shape is rather constant, Asajirus species almost always have an oval or eggshaped body, and Oligotrema species are usually slightly more elongated and the siphons are situated on opposite ends of the body and directed away from each other. The test, a protective layer secreted by epithelium, is covered by short hair-like processes with adhering sand, mud particles, or, more often, tests of foraminiferans. The cloacal siphon is always small and inconspicuous. The branchial siphon is large, situated on the anterior end or on a side of the body, and surrounded by six large lobes. The opening of the branchial siphon leads to the buccal cavity lined with the test; the buccal cavity is homologous to the branchial siphon of other ascidians. As in all tunicates, The pharynx is perforated, but unlike in other ascidians, is small and has only a few openings leading to a system of thin-walled chambers that open into the atrial cavity by ciliated stigmata. Some species have a very large globular stomach, which occupies most of the internal space of the animal. All species have a kidney, a large thin-walled excretory organ, similar to the kidney of ascidians in the family Molgulidae (suggesting a possible relationship with this family).

All Hexacrobylidae are relatively small, from less than 1 in (3 mm) up to 2.4 in (6 cm), but usually they are usually no more than 1.4 in (1 cm) in greatest dimension.

Distribution

Sorberaceans are widely distributed in bathyal and abyssal depths in all oceans, but records are not numerous. They have not been recorded in the high Arctic and North Pacific Oceans, but deepwater tunicates in these regions have been less intensively sampled.

Habitat

Most specimens of sorberaceans were collected in deep waters at 3,280–16,400 ft (1,000–5,000 m). However, in 1903 a single specimen of Oligotrema psammites was recorded at 308 ft (94 m), and in 1990 another specimen of the same species was recorded at 886 ft (270 m). The deepest record is of a single specimen of Asajirus indicus at 25,853 ft (7,880 m); only four other tunicates have been recorded at such great depths. The bottom at abyssal depths is almost always soft and covered by thick layer of mud, sorberaceans live unattached on the mud.

Behavior

Sorberaceans do not survive capture, and no one has directly observed their behavior. Although they are not firmly attached to the substratum, it is unlikely that they are mobile, as their body covered by hairs with attached sediments suggests that they are anchored in the mud and cannot actively move. Their muscular branchial siphon has fingerlike lobes on the end, suggesting that it can operate as a hand to capture small invertebrates.

Feeding ecology and diet

Unlike most other benthic tunicates sorberaceans are carnivorous and not filter feeders. The specimens examined had an almost-empty digestive tract, so they are certainly not detritus-feeders, otherwise the digestive tract would be full of sediments. Possibly they can actively search and capture small moving invertebrates using their muscular (and sometimes eversible) branchial siphon. The large globular stomachs of some species suggest that the prey may be relatively large, and small copepods, isopods, ostracods, nematodes, and polychaetes were found in the stomachs of some specimens.

Reproductive biology

Almost nothing is known about sorberacean reproduction. As with all other benthic tunicates, they are hermaphrodites. All species are apparently oviparous, and larvae are not known. The youngest recorded specimens, 0.02 in (0.5 mm) in diameter, already resembled adult forms.

Conservation status

No species of Sorberacea are listed by the IUCN.

Significance to humans

None known.

Species accounts

List of Species

No common name

Asajirus indicus

family

Hexacrobylidae

taxonomy

Hexacrobylus indicus Oka, 1913, Sri Lanka, Indian Ocean.

other common names

None known.

physical characteristics

Body oval, not usually exceeding 0.8 in (2 cm) high. Atrial siphon relatively long, but it is internal, and thus can be seen only on dissected specimens after test is removed. On surface of test, only small inconspicuous atrial opening is present on top of body. Branchial siphon is large and muscular. On preserved specimens, siphon is internal, lying wholly within the body, but it is not known whether this is normal in living specimens. Branchial siphon opens on anterior part of the body by wide slit surrounded by six wide lobes. Body coated by fine short hairs that are significantly longer on the posterior end of body, suggesting specimens sit vertically at the bottom.

distribution

All oceans except central Arctic and North and East Pacific Oceans.

habitat

Lives unattached on soft muddy bottom of continental slopes and abyssal plains and trenches; recorded from 1,970 to 24,540 ft (600 to 7,480 m) deep.

behavior

Nothing is known.

feeding ecology and diet

Carnivorous, including small invertebrates.

reproductive biology

Nothing is known.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Oligotrema sandersi

family

Hexacrobylidae

taxonomy

Gasterascidia sandersi Monniot and Monniot, 1968, northwest Atlantic Ocean.

other common names

None known.

physical characteristics

Small, not exceeding 0.2 in (5 mm). Oval body covered by sparse hairs with attached mud particles and foraminiferan tests. Two prominent siphons on opposite sides of body. Highly muscular, completely eversible branchial siphon longer than atrial siphon, has six fingerlike lobes. Stomach very large, occupies most of body.

distribution

Widely distributed in Atlantic Ocean, not recorded in other oceans.

habitat

Abyssal; recorded at about 4,920–16,400 ft (1,500–5,000 m) deep.

behavior

Nothing is known.

feeding ecology and diet

Carnivorous, including small invertebrates.

reproductive biology

Nothing is known.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Resources

Periodicals

Monniot, C., and F. Monniot. "Nouvelles Sorberacea (Tunicata) profondes de l'Atlantique Sud et l'Océan Indien." Cahiers de Biologie Marine 25 (1984): 197–215.

——. "Revision of the Class Sorberacea (Benthic Tunicates) with Descriptions of Seven New Species." Zoological Journal of the Linnean Society 99, no. 3 (1990): 239–290.

Monniot, C., F. Monniot, and F. Gaill. "Les Sorberacea: une nouvelle classe de tuniciers." Archives de Zoologie expérimentale et générale 116, no. 1 (1975): 77–122.

Kott, P. "The Australian Ascidiacea." Memoirs of the Queensland Museum 32, no. 2 (1992): 621–655.

——. "The Family Hexacrobylidae Seeliger, 1906 (Ascidiacea, Tunicata)." Memoirs of the Queensland Museum 27, no. 2 (1989): 517–534.

Karen Sanamyan, PhD